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Cite this: Chem. Soc. Rev., 2022,
51, 9262

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Rare-earth based tetrapyrrolic sandwiches:
chemistry, materials and applications
Alexander G. Martynov, a Yoji Horii, b Keiichi Katoh, c Yongzhong Bian,
Jianzhuang Jiang, *de Masahiro Yamashita*f and Yulia G. Gorbunova *agde

The unique properties of natural tetrapyrrolic compounds have inspired the rapid growth of research interest in thedesign and synthesis of artificial porphyrinoids and their metal complexes as a basis of modern functional materials.
A special role in the design of such materials is played by sandwich complexes formed by tetrapyrrolic macrocycles
with rare earth elements, especially lanthanides. The development of synthetic approaches to the functionalizationof tetrapyrrolic compounds and their rare earth complexes has facilitated the intensive development of new
applications over the last decade. As a way of expanding the functionalities of rare earth complexes, sophisticatedexamples have been obtained, including mixed-ligand complexes, p-extended analogues, covalently linked andfused sandwiches, complexes with less-common tetrapyrrols, sandwiches with non-tetrapyrrolic macrocycles andReceived 5th July 2022
DOI: 10.1039/d2cs00559j

even complexes containing up to six stacked ligands. This review intends to oﬀer a general overview of the
preparation of such sophisticated REE tetrapyrrolic sandwiches over the last decade as well as emphasizes thecurrent challenges and perspectives of their application in areas such as single-molecule magnetism (SMM), organic

rsc.li/chem-soc-rev

field-eﬀect transistors (OFET), conductive materials and nonlinear optics (NLO).

aA.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 119071 Leninskiy pr., 31, bldg.4, Moscow, Russia
Department of Chemistry, Faculty of Science, Nara Women’s University, Nara 630-8506, Japanc
Department of Chemistry, Graduate School of Science, Josai University, 1-1 Keyakidai, Sakado, Saitama 350-0295, Japand
Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, School of Chemistry and BiologicalEngineering, University of Science and Technology Beijing, Beijing, Chinae
Daxing Research Institute, and Beijing Advanced Innovation Center for Materials Genome Engineering, University of Science and Technology Beijing, Beijing, China.
E-mail: jianzhuang@ustb.edu.cnfDepartment of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-Ku, Sendai 980-8578, Japan.
E-mail: yamasita@agnus.chem.tohoku.ac.jp
gN.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 119991 Leninskiy pr., 31, Moscow, Russia. E-mail: yulia@igic.ras.rubAlexander G. Martynov graduatedfrom the Higher Chemical Collegeof the Russian Academy of Sciencesin 2006. He received his PhD and
Dr Hab. degrees in 2008 and 2020.
In 2022 he was awarded the
Moscow government prize foryoung scientists. In 2022 hebecame a Professor of the Russian
Academy of Sciences. Currently heis a leading researcher at the
Frumkin Institute of Physical
Chemistry and Electrochemistry
Alexander G. Martynov
RAS. His research interests are in
the areas of spectroscopy of tetrapyrrolic macrocycles and tetrapyrrolbased materials with tuneable properties including REE sandwich
complexes for magnetic applications, agents for photodynamic
therapy and carbene transfer catalysts.

9262 | Chem. Soc. Rev., 2022, 51, 9262–9339

Yoji Horii was born in Gumma,
Japan in 1989. He received his
Bachelor’s (2012), Master’s (2014)
and PhD (2017) degrees at Tohoku
University under the supervision ofProfessor Masahiro Yamashita. He
worked on magnetism and electronic states of phthalocyaninatolanthanide single-molecule magnets. He moved to Osaka
University as the postdoctoral
fellow in April 2017. In 2019, hemoved to Tohoku University as a
Yoji Horii
postdoctoral fellow. Since April
2020, he has been working as an assistant professor at Nara
Women’s University. His current research interests include molecularmagnetism, two dimensional nanomaterials and soft materials.

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1. Introduction
Just as it is diﬃcult to overestimate the role of natural porphyrins in the functioning of living beings, it is also diﬃcult todisagree with the fact that synthetic porphyrinoids have greatly
contributed to the development of modern materials science.
In particular, this has been made possible by a variety of unique

Chem Soc Rev
photophysical, electrochemical, catalytic, magnetic, and otherproperties provided by both the tetrapyrroles themselves andthe metal ions placed in their cavities.
Among various coordination compounds formed by tetrapyrrole macrocycles, the complexes with rare earth elements(REE) attract special attention.1–6 Due to large ionic radii andhigh coordination numbers, these elements form sandwich

Keiichi Katoh is currently an
associate professor in the
Department of Chemistry of
Josai University where he isleading research activities ininorganic
chemistryandmagnetism. He received his PhD
in 2003 from the Graduate
University for Advanced Studies(Institute for Molecular Science).
After being a post-doctoral fellow
at Hokkaido University (ResearchInstitute for Electronic Science)
Keiichi Katoh
until 2004, he worked as a postdoctoral fellow at Kyoto University (Institute for Chemical
Research) until 2008, and he joined the Prof. Yamashita group as
a post-doctoral fellow at Tohoku University in 2008. From 2009 to
2020, he was appointed to an assistant professor in the Departmentof Chemistry of Tohoku University. He works on the development ofsolid state spin systems based on molecular magnetic materials.

Yongzhong Bian was born inShanxi, China. He received his
BSc (1998) and PhD (2005)(with Prof. Jiangzhuang Jiang)
from Shandong University. He
carried out a JSPS postdoctoral
fellowship (2005–2007) at the
University of Tokyo with Prof.
Takuzo Aida. He became an
associate professor at Shandong
University in 2007, and moved to
the University of Science andTechnology Beijing as a full proYongzhong Bian
fessor in 2009. His current
research centers on porphyrin chemistry, encompasses applications
in Fluorescent chemosensing and bioimaging, photodynamic therapy, photochemical catalysis and molecular recognition.

Jianzhuang Jiang was born in
Heilongjiang, China. He received
his BSc (1985), MSc (1988), andPhD (1993) (with Tsinglien
Chang)
fromthe
PekingUniversity. During his doctoral
study (1990–1992), he obtained
a Fellowship from the Ministry of
Culture, Science, and Sport ofJapan and carried out his PhD
work at the Osaka University
under the guidance of Kenichi
Machida and Ginya Adachi. He
Jianzhuang Jiang
became a Postdoctoral Fellow at
Peking University with Tsinglien Chang (1993–1994), a Visiting
Scholar at The Chinese University of Hong Kong with Dennis K. P.
Ng and Thomas C. W. Mak (1995–1996), and a Postdoctoral Fellow
at the Queensland University of Technology with Dennis P. Arnold(1998–2000). He joined Shandong University and the University of
Science and Technology Beijing in 1996 and 2008, respectively. He
has been working on porphyrin- and phthalocyanine-basedmaterials for more than thirty years to develop new singlemolecule magnets, organic field-effect transistors, opticalmaterials, energy materials, and porous materials.

Masahiro Yamashita received his
BSc degree in 1977, MSc in 1979,and DSc in 1982 from Kyushu
University. After his graduation,
he joined the Institute for
Molecular Science (IMS). In
1983, he was appointed as an
Assistant Professor at Kyushu
University. In 1988, he wasappointed as an Associate
Professor at Nagoya University,and he was promoted to a fullProfessor at the same university
Masahiro Yamashita
in 1998. He was a full Professor
at Tokyo Metropolitan University from 2000 to 2004. He moved to
Tohoku University as a full professor in 2005. He has been honoredwith the Inoue Scientific Award (2002), the Chemical Society ofJapan Award for Creative Work (2005), the Award of Japan Societyof Coordination Chemistry (2014), Mukai Award (2019), and theChemical Society of Japan Award (2020). He is now an Associate
Member of the Science Council of Japan. He is a Fellow of the Royal
Society of Chemistry (FRSC).

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Fig. 1 Double- and triple-decker sandwich complexes formed by REE
ions with tetrapyrrolic macrocycles, X = C-Ar, C-Alk, and N.

double- and triple-decker complexes with porphyrins (Por),phthalocyanines (Pc) and related macrocycles (Fig. 1), which,with some exceptions, are not observed for the other metals of
the periodic table. Tightly coupled p-systems of stacked aromatic macrocycles provide these sandwiches with unique optical and electrochemical properties, and the presence of
4f-electron shells in paramagnetic lanthanide (Ln) metal centres expands the area of their application to the actively developing and promising fields of molecular magnetism andspintronics.7–11
Interestingly, the somewhat detective chemistry of sandwich complexes with tetrapyrrolic ligands started almost

Yulia G. Gorbunova is a Fullmember of Russian Academy of
Sciences (2022) and main
researcher at the Russian
Academy of Sciences (KurnakovInstituteofGeneralandInorganic Chemistry RAS (IGIC
RAS) and Frumkin Institute ofPhysicalChemistry
andElectrochemistry
RAS).
She
graduated in Chemistry from theLomonosov
Moscow
State
University
andreceived
her
PhD
Yulia G. Gorbunova(1995) and habilitation (2006)
from IGIC RAS. Her current research interests are related to thechemistry of tetrapyrrolic compounds as the basis of functional
materials, including molecular switchers, nonlinear optics,
electrochromic materials, SMM, sensors and hybrid organoinorganic nanomaterials. She was a recipient of the Award ofEuropean Academy of Science for young researcher (2001), theRussian Government prize in the field of science and techniques(2002) and the Chugaev Award in coordination chemistry (2010).
In 2016 Prof. Gorbunova was awarded by order ‘‘Chevalier dansl’Ordre national des Palmes Academiques’’ by the Government ofFrance.

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Review Article
simultaneously with rise of phthalocyanines, although the firstelement to form double-decker complexes – tin(IV) – did notbelong to rare earth elements. The synthesis of its bisphthalocyaninate, Sn(Pc)2, was reported in 1936 by Linstead et al.12 The
authors used the reaction of (Pc)SnCl2 and Na2(Pc), and thisraise-by-one-story approach much later became the main way tovarious heteroleptic and mixed ligand sandwiches.
Much later, in 1965 Kirin and Moskalev et al. reported
seminal work on the template reaction between REE acetatesand melted phthalonitrile.13 The authors isolated surprisingly
stable green and blue complexes, where metal ions were
sandwiched between two Pc ligands – M(Pc)2; however, at thattime the details of their electronic structure and the nature of
the intriguing redox behaviour remained unclear. In particular,
thin films formed by bisphthalocyaninates were able to reversibly change colour when a potential was applied or when thesefilms were treated with volatile oxidants and reducing agents.
The electrochromic behaviour of bisphthalocyaninates was
explained in 1979 by Corker et al.14 who used UV-Vis andEPR spectra to show that M(Pc)2 undergoes ligand-centred
redox-triggered interconversion between three forms oftrivalent metal complexes – anionic [M3+(Pc2)2], neutral[(Pc )M3+(Pc2)]0 and cationic [M3+(Pc)2]+ forms. Due to the
pronounced difference in spectral signatures these three forms
possessed different colours, and for many years they were
known as blue, green and red forms, respectively. Apart fromchanges in UV-Vis spectra, switching between redox statescaused modulation of NIR absorbance, and the nature of
NIR bands of neutral green complexes, has been discussed in
the works of Simon et al.,15 Aroca et al.,16 Tomilova andLukyanets et al.17
Importantly, under harsh conditions of template condensation, apart from double-decker complexes, REE ions can also
form blue triple-decker sandwiches M2(Pc)3,18,19 where Pc
ligands are isoelectronic with those in blue forms of bisphthalocyaninates. Because of the similarity of their optical properties and lack of available mass-spectral characterization thesecomplexes were sometimes mistaken for each other. These
contradictions have been eliminated by single-crystal X-ray
assignment performed in 1980 for neodymium bisphthalocyaninate, Nd(Pc)2 by Kasuga et al.,20 and in 1999 for dilutetium tris-(tetra-15-crown-5-phthalocyaninate), Lu2[(15C5)4Pc]3by Troyanov et al.21
The survey for milder conditions where double- and tripledeckers could be selectively obtained, aﬀorded several convenient protocols, which are commonly used in synthetic practice
nowadays. These protocols include template condensation ofphthalonitriles and REE salts in high boiling alcohols in thepresence of DBU as a phthalonitrile activator as suggested by
De Cian et al.,22 and also direct reactions of pre-synthesized
phthalocyanine ligands with metal salts as suggested by Takahashi et al.23,24 While the former method yields double-decker
sandwiches, the latter method furnishes both double- andtriple-decker complexes, and later studies showed that the
selectivity of complex formation can be governed either by
the presence or absence of DBU, respectively.25,26

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In the case of phthalocyanines both template and direct
methods are equally applicable for the synthesis of REE sandwiches, however, in the case of porphyrins only direct method
can be used to synthesize double- and triple-decker complexes.
First example of homoleptic REE bisporphyrinates was reported
in 1977 by Buchler et al. with the example of quadrivalentcerium complex Ce(TTP)2 formed upon the reaction ofCe(acac)3 with tetra-p-tolylporphyrin, H2(TTP) in refluxing
1,2,4-trichlorobenzene (TClB).27 The authors noticed a remarkable analogy between the structure and the redox-properties ofthis complex and the ‘‘special pair’’ formed by chlorophyl
molecules in the photosynthetic systems of green plants.28
Later more double- and triple-decker porphyrinates were isolated and characterized,29–31 and it was found that porphyrin
ligands tended to form sandwiches only with early and middle
lanthanides, while Pc ligands could form sandwiches with all
REE ions, including the relatively small Sc3+ ions.32,33
Finally, REE complexes provided the marriage of porphyrins
and phthalocyanines in mixed ligand sandwiches, where bothtypes of ligands are combined and strongly coupled due tointramolecular orbital interactions providing them with peculiar optical properties of panchromatic absorbers. Synthesis
and crystallographic characterization of the first example ofsuch complexes were reported in 1986 by Moussavi et al. withthe example of mixed ligand triple-decker complexes with tetrap-anisylporphyrin (Pc)M(Pc)M(TAnP), where M = Nd, Eu, and
Gd.34
The rapid development of the chemistry of sandwich REE
complexes was driven both by advances in spectroscopy andphysicochemical characterization of these challenging objects
and breakthrough discoveries which aﬀected many areas ofmodern chemistry. Among them we should inevitably mention
the first organic semiconductor – lutetium bisphthalocyaninate, Lu(Pc)2, described for the first time in 1987 by Turek
et al.,35 and especially the first example of a 4f single moleculemagnet – terbium(III) bisphthalocyaninate, Tb(Pc)2, reported in2003 by Ishikawa et al.36
Because of this, the largely fundamental interest in thesecompounds was gradually replaced by a focus on appliedresearch where archetypal double- and triple-decker complexes
M(Pc)2 and M2(Pc)3 coexist with more sophisticated compounds, including mixed-ligand complexes, p-extended analogues, covalently linked and fused sandwiches, complexes with
less-common tetrapyrrols, sandwiches with non-tetrapyrrolicmacrocycles and even complexes containing up to six stacked
ligands obtained by coordination or supramolecular bonding.
Moreover, these complexes have been recognized as molecularswitches whose properties can be altered by the reversible selfassembly processes, protonation, redox reactions, etc., opening
the way to the creation of ‘‘smart’’ materials.
These sophisticated complexes began to attract the mostattention over the last decade; thus, the aim of the present
review is mainly to summarize the advances in the chemistry of
REE tetrapyrrolic sandwiches over this period to supplement
the previously published comprehensive reviews1–6 with new
data. Another goal of this review is to emphasize the current

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challenges and perspectives of sandwich complex applicationin various fields such as single-molecule magnetism (SMM),
organic field-effect transistors (OFET) and conductive materials, and nonlinear optics (NLO).

2. Advances in the synthesis offunctionalized multiple-decker
sandwiches
2.1 New homo- and heteroleptic double- and triple-decker
complexes bearing symmetrically substituted tetrapyrolicligands
From the synthetic viewpoint, sandwich complexes with symmetrically substituted tetrapyrrolic ligands are the most available objects, particularly due to facile synthesis and separation
procedures. The following section will include various classesof homo- and heteroleptic sandwiches with symmetricalligands. In the latter case, ligands belong to the same classesof tetrapyrrolic macrocycles.
2.1.1 Aryloxy- and aryl-substituted sandwich phthalo- andnaphthalocyaninates. Over the last decade among other sandwich complexes with symmetrical ligands special attention was
paid to aryloxy-substituted phthalocyaninates, particularly due
to high solubility, which contributes to the processing of thinfilm materials and devices. Aryloxy-groups reveal electronwithdrawing behaviour,37 making their reduction easier andtheir oxidation more diﬃcult. Finally, these substituents participate in the molecular packing of complexes governing conductivity and other useful properties of materials basedon them.
For example, Kong et al. used template condensation of 4,5di(a-naphthoxy)phthalonitrile with Eu(acac)3 and DBU in
refluxing n-PentOH to make a double-decker complex
Eu[(NaphO)8Pc]2 (Scheme 1) which was used as a componentof a flexible ambipolar OFET,38 and it also revealed sensing
properties towards NO2 and NH3 (cf. Section 4).39 To synthesize
a triple-decker complex Eu2[(NaphO)8Pc]3, raise-by-one-story
approach was applied, namely octa-naphthyloxy-phthalocyanine was condensed with the double-decker complex(Scheme 1).40 The resulting trisphthalocyaninate could form
self-assembled microstructures whose morphology and n-/pmobilities could be controlled by solvent–vapour annealing
procedure (cf. Section 4.2). Similar approaches were used tosynthesize various examples of other aryloxy-substituted sandwiches, including heteroleptic complexes, which were designed
as high-performance air-stable ambipolar organic thin filmtransistors,41–44 optical limiters45–48 and single moleculemagnets.49
Phenyl-substituted phthalo- and naphthalocyanines
were used by Dubinina et al. for the synthesis of REE
complexes.50,51 The selectivity of complex formation upon
direct metalation of these ligands depended on conditions
aﬀording either monophthalo- and naphthalocyaninates or
sandwich complexes (Scheme 2). While double-decker
complexes were obtained for both types of macrocycles,

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Scheme 1

Synthesis of a-naphthyloxy-substituted Eu(III) bis- and trisphthalocyaninates.38,40

Scheme 2

Synthesis of lanthanide complexes with phenyl-substituted phthalo- and naphthalocyanines.50,51

triple-decker complexes could be obtained by only using phthalocyanine ligands.
Single-crystal XRD studies of Lu(Ph8Pc)2 revealed intramolecular edge-to-face p–p stacking interactions between peripheral phenyl groups, reinforcing the overall intramolecularinteractions between Pc ligands (Fig. 2).50 In turn, it resulted ina strong hypsochromic shift of NIR bands in UV-Vis-NIR spectraof bisphthalocyaninates – for example the intervalence band
shifted from 1575 nm in the spectrum of Tb(tBu4Pc)2 to
1433 nm in the spectrum of Tb(Ph8Pc)2. In contrast, benzoannelation going from phthalo- to naphthalocyaninates caused
bathochromic shifts of intervalence bands by ca. 200 nm.

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Measurements of conductivity of thin drop-casting films
formed by phenyl-substituted terbium(III) phthalocyaninates
showed that in the series of mono-, bis- and trisphthalocyaninates the complex Tb(Ph8Pc)2 had the largest conductivity of
5  1010 S cm1, which is however much smaller than thevalues obtained for unsubstituted double-deckers.51 Optical
microscopy data showed that such a low conductivity value
was partially caused by film inhomogeneity. AFM studies offilms, formed by triple-decker complex Tb2(Ph8Pc)3 showedthat it forms ring-shaped nanostructures – nano-coffee rings,
which appear due to an increase in the surface tension of thesolutions of this complex in comparison with mono- andThis journal is © The Royal Society of Chemistry 2022

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Fig. 2 Top view of the X-ray structure of Lu(Ph8Pc)2 (CCDC PIXGUA).
Dark blue lines show edge-to-face p-stacking interactions.50

bisphthalocyaninates. This feature can be used for nanopatterning of various substrates.
2.1.2 Crown-substituted sandwich phthalocyaninates.
Crown-phthalocyanines still attract continuing interest, although
they are known since late 1980s,52,53 and lutetium(III) bis(tetra-15crown-5)phthalocyaninate, Lu[(15C5)4Pc], was actively studied in
1990s as a potential component of ionoelectronic materials.54,55
In subsequent years, homoleptic crown-substituted doubledeckers were synthesized by Gorbunova et al. for almost all rareearth elements including the smallest Sc3+ ions.56,57 The availability of this series allowed the smooth changes of major
physical-chemical properties following f-contraction to berevealed,58 although discrepancy was observed in the case of
cerium complexes which can exist in tri- and quadrivalent statesof metal centres.59
Attention to crown-substituted sandwiches was attracted
mainly in the context of fabrication of electronic devices dueto their ability to form thin films with well-definedmorphology.58,60 While investigating the formation of ultrathin
Langmuir films by crown-substituted bisphthalocyaninates,
Selektor et al. discovered the phenomenon of orientationinduced redox isomerism in planar supramolecular systems
with Ce[(15C5)4Pc]2 as the example.61 Depending on thesurface pressure at the water/air interface the complex underwent valence tautomeric interconversion: Ce4+[L2]2 2[L2]Ce3+[L ]; moreover, films with diﬀerent redox states ofmetal centres could be transferred from water to solid surfaces
to characterize the valence state by various spectroscopic techniques. The phenomenon was also found to be general for mostof bisphthalocyaninates, which are formed by lanthanides
exhibiting di- and trivalent states – Sm, Eu, Er, Tm, andYb.62,63
Homonuclear trisphthalocyaninates of strongly paramagnetic lanthanides – Ln2[(15C5)4Pc]3, Ln = Tb, Dy, Ho, and Tm
were studied by variable-temperature NMR showing the possibility of their application as thermosensors in the physiologicaltemperature range (303–323 K)64,65 due to the sensitivity oflanthanide-induced shifts to temperature. The Tb(III) complex

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was found to have the highest sensitivity of this shift among thereported paramagnetic complexes of 4f and d elements with
polydentate ligands in organic media.
Several types of crown-substituted complexes were synthesized to study single-molecule magnetism,66,67 which is particularly interesting for this type of REE sandwiches since it canbe tuned by the interaction of crown-ether macrocycles withalkali metal cations. Such tuning was demonstrated for the firsttime by Horii et al.67 with heteroleptic terbium(III) complex[(15C5)4Pc]Tb(Pc) synthesized from (Pc)Tb(acac), 4 0 ,5 0 dicyanobenzo-15-crown-5 and DBU in hexanol as the example(cf. Section 3.3.1 for more details on SMM studies).
Alternative one-pot strategy towards this complex was
proposed by Martynov et al.68 It was based on the generationof (Pc)Tb(acac) by transmetalation of a lanthanum(III) complex
La(Pc)2 with Tb(acac)3nH2O and the reaction of this intermediate with tetra-15-crown-5-phthalocyanine, H2[(15C5)4Pc], in thepresence of DBU in refluxing 1-chloronaphthalene (ClN) and
OctOH mixture (Scheme 3a). The yield of the complex was 24%,and the presence of DBU was shown to be crucial for theformation of the double-decker complex – in the absence of
DBU the mixtures of homonuclear triple-decker complexes(Pc)Tb[(15C5)4Pc]Tb(Pc) and [(15C5)4Pc]Tb[(15C5)4Pc]Tb(Pc)were obtained.69,70
Original variant of the raise-by-one-story approach was
proposed to synthesize heteronuclear triple-decker complexes,[(15C5)4Pc]M*[(15C5)4Pc]M(Pc), using La(Pc)2 as a donor of
unsubstituted phthalocyaninate-dianion (Scheme 3b).71,72
Thus, its reaction with crown-substituted bisphthalocyaninates, M*[(15C5)4Pc]2 and M(acac)3nH2O, in refluxing ClN
resulted in remarkably rapid – 10–15 min – formation of thetarget heteronuclear complexes with a wide range of pairs of
dia- and paramagnetic metal centers (M a M* = Nd, Sm, Eu,
Gd–Yb, Y) in high yields. SMM properties of homonuclear diterbium(III) complex and heteronuclear terbium(III)/yttrium(III)complexes were studied and the effects of intramolecularferromagnetic interactions together with the impact of coordinating surrounding were revealed.66
A heteroleptic octopus-type Tb(III) bisphthalocyaninate bearing a 15-crown-substituted ligand and a ligand with eightthioacetate-terminated flexible tentacles was synthesized by
Shokurov et al. via the raise-by-one-story approach startingfrom tetrahydropyranyl-protected phthalocyanine and halfsandwich Tb(III) crown-phthalocyaninate followed by the cleavage of protective groups, the introduction of iodine atoms andtheir nucleophilic substitution with thioacetate residues(Scheme 4).73 The resulting octopus formed a self-assembled
monolayer (SAM) on gold with a face-to-face orientation of themolecules, serving as an anchor for further surface modification, namely the binding of additional crown phthalocyanine
molecules via potassium ion bridges. The enhanced redoxswitching ability of the bilayer thus formed was demonstrated(cf. Section 4.3).
2.1.3 Chiral sandwich phthlocyaninates. An interesting
example of aryloxy-substituted sandwiches was synthesized by
Zhou et al. via template condensation of phthalonitrile,

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Scheme 3 Transmetalation of La(Pc)2 in the synthesis of heteroleptic crown-substituted bisphthalocyaninates (a) and heteronuclear trisphthalocyaninates (b).66,68,71,72

Scheme 4

Synthesis of heteroleptic octopus-type Tb(III) bisphthalocyaninate.73

annelated with enantiomerically-pure R- or S-binaphthyloxy(BNPO) groups (Scheme 5a).74 Studies of the resulting

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complexes Y[(R-/S-BNPO)4Pc]2 using CD-spectroscopy revealed
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Scheme 5 Synthesis of chiral bisphthalocyaninates on the examples of Y[(BNPO)4Pc]2 (a, only S-enantiomer is shown),74 and menthyl-substitutedbisphthalocyaninates M[(MenthO)4Pc]2 (b).75

phthalocyanine chromophores, and the eﬃciency of this transfer was much more pronounced in comparison with anotherexample of chiral bisphthalocyaninates, M[(MenthO)4Pc]2,
bearing D- or L-menthyl groups (Scheme 5b).75 The latter complexes were prepared by Lv et al. via direct metalation of thecorresponding methyl-substituted phthalocyanines withM(acac)3nH2O (M = Eu, Lu, Y) in n-PentOH. The latter example
also emphasized the role of intramolecular p–p interaction
since the monomeric chiral ligand H2[(D-/L-MenthO)4Pc] wasCD-inactive.
Another naturally occurring chiral terpene, ()-myrtenal,
was used by Zheng et al. to synthesize the dicyanobenzoannelated pinene derivative which underwent template condensation to furnish double-decker complexes M[(pinene)4Pc]2,
M = Eu, Er, and Lu (Scheme 6).76 Introduction of bulky
conformationally rigid hydrocarbon substituents was madewith the aim to decrease both inter- and intramolecular interactions between Pc ligands. Such tuning of intermolecularinteractions decreased the aggregation of complexes andenhanced their film-forming ability. Decrease of intramolecularinteractions resulted in a significant red shift of the intervalence band in the NIR region in comparison with other doubledecker complexes, improving the electrochromic properties ofcompounds thus modified. Finally, chirality of complexes was
manifested in their CD-spectra.
Terbium(III) bisphthalocyaninate substituted with chiral (S)2-(dodecyloxy)propoxy groups was synthesized by Gonidec
et al. as a liquid crystalline material with tuneable magnetic

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Scheme 6 Key steps leading to pinene-annelated phthalonitrile andbisphthalocyaninates (DMAD – dimethyl acetylene dicarboxylate). Only
one isomer of double-decker complexes is shown.76

properties.77 This double-decker complex was synthesized by
the direct metalation of the corresponding phthalocyanine(Scheme 7) with lithium bis(trimethylsilyl)amide as a basic
reagent promoting complex formation. Similar to mentylsubstitued complexes, the metal-free ligand was CD-silent,
while the double-decker complex showed a pronounced CD
signal in the Q-band region. Also, the complex showed mesogenic behaviour and both crystalline and disordered states ofChem. Soc. Rev., 2022, 51, 9262–9339 | 9269

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Scheme 8 Synthesis of bisphthalocyaninates based on octa-(di-nbutylamino)phthalocyanine.81,82
Scheme 7 Synthesis of mesogenic terbium(III) bisphthalocyaninate, bearing (S)-2-(dodecyloxy)propoxy-substituents.77

the same specimen could be kinetically trapped upon measurements of SMM behaviour at low temperatures. Both structuralstates showed markedly diﬀerent magnetization dynamics
revealing important influence of the structural environmentof a single-molecule magnet taking advantage of its phasebehaviour.
2.1.4 Sandwich phthalocyaninates bearing strong electron
donors. High sensitivity of phthalocyanines to electronic eﬀects
of peripheral substituents allows the tuning of the properties of
their complexes, thus over the last decade extreme samples
have been reported, bearing particularly strong electron donors
or acceptors.78 Among the former, the introduction of di-nbutylamino-groups had a profound eﬀect on the electronic
structure of phthalocyanines – for example, octa-substituted
metal-free ligand H2[(Bu2N)8Pc] revealed a significant decreaseby 0.26–0.35 V for the HOMO–LUMO gap in comparison with
H2(Pc) due to the effective p–p conjugation between the peripheral nitrogen atoms and the Pc chromophore.79 This phthalocyanine was also used to synthesize various double-decker
complexes (Scheme 8). Its direct metalation with M(acac)3
nH2O, in the TClB/OctOH (5 : 1 v/v) mixture yielded doubledecker complexes in mediocre yields.80
The complex Tb[(Bu2N)8Pc]2 revealed excellent SMM behaviour which could be further improved by the synthesis of aheteroleptic complex [(Bu2N)8Pc]Tb(Pc) (cf. Section 3.2.2).81
This complex was synthesized by the raise-by-one-story
approach involving the metalation of H2[(Bu2N)8Pc] withTb(acac)nH2O in refluxing OctOH and in situ reaction of thegenerated Tb(III) monophthalocyaninate with Li2(Pc). In thesame manner, erbium(III) complex [(Bu2N)8Pc]Er(Pc) was alsosynthesized and its anionic form was a first sandwichtype phthalocyanine-based erbium(III) compound with fieldinduced slow relaxation.82

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Chiral heteroleptic complex was synthesized in a similarmanner by the reaction of [(Bu2N)8Pc]Tb(acac) with
H2[(BNPO)4Pc],83 and it also revealed excellent SMM characteristics. Such superior SMM behaviour of amino-substitutedcomplexes was explained using DFT calculations in terms of
the electrostatic eﬀects of electron-rich ligands.
2.1.5 Sandwich phthalocyaninates bearing strong electron
acceptors. Apart from spectacular red–green–blue colourchange upon oxidation and reduction of sandwich Pcs rendering them as components of electrochromic materials, switching
between redox states of sandwiches can be used to tune theirmagnetic properties.84,85 Thus, introduction of strong electron
acceptors into double-decker complexes was used to stabilize
their reduced forms and study their SMM performance. Starting from a series of dicyanophthalimides Gonidec et al. synthesized electron-deficient double-decker complexes stabilized intheir anionic forms Tb[(R-imide)4Pc]2TBA+ (Scheme 9).86 Their
SMM properties were virtually identical to those of the parent
Tb(Pc)2TBA+ complex. However, while the anionic forms of
unsubstituted double-decker can be readily oxidized, often not
on purpose, the imide-substituted compounds were stable in
the anionic forms in a wide range of potentials, rendering themas components of hybrid magnetic materials without undesired
charge transfer interactions between the substrates and SMMs.
Another extremely electron-deficient Tb(III) bis(octaisopropyl)phthalocyaninate where all hydrogen atoms were
replaced with fluorine atoms was studied by Gonidec et al.87
It could be electrochemically reduced to di- and trianionicforms, and magnetic characteristics of these forms were studied by low-temperature MCD (cf. Section 3.4).
2.1.6 Halogenated sandwich phthalocyaninates. Several
examples of halogenated sandwich complexes were synthesizedby template condensation of the corresponding phthalonitriles
in the presence of metal salts. For example, the short-run
reaction of 4-iodophthalonitrile with lanthanide acetates inThis journal is © The Royal Society of Chemistry 2022

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Scheme 9

Chem Soc Rev

Synthesis of the imide substituted double-decker complexes.86

melt at 380 1C for 5–6 min yielded a series of iodinatedcomplexes M(I4Pc)2, M = Sm-Dy in 13–25% yields; the abilityof complexes to absorb gaseous NH3 was studied by -Senocak
et al.88 The yield of Tb(I4Pc)2 could be improved to 42% when 4iodophthalonitrile was condensed in the presence of Tb(acac)3
nH2O and DBU in refluxing PentOH. Under the same conditions, condensation of this phthalonitrile with terbium(III)monophthalocyaninate [(tBuPhO)8Pc]Tb(acac) yielded the heteroleptic complex (I4Pc)Tb[(tBuPhP)8Pc]. Functionalization ofiodinated sandwich using the Sonogashira reaction with 4pentyn-1-ol was also studied.89
Neodymium(III) bis(octachlorophthalocyaninate), Nd(Cl8Pc)2,
was synthesized by Kuzmina et al. via two alternative strategies.
Template condensation of 4,5-dichlorophthalonitrile with
Nd(OAc)3H2O and DBU in refluxing PentOH yielded only 10%of the target complex; however, its yield could be improved to 87%
using the raise-by-one-story reaction between H2(Cl8Pc) and(Cl8Pc)Nd(OAc) and MeOLi in a refluxing mixture of trichlorobenzene and cetyl alcohol. This sandwich complex revealed improved
optical limiting behaviour in comparison with nonhalogenatedderivatives due to the heavy atom eﬀect, although halogenatedmonophthalocyaninates turned out to be better optical limiters
in comparison with sandwich complexes.90 Interestingly, 4,5dibromophthalonitrile did not yield any traces of double-decker
complexes under the conditions of template synthesis.91
2.1.7 Sandwich phthalocyaninates with substituents in
nonperipheral positions. All the aforementioned sandwiches
contained substituents in the peripheral positions of the Pc
rings, while nonperipherally-substituted complexes still remain
relatively unexplored, although sterical hindrance caused byproximate nonperipheral substituents can have a profoundinfluence on the structure and properties of such complexes,
including particularly strong basicity of meso-nitrogen atoms
and near-IR absorbance.92
Sandwiches based on tetra-a-substituted Pc ligands werereported for the first time by Wang et al. in 2005 with tetra[a-(3-pentyloxy)]phthalocyaninates, M[(a-Et2CHO)4Pc]2, M = Eu,
Lu, and Y as examples (Fig. 3a).93 These complexes wereThis journal is © The Royal Society of Chemistry 2022

Fig. 3 Homoleptic nonperipherally-substituted bisphthalocyaninates: (a)C4h isomer of M[(a-C5H11O)4Pc]2 and X-ray structure of the Y(III) complex(CCDC NAZTEO),93 and (b) meso-protonated complex HM[(a-BuO)8Pc]2
with the X-ray structure of the Eu(III) complex (CCDC XUXCOJ01).95
Substituents in the X-ray structures are truncated to oxygen atoms;
hydrogen atoms are omitted.

synthesized in a straightforward manner by the reaction of
metal-free phthalocyanine with the corresponding M(acac)3
nH2O in refluxing OctOH, and the major C4h isomer of
europium(III) complex was characterized using XRD, whichshowed its elegant pinwheel-like structure. SMM behaviour of
terbium(III) complex was also studied, showing the clear dependence of the magnetic properties of the complex on its redox
state (cf. Section 3.4).94
The synthesis of complexes with octa-a-substituted ligands(Fig. 3b) turned out to be more challenging. In 2010 Gao et al.
showed that the reaction of H2[(a-BuO)8Pc] with M(acac)3nH2O
and DBU in OctOH did not yield any traces of sandwich
complexes; however, it was found that crown-ethers catalyse
this reaction, and the best yield of the complex HM[(aBuO)8Pc]2 – 36.5% – was obtained in the presence of dibenzo18-crown-6.95 Crystallographic characterization of this complex
revealed severe distortion of Pc ligands because of the stericalhindrance, caused by the eight proximate butoxy-groups as wellas the presence of a proton at one of meso-nitrogen atoms.
Localization of this proton in solution was shown by Damjanović et al. using the 1H-NMR study of the paramagnetic
terbium(III) complex HTb[(a-BuO)8Pc]2,96 and the impact ofprotonation on the SMM behaviour of this complex was also
demonstrated by Horii et al. (cf. Section 3.5).97
2.2 Sandwich complexes with low-symmetry phthalocyanine
ligands
Lowering the symmetry of phthalocyanines is a powerful tool totune their physical-chemical properties,98 which is particularly

