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8631

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Germanium- and tin-bridged diazulenylmethyl
cations: eﬀects of group 14 elements on the
structure and properties of π-extended cations†
Masahito Murai,

Received 18th May 2023,
Accepted 20th May 2023

*a Mako Ito,a Satoshi Takahashia and Shigehiro Yamaguchi*a,b

Diazulenylmethyl cations bridged with a germanium and tin moiety were synthesized. In these cations,

DOI: 10.1039/d3dt01491f

the nature of these elements has impacts on the chemical stability and photophysical properties. Upon
aggregation, these cations exhibit absorption bands in the near-infrared region, which are slightly blue-

rsc.li/dalton

shifted compared to those of silicon-bridged congeners.

Cationic π-conjugated compounds have attracted considerable
attention due to their wide range of applications1 including
conducting materials,1c,d dyes for photothermal conversion
and bioimaging,1g and sensitizers for photoreactions.1e Whilevarious types of cationic π-systems have been developed todate, their properties largely depend on the nature of theπ-skeleton employed and fine-tuning of suitable counter
anions. A key issue in the design of novel classes of cationicπ-skeletons is the need to attain eﬀective charge delocalizationthat can furnish high chemical stability and unusual electronic
structures.2 Another important matter for imparting
π-skeletons with valuable functionality is controlling the molecular arrangement in the aggregated and solid state.3
In this context, we have recently disclosed silicon-bridged
diazulenylmethyl cations 1 as a new class of stable carbocations (Fig. 1),4 where the positive charge is eﬀectively delocalized over the π-skeleton, reflecting the highly polarizable character of azulenyl moieties.5,6 Specifically, the connection of
two azulenyl groups at the 1-position to the carbocation
center, while reinforcing the skeleton in a planar fashion witha Si-bridge, is crucial for the high chemical stability of thesystem. This stabilization mode is diﬀerent from that found inconventional cationic π-systems, where nitrogen- and oxygenbased electron-donating groups are usually used.2 The Si-a
Department of Chemistry, Graduate School of Science and Integrated ResearchConsortium on Chemical Science (IRCCS), Nagoya University, Furo, Chikusa, Nagoya
464-8602, Japan. E-mail: masahito.murai@chem.nagoya-u.ac.jp,
yamaguchi@chem.nagoya-u.ac.jpbInstitute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo,
Chikusa, Nagoya 464-8601, Japan† Electronic supplementary information (ESI) available: Experimental and computational details, characterization data, photophysical and spectral data, as wellas crystallographic data for 2PF6. CCDC 2252079. For ESI and crystallographic
data in CIF or other electronic format see DOI: https://doi.org/10.1039/
d3dt01491f

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bridge also aﬀects the molecular arrangement in the condensed state.7 The observed oﬀset π-stacked arrangement isdue to the steric congestion of the substituents on the siliconbridge and the positioning of the counter anions.
Consequently, J-type aggregates3b with sharp absorption andemission bands in the near-infrared (NIR) region were formed.8
Such NIR chromophores have a promising variety of prospective
applications in fields, such as aggregation-induced staining forbioimaging.1g,8b,d Hence, with this prominent π-skeleton in
hand, further diversification of its properties became our next
subject. A promising approach to achieving this objective would
be via the introduction of other main group elements at thebridging position.9 Herein, we have introduced the heavier
group 14 elements, Ge and Sn,10,11 into diazulenylmethyl
cations. Relative to silicon,10 these heavier elements are characterized by larger atomic radii, lower electronegativity, less
eﬀective orbital interactions with the π-conjugated skeleton, andheavy-atom eﬀects. We synthesized Ge- and Sn-bridged diazulenylmethyl cations 2 and 3 (Fig. 1), and investigated the eﬀects ofthese heavier elements on the stability, structures, and properties in solution as well as in the aggregated and solid states.
Following the synthetic method reported for the Si-bridged
diazulenylmethyl cation 1,4 Ge- or Sn-bridged 2PF6 and 3PF6
were synthesized in three steps from 2-iodoazulene(Scheme 1). The key step is the cyclization of diazulenyl-substituted precursors 4 and 5 under Vilsmeier formylation con-Fig. 1 Design and structures of diazulenylmethyl cations 1–3 withgroup 14 elements as bridging moieties.