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important in the field of nonlinear optics as well as in the
fabrication of hybrid materials, where it allows the introductionof anchor groups which aﬀord immobilization of complexes on
various supports.
2.2.1 Rational template synthesis of A7B-type bisphthalocyaninates. While low-symmetry phthalocyanines are typicallysynthesized by template cross-condensation of two diﬀerent
phthalonitriles, this approach has only limited applicability inthe case of sandwich complexes since such condensation leads
to enormous number of various products; however, it still
can be applied when only the A7B scaﬀold is required.
This was showed for the first time with A7B-type complex[(15C5)Pc]Lu(Pc) as the example, which was synthesized in asmall yield of 3% by cross-condensation of unsubstituted and
15-crown-5-substituted phthlonitriles.99
This approach was optimized by Alpugan et al. with theexample of europium(III)-templated cross-condensation of 4,5di(n-hexylthio)phthalonitrile as component A with phthalonitriles, bearing one or two 5-hydroxypentylthio-groups as component B (Scheme 10).100 Using the stoichiometric ratio of
precursors yielded a mixture of six products with laborious
separation issues. However, due to similar reactivities of bothcomponents, the distribution of products is expected to beclose to statistical, thus increasing the ratio of A : B the yield of
products, containing more than one B unit can be decreased,
and the optimal ratio of 10 : 1 was selected to obtain two main
products – complexes Eu(A4)2 and (A4)Eu(A3B), whose separation was facilitated by increased polarity of the asymmetriccompound. Reproducible 10–12% yields were obtained for A7Btype complexes based on hydroxylated phthalonitrile. Further
functionalization of complexes was achieved by their mesylation and substitution of mesyl groups with azide as a possible
substrate for click-chemistry.
2.2.2 Direct synthesis of homoleptic complexes with lowsymmetry tetrapyrrolic ligands. Application of template reactions for the preparative synthesis of sandwich

phthalocyaninates with more than one low-symmetry ligand
is prohibitive, thus their synthesis is performed by the direct
metalation of low-symmetry ligands.
Direct metalation of A3B-type ligand was attempted by
Oluwole et al. for the synthesis of homoleptic europium(III)bis- and trisphthalocyaninates bearing diethyleneglycol chainswith terminal OH groups (Scheme 11).101 Unexpectedly, direct
metalation of this ligand with Eu(OAc)3 in refluxing OctOH
neither in the absence nor in the presence of DBU failed to
produce target sandwiches, likely due to side processes associated with the interaction of diethyleneglycol groups with Eu3+ions. However, protection of OH-groups with dihydropyran instarting phthalocyanine prior to complex formation allowedsmooth conversion of the protected derivative into doubledecker and triple-decker complexes, and the selectivity wascontrolled by the addition of DBU. Deprotection of THPOgroups yielded OH-substituted sandwiches, which were immobilized on the surface of quantum dots to give the firstexamples of hybrid optical limiters based on sandwich REE
complexes.
A similar necessity to introduce protective groups wasobserved by Pushkarev et al. with the example of metalationof the phthalocyanine ligand bearing one phenolic OH group

Scheme 10 Selective synthesis of a functionalized A7B-type europium(III)bisphthalocyaninate.100

Scheme 11 Synthesis of sandwich europium(III) complexes bearing
diethyleneglycol substituents with terminal OH-groups.101

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conjugated with the Pc system (Scheme 12a).102 Degradation of
the unprotected ligand was observed upon metalation witheuropium and lutetium acetylacetonates in high boiling solvents; however, the benzylated derivative smoothly produced
double-decker complexes where benzyl protections were
removed by treatment with sulphuric acid.
This approach allowed Pedrini et al. to synthesize homoleptic terbium(III) bisphthalocyaninate with A3B-type ligands
bearing undec-10-en-1-yloxy groups, which were used to obtaincovalent 1D architectures using diene metathesis (Scheme 12b).103
This reaction turned out to be concentration-dependent – theproduct of intramolecular cyclisation of two o-alkenyl chains was
obtained when metathesis was performed in dichloromethane
solution at 2.4 mM concentration. Increasing the concentration
by an order of magnitude afforded the mixture of oligomers
containing species from dimer to pentamer according to MALDITOF mass-spectrometry. Importantly, the magnetic properties ofthis mixture were only slightly modified after the formation of thebond between molecules, highlighting the potential of this reaction
scheme to form multinuclear magnetic assemblies.
Studies of metalation of covalently bonded dimeric phthalocyanines in the synthesis of sandwich complexes revealed
that this reaction can proceed either in the intra- or intermolecular mode leading to mononuclear complexes or oligomericassemblies (Scheme 13).
For example, clamshell-type diphthalocyanine H4[c-Pc2] withan o-xylyl bridge studied by Pushkarev et al. underwent mainly

Chem Soc Rev
intracavity metalation upon interaction with lanthanide acetylacetonates (Scheme 13a), although some oligomers were detectedby MALDI TOF mass-spectrometry.104 To form well-defined
dimeric bisphthalocyaninates the starting clamshell ligand canbe treated with lanthanide monophthalocyaninates.105,106 With aditerbium complex (tBu4Pc)Tb[c-Pc2]Tb(tBu4Pc) as the example
ferromagnetic intermetallic interactions were observed leading toelongation of magnetic relaxation times in comparison with themonomeric terbium complex Tb(tBu4Pc)2 (cf. Section 3.3.1).105
The balance of intra- vs. intermolecular complexation wasstudied by Birin et al. for butoxy-substituted diphthalocyaninewith 24-crown-8 bridge H4[(24C8)Pc2].107 Its interaction withM(acac)3, M = La, Ce, and Eu, and DBU in ClN yielded mixturesof mononuclear complexes and olygomers, and the yields ofmonomeric intercavity complexes (Scheme 13b) increased withthe decrease of the metal size, gaining 40% for the Ce(IV)complex. The latter complex revealed unusual solvatochromic
behaviour when going from chloroform to toluene, which was
not observed in the case of homoleptic symmetricalCe[(15C5)4Pc]2. Such behaviour was also studied by NMR
spectroscopy which revealed solvent-induced changing of sandwich conformation.
Intracavity complexes of cerium(IV) with diporphyrins withchiral BINOL bridges Ce[(BINOL-n)Por2], n = 1 or 2
(Scheme 13c), were synthesized by Lu et al. using metalationof the corresponding ligands with cerium(III) acetylacetonate intrichlorobenzene in the presence of metal lithium, which was

Scheme 12 Synthesis of sandwich complexes starting from hydroxy-substituted phthalocyanine ligands.102,103

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Scheme 13 Examples of sandwich complexes formed with dimeric tetrapyrrols: (a) intra-104 vs. intermolecular105,106 binding of lanthanides withclamshell-phthalocyanine H4[c-Pc2]; (b) 24-crown-8-interlocked bisphthalocyaninates M[(24C8)Pc2];107 and (c) binaphthol-strapped chiralbis(porphyrinato)cerium(IV) complexes.108

added to activate the porphyrin.108 CD measurements evidenced that (R)- and (S)-BINOL-strapped complexes formed
perfect mirror spectra with negative and positive signs in the
Soret band regions respectively. This observation suggests that
the C2-chirality of a BINOL strap is transcribed to the doubledecker core as a defined chiral twist in the inter-porphyrinarrangement. The intensity of signals also depended on thelength of the aliphatic bridge – (CH2CH2)n between BINOL and
CePor2 units, indicating the possibility to tune the efficiency of
the intramolecular chirality transfer.
Homoleptic complexes with low-symmetric ABAB-type
Pc ligands have been designed as NLO materials, bearing in
mind the concept of octupolar molecules, which can be

Scheme 14

approximated by a cube with alternating donor and acceptor
groups in the corners (Scheme 14). With this aim, doubledeckers were synthesized by direct metalation of the corresponding Pcs bearing either four hexylthio-groups –H2[(SHex)4Pc],109,110 or alternating pairs of a-PentO and b-Cl
substituents H2[(a-PentO)4Cl4Pc].111,112
Homoleptic terbium(III) bis-porphyrinate HTb(DADAPor)2
was synthesized by the reaction of metal-free tetra-mesosubstituted porphyrin bearing alternating donor- and acceptor
p-diphenylamino-phenyl and p-carboxymethylphenyl-ethynyl
groups (Scheme 15).113 Although X-ray structure of thiscomplex was not obtained, DFT calculations suggested that
intramolecular interactions between donor and acceptor

Synthesis of octupolar bisphthalocyaninates based on ABAB-type ligands.109–112

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Scheme 15 Synthesis of octupolar bisporphyrinate with ABAB-type
ligands.113

fragments should stabilize distorted cuboid conformation ofthis sandwich with a twist angle near 171. This structuralfeature provided the studied complex with the largest oﬀresonant first hyperpolarizability as evaluated by harmoniclight scattering measurements. Details on NLO studies ofsynthesized octupolar sandwiches are given in Section 5.1.
2.2.3 Heteroleptic complexes with low-symmetry phthalocyanine ligands. Heteroleptic complexes bearing only one lowsymmetry ligand are convenient components of covalent or
supramolecular assemblies with nanomaterials. Their synthesis is typically accomplished via the raise-by-one-story
approach, where monophthalocyaninate with one ligand reacts
either with another ligand or with phthalocyanine precursors.
For example, bisphthalocyaninates bearing one pyrenecontaining A3B ligand were synthesized by Klyatskaya et al.
starting from Li2(Pc) which was treated with M(acac)3 in ClN
generating (Pc)M(acac) species, M = Tb, Dy, and Ho.114,115 Theirfurther interaction with dilithium complex of pyrenylsubstituted phthalocyanine Li2[(Pyrene)Pc] yielded 20–24% of
the target heteroleptic complexes [(Pyrene)Pc]M(Pc), whichwere separated from homoleptic complexes by column chromatography (Scheme 16). Isolation of the heteroleptic tripledecker terbium(III) complex with one terminal unsubstituted

Scheme 16

Chem Soc Rev
ligand and two adjacent pyrenyl-substituted ligands was also
reported; the order of ligands was determined by collisioninduced dissociation using Fourier transform ion cyclotron
resonance and an Orbitrap mass spectrometer.116
Due to the presence of pyrenyl group in the double-decker
terbium(III) complex, it could be anchored to the surface ofcarbon nanotubes via p–p interactions. The anisotropy energy
barrier and the magnetic relaxation time of the resulting
conjugate are both increased compared with those of purecrystalline [(Pyrene)Pc]Tb(Pc).114 Also this conjugate behaved
as an original spin-valve device, where localized magnetic
moments lead to a magnetic field dependence of the electronictransport through single-walled carbon nanotubes, resulting inmagnetoresistance ratios up to 300% at temperatures lower
than 1 K (cf. Section 3.8).117
Interestingly, the ability of the pyrenyl group to participate
in stacking interactions governed the crystal packing of the[(Pyrene)Pc]Tb(Pc) complex – its molecules formed head-to-taildimers with the shortest intermolecular distances of 3.345–
3.369 Å which is in good agreement with the general values
observed for p–p stacking (Fig. 4).115
Heteroleptic bisphthalocyaninates, bearing one fullereneappended ligand, were synthesized by Ballesteros et al. startingfrom a A3B-type ligand with one benzylic alcohol group

Fig. 4 Structure of a head-to-tail supramolecular dimer, formed in crystal
packing by molecules of [(Pyrene)Pc]Tb(Pc) (CCDC LOGBEQ).115 Hexyl
substituents, hydrogen atoms and solvate molecules are omitted forclarity. The shortest intermolecular distances are shown with blue dottedlines.

Synthesis of pyrenyl-substituted bisphthalocyaninates [(Pyrene)Pc]M(Pc).114,115

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Scheme 17

Review Article

Synthesis of fullerene-substituted bisphthalocyaninates [(BnOCO-C60)Pc]M(Pc).118

H2[(BnOH)Pc] whose reaction with acetylacetonates in odichlorobenzene yielded the corresponding monophthalocyaninates [(BnOH)Pc]M(acac), M = Sm, Eu and Lu (Scheme 17).118
These complexes were used as templates in reactions withphthalonitrile to construct unsubstituted Pc ligands yielding
heteroleptic sandwiches which were esterified with fulleropyrrolidine carboxylic acid C60-COOH. Photophysical characterization of the resulting complexes [(BnOCO-C60)Pc]M(Pc)showed that their components totally lack electronic interactions in the ground state; however, photoexcitation of the C60
fragment at 387 nm lead to a fast and long-lived charge transferfrom M(Pc)2 to C60 p-systems. In contrast, the short-lived natureof the bisphthalocyaninate excited states fails to trigger analogous charge-transfer reactions upon photoexcitation of thismoiety.
Monophthalocyaninates can act as templates in crosscondensation of pairs of diﬀerent phthalonitriles A and B
paving the way to heteroleptic complexes where one of theligands is represented by various combinations AnB4n, n = 0–4.
This one-pot approach was used by Pan et al. to synthesize aseries of tetrathiafulvalene(TTF)-fused heteroleptic europium(III) double-deckers [(TTF)nPc]Eu(Pc).119 With this aim(Pc)Eu(acac) reacted with the mixture of unsubstituted andTTF-fused phthalonitriles (Scheme 18) and the mixture of theresulting complexes could be successfully separated by sizeexclusion chromatography together with chromatography on
silica to separate the cis- and trans-isomers of [(TTF)2Pc]Eu(Pc).
Introduction of the TTF units onto double-decker complexes
had strong effects on their UV-Vis spectra and electrochemical
properties, revealing the significant contribution of these units
to the electronic structure of double-deckers.

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Scheme 18 One-pot synthesis of a series of tetrathiafulvalene (TTF)fused bisphthalocyaninates [(TTF)nPc]Eu(Pc), n = 0–4.118

Unprecedent heteroleptic complexes with open-chain
triisoindole-1-one ligands (R-TIO)Lu(Pc), R = Me or iPr, were
unexpectedly isolated by Wang et al. from the conventional raiseby-one-story synthesis of heteroleptic bisphthalocyaninates via
template condensation of bulky diaryloxy-phthalonitriles withThis journal is © The Royal Society of Chemistry 2022

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Scheme 19

Synthesis of heteroleptic double-deckers with triisoindole-1-one ligands.120

lutetium(III) monophthalocyaninate and DBU in refluxing pentanol (Scheme 19).120 It was found that the target [(R2PhO)8Pc]Lu(Pc) complexes were sole products of template condensation
only when it was performed under strictly anaerobic conditions.
However, the presence of even 5 ppm of oxygen in inert gas media
terminated the reaction at the stage of uncyclized isoindole
oligomeric derivatives rather than the phthalocyanine chromophores. The yields of TIO-containing sandwiches could be
increased up to 25.8% when the reactions were performed in apure oxygen atmosphere. Experiments with labeled 18O2 showedthat the carbonyl group in the TIO ligand contains 16O atom,suggesting that it comes from the solvent. TIO ligands werestabilized both by bulky aryloxy-groups as well as by halfsandwich part of the molecule, and demetalation of these complexes resulted in complete degradation of triisoindole-1-ones.
2.3

Chem Soc Rev

Mixed ligand sandwich complexes

In hierarchy of sandwich complexes mixed ligand double- andtriple-deckers combining tetrapyrrolic macrocycles from diﬀerent families (Por, Pc, Nc, etc.) attract special attention due towider possibilities of tuning of both structural and functional
characteristics in comparison with homoligand complexes.
Among the most important features in the context of thepresent review we should mention specific acid–base equilibria
in double-decker Por-Pc complexes (cf. Section 3.5). Panchromatic absorbance appearing from orbital interactions between
stacked ligands is particularly important for the elaboration ofbroadband light harvesting materials.121 Combination ofligands with different sterical demands in sandwich complexes
also affords precise control of twist angles between theseligands which in turn rules the shape of the coordination
polyhedron of the metal centre bound to these ligands andaffects its SMM behaviour (cf. Section 3.2.1).
2.3.1 Mixed complexes containing porphyrin and phthalocyanine ligands. Mixed ligand sandwiches containing both
porphyrin and phthalocyanine macrocycles have been known
for several decades starting from the seminal work of Moussavi
et al. in 1989.34 A new round of interest in these complexes
began in 2012 when Sakaue et al. reported the synthesis,
structural characterization and magnetic relaxation studies ofThis journal is © The Royal Society of Chemistry 2022

homo- and heteronuclear triple-deckers (TAnP)M*(Pc)M(Pc),
M* = M = Tb and M* a M = Tb/Y.122 These complexes wereprepared in modest yields of 2.3–4.2% by generation of
REEmonoporphyrinates
fromtetra-p-anisyl-porphyrin
H2(TAnP) and M*(acac)3 with subsequent interaction withM(Pc)2 in refluxing trichlorobenzene (Scheme 20). XRD characterization of complexes revealed different symmetries ofcoordination surrounding of metal centres M and M*, which
has a profound effect on the dynamics of magnetization
relaxation (cf. Section 3.2.1).
Interest in studies of mixed ligand sandwiches requires
eﬃcient synthetic pathways with improved yields, which canbe realized either by optimization of experimental protocols, orby application of auxiliary functionalities in macrocycles, governing the regioselectivity of complex formation.
With the aim to synthesize early lanthanide complexes
based on tetra-meso-aryl-porphyrins and crown-phthalocyanine Birin et al. proposed one-pot procedures implying
reactions between corresponding porphyrins, M(acac)3 and15-crown-5-phthalonitrile, the latter undergoes template

Scheme 20 Synthesis of homo- and heteronuclear mixed ligand complexes (TAnP)M*(Pc)M(Pc), M* = M = Tb or M* a M = Tb/Y.122

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Scheme 21 Synthesis of crown-substituted mixed ligand sandwich complexes.123–129

condensation with simultaneous formation of sandwich complexes (Scheme 21).123–126 The balance of concurrent stages wasstudied127 and the optimal conditions were found leading
either to mixtures of double- and triple-decker complexes or
selectively to triple-decker complexes with a Por–Pc–Por ligand
arrangement. The crucial role of electron-donating crown-ether
groups in the synthesis of such triple-deckers was demonstrated, when unsubstituted phthalonitrile was used insteadof crown-substituted precursor; the synthesis terminated at thestep of mixed-ligand double-decker complex.128 Going fromearly to late lanthanides drastically decreased the yield of
triple-deckers, thus, to synthesize a terbium(III) complex it
was necessary to use the raise-by-one procedure with theaddition of terbium(III) porphyrinates to pre-synthesized
double-decker complexes.129
Classical conditions of the raise-by-one-story approach to
Por–Pc–Pc mixed ligand complexes via the interaction of monoporphyrinates with bisphthalocyaninates in high-boiling solvents turned out to be inapplicable in the synthesis ofcomplexes with phthalocyanines bearing bulky substituents.
To circumvent this drawback Jin et al. used microwave activation, the thus optimized conditions allowed the synthesis ofseries of complexes in high yields (Scheme 22).130 Importantly,
MW activation allowed the speeding-up of the synthetic procedures, thereby minimizing possible thermolysis of labile compounds. This benefit allowed the synthesis of complex with freemeso-positions in porphyrin ligands, while under classical9278 | Chem. Soc. Rev., 2022, 51, 9262–9339

Scheme 22 Microwave-mediated synthesis of lanthanide porphyrin/
phthalocyanine triple-deckers bearing bulky substituents.130

solvothermal synthesis of similar complexes starting from
5,15-diphenylporphyrin its degradation was observed under
prolonged refluxing of reaction mixtures.128
Straightforward synthesis of mixed ligand triple-deckers
leading to the regioselective Por–Pc–Por arrangement of ligands
was achieved by González-Lucas et al.131 who applied a diporphyrin with a flexible decamethylene spacer (Scheme 23). This

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Scheme 23 Synthesis of sandwich mixed ligand complexes with spacered diporphyrin and X-ray structure of dilanthanum(III) complex (CCDC LUVWUX).
Only one of the symmetry-imposed disorder components is shown. Hydrogen atoms and solvent molecules have been omitted for clarity.131

pincer ligand sequentially captures lanthanide ions and phthalocyanines to efficiently form desired triple-decker complexes
in high yield when it reacts with stoichiometric amounts ofLa(acac)3 and H2(Pc). In a similar manner, complexes with
Pr(III), Nd(III), Sm(III) and Eu(III) were synthesized; however,attempts to synthesize complexes with a smaller metal centre –Dy(III) resulted in the formation of a double-decker complex
linked with a free-base porphyrin residue. Reaction of thiscomplex with La(acac)3 resulted in the formation of an intermediate heteronuclear La(III)/Dy(III) complex which howeverunderwent complete transmetalation with the formation of ahomonuclear lanthanum(III) complex.
To broaden the range of useful properties of mixed ligand
complexes, they can be functionalized with redox-active
groups or hydrogen-bond donors which can form supramolecular aggregates of various architectures via self-assemblingprocesses.
A series of ferrocene-decorated mixed ligand sandwiches
were synthesized by Zhu et al. using the raise-by-one-story
approach starting from 5,15-diferrocenylporphyrin in 20–5%
yields for double-decker complexes and 70% for a triple-decker
sandwich (Fig. 5).132 Electrochemical studies of these complexes revealed two consecutive Fc-based one-electron oxidation waves, suggesting the effective electronic coupling between
these units which could be finely tuned by structural modifications of the sandwich framework. Thus, the separation between
these waves increased with the decrease of the REE ionicradius, but it decreased when going from a double- to tripledecker complex.
Mixed ligand europium(III) double-deckers bearing from one
to four n-octylamino-groups (Fig. 6) were synthesized and usedas tectons for self-assembled nanostructures, formed upon
injection of concentrated solutions of complexes in toluene
into excess of methanol.133,134 The morphology of the resulting
aggregates was studied by various methods, and it wasThis journal is © The Royal Society of Chemistry 2022

Fig. 5 Ferrocene-decorated double- and triple-decker mixed ligand
complexes.132

Fig. 6 Amino-substituted mixed ligand complexes (A1-TPP)Eu(Pc): R1 =NHOct, R2 = R3 = H; (A2-TPP)Eu(Pc): R1 = R2 = NHOct, R3 = H; (A4TPP)Eu(Pc): R1 = R2 = R3 = NHOct.133,134

demonstrated that it governed their functional characteristics.
For example, in the case of (A1-TPP)Eu(Pc) and (A4-TPP)Eu(Pc)this procedure resulted in the fabrication of uniform well-Chem. Soc. Rev., 2022, 51, 9262–9339 | 9279

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Scheme 24

Review Article

Template (i) vs. direct (ii) synthesis of complexes containing phthalo- and naphthalocyanine ligands.43,136,137

defined nano-rods and nano-sheets with conductivities of
3.85  105 and 2.48  105 S m1, respectively. This diﬀerence
was attributed mainly to the more eﬀective intermolecularp-electron delocalization in nano-sheets which was concluded
from the analysis of X-ray diﬀraction patterns of thenanostructures.134
2.3.2 Mixed ligand complexes with naphthalocyanines.
Introduction of naphthalocyanines into mixed ligand complexes aﬀords strong dissymmetrisation of electronic density
due to anisotropic expansion of the p-system, which can beused for grafting of molecules in a well-defined fashion onvarious supports. For example, Komeda et al. absorbed(Nc)Tb(Pc) on the Au(111) surface in segregated Pc- or Nc-up
modes to study Kondo resonance originating from interactions
between conduction electrons and a localized spin (cf. Section
3.7).135 Katoh et al. showed that the cationic complex(Nc)Tb(Pc)+PF6 forms tetragonal crystals with perfect collinear
arrangement of metal centres with eﬃcient intermolecularferromagnetic coupling (cf. Section 3.3.3).136
Mixed-ligand Pc–Nc complexes can be synthesized using theraise-by-one-story approach. Thus, neutral complex (Nc)Tb(Pc)
was synthesized in a mediocre yield by template condensationof 2,3-naphthalonitrile in the coordination sphere of
terbium(III) monophthalocyaninate in the presence of DBU
and ammonium molybdate (Scheme 24i).136 High quality crystals of its cationic complex were obtained by the electrochemical oxidation of neutral (Nc)Tb(Pc)0 in dichloromethane in thepresence of NBu4PF6 electrolyte.
The interaction of naphthalocyanine ligands with monophthalocyaninates using lithium methoxide in the mixture of
TClB and cetyl alcohol allowed Dubinina et al. to optimize thesynthesis of mixed ligand phthalo-/naphthalocyaninates(Scheme 24ii).43,137 It aﬀorded a series of complexes containing
various substituents in both Pc and Nc rings, and the yields ofsandwiches were high even for the late lanthanides. AFM
studies of thin films formed by lutetium(III) Pc–Nc complexes
evidenced that their tendency to aggregate depended on the9280 | Chem. Soc. Rev., 2022, 51, 9262–9339

Scheme 25 Synthesis of double-decker complexes with octaethylporphyrazine and the X-ray structure of the complex Tb(OEPz)2 (CCDCQOQPIX).140 Hydrogen atoms and solvate molecules are not shown forclarity.

substitution patterns, gaining maximum for (Ph8Nc)Lu(Ph8Pc),
and in general the series of complex behaved as semiconductors with small activation energy.43
A mixed ligand europium(III) complex with decylthiosubstituted naphthalocyanine and push-pull porphyrin(Fig. 7a) was synthesized by Stefak et al., who used the templatecondensation of 6,7-di(n-decylthio)-2,3-naphthalonitrile in thepresence of the corresponding monoporphyrinate.138 This
complex was suggested as a rotor which can be absorbed on
the Cu(111) surface in a porphyrin-up fashion, forming a network where molecules undergo simultaneous and coordinatedrotational switching under the influence of an STM tip-induced
electric field by applying biases above 1 V at 80 K (Fig. 7b).139

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Fig. 7 (a) Donor–acceptor porphyrinate rotor mounted on anaphthalocyaninato-europium(III) complex.138 (b) Behaviour of a supramolecular network of a donor–acceptor double-decker complex inmonolayer, formed by complex on the Cu(111) surface.139

2.4

Sandwiches with less-common tetrapyrrolic ligands

Advances in the synthesis of novel tetrapyrrolic ligands were often
followed by the introduction of these ligands into the chemistry of
sandwich REE complexes. The following section will include
examples of double- and triple-deckers with porphyrazines including hetero-annulated derivatives, tetrabenzoporphyrins, andcore-modified macrocycles – tetrabensotriazaporphyrins (TBTAP),
corroles and N-confused porphyrins. Most of these complexes are
yet to be comprehensively explored in terms of applicability, thus
this section can give a summary inspiring their further study.
2.4.1 Sandwich complexes with porphyrazine ligands. In
contrast to porphyrins and especially phthalocyanines attention to porphyrazines in context of sandwich REE complexes
still remains very modest.

Chem Soc Rev
A series of neutral double-decker complexes with anoctaethylporpyrazine ligand (OEPz) with Tb(III), Dy(III), Gd(III)and Y(III) metal centres was reported in 2014 by Giménez-Agulló
et al. Sandwiches were synthesized starting from H2(OEPz)
using microwave activation.140 Interestingly, the synthetic protocol included the necessity to add tetrabutylammonium
benzoate as an auxiliary reagent – in its absence doubledecker complexes could not be obtained. The authors claimed
that this salt was needed to stabilize the intermediate – anionicform [M3+(OEPz2)2]TBA+, which underwent oxidation to theneutral radical form during reaction workup.
X-ray structure of the complex showed that the molecule
adapts staggered conformation with SAP surrounding of themetal centre which is a prerequisite of SMM behaviour of Tb(III)and Dy(III) complexes. Indeed, AC magnetometry showed that
Tb(OEPz)2 is characterized by high value of magnetization
relaxation barrier comparable with the value of Tb(Pc)2.
Sandwich REE complexes with heterocycle-annelated macrocycles were reported for the first time by Tarakanova et al.
in 2014141 with complexes with tetradiazepinoporphyrazine(DZPz) ligands as examples. High CH-acidity of the peripheral
diazepine rings made them eﬃcient hydrogen bond donors
thus the macrocycles themselves were found to form highly
stable dimers in solutions.142 Staggered conformations of thesedimers reminded the arrangement of tetrapyrrols in REE
sandwiches; therefore, additional stabilization of a doubledecker scaﬀold could be expected.143 And indeed, it was shownthat direct metalation of H2[(tBuPh)8DZPz] with M(acac)3and DBU (Scheme 26a) smoothly yielded double-decker complexes at the temperature of refluxing o-dichlorobenzene(180 1C), where classical phthalocyanines selectively form
only monophthalocyaninates.141,143–145 Importantly, diazepine
rings were susceptible to hydrolysis in alkaline media, which
made template synthesis of REE complexes with DZPz
ligands less efficient than direct metalation.141 DFT modellingof the resulting double-deckers suggested the presence of

Scheme 26 (a) Synthesis of double-decker complexes with tetradiazepinoporphyrazine ligands. (b) Optimized structure of Lu(Ph8DZPz)2 in top andside views, showing hydrogen bonds between CH2 groups of diazepine rings and meso-N atoms. Other hydrogen atoms are omitted for clarity. Modelwas built using Cartesian coordinates, provided in the ESI in ref. 143.

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intramolecular hydrogen bonds between endocyclic CH-groupsof diazepine rings and meso-nitrogen atoms of opposite ligands(Scheme 26b).
Among other diﬀerences in the reactivity of DZPz and Pc
ligands the stabilization of anionic forms of double-decker
complexes should be emphasized. In contrast to M(Pc)2 whichreadily undergoes aerobic oxidation to neutral forms, thesynthesized complexes M[(tBuPh)8DZPz]2 remained in thisform during chromatographic purification, and the additionof strong oxidants like bromine was needed to generate neutral
forms which revealed characteristic intervalence bands in the
NIR region.143 The cerium complex fell out of this trend and its
double-decker complex was stabilized in the neutral form witha quadrivalent metal centre.145 Its electrochemical reduction totrivalent state was characterized by slow electron transfer
kinetics which was explained by strong structural rearrangement of the sandwich molecule following the electron transfer
step as well as shielding of the metal center by the bulky
macrocycles.
Interestingly, solutions of anionic double-deckers with DZPz
ligands were stable for several months showing no evidence of
aggregation or decomposition; however, in the case of neutral
forms evolution of their UV-Vis spectra was observed, depending on the nature of the metal centre. First of all, upon storageof solutions of M[(tBuPh)8DZPz]20 in CH2Cl2 the hypsochromic shift and broadening of Q-bands were observed, and DLS
studies of aged solutions revealed the formation of aggregateswith a hydrodynamic radius of 20 nm. The tendency of neutral
forms to aggregate was explained by the possibility of inversionof diazepine moieties leading to the reorientation of theintramolecular hydrogen bonds in the double-decker followedby the formation of intermolecular hydrogen bonds. In turn,it weakened the stabilizing eﬀect of intramolecular H-bonding,and it resulted in lowering of stability in early lanthanide
complexes – slow demetalation of La[(tBuPh)8DZPz]20
was observed even after 1 day of solution storage, while
lutetium(III) complex was stable towards demetalation ever after
five days.143
In the case of mixed ligand complexes [(tBuPh)8DZPz]M(Pc),
M = La, Eu and Lu, spontaneous aggregation due to inversion ofdiazepine ring was observed even for anionic forms.146 In
contrast to homoleptic complexes these double-deckers could
form only dimeric species, which were observed by NMR
spectroscopy. Tendency to aggregation followed the growth of
the ionic radius of lanthanide ion due to an increase ofinterligand distance, weakening of intramolecular CH. . .N
interactions and facilitation of diazepine ring inversion. The
same trend was observed in the case of neutral forms generated
during spectroelectrochemical studies. Thus, the intermolecular self-assembly of sandwich complexes containing DZPz decks
can be controlled by changing the nature of both peripheral
substituents and REE ions.
Soluble tetrathieno[2,3-b]porphyrazine annelated withcyclohexane rings was used by Dubinina et al. to synthesize
lutetium(III) complexes expecting that the presence of sulphurcontaining heterocycles can provide the resulting complexes

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Review Article

Scheme 27 Synthesis of lutetium(III)tetrathieno[2,3-b] porphyrazine.147

complexes

withsubstituted

with improved NLO properties.147 Starting metal-free ligand
was synthesized from tiophenedicarbonitrile by Zn-templatedcondensation and demetalation. Reactivity of the resulting
ligand was quite unusual – upon its reaction with Lu(OAc)3
4H2O and MeOLi in refluxing mixture of TClB and cetyl alcohol
only mono- and triple-decker complexes were obtained,
which were separated by preparative TLC (Scheme 27).
Although the formation of double-decker was detected bymass-spectrometry, it could not be isolated because of its low
stability associated with the presence of p-radical.
2.4.2 Sandwich complexes with tetrabenzoporphyrin andtetrabenzocorrin ligands. Tetrabenzoporphyrins (TBP) were
known since 1938;148 however, their complexes with REE were
described for the first time only in 2012. The studies were
inspired by interest in the properties of porphyrin complexes
with an expanded aromatic system. These and more recent
results are summarised in Scheme 28.
Thus, interaction of tetra(bicyclo[2.2.2])octadienoporphyrinwith Gd(OAc)3 in refluxing TClB was studied by Xu and Liu
et al.149 Under these conditions metalation of tetrapyrrolicmacrocycles was accompanied by retro Diels-Alder aromatization producing TBP ligands. Heating of the reaction mixture for
15–17 h resulted in the formation of a double-decker complex
Gd(TBP)2, and increase of the reaction time to 45–48 h resulted
in the predominant formation of a triple-decker complex
Gd2(TBP)3.150,151 Magnetic properties of complexes werestudied revealing antiferromagnetic interactions between pand f-electrons in Gd(TBP)2 and between two Gd3+ ions in thecase of Gd2(TBP)3.
Interaction of tetrabenzoporphyrin H2(TBP) itself withM(acac)3, M = La, Nd, Gd, Dy, and Lu was used by Galanin
et al. to generate half-sandwich complexes (TBP)M(acac) which
reacted with Li2(Pc) to form mixed ligand double-deckers

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Scheme 28

Synthetic pathways to sandwich complexes with benzoporphyrin and corrin ligands.

(TBP)M(Pc) in 48–74% yields.152 A similar strategy was applied
to synthesize complexes starting from tert-butyl-substituted
benzoporphyrin H2[(tBu)4TBP].153
Original strategy leading to homoleptic complexes M(TBP)2,
M = Gd, Er, and Lu, was proposed using a dipyrrolic precursor(PhIm)2 prepared from readily available phthalimide PhIm.154
This procedure compares favourably with the time-consuming
protocol using tetra(bicyclo[2.2.2])octadienoporphyrin due tothe availability of starting compounds.
Application of acyclic olygopyrrolic precursors aﬀords more
options in the synthesis of various porphyrinoid derivatives.
Condensation of 2-methylquinoline with a tripyrrolic compound (PhIm)3 prepared from phthalimide aﬀorded meso(quinolin-5-yl)tetrabenzoporphyrin H2(QnlTBP) which was also
used to synthesize mixed ligand complexes.155 The aforementioned dipyrrolic compound (PhIm)2 can be dimerized in thepresence of lanthanide acetates and hydrazine with the formation

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of complexes with the 1-tethyltetrabenzo-octadehydrocorrin
ligand.156 Depending on the ratio of reagents, either halfsandwiches or homoleptic double-decker complexes could beobtained. The reaction of the former complex with phthalonitrile
can be used to synthesize phthalocyanine-corrin mixed ligand
double- and triple-decker complexes.
2.4.3 Sandwich complexes with tetrabenzotriazaporphyrin
ligands. The tetrabenzotriazporphyrin (TBTAP) ligands areintermediate macrocycles between classical porphyrins andphthalocyanines, possessing a permanent dipole moment,
which may be responsible for the emergence of new properties.
REE complexes with these ligands were reported by Pushkarev
et al. on the examples of homo- and heteroleptic lutetium(III)157,158
and europium(III) double-decker complexes which were synthesized by direct metalation of meso-phenyl-TBTAP macrocycle
using lithium methoxide as a basic reagent (Scheme 29a). Apartfrom a homoleptic double-decker complex, small amount ofChem. Soc. Rev., 2022, 51, 9262–9339 | 9283

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Scheme 29 (a) Synthesis of homo- and heteroleptic complexes with meso-phenyl-tetrabenzotriazaporphyrin, H2(Ph-TBTAP).157,158 (b) Side products ofdephenylation. (c) X-ray structure of the complex Eu(Ph-TBTAP)2 (CCDC ZUNJID).158 Hydrogen atoms are not shown for clarity, meso-phenyl groups
are highlighted with blue colour. (d) Anti- vs. syn-conformers of M(Ph-TBTAP)2 complexes.

triple-decker sandwich was isolated in the course of synthesis ofEu(Ph-TBTAP)2, and moreover, prolonged heating of reaction
mixtures resulted in partial dephenylation of TBTAP ligands(Scheme 29b), which was never observed previously in the chemistry of meso-aryl-substituted tetrapyrroles.
Comparison of UV-Vis spectra of M(Pc)2, (Ph-TBTAP)M(Pc)and M(Ph-TBTAP)2 revealed that replacement of one or two
meso-nitrogen atoms with C-Ph groups resulted in gradualperturbation of electronic systems, leading to complicatedspectral patterns with split Q- and intervalence bands, which
is not typical for symmetrical complexes. Electrochemical studies of new complexes evidenced that the introduction of two
bulky phenyl rings has a profound eﬀect on frontier orbital
levels due to weakening of the intramolecular interaction
between macrocyclic ligands.
Single-crystal X-ray characterization of Eu(Ph-TBTAP)2 showedthat the molecule adapt syn-conformation (Scheme 29c and d),although conformational analysis performed by DFT calculations
suggested that anti-form should be more stable and this form isobserved as a sole state in solution according to NMR studies.
Thus, a less stable conformer is formed in the solid state due tothe advantage in energy of the molecular packing.
2.4.4 Sandwich complexes with corrole ligands. Corroles asring-contracted porphyrinoids have been known since 1964;
however, their fascinating properties were revealed mainly inthe 1990s when straightforward synthetic procedures towards
this class of macrocycles were proposed by the groups of Grossand Paolesse.159,160
First examples of REE complexes with corroles were synthesized only in 2013 by Buckley et al. either by metalation of trismeso-aryl-corrole with M[N(SiMe3)2]3, M = La and Tb or by thereaction of lithium corrolate with GdCl3.161 In both the cases
half-sandwiches were isolated, where the coordination sphereof lanthanide ions was saturated with neutral ligands –dimethoxyethane or trimethyltriazanonane. A similar procedure was used to synthesize cerium complexes with corrole
ligands and axial ligand-dependent self-assembling of theresulting half-sandwiches was studied.162

9284 | Chem. Soc. Rev., 2022, 51, 9262–9339

Mixed ligand complexes containing corroles and phthalocyanine ligands were synthesized by the reaction of presynthesized monophthalocyaninates with corroles in thepresence of DBU in octanol (Scheme 30).163–168 Several substituted macrocycles were introduced into this reaction, butinvariantly on substituents and REE ions in all cases
mixed ligand triple-deckers were isolated with inner Cor
and terminal Pc ligands. Moreover, reaction of halfsandwich corrole complex with metal-free phthalocyanineand one-pot reaction of corrole, metal-free Pc and metal salts
both yielded the identical triple-deckers and their yields weresystematically larger than that when the first method wasused.166
Some of the synthesized complexes were characterized by
XRD revealing that in contrast to other symmetrical tripledeckers two outer phthalocyanine ligands were not staggered,and coordination polyhedra of two metal centres were slightly
diﬀerent, although in all cases they were close to distorted
square prisms.163,164,167 This distortion of coordination surrounding manifested in fast relaxation of magnetization
because of QTM (the quantum tunnelling of the magnetisation;cf. Section 3.1), as evidenced from AC magnetic susceptibility
measurements performed for dysprosium(III) complexes
Dy2[(PentO)8Pc]2[(p-XPh)3Cor], X = F or Cl.164
The presence of three protons in corroles suggests that theclosed-shell form of triple-decker complexes should be anionicin nature, which makes them diﬀerent from trisphthalocyaninates, where the charge of two trivalent REE ions is balanced by
the charge of three dianionic ligands. Thus, to stabilize thenegative charge of the (M3+)2(Cor3)(Pc2)2 species a counterion
is needed. NMR studies of the europium(III) complex suggested
that the charge is balanced by a proton, which is likely to bedelocalized over the isoindoline nitrogen atoms of one of thephthalocyanine ligands.163 Air oxidation of the protonatedforms with the formation of paramagnetic complexes withdelocalized p-electron was observed, which was followed bydisappearance of NMR spectra and appearance of intervalencebands in UV-Vis-NIR spectra at ca. 2000 nm.167

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Scheme 30 (a) Synthesis of mixed ligand complexes with phthalocyanine and corrole macrocycles. (b) X-ray structure of the complex Eu2[(OctO)8Pc]2[(p-ClPh)3Cor] (CCDC HOKQOP).163 Hydrogen atoms and substituents in top views are not shown for clarity.

Rich electrochemistry of studied complexes was demonstrated, and up to five oxidized states and three reduced states
were observed rendering them as potential molecular materials
for information storage devices.163,166–168 The potentials ofredox transitions could be finely tuned both by variation of
metal centres and substituents in the para-positions of mesophenyl groups, due to linear correlations between the sums ofHammett constants s of these substituents and electrochemical characteristics of complexes.168
Comparative studies of self-assembling of two europium(III)complexes Eu2(R8Pc)2[(p-ClPh)3Cor], where R = H or octyl
revealed that substituents in Pc ligands aﬀected the morphology of aggregates formed by the phase transfer method upon
mixing of solutions of complexes in chloroform with excess of
methanol.165 TEM and SEM studies of the resulting aggregates
showed that the complex with unsubstituted Pc ligands forms
nanobelts of 1.5–2.0 mm length and 100 nm width, while theoctyloxy-substituted complex forms large scale sheet-likenanostructures with an average length of ca. 500 nm and awidth of 300 nm. NLO studies of these complexes showedsignificant nonlinear reverse saturation absorption and selfdefocusing behaviour.
Altogether, these results suggest that these corrolecontaining mixed-ligand complexes can be considered as promising candidates for future nano-electronic and optical device
applications.
2.4.5 Sandwich complexes with N-confused porphyrin
ligands and heteroanalogues of porphyrins. Among coremodified derivatives of porphyrins special attention is paid toN-confused (or inverted, iPor) analogues with endocyclic CH
and exocyclic NH groups. They have been known since theThis journal is © The Royal Society of Chemistry 2022

seminal work of Furuta et al. reported in 1994,169 andthese macrocycles found application in the construction ofmultinuclear assemblies due to their versatile binding
modes.170
First REE sandwiches with N-confused porphyrins weresynthesized in 2012 by Cao et al. on the examples of mixed
ligand double-deckers of iPor-Pc type using the typical raise-byone-story approach with monophthalocyaninate and metal-freeN-confused porphyrin (Scheme 31a).171 For the comparison ofspectral properties, N-methylated iPor complex was alsosynthesized.
In 2014 cross-condensation of common and N-confused
porphyrins with La(III) and Pr(III) acetylacetonates was reported,leading to heteroleptic double-decker complexes of Por-iPor
type (Scheme 31b).172 Homoleptic bisporphyrinates were isolated as side products, while formation of homoleptic complexes with N-confused ligands was not observed. Interestingly,
heteroleptic complexes exhibited NIR band at ca. 850 nm,
which was absent in homoleptic bisporphyrinates.
Another type of porphyrin core modification, namely replacement of one of pyrrolic nitrogen atoms with oxygen orsulphur was used for spectacular tuning of magnetic propertiesof mixed ligand double-deckers. This was demonstrated on theexamples of dysprosium(III) complexes with phthalocyanineand oxa- or thiaporphyrin ligands [(tBuPh)4XPor]Dy(Pc), X = O
or S (Scheme 32).173 It was demonstrated that although Dy–X
bonds are longer than Dy–N bonds, the symmetries of coordination polyhedra did not change upon going from classicalporphyrin to its oxa- and thia-analogues. However, the replacement of one of heteroatoms significantly enhanced the anisotropy of the Dy(III) ion providing the complex with one of theChem. Soc. Rev., 2022, 51, 9262–9339 | 9285

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Scheme 31 (a and b) Synthesis of heteroligand double-decker complexes with N-confused ligands. (c) X-ray structures of [(tBuPh)4iPorH]Lu(Pc)(CCDC YEFZIU), [(tBuPh)4iPorH]Eu(Pc)MeOH (CCDC YEFZAM)171 and [(p-ClPh)4iPorH]Pr[(tBuPh)4Por] (CCDC BODFIL).172 Solvate molecules and mostof hydrogen atoms are not shown for clarity.

Scheme 32 Synthesis of mixed ligand double-decker complexes withoxa- and thiaporphyrin ligands and X-ray structure of [(tBuPh)4SPor]Dy(Pc) (CCDC AHOPUK).173 Hydrogen atoms and solvent molecules are
not shown for clarity.

largest barriers ever reported for dysprosium(III) complexes withtetrapyrrolic ligands (cf. Section 3.2.2).
2.5 Complexes based on half-sandwiches with axiallycoordinated non-tetrapyrrolic ligands
Due to large coordination numbers of REE ions, their halfsandwich complexes tend to have labile coordination spheres.

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This phenomenon is at the heart of the raise-by-one-story
approach leading to the aforementioned heteroleptic andmixed ligand sandwiches, but it is possible to combine ligandsof completely diﬀerent natures in a molecule as part of thisapproach.
Thus, the present section will include sandwich REE complexes, where porphyrin or phthalocyanine ligands coexist withligands of non-tetrapyrrolic nature. Although they only resemble sandwich structures of archetypal double- and triple-decker
complexes, we included them in this section due to the increasing interest in such type of objects providing pathways to novel
fluorescent materials, optical limiters, molecular magnets, etc.
Finally, some of the structures are curious by themselves,
broadening the rich coordination chemistry of both REE andtetrapyrrolic macrocycles.
2.5.1 Sandwich complexes with Schiﬀ bases axially coordinated to REE monophthalocyaninates. Starting from 2013 thescope of sandwich-type tetrapyrrole-based rare-earth molecularmaterials was extended by monophthalocyaninates with axiallycoordinated Schiﬀ base (SB) ligands. The summary of thereaction studied on the example of the ligand, based on ophenylenediamine and methoxy-substituted salicylic aldehyde,
H2(SB) is given in Scheme 33.
First examples of such complexes were prepared by Wang et al.
from (Pc)M(acac), M = Dy and Y and H2(SB).174 Depending on thebase used for deprotonation of H2(SB) complexes of diﬀerent
nuclearities were obtained. Performing this reaction with Ca(OH)2
aﬀorded a quadruple-decker complex where the [Ca(MeOH)2]2+
group bridged two (Pc)M(SB) units (Scheme 33a), while dinuclear double-deckers (Pc)M[(SB)K(MeOH)] was isolated in theThis journal is © The Royal Society of Chemistry 2022

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Scheme 33 Synthesis of mixed-ligand complexes based on half-sandwich phthalocyaninates and Schiﬀ base H2(SB) with X-ray structures of thecorresponding complexes – from the left to the right: (Pc)2Dy2(SB)2Ca(MeOH)2 (CCDC FEVDOB),174 (Pc)Dy(SB)K(MeOH) (CCDC FEVDUH),174(Pc)Tb(SB)Zn(Py)2NO3 (CCDC YIMMUE)175 and (Pc)2Dy2(CB)(H2O) (CCDC XILTOD).176

presence of KOH (Scheme 33b). The latter complex dissociatedwith the formation of double-decker species upon treatment with
K2CO3 (Scheme 33c). Fast QTM relaxation was observed in zerofield AC magnetic susceptibility measurements for dysprosium(III)complexes of both types, although long-distance magnetic interactions between two paramagnetic centres in quadruple-decker
complex was detected (cf. Section 3.2.2).
Later, Ma et al. synthesized terbium(III) complexes with all
alkali metal cations [(SB)M(MeOH)]Tb(Pc), M = Li+ to Cs+ andthe possibility to exchange these cations with the Zn2+ ion wasdemonstrated (Scheme 33d), providing an interesting example
of polyfunctional complex with tuneable properties.175
Slow diﬀusion of the acetonitrile solution of H2(SB) into thesolution of (Pc)M(acac), M = Gd and Dy, in chloroform aﬀorded
another type of triple-decker complex with bridging (SB)2 unit
and chemically non-equivalent REE ions – M2(Pc)2(SB)(H2O)(Scheme 33e).176 One of metal centres was in octahedralsurrounding and another one was hepta-coordinated by four
isoindole nitrogen atoms of (Pc)2, two oxygen atoms of a
Schiff-base ligand, and one oxygen atom of water molecule.
Homoleptic binuclear dysprosium(III) complex with the H2(SB)ligand had the same binding mode; however, incorporation of
the phthalocyanine ligand had a profound effect on SMM
properties of the mixed-ligand sandwich in comparison withThis journal is © The Royal Society of Chemistry 2022

Dy2(SB)3(H2O) in terms of ferromagnetic coupling between
paramagnetic ions and energy barrier, Ueff.
Analogous binuclear complexes were synthesized by Li et al.
based on a family of chiral ligand prepared by the condensationof 1,2-diphenylethylenediamine and 5-chlorosalicylic aldehyde.
Photophysical characterization of such complexes showed theirpotential in future nonlinear optical applications such as
wavelength conversion and optical switching.177,178
By varying the ratio of half-sandwich phthalocyaninate andSchiﬀ base, complexes with diﬀerent stoichiometries of ligands
can be obtained. This was shown by Gao et al. on the example of
the reaction between one equivalent of dysprosium(III) phthalocyaninate and two equivalents of tetrathiafulvalene-fusedSchiﬀ base H2(SB-TTF) yielding a binuclear complex
Dy2(Pc)(SB-TTF)2(MeOH) (Scheme 34a).179 Due to the intrinsic
dipole moment, this complex could be absorbed on the surfaceof HOPG either in Pc-up or S-TTF-up fashion depending on theapplied potential, which was demonstrated by scanning tunneling microscopy at the single molecule level, rendering thiscompound as a component of functional materials fabricatedby self-assembly.
Gao et al. used dysprosium(III) or erbium(III) monophthalocyaninates generated in situ as templates for condensation of
2,6-diformyl-4-methylphenol and 1,3-diaminopropane to form

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Scheme 34 (a) Synthesis of a mixed-ligand complex with tetrathiafulvalene-fused Schiﬀ base with the X-ray structure of complex Dy2(Pc)(SBTTF)2(MeOH) (CCDC WONKIV).179 (b) Synthesis of sandwiches based on closed-macrocyclic Schiﬀ base and REE monophthalocyaninates with the X-ray
structure of complex Dy3(Pc)2(mSB)(OMe)2 (CCDC IKAQAO, only one of two crystallographically independent molecules is shown).180 Hydrogen atoms
are omitted for clarity.

unusual trinuclear triple-decker sandwiches with a macrocyclic
Schiﬀ ligand and bridging acetate and methoxide counterions
M3(Pc)2(mS)(OAc)(OMe)2 (Scheme 34b).180 Interestingly, in the
crystal lattice these complexes existed as pairs of symmetricallyunrelated molecules with slightly diﬀerent symmetries of coordination polyhedra. This feature resulted in multiple magnetic
relaxation modes found by AC magnetic susceptibility measurements of the dysprosium(III) complex. Pronounced intramolecular interactions between proximal metal centres wasobserved – ferromagnetic in the case of dysprosium and antiferromagnetic in the case of erbium (cf. Section 3.2.2).
2.5.2 Monoporphyrinato- and monophthalocyaninatocomplexes with axially coordinated cyclic ligands. Sandwiches
formed by heavy lanthanide monoporphyrinates181 andmonophthalocyaninates182 with cyclen ligands coordinated tometal centres were reported by Santria and Kizaki et al. These
complexes with well-defined coordination spheres of metal
ions were synthesized by simple reactions of (TPP)MCl or(Pc)MCl, M = Tb–Yb and Y with excess of cyclen in methanol
or THF, respectively (Scheme 35), and single crystals of allcomplexes were characterized by XRD to show that outer-spherechloride anions do not bind to metal ions, thus, they adapt atetragonal SAP symmetry of coordination polyhedra formed
solely by pairs of N4 ligands. The smooth changes of structuralparameters of both families of complexes depending on theionic radii of M3+ were demonstrated.
Ground multiplet states of a series of porphyrin complexes[(TPP)M(Cyclen)]+Cl were determined by NMR and magnetic
susceptibility data together with CASSCF calculations, giving
insight into their electronic structures.183 Intramolecular magnetic interactions between localized 4f-electronic systems andphotoexcited Por or Pc p-systems in terbium(III) Por/cyclen andPc/cyclen complexes was studied by variable-temperature
variable-field magnetic circular dichroism spectroscopy

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Scheme 35 Synthesis of REE porphyrinates and phthalocyaninates withaxially-coordinated cyclen ligands with X-ray structures of[(TPP)Tb(Cyclen)]+Cl (CCDC LEBMUD) and [(Pc)Tb(Cyclen)]+Cl (CCDC
WENBID).181,182 Hydrogen atoms are omitted for clarity.

providing experimental data to validate the theoretical models
describing such interactions.184,185
Interesting sandwich complexes with Z5-Cp and Z5-Me5Cp
ligands sitting-atop scandium(III) phthalocyaninate weresynthesized from (Pc)ScCl by Platel et al. (Scheme 36).186 As afirst step of this work, template condensation of phthalonitrilein the presence of ScCl3 in ClN was performed, yielding
inseparable mixture of target (Pc)ScCl with ca. 15% of
unreacted ScCl3. Treatment of this mixture with LiCH(TMS)2
in THF aﬀorded the formation of a soluble binuclear complex(Pc)ScCl2Li(THF)2 which could be crystallized as an individualcompound. Its reaction with NaCp or LiCp* in THF provided

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Scheme 36 Synthesisofscandiumphthalocyaninates–(Pc)ScCl2Li(THF)2 (CCDC DUDTED) and (Pc)Sc(Cp) (CCDC DUDTIH).186
Hydrogen atoms are omitted for clarity.

(Pc)Sc(Cp) and (Pc)Sc(Cp*), respectively, as first representativesof metallophthalocyanine-ocenes.
Another class of macrocyclic ligands which was recently
introduced into sandwich REE chemistry is hemiporphyrazin,
Hp. Synthesis of this 20p-electron antiaromatic relative of
azaporphyrins was reported for the first time in 1952 by Elvidgeand Linstead.187 However, REE complexes with Hp werereported for the first time only in 2017.188 The complexes(HHp)M(Pc), M = Eu, Lu, were obtained by Liu et al. using
reactions of the corresponding (Pc)M(acac) with H2(Hp) and
DBU in refluxing pentanol. Metalation of hemiporpyrazine withM(acac)3nH2O under the same conditions yielded doubledecker complex (HHp)M(Hp). ESR and NMR studies of theresulting sandwiches evidenced that they were diamagnetic in

Scheme 37 Synthesis of complexes with hemiporphyrazine ligand188,189
and X-ray structures of (HHp)Eu(Pc) (CCDC GAWCAL) and (HHp)Eu(Hp)(CCDC GAVXUZ).188 Hydrogen atoms are omitted for clarity.

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Chem Soc Rev
contrast to most bisphthalocyaninates existing as free radicals.
The proton needed for such state was localized by NMR
spectroscopy, XRD and quantum-chemical calculations – allmethods evidenced that the proton is bound to the pyridine
nitrogen atom. Quantum-chemical calculations of shielding
surfaces of both types of complexes revealed that in homoleptic
complexes hemiporphyrazine ligands retained their antiaromatic nature; however, combination of antiaromatic Hp andaromatic Pc species provided molecules with overall aromatic
behaviour.
XRD characterization of both types of Hp complexes
revealed nearly ideal SAP surrounding of metal centres,
although somewhat distorted because of the protonation of
one of the pyridine groups. Nevertheless, measurements ofmagnetic properties of (HHp)Dy(Pc) and (HHp)Dy(Hp) revealed
excellent SMM properties (cf. Section 3.2.2).
2.5.3 Heteronuclear complexes with metalloligands. Halfsandwich porphyrinates and phthalocyaninates were used toform heteronuclear complexes with metalloligands. For example, in 2013 Gao et al. reported solvothermal one-pot reactionsof metal-free H2(TPP) or H2(Pc) with Gd(III), Tb(III), Dy(III) andHo(III) acetylacetonates, and sodium salt of Kläui’s, Na[(g5QO)(OEt)2}3].190 These reactions yielded sevenCp)Co{P(Qcoordinate complexes with axial tripodal ligands (Scheme 38a
and b). Tb(III) and Dy(III) complexes showed field-induced slow
relaxation of magnetization; these complexes were convenientmodels demonstrating the correlation between the magnetic
relaxation properties and subtle distortions of the coordination geometry of the paramagnetic lanthanide ions (cf.
Section 3.2.2).

Scheme 38 Synthesis of heteronuclear complexes based on REE halfsandwiches. (a and b) Synthesis of Dy(III) porphyrinate (CCDC LIMDOC)
and phthalocyaninate (CCDC LIMCUH) with a Kläui ligand. (c) Synthesis ofTb(III) phthalocyaninate bearing a polyoxotungstate ligand (Pc)M(PW11O39)6(NBu4+)6 (CCDC GUZFOZ), and the X-ray structure of anionicpart of the complex (Pc)Tb[PW11O39]6. Solvate molecules, hydrogenatoms and counterions are omitted for clarity.

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Later, half-sandwich Yb(III) with porphyrin derivatives andaxial Kläui’s ligands attracted great interest as highly near-IR
emissive complexes with unprecedented quantum yields up to
63%,191 which contributes to the design of molecular probes
for near-IR imaging.192
In 2020 Sarwar et al. reported the synthesis of Tb(III),
Dy(III) and Y(III) phthalocyaninates with a monolacunarya-Keggin polyoxotungstate ligand [PVWVI11O39]7.193 With thisaim, the corresponding monophthalocyaninates (Pc)M(OAc)
reacted with (NBu4)4H3[PW11O39] in the presence of triethylamine and NBu4Br under mild conditions, yielding anioniccomplexes with tetrabutylammonium counterions (Pc)M(PW11O39)6(NBu4+)6 (Scheme 38c). Their characterisation
included high resolution mass spectrometry, synchrotronbased single-crystal X-ray diﬀraction and magnetic studies
revealing slow relaxation of the magnetisation for the Dy(III)derivate (cf. Section 3.2.2).
2.5.4 Inverted sandwich complexes. Final class of complexes in the following section – inverted sandwiches do notrefer to sandwich complexes in the common sense, since they
contain only one phthalocyanine ligand as a bridge between
two M(dpm)2+ groups, where dpm – dipivaloylmethanate
anion. The term ‘‘inverted’’ refers to the fact that one macrocycle is sandwiched between two REE ions, as opposed tothe conventional sandwiching of metal ions between two
macrocycles.
Thus, these complexes were reported for the first time in1983 in a short communication by Sugimoto et al., describing
the reaction between Li2(Pc) and M(dpm)3 in refluxing THF
under anaerobic conditions.194 This reaction aﬀorded a seriesof (l-Pc)[M(dpm)2]2, where M = Sm to Yb and Y. The X-ray
structure of the samarium(III) complex with benzene solvate
molecules was determined (Fig. 8), showing the extremely rare
representative of binuclear complexes formed by a single tetrapyrrolic macrocycle. A bit later magnetic properties of theseunique complexes were studied by Maeda et al., showing thefirst evidence of intramolecular spin–spin exchange interaction
between trivalent REE ions.195,196 However, for many years that
followed, these complexes attracted no attention.

Fig. 8 X-ray structure of (l-Pc)[Sm(dpm)2]2 (CCDC CALWIV).194 Solvate
molecules and hydrogen atoms are not shown.

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Only in 2014 detailed characterization of magnetic properties of (l-Pc)[M(dpm)2]2, M = Tb and Dy, was performed by
Tsuchiya et al., showing their striking diﬀerence from common
binuclear triple-deckers.197 Unexpectedly, antiferromagnetic
interaction between metal centres was found in contrast to
ferromagnetic coupling in the corresponding trisphthalocyaninates. NMR studies showed that aromatic proton resonanceswere shifted down-field – again, it was in sharp contrast with
M2(Pc)3 with large up-field lanthanide-induced shifts. These
intriguing results were explained assuming the ligand fieldpotential of the inverted-sandwich complexes gave strong
easy-plain-type magnetic anisotropy, where the Jz value of thelowest sublevel has a small value, while trisphthalocyaninateshave a strong easy-axis-type magnetic anisotropy. The benzene
solvates (l-Pc)[M(dpm)2]22C6H6 showed interesting behaviour
with respect to desolvation and exchange of solvation molecules to dichloromethane, affording the pathway for fine tuning of magnetic properties (cf. Section 3.6).198–200
Interesting properties of inverted sandwiches could encourage synthetic studies to broaden this class of REE complexes;
however, to date, only dpm ligands could stabilize their unusual architecture. To the best of our knowledge, the sole
reported attempt to exchange these anions was made by Ge
et al. when the complex (l-Pc)[Dy(dpm)2]2 was treated with 3methoxy- or 3-ethoxysalicyllic aldehydes H(LR), R = OMe,
OEt.201 However, it resulted in the deterioration of the inverted
sandwich scaﬀold with the formation of a new type of tripledeckers (l-LR)2Dy2(Pc)2(H2O) where two (DyPc)+ units werebridged by a pair of coplanar deprotonated LR ligands(Scheme 39a). Although at the first glance both complexes
had similar structures invariant to substituents in bridging
ligands, their magnetic properties were markedly diﬀerent.
Magnetic studies indicated that the MeO-substituted complex
behaved as a zero-field single-molecule magnet with a higher
energy barrier, while EtO-substituted complex exhibited a fast
tunneling relaxation process (cf. Section 3.2.2). Detailed analysis of the XRD data revealed that one of MeO-groups

Scheme 39 (a) Transformation of inverted sandwich (l-Pc)[Dy(dpm)2]2
into a triple-decker mixed ligand complex (l-LR)2Dy2(Pc)2(H2O) uponinteraction with 3-alkoxysalicylic aldehydes, H(LR), R = OMe, OEr. (b)Structure of the middle layers formed by deprotonated ligands LR,coordinated to dysprosium ions (CCDC MESMIJ, R = Me and MESMOP,
R = Et).201

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coordinated to dysprosium(III) ion, while oxygen atoms of bothethoxy-groups remained in the non-coordination mode
(Scheme 39b). This diﬀerence in the substitution of ligands
in the middle layer leads to perturbation of the main magnetic
axes, consequently changing the coupling interaction andmodulating the magnetic behaviour for this Dy2 system.
2.6

Sandwich complexes that go beyond triple-deckers

Simple electrostatic considerations suggest that triple-deckers
formed by doubly-deprotonated porphyrins and phthalocyanines with a pair of trivalent REE cations are the largest
possible electroneutral species. However, there is interest inlarger sandwiches, which can fill in the gap between discrete
double-/triple-decker complexes and infinite lattices formed bythese complexes in solid functional materials. The following
section will describe some approaches which were developed
over the last decade to obtain discrete multideckers, avoiding
electrostatic limitations.
2.6.1 Discrete sandwiches containing from four to six
decks bridged through REE(III) and Cd(II) metal centres. Fromthe electrostatic viewpoint electroneutral trisphthalocyaninates
can be considered as complexes formed by anionic doubledeckers [M(Pc)2] and cationic half-sandwiches. Thus, bindingof two [M(Pc)2] species with an appropriate divalent cation
could be used as a pathway to a quadruple-decker complex. In
2010 this idea was realized by Fukuda et al. on the exampleof the reaction between [Lu(Pc)2](NBu)4 and cadmium
acetate (Scheme 40a).202 Heating of solid reagents at 400 1C
followed by chromatographic separation aﬀorded 3.4% of aChem Soc Rev
quadruple-decker complex (Pc)Lu(Pc)Cd(Pc)Lu(Pc), which wascharacterized by elemental analysis, mass-spectrometry and
NMR spectroscopy. The complex underwent eight reversible
one-electron redox-processes – four oxidation reactions andfour reduction reactions, while the parent lutetium bisphthalocyaninate showed only two oxidation reactions and three
reduction reactions in the same potential range; moreover, in
this case the gap between first reduction and oxidation potentials was wider than in the case of a quadruple-decker complex,
revealing the stacking effect which stabilized the redox forms of
the extended complex. Spectroelectrochemical studies also
revealed exceptionally low-lying p–p* excited states of theoxidized form of a quadruple-decker complex – the corresponding absorbance bands appeared at ca. 1000 cm1, where
normally only finger-print vibration bands are observed in IR
spectra.203 These experimental observations were interpretedusing molecular orbital calculations.204
Both stability and yields of quadriphthalocyaninates could
be improved when electron-rich BuO-substituted ligands wereused instead of unsubstituted phthalocyaninates. Thus, Wanget al. showed that the reaction of neutral Dy[(BuO)8Pc]2 withCd(OAc)22H2O in refluxing TClB aﬀorded target substituted
quadruple-decker complex Dy2Cd[(BuO)8Pc]4 in yield as high as
43% (Scheme 40b).205 In a similar manner, Eu(III) complex withn-hexylthio-substituents Eu2Cd[(HexS)8Pc]4 was synthesizedstarting from the corresponding double-decker in 84% yield(Scheme 40c).206 Both the OFET and gas sensing properties
were studied on the self-assembled film of the latter complex(cf. Section 4).

Scheme 40 (a–c) Synthesis of discrete quadruple-decker complexes Lu2Cd(Pc)4,202 Dy2Cd[(BuO)8Pc]4,205 and Eu2Cd[(HexS)8Pc]4206 via connectionof two double-decker molecules with Cd(II) ions. (c) Structure of complex Dy2Cd[(BuO)8Pc]4 (CCDC IYOCIJ),205 the structure of only one of two
crystallographically independent molecules is shown, hydrogen atoms and solvent molecules are omitted.

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Single crystals of the Dy(III) complex were obtained by Wanget al., and XRD provided a first ultimate proof of a quadrupledecker structure (Scheme 40d).205 Separation between Dy(III)ions – ca. 6.65 Å was expectedly much larger than in tripledecker complexes; however, it was still enough to detect notable
intramolecular interaction between paramagnetic metal centres, which was responsible for the fast quantum tunnelling ofmagnetization in a zero DC field (cf. Section 3.2.4). Interestingly, introduction of alkoxy and alkylthio groups provided
complexes with different conformations. In the case of the
BuO-substituted complex both Dy(III) and Cd(II) metal centres
were in distorted prismatic surrounding with a twist angle ofca. 201,205 while in HexS-substituted complex Eu(III) and Cd(II)had lightly deviated SAP coordination geometries with twist
angles of ca. 43 and 391.206
The capability of cadmium bridge to connect pairs of
diﬀerent double-deckers was tested by Wang et al. on theexample of homoleptic bisphthalocyaninates M(Pc)2 andmixed-ligand complexes (TClPP)M(Pc), M = Eu and Y.207 In
principle, this reaction could lead to isomeric quadruple-decker
complexes with diﬀerent sequences of tetrapyrrols – Pc–Pc–Pc–
Por or Pc–Pc–Por–Pc, but both NMR and X-ray crystallography
showed that only one type of complex with terminal porphyrin
ligand was formed. The complexes were isolated in 9–14%
yields. Similarly to homoleptic quadriphthalocyaninates, thesynthesized mixed-ligand complex had rich electrochemistry
associated with four ligand-centred oxidation and three
reduction processes, whose potentials showed dependence on
the metal centre.
No surprise, extended sandwiches became attractive objects
to study long-distance interactions between paramagnetic
metal ions in the context of elaboration of SMM materials

Review Article
and understand mechanisms or magnetization relaxation.
Thus, the approach to quadriple-deckers was extended to evenhigher sandwiches. Wang et al. showed that reactions of
M[(BuO)8Pc]2 with one or two equivalents of H2[(BuO)8Pc]
and Cd(OAc)22H2O allowed the introduction of one or two
additional phthalocyanine layers between REE-containing moieties, aﬀording therefore quintuple- and sextuple-decker sandwiches (Scheme 41).208,209 Moreover, this strategy allowed thesynthesis of heteroleptic quintuple-decker sandwiches with
precise sequences of unsubstituted and octa-pentyloxysubstituted ligands – either Pc–Pc–Pc*–Pc–Pc or Pc–Pc*–Pc*–
Pc*–Pc.210 Single-crystal XRD studies of complexes evidenced ofsmooth elongation of distances between REE ions going from
triple- to sextuple-deckers, providing more objects for studies oflong-range magnetic interactions (cf. Section 3.2.4).211–214
In principle, this approach can be used to obtain multidecker complexes with any number of tetrapyrrolic ligands,
since further sequential addition of ‘‘CdPc’’ fragments does not
violate the electroneutrality requirement in the resulting
extended sandwiches. However, the problems with isolationof required multi-decker from the mixture of complexes arehighly likely to arise.
2.6.2 Sandwich complexes based on fused dimeric tetrapyrrols. Synthesis of binuclear fused phthalocyanines [f-Pc2]
sharing common benzene rings was reported for the first timeby Kobayashi et al. in 1994 via the cross-condensation ofdiiminoisoindolines derived from phthalonitriles and pyromellitic tetranitriles.215 Almost 20 years later Wang et al. recognized f-Pc ligand as a convenient scaﬀold to form biradical
sandwich complexes which can be used to clarify the role ofradicals in 2p–4f integrated systems and study the magnetic
properties of polynuclear systems (cf. Section 3.3.2).216

Scheme 41 Synthesis of discrete quintuple-decker complexes M2Cd2(Pc)5208 and sextuple-decker complexes M2Cd3[(BuO)8Pc]6.209 Butoxy-groups inresulting multi-deckers are not shown.

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Thus, in 2013 Wang et al. showed that Y(III)- or Dy(III)templated cross-condensation of bis(diiminoisoindoline) with
4,5-dibutoxyphthalonitrile in refluxing pentanol in thepresence of DBU furnished target fused bisphthalocyaninates(Pc)M[f-Pc2]M(Pc)OBu in 2.0–4.3% yields (Scheme 42a).216 In
the same manner Tb(III) complex was synthesized by Morita
et al. in 2.5% yield using hexanol as a solvent for templatecondensation.217 Using 4,5-di(ethylthio)phthalonitrile inthis procedure allowed Wei et al. to increase the yield of the
fused bisphthalocyaninate (Pc)M[f-Pc2]M(Pc)SEt to 24.8%(Scheme 42b).218
Fused complexes with higher nuclearity could be synthesized using variants of the raise-by-one-story approach. Thus,
Motita and Katoh et al. treated metal-free BuO-substituted
fused phthalocyanine H4[f-Pc2]OBu with double-deckers
and Tb(III) or Dy(III) acetylacetonates in TClB to synthesize
tetranuclear Tb(III)219 and Dy(III)220 complexes (Pc)2M2[f-Pc2]M2(Pc)2OBu (Scheme 42c). Alternatively, Wei et al. treatedSEt-substituted fused Eu(III) bisphthalocyaninate with
H2[(SEt)8Pc] and Eu(acac)3 to synthesize tri- and tetranuclear
complexes (Pc)M[f-Pc2]M2(Pc)2SEt and (Pc)2M2[f-Pc2]M2(Pc)2SEt
which could be separated by chromatography (Scheme 42d).218
In all cases the yields of isolated complexes were relatively
small, yet, it was enough to characterize their properties anddemonstrate their applicability.

Chem Soc Rev
Fused bi-, tri- and tetranuclear substituted complexes wereused as redox-active components in sensors to NO2 and NH3
revealing excellent sensitivity to these gasses.218,221 The film
formed by the tetranuclear SEt-substituted complex was one of
the best gas sensors obtained so far for all solution-processedorganic semiconductor films in dry air at room temperature.218
Binuclear SEt-substituted complex was also used as a component of an electrochemical sensor for simultaneous detectionof acetaminophen and ascorbic acid.222 SHex-substituted tetranuclear complex was found to form well-defined single crystal
microsheets upon self-assembling in solution, which were used
to fabricate OFET with high hole and electron mobilities of 18
and 0.3 cm2 V1 s1, respectively, representing the best resultfor phthalocyanine-based OFETs at that time (cf. Section 4).223
A unique example of fused bisporphyrinate synthesizedfrom b-b/meso–meso/b-b triply-linked diporphyrin H4[f-Por2]
was reported by Lee et al. in 2019.224 Starting ligand was treatedwith LiHMDS to generate a tetralithium intermediate which
reacted with half-sandwiches (TPP)M(acac) to furnish fused
sandwiches (TPP)M[f-Por2]M(TPP), M = Tb and Y in small yields(Scheme 43). Complexes were isolated in neutral states exhibiting an open shell biradical character, as determined frommagnetic susceptibility measurements and theoretical calculations. The Tb(III) complex exhibited rich electrochemistry, having more than eight clear peaks in the voltammetry curves;

Scheme 42 Synthesis of fused sandwich phthalocyaninates. (a and b) Binuclear complexes (Pc)M[f-Pc2]M(Pc)XR, XR = OBu or SEt. (c) Tetranuclear
complexes (Pc)2M2[f-Pc2]M2(Pc)2OBu. (d) Tri- and tetranuclear complexes (Pc)2M2[f-Pc2]M(Pc)SEt and (Pc)2M2[f-Pc2]M2(Pc)2SEt. Superscripts OBu and SEt
indicate that all accessible peripheral positions of the benzene rings contain the corresponding substituents.