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Scheme 1

Dalton Transactions

Synthesis of 2 and 3.

ditions,4 to produce 2Cl and 3Cl, respectively. While the reaction of 4 at 60 °C selectively aﬀorded 2Cl, similar to the synthesis of the silicon congener 1Cl, a lower reaction temperature
was required for the cyclization of 5 in order to prevent the formation of unidentified products that are likely formed via cleavage of the C–Sn bond. Subsequently, 2Cl and 3Cl were treatedwith KPF6 to aﬀord 2PF6 and 3PF6, which were isolated as deep
blue solids.
Whereas 2 and 3 are formally classified as secondary carbocations, they are stable toward air and moisture and they canbe stored in air without any precautions irrespective of thecounter anions. The chemical stability of 2PF6 and 3PF6 wasassessed by determining their pKR+ values, which represent
their aﬃnity toward a hydroxide ion.12 The pKR+ values were
estimated to be 10.5 and 11.5 for 2PF6 and 3PF6, respectively(Fig. S1†). These values are higher than those of Si-bridged
1PF6 ( pKR+ = 9.7)4 and trioxatriangulenium hexafluorophosphate ([TOTA][PF6]: pKR+ = 9.3),12 the latter of which is a
representative stable trityl cation (the structure is shown in
Fig. S1†). This comparison demonstrates the importance ofthe nature of heavier elements for chemical stability.
Single crystals of 2PF6 suitable for X-ray diﬀraction analysis
were obtained by the slow evaporation of a CH2Cl2/hexane
solution at room temperature (Fig. 2a). In the crystalline state,

Fig. 2 (a) The molecular structure of 2PF6 with thermal ellipsoids at
50% probability and (b) molecular arrangement of 2PF6 in the crystal
state. Blue: Ge, orange: P, yellow: F, white: H, gray: C. While the unit cellof 2PF6 contains two crystallographically independent molecules, only
one of them is shown in (a). The relevant distances for the F⋯H–C
hydrogen bonds and the representative C⋯C distance between the two
adjacent seven-membered rings are shown.

8632 | Dalton Trans., 2023, 52, 8631–8635

2PF6 exhibits a curved π-conjugated skeleton, in which the PF6−
anion sits in the bay area with hydrogen bonds between the
fluorine and the hydrogen atoms of the two azulene andmethene moieties. Similar to the structure of 1PF6,4 the bondlength alternation in the seven-membered ring of 2PF6 is small(1.377(8)–1.403(8) Å), whereby the C1–C2 bond (1.372(9) Å)exhibits a pronounced double bond character (Fig. S2†). This
structure stands in contrast to the structure of the parent
azulene, which exhibits a more pronounced bond length alternation in its seven-membered ring (1.3679(8)–1.4155(8) Å).13
The relatively small bond length alternation in 2PF6 reflects alarge contribution of an aromatic tropylium ion, which is
responsible for the eﬀective delocalization of the positive
charge and thereby its high chemical stability. The most significant diﬀerence is the geometry of the central six-memberedring, wherein the Ge atom sits slightly above/below the plane
formed by the remaining five carbon atoms, while 1PF6 exhibitsa more planar geometry. This diﬀerence likely results from thelonger endocyclic Ge–C bond lengths (Ge–C2, 1.946(6) Å) andthe smaller C2–Ge–C2′ angle in 2PF6 (98.5°) compared to thosein 1PF6 (Si–C2, 1.862(3) Å; C2–Si–C2′, 99.8°).4
In the crystal state, molecules of 2PF6 adopt an oﬀset
π-stacked arrangement, in which the outer seven-memberedrings overlap with the closest interfacial distances of 3.39 and3.42 Å (Fig. 2b). This packing mode is similar to that observedfor 1PF6,4 which implies that this is a common feature of thestructurally constrained diazulenomethene skeleton that isprobably a result of the location of the PF6− anion and thesteric repulsion of the vertically oriented substituents on the
Ge or Si bridge (Fig. 2 and S6†).7
The electronic eﬀects resulting from the Ge- and Sn-bridges
were studied by measuring the photophysical and electrochemical properties of 2PF6 and 3PF6. In the UV-visible absorption spectra of 2PF6 and 3PF6 in CH2Cl2, intense absorption
bands were observed with maximum wavelengths (λabs) at 639
and 637 nm, respectively, which are slightly blue-shifted compared to that of 1PF6 (λabs = 646 nm) (Fig. 3a). The λabs of 2PF6
and 3PF6 did not significantly change upon changing thesolvent from CH2Cl2 to THF to DMSO (Table S2†). It should be

Fig. 3 (a) UV–vis absorption spectra of 1PF6 (orange), 2PF6 (green), and3PF6 (blue) in CH2Cl2. (b) Kohn–Sham molecular orbitals and electronictransitions of 1–3 calculated at the M06/6-311+G(d,p) (for Si, Ge, C, andH) and the LANL2DZ (for Sn) levels of theory with CH2Cl2 using thepolarizable continuum model. The red values in (b) represent the percentage probability for each S0–S1 electronic transition.