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Scheme 43 Synthesis of fused sandwich bisporphyrinates (TPP)M[fPor2]M(TPP).224

moreover, it has a very small HOMO–LUMO energy gap asevidenced from optical (3700 nm, 0.33 eV) and electrochemicalmeasurements (3400 nm, 0.36 eV). These features together withhigh stability render these biradicals as promising candidates
for use in molecular electronics, optics, and spintronics.

3. Development of the functionalities
of sandwich tetrapyrrole rare-earth
single-molecule magnets
In this section, we will provide a comprehensive survey ofseveral types of sandwich tetrapyrrole rare-earth (RE) singlemolecule magnets (SMMs) that use a supramolecular approach,combined with a discussion of their functionalities. Importantly, the properties of these molecules are changed significantly by the formation of associations among such molecules.
The chemical and physical properties of molecular assemblies
composed of a finite number of molecules are also aﬀected byvarious intermolecular interactions. Currently, researchers arealso conducting studies on applying self-assembly strategies
that actively apply coordination bonds to molecular magnetic
materials. In 2002, Wernsdorfer et al. reported that the introduction of hydrogen bonds and van der Waals forces into atetranuclear Mn complex SMM (abbreviated as [Mn4O3Cl4(O2CEt)3(py)3] = Mn4) resulted in dimers of the type Mn4  Mn4,
which exhibit superexchange interactions and suppress thequantum tunnelling of magnetisation (QTM; cf. Section
3.1).225 This phenomenon was achieved via the superexchangeinteraction between the SMMs, which act as an exchange bias
(Hbias E M2J/gmB) on the QTM that occurs at a zero magnetic
field, resulting in a spin-sublevel energy mismatch. Thus, themagnetic interaction acts as a perturbation between the SMMs
that aﬀects the QTM that occurs at a near-zero magnetic field.
In other words, it is also possible to control the properties of9294 | Chem. Soc. Rev., 2022, 51, 9262–9339

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SMMs using intra/intermolecular magnetic interactions
between SMMs. Moreover, to control the magnetic interactions
between SMMs, it is possible to use not only hydrogen bonds
but also p–p interactions, CH–p interactions, and inclusion(host–guest) interactions associated with ion and molecular
recognition. Controlling the spin relaxation time (t) by utilizing
the interactions between the infinite number of intramolecularvibrational modes of SMMs and spin excitation is essential for
spintronics. SMMs (molecular crystal) have weak intermolecular interaction and low phonon energy in the acoustic mode.
Therefore, the spin-acoustic mode interaction is restricted. On
the other hand, by introducing a strong intermolecular interaction into the SMMs, the phonon energy in the acoustic modecan be increased, and interaction with the spin can be
expected. In other words, from the viewpoint of controllingmagnetic relaxation, the approach of introducing a molecularassembly structure into SMMs is one method (vide infra).
Furthermore, ligand modifications, protonation–deprotonation reactions, and redox reactions also cause dramatic changes
in SMM characteristics by inducing substantial changes in theircoordination environments. Sandwich tetrapyrrole rare-earthtype SMMs have attracted attention for potential use in thefields of spintronic devices and/or quantum technology (QT).
However, the performance of these devices depends on thenature of the ground-state electronic structures of the SMM
units. By using supramolecular approaches to prepare sandwich tetrapyrrole rare-earth-type SMMs, their ground-state electronic structures and functionality can potentially be controlled(Fig. 9).
3.1

Brief outline of single-molecule magnets

In 1993, Sessoli, Gatteschi, and co-workers reported that thecluster [Mn12O12(AcO)16(H2O)4]2AcOH with S = 10 (henceforthabbreviated as Mn12; Fig. 10a), which contains twelve Mn ions(eight Mn3+ with S = 2 and four Mn4+ with S = 3/2), exhibitedsuperparamagnetic properties; they referred to Mn12 as an
SMM because it exhibited both single molecule properties
and ferromagnetic properties (Fig. 10c).226,227 However, REEtype SMMs are fundamentally diﬀerent from transition-metal(TM) SMMs, because their SMM behaviour originates from the felectrons (Fig. 10b). Since the orbital angular momentum L
does not disappear for f electrons, ground-state multiplets
exhibit a total angular momentum J due to spin–orbit coupling(SOC). When the ground-state multiplet is placed in a ligand
field (LF) environment, the degeneracy splits into the 2J + 1
sublevels (Fig. 10d). As a result, double-well potentials form,
and SMM behaviour emerges (vide infra).36,226 In general, thesublevel splitting in REE-type SMMs (102–103 cm1)5 is muchlarger than that of TM-type SMMs (ca. 70 cm1),228 indicatingthat REE-type SMMs are more useful in terms of their SMM
characteristics, such as large magnetic anisotropy and Ueff
(Fig. 10c and d).229,230
In the last 20 years, SMMs with high operating temperature
have been synthesized by controlling the LF and/or crystal field(CF) around the metal ion, even in single-ion magnets(SIMs).234 Since the Mn12 cluster complex has been shown toThis journal is © The Royal Society of Chemistry 2022

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Fig. 9 Brief overview of rare-earth sandwich tetrapyrrole SMMs. Double-decker complexes with various kinds of porphyrinoids act as SMMs. The
magnetic properties of the double-decker SMMs can be reversibly tuned by redox reactions and protonation–deprotonation reactions. Various
approaches for connecting/arranging SMMs have been used to introduce Ln–Ln interactions and tune the coordination environment.

Fig. 10 Molecular structures of (a) Mn12 and (b) Tb(Pc)2 based on the cif
files from ref. 231 and 232. Schematic energy diagrams of (c) Mn12 and (d)Tb(Pc)2. In the case of Mn12, eight Mn3+ and four Mn4+ ions with d-orbital
splitting (LF splitting) couple with each other (exchange interactions) toform an S = 10 state. The L of Mn3+ and Mn4+ is quenched due to the large
LF splitting. The zero-field splitting (D = –0.46 cm1) of S = 10, whichmainly originates from second-order SOC, results in a Ueff of ca. 40 cm1for spin reversal between mS = +10 and mS = 10. In the case of Tb(Pc)2,
S = 3 and L = 3 couple with each other (first-order SOC) to form 7F6. LF
splitting of the Tb3+ centre results in a Ueff of ca. 400 cm1 for spin reversal
between mJ = +6 and mJ = 6. mJ vs. energy plots were constructed fromref. 233.

exhibit the SMM phenomena, 3d to 5f metal complexes havebeen investigated as SMMs (henceforth, the term SMM will be
used to refer to SIMs as well).228,230,235–244 SMMs have a fixed
number of metal ions and an ordered magnetic ion structure,and exhibit uniaxial magnetic anisotropy due to the introduction of an LF and SOC.226,227,229,245 A brief description ofmagnetic anisotropy is given here. When a magnetic material

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is magnetised in various directions, there are directions inwhich it is easy to magnetise (easy axis of magnetisation) anddifficult to magnetise (hard axis of magnetisation). When the
direction is in one direction (uniaxial), it is called ‘‘uniaxial
magnetic anisotropy’’. This magnetic anisotropy is caused by
the orientation of magnetic moments of the atoms which have
the CF and SOC that make up the magnetic material. Thatis, the stable direction of orbital angular momentum in the
crystal affects the direction of spin, which is the source ofmagnetism. As described later, the 4f orbital shape of rare earth
metal ions has a very strong directivity, so it is possible tocontrol the magnetic anisotropy by controlling the appropriate
CF (cf. Section 3.2). A characteristic of Mn12 is that the JahnTeller axis of Mn3+ ions with magnetic anisotropy is almost
parallel to the S4 axis, showing large uniaxial magnetic anisotropy as a single molecule.226 It is possible to control themagnetic properties of metal complexes, such as their electronic state, LF, and magnetic anisotropy, by changing the metal
ions and coordinating ligands. In particular, it has been shownthat the LF directly affects the magnetic properties. Classicalmagnets exhibit large magnetisation via three-dimensional
interactions among the electron spins of the metal ions, but
SMMs involve the uniaxial magnetic anisotropy of one molecule. It is thought that SMMs are also ‘‘single-domain magnets’’, as intermolecular magnetic interactions are ignored.
Using the eﬀective spin Hamiltonian, we discuss the uniaxial magnetic anisotropy and Ueﬀ in SMMs. The concept of ^ was introduced by M. H. L.
eﬀective spin Hamiltonian H
Pryce which has only the spin operator being considered. When
incorporating the CF and SO into the eﬀective spin Hamiltonian, it can be expressed by D (uniaxial crystal field constant)and E (biaxial crystal field constant). Here, the spin Hamilto^ ¼ DS^z2 with the D. In the absence of annian is given by H

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external DC magnetic field (H = 0; H and Hdc are used interchangeably), the Zeeman term goes to zero, so DmS2 (mS is the zcomponent of S) is the splitting energy at zero magnetic field(zero-field splitting, ZFS). For the reasons mentioned above, thesplitting width of the ground and highest energy levels isdefined as Ueﬀ = |D|S2 for TM-type SMMs, where the D value
is the ‘‘zero-field splitting constant’’ and the S is the spinquantum number of the ground state, which mainly originatesfrom second-order SOC (the Ueﬀ of REE-type SMMs is explainedin the next section). For SMMs, the degeneracy of the total spinstate Stotal is split into 2S + 1 by LF and SOC, a quantizeddouble-well potential is formed by the uniaxial magnetic anisotropy, and the spin inversion between spin-up and spindownstate caused by the spin sublevel shows Arrhenius-likerelaxation (t = t0 exp(Ueﬀ/T), Fig. 10c). Here, t represents themagnetic relaxation time, t0 is the frequency factor, Ueﬀ is theenergy barrier associated with spin inversion, and T is thetemperature. At temperatures higher than Ueﬀ associated withspin inversion, the SMM behaves as a paramagnetic material,
but when the temperature decreases and the Ueﬀ cannot be
overcome, the magnetic moment (spin) is frozen. The temperature at which the t reaches 100 s is defined as the blocking
temperature (TB). SMMs behave like a magnet which showsmagnetic hysteresis with coercivity (HC) below TB due to long t
values. In addition, the field cooled (FC) and zero-field cooled(ZFC) measurements of the SMM diverge at TB. The magnitudeof magnetic anisotropy is possible to estimate from the M
versus HT1 plot at diﬀerent T values. If SMMs have large
uniaxial magnetic anisotropies and/or the population of lowlying excited states, M versus HT1 shows non-superposedmagnetisation curves.
The temperature change in the spin system reaches thermal
equilibrium through heat exchange between the spin and thelattice, and finally moves to the heat bath. Therefore, theinformation for slow magnetic relaxation can be obtained fromalternating current (AC) magnetic measurements. The
measurement data are analyzed by using a generalized Debye
model containing the dispersion coeﬃcient a. It is possible toestimate values of t. The magnetic relaxation undergoes asingle relaxation process at a = 0, a Cole–Cole plot (w00 versus
w 0 plot) is semicircular.246 In addition, magnetic relaxation is aphenomenon associated with spin–phonon coupling, in which
the lattice vibration of the molecular crystal is mainly dominated by intramolecular vibration. SMMs undergo three typesof magnetic relaxation processes due to spin–lattice interactions, i.e., (1) direct, (2) Raman, and (3) Orbach processes.247
The direct process is caused by the absorption or emission of aphonon (h o), while the Raman process is due to phonon
scattering between the ground state and the virtual transition
to higher energy levels.248–250 Numerical simulation of the T
dependence of t is carry out using a combination of themagnetic relaxation processes.247,251,252
Furthermore, at extremely low temperatures (e.g. 40 mK),
spin inversion via QTM occurs due to the quantum tunnelingeﬀect when the energies of the spin sublevels are equal, and thesteps in the magnetic hysteresis curves (M versus H) are

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observed using a micro-SQUID (Fig. 12c).227,245,253 The origin
of QTM in SMMs can be understood via an energy level diagram
for a system with a tunnelling gap (DEtunnel) by the hyperfine(hf) interaction between the electronic spin and the nuclear
spin I. QTM is complicated due to the interactions of theground state with the I, which cause energy level crossing(Fig. 12b).253–255 It is possible to explain QTM using theLandau–Zener–Stückelberg (LZS) model to the ground state
mS levels.245 Quantum phenomena, such as the QTM
observed in SMMs, have been studied in relation to the fieldof quantum logic (e.g., quantum spin coherence,256 quantum
phase transition,257 Berry phase interference,258 and quantum
computers259). Interest in the use of molecules as spin qubits is
growing rapidly, and several perspective articles and reviews
focusing on such molecules have been publishedrecently.260–263 Using SMMs in information storage devices
would allow memory capacity to be expanded quite dramatically. Thus, the development of multi-functional SMMs can be
expected to continue for several years to come.264 In addition,
single-crystal X-ray diffraction analysis and/or theoretical calculations (DFT and ab initio; e.g. CASSCF/RASSI-SO/SINGLE_ANISO calculations)265 for the structure optimisation and theground state are the most effective methods for investigating
the coordination environment of Ln3+ ions in SMMs. Additionally,
electron spin resonance (ESR), nuclear magnetic resonance(NMR), inelastic neutron scattering, and muon spin relaxation(mSR) are useful in the investigation of the SMM phenomena.245 It
is possible to evaluate the SMM characteristics by using a variety
of measurements together with magnetic measurements. Thus,
the field of molecular magnetism, which includes SMMs, hasbeen extensively studied recently in physics and nanotechnology
contexts, particularly in the areas of molecular spintronics andquantum technology (QT) (Fig. 11).266
For more information, several books and reviews suggested by
the authors are included in the reference. In particular, D.
Gatteschi, R. Sessoli and J. Villain have published a groundbreaking book on SMMs in 2006, which covers everything from basic
principles of physical phenomena to measurement techniques.245
3.2 Brief overview of rare-earth sandwich tetrapyrrole singlemolecule magnets
Phthalocyanines feature prominently in diverse research fields,
including molecular conductors, solar cells, liquid crystals,
pharmaceuticals, and biological chemistry,271,272 as they arehighly soluble in several organic solvents, chemically stable androbust, as well as commercially available.273–275 In addition, in
recent years, tetrapyrrole has attracted attention as a ligand for
SMMs.9,36
In order to identify an underlying cause for SMM properties,
we review the coordination environment and SMM propertiesof Ln(Pc)2 with a D4d symmetry. The LF potential around a Tb3+
ion (7F6) with a total angular momentum (J) of 6 splits theground-state multiplet so that the lowest sublevel has
the largest z-component of J (mJ = 6) and a large energy gap(ca. 400 cm1) to the next-closest sublevel (mJ = 5), which iscorrelated with Ueﬀ (Fig. 10d).36 In 2003, Ishikawa et al. first

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Fig. 11 Quantum technology actively using SMMs. Rare-earth sandwich tetrapyrrole SMMs (Tb(Pc)2) have excellent chemical and physical stability, whichmake it possible to use them in quantum devices. Reprinted with permission
from (a) ref. 267. Copyright 2010, American Chemical Society. Reprinted from (b)ref. 268. Copyright 2016, Royal Society of Chemistry. Reprinted from (c) ref. 269.
Copyright 2011, MDPI. Reprinted with permission from (d) ref. 270. Copyright
2014, American Association for the Advancement of Science.

Chem Soc Rev
tional modes upon ligand modification of the [Dy(Cp)2]+(Cp = cyclopentadienyl) complex.234,282,283 The high Ueﬀ of thiscomplex originates from the Orbach process via 4th excited LF
levels, meaning that those via lower excited states are eﬀectively
suppressed. The control of the coupling between spin andvibrations is therefore extremely important for achieving
high-performance SMMs.
There have been many attempts, both experimental andtheoretical, to elucidate the relationship between coordinationgeometry and LF parameters, which have a great influence on
the ground-state multiplet structure and magnetic anisotropy(cf. Sections 3.2.1 and 3.2.2).229 Contractions of square antiprismatic (SAP) coordination environments aﬀect the LF of
Ln3+ ions, thus bringing about highly significant changes
in the SMM properties.36,84,233,242,252,276,284–287 In general,the LF (or CF) parameter in the Hamiltonian is defined asfollows:^ LF ¼
H

6 X
k
X

q

^ ;
Bqk O
k

k¼2 q¼kreported the SMM properties of (NBu4+)[Tb(Pc)2], whichcontains a single rare-earth 4f ion (Ueﬀ = 230 cm1; t0 =6.25  108 s; HC E 0.01 T@0.04 K).36,276 For the neutral(one-electron-oxidized) species Tb(Pc)02 with an unpaired
p-radical (S = 1/2) on the Pc ligands, the Ueﬀ is 410 cm1.277
In comparison to (NBu4+)[Tb(Pc)2] and Tb(Pc)02 , the cationicspecies Tb[(EtO)8Pc]2+ has a much larger Ueﬀ (550 cm1).84 The
cationic species has a shorter interligand distance than theneutral and anionic ones because the electrons are removed
from antibonding molecular orbitals.84 In addition, structuralchanges in Tb[(BuO)8Pc]2+ are induced by oxidation from its
neutral/anionic species, whereby the interligand distancedecreases upon oxidation (cf. Section 3.4).278 Thus, LF contractions cause strong LF splitting, which directly aﬀects the Ueﬀ
value. This result quantified LF contractions in the cationicspecies. Later, the group of complexes that exhibits SMM
characteristics with a mononuclear d/f metal ion also became
known as single-ion magnets (SIMs). In particular, Ln(Pc)2based SMMs are attractive complexes that can be developed
using various approaches, because it is possible to control the
distance between the Ln3+ ions, the coordination environment,
and the electronic state by chemically modifying the Pc
ligands.3,4,279 It should be mentioned here that the relationshipbetween the magnitude of the LF splittings and the Ueﬀ is notstraightforward. In case of Tb(Pc)2, the Ueﬀ coincides with theenergy diﬀerence between ground mJ = 6 and 1st excited mJ =
5 states (Fig. 10d). The reason why the magnetic relaxations
via 2nd or higher LF level as seen in Mn12 do not occur in theseries of Tb(Pc)2 remains unclear to date. However, recent
research studies on the SMMs indicate the importance of thevibrations of atoms and molecules in the crystalline
lattice.248,280,281 For instance, SMMs with a high Ueﬀ(1541 cm1) and record TB value (80 K) were reported by Guo
et al. in 2018, which was achieved by modifying the vibra-

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where Bqk is the LF parameter and Ôqk is the Stevens
operator.288–291 There are noticeable differences in SMM properties due to the presence or absence of particular off-diagonal
LF terms (Bqk, k = 2, 4, 6; q a 0), which are the non-axial LF
parameters. As a specific example, it has been reported that the
LF parameters of the square prismatic (SP) coordination structure with D4h symmetry and those of the ideal SAP coordination
structure with D4d symmetry are different. In the SAP geometry,
only the axial anisotropy LF parameters occur (Bqk, k = 2, 4, 6; q =0).252,276,284,288,291–293 On the other hand, the SP geometry
contributes not only to the axial LF parameters, but also to
the transverse anisotropy LF parameters B44 and B46. In the caseof [Tb(Pc)2], the LF parameters (Bqk/cm1) are as follows: realgeometry (B02 = 414, B04 = 228, B06 = 33, B44 = 10)252 and ideal
geometry (B02 = 574, B04 = 116, B06 = 20).292 In the case of[Dy(Pc)2], the Bqk are as follows: real geometry (B02 = 395, B04 =220, B06 = 40, B44 = 10).284,287 The fourth-range extradiagonal
parameters (B44 and/or B46) exert a greater influence on both thehyperfine structure of the ground state and the ground-state
multiplet structure.122,276,294–296 Moreover, as the structure
deviates from D4d symmetry, the twist angle (y) and the zenithal
angles (j) between the C4 axis and the direction of the Ln3+–Nisocoordination bond exert great influence on the ground state
and the first excited state of the octacoordination geometry of
the Ln(Pc)2 complex as determined using the theoretical
approach.286,287 Several groups have reported the mJ groundstates for different j and Ln3+–Niso distances. It is clear that the
Tb3+ ground-state doublet will be |mJ| = 6 at j o 551. However,taking a Ln3+–Niso distance of 2.3 Å as an example, the |mJ| = 15/
2 ground state of Dy3+ will no longer be the lowest at j 4491.286 Previous studies have reported that the lowest |mJ|
ground state of the Tb(Pc)2 system is the |mJ| = 6 state, andthat of the Dy(Pc)2 system is the |mJ| = 13/2 state.233 These high|mJ| ground states have high magnetic anisotropy, which isChem. Soc. Rev., 2022, 51, 9262–9339 | 9297

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advantageous for SMM design. Consequently, the distortion of
the coordination environment is reflected in the SMM characteristics of rare-earth sandwich tetrapyrrole SMMs. The protonation–deprotonation reactions of the double-decker SMMs
cause significant changes in the coordination environment
around the Ln3+ ions, enabling reversible switching of the
SMM properties (cf. Section 3.5).297 The SMM properties of
Ln3+ sandwich complexes are also sensitive to the nuclear spinsof the Ln3+ ions (cf. Section 3.2.3)276 and Ln–Ln magnetic
interactions (f–f interactions; cf. Sections 3.2.4 and 3.3).298
Various approaches are available to induce f–f interactions toimprove SMM properties (cf. Sections 3.3.1–3.3.4).
Ln(Pc)2 exhibits excellent thermal stability and is attracting
attention as an SMM to which the vacuum deposition method
can be applied; they can be deposited on various substrates inan ultra-high vacuum (UHV) environment. Ln(Pc)2-based SMMs
have excellent physical and chemical stability, and manygroups, including ours, are investigating these SMMs as
molecular-memory and spintronic device materials (cf. Section
3.8).117,135,232,267,299–306 The unpaired p-radical (S = 1/2) on thePc ligands in Tb(Pc)2 interacts with the conduction electrons,
and Kondo resonance has been observed using scanning tunnelling spectroscopy (STS).302 Furthermore, the Kondo resonance largely depends on the twist angle (y) of the tetrapyrrole
ligand and the electronic structure due to the delocalized p spinstate, demonstrating the manipulation of a single spin with aconduction electron.135,302,307 In addition, Ln-Pc-based SMMs
are attractive materials for quantum mechanical devices thatcan be developed based on the separation between the groundstate doublet mJ and the excited states, as well as the hyperfine(hf) interaction between the electronic spin and the nuclear
spin I, because it is possible to control the hyperfine QTM (hfQTM).269,270,303,308–315 The important point here is that eachspin (S, J, I) in Tb(Pc)2 shows a cascade phenomenon throughspin interactions. Here, Tb(Pc)2 incorporated in a transistor
structure is in contact with the source, drain, and gate electrode. The various spins (S, J, I) interact with the injected
conduction electrons via the delocalized p electrons, and the
conduction electrons recognize changes in the hf-QTM state
during magnetic field sweeps.270 This represents a readout
mechanism for the nuclear spin state, in which the transport
properties of Tb(Pc)2 are controlled by the conduction electrons.
Thus, Tb(Pc)2 SMMs have all the properties necessary to serve as afunctional component of quantum-mechanical devices (Fig. 12).
3.2.1 Eﬀect of the coordination environment around theLn3+ ions on the SMM properties. In this section, we summarize the theoretical and experimental research that has elucidated the relationship between the coordination geometries
around the Ln3+ ions and the properties of the resulting SMMs.
Ishikawa et al. reported that the wavefunctions of the LF
sublevels, which are strongly correlated with the SMM properties, are sensitive to the coordination geometry around the Ln3+
ions.233 In double-decker complexes, a Ln3+ ion is coordinatedby the eight pyrrole N atoms of the porphyrinoids, and thecoordination geometries are evaluated via the twist angle ybetween the upper and lower ligands. In this case, there are two

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Fig. 12 Molecular qubit based on a Tb(Pc)2 SMM. (a) Schematic diagramof the qubit detection. Tb(Pc)2 is placed between Au electrodes. (b) In the
Zeeman diagram of Tb(Pc)2, the strong hyperfine coupling between themJ = 6 and the nuclear spin (I = 3/2) forms four avoided level crossings
with a tunnel gap (DE E 1 mK) and four diﬀerent qubit states (|–3/2i, |–1/2i,|+1/2i, and |+3/2i). (c) Quantum tunnelling phenomenon due to hyperfine
coupling in which the nuclear spin is observed in the magnetisation curveof Tb(Pc)2. Reprinted from ref. 308 with permission from the Royal Societyof Chemistry.

extreme coordination geometries, i.e., the square-prism (SP)geometry (y = 01) and square antiprism (SAP) geometry (y = 451).
In the case of the SP geometry, the LF Hamiltonian ĤLF
contains non-zero B44Ô44 and B46Ô46 terms:
ĤLF = B02Ô02 + B04Ô04 + B44Ô44 + B06Ô06 + B46Ô46
where the Bqk terms represent the LF parameters and the Ôqk
terms represent the Stevens operators. Stevens operators with
non-zero q values act to mix |mJi and |mJ  qi. Therefore, the B44
Ô44 and B46Ô46 terms mix the |mJi and |mJ  4i; this mixing
enhances the QTM. In contrast, the ĤLF of the ideal SAP
geometry does not contain an operator that gives oﬀ-diagonal
0 
0 
terms between |mJi and jmJ i mJ amJ :ĤLF = B02Ô02 + B04Ô04 + B06Ô06.
0

Therefore, there is no mixing between |mJi and jmJ i in theideal SAP geometry, suppressing QTM. It is not easy to construct the SP geometry in double-decker complexes without
replacing the Ln3+ ions.316 Instead, SP and SAP geometries
around Ln3+ ions have been realized by using porphyrinato–
phthalocyaninato triple-decker complexes (Fig. 13).
In 2012, Sakaue et al. reported the crystal structure and SMM
properties of the heteroligand triple-decker complex(TAnP)Ln(Pc)Ln*(Pc), Ln* = Ln = Tb, or Ln* a Ln = Tb, Y(Scheme 20 and Fig. 13a).122 The metal center sandwiched by
two Pc ligands (Ln*) shows SAP geometry, whereas that sandwiched by Pc and TAnP, namely, Ln, shows SP geometry. It is
possible to synthesize heterometal complexes (TAnP)Y(Pc)Tb(Pc) and (TAnP)Tb(Pc)Y(Pc), in which one of theThis journal is © The Royal Society of Chemistry 2022

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Fig. 13 X-ray structures of mixed ligand complexes, showing coordination surrounding symmetries for diﬀerent metal centres: (a) (Pc)Tb(Pc)Tb(TAnP)(CCDC SAZLIQ);122 (b) (TPP)Tb(Pc)Tb(Pc) (CCDC IJOSUX)317 and (c) (TClPP)Dy[(PhO)8Pc]Dy[(PhO)8Pc] (CCDC MIKSEG).318 Hydrogen atoms and solvate
molecules are omitted for clarity.

metal ions is replaced with a diamagnetic and isostructural Y3+
ion. The Tb3+ ion of (TAnP)Y(Pc)Tb(Pc) adopts SAP geometry,
whereas that of (TAnP)Tb(Pc)Y(Pc) adopts SP geometry. The
maxima of the imaginary part of the AC magnetic susceptibility 00 
wM for (TAnP)Y(Pc)Tb(Pc) occur at much lower frequencies
than those of (TAnP)Tb(Pc)Y(Pc) (Fig. 14b). In other words,(TAnP)Y(Pc)Tb(Pc) shows much longer t values than
(TAnP)Tb(Pc)Y(Pc). These diﬀerences have been ascribed tothe diﬀerences in the coordination geometry around the Tb3+
ions. As the Tb3+ ion of (TAnP)Y(Pc)Tb(Pc) adopts SAP geometry, mixing between LF sublevels and the accompanying QTM is
suppressed, which results in long t values.
In 2016, Katoh et al. reported a detailed study on the crystal
structures and the SMM properties of triple-decker complexes
composed of two Tb3+ ions, TPP ligands, and Pc.317 In case of
the triple-decker complex (TPP)Tb(Pc)Tb(TPP), the coordination geometry of the two Tb3+ ions is close to the SP geometry

00

Fig. 14 (a) wM vs. frequency plots for (TAnP)Y(Pc)Tb(Pc) (upper) and(TAnP)Tb(Pc)Y(Pc) (lower). (b) Coordination geometry and y vs. Ueﬀ plots
for the series of Tb3+ triple-decker complexes. Reproduced from (a) ref.
122 and (b) ref. 317 with permission from the Royal Society of Chemistry.

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(y = 41), and the Ueﬀ is ca. 50 cm1. Another triple-decker
complex, (Pc)Tb(Pc)Tb(TPP), showed two diﬀerent types ofcoordination geometry around the Tb3+ ions (Fig. 14b), and00
thus two peaks in the wM T vs. T plots. The coordinationgeometry around the Tb3+ (Fig. 13b) sandwiched by two Pc
ligands is similar to the SAP geometry (y = 381), whereas that of
the Tb3+ sandwiched by a TPP and a Pc ligand is similar to the
SP geometry (y = 141). The y value of the phthalocyaninato
triple-decker complex Tb2[(BuO)8Pc]3 is 321.319 By comparing
the magnetic properties of this series of triple-decker complexes, Katoh et al. demonstrated the existence of a linear
relationship between the Ueﬀ and y values (Fig. 14b). This result
is consistent with the degree of mixing of the oﬀ-diagonal LF
terms (B44Ô44 and B46Ô46) due to the deviation from the SAP
geometry.
The trend for Dy3+ complexes is similar to that for Tb3+
complexes. In 2013, Kan et al. reported a series of Dy3+ tripledecker complexes composed of phthalocyaninato and porphyrinato ligands (Fig. 13c).318 The y value of the Ln3+ between aphthalocyaninato and a porphyrinato ligand is narrow (101),
whereas the y value of Ln3+ surrounded by two phthalocyaninato ligands is wide (251). The mono-Dy3+ complex with a widey value, (TClPP)Y[(PhO)8Pc]Dy[(PhO)8Pc], shows slow magnetic
relaxation without a bias DC magnetic field. In contrast,(TClPP)Dy[(PhO)8Pc]Y[(PhO)8Pc] does not show slow magnetic
relaxation even in a DC field. The homonuclear Dy3+ complex(TClPP)Dy[(PhO)8Pc]Dy[(PhO)8Pc] shows field-induced SMM
properties. These results demonstrate that wider y values
improve the t values.
In 2012, Wang et al. reported the structural and magnetic
properties of the Dy3+ double-decker complexes (TClPP)Dy(Pc)and (TClPP)Dy[(a-Et2CHO)4Pc] (Fig. 15).320 The y value of(TClPP)Dy(Pc) is 441, whereas that of (TClPP)Dy[(aEt2CHO)4Pc] is 381. (TClPP)Dy(Pc) shows clear peaks in the00

wM vs. T plots in comparison with (TClPP)Dy[(a-Et2CHO)4Pc],

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Fig. 15 X-ray structures of mixed ligand complexes (TClPP)Dy(Pc) (CCDC
MAVQIL)317 and (TClPP)Dy[(a-Et2CHO)4Pc] (CCDC MAVQOR)321 together
with the symmetries of coordination surrounding lanthanide ions. Hydrogen atoms and solvate molecules are omitted for clarity.

indicating that QTM is suppressed in (TClPP)Dy(Pc) due to the
SAP coordination geometry.
3.2.2 Eﬀect of peripheral and coordinating atom substitution on the SMM properties. Atom substitution at the peripheral positions of the porphyrinoid ligands aﬀects the SMM
properties through changing the coordination geometry andelectrostatic potential of the coordinating N atoms.
In 2013, Ganivet et al. reported a series of Tb3+ doubledecker complexes composed of octa(p-tert-butyl phenoxy)phthalocyanine H2[(tBuPhO)8Pc], tetra(tert-butyl)phthalocyanine
H2(tBu4Pc) and unsubstituted phthalocyanine H2(Pc).49 The
heteroleptic ligand complexes in which one of the Pc ligands
is unsubstituted show higher Ueﬀ values (642–652 cm1) thanthe homoleptic ones (394–504 cm1). Ganivet et al. proposedthat electron-donating substituents increase the Tb–N distances, thus pushing the Tb3+ toward the unsubstituted phthalocyanine and hence increasing the Ueff. This concept was
subsequently extended by Chen et al. in 2017. These authors
reported a high Ueff value (652 cm1) and a magnetic hysteresisof up to 30 K at a sweep rate of 200 Oe s1 for the double-decker
complex [(Bu2N)8Pc]Tb(Pc), in which one of the Pc ligands issubstituted with alkylamino groups (Scheme 8).81 DFT calculations of the optimized structures for [(Bu2N)8Pc]Tb(Pc) and thecorresponding homoligand complex Tb[(Bu2N)8Pc]2 revealed
that the electrostatic potential of (Bu2N)8Pc is greater in theheteroligand Pc/(Bu2N)8Pc complex than in the homoligand(Bu2N)8Pc complex (Fig. 16a). This result supports the enhancement of Ueff in heteroligand complexes.
The SMM properties of the porphyrazine (Pz)-based doubledecker complexes Tb(OEPz)2 and Dy(OEPz)2 were reported by
Gimnez et al. (Scheme 25).140 The Ueﬀ values of Tb(OEPz)2(266 cm1) and Dy(OEPz)2 (22 cm1) are similar to those of
their Pc analogues. However, Pz complex benefits from its Pc
counterpart due to easier processing. They have a higher
solubility of up to 50 mM in CH2Cl2 and they can be readily
sublimed under relatively mild conditions, that is, 360 1C and
10 mbar. Tb(OEPz)2 and Dy(OEPz)2 were evaporated using

9300 | Chem. Soc. Rev., 2022, 51, 9262–9339

Fig. 16 (a) Electrostatic potential around Tb3+ in the Tb[(Bu2N)8Pc]2 and[(Bu2N)8Pc]Tb(Pc) double-decker complexes. (b) Magnetic hysteresis
curves for atom-replaced double-decker complexes. [(tBuPh)4SPor]Dy(Pc) show larger magnetic hysteresis (right) than [(tBuPh)4OPor]Dy(Pc)(left) at a sweep rate of 200 Oe s1. Adapted with permission from (a) ref.
81. Copyright 2017, American Chemical Society. Reproduced from (b) ref.
173 with permission from the Royal Society of Chemistry.

organic molecular beam epitaxy at 550 K and absorbed intact
on the surfaces of Au(111), Ag(111) and Cu(111) in a flat-on
fashion, forming hexagonal close-packed layers.322 Systematic
STM studies with multi- and monolayer XPS and complementary DFT calculations were performed showing the preservationof the electronic properties of both ligands and lanthanide
centres upon adsorption. Persistence of slow dynamics in
Tb(OEPz)2 single molecule magnets embedded in conducting
polymers was confirmed by muon spin relaxation on theexample of thin films of PEDOT:PSS mixed with the complex,
suggesting that the resulting hybrid material can be interestingfor organic spintronics.323
Replacing the coordinating N atoms of the porphyrinoidwith another element modulates the LF splitting of the Ln3+
ions. In 2015, Cao et al. reported the magnetic propertiesof [(tBuPh)4OPor]Dy(Pc) and [(tBuPh)4SPor]Dy(Pc), in which
one of the coordinating N atoms of the porphyrin unit isreplaced with an O or an S atom, respectively (Scheme 32 andFig. 16b).173 Importantly, the Ueﬀ values of [(tBuPh)4OPor]Dy(Pc)(95 cm1) and [(tBuPh)4SPor]Dy(Pc) (135 cm1) are larger thanthose of the non-replaced analogue [(tBuPh)4Por]Dy(Pc) (Ueﬀ =28 cm1). In addition, the Ueﬀ of [(tBuPh)4SPor]Dy(Pc) is thelargest among the Dy3+-porphyrinoid sandwich complexes. The
diﬀerence in the Ueﬀ values between [(tBuPh)4OPor]Dy(Pc) and[(tBuPh)4SPor]Dy(Pc) are also evidenced by the magnetic hysteresis curve, in which the latter complex exhibits larger magnetic
hysteresis than former complex does (Fig. 16c). Ab initio calculations revealed that the ground-state doublet of [(tBuPh)4Por]Dy(Pc) has the second largest mJ value of 13/2, whereas thoseof [(tBuPh)4OPor]Dy(Pc) and [(tBuPh)4SPor]Dy(Pc) have an mJ