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noted that 2PF6 has a larger molar absorption coeﬃcient (ε)
than 1PF6 and 3PF6. The cyclic voltammogram of 2PF6 in CH2Cl2
showed reversible redox waves for a one-electron reduction,
similar to that of 1PF6, while that of 3PF6 only showed an irreversible reduction wave (Fig. S8†). The cathodic peak potentials (Epc) follow the order 1PF6 (−0.73 V, E1/2 = −0.69 V) < 2PF6(−0.79 V, E1/2 = −0.74 V) < 3PF6 (−0.84 V) relative to the ferrocene/ferrocenium couple (Fc/Fc+). These results suggest thatthe energy levels of the LUMOs of cationic diazulenylmetheneskeletons increase upon introducing progressively heavier
bridging elements.
To gain insights into the eﬀects of bridging elements on
the structures and electronic properties, density-functional
theory (DFT) calculations were conducted for 1, 2, and 3
without including the counter anions (Fig. S6†) at the M06/6311+G(d,p) (for Si, Ge, C, and H) and LANL2DZ (for Sn) levelsof theory in CH2Cl2 using the polarizable continuum model(PCM). The optimized structure for 2 exhibits an endocyclic
Ge–C bond length of 1.953 Å and a C–Ge–C angle of 98.3°,
both of which are comparable to those obtained from the X-ray
diﬀraction analysis (Fig. S2†). Although a satisfactory level ofcrystal data could not be obtained for 3, the structural optimization revealed an endocyclic Sn–C bond length of 2.143 Å anda C–Sn–C angle of 93.5°, which are much longer and smaller
than the corresponding values of 1 and 2 (Fig. S3†). This is an
important structural eﬀect caused by the bridging elements.
The HOMO and LUMO energy levels increase in the order 1< 2 < 3, albeit the diﬀerences between 1–3 are subtle (Fig. 3b).
This tendency itself is consistent with the Epc values obtained
from cyclic voltammetry. Notably, while a σ*–π* orbital interaction is observed between the exocyclic SiMe2 moiety and thediazulenylmethene π-skeleton in the LUMO of 1,14 no such
orbital interaction is observed in the LUMOs of 2 and 3 when
the MOs are represented with a threshold value of 0.017(Fig. 3b and S4†).
Electrostatic potential (ESP) maps for 2 and 3 obtained at
the same level of theory showed that a positive charge is eﬀectively delocalized over the concave area of the molecules(Fig. S5†), which is similar to that for 1. The natural charge at
the methene carbon atom for each compound was obtained bynatural population analysis (NPA). The obtained values for 1(−0.026), 2 (−0.026), and 3 (−0.028) are comparable, whichsuggests that the impact of the heavier bridging elements on
the delocalization of the positive charge is limited. On theother hand, slight diﬀerences between the LUMO levels of 1, 2,
and 3 might be due to the diﬀerences in their stabilities as
demonstrated by the assessment of their pKR+ values (Fig. S1†),albeit a detailed rationale for these diﬀerences remains
unclear at this stage (vide supra).
Time-dependent (TD) DFT calculations at the same levelfurnished the energies for the electronic S0–S1 transitions for
1–3. The energy values are consistent with the order of theexperimentally observed hypsochromic shifts of λmax in theorder 1 > 2 > 3 (Fig. 3b). These calculations also show that 2
exhibits the highest oscillator strength ( f ), which is consistent
with the fact that 2PF6 has the largest ε value among the three

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Paper
compounds. It should also be noted here that S0–S1 transitionsfor 1–3 consist of HOMO–LUMO and HOMO–LUMO+1 transitions. Given that the contribution of the HOMO–LUMO+1
transition decreases, the oscillator strength value, and thereby
the molar absorption coeﬃcient, tend to increase. The HOMO
and LUMO+1 of 1–3 consist of a combination of the HOMO
and LUMO of the two parent azulene units. Thus, the S0–S1
transition in 2 has the least azulene character, which is most
likely responsible for it having the largest molar absorption
coeﬃcient among the three compounds.
Even though the long E–C bonds in 2PF6 and 3PF6 result in
distorted structures (Fig. S3†), these compounds retain the
J-type aggregation behavior in solution, which is accompanied
by absorption bands in the NIR region as observed for 1PF6.3b,4When the methylcyclohexane (MCH) content of a 1,2-dichloroethane (DCE)/MCH solution of 2PF6 was increased, the intensity of the original absorption band at 646 nm gradually
decreased and a new broad band emerged at around 800 nm.
The new absorption band became more prominent in a 20 : 80(v/v) DCE/MCH solution (λmax = 810 nm) and further blueshifted to 773 nm in a 10 : 90 DCE/MCH solution (Fig. 4b). The
powder X-ray diﬀraction pattern of the resulting aggregates of
2PF6, which were collected by filtration from the 10 : 90 DCE/
MCH solution, showed good agreement with a pattern simulated from its crystal packing arrangement (Fig. S9†). This
result implies that the oﬀset molecular arrangement observedin the crystalline state is also formed, at least partly, in theFig. 4 UV-vis-NIR absorption spectra of (a) 1PF6, (b) 2PF6, and (c) 3PF6 invarious ratios of 1,2-dichloroethane (DCE)/methylcyclohexane (MCH)(v/v). Fluorescence spectra recorded in a 10 : 90 (v/v) DCE/MCH mixed
solvent are shown as blue dashed lines. (d) Comparison of the S0–S1
electronic transition for the dimeric motifs of 1 and 2 together with theirKohn–Sham HOMOs and LUMOs calculated at the M06/6-311+G(d,p)
level of theory. The geometries for the calculations were taken fromtheir crystal packing structures and the counter ions have not been
included.