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value of 15/2. The coordination of the O and S atoms to
Dy3+ is weaker than that of the N atoms, as evident from theexperimental bond lengths and calculated charge distributions.
The decrease in the LF along the Dy3+–X (X = O, S) direction
stabilizes the mJ = 15/2 states and thus enhances the SMM
properties.
In 2018, the SMM properties of hemiporphyrazinato doubledecker complexes (Scheme 37) were reported by Liu et al.189 The
homoleptic bis(hemiporphyrazinato) complex (HHp)Dy(Hp)and heteroleptic complex (HHp)Dy(Pc) show Ueﬀ values of
56 cm1 and 40 cm1, respectively. These Ueﬀ values are larger
than those of Pc-based Dy3+ double-decker complexes. The loneelectron pair on N in the pyridine in Hp is more localized incomparison with that of the isoindole N atom of Pc. As a resultof the stronger LF of Hp compared to that of Pc, the SMM
properties of the Hp-based complexes are superior to those of
the Pc-based complexes.
SMMs that combine porphyrinoid and non-porphyrinoid
ligands are rare. In 2013, Wang et al. reported triple-decker
SMMs composed of Pc and the Schiﬀ-base ligand SB. Reactionof the half-sandwich complex [(Pc)Dy(acac)] with H2(SB) aﬀords
the triple-decker complex (Pc)2Dy2(SB)(H2O) (Scheme 33),
which shows a Ueﬀ value of 19.8 cm1.176 The two Dy3+ ionsof (Pc)2Dy2(SB)(H2O) are crystallographically independent,
whereby one adopts a SAP geometry that comprises four N
atoms of Pc and two N and two O atoms of SB, while the otheradopts a seven-coordinated geometry constructed by four N
atoms of Pc, two O atoms of SB, and one oxygen atom of a watermolecule. In 2014, Gao et al. reported a similar triple-decker
SMM, Dy2(Pc)(SB-TTF)2(MeOH) (Scheme 34a), with a Ueﬀ valueof 20 cm1, which forms assembled structures on highlyoriented pyrolytic graphite (HOPG).179 The same author
reported the SMM properties of the trinuclear Dy3+ complex
Dy3(Pc)2(mS)(OAc)(OMe)2 (Scheme 34b) composed of two Pc
ligands and macrocyclic Schiﬀ base liagnd(mS), which exhibits
zero-field SMM behavior with a Ueﬀ value of 42 cm1 andferromagnetic interactions. In contrast, its Er3+ analogue
Er3(Pc)2(mS)(OAc)(OMe)2 exhibts neither SMM properties nor
ferromgnetic interactions.180 A similar analogue of the dinuclear Dy3+ complexes (l-LR)2Dy2(Pc)2(H2O) composed of salicyclic aldehydes H(LR) (R = OMe and OEt) and Pc reported by Ge
et al. (Scheme 39) exhibits zero-field SMM behavior with the Ueﬀ
values ranging from 27 (R = OEt in the presence of a DC
magnetic field) to 47 cm1 (R = OMe).201 The MeOsubstituted complex exhibits better SMM properties than the
EtO-subsituted one does because coordination geometry of thelatter one deviates from the SAP geometry, which is caused by along bond length between the ethocy O atom and Dy3+ ion. As
the result of diﬀerence in coordination geometry and LF,
methoxy-substituted complex exhibits intramolecular ferromagnetic interactions between two Dy3+ ions. Reaction of thehalf-sandwich complex [Dy(Pc)(OAc)] with the polyoxotungstate[PW11O39]6 aﬀords (Pc)Dy[PW11O39]6(NBu4+)6, which shows a
Ueﬀ value of 34 cm1 at a bias DC field of 500 Oe(Scheme 38c).193 SMMs composed of Kläui’s tripodal ligand
and a porphyrinoid, (LCo)Ln(Pc) and (LCo)Ln(TPP) (Ln = Tb andThis journal is © The Royal Society of Chemistry 2022

Chem Soc Rev
Dy) (Scheme 38a and b), were reported by Gao et al.,190 andtheir Ueﬀ values ranged from 6 to 17 cm1.
3.2.3 Quantum tunnelling of the magnetisation driven by
nuclear spin. Quantum tunnelling of magnetisation (QTM) is
an emblematic phenomenon originating from the quantum
mechanical properties of SMMs. QTM was observed for the firsttime in the Mn12 cluster SMM.255 This cluster has a total spinquantum number S = 10, and its degree of degeneracy is 21, i.e.,
21 of the magnetic states mix with each other at a certain
magnetic field. The occurrence of QTM was observed as steps in
the magnetic hysteresis curve of the Mn12 cluster. The energy
separation of 21 of the magnetic states of the Mn12 cluster isonly a few wavenumbers (Fig. 10c). Therefore, the application ofa relatively weak magnetic field can induce the mixing ofmagnetic states, making it possible to observe the steps in
the hysteresis curve of the Mn12 cluster. In contrast, the energy
separation of the LF sub-levels of [Tb(Pc)2] (and [Dy(Pc)2]) is
too great (several hundred wavenumbers) for mixing to be
induced between the 13 (16 for [Dy(Pc)2]) mJ states(Fig. 10d). In contrast, Ishikawa et al. reported that doping
excess amounts of the tetrabutylammonium (Bu4N+) salts of[Tb(Pc)2], [Dy(Pc)2], and [Ho(Pc)2] into a diamagnetic matrix([Y(Pc)2]) led to the observation of steps in the magnetic
hysteresis at dozens of millikelvin (Fig. 12c).276,324 These steps
originate from the QTM among the ground-state LF sublevels
coupled with the nuclear spins of Ln3+ (hyperfine coupling). In
the case of [Tb(Pc)2], the lowest energy doublet |mJi = |6i
couple with the nuclear spin I = 3/2 of 159Tb (natural abundanceof 100%) to form the |mJ,mIi (mI = 3/2, 1/2) states (Fig. 12b).
The steps in the magnetic hysteresis of [Tb(Pc)2] originate
from the QTM among those eight hyperfine states. The steps in
the magnetic hysteresis curves of [DyPc2] are complicated andunclear compared to those of [TbPc2] due to the sevenisotopes of Dy, namely, 156Dy, 158Dy, 160Dy, 161Dy, 162Dy,
163
Dy, and 164Dy. Moreno-Pineda et al. reported the magnetic
hysteresis of (Et4N+)[163Dy0.05Y0.95(Pc)2] (I = 5/2) and(Et4N+)[164Dy0.05Y0.95(Pc)2] (I = 0).311 163Dy has a nuclear spin,
whereas 164Dy does not. The magnetic hysteresis curves of(Et4N+)[163Dy0.05Y0.95(Pc)2] show clearer steps compared tothose of the mixture of nuclear isomers (Fig. 17a). Moreover,the multiple steps in the hysteresis curves disappear in(Et4N+)[164Dy0.05Y0.95(Pc)2] (Fig. 17b). This is direct evidence
that nuclear spins correlate with the QTMs.
Nuclear-spin-driven QTM has also been observed in the Tb3+
triple-decker complex (Pc)Tb(Pc)Tb[(Hex)8Pc], which contains
two Tb3+ ions (Tb(1) and Tb(2)) (Fig. 17c–f).314 As will be
discussed later, magnetic interactions exist between the Tb3+
ions in the triple-decker complex (cf. Section 3.2.4). Therefore,
QTM occurs among |m1J , m1I , m2J , m2I i. The number of possible
quantum states d for the triple-decker complex (d = (2I + 1)2 =16) is larger than that for [Tb(Pc)2] (d = (2I + 1)1 = 4). The
strategy of using Ln3+–Ln3+ interactions is promising for theconstruction of multiple qubits, which is required for quantum
computers.
3.2.4 f–f interactions elucidated using multiple-decker
complexes. In addition to double-decker complexes, multiple-

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Fig. 17 Field dependence of the magnetisation at 0.03 K with the field applied parallel to the easy axis of magnetisation for (a)(Et4N+)[163Dy0.05Y0.95(Pc)2] (I = 5/2) and (b) (Et4N+)[164Dy0.05Y0.95(Pc)2] (I = 0). Insets show magnifications in the 30 mT region. (c) Crystal structureof triple-decker complex (Pc)Tb(Pc)Tb[(Hex)8Pc]. (d) Hysteresis loop of (Pc)Tb(Pc)Tb[(Hex)8Pc] recorded with a field sweep of 0.140 T s1. (e) First field
derivative for a field sweep from 1 to +1 T of the data in (d), and (f) Zeeman diagram with the field parallel to the magnetic easy axes. Adapted withpermission from (a and b) ref. 311. Copyright 2017, Wiley-VCH. Adapted with permission from (c–f) ref. 314. Copyright 2018, American Chemical Society.

decker complexes that contain two lanthanoid ions Ln3+ also
show SMM properties. The main differences between the
double-decker and multiple-decker complexes are the existenceof intramolecular Ln3+–Ln3+ interactions in the latter case.
Magnetic interactions between Ln3+ ions in binuclear metal
complexes were reported by Ishikawa et al.298 before the firstreport of rare-earth-based SMMs in 2003.36 The authors synthesized a series of phthalocyaninato triple-decker complexes(Pc)Ln(Pc)Ln 0 [(BuO)8Pc] (Ln = Ln 0 = Tb, Dy, Ho, Er, Tm, Yb),
denoted as [Ln,Ln 0 ] (Fig. 18). A comparison of the DC magnetic
properties of [Ln,Ln 0 ], [Y,Ln 0 ], and [Ln,Y] (in the latter two, one
Ln3+ is replaced with the diamagnetic and isostructural Y3+

Fig. 18 Molecular structures and DwMT values of (Pc)Ln(Pc)Ln 0 [(BuO)8Pc]
denoted as [Ln,Ln 0 ]. The red curves show theoretical data. Adapted withpermission from ref. 298. Copyright 2003, American Chemical Society.

9302 | Chem. Soc. Rev., 2022, 51, 9262–9339

ion), enables the observation of Ln3+–Ln3+ interactions with theincrease/decrease in DwMT values:o
1n ½Y;Ln0 ½Ln;Ln0 ½Ln;Y
DwM T ¼ wM
T  wMT þ wM T ;2½Ln;Ln0 ½Y;Ln0 ½Ln;Y

, wM
and wM
represent the wM values forwhere wM
[Ln,Ln 0 ], [Y,Ln 0 ], and [Ln,Y], respectively. Ishikawa et al.
revealed that the behaviour of the DwMT values can be explained
solely by the magnetic dipolar term, indicating that the magnetic f–f interactions originate from magnetic dipole–dipole
interactions. Increases in the DwMT in Tb3+, Dy3+, and Ho3+
complexes with decreasing temperature indicate the existenceof ferromagnetic interactions. On the other hand, the Er3+ andTm3+ complexes show antiferromagnetic interactions, whichwere observed as decreases in DwMT. The DwMT values of the
Yb3+ complex showed only negligible temperature dependence.
This complicated behaviour can be interpreted in terms ofdipole–dipole interactions. As discussed in the section on
double-decker complexes (cf. Section 3.2), Tb3+, Dy3+, andHo3+ ions show easy-axis magnetic anisotropy when sandwiched by Pc ligands. In other words, the magnetic dipoles ofthese ions in the [Ln,Ln 0 ] complexes tend to be aligned along
the C4 axis (perpendicular to the Pc plane). Therefore, head-totail ferromagnetic arrangement of the magnetic dipoles isfavored in Tb3+, Dy3+, and Ho3+ complexes. In contrast, Er3+
and Tm3+ sandwiched by Pc ligands show easy-plane magnetic
anisotropy. As a result, parallel and antiferromagnetic arrangement of the magnetic dipoles is favored in Er3+ and Tm3+
complexes. The DwMT of the Yb3+ complex is less temperaturedependent than those of other [Ln,Ln 0 ] complexes because ofThis journal is © The Royal Society of Chemistry 2022

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the weak magnetic anisotropy of Yb3+. The use of diamagnetic
and isostructural Y3+ ions demonstrated by Ishikawa et al. is asmart method to accurately determine the effect of magnetic
Ln3+–Ln3+ interactions that has been used for a wide variety of
Ln3+-based SMMs.
The next question is how long the Ln3+–Ln3+ distance must be
in order to induce f–f interactions. The Ln3+–Ln3+ distance of thePc triple-decker complex estimated using DFT calculations is3.57 Å (this value is consistent with the crystal structure analyses
for triple-decker analogues).298 Increasing the Ln3+–Ln3+ distanceshould affect the magnitude of the f–f interactions.
In 2010, Fukuda et al. reported the first example of a discrete
quadruple-decker complex composed of four Pc ligands.202 The
reaction of the anionic Lu3+ double-decker complex [Ln(Pc)2]
with Cd2+ acetate aﬀorded (Pc)Lu(Pc)Cd(Pc)Lu(Pc), which isreferred to as [LnCdLn] (Fig. 19a). As the two Ln3+ ions are
separated by a diamagnetic Cd2+ ion, the Ln3+–Ln3+ distance islonger than those in triple-decker complexes. Fukuda et al.
investigated the Ln3+–Ln3+ interactions in [LnCdLn] (Ln = Tb,
Dy, Er)325 using the same method as for the series of [Ln,Ln 0 ]complexes298 and revealed the existence of f–f interactions,although their magnitude was low compared to those observedin [Ln,Ln 0 ]. In the case of [TbCdTb] and [DyCdDy], an increasein DwMT is observed, indicating intramolecular ferromagnetic
interactions between the Ln3+ ions (Fig. 19b). As discussed in
the section on [Ln,Ln 0 ], the head-to-tail arrangement of themagnetic dipoles is stabilized when axially anisotropic Ln3+
ions (Tb3+ and Dy3+) are involved. On the other hand, [ErCdEr]
shows a decrease in DwMT due to the easy-plane magnetic
anisotropy of the Er3+ ions.

Chem Soc Rev
Wang et al. and Katoh et al. have reported the crystal
structures of Dy3+ and Tb3+ quadruple-decker complexes
composed of (BuO)8Pc ligands, Tb2Cd[(BuO)8Pc]4 andDy2Cd[(BuO)8Pc]4.205,211 On account of the low solubility ofthe quadruple-decker complexes composed of unsubstituted
Pc ligands, peripheral n-butoxy substitution is mandatory toobtain single crystals suitable for X-ray diﬀraction analysis. The
strategy of using Cd2+ ions to elongate the stacked p-system wasfurther extended by Wang et al. Their group succeeded in
obtaining a series of stacked p-systems: the quintuple-decker
complexes Ln2Cd2[(BuO)8Pc]5208 and sextuple-decker complexes Ln2Cd3[(BuO)8Pc]6.209 Horii et al. reported the crystal
structures of the Dy3+ multiple-decker complexes composedof [(BuO)8Pc] ligands (Fig. 19a) and confirmed that the
Dy3+–Dy3+ distances are 9.8 Å for the quintuple-decker complex
Dy2Cd2[(BuO)8Pc]5 and 13.0 Å for the sextuple-decker complex
Dy2Cd3[(BuO)8Pc]6.213,326 Very weak f–f interactions exist in
both complexes, as demonstrated by the increase in their DwMT
values (Fig. 19a right). These f–f interactions can be used toalter their SMM properties, as discussed in the following
section.
3.3

Using f–f interactions to improve SMM properties

In 2002, Wernsdorfer et al. reported the magnetic properties ofa SMM dimer composed of the TM-based cluster Mn4.225 The
step at zero DC magnetic field in the magnetic hysteresis curveof the monomeric SMM Mn4 disappears in the dimerized
SMMs Mn4  Mn4 due to the suppression of QTM. The mechanism of the suppression of QTM can be explained as follows. If
the probability of QTM between the up (|mi) and down spin

Fig. 19 (a) Synthesis and crystal structures of a series of phthalocyaninato multiple-decker complexes; representative molecular crystal structures of the
Dy3+ complexes Dy2Cd[(BuO)8Pc]4, Dy2Cd2[(BuO)8Pc]5 and Dy2Cd3[(BuO)8Pc]6; n-butoxy groups in the crystal structures are omitted for clarity. DwMTvs. T plots for the Dy3+ multiple-decker complexes were constructed using data from ref. 211, 213 and 326. (b) DC magnetic susceptibilities of quadrupledecker complexes that contain Tb3+, Dy3+ and Er3+ ions. Reproduced from (b) ref. 325 with permission from the Royal Society of Chemistry.

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Review Article(|ki) states of one SMM is PQTM, the QTM probability between
the ferromagnetically coupled states (|m,mi and |k,ki) in the
SMM dimer is expressed as PQTM2, because QTM between thesestates requires simultaneous spin reversal. Since PQTM2 is muchsmaller than PQTM, QTM between the coupled spin states is aforbidden transition. Therefore, the coupling of two SMMs
suppresses the QTM at zero bias DC magnetic field. This
strategy (exchange bias eﬀect) is important when considering
SMMs as a possible candidate for information storage, because
it can improve data stabilisation at zero DC field. The f–f
interactions in multiple-decker complexes can also be used asa source of exchange bias. The effect of f–f interactions on the
dynamic magnetic properties of rare-earth SMMs was reported
by Ishikawa et al.327 They prepared the homodinuclear Tb3+
triple-decker complex (Pc)Tb(Pc)Tb[(BuO)8Pc] (denoted as[Tb,Tb]) and the two heterodinuclear complexes (Pc)Y(Pc)Tb[(BuO)8Pc] (denoted as [Y,Tb]) and (Pc)Tb(Pc)Y[(BuO)8Pc](denoted as [Tb,Y]), as summarized in Fig. 18. The mono-Tb3+
complexes [Y,Tb] and [Tb,Y] show clear DC magnetic field (Hdc)00dependences in their wM vs. T plots; the peak maxima shift to ahigher temperature with increasing Hdc magnitude (Fig. 20a).
00
In contrast, the wM vs. T plots for [Tb,Tb] are Hdc-independent
in the field range of 0 to 3000 Oe (the right side of Fig. 20a). The
two peaks in the plots of [Tb,Tb] are due to the two inequivalent

Tb3+ ions. The increase in the temperatures of the maxima of[Y,Tb] and [Tb,Y] corresponds to the suppression of QTM by
applying an Hdc. The QTM occurs among the energeticallydegenerate states. However, applying an Hdc resolves thedegeneracy of the ground-state doublet |6i due to Zeeman
splitting. Therefore, the QTM of [Y,Tb] and [Tb,Y] is suppressedwith increasing Hdc, causing the remarkable Hdc dependences
00

of wM . In contrast, the Tb3+ ions of [Tb,Tb] are affected by themagnetic dipolar field from the other Tb3+ ion. Therefore, theTb3+ ions of [Tb,Tb] act as if a DC magnetic field is applied evenin the absence of an external Hdc. Therefore, the QTM in[Tb,Tb] is suppressed without an Hdc, resulting in weak Hdc
00
dependence in its wM vs. T plots.
The Zeeman diagrams of the triple-decker complexes
reported by Sakaue et al. aid in the understanding of therelaxation mechanism of [Tb,Tb].122 Fig. 20b is a schematic
illustration of the Zeeman diagrams of various multiple-decker
Tb3+ complexes.212 Due to the ferromagnetic dipole–dipole
interactions between the Tb3+ ions in the triple-decker complex[Tb,Tb], the lowest energy states are |mJ, mJi = |6, 6i at zeroHdc. The antiferromagnetically coupled doublet |6, 86i islocated at ca. 3 cm1. Due to the hyperfine coupling between J
and I, the Zeeman diagram of |6, 6i and |6, 86i shows
multiple lines, which are depicted as bands in Fig. 20b. At zero00

Fig. 20 (a) DC magnetic field (Hdc) dependences of the wM vs. T plots at an AC frequency of 997 Hz for the triple-decker complexes [Y,Tb], [Tb,Y] and00[Tb,Tb]. (b) Schematic illustration of the Zeeman diagrams of Tb3+ multiple-decker complexes. (c) Hdc dependences of the wM vs. n plots for the series ofbutoxy-substituted Tb3+ multiple-decker complexes at 4 K based on the data from ref. 319, 328 and 211. Adapted with permission from (a) ref. 327.
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DC magnetic field, |+6, +6i and |6, 6i are degenerate.
However, QTM between |+6, +6i and |6, 6i is prohibited because this transition requires simultaneous spinreversal. The Zeeman diagram also predicts energy crossing
between |6, 86i and |+6, +6i at Hdc = 3000 Oe, indicatingthat the QTM between |6, 86i and |+6, +6i is enhanced at
3000 Oe.
In the case of the quadruple-decker complexes, the magnitude of the f–f interactions decreases and the Hdc dependencesof t are changed. Fukuda et al. reported the Hdc dependences of
the t values for homonuclear ([TbCdTb]) and heteronuclear([TbCdY]) quadruple-decker complexes.329 [TbCdTb] has f–f
interactions, whereas [TbCdY] does not. At zero Hdc, [TbCdTb]
shows longer t values than [TbCdY]. However, at Hdc = 1000 Oe,
the t values of [TbCdTb] become faster than those of [TbCdY].
Further increasing the Hdc causes the t of both complexes tobecome similar. This complicated modulation of t can again be
explained using a Zeeman diagram. Since the magnetic easy
axis of the Tb3+ ion in the multiple-decker complex is parallel to
the C4 axis, the ferromagnetic head-to-tail arrangement of themagnetic dipoles is stabilized in [TbCdTb]. Therefore, theground and first excited states of [TbCdTb] are |6, 6i and|6, 86i, respectively. Compared to the triple-decker complex,
the energy difference between |6, 6i and |6, 86i is smallerin [TbCdTb] due to the weaker f–f interactions. The groundstate doublet |6, 6i shows Zeeman splitting, whereas |6,
86i does not. Due to the hyperfine interactions, |6, 6i and|6, 86i are shown as crowded lines. At zero Hdc, the QTM in[TbCdTb] is suppressed due to the exchange bias effect, which
prolongs the t values. However, |+6, +6i and |6, 86i cross ataround 1000 Oe (middle plot in Fig. 20b). Therefore, the QTM
between these is enhanced, decreasing the t values of[TbCdTb]. This is the reason for the magnetic field dependences of the t values in [TbCdTb] and [TbCdY]. A similar trend
has also been observed in the Dy3+ analogues. Katoh et al.
reported that the homonuclear Tb3+ triple-decker complex
Tb2[(BuO)8Pc]3 shows two peaks in the imaginary part of theAC magnetic susceptibility in the presence of a non-zero Hdc(3000 Oe) as summarized in the left side of Fig. 20c.319 In
contrast, quadruple-decker Tb2Cd[(BuO)8Pc]4328 and quintupledecker Tb2Cd2[(BuO)8Pc]5211 complexes that contain two Tb3+
ions show dual magnetic relaxations at zero Hdc. Importantly,
the dual magnetic relaxations become a single one in thepresence of a non-zero Hdc (2000 Oe). The complicated magnetic behaviour in these complexes originates from the differences in the magnitude of the f–f interactions. The dual
magnetic relaxations of Tb2[(BuO)8Pc]3 have been observed at
3000 Oe, where level-crossing between |+6, +6i and |6, 86i
occurs. Therefore, one of the dual magnetic relaxations in
Tb2[(BuO)8Pc]3 is attributed to the QTM and spin-lattice relaxations among |+6, +6i and |6, 86i. In the case of
Tb2Cd[(BuO)8Pc]4 and Tb2Cd2[(BuO)8Pc]5, |6, 6i and |6,
86i show crossing at zero Hdc because of the weaker f–f
interactions (small energy difference between |6, 6i and|6, 86i), as shown in Fig. 20b. Therefore, magnetic relaxations among these states occur and are observed as the dualThis journal is © The Royal Society of Chemistry 2022

Chem Soc Rev
magnetic relaxations at zero Hdc. Increasing Hdc resolves thedegeneracy of |6, 6i and |6, 86i, resulting in the observation of single magnetic relaxation.
The eﬀect of the f–f interactions on the dynamic magnetic
properties has been reported in Dy3+ complexes. Katoh et al.
reported the SMM properties of the triple-decker complexes
Dy2[(BuO)8Pc]3 and DyY[(BuO)8Pc]3.330 Homonuclear Dy2[(BuO)8Pc]3
has intramolecular f–f interactions, whereas DyY[(BuO)8Pc]3 doesnot. The magnetic relaxation time t of Dy2[(BuO)8Pc]3 is 1.30 
104 s at 1.82 K. This value is one order of magnitude greater thanthat for DyY[(BuO)8Pc]3 (3.20  105 s at 1.9 K), indicating that
f–f interactions act as an exchange bias to suppress the QTM
at zero Hdc.
Horii et al. reported the exchange bias eﬀect in Dy3+
quadruple-decker (Dy2Cd[(BuO)8Pc]4), quintuple-decker (Dy2Cd2[(BuO)8 Pc]5 ), and sextuple-decker (Dy 2 Cd 3 [(BuO) 8 Pc] 6)complexes.213,326 Similar to the triple-decker complexes
composed of two Dy3+ ions, the series of multiple-decker
complexes shows longer t values than the mono-Dy3+ complex
due to the f–f interactions in the former complex. The exchange
bias effect is observed in Dy2Cd3[(BuO)8Pc]6, in which theintramolecular Dy3+–Dy3+ distance is ca. 13 Å.
3.3.1 Connecting double-decker SMMs without using Cd2+
ions. The exchange bias eﬀects of the multiple-decker complexes prolong the t values. However, double-decker complexes
show longer t values than multiple-decker complexes. The
short t values in the latter case are associated with the changes
in the coordination geometry around Ln3+ that are induced by
the coordination of the metal ions. In the case of the tripledecker complex, the inner Pc ligand is coordinated by two Ln3+
centres. Therefore, the coordination geometry around Ln3+
becomes unsymmetrical, enhancing the QTM (PQTM becomeslarge). Thus, it is desirable to induce the exchange bias eﬀect
while keeping the SAP geometry of the double-decker complex(i.e., keeping PQTM small).
The use of the clamshell-type phthalocyanine331 reported
by Tolbin et al. enables the construction of a discrete stacked
p-system without using metal coordination.104 Horii et al.
reported a clamshell-type quadruple-decker complex(Pc)Tb[c-Pc2]Tb(Pc) in which the two Tb3+ double-decker units(tBu4Pc)Tb(Pc) are connected by a benzyloxy linker (Scheme 13
and Fig. 21a).105 Ferromagnetic f–f interactions are present
between the two double-decker units, as evidenced by theincrease in the wMT value at low temperatures because thehead-to-tail arrangements of the magnetic dipoles are stabilized. The magnetic hysteresis of (Pc)Tb[c-Pc2]Tb(Pc) at a zeroDC magnetic field is wider than that of the monomer doubledecker complex (tBu4Pc)Tb(Pc), indicating that the exchange
bias is effective in prolonging the t values. The reaction of(Pc)Tb[c-Pc2]Tb(Pc) with Cd2+ acetate affords the Cd2+containing complex (Pc)Tb[c-Pc2Cd]Tb(Pc). Although ferromagnetic interactions between the Tb3+ ions exist in (Pc)Tb[c-Pc2Cd]Tb(Pc), it shows a butterfly-shaped magnetic hysteresis curve in which the curve is closed at zero Hdc. These results
indicate that the QTM is enhanced (increased PQTM value) by
the coordination of a Cd2+ ion.

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Horii et al. used supramolecular assembly reported by
Ishikawa et al.332 to introduce f–f interactions.67 The crownether-substituted double-decker complex [(15C5)4Pc]Tb(Pc) forms
a supramolecular quadruple-decker complex [K4{[(15C5)4Pc]Tb(Pc)}2]4+ in the presence of excess potassium acetate(Fig. 21b). Quantitative conversion from [(15C5)4Pc]Tb(Pc) to[K4{[(15C5)4Pc]Tb(Pc)}2]4+ was confirmed by paramagnetic
1
H NMR analysis and absorption spectra. In addition, ferromagnetic Tb3+–Tb3+ interactions exist in [K4{[(15C5)4Pc]Tb(Pc)}2]4+. At
1.8 K, the t value of [K4{[(15C5)4Pc]Tb(Pc)}2]4+ is 1000 times longer
than that of [(15C5)4Pc]Tb(Pc), indicating that the exchange bias
suppresses QTM.
Recently, the crystal structures of the triple-decker complexes dimerized via analogous supramolecular associations
using crown ethers and K+ ions were elucidated by Martynov
et al.333 The triple-decker complex [(15C5)4Pc]Tb(Pc)Tb(Pc)forms a supramolecular dimer in the presence of K+ ions(Fig. 22a). Such dimerization did not alter the symmetry ofReview Article
coordination surrounding of metal centres, and the Ln  Ln
distances ranging from 3.5 to 13.2 Å renders these complexes asconvenient models to study long-range interactions between
paramagnetic metal centres aligned in a collinear fashion. In
the case of [(15C5)4Pc]Tb[(15C5)4Pc]Tb(Pc), the reaction withfour K+ ions does not induce dimerisation but instead changes
the coordination geometry of Tb* from SAP to SP which wasdemonstrated by crystallographic characterization of isostructural Y3+ complex and its supramolecular inclusion compound(Fig. 22b).334 Therefore, the approach of using (15C5)4Pc is
promising not only for introducing f–f interactions, but also fordirectly changing the LF. Although the SMM properties of[(15C5)4Pc]Tb(Pc)Tb(Pc) and [(15C5)4Pc]Tb[(15C5)4Pc]Tb(Pc)have not yet been reported, modulation of the magnetic anisotropy, which may be correlated with the LF, was demonstrated
via NMR analyses.
The connection of the double-decker SMM H[(a-BuO)8Pc]Tb(Pc) by Na+ was reported by Chen et al. (Fig. 21c).335,336

Fig. 21 (a) Molecular structures and magnetic hysteresis curves of the clamshell-type quadruple-decker complexes. (b) Supramolecular dimerisation of00
the crown-ether-substituted SMM [(15C5)4Pc]Tb(Pc). The maxima in the wM vs. n plots shift toward the low n region upon dimerisation. (c) Dimerisation+of the double-decker SMM H[(a-BuO)8Pc]Tb(Pc) (CCDC FIJBEI) by Na ions yielding dimer Na2{[(a-BuO)8Pc]Tb(Pc)}2 (CCDC FIJBIM). Reproduced from(a) ref. 105 with permission from the Royal Society of Chemistry. Adapted with permission from (b) ref. 67. Copyright 2018, Wiley-VCH.

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Fig. 22 X-ray structure of the supramolecular assemblies formed by heteroleptic Y3+ crown-phthalocyaninates. (a) Dimer formed by [(15C5)4Pc]Y(Pc)Y(Pc) (CCDC SULZIL) in the presence of KBPh4. (b) Intercalation of potassium cations between crown-substituted ligands of [(15C5)4Pc]Y[(15C5)4Pc]Y(Pc) (CCDC ITUJEP and ITUGUC)333,334 without the formation of supramolecular dimers. Tetraphenylborate ions, hydrogen atoms and solvent
molecules are not shown for clarity.

Compared to the monomeric complex H[(a-BuO)8Pc]Tb(Pc)(Ueﬀ = 125 cm1), the corresponding dimer complex Na2{[(aBuO)8Pc]Tb(Pc)}2 shows a larger Ueﬀ value (367 cm1). The DC
magnetic measurements imply that the Tb3+–Tb3+ magnetic
interactions in Na2{[(a-BuO)8Pc]Tb(Pc)}2 are very weak. Therefore, the change in the stacking angle from y = 281 to y = 341
upon Na+ coordination is the main reason for the increasein Ueff.
3.3.2 Rare-earth-fused phthalocyaninato sandwich complexes. The distance between spins and their placement areimportant factors that influence SMM characteristics. QTM canbe eﬀectively suppressed by arranging the Tb(Pc)2 units on themagnetic anisotropy axis (C4 rotation axis) at the distance atwhich the magnetic dipole interactions are maximized (closest
Tb  Tb distance: ca. 0.62 Å).67,105 To put it simply, the verticalstacking of Tb(Pc)2 improves SMM characteristics and suppresses the QTM. This raises the naive question, ‘‘How does
the geometry of the Ln(Pc)2/Ln2(Pc)3 units aﬀect the SMM
characteristics?’’ Klyatskaya et al. reported that eﬀective
through-space p–p interactions between pyrene-substituted
unsymmetrical complexes [(Pyrene)Pc]Ln(Pc), Ln = Tb, Dy,
and Ho lead to the formation of head-to-tail p dimers withclosest Tb  Tb distance: ca. 13.08 Å (Fig. 4).115 We now
summarize the research results related to the effects ofthrough-bond interactions in Ln3+–Pc sandwich complexes on
their SMM properties.
In 2013, Wang et al. prepared the first biradical dinuclear
Dy3+ triple-decker SMM with a fused phthalocyanine ligand(Pc)Dy(f-Pc2)Dy(Pc)OBu (Scheme 42 and Fig. 23a).328 The distance between the Dy3+ ions coordinated to the fused ring Pc4is 10 Å or more and p–f interactions are observed. The p-bridge

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Fig. 23 Crystal structure of (a) (Pc)Ln(f-Pc2)Ln(Pc)OBu (Ln3+ = Dy and Tb)
and (d) (Pc)2Ln2(f-Pc2)Ln2(Pc)2OBu (Ln3+ = Tb and Dy) and the microSQUID-measurement magnetization decay curves (b) and hysteresis loops
for oriented crystalline samples of (c) (Pc)Tb(f-Pc2)Tb(Pc)OBu and (e)(Pc)2Dy2(f-Pc2)Dy2(Pc)2OBu. Molecular structures of (a) and (d) based on
the cif files from ref. 217 and 220. Protons and n-butoxy groups in (Pc)Ln(fPc2)Ln(Pc)OBu and (Pc)2Ln2(f-Pc2)Ln2(Pc)2OBu are omitted for clarity.
Adapted with permission from (a and b) ref. 217. Copyright 2018, American
Chemical Society. Adapted with permission from (d and e) ref. 220.
Copyright 2018, Wiley-VCH.

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delocalized biradical antiferromagnetic interactions effectively
suppress the QTM of the Dy3+-ion-based SMMs, and at temperatures of up to 5 K, butterfly-type magnetic hysteresis with
HC was observed.
In 2018, Morita et al. prepared the first dinuclear Tb3+
fused-Pc4 triple-decker SMM (abbreviated as (Pc)Tb(fPc2)Tb(Pc)OBu).217 The complex shows an open hysteresis loopwith HC = 180 Oe at 1.8 K (these values were obtained from themagnetic data for a magnetically diluted sample). From thecrystal structure, the twist angle (y) between the Pc ligands wasdetermined to be 451, which indicates an SAP coordination
environment. The strict SAP coordination caused the absenceof the oﬀ-diagonal LF terms (Bqk; q a 0), which eliminates thetunneling gap (DE) via level-crossings in the ground state,
meaning that QTM processes are suppressed. In addition,
solution 1H NMR measurements of (Pc)Tb(f-Pc2)Tb(Pc)OBu
show that the coordination environment of the Tb3+ ions aﬀects
the anisotropic magnetic susceptibility (wax). Incorporating
radicals improves the magnetic properties, as radical-bridged
metal complexes exhibit slow magnetic relaxation dynamics
due to suppressed QTM, exhibiting high TB values due to strong
exchange coupling Jex in comparison with non-radical
systems.239,337–339 However, it is not necessary to incorporate
strong magnetic interactions between the Ln3+ ions and radical
ligands, as shown by (Pc)Tb(f-Pc2)Tb(Pc)OBu. Although theinduced intramolecular magnetic interactions using the molecular design of (Pc)Tb(f-Pc2)Tb(Pc)OBu are weak, these interactions play a role in suppressing QTM processes. This prediction
was supported by DFT calculations, which were used to analyse
the exchange couplings in the complex. The obtained results
indicated the presence of magnetic interactions among the four
spin centres, including indirect Tb3+–Tb3+ interactions throughp-radicals. On the other hand, hyperfine couplings (0.015 cm1)
were observed for an oriented crystalline sample in microSQUID measurements (Fig. 23b and c), indicating that theinter- and intramolecular magnetic interactions were very weak,with smaller couplings than that for Tb(Pc)2 (0.017 cm1).276
Magnetic hysteresis loops with Hc = 860 Oe at 1.8 K wereobserved for a crystalline sample of (Pc)Tb(f-Pc2)Tb(Pc)OBu(Fig. 23). Considering the relationship between the moleculararrangement and Hc, the weak intermolecular magnetic interactions are more effective in suppressing QTM. These results
indicate that the collinearity of magnetic dipole–dipole interactions strongly affects the suppression of QTM.
In 2013, Morita et al. prepared the first tetranuclear Tb3+
fused-phthalocyaninato quintuple-decker SMM (Fig. 23c)
by using a fused phthalocyaninato ligand to control the
spatial arrangement of the Tb3+ ions in the SMM (Pc)2Tb2(f-Pc2)Tb2(Pc)2OBu.219 In DC magnetic susceptibility measurements, ferromagnetic interactions among the four Tb3+ ionsin (Pc)2Tb2(f-Pc2)Tb2(Pc)2OBu were observed, with two kindsof magnetic dipole–dipole interactions. (Pc)2Tb2(f-Pc2)Tb2(Pc)2OBu can be described as a weakly ferromagnetically coupleddimer of the SMM Tb2[(BuO)8Pc]3319 with strong dipole–dipole
interactions in the triple-decker moieties and weak dipole–
dipole interactions through the fused-Pc4 linking the two

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Tb2[(BuO)8Pc]3 units. Dual magnetic relaxation processes wereobserved in (Pc)2Tb2(f-Pc2)Tb2(Pc)2OBu, similar to those in otherdinuclear Tb3+–Pc complexes.211,317,340 This provides clear evidence that the magnetic relaxation phenomena depend heavily
on the magnetic dipole interactions between the Tb3+ ions in(Pc)2Tb2(f-Pc2)Tb2(Pc)2OBu.
In 2018, Katoh et al. reported the magnetic properties
and spin-relaxation processes of the tetranuclear Dy3+ fusedphthalocyaninato quintuple-decker SMM (Pc)2Dy2(f-Pc2)Dy2(Pc)2OBu and its crystal structure (Fig. 23d and e).220 The
structure of (Pc)2Dy2(f-Pc2)Dy2(Pc)2OBu can be regarded as a
dimer of Dy2[(BuO)8Pc]3 SMMs341 with diﬀerent magnetic
relaxation behaviour corresponding to the octacoordinationgeometry sites Dy1 with a C4 symmetry (y1 = 231) and Dy2 with a
D4d symmetry (y2 = 451). The lowest Kramers doublet (KD) of(Pc)2Dy2(f-Pc2)Dy2(Pc)2OBu is thought to be a mixed state ofmJ = 13/2 and 11/2. At an Hdc of 1750 Oe and T range of 1.8–
3.75 K, the QTM is suppressed, and the direct process isenhanced; these depend on the different coordination environments. Consequently, the coordination geometry plays a moresignificant role in the spin-relaxation phenomena of (Pc)2Dy2(f-Pc2)Dy2(Pc)2OBu than the dipolar bias between Dy3+
ions.341,342 The different conclusions for the Tb3+ and Dy3+
systems are attributable to the ground mJ state for theircoordination geometries.36,84,233,242,252,276,284–287
These studies show that the SMM properties can be finetuned by introducing magnetic dipole–dipole interactions in acontrolled SMM spatial arrangement. In addition, we can
expect to obtain nuclear spin molecular qubits using the spincascade |S = 1/2i8 J = 6i8I = 3/2i of the Tb(Pc)2 system by taking
advantage of the more complex intramolecular magnetic interactions in (Pc)Tb(f-Pc2)Tb(Pc)OBu.270,303,308
3.3.3 Crystal engineering of rare-earth-phthalocyaninato
sandwich complexes for one-dimensional spin arrangement.
Self-assembly via coordination bonds and supramolecularapproaches are eﬀective methods of controlling the geometricspin arrangement. It is possible to rationally design moleculebased magnetic materials with zero-dimensional (0D), onedimensional (1D), two-dimensional (2D), and three-dimensional(3D) structures with a variety of properties, such as electronic andionic conduction, gas adsorption, and stimulus-induced switching characteristics.343 This concept has been a huge success in
improving the functionalities of Ln(Pc)2-based SMMs.
Tb(Pc)02 can be used in molecular spintronics due to its SMM
characteristics and its physical and chemical stability.344 Investigating the influence of the direction of the magnetic dipole
interactions acting among the Tb(Pc)02 SMMs in the bulk solidon the magnetic relaxation behaviour is important for controlling its magnetic properties, which depend on the moleculararrangement.345,346 In 2017, Yamabayashi et al. showed that thearrangement of the Tb(Pc)02 SMMs in crystals aﬀects the groundstate, and that QTM is suppressed at low temperatures.347 As aresult, it has been suggested that QTM depends on the orientation angle (s) of the uniaxial magnetic anisotropy with respect
to the vector connecting the Tb3+ ions. In addition, QTM is

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suppressed in the Tb(Pc)2 dimer when the Tb(Pc)2 units areconnected with a spacer acting as a Hbias via through-space
magnetic dipole interactions (dipolar bias).105 Moreover, it ispossible to eliminate the influence of the transverse field byarranging the Tb3+ ions in the direction of the C4 axis.97 The
probability of QTM occurring in one Tb(Pc)2 unit is PQTM,
and the probability of QTM occurring simultaneously in a
Tb(Pc)2 dimer is PQTM2, indicating that QTM is eﬀectively
suppressed.
Magnetic dipole interactions are dominant in quasi-1D
molecular magnetic materials, in which (Nc)Tb(Pc) SMM units
adopt a structure similar to that of Tb(Pc)2 SMMs (Fig. 24b). In
2018, Katoh et al. reported that the magnetic properties of(Nc)Tb(Pc)0/+ (neutral (Nc)Tb(Pc)0: Ueﬀ = 652 cm1 with t0 =2.5  1012 s; cationic (Nc)Tb(Pc)+: Ueﬀ = 584 cm1 with t0 =5.5  1012 s) with 1D structures are significantly diﬀerent fromthose of a magnetically diluted sample ((Nc)Tb0.1Y0.9(Pc)0: Ueﬀ =342 cm1 with t0 = 1.1  108 s).136 In particular, the t of(Nc)Tb(Pc)+ (22 s at 1.8 K) in the low-temperature region is fiveorders of magnitude slower than that of (Nc)Tb0.1Y0.9(Pc)0(1.2 ms at 1.8 K). Furthermore, the HC of (Nc)Tb(Pc)+ wasretained at temperatures of up to ca. 20 K, which is the TB.
The single-ion anisotropy of the Tb3+ ions in a 1D structure andthe magnetic dipole interactions acting among the molecules
determine the direction of the magnetic properties. These
results show that the spin dynamics can be improved bymanipulating the arrangement of the SMMs in the solid state.
There are clear advantages to using quasi-1D SMM systems withmagnetic dipole interactions such as (Nc)Tb(Pc)0/+. This strategy could lead to dramatic changes in the spin-blocking phenomena, because t is slower at a zero magnetic field. In otherFig. 24 (a) Crystal structure of [(Nc)Tb(Pc)]+PF6 (CCDC YICQIN). Hydrogen atoms and the PF6 anion are omitted for clarity. (b) 1D structure of[(Nc)Tb(Pc)]+PF6 along the c-axis. (c) MH measurement hysteresis loopwith a coercivity (HC) of 150 mT (1500 Oe) for a single crystal of[(Nc)Tb(Pc)]+PF6 at 1.8 K (H//c-axis). The data were normalized to thevalue in a field of 7 T. (d) 1D structure of [Dy(Pc)2]Ix along the c-axis.
Hydrogen atoms are omitted for clarity. (e) MH curve of [Dy(Pc)2]Ix for apolycrystalline sample at 2.2 K (top). Magnetoresistance (MR) measurement of [Dy(Pc)2]Ix for a single crystal at 2.2 K. Adapted with permission
from (a–c) ref. 136 and (d and e) ref. 348. Copyright 2018 and Copyright2021, Wiley-VHC.

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words, since the probability of QTM occurring in (Nc)Tb(Pc)+ is
PQTMn (n 4 2), the QTM is eﬀectively suppressed in a 1D
system. Therefore, this relatively simple molecular-design
method can be used to construct a variety of Ln(Pc)2-SMMbased functional molecular magnetic materials, and new functionalities, such as electronic and ionic conduction, gas adsorption, and various switching characteristics, can be incorporated
by controlling the pore size.
These Ln(Pc)2-SMM-based materials are expected to exhibit
galvanomagnetic eﬀects due to the correlation between thelocalized f electrons and conduction electrons. There have been
several reports of partially oxidized 1D [Ln(Pc)2]Ix complexes(Ln = Ce, Pr, Sm, Ho, and Yb).349–353 In 2021, a 1D arrangementof a partially oxidized Dy3+ double-decker SMM, [Dy(Pc)2]Ix(1.95 o x o 2.26), was reported by Sato et al. (Fig. 24d).348
Comparison of the Arrhenius plots with those of a magneticallydiluted sample [Dy0.02Y0.98(Pc)2]Ix revealed that the t for[Dy(Pc)2]Ix is four times slower at 1.8 K ([Dy(Pc)2]Ix: 0.0050 s;[Dy0.02Y0.98(Pc)2]Ix: 0.0013 s), and that its QTM at low temperatures is suppressed by the dipolar bias in the 1D array. In[Dy(Pc)2]Ix, the Pc ligands were partially oxidized, and the I3
ion was present as a counter ion. Based on the polarised
reflection spectra of [Dy(Pc)2]Ix, the p-radicals were delocalized
in a 1D column and behaved as a conductor even at low
temperatures. Given that conductivity was observed at temperatures lower than TB, the negative magnetoresistance (MR)effects of ca. 1% corresponding to the magnetic hysteresis of
the SMM were observed when the galvanomagnetic characteristics were investigated at 2.2 K. This indicated that thegalvanomagnetic properties between the SMMs and the electrical conduction characteristics due to the p–f interactions
could be observed. Our groups are continuing to investigate thegalvanomagnetism due to p–f interactions in partially oxidized[Ln(Pc)2]Ix 1D complexes. This strategy may pave the way for theconstruction of electro-conductive SMMs as molecular spintronic devices via a spin-cascade system.
3.3.4 Cocrystals of rare-earth phthalocyaninato sandwich
complexes and fullerenes. In this section, we review the eﬀects
of supramolecular assembly with fullerenes on the SMM properties. Supramolecular complexes between porphyrin (Por2)
moieties and (C60-Ih)[5,6]fullerene (C60), and/or fullerene derivatives, have been investigated in detail with respect to host–
guest chemistry and electronic structures.354–359 In fullerene–
porphyrin systems, a close contact between the molecules andselective supramolecular interactions is generated. C60 regioselectively binds at the centre of the porphyrin complex via amolecular recognition process supported by the non-planar
deformation of the porphyrin ligand, which matches the convex
shape of C60. Since the LF around the Ln3+ ions affects theircharge density, researchers have focused on ways to control
the magnetic properties of Ln3+ SMMs via supramolecularassembly with a fullerene. The coordination geometry significantly affects the spin-relaxation phenomena of Ln3+–Pc/Por
multiple-decker
complexes.105,122,317,341
In
particular,
Ln2(Pc)3 complexes have an important advantage overLn(Pc)2 complexes as the magnetic relaxation processes

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correspond to the Zeeman splitting when two 4f spin systems
are present.67,211,219,220,319,326,330,340–342
In 2014, Wang et al. reported cocrystals of C60 with the SMM(Pc)Dy(TCIPP), along with the relationship among the coordination environment, intermolecular interactions, and the SMM
behaviour.360 Focusing on the twist angle y between theligands, it has been reported that the initial angle (43.601 for(Pc)Dy(TCIPP)) does not change dramatically after cocrystallisation (41.221/44.451 for (Pc)Dy(TCIPP)0.5C60, 44.411
for (Pc)Dy(TCIPP)C60, and 44.351 for (Pc)Dy(TCIPP)2C60). However, the frequency (n) dependences of the alternating current(AC) measurements changed due to the diﬀerent moleculararrangements, depending on the number of incorporated C60
molecules.
Combining a triple-decker SMM with C60 can be expected toreduce the overall symmetry and thus reveal the eﬀects of the
LF term. In 2020, Katoh et al. reported the dinuclear Tb3+ tripledecker complex (TPP)Tb(Pc)Tb(TPP), which contains porphyrinunits on the outside to bind with C60 (henceforth abbreviatedas [(TPP)Tb(Pc)Tb(TPP)]C60).321 The supramolecular complex[(TPP)Tb(Pc)Tb(TPP)]C60 (Fig. 25a and b) was prepared byassembling C60 with the dinuclear Tb3+ triple-decker complex

Review Article(TPP)Tb(Pc)Tb(TPP) with quasi-D4h symmetry to investigate therelationship between the coordination symmetry and SMM
properties (Fig. 25e and f). The two Tb3+ sites of(TPP)Tb(Pc)Tb(TPP) are equivalent, and y was determined tobe 3.621 (Fig. 25d). On the other hand, the two Tb3+ sites of[(TPP)Tb(Pc)Tb(TPP)]C60 are not equivalent. The y values forsites Tb1 and Tb2 were determined to be 3.671 and 33.81,
respectively, due to a change in the coordination symmetry of(TPP)Tb(Pc)Tb(TPP) upon association with C60 (Fig. 25c).
At 1.8 K, (TPP)Tb(Pc)Tb(TPP) and [(TPP)Tb(Pc)Tb(TPP)]C60
undergo diﬀerent magnetic relaxations, and the changes in
the ground state aﬀect the spin dynamics. (TPP)Tb(Pc)Tb(TPP)and [(TPP)Tb(Pc)Tb(TPP)]C60 relax via QTM at zero Hdc, andthe H dependences of their t values are similar at H 4 1500 Oe.
On the other hand, at H o 1500 Oe, their t values show diﬀerent
behaviour, since the oﬀ-diagonal terms (Bqk; q a 0) aﬀect themagnetic relaxation mechanism. Based on the temperature and H
dependences of t, the spin dynamics can be explained by spin–
phonon interactions together with the direct and Raman mechanisms. Thus, a supramolecular approach can be used to control themagnetic anisotropy along the C4 rotation axis and the spindynamic properties in Ln3+–Pc multiple-decker complexes.

Fig. 25 Cocrystals of rare-earth phthalocyaninato sandwich complexes and fullerene derivatives. (a–c) Crystal structure of [(TPP)Tb(Pc)Tb(TPP)]C60.
The inclusion of C60 changes the coordination environment of the Tb3+ ion and affects the SMM characteristics. (d) Coordination environment of(TPP)Tb(Pc)Tb(TPP) before the subsumption of C60. The micro-SQUID-measurement hysteresis loops (e) and alternating current (AC) magnetic
susceptibility data (f) of [(TPP)Tb(Pc)Tb(TPP)]C60. (g) The cocrystal [(Pc)Tb(OEP)]3(Li+@C60PF6)2 forms a quasi-kagome lattice due to the two kinds ofisosceles triangles (A and B). Protons and crystal solvents of [(TPP)Tb(Pc)Tb(TPP)]C60 and [(Pc)Tb(OEP)]3(Li+@C60PF6)2 are omitted for clarity. Adaptedwith permission from (a–f) ref. 321. Copyright 2020, Wiley-VHC. Reproduced from (g) ref. 361 with permission from the Royal Society of Chemistry.

9310 | Chem. Soc. Rev., 2022, 51, 9262–9339

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In 2020, Iwami et al. reported the cocrystallisation of alithium-ion-encapsulating fullerene (Li+@C60), C60 and C70
with a (Pc)Tb(OEP) SMM (Fig. 25g).361 The OEP unit interactedwith the fullerene derivatives to form five diﬀerent cocrystals:[(Pc)Tb(OEP)]3(Li+@C60PF6)2, [(Pc)Tb(OEP)](Li+@C60BF4),[(Pc)Tb(OEP)]C70C6H14, [(Pc)Tb(OEP)]C60C6H14, and [(Pc)Tb(OEP)]C60Et2O. For the cocrystals containing Li+@C60 ([(Pc)Tb(OEP)]3(Li+@C60PF6)2 and [(Pc)Tb(OEP)](Li+@C60BF4)), the use of
diﬀerent counter anions resulted in diﬀerent crystal packings.
In particular, [(Pc)Tb(OEP)]3(Li+@C60PF6)2 was found to have aunique quasi-kagome lattice packing that induces intermolecularferromagnetic interactions. Depending on the crystallisation conditions, diﬀerent packings, i.e., 1D column packings for[(Pc)Tb(OEP)](Li+@C60BF4) and [(Pc)Tb(OEP)]C70C6H14 and
AABB-type arrays of (Pc)Tb(OEP) and C60 dimers for[(Pc)Tb(OEP)]C60C6H14 and [(Pc)Tb(OEP)]C60Et2O, were formed.
In addition, the magnetic properties of the (Pc)Tb(OEP) units weremodulated via cocrystallisation.
The relationship between the changes in the coordinationgeometry of the same molecule and the SMM properties
induced via a supramolecular approach was also investigated.
The corresponding results showed that the magnetic anisotropy along the C4 rotation axis of the metal sites in the
dinuclear Ln3+–Pc multiple-decker complexes could be controlled by forming a supramolecular complex, which could leadto switchable SMMs.
3.4

Correlation between redox states and SMM properties

Porphyrinoids are redox-active ligands due to their extended
p-systems and high redox stability. Therefore, sandwich complexes composed of porphyrinoids are also redox active. In thissection, we will explain the relationship between redox statesand SMM properties. The Ln3+ ions (except for Ce3+) are inessence redox inert. Therefore, the redox reactions of the
sandwich complexes presented here occur on the porphyrinoid
ligands.
Ishikawa and Takamatsu et al. reported that the Ueﬀ valuesof the anionic forms of the double-decker complexes (NBu4+){Tb[(EtO)8Pc]2} and (NBu4+){Dy[(EtO)8Pc]2} are enhanced intheir corresponding cationic forms, {Tb[(EtO)8Pc]2}+(SbCl6)84
and {Dy[(EtO)8Pc]2}+(SbCl6).362 The mechanism of the structural changes due to the redox reactions can be explained using
molecular orbital theory363 (Fig. 26a and b). As was briefly
introduced in Section 3.2, the oxidized species of Tb3+–Pc andDy3+–Pc double-decker complexes tend to show improved SMM
properties compared to the reduced ones due to the shortligand-to-metal distance in the oxidized form, which result ingreater LF splitting and Ueff values. The molecular orbitals of
the sandwich complexes are constructed from the combinationof the Pc ligands. The stacked dimerisation shown in the
double-decker complexes constructs the antibonding HOMO
orbital and the bonding HOMO1 orbital (Fig. 26a). The
removal of electrons (oxidation of the complex) increases thebond order between the ligands, causing axial compression anda decrease in the interligand distance. Although this mechanism is widely accepted, recent ab initio calculations based on

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Chem Soc Rev

Fig. 26 (a) Molecular orbitals of [Ln(Pc)2]. Removal of two electrons
from the antibonding HOMO aﬀords the cationic form [Ln(Pc)2]+ with a00
shorter Ln3+–Pc distance. (b) wM vs. T plots for anionic((NBu4+){Tb[(EtO)8Pc]2}) and cationic ({Tb[(EtO)8Pc]2}+(SbCl6)) Tb3+
double-decker complexes. (c) Magnetic hysteresis behaviour of a Tb3+
double-decker complex in a series of redox states ([Tb(iprPc)2]/0/+) as
captured using MCD spectroscopy. (d) Structure of the perfluoro doubledecker complexes [Tb(F64Pc)2]/2/3. Adapted with permission from (a)ref. 363. Copyright 2006, Elsevier Ltd. Adapted with permission from (b)ref. 84 (c) ref. 366 and (d) ref. 87. Copyright 2007, Copyright 2010 and
Copyright 2013, American Chemical Society.

the CASSCF/RASSI-SO method have indicated that the magnitude of the LF splitting does not strongly depend on the
structural changes induced by the redox reactions, and thatthe calculated energy levels of the first excited doublets(|mJi = |5i) for Tb3+ double-decker complexes are ca.
300 cm1.293,364,365 Therefore, the 8% increase in the Ueff of(NBu4+){Tb[(EtO)8Pc]2} due to oxidation may arise from thermally assisted QTM via higher excited LF sublevels.
In 2010, Gonidec et al. reported the magnetic properties ofthe various redox states of the double-decker complex[Tb(iprPc)2]/0/+ (iprPc = 2,3,9,10,16,17,23,24-octa(isopropylidenedioxy)phthalocyaninato) as detected using magnetic circulardichroism (MCD) spectroscopy (Fig. 26c).366 The advantage of
MCD is the measurement of the magnetic properties of thespecimen in frozen solution states. Therefore, the eﬀects of theintermolecular magnetic interactions and crystal packing, which
could potentially aﬀect the magnetic properties, can be minimized. The MCD intensity vs. magnetic field plots at 1.5 K show
magnetic hysteresis for all redox states due to magnetisation
blocking. [Tb(iprPc)2] and [Tb(iprPc)2]+ show butterfly-shaped
hysteresis curves, and the opening of the curve is larger in[Tb(iprPc)2]+. Importantly, [Tb(iprPc)2]0 (a ligand p-radical
complex) shows clear opening of the hysteresis curve at zero DC
magnetic field. These results clearly indicate that the ligand
p-radical suppresses QTM.277
In 2010, the same group reported the magnetic properties(MCD spectra) of the highly reduced double-decker complex[Tb(F64Pc)2]/2/3 (F64Pc = 1,4,8,11,15,18,22,25-octakis-fluoro2,3,9,10,16,17,23,24-octakis-perfluoro(isopropyl)phthalocyaninato)
stabilized by electron-withdrawing fluoro groups (Fig. 26d).87
Magnetic hysteresis behaviour was observed in all the
reduced states. Among these, [Tb(F64Pc)2]2 showed theChem. Soc. Rev., 2022, 51, 9262–9339 | 9311

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largest magnetic hysteresis. The non-p-radical complexes([Tb(F64Pc)2] and [Tb(F64Pc)2]3) showed narrowing of
the hysteresis curve at zero DC magnetic field, whereas
the p-radical species [Tb(F64Pc)2]2 did not show such behaviour. Accordingly, the results of this study corroborate suppression of the QTM at the zero DC field due to a ligand p
radical.
The first crystal structure characterisation of the dianionic double-decker complex [Cryptand(Na+)]2[Tb(Pc)2]2 wasreported by Konarev et al. in 2019.85 The existence of the ligand
p-radical was confirmed based on near-IR absorption and EPR
analyses. The interligand distance in [Cryptand(Na+)]2[Tb(Pc)2]2 (3.258 Å) is shorter than that (3.294 Å) in themonoanionic form (PPN+)[Tb(Pc)2] (PPN+ = bis(triphenylphosphoranylidene)ammonium). Interestingly, [Cryptand(Na+)]2[Tb(Pc)2]2 does not show slow magnetic relaxation or magnetic
hysteresis at 1.9 K, which may be related to the relatively strong
intermolecular antiferromagnetic interactions confirmed by theDC magnetic susceptibility measurements.
In 2020, Horii et al. reported a comprehensive study on the
structural and magnetic properties of multiple-decker Tb3+(BuO)8Pc complexes (triple- (Tb2[(BuO)8Pc]3), quadruple(Tb2Cd[(BuO)8Pc]4), and quintuple-decker (Tb2Cd2[(BuO)8Pc]5)
complexes) in highly oxidized states.214 Crystal structure analyses and DFT calculations revealed that the multiple-decker
complexes show decreasing interligand distances with successive oxidations. More importantly, paramagnetic NMR analyses
in the solution state revealed that the axial component of themagnetic susceptibility (wax) also decreases with successiveoxidation (Fig. 27a). This behaviour stands in clear contrast
to that of the double-decker complexes, in which wax increases
with oxidation.278 In other words, the Tb3+ ions in the multipledecker complexes show decreased magnetic anisotropy uponoxidation. A similar trend was confirmed in the AC magnetic
measurements of the oxidized species characterized by X-ray
analyses (Fig. 27b). In the case of the triple-decker complex
Tb2[(BuO)8Pc]3, the oxidation from the neutral to the twoelectron-oxidized state {Tb2[(BuO)8Pc]3}2+(SbCl6)2 shifts the00

maxima of the wM vs. AC frequency (n) plots to higher n. In
addition, the Ueﬀ decreases from 233 cm1 (Tb2[(BuO)8Pc]3) to
167 cm1 ({Tb2[(BuO)8Pc]3}2+(SbCl6)2) upon oxidation. In thecase of the multiple-decker complexes, oxidation causes a
decrease in the interligand distances as well as a decrease in
the intermetal distances (Fig. 27c). Due to the approach of thepositively charged metal ions along the axial direction, the axial
LF is decreased. This is the reason for the decrease in Ueﬀ uponoxidation of the multiple-decker complexes.
3.5

Proton-induced switching of the SMM properties

Switching of the SMM properties via external stimuli has great
importance for the use of SMMs as information-storage devices
due to the requirement to eventually erase stored information.
Tanaka and Inose et al.297,367 reported the switching of
SMM properties via the protonation/deprotonation of the
Tb3+-porphyrinato double-decker complex HTb(TPP)2. In the9312 | Chem. Soc. Rev., 2022, 51, 9262–9339

Fig. 27 (a) wax values for the oxidation states of a (BuO)8Pc multiple00
decker complex. (b) wM vs. frequency (n) plots for the redox series ofdoubleandtriple-decker
complexes(Tb2[(BuO)8Pc]3
and{Tb2[(BuO)8Pc]3}2+(SbCl6)2). (c) Crystal structures of Tb2[(BuO)8Pc]3and {Tb2[(BuO)8Pc]3}2+(SbCl6)2 from the cifs in ref. 319 and 214. nButoxy groups are omitted for clarity. The triple-decker complex showsa decrease in axial magnetic anisotropy upon oxidation due to thedecrease in the LF with the approach of a positively charged Ln3+ ion
along the axial direction. Adapted with permission from (a) ref. 214.
Copyright 2020, Wiley-VHC.

protonated form of HTb(TPP)2, H+ coordinates to one pyrrole N
atom (Fig. 28a). The distance between Tb3+ and the protonatedN atom is 2.840 Å; this value is significantly longer than thebond lengths between Tb3+ and the non-protonated N atoms(2.430–2.627 Å). In other words, the Tb3+ ion in HTb(TPP)2
adopts a heptacoordinated structure. The significant deviationfrom the SAP geometry in HTb(TPP)2 results in the absence of
SMM behaviour. When the coordinated proton is removed by
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), the deprotonatedform (HDBU+)[Tb(TPP)2] is obtained. (HDBU+)[Tb(TPP)2]
crystallizes in the space group I4/mmm, and the eight N atoms
coordinating to the Tb3+ ion are crystallographically equivalent.
Therefore, the Tb3+ of (HDBU+)[Tb(TPP)2] adopts an octacoordinated SAP geometry, which is one of the most ideal symmetries for SMMs. As a result, (HDBU+)[Tb(TPP)2] shows slow
magnetic relaxations and butterfly-shaped hysteresis, which
are characteristics of SMMs. The conversion from (HDBU+)[Tb(TPP)2] to HTb(TPP)2 is achieved by the addition of acetic
acid. Therefore, the protonation/deprotonation process is reversible, enabling the reversible switching of the SMM properties
by a single proton.
The protonation/deprotonation of the phthalocyaninatoporphyrinato heteroligand double-decker complex H(Pc)Tb(TPP)
was reported by Tanaka et al. (Fig. 28b).368 This complex is
obtained as the protonated form H(Pc)Tb(TPP), in which theproton coordinates to the meso-N of the Pc ligand. Since theThis journal is © The Royal Society of Chemistry 2022

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Fig. 28 (a) Proton-induced on/oﬀ switching of SMM properties (HTb(TPP)2 and (HDBU+)[Tb(TPP)2]). (b) Protonation/deprotonation of the porphyrinato–phthalocyaninato Tb3+ double-decker complex H(Pc)Tb(TPP) and (c) phthalocyaninato–phthalocyaninato double-decker complex HTb[(aBuO)8Pc]2. (d) Magnetic hysteresis curves of the protonated (HTb[(a-BuO)8Pc]2) and deprotonated form ((HDBU+){Tb[(a-BuO)8Pc]2}) of the doubledecker complex. Reproduced from (a) ref. 297 with permission from the Royal Society of Chemistry. Adapted with permission from (d) ref. 97. Copyright2018, American Chemical Society.

protonation/deprotonation occurs on the non-coordinating N
atom, the changes in the SMM properties induced by deprotonation are not significant compared to those of the homoporphyrinato complexes. However, the protonated form shows
additional magnetic relaxations at low temperatures.
The protonated forms of homo-phthalocyaninato Ln3+
double-decker complexes (Fig. 3) were obtained using the[(a-BuO)8Pc] ligand,95 which has eight n-butoxy groups at thenonperipheral positions of Pc, causing a saddle-shaped distortion of the p-structure (Fig. 3). This distortion causes the mesoN atoms of [(a-BuO)8Pc] to be more basic than those in[(b-BuO)8Pc]. In 2018, Horii et al. reported the modulation of
the SMM properties of HTb[(a-BuO)8Pc]2 via a protonation/
deprotonation reaction (Fig. 28c).97 The protonation of themeso-N atoms in HTb[(a-BuO)8Pc]2 causes asymmetric distortion as well as a low-symmetry structure, whereas deprotonation using DBU aﬀords (HDBU+){Tb[(a-BuO)8Pc]2}, which is
distorted in a highly symmetric windmill-like fashion. The
deprotonated form shows larger magnetic hysteresis than theThis journal is © The Royal Society of Chemistry 2022

protonated one does. Similar trends were observed in magnetically diluted samples, indicating that the diﬀerences between
the magnetic properties of the protonated and deprotonatedforms originate from the molecular structures rather than theintermolecular magnetic interactions.
The following two examples also show changes in their SMM
properties as a result of deprotonation. However, oxidationfrom the anionic (and protonated) form to the neutral (anddeprotonated) form is the driving force for the deprotonation. In 2019, Chen et al. reported the magnetic properties ofHTb[(a-C5H11O)4Pc]2 with 3-pentyloxy-substituents (Fig. 3a).94
The Ueﬀ value for HTb[(a-C5H11O)4Pc]2 is 300 cm1. On theother hand, the Ueﬀ value for the neutral and deprotonatedform Tb[(a-C5H11O)4Pc]2 obtained using 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) is 324 cm1, indicating that
the SMM properties are enhanced by deprotonation. In contrast
to the previous examples, the presence of the ligand p-radical inTb[(a-C5H11O)4Pc]2 was confirmed by absorption spectroscopy.
Therefore, Chen et al. concluded that the enhancement of theChem. Soc. Rev., 2022, 51, 9262–9339 | 9313

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Fig. 29 Molecular structures of Dy[(BNPO)2Pc](TClPP).

SMM properties stems from the radical–f interactions andenhanced LF due to the shorter ligand-to-metal distances in
the neutral form.
In 2019, Sun et al. reported the negative eﬀect of deprotonation on SMM properties by comparison of the chiral(phthalocyaninato)(porphyrinato) Dy3+ double-decker complexes (R)/(S)-H[(BNPO)2Pc]Dy(TClPP) and the corresponding
neutral unprotonated species (R)/(S)-[(BNPO)2Pc]Dy(TClPP)(Fig. 29).369 The AC magnetic susceptibilities of the protonatedform (R)/(S)-[(BNPO)2Pc]Dy(TClPP) show clear frequency dependences, whereas (R)/(S)-[(BNPO)2Pc]Dy(TClPP) shows no frequency dependence even under a bias DC field (Hdc = 2000 Oe).
DFT calculations of (R)/(S)-H[(BNPO)2Pc]Dy(TClPP) and(R)/(S)-[(BNPO)2Pc]Dy(TClPP) revealed that the ionic part of
the N–Dy bond decreases due to oxidation (deprotonation).
This decrease in the electrostatic interactions between theligand and Dy3+ decreases the uniaxial magnetic anisotropy of(R)/(S)-[(BNPO)2Pc]Dy(TClPP).
3.6 Influence of solvation eﬀects in the crystal lattice on SMM
properties
The benzene solvates (l-Pc)[Ln(dpm)2]22C6H6 showed interesting behaviour with respect to desolvation and exchange ofsolvation molecules. Thus, heating of solvates with Ln = Sm,
Tb, Dy198 and Er199 at 100 1C was shown to cause desolvation
without disintegration of crystals. This procedure had dramatic
influences on the structural and magnetic properties of complexes which were demonstrated on a dysprosium analogue.
The solvated complex contains one crystallographically independent metal center, which exhibits a single magnetic relaxation process with an energy barrier (Ueﬀ) of 39 cm1. Whendesolvated, the complex had two diﬀerent types of metal
centres with slightly diﬀerent surrounding, and it exhibitedfield-induced SMM behaviour with two thermally activatedmagnetic relaxation processes with Ueﬀ which reach 44 cm1and 76 cm1, respectively. Soaking of desolvated crystals inbenzene recovered (l-Pc)[Ln(dpm)2]22C6H6 which was confirmed by X-ray powder diﬀractions and elemental analyses.
This work confirmed that the solvated molecules can finely
tune the magnetic relaxation mechanisms. Further development of this approach by Ge et al. showed that benzene can be
replaced with dichloromethane in the course of ‘‘crystal to9314 | Chem. Soc. Rev., 2022, 51, 9262–9339

Fig. 30 Interconversion between solvates (l-Pc)[Er(dpm)2]22C6H6(CCDC 1494900) and (l-Pc)[Er(dpm)2]22CH2Cl2 (CCDC 1494901) causing drastic change of magnetic relaxation as seen from the corresponding
out-of-phase (w00 ) AC susceptibility plots under 600 Oe DC field. Adaptedwith permission from ref. 200. Copyright 2017, American Chemical
Society.

crystal’’ transformation.200 Again, the change of solvation state
had influence on the magnetic properties, which was demonstrated on the example of (l-Pc)[Er(dpm)2]22C6H6 undergoingtransformation to (l-Pc)[Er(dpm)2]22CH2Cl2.200 Magnetic
susceptibility measurements showed that the two solvates
exhibited dramatically diﬀerent relaxation behaviour (Fig. 30).
While benzene solvate exhibited fast relaxation and an estimated energy barrier of 2.6 cm1 under 600 Oe DC field, thedichloromethane solvate behaved as a field-induced SMM witha higher energy barrier (Ueﬀ) of 34.3 cm1. The source of thesemagnetic diﬀerences was tentatively assigned to subtle structural diﬀerences caused by the change of the crystal lattice, asconfirmed by ab initio calculations.
Ruan et al. showed that crystallization of Tb(Pc)2 from
CHCl3/MeOH or CH2Cl2/MeOH mixtures aﬀorded orthorhombic crystals Tb(Pc)2 (space group P212121) and Tb(Pc)2CH2Cl2(Pnma) solvates respectively.370 Although the structural diﬀerences in magnetically active sandwich molecules were only
subtle, absorption of CH2Cl2 in the crystal lattice had a profound eﬀect on magnetic relaxation, resulting in a butterflyshaped hysteresis loop in the case of solvate up to 10 K, while
Tb(Pc)2 showed almost no opening even at 2 K. These results
are consistent with the conclusions of Yamabayashi et al.,shown in Section 3.3.3.347
In the context of this result, an important study by Dailey
and Besson on the selective crystallization of four possible
polymorphs of M(Pc)2 complexes should be mentioned.371
The authors proposed reproducible and selective crystallization
techniques aﬀording pure a-, g- and d-phases together with theCH2Cl2 solvate of complexes depending on the solvent, applied
for crystallization. Systematic crystallographic and UV-Vis characterization of complexes allowed some of the confusion overchemical composition in previously published literatureThis journal is © The Royal Society of Chemistry 2022

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associated with interconversion between neutral and ionicforms of bisphthalocyaninates to be explained.
3.7

Magnetic studies on surfaces

The properties of molecules significantly change not only
through aggregation but also through the interface oforganic–inorganic hybrid materials. One of the goals of theresearch involving sandwich-type tetrapyrrole rare-earth metal–
inorganic hybrid materials is their application in spintronic
devices.344 In the case of Tb(Pc)02 , the magnetic species arecomposed of the f electron of the central Tb3+ ion and the pradical on the Pc ligands. The Pc ligands are almost flat, andthe direction of the magnetic moment (axis of easy magnetisation) of the Tb3+ ion is perpendicular to the Pc ligand withuniaxially anisotropy, indicating that they are single-moleculemagnets.36 Furthermore, since they are thermally stable, they
can be deposited on various substrates in an ultra-high vacuum(UHV). Several groups have reported on the structures andphysical properties of sandwich-type tetrapyrrole rare-earth
complexes on various substrates. In this section, we give an
overview of the morphologies and magnetic properties ofdouble-decker and triple-decker complexes on different substrates. Since 2008, many examples of Ln(Pc)2 and/or Ln2(Pc)3complexes on various substrates, including non-magnetic
metals (Cu(100),267 Cu(111),372,373 Cu(001),374 Au(111),232,302
Ag(111),375 Ir(111),376 MgO/Ag(100),377 CuO/Cu(110)378), magnetic metals (Co/Ir(111),379 Co/Cu(100),380 Co/Au(111),299 Ni/
Cu(100),381 Ni/Ag(100),381 Fe/Cu(100),382 FeMn/Cu(100),382
CoO/Ag(100),383 Mn/Ag(100),383 Ni(111)384), carbon-based substrates (HOPG,385–387 graphite(0001),388 graphene/SiC(0001),389
graphene/Ni(111)390), silicon-based substrates (SiO2(100)391),
perovskite (LaSrMnO3/SrTiO3,380 SrVO3(001)392), and composite
substrates,393,394 have been reported. Moreover, materials of
sandwich-type tetrapyrrole rare-earth metal complexes combined with carbon nanotubes (CNT)/fullerene are shown in
Chapter 3.8. In general, eight lobes for both Ln(Pc)2 and/or
Ln2(Pc)3 are observed in topographic STM images,301 whichreflects the electronic state of the Pc ligand on the vacuum side.
On the other hand, for Ln(Pc)x/Au(111), four lobes are observedin the topographic image.232 In addition, the STM heightprofiles for Ln(Pc)x, Ln(Pc)2, and Ln2(Pc)3 on substrates reflect
the molecular structure (crystal structure).301
In recent years, physical properties of surfaces have been
actively studied to advance the research on applications, andtogether with the progress of nanotechnology, the academic
and social importance have increased. Precise control is possible at the atomic level, and organic–inorganic hybrid compounds can be prepared with the surfaces acting as reaction
fields. Atoms and molecules adsorbed on surfaces can arrange
into various dimensional structures (zero dimensional (0D),
one dimensional (1D), two dimensional (2D), and three dimensional (3D)) which exhibit different properties. For example, 2D
substances have multifunctional properties. Due to the electronic state of graphene, spin-polarised electron bands form fornon-magnetic materials. In addition, graphene is used forThis journal is © The Royal Society of Chemistry 2022

Chem Soc Rev
molecular spintronics because it utilizes spin–orbit interactions which increases when the p–d orbitals mix at the interface
with magnetic metal electrodes.395 At the interface, chargetransfer bring about the alteration of the electronic structure,
thereby affecting or even quenching the magnetic moment of
the molecules. Recently, such spin current control at the interface at the molecular level has become known as spinterface.396
In principle, it is possible to use the spin for quantum information processing (QIP), and spin detection and evaluation areimportant for realising a quantum computer (QC) and/or
spintronic devices.258 Electron spin resonance (ESR), reportedby E. Zaboisky in 1945, is one such detection method.
At the same time, scanning tunnelling microscopy (STM)
and scanning tunnelling spectroscopy (STS) can be used to
identify one magnetic atom adsorbed on a substrate surface
and directly observe the density of states (DOS). Thus, they havebeen attracting attention as a method for single spin detection.
STM and STS play a major role in directly investigating therelationship between morphology and electronic structure. For
example, for CoPc adsorbed on an Au surface, a symmetricKondo resonance is observed because only the direct tunnel
process from the probe to the substrate is suppressed by the Pc
ligand.397 It is possible to study the interactions between
magnetic atoms and non-magnetic/magnetic metals or thosebetween magnetic atoms on an atomic scale.398 The Kondo
eﬀect, which is the principle of Kondo resonance, was proposedby Jun Kondo in 1964. As the temperature of a metal decreases,
its electrical resistance decreases. However, the electrical resistance increases when the temperature is lowered below acertain temperature due to trace amounts of magnetic impurities contained in the metal. This is a phenomenon in which
the spins of metallic conduction electrons and the spins ofmagnetic impurities finally form a Kondo–Yoshida singlet,
which has a higher resistance than the metallic state and theelectrical resistance remains constant. In this process, as aresult of the spin exchange process between the conduction
electrons and the magnetic impurities, a resonance state isobserved as a sharp peak at the Fermi level (0 V). The temperature at which the electrical conductivity becomes constant at
low temperatures is defined as the Kondo temperature (TK),
and in the case of STS, it is proportional to the peak width at
half maximum of the Kondo resonance obtained from tunnelling current spectroscopy. By applying the Kondo effect to the
conduction electrons, it is possible that the conductancechanges, and this phenomenon has been attracting attention
again recently because of its high utility in spintronics. As aresult, the Kondo resonance should be observed even forM(Pc)2, which has a spin source.
A Kondo resonance for a delocalized p radical on the ligand
due to the electronic state of Tb(Pc)2 adsorbed on a substrate
has been reported, and it is clear that the Kondo resonanceobserved on Au(111) depends on the molecular conformation(Fig. 31a–c and 32a, b).302,399 In other words, the Kondo
resonance can be switched off by changing y by o81. In
addition, when Cs is deposited on Tb(Pc)2/Au(111), the electronic structure of the molecule changes.400 This is probably

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significantly due to the addition and desorption of proton at
the isoindole nitrogen (Niso) position, meaning that the Kondo
resonance could be switched on and off (Fig. 31i–n).307 Moreover, Kondo resonances have been detected for Tb(Pc)x/Au(111)and Tb2(Pc)3/Ag(111) (Fig. 32d).232,401 However, a Kondo resonance for Y2(Pc)3/Au(111) has not been observed under thesame conditions as those used for Y(Pc)2/Au(111) (Fig. 32c).302
It has been shown that interactions with a Cu(111) substratequench the unpaired p electron of Tb(Pc)02 , explaining the lackof a Kondo resonance. In addition, it has been shown that theunpaired p electron expected for an isolated molecule ofDy(Pc)02 is quenched upon adsorption onto Cu(001). In thecase of Nd(Pc)2/Cu(100), quenching occurs due to charge
rearrangement at the interface.402 On the other hand, a 4f
metal centre of Dy(Pc)2/Cu(001) contributes to the Kondo
resonance.374 From density functional theory (DFT) calculations, the Kondo resonance occurs due to the coupling between
the Dy3+ ion and the Cu electron bath which is mediated bystrong hybridization between Dy d and f orbitals.
Research on the Kondo resonance induced at the interface
between a light rare-earth tetrapyrrole sandwich compounds

Fig. 31 STM image of the sandwich-type tetrapyrrole rare-earth metal
complexes on the Au(111) surface. (a) STM image of an Isolated Tb(Pc)2
molecule. (b) Molecular structure of Tb(Pc)2 which is shown in the upper(lower) Pc ligand is coloured in blue (silver). (c) A simulated STM image ofTb(Pc)2 obtained by using DFT calculations. (d) Molecular structure of(Nc)Tb(Pc). (e–h) STM and DFT simulated STM images of isolated Pc-up
and Nc-up ligands of (Nc)Tb(Pc). White bars in (a), (e), and (g) are 1 nm. (iand j) A schematic model of the operation caused by the injection oftunnelling electrons into a (HOEP)Tb(OEP). (i) Proton (H) atom hoppingand (j) H atom desorption. (k–n) STM and DFT simulated STM images ofisolated (k and m) Tb(OEP)2 and (l and n) (HOEP)Tb(OEP). Topographic
images, which is similar to those in (k) and (l), observed by the operations in(i) and (j). Reproduced from (a–c) ref. 302. Copyright 2011, Nature
Research. Adapted with permission from (d–h) ref. 135. Copyright 2013,
American Chemical Society. Reproduced from (i–n) ref. 307. Copyright2018, Royal Society of Chemistry.

because charge transfer occurs when Cs is added, switching on
the Kondo resonance. On the other hand, for Tb(Pc)2/Ag(111),
Kondo resonance is silent in the first layer of Tb(Pc)2 but activein that of the second layer.375 The p radical recovers bydecoupling at the interface. Since the Kondo resonance is silent
even for Tb(Pc)2/Cu(111), the interactions between the substrate and Tb(Pc)02 are important for understanding the Kondo
resonance.372 In other words, it is possible to control the spinstate of Tb(Pc)02 via the interface. In addition, since Kondo
resonances depend on the electronic state of ligands, the TK
values of delocalized p radicals of Nc2 (naphthalocyaninato)
and Pc2 ligands of (Nc)Tb(Pc)/Au(111) differ (Fig. 31d–h).135
Furthermore, for Tb(OEP)2 (OEP2 = 2,3,9,10,16,17,23,24-octaethylporphyrinato)/Au(111), the electronic structure changes

9316 | Chem. Soc. Rev., 2022, 51, 9262–9339

Fig. 32 STS measurement of the sandwich-type tetrapyrrole rare-earth
metal complexes on a metal substrate near the Fermi level. (a) Change inthe Kondo peak height of Tb(Pc)2/Au(111) when the tip position is moved
along the blue line (bottom) in the STM topography panel. The diﬀerent
colours correspond to the dI/dV intensity, described above the figure. (b)dI/dV spectra measured at the upper Pc ligand of Tb(Pc)2/Au(111) at
positions I (centre) and II (ligand) of the molecule. (c) dI/dV spectra andSTM topography of Y(Pc)2/Au(111) (I) and Y2(Pc)3/Au(111) (II). (d) STM
topography (i) and topography with staggered three Pc ligands (iii) ofTb2(Pc)3/Ag(111). (ii) Map of the fitted Kondo temperature (TK) on a single
Tb2(Pc)3/Ag(111). (iv) Averaged point spectra showing the Kondo peak (redx’s on ii) on a single Tb2(Pc)3/Ag(111). Reproduced from (a–c) ref. 302.
Copyright 2011, Nature Research. Reproduced from (d) ref. 401. Copyright2018, Royal Society of Chemistry.

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has been reported. For (Pc)Ce(TPP)0 and Ce(Pc)02 , the formal
charge of Ce4+ resulting from the donation of four negative
charges to both Pc2 and TPP2 anions causes a 4f0 electronic
configuration.403,404 In this case, no Kondo resonance is
expected. However, (Pc)Ce(TPP)/Ag(111) exhibits a Kondo resonance but (Pc)Ce(TPP)/Cu(111) does not.403 Thus, a formal
3 +/4+ oxidation state would result in one electron in the 4f
orbital and one unpaired p electron in a radical state ( )/2 inthe ligand for a valence fluctuating system, such as Ce(Pc)02 , inwhich the Ce ion has a valence charge of about 3.57 estimatedfrom Ce LIII-edge X-ray absorption near edge structure spectrum (XANES).403 These systems with fluctuating valences could
lead to a ligand-spin induced Kondo resonance similar to thoseobserved for Cu(Pc)/Ag(100) and Tb(Pc)x/Au(111).405 Intermediate valence states make it diﬃcult to explain the presence of a
Kondo resonance. Strong hybridization of the molecular orbitals and substrate could result in the quenching of the spin at
the interface, eliminating the Kondo resonance. In addition,
ligand–ligand interactions in Pc based molecules have beenshown to reduce molecule–substrate coupling via the modification of the molecule/substrate distance or charge transfer andtherefore to reduce or even quench TK.135,405 For example, thecompetition between the Kondo effect and Ruderman–Kittel–
Kasuya–Yosida (RKKY) coupling in 1D and 2D lattices has beenreported.135,406 Thus, theoretical and experimental results areneeded to fully understand the spin states at the interface
involved in the Kondo process.
The magnetic anisotropy and coercivity can be investigatedusing soft X-ray magnetic circular dichroism (XMCD) measurements on Ln(Pc)2 (Ln3+ = Tb and Dy) on a substrate (Fig. 33).267
Moreover, the magnetic properties of rare-earth metal ions
three-dimensionally surrounded by organic ligands in anelement-selective manner can be analysed, and the origin of
the magnetic anisotropy of SMMs can be determined from theviewpoint of their electronic states. Soft XMCD can be used on asingle atomic layer when synchrotron radiation is used. The
uniaxial magnetic anisotropy of the Tb3+ ion of Tb(Pc)2 can be
determined by measuring the change in the XMCD spectrum
when the magnetic field is applied in diﬀerent directions(vertical and in-plane directions to the substrate). By performing sum-rule analysis on the obtained spectrum, magnetic
hysteresis can be obtained from the quantitative evaluation of
the Tb 4f magnetic moment and the magnetisation curve foreach element. In addition, by applying the sum rule to the
XMCD (and/or XAS) spectra, the orbital magnetic moment Lz
and spin magnetic moment 2Sz of the Tb3+ ions can becalculated separately. Studies on the magnetic properties ofLn(Pc)2 (Ln3+ = Tb and Dy) on various substrates using XMCD
have been reported.345,380,387,391 The ferromagnetic Ni
layer forms out-of-plane (OP)/Cu(111) and in-plane (IP)/Ag(100), depending on the type of substrate (Fig. 33d–g).381
When Tb(Pc)2 is adsorbed on these Ni layers, the elemental
decomposition hysteresis loop changes according to the magnetisation direction of the ferromagnetic Ni layer due to themagnetic interaction between Tb(Pc)2 and Ni layer. Tb(Pc)2/Fe/
Cu(111) has the same antiferromagnetic relationship as the Ni

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system.382 When Li is deposited, the ferromagnetic exchangeinteractions change probably because charge transfer occurs
when Li is added. Furthermore, it has been reported that theSMM characteristics of Tb(Pc)2 and Dy(Pc)2 adsorbed on the
MgO substrate are better (Fig. 33c).377,407 The improvement in
the SMM properties is believed to be due to the effects of spinphonon coupling on the magnetic stabilities of surfaceadsorbed SMMs. This suggests that the use of substrates withlow phonon density of states is beneficial for the constructionof molecular-based spintronics devices and for further
increases in the blocking temperature (TB) of surfaceadsorbed Ln(Pc)2 (Ln3+ = Tb and Dy).
Spintronics of SMMs, like Tb(Pc)2 on a substrate, has been
prepared using the spin state control. With respect to Tb(Pc)2adsorbed on a ferromagnetic Co layer grown on an Ir(111), thespin-polarised tunnel current changes according to the application direction (parallel/antiparallel) of the external magnetic

Fig. 33 XMCD hysteresis curves for Tb(Pc)2 on a metal substrate. (a)Schematic illustration of a Tb(Pc)2 on an ultrathin MgO film on Ag(100).
(b) STM image of a 2D film of Tb(Pc)2 on a double-layer film of MgO on
Ag(100). (c) Hysteresis loop obtained with XMCD at 3 K and the best fit
calculations of magnetisation dynamics for Tb(Pc)2/MgO/Ag(100) andTb(Pc)2/Ag(100). (d) Schematic illustration of the magnetic anisotropy axisof Tb(Pc)2/Ni(OP)/Cu(100) and (f) elemental Ni and Tb resolved hysteresis
loops from XMCD at 8 K. (e) Schematic illustration of the magnetic
anisotropy axis of Tb(Pc)2/Ni(IP)/Ag(100) and (g) elemental Ni and Tb
resolved hysteresis loops from XMCD at 8 K. Adapted with permission from(a and b) ref. 377. Copyright 2016, Wiley-VCH. Reproduced from (c) ref.
407. Copyright 2019, Wiley-VCH. Reproduced from (d–g) ref. 305. Copyright 2016, Royal Society of Chemistry.

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field (Bext) (Fig. 34a).379 It depends on the magnetisation direction of the chip and the sample, and the pattern of spinpolarisation of the molecular orbital is observed (Fig. 34b–d).
It has been shown that the spin of Tb(Pc)2 is antiferromagnetically coupled to the magnetisation of Co-islands grown on
Au(111) regardless of the adsorption site of the Tb(Pc)2.299 Bothedge and flat-lying molecules exhibit a stable spin direction
opposite to the magnetisation direction of the Co-islands towhich they are attached (Fig. 34e). More importantly, the spinstate of Tb(Pc)2 can be switched independently of the Co island
by applying Bext (Fig. 34f). The magnetic asymmetries of edge
and flat molecules are reversed due to the difference in the DOS

Review Article
due to the Tb(Pc)2-Co-island interactions between the twodifferent adsorption configurations. Stable spin polarisation
is inferred from the substantial hysteresis observed in themagnetisation curve of the Tb(Pc)2 SMM even with the frozen
magnetization of the Co island (Fig. 34g). The observation ofsignificant openings of the hysteresis in the magnetisation
curves suggests suppression of the quantum tunnelling of themagnetisation (QTM) process.
SMMs and/or molecular nano magnets (MNM) havepotential for technological applications in magnetic storage
and for implementing qubits in the quantum computing(QC).266 The application in magnetic storage requires highanisotropic energy barriers for spin reversal (Ueﬀ) and QTM
suppression, whereas QC requires a QTM process, such as aspin cascade |S = 1/2i8J = 6i8I = 3/2i in the Tb(Pc)02 system, for
qubit reading and manipulation (the spin cascade mechanism
for qubit is described in Section 3.2).270 Therefore, both applications can be achieved using organic–inorganic hybrid materials by developing viable means for coordinating the QTM
process in a controlled manner. An approach that uses the
hybrid materials to control and enhance the properties ofsandwich-type tetrapyrrole rare-earth metal SMMs will accelerate further research towards the realisation of spintronic
devices made of functional sandwich-type tetrapyrrole rareearth metal complexes.
3.8 Rare-earth phthalocyaninato sandwich complex/carbon
nanotube hybrid materials

Fig. 34 Spin state control of Tb(Pc)2 on a ferromagnetic Co layer. (a)Schematic illustration of the SP-STM experimental setting of a Tb(Pc)2/Co/
Ir(111) by applying an external magnetic field (Bext) which is parallel (right)
and antiparallel to the magnetisation direction (left). (b) Schematic illustration of the spin-split on the LUMO of Tb(Pc)2 on the ferromagnetic
Co layer. (c) Spin-resolved point-mode spectroscopy. The parallel(blue line) and the antiparallel magnetisation directions (red line) (above).
Diﬀerence in the spectroscopic curves (below). (d) The spin polarisationof the molecular orbitals which have opposite spin polarisations: red(U = +1.0 V) and blue (U = +1.3 V). (e) STM topographic image ofTb(Pc)2/Co/Au(111), which is grouped in islands Co 1 and Co 2.
(f) Conductance mapping for Tb(Pc)2/Co/Au(111) obtained with Vs =
600 mV and Bext = +1 T (i) and 1 T (ii). (iii) Diﬀerence mapping
DG (+1 T, 1 T). The scale of dI/dV (top) and the DG (bottom) are shownin the colour bar scales. (g) dI/dV signal measured as a function of external
magnetic field (Bext) for Tb(Pc)2/Co/Au(111), which are shown in (f). The
hysteresis loops of dI/dV shown in both edge and flat-lying molecular
positions. Adapted with permission from (a–d) ref. 379. Copyright 2012,
Nature Research. Adapted with permission from (e–g) ref. 299. Copyright
2019, American Institute of Physics.

9318 | Chem. Soc. Rev., 2022, 51, 9262–9339

The internal nanospace of carbon nanotubes (CNTs) can be
regarded as a reaction vessel that is 1/1000 smaller than amicroreactor.408 Therefore, these chemical reaction fields,
which have a diameter range of 1–3 nm (100 nm to 0.1 mm
in length), are expected to have a great influence on reaction
chemistry and/or nanoelectronics research in the 21st
century.409–412 It has been shown that atoms and molecules
can be contained in the nanospace of CNTs.413–418 Focusing on
the use of the stable one-dimensional nanospace of a CNT as ahost for host–guest structures, various SMM–CNT hybrid materials (i.e., with single-walled CNTs (SWCNTs): internal diameterof ca. 3 nm and multi-walled CNTs (MWCNTs): internal diameters of (5–50 nm)) have been reported for nanoelectronics
and spintronics research.344,419–421
In 2009, Kyatskaya et al. first reported a [(Pyrene)Pc]Tb(Pc)–
SWCNT hybrid material, in which a [(Pyrene)Pc]Tb(Pc) (one Pc
ligand is substituted with three hexyl groups and one 4-(4pyren-1-ylbutoxy) group) was attached to the outside of a
SWCNT, along with its magnetic properties.114 [(Pyrene)Pc]Tb(Pc)–SWCNT shows a Coulomb blockade in which a
Tb(Pc)2 derivative and a SWCNT act as a quantum dot in a fieldeffect-transistor (FET) device. In the spin valves of [(Pyrene)Pc]Tb(Pc)–SWCNT, the maximum magnetic resistance ratio
between the parallel state and the antiparallel state is approximately 300% at submillikelvin temperatures.117,312 In 2011, in a
related study, Urdampilleta et al. reported hybrid devices thatconsist of chemical vapor deposition (CVD)-grown CNT

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transistors decorated with Tb(Pc)2 SMMs.269,309 The devices
were achieved by tailoring the supramolecular p–p interactions
between the Tb(Pc)2 SMMs and CNTs (Tb(Pc)2–CNT) (Fig. 35a).
Magnetoresistance measurements revealed steep steps, whichwere related to the magnetisation reversal of the individual
Tb(Pc)2 SMMs. The electron-transport properties of thesedevices are strongly dependent on the relative magnetisation
orientation of the grafted SMMs. In this material, the Tb(Pc)2
SMMs act as a local spin polarizer and analyser on the CNT
electron-conduction channel. As a result, reluctance ratios of
up to several hundred percent were observed at submillikelvintemperatures. These results suggest that a spin valve with alarger magnetoresistance ratio can be prepared by using SMMs
as the spin source instead of ferromagnets. These systems were
demonstrated to detect differential conductance (dI/dV) by
taking advantage of the switching of the QTM steps by thecorresponding magnetic field (m0Hz o 50 mT). In otherwords, readout of the nuclear spin state was realized using
Tb(Pc)2 SMMs.
In 2019, Abd El-Mageed et al. reported a supramolecular
structure formed by a Tb(BDP)2–SWCNT hybrid material, in
which Tb3+-5,15-bisdodecylporphyrin (BDP) double-decker
SMMs were attached to the outside of an SWCNT.423 Scanningtunneling microscopy (STM) confirmed an ordered, selforganized spiral array on the SWCNT surface stabilized by
non-sharing (p-stacking) interactions. In addition, atomic force

Chem Soc Rev
microscopy (AFM) images exhibited a distinctive debundlingeffect due to the adsorption of the Tb(BDP)02 SMMs on the
SWCNT surface with some helical array structures. The SMM
properties of Tb(BDP)2–SWCNT were investigated, and the
hybrid materials interestingly showed larger butterfly-shaped
hysteresis loops than the Tb(BDP)02 SMMs alone, indicating aslow t for Tb(BDP)2–SWCNT. So far, electron-transport properties have not yet been reported for Tb(BDP)2–SWCNT.
In 2021, Katoh et al. first reported the encapsulation ofTb(Pc)2 SMMs in the internal nanospace of SWCNTs(Tb(Pc)2SMM@SWCNT).422 The magnetic and electronic properties of the Tb(Pc)2SMM@SWCNT hybrids (Fig. 35e) were
investigated in detail using DC and AC magnetic susceptibility
measurements, transition electron microscopy (TEM), scanning electron microscopy (SEM), STM, and STS. By arranging
the Tb(Pc)2 SMMs in the 1D internal nanospaces of the
SWCNTs, it is possible to investigate the essential SMM characteristics of Tb(Pc)2 without considering LF eﬀects. Moreover,it appears that the electron correlation between Tb(Pc)2 and the
SWCNT can aﬀect the electron transport and/or electromagnetic properties. Since bandgap modulation was observed dueto the encapsulation of Tb(Pc)2 in SWCNTs, these materials
should be useful in FETs. Furthermore, as the stable internal
nanospace of SWCNTs is used, it is thought that the density of
the SMMs in the SMM–SWCNT hybrid material can be controlled, and the hybrids should be usable as spin valves. This
strategy may pave the way for the construction of SMM–SWCNT
hybrid materials for operable molecular spintronic devices
using the quantum effects of individual SMMs.

4. Semiconducting materials based on
REE sandwich complexes

Fig. 35 Supramolecular spin valves based on the Tb(Pc)2–SWCNT hybrid
device. (a) Schematic illustration of the device. (b) Conductance hysteresis
loops of Tb(Pc)2–SWCNT. dI/dV measured at 40 mK with an in-planemagnetic field applied along the easy axial direction (01). Bias and temperature dependences of the conductance hysteresis loops of Tb(Pc)2–
SWCNT. (c) Scale map of the dI/dV hysteresis as a function of the in-planemagnetic field and source drain voltage (Vds). (d) Temperature dependenceof the conductance hysteresis loops. (e) Supramolecular structure of theTb(Pc)2@SWCNT hybrid material. TEM image and schematic illustration ofTb(Pc)2@SWCNT. (f) STM image showing empty (red dot) and filled (blackdot) regions. Comparative STS spectra measured at the red and black
dotted points in STM image. The STS of Tb(Pc)2 adsorbed on an Au(111)
surface is illustrated as plot III (blue) in (f) as a reference. Reprinted from (a–d) ref. 269. Copyright 2011, MDPI. Reproduced from (e and f) ref. 422.
Copyright 2021, Royal Society of Chemistry.

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Discovery of semiconducting properties of phthalocyanines
reported by the group of J. Simon in 1987 on the examples ofradical complexes LiPc and Lu(Pc)2 is definitely one of important milestones not only for tetrapyrrolic compounds, but for
all organic electronics in general.35 This discovery nearly coincided with the first report on organic field eﬀect transistors(OFETs) made in 1986 by Tsumura et al. with polytiophene
conducting layer,424 thus no surprise that the same year Lu(Pc)2
was introduced into OFET technology.
OFETs attract increasing research interest since they werefirst reported due to their potential for application inflexible displays, integrated circuits and low-cost electronic
devices. A typical OFET device consists of an organic semiconducting layer, a dielectric layer, and three electrodes(source, drain, and gate electrodes). Three key parameters
define the performance of an OFET device, namely charge
carrier mobility (m), current on/oﬀ ratio, and threshold voltage(VT). The organic semiconducting layer is the most importantpart to determining the OFET performance, though otherfactors, such as the device configuration, the interfaces, andthe materials for electrodes and insulators, also can aﬀect theperformance.425,426

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p-Conjugated tetrapyrrole derivatives, including porphyrins(Pors) and phthalocyanines (Pcs), are important organic semiconducting materials for OFETs because of their unique molecular and electronic structures.427,428 For rare-earth based
tetrapyrrolic sandwiches, the molecular energy levels are highly
disturbed by the intramolecular p–p interactions between tetrapyrrolic rings, typically possessing destabilized HOMOs andstabilized LUMOs, thus having decreased molecular energy
gaps in comparison to the constituent monomeric tetrapyrroles. This is desirable for improving the semiconducting
properties of tetrapyrrolic materials, as demonstrated by Guillaud et al. for Lu(Pc)2 and Tm(Pc)2 based OFETs.429 The
molecular energy levels of tetrapyrrolic sandwiches can befurther tuned by choosing different ligands and rare-earth
metal ions, adjusting the number of deckers, and incorporating
various substitutes on tetrapyrrolic ligands.1,7 The intermolecular interactions and solubility of sandwiches are also adjustable by such chemical modifications, which can provide
further opportunity for the fabrication of solution-processable
and high quality films for OFET devices.
Moreover, specific substitution of sandwich complexes may
allow for the formation of supramolecular structures thatprovide intrinsic dielectric insulation at the molecular level.
For example, ultrathin solid films of structurally similar octabutoxy and tetra-crown-ether substituted Eu(III) bisphthalocyaninates, Eu[(BuO)8Pc]2 and Eu[(15C5)4Pc]2, were formed andtheir transversal conductivity and, consequently, electrochemical activity were studied in an aqueous electrolyte.430 Excellent
transfer of charge through monomolecular ultrathin films ofcrown-substituted complex was observed enabling its redox-

Table 1

multistability, which can be utilized in molecular logic andinformation storage devices. However, transversal charge transfer was inhibited in the films formed by octa-butoxy-substitutedcomplex, which might be promising for OFET applications.
Depending on the correlation between the molecular energy
levels of the semiconducting layer and the work function ofsource–drain electrodes, holes or electrons can be preferentiallyinjected from the source electrode to the semiconducting layer,
and transported to the drain electrode, leading to p- or n-typetransporting modes, respectively. If both types of charge carriers can be injected and transported, an ambipolar OFET
device is achieved, which, however, is of great importance incomplementary metal oxide semiconductor (CMOS) logic
circuits.431 Most tetrapyrrolic sandwiches can exhibit p-type
OFET performance in air with usual source–drain electrodes(such as Au, with a work function of 5.1 eV), because of thesuitable HOMO energy levels of sandwiches for hole injection(Table 1). However, unipolar n-type operations are often
detected under an inert (or reducing) gas atmosphere, becauseof the relatively high LUMO energy levels of sandwiches, whichcannot resist the air-derived electron traps. On the other hand,due to the versatility of tuning the HOMO and LUMO energy
levels of tetrapyrrolic sandwiches by molecular design,432
single-component air-stable ambipolar OFET devices basedon tetrapyrrolic sandwiches have become the research focus
in recent years (Table 2).
4.1

p-Type OFETs

In 2005, Su et al. reported LB film-based OFETs by using
heteroleptic bis(phthalocyaninato) rare earth complexes

Performance of sandwich complexes based p-type OFET devices

Compounds/deposition method

Hole mobility[cm2 V1 s1] VT [V] Ion/Ioﬀ
4[(OctO)8Pc]Tb(Pc)/LB
6.4  10
1.7  103[(OctO)8Pc]Lu(Pc)/LB
Lu[(a-Oct)8Pc]2/SC
1.5  103
8.0  103
Lu[(a-Oct)8Pc]/SC, TA
7.47  105
Gd[(a-C8H17)8Pc]2/SC
1.96  105
Gd[Pc(a-C8H17)8]2/SC
0.60[(OctO)8Pc]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB
0.40[(OctO)8Pc]Ho[(15C5)4Pc]Ho[(15C5)4Pc]/LB
0.24[(OctO)8Pc]Lu[(15C5)4Pc]Lu[(15C5)4Pc]/LB
0.0032[(BuO)8Pc]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB
0.014[(HexO)8Pc]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB[(DecO)8Pc]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB
0.21[(C12H25O)8Pc]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB
0.053
0.46
[(HexO)8Pc]Eu[(DEGOMe)8Pc]Eu[(DEGOMe)8Pc]/LB
0.17[(OctO)8Pc]Eu[(DEGOMe)8Pc]Eu[(DEGOMe)8Pc]/LB[(DecO)8Pc]Eu[(DEGOMe)8Pc]Eu[(DEGOMe)8Pc]/LB
0.096[(C12H25O)8Pc]Eu[(DEGOMe)8Pc]Eu[(DEGOMe)8Pc]/LB 0.014[(OctO)8Pc]Eu[(12C4)4Pc]Eu[(12C4)4Pc]/LB
0.03
0.31
0.33
0.02
[(OctO)8Pc]Eu[(18C6)4Pc]Eu[(18C6)4Pc]/LB
0.28
0.78
[(Dec)4Por]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB
0.04
0.05
[(p-PentOPh)4Por]Eu[(15C5)4Pc]Eu[(15C5)4Pc]/LB
0.03
1–2  105
Tb2[(BuO)8Pc]3/DC

9320 | Chem. Soc. Rev., 2022, 51, 9262–9339

L and W [mm]4Source–drain Gate
electrodeelectrode Insulator

5.5  10 240 and 28 600 Au
102
25 103
10 and 2000
Au/Cr
12.5 105
10 and 2000
Au/Cr
28.9 103
5 and 20 000 Au
2
27.6 2  10
20 and 20 000 Au—
1.4  105 240 and 28 600 Au
—
—
—
—
—
9.3  10 240 and 28 600 Au—
8.2  103—
9.7  104—
3.9  10—1.01  102 240 and 28 600 Au—
1.02  103—
3.85  104—
1.15  102—
2.32  103 240 and 28 600 Au—
8.59  104—
7.91  105—
5.5  103 240 and 28 600 Au—
1.73  105
3.84 2.20  104 240 and 28 600 Au
4.02 4.94  103
1.19 1.48  104
4.34 1.03  104——
20 and 150
—

Si

SiO2

Ref.
433

n-Type Si SiO2/OTS
n-Type Si SiO2/OTS
Si
SiO2/OTS
Si
SiO2/OTS
Si
SiO2
Si
Si
Si
SiO2

434

Si

SiO2

438

Si

SiO2
439
SiO2/HMDS
SiO2/OTS
SiO2
SiO2/HMDS
SiO2/HMDS 440
SiO2/OTS
SiO2/HMDS
SiO2/OTS
SiO2
441

Si
Si

Si

435
436
437

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Table 2Chem Soc Rev

Performance of sandwich complexes based ambipolar-type OFET devices

ElectronHole mobility Ion/ mobilityIon/
2 1 1[cm V s ] Ioﬀ [cm2 V1 s1] Ioﬀ VT [V] L and W [mm]Source–drain
Gateelectrode electrode Insulator Ref.

——
240 and 28 600

Au
Au
Au

—
—
Si

—
—
SiO2

240 and 28 600
240 and 28 600
—

Au
Au
Au

Si
Au
Si

SiO2
PMMA
SiO2/
HMDS

[(PhS)8Pc]Eu(Pc)/QLS film[(PhS)8Pc]Ho(Pc)/QLS film(Pc)Eu[f-Pc2]Eu(Pc)OBu/QLS film

0.034
0.17
1.7  104 inair—
0.10 in air
104
4
1.1  10 in —air
4.9  107 inair
4.4  103
2.6  1030.8 in air—

240 and 28 600

Au

Si

221

(TPP)Eu[(PhO)8Pc]Eu[(PhO)8Pc]/QLS film

240 and 28 600

AuSi

1.1 + W = 28.6 mm, L = Au
13
240 mm, W/L =119—
240 and 28 600 Au

Si

SiO2/
HMDS
SiO2/
HMDS
SiO2/
HMDS

Si

SiO2/
HMDS

42

Au

Si

SiO2/
HMDS

40

Au

Si

SiO2/
HMDS

445

—
—

—
Au

—
Si

206
223

—

Au

Si

—
SiO2/
HMDS
SiO2/
HMDS

102 —
240 and 28 600— 20 + 240 and 28 600
38— 13 +
25
102– —
240 and 28 600
3
10103– —
104
102– —
103
103– —
104

Au
Au

Si
Si

SiO2
SiO2

447
448

Au

Si

SiO2/
HMDS

449

Compounds/deposition method
Y(Pc)2/DFT
La(Pc)2/DFT[(PhO)8Pc]Ho(Pc)/QLS film
Ho[(PhO)8Pc]2/QLS film
Eu[(NaphO)8Pc]2/QLS film
Eu[(SPh)8Pc]2/QLS film
Ho[Pc(SPh)8]2/QLS film

0.031
0.088
—

— —
— —
103 210.54 in N2
0.01 in N2
0.11 in N2

4

10 12
102
— —

3.0  103 in
N2
2.3 in N2—

0.04 in air

105 0.08 in N2

105

Eu2[(p-FPhO)8Pc]3/QLS film

0.24

—

0.042

—

[(PhO)8Pc]Eu[(PhO)8Pc]Eu(Pc)/QLS film

0.68

—

0.014

—

Eu2[(PhO)8Pc]3/QLS film
Eu2[(NaphO)8Pc]3/QLS film

0.041
2.5  106

— 0.0026
102 2.3  106

Solvent-vapor annealing 60 1C
Solvent-vapor annealing 80 1C

2.6  107
4.9  104102 2.3  108
103 7.1  105

Solvent-vapor annealing 100 1C
0.11
Solvent-vapor annealing 120 1C
1.1  102
RCCOOH)Por]/ 7.0  107[(PhO)8Pc]Eu[(PhO)8Pc]Eu[(CRQLS film
RCCOOH)Por]/ 0.11[(PhO)8Pc]Eu[(PhO)8Pc]Eu[(CRnanoribbon
5.6  103
CdEu2[(HexS)8Pc]4/QLS film
18(Pc)2Eu2[f-Pc2]Eu2(Pc)2SHex/single crystal
microsheets[(Py4Por)Eu[(PhO)8Pc]Eu[(PhO)8Pc]]/QLS film 6.9  107

105 0.06
104 9.6  103
102 7.5  107[(Py4Por)Eu[(PhO)8Pc]Eu[(PhO)8Pc]]/CdS
hybrid QLS film
(TClPP)Y(Pc) and fullerene C60 cocrystals(TFPP)Eu[(PhO)8Pc]Eu[(PhO)8Pc]/QLS film

106 4  104
10 1.2  103
103– 0.3
104
102 3.07  105
4

10

3.72
6.0  105102 2.22— 1.4  104

(TFPP)Eu[(PhO)8Pc]Eu[(PhO)8Pc]/QLS film/
CuPc VCD film:[(p-HOPh)4Por]Eu(Pc)Eu(Pc)/QLS film

0.16

—

[(p-HOPh)4Por]Eu(Pc)Eu(Pc)/rGO/QLS film

25.53[(p-HOPh)4Por]Eu[(PhO)8Pc]Eu[(PhO)8Pc]/
QLS film[(p-HOPh)4Por]Eu[(PhO)8Pc]Eu[(PhO)8Pc]/
rGO/QLS film

2.14  106

8

4.85  10

30.9

0.01

10 – 2.33  108
103
103– 0.77
104
102– 2.33  108
103
103– 39.6
104[(OctO)8Pc]M(Pc), M = Tb and Lu as the electroactive components, which showed hole mobilities of 6.4  104 and 1.7 
103 cm2 V1 s1, respectively, on a SiO2 dielectric layer with
top contacted Au source–drain electrodes.433 Chen et al.
designed amphiphilic tris(phthalocyaninato) rare earth complexes [(OctO)8Pc]M[(15C5)4Pc]M[(15C5)4Pc], M = Eu, Ho andLu, and fabricated LB film-based OFETs with SiO2/Si substrates
and top contacted Au source–drain electrodes.436 The devices
exhibited highly improved hole mobilities in the range of 0.24–

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104 3 +17
10 —3
10 – —
104
102 —
10

0.30
2

— —
102 8 + 240 and 28 600
10102 1 + 3
2
10 20 +18
105 3 + 9
104 4 + 5
102 8 + 8 240 and 28 600

3

0.09

3 +
12
—

432
442
38
443

444
44

446

—

0.60 cm2 V1 s1 with an on/off current ratio of 105, which was
ascribed to the intramolecular p–p stacking among Pc rings andthe J-aggregation in the LB films.437
In order to study the substituent eﬀect on the assemblingproperties and OFET performance of tris(phthalocyaninato)
triple-deckers [(CnH2n+1O)8Pc]Eu[(15C5)4Pc]Eu[(15C5)4Pc], n =4, 6, and 8, diﬀerent lengths of alkoxy chains were connected
to one of the outer phthalocyanine ligands.437 The results
indicated that the molecular packing and intermolecular

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interaction in LB films are closely related to the length ofhydrophobic side chains, thus leading to the change of holemobilities within 0.0032–0.60 cm2 V1 s1.
A similar trend was observed for another series of amphiphilic tris(phthalocyaninato) rare earth complexes [(CnH2n+1O)8Pc]Eu[(DEGOMe)8Pc]Eu[(DEGOMe)8Pc], DEGOMe = (OC2H4)2OCH3,
n = 6, 8, 10 and 12.438 In the OFET devices with bottom contacted
Au source–drain electrodes, the LB films of those triple-deckers also
displayed hole mobilities in the range of 0.014–0.46 cm2 V1 s1,
depending on the length of hydrophobic alkoxy chains.
The hydrophilic sides of tris(phthalocyaninato) rare earth
complexes were also modified with diﬀerent crown ether
substituents.439 Replacing the 15-crown-5 rings of [(OctO)8Pc]M[(15C5)4Pc]M[(15C5)4Pc] (Fig. 36a) with 12-crown-4 or 18crown-6 moieties resulted in diﬀerent packing modes in LB
films. Thus, 12-crown-4 and 15-crown5-substituted complexes
formed edge-to-edge connected J-aggregates, providing acharge transfer channel parallel to the Pc rings. In contrast,
18-crown-6-substituted complex formed face-to-face connected
H-aggregates in LB films, providing a channel along the longaxis of the assemblies (Fig. 36b and c). In addition, the authors
found that the surface treatment of the SiO2/Si substrate byhexamethyldisilazane (HMDS) or octadecyltrichlorosilane(OTS) can significantly improve the quality of the LB film, thus
leading to an increased carrier mobility.
Mixed-ligand (phthalocyaninato)(porphyrinato) Eu(III) tripledeckers (Por*)Eu[(15C5)4Pc]Eu[(15C5)4Pc] where Por* = tetrameso-decylporphyrinateortetra-meso-(p-pentyloxyphenyl)
porphyrinate were synthesized and tested in LB film-based
OFET devices.440 These triple-deckers adopt a ‘‘face-to-face’’connected ‘‘edge-on’’ orientation on HMDS- or OTS-treated

Review Article
SiO2/Si surfaces, leading to the eﬀective intramolecular p–p
stacking in films. As a result, the hole mobility of OFET devices
can reach 0.78 cm2 V1 s1, with an on/off ratio of 2.2  104 anda threshold voltage of 3.84 V.
In addition to LB films, drop-casted and spin-coated filmsof tetrapyrrolic sandwiches have also been applied for OFET
devices. By using spin-coated films of a liquid crystallinebis(phthalocyaninato) lutetium complex Lu[(a-Oct)8Pc]2, Ray
et al. fabricated bottom gated OFET devices, which showed a holemobility of 1.5  103 cm2 V1 s1 with an on/oﬀ current ratio of
103 at a threshold voltage of 25 V.434 After thermal annealing,
the hole mobility was improved to 8.0  103 cm2 V1 s1,
together with an increased on/oﬀ ratio by two orders of magnitude
and a reduced threshold voltage to half of that of the as-depositedfilms. Recently, through the study of spin-coated films of thegadolinium complex Gd[(a-Oct)8Pc]2,435 Ray et al. revealed that the
OFET performance decreases along with scaling down the channel length of devices. The electronic transport properties of dropcasted films of a triple-decker complex Tb2[(BuO)8Pc]3 werereported by Yamashita et al., which also show p-type mobilities
in the range of 1–2  105 cm2 V1 s1.441
4.2

Ambipolar OFETs

The ambipolar semiconducting properties of unsubstitutedbis(phthalocyaninato) rare earth complexes were initiallyreported by Simon and co-workers in 1990s.450 The highintrinsic conductivity of Lu(Pc)2 and Tm(Pc)2 was ascribed to
the narrow molecular energy gaps associated with their radical
nature.429 The ambipolar charge transfer properties of M(Pc)2,
M = Y or La were also studied by density functional theory (DFT)
calculations, which suggest that the delocalized radical can

Fig. 36 (a) Amphiphilic Eu(III) crown-phthalocyaninates [(OctO)Pc]Eu[(crown)4Pc]Eu[(crown)4Pc] and schematic arrangement of complexes[(OctO)8Pc]Eu[(12C4)4Pc]Eu[(12C4)4Pc] – (b) and [(OC8H17)8Pc]Eu[(18C6)4Pc]Eu[(18C6)4Pc] – (c) in the LB film-based OFET devices. Adapted withpermission from ref. 439. Copyright 2009, American Chemical Society.

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induce a high HOMO level and a low LUMO level, leading to asmall ionization potential and a large electronic aﬃnity. As aresult, small injection barriers for both hole and electron fromsource–drain electrodes (Au) can be expected for these neutral
double-deckers.432 The DFT study also revealed small charge
reorganization energy and large transfer integral values in the
crystal of M(Pc)2, thus predicted promising ambipolar chargetransfer properties. However, in experimental studies, singlecomponent bis(phthalocyaninato) rare earth complex-based
OFET devices usually show p-type transporting properties inair, the corresponding n-type operations only can be detectedunder an inert or reducing atmosphere.
By connecting electron withdrawing phenoxyl groups onto
one or both Pc rings of rare earth double-deckers, heteroleptic[(PhO)8Pc]Ho(Pc) and homoleptic Ho[(PhO)8Pc]2 wereobtained, which exhibit p- and n-type transporting properties,
respectively, in the quasi-Langmuir–Shäfer (QLS) film-based
OFET devices.442 Introducing other electron withdrawing
groups, such as naphthoxyl38 and phenylthiol,443 to the Pc ringsof rare earth double-deckers can lead to transforming theirsemiconducting nature from p-type to ambipolar type by tuning
their HOMO/LUMO energy levels.37 A dimeric phthalocyanine
ligand was incorporated into rare earth sandwiches, leading to
the formation of a yttrium(III) double-decker dimer (Pc)Y[fPc2]Y(Pc)OBu (cf. Section 2.6.2),216 which exhibits a unique biradical state and can be fabricated into high quality QLS films, thus
displaying very high carrier mobilities of 2.3 and 0.8 cm2 V1 s1for electrons and holes in air and N2, respectively (Table 2).221
Molecular orbital (MO) levels of sandwiches can be adjusted
by the addition of another tetrapyrrolic ring to double-deckers.
In this way, a mixed (phthalocyaninato)(porphyrinato) europium(III)triple-decker complex (TPP)Eu[(OPh)8Pc]Eu[(OPh)8Pc] was preparedand fabricated into QLS film-based OFET devices, which displayedbalanced hole and electron mobilities in air and N2, respectively.444
Tetrapyrrolic sandwich-based air-stable ambipolar OFET
devices were first achieved by using a partially fluorinatedtris(phthalocyaninato) europium complex Eu2[(p-FPhO)8Pc]3.44
The introduction of electron-withdrawing 4-fluoro-phenoxy
groups on the peripheral sites resulted in finely controlled
HOMO (5.27 eV) and LUMO (4.17 eV) levels; both facilitatecharge carrier injection from Au electrodes. In particular, the
LUMO level is suitable for electron transport under ambientconditions. The triple-decker molecules also form H-aggregates
and adopt an ‘‘edge-on’’ conformation in the QLS films,
which can provide intense intermolecular interaction andp-electron delocalization. As a result, the QLS film-based
single-component OFET devices display an average hole mobility of 0.24 cm2 V1 s1 and an average electron mobility of
0.042 cm2 V1 s1 in air, with relatively low threshold voltagesof 1.1 and +13 V for holes and electrons, respectively. Bytaking a similar optimizing strategy, tris(phthalocyaninato)
europium complexes [(PhO)8Pc]Eu[(PhO)8Pc]Eu(Pc), Eu2[(PhO)8Pc]3
and Eu2[(NaphO)8Pc]3 were obtained. The HOMO and LUMO levelsof these triple-deckers were also successfully tuned into the range
for air-stable ambipolar semiconductors by the introduction ofelectron-withdrawing phenoxy42 or naphthoxy subsititutes.40

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Chem Soc Rev
Triple-decker complex Eu2[(NaphO)8Pc]3 was used to fabricate ambipolar OFET device by QLS technique, and nanoparticles of the pristine QLS film could be converted into
well-defined microstructures using solvent vapour annealing
with o-dichlorobenzene (Fig. 37).40 Fragment-like irregularpolygons, quasi-four-corner sheets, 2D squares, and clovers
were obtained at diﬀerent temperatures (60, 80, 100 and
120 1C, respectively), providing access to the tuning of OFET
device performance due the carrier mobilities depending on
morphologies and crystallinities. The best result was achieved
for the most crystalline 2D squares-based devices with thecarrier mobilities of 0.11  0.03 cm2 V1 s1 for holes and0.06  0.02 cm2 V1 s1 for electrons.
A further modification was performed by replacing one ofthe outer Pc ligands with an amphiphilic low-symmetry porphyrin ring (Fig. 38a).445 The resulting triple-decker can form 1D nanoribbons via a ‘‘phase-transfer’’ process (Fig. 38b), whichshow improved air-stable ambipolar semiconducting properties
with mobilities of 0.11 and 4  104 cm2 V1 s1 for holes andelectrons, respectively, 3 to 6 orders higher than those of noncrystalline QLS film (Fig. 38c).
Further extension of sandwiches was beneficial for their
OFET behaviour. Thus a quadruple-decker complex
Eu2Cd[(HexS)8Pc]4 exhibited air-stable ambipolar semiconducting characters due to the extended p-system and narrowed
bandgap206 The best result for phthalocyanine-based OFETs wasachieved with a dimeric phthalocyanine ligand involved sandwichtype triple-decker complex (Pc)2Eu2[f-Pc2]Eu2(Pc)2SHex.223 This special sandwich can be fabricated into single crystal microsheets,
which display high hole and electron mobilities of 18 and0.3 cm2 V1 s1, respectively, with on/oﬀ ratios of 103–104.
Another approach for ambipolar OFETs is the fabrication of
bicomponent films and/or cocrystals with triple-decker complexes as one of the components. For example, bicomponent
thin film transistors of (TPyP)Eu[(OPh)8Pc]Eu[(OPh)8Pc]/CdS

Fig. 37 The carrier mobilities for holes and electrons in pristine QLS filmof Eu2[(NaphO)8Pc]3 and microstructures obtained by solvent vapour
annealing at diﬀerent temperatures together with SEM images of thecorresponding films. Adapted from ref. 40, Copyright (2018) with permission Elsevier.

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4.3 Supramolecular assembling of semiconductive materials
based on sandwich crown-substituted complexes

Fig. 38 (a) Amphiphilic mixed-ligand Eu(III) complex and SEM images offilms showing crystallinity of films formed by the ‘‘phase transfer’’
process – (b) and non-crystalline films obtained by the quasi-Langmuir–Shäfer process – (c) Adapted with permission from ref. 445. Copyright
2016, American Chemical Society.

show highly improved charge mobilities for holes (0.09 cm2 V1 s1)and electrons (0.01 cm 2 V1 s 1) relative to those of
single-component thin film transistors of (TPyP)Eu[(PhO)8Pc]Eu[(PhO)8Pc] for holes (6.9  107 cm2 V1 s1)and electrons (3.07  105 cm2 V1 s1).446 OFET devices basedon the cocrystals of (TClPP)Y(Pc)Y(Pc) and C60 also display very
high mobilities of 3.72 and 2.22 cm2 V1 s1 for holes andelectrons, respectively, which is proposed to be due to similar
LUMO energy levels of the two components and the low
reorganization energy values for holes and electrons at theinterface.447 On the other hand, ambipolar OFETs have beenachieved with a bilayer heterojunction configuration by using
triple-decker complexes as air-stable ambipolar semiconductors. The QLS films of (TFPP)Eu[(PhO)8Pc]Eu[(PhO)8Pc] wereobtained on vacuum deposited (VCD) CuPc templates, andfabricated into two-component bilayer thin film transistors.448
The devices possess charge mobilities of 0.16 and0.30 cm2 V1 s1 for holes and electrons, respectively, whichare 103–105 times of those of the single-component based
devices. The improvement of the performance can be attributed
to the heterojunction effect and the H-aggregation of tripledeckers on CuPc films. Bilayer heterojunction FETs were alsofabricated by QLS films of [TP(OH)P]Eu(Pc)Eu(Pc) and[TP(OH)P]Eu[(PhO)8Pc]Eu[(PhO)8Pc], as the top layers, andreduced graphene oxide (rGO) film as the sublayers. The
obtained devices exhibit very high and balanced carrier mobilities of 30.9 and 39.6 cm2 V1 s1 for holes and electrons,
respectively, again due to the heterojunction effect.449

9324 | Chem. Soc. Rev., 2022, 51, 9262–9339

The relationship between morphology and conductivity oflayers formed by tetrapyrrole sandwiches in electronic devices
makes it possible to apply supramolecular tools to control thesefactors. For this purpose, molecular blocks capable of exhibiting various types of intermolecular interactions, such as hydrophobic and stacking interactions, as well as hydrogen bonds,
were synthesized and studied. However, the assembly of suchmolecules is a trial-and-error method, since the morphology
and conductivity of the film cannot be predicted a priori. To
some extent, this problem can be solved by applying cationinduced assembly of crown-substituted phthalocyanines, where
well-established cooperative eﬀects govern the formation ofwell-defined supramolecular architectures (cf. Section 3.3.1).
Thus, Zvyagina et al. used homoleptic crown-substitutedbisphthalocyaninates M[(15C5)4Pc]2, M = Ce, Tb, and Lu as
potential molecular blocks for cation-induced formation of 1D
supramolecular polymers {M[(15C5)4Pc]24+}n where enhanced
conductivity was expected (Fig. 39a).451 Noteworthy, this possibility was envisaged in 1994 by Toupance et al. in one of keystone
papers on the construction of ionoelectronic materials based oncrown-substituted phthalocyanines; however, experimental verification of this hypothesis was reported only in 2021.
Indeed, interaction of Lu(III) complex with KBPh4 in chloroform–acetonitrile mixture resulted in the intermolecular binding of potassium cations which connect neighbouring doubledeckers in a cofacial manner, yielding nanowires with a thicknessof 10–50 nm and a length of up to 50 mm. The four-probe

Fig. 39 (a) Modes of interaction of M[(15C5)4Pc]2 with potassium
cations – intramolecular for M = Tb and Ce vs. intermolecular for M = Lu.
(b) The currentvoltage characteristic of the drop-cast films of the assemblies formed by M[(15C5)4Pc]2 and KBPh4. Inset shows SEM image of
nanowires formed by Lu(III) complex. (c) Schematic illustration of the mechanism of alignment of the {Lu[(15C5)4Pc]24KBPh4}n nanowires. Adapted withpermission from ref. 451. Copyright 2021, American Chemical Society.

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Fig. 40 Formation of a bilayer composed of H2[(15C5)4Pc] molecules appended to octopus-Pc SAM on gold via supramolecular assembly. Adaptedwith permission from ref. 73. Copyright 2022, Wiley-VCH.

technique in a four-stripe layout revealed the superior conductivity of these wires up to 11.4 S cm1, the highest among them
being reported for phthalocyanine assemblies (Fig. 39b). The
electric properties of these 1D assemblies made it possible touse technologically feasible methods for their controlled alignment on surfaces under an applied electric field (Fig. 39c).
In striking contrast, cerium- and terbium-based complexes
underwent mainly intramolecular cation binding without theformation of conductive nanowires (Fig. 39a and b). Apparently,
this is associated with a larger interligand distance in thesecomplexes allowing potassium cations to intercalate between
crown-substituted ligands.452 The films, formed by the resulting assemblies had 8 orders of magnitude lower conductivity
due to the lack of intermolecular interactions.
The way to controllable immobilization of supramolecularassemblies onto the electroactive surfaces was paved by Shokurov et al., who used the octopus-like double-decker anchor,
octopus-Pc (Scheme 4) which can form self-assembled monolayer (SAM) on the surface of Au electrodes (Fig. 40).73 Face-on
orientation of the anchors allowed for the subsequent bindingof crown-phthalocyanine molecules H2[(15C5)4Pc] via potassiumion bridges. This chemistry was utilized to form a heterogeneous
bilayer, in which a single molecule thick adlayer brought anadditional redox-state to the system, thus expanding the multistability of the system as a whole. All four redox states available tothis system exhibited characteristic absorbance in the visible
range, allowing for the switching to be easy read out using opticaldensity measurements. It is expected that the proposed approach
can be used in a wide range of switchible materials – singlemolecule magnets, conductive and optical devices, etc.

5. Non-linear optical materials basedon REE sandwich complexes
Metalloporphyrins (MPor) and metallophthalocyanines (MPcs) are
important organic nonlinear optical (NLO) materials due to their
two-dimensional p-system.453,454 Among practically relevantThis journal is © The Royal Society of Chemistry 2022

properties optical limiting (OL) is particularly interesting as it
can be used to protect eyes from damage caused by laser
irradiation.
Upon the formation of sandwich complexes with rare earth
metals, the p-conjugation systems can be further extended
along the axial direction of tetrapyrrol plane, leading to threedimensionally conjugated molecules, which is desirable for
improving the NLO response by regulating the intramolecularcharge transfer transitions and/or constructing dipolar andmultipolar systems.3
5.1

Second-order nonlinear optical materials

For materials having second-order NLO responses at themolecular level, noncentrosymmetric molecules with strong
intramolecular dipolar charge distribution are required. Heteroleptic double-decker tetrapyrrolic complexes generally meetthis requirement. However, reports in this field are still very
rare. The sole example of heteroleptic double-decker complexes
studied for the second-order NLO properties is a mixed porphyrin and tetraazaporphyrin zirconium(IV) complex (OEP)Zr(OETAP).455 A negative hyperpolarizability b [(83  43) 
1030 esu] was obtained at 1907 nm by the electric fieldinduced second harmonic (EFISH) measurements.
An interesting trial for second-order NLO materials has been
extended to homoleptic double-decker complexes with ‘‘cubic’’octupolar architectures (cf. Section 2.2.2 for structures of complexes). In this regard, Ayhan et al. developed ABAB homolepticbis(phthalocyaninato) rare earth complexes M[(SHex)4Pc]2, M =Nd, Eu, Dy, Y and Lu,109,110 which represent a new class of cubic
molecules with an octupolar charge distribution. Their secondorder NLO properties were studied by Hyper–Rayleigh Scattering (HRS) measurements at 1907 nm in chloroform solution.
The first molecular hyperpolarizabilities (bHLS) were evaluatedto be in the range of 3010  1030 esu to 5760  1030 esu forY(III) and Lu(III) complexes, respectively, which are among thelargest values ever reported for nondipolar molecules. Furthermore, it was found that the bHLS values of these complexes are
depended on the number of 4f electrons and ionic radii of the
sandwiched lanthanide(III) ions, thus providing opportunity toChem. Soc. Rev., 2022, 51, 9262–9339 | 9325

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regulate the second-order NLO responses. Another ABAB homoleptic bis(phthalocyaninato) rare earth complex Lu[(aPentO)4Cl4Pc]2, containing alternating pentyloxy and chloro
substituents on phthalocyanine ligands, was reported by Cao
et al.,111 which also showed a high molecular hyperpolarizability (bHLS = 1240  1030 esu at 1907 nm, in chloroform).
By applying a porphyrin ligand with di(phenylamino)phenyl
and carboxymethyl–phenylethynyl meso-substituents as typicalelectron-donating and electron-accepting groups, respectively,
Sun et al. synthesized the first octupolar bis(porphyrinato) rareearth complex [HTb(DADAPor)2] (Scheme 15).113 Because of the
reduced protonated nature, this compound does not have any
absorption in the near-IR range beyond 800 nm. Consequently,
an off-resonant hyperpolarizability (bHLS = 1700  1030 esu at1907 nm, in chloroform) was obtained for this compound,which is the largest off-resonant hyperpolarizability for octupolar molecular materials.
Density functional theory (DFT) studies were also conducted
to clarify the nature of the second-order NLO properties ofoctupolar homoleptic bis(phthalocyaninato) rare earth
complexes.456 A series of cubic molecules based on thebis(phthalocyaninato) yttrium Y(Pc)2 skeleton, containing two
couples of electron-donating and electron-withdrawing groups
in an alternate manner (Fig. 41a), were investigated. According
to the calculation results: (1) the static hyperpolarizability(bHLS, 103–104 a.u.) increases along with increasing electronegativity of the peripheral substituents; (2) a dynamic perturbation (excitation) will induce a resonance effect with a muchhigher dynamic bHLS (104–106 a.u.), which is also positively
correlated with the electronegativity of peripheral substituents;(3) the molecular conformation affects the second-order NLO
properties. The maximal static bHLS appears at a rotation angle
y of 701, but not 901, because Y(Pc)2 molecule employs a cuboid
instead of a standard cube (Fig. 41b and c). These findings are

Review Article
useful for the design of new octupolar NLO materials, giventhat the theoretically predicted maximal bHLS values can reach
105  1030 esu for such octupolar molecules.
5.2

Third-order nonlinear optical materials

The tetrapyrrol macrocycles are very suitable for the third-order
NLO materials due to three special advantages, including (1)the large number of free p electrons playing the role ofoscillators driven by an external periodically vibrating electromagnetic field; (2) the extended delocalized p-system acting asa broad oscillating space; and (3) the huge polarizing/hyperpolarizing moments due to the long distance between the peripheral electron-donating and withdrawing substituents. The
double-decker and multi-decker tetrapyrrols further improves
the total three advantages in comparison with the singledecker ones.
The study of third-order NLO materials based on doubledecker phthalocyanines started from 1990s. Shirk and coworkers investigated the third-order NLO performance of
double-deckers M(Pc)2 (M = Sc, Y, Eu, Gd, Lu) using thedegenerate four-wave mixing (DFWM) method.457 The secondorder hyper-polarizabilities (g) and third-order susceptibility
w(3) values of M(Pc)2 are about an order of magnitude higher
than those of typical monomeric metallophthalocyanines.
Among these double-deckers, the third-order NLO performanceof Sc(Pc)2, Lu(Pc)2 and Yb(Pc)2 are better than others. This
therefore suggests that the large increase in the hyperpolarizabilities of bis(phthalocyaninato) complexes over those of themonophthalocyaninato compounds could not be simply attributed to the extended delocalization length from monomers todouble-deckers.
In the past 20 years, the Z-scan method was widely used inmost third-order NLO experiments. Sheng and coworkers studied a series of homoleptic bis(phthalocyaninato) rare earth

Fig. 41 (a) Molecular structures of the series of bis(phthalocyaninato) yttrium double-decker skeletons used for DFT modelling. (b) Plot of the calculated
static bHRS as a function of the rotation angle y for the complex Y(Pc4)2. (c) Cuboid structure of the bis(phthalocyaninato) yttrium double-decker skeleton
with a rotation angle of 901. Adapted with permission from ref. 456. Copyright 2015, Wiley-VCH.

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complexes M[(OctO)8Pc]2, M = Pr, Ce, Sm, and Lu.458–460 The
NLO performance of these compounds are tested using theexciting laser wavelength of 800 nm (Z-scan, 170 ns). The twophoton absorption cross-section s2PA values of this series are in
the range of (3–254)  103 GM, showing their strong nonlinear
optical response. More importantly, the authors measured thereal part of the second-order molecular hyperpolarizability (gR)of the neutral complex Pr[(OctO)8Pc]20 together with its anion
Pr[(OctO)8Pc]2 and cation Pr[(OctO)8Pc]2+. They found that thegR values of neutral, anionic and cationic forms are 440 
1032, 180  1032, and 220  1032 esu, respectively.460 This isa key experimental phenomenon which shows the free single
electron in the neutral radical molecule can be used as a good
oscillator in the NLO response. When this free single electron ispaired in the anionic form or removed in the cationic form, the
NLO response will be significantly weakened. Following asimilar mentality, Plekhanov et al. examined another complex
Lu[(SHex)8Pc]2.461 Using a similar Z-scan test (300 ns) at an
exciting laser wavelength of 1550 nm, the complex achieved aremarkable nonlinear refractive index (n2) value of (300  3) 
105 cm2 GW1. They found that the n2 for unsubstitutedcomplex Lu(Pc)2 is only (228  3)  105 cm2 GW1, which isa quarter lower than that of Lu[(SHex)8Pc]2. This comparative
investigation revealed that the peripheral electron-withdrawing
substituents are beneficial to the NLO response because they
could provide more free electrons to the p conjugated system.
Karpo et al. studied the NLO properties of a series of butylsubstituted complexes M(Bu8Pc)2, M = Eu, Dy, Er, and Lu in
THF solution using the open aperture z-scan technique with
350 ps laser pulses at 532 nm.462 All the compounds exhibitedreverse saturable absorption (RSA), and application or rateequations formalism evidenced that RSA is mainly contributedby the triplet state absorption. Absorption cross-sections fromthe ground state (s0), excited singlet and triplet (s1 and s2) werededuced, evidencing that s2 c s0 4 s1, the largest ratio s2/s0
was found for erbium complex.
Sekhosana et al. conducted a series of comparative
studies on the NLO properties of double-decker phthalocyanines with electron-withdrawing pyridin-4-yloxy substituents(PyO).47,463–466 They found that the imaginary part of thethird-order susceptibility w(3)value is 106  1012 esu forI12
Yb[(a-OPy)4Pc]2 and 338  10
esu for Yb[(b-OPy)4Pc]2 (Y20)
under the Z-scan test (10 ns, in DMSO) at the wavelength of
532 nm. When the –OR substituents are linked onto themacrocycle a-sites, there will be a p–p-super-conjugation
between the pz(O) and p(Pc) orbitals. When the –OR substituents are connected on the b-sites instead of the a-sites, theoscillation space of p electrons will be more effectively
extended, which is positive to the periodic oscillation in the
nonlinear forced vibration.
Another interesting discovery is that the w(3)value for theIneutral complex Nd[(tBuPhO)4Pc]0 is 377  1012 esu, but it
gains spectacular increase to 1280  1012 esu for the ioniccomplex Nd[(tBuPhO)4Pc]Nd(OAc)2+.47 Although the lattercomplex does not have unpaired p-electrons, a huge positive/
negative ion-pair is constructed by the bisphthalocyaninate

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Chem Soc Rev
anion and its counter-ion, rendering it as a (d+)–(d) pair witha very strong dipole moment, which shows an improved NLO
property.
Throughout the investigations on various RE(Pc)2 molecules, it is easy to find out two rules to get the double-decker
tetrapyrrols with high NLO performance: (1) the neutral radical
M(Pc)20 is a good NLO material due to the additional free
single p electron. When this radical complex is oxidized/
reduced and then this free single p electron is removed/paired,it lowers the overall NLO response. However, it still can be
improved by ion pair formation.47 (2) The peripheral electrondonating substituents, especially the –OR groups, are beneficial
to NLO response because they could provide more free electrons to the p conjugated system. If the –OR groups are linking
at the b-sites, they will provide an extended oscillating space,
and thus an improved NLO response. In comparison with these
positions, the efficiency of substitution at a-sites is not obvious.
Sheng and coworkers also tested a series of heteroleptic
double-deckers [(OctO)8Pc]M(Pc), M = Y, Ho, Er, Yb, and foundthat their s2PA is in the range of (126–183)  103 GM,467 whichis still at the same level of M[(OctO)8Pc]2, M = Pr, Ce, Sm, andLu series.458–460
Jiang and coworkers explored binuclear phthalocyaninecontaining sandwich-type rare-earth complexes.468 They
revealed that the w(3)value of (Pc)M[f-Pc2]M(Pc)SHex (18.6 I1210esu) is higher than that of Pr[(SHex)8Pc]2 (15.6  1012 esu),
due to the enlarged p-conjugation by incorporating a binuclear
phthalocyanine ligand.
Chen et al. conducted another comparative study on a seriesof porphyrin-appended europium(III) bis(phthalocyaninato)
complexes (Fig. 42a).469 The w(3)I values for [(a-ZnPor)Pc]Eu(Pc),[(b-ZnPor)Pc]Eu(Pc) and [(b-ZnPor)2Pc]Eu(Pc) are 111  1022,74  1022, and 133  1022 m2 V2 which are in cases smaller22
than w(3)m2 V2).
I for the unsubstituted Eu(Pc)2 complex (194  10
This result shows that the using of larger ligands is not always
beneficial to NLO properties. In the porphyrin-appended Pc
ligands, there is no unified conjugating system between the Por
and Pc macrocycles, which cannot provide a channel for electron
oscillation. So the eﬀective electron oscillating space is still
located at the Eu(Pc)2 part.
In order to improve the third-order NLO responses of rareearth based tetrapyrrolic sandwiches, multi-deckers were constructed to extend the delocalized p-system and increase theoscillating space. However, the w(3)I value of a homoleptic tripledecker Nd2[(b-tBuPhO)4Pc]3 is found to be 350  1012 esu,470
which is slightly lower than that of the corresponding double12decker Yb[(b-tBuPhO)4Pc]2 (w(3)esu) in the sameI = 420  10
1
Z-scan test (10 ns, 0.3 mmol L in DMF).465 Similar results
were also obtained in the comparison of mixed (phthalocyaninato)(porphyrinato) europium triple- and double-decker complexes, in which gR for [(Pyr)4Por]Eu(Pc)Eu(Pc) is also lower
than gR for [(Pyr)4Por]Eu(Pc) (Fig. 42b and c).31
However, Oluwole et al. showed that symmetricalEu2[(BuO)8Pc]3 has a higher w(3)value in comparison with
I
Eu[(BuO)8Pc]2.101 NLO response and OL properties can be
improved by lowering the molecular symmetry via theChem. Soc. Rev., 2022, 51, 9262–9339 | 9327

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Fig. 42 Examples of p-extended sandwich complexes used for NLO studies: (a) Eu(III) bisphthalocyaninates with appended porphyrin units;469 (b and c)double- and triple-decker complexes with meso-pyrenyl-substituted porphyrin ligands.31

introduction of diethyleneglycol substituents (Scheme 11),which in turn can be used to graft complexes to quantum dot
surfaces gaining even more spectacular improvement of OL
characteristics.
The third-order NLO properties of multi-decker sandwiches
with more than a three layered structure were also examined byusing Z-scan technique.48,209,471–473 Along with expanding thep–p interaction system in the order of triple-, quadruple-,quintuple-, and sextuple-deckers, as expected, the NLO
responses are gradually improved due to the extended oscillat12
ing space. Remarkable w(3)and
I and gR values of 4910  10
30
12 700  10
esu were recorded for a sextuple-decker
Nd3Cd2[(tBuPhO)4Pc]6 in the Z-scan test (10 ns, 532 nm,
3.0  104 M in CH2Cl2),48 suggesting the great potential for
application.

While solution measurements provide fundamental NLO
characteristics of synthesized complexes, their practical application requires immobilization of molecules on surfaces orincorporation into polymeric matrices for coating fabrication.
The latter approach was used by Liu et al. to form opticallimiters based on heteroleptic sextuple-decker complex(Pc)Sm(Pc)Cd(Pc*)Cd(Pc*)Cd(Pc)Sm(Pc), where Pc* = (BuO)8Pc.474
Due to RSA mechanism this complex showed excellent OL
characteristics in chloroform solution (Fig. 43a), which
inspired the authors to probe its practical application. Thus,
this complex was dispersed in gel glass formed by methyltriethoxysilane polycondensation (Fig. 43b), aﬀording materials
capable of eﬃcient optical limiting – while linear transmittanceof a glass sample containing 1 mass% of complex was 84%(Fig. 43c), the minimum normalized transmittances decreased
to 10% (Fig. 43d).

6. Conclusions

Fig. 43 (a) NLO properties of (Pc)Sm(Pc)Cd(Pc*)Cd(Pc*)Cd(Pc)Sm(Pc) in
CHCl3 at a concentration of 2.0  105 mol L1 at diﬀerent incident
intensities. (b) Photographs of the gel glass of the complex with diﬀerent
concentrations, G1–G5 where the number corresponds to weight percentage of the complex. (c) Transmittance spectra of glasses G1–G5. (d)NLO properties of gel glass G1 at different incident intensities. Reproduced
from ref. 474 with permission from the Royal Society of Chemistry.

9328 | Chem. Soc. Rev., 2022, 51, 9262–9339

The data presented in this review showed that tetrapyrrole
sandwich complexes based on rare earth elements are a uniquemolecular platform for targeted design of functional materials.
Over the last 20 years, the field of REE sandwich complexes
has made great progress in both basic and applied research.
The variety of architectures and properties of sandwiches
presented in this review shows fantastic opportunities for theirapplication in advanced technologies, particularly SMM-based
quantum information storage and processing, where spins areused as information carriages and quantum eﬀects play animportant role.
We can expect that the prospects for the use of REE tetrapyrrolic sandwiches will greatly expand in the coming years
both with the further development of their synthetic chemistry
and further elaboration of approaches to hybrid materials andthin film-based devices. The latter aspect is particularly important in terms of application, as the requirements in small
amounts of complexes needed for the fabrication of suchdevices partially alleviate the need in large scale synthesis.

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Thus, despite the rather large number of papers alreadypublished and reviewed, this area of research still has many
avenues for further development, so the authors are confident
that this review will encourage scientists from various fields ofchemistry and materials science to tackle this exciting andpromising topic.

(2202028). J. Jiang thanks the Financial support from the
National Natural Science Foundation of China (22235001,
22175020), M. Yamashita thanks the JSPS KAKENHI Grant
Number JP19H05631, JP20225003, CREST (JPMJCR12L3) from
JST, the National Natural Science Foundation of China (NSFC,
22150710513), and the 111 project (B18030) from China.

List of abbreviations
3nCn, n = 4, 5, 6
acac
ClN
c-Pc2
DClB
dpm
DZPc
f-Pc2
Hp
LF
Ln
Menth
Naph
Nc
NLO
OFET
P or Por
Pc
Pz
QT
QTM
REE
SB
SMM
SOC
SWNT
TAnP
TBP
TBTAP
TClB
TClPP
TM
TTP

3n-crown-n-ether
Acetylacetonate
Chloronaphthalene
Clamshell-bisphthalocyanine
Dichlorobenzene
Dipivaloylmethanate
Diazepinoporphyrazine
Fused bisphthalocyanine
Hemiporphyrazine
Ligand-field
Lanthanide
MenthylNaphthyl
Naphthalocyanine
Nonlinear optics
Organic field-eﬀect transistor
Porphyrin
Phthalocyanine
Porphyrazine
Quantum technology
Quantum tunneling of magnetization
Rare earth element
Schiﬀ baseSingle-molecule magnet
Spin–orbit coupling
Single-wall nanotube
Tetra-p-anisylpoprhyrin
Tetrabenzoporphyrin
Tetrabenzotriazporphyrin
1,2,4-Trichlorobenzene
Tetra-p-chlorophenylpoprhyrin
Transition metal
Tetra-p-tolylporphyrin

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
A. G. Martynov (Chapter 2) thanks Russian Science Foundation(18-73-10174-P). Y. Horii thanks the JSPS KAKENHI Grant
Number JP21K14645. K. Katoh thanks the JSPS KAKENHI Grant
Number JP24750119, JP15K05467. Y. Bian thanks the Financial
support from the National Natural Science Foundation ofChina (22271012) and Beijing Natural Science Foundation

This journal is © The Royal Society of Chemistry 2022

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