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aggregated state in solution. Compound 3PF6 also exhibited abathochromically shifted absorption band (λmax = 751 nm)
when the DCE/MCH ratio was increased to 10 : 90 (Fig. 4c). The
λmax values of the aggregates exhibited hypsochromic shifts that
follow the order 1PF6 > 2PF6 > 3PF6. Although there is no direct
evidence due to the lack of crystal data for 3PF6, we infer fromthe crystal structure observed for 2PF6 that the heavier elements
distort the central six-membered ring, which reduces the degreeof overlap of the adjacent molecules in the aggregated state,thus resulting in a blue-shifted absorption band in the NIR
region. Notably, the diﬀerence in the λmax in the aggregatedstate between 3PF6 and 1PF6 is 42 nm, while the diﬀerence
between their monodispersed states is only 9 nm (Fig. 3a).
To gain insight into the bathochromic shift of the absorption bands upon aggregation, DFT calculations were conducted at the M06/6-311+G(d,p) level for two adjacent molecules of 2, the coordinates of which were taken from thecrystal packing structure of 2PF6 (Fig. 4d). Compared to themonomer (Fig. 3b), the dimeric motif of 2 is characterized by
HOMO and LUMO energy levels that are increased by 0.42 eV
and 0.24 eV, respectively, and thereby the corresponding transition energy is by 0.198 eV lower, which is in agreement with abathochromic shift of 44 nm. This calculation also showed a significant increase in the oscillator strength ( f ) for the dimericmotif. The oﬀset arrangement of the electronic transition dipoleof 2, which is oriented along the long axis of the molecule(Fig. S4†), should be relevant to the increased f value, despite
the limited overlap of the molecules only in the outer sevenmembered ring (Fig. 2b). The diﬀerence between the absorption
wavelengths of the aggregated states of 1PF6 and 2PF6 can also be
reproduced by comparing the calculation results for the dimericmotifs of 1 and 2 (Fig. 4d). In the LUMO of the dimer of 2, inphase orbital interactions across the two molecules wereobserved (see the dotted area in Fig. 4d), the degree of which
seems smaller than that of the dimer of 1.
Although monodispersed 2PF6 and 3PF6 in solution are
essentially non-emissive, fluorescence was observed upon theformation of J-type aggregates in their 10 : 90 DCE/MCH solutions (Fig. 4a–c). The emission maximum wavelengths of 2PF6
and 3PF6 were 828 and 820 nm, respectively, which wereslightly blue-shifted compared to that of 1PF6 (845 nm). The
Stokes shifts for 2PF6 (860 cm−1) and 3PF6 (1121 cm−1) werelarger than that for 1PF6 (776 cm−1), which might be related to
the loosely packed aggregated structure in 3PF6 that occurs dueto the distorted geometry of its central six-membered ring
(Fig. S3†). The quantum yields of the aggregates of 2PF6 and3PF6 in 10 : 90 DCE/MCH (ΦF < 0.01) are lower than the previously reported value of the Si-bridged congeners (ΦF ≈0.04).4
In conclusion, Ge- and Sn-bridged diazulenylmethyl cations
were synthesized and the eﬀects of these main group bridges
on the stability and photophysical properties were studied.
The incorporation of these heavier elements increases thestability of the resulting cationic π-systems. Among the derivatives, Ge-bridged 2PF6 showed the largest molar absorption
coeﬃcient in its UV-visible absorption spectrum. The use of8634 | Dalton Trans., 2023, 52, 8631–8635

Dalton Transactions
heavy group 14 elements as bridging moieties also gave rise to acentral six-membered ring with a distorted geometry. This geometry, which deviates from planarity, may determine how
closely the π-stacked structures align and thus govern the magnitude of the hypsochromic shift of their absorption bands inthe aggregated state. Despite the incorporation of heavier
elements, aggregates of these cationic π-systems are capable ofemitting fluorescence, albeit their emissions are generally faint.

Conﬂicts of interest
There are no conflicts to declare.

Acknowledgements
This work was financially supported by CREST (JPMJCR21O5)
from the Japan Society and Technology Agency (JST). The synchrotron X-ray crystallography experiment for 2PF6 was performed at the BL02B1 beamline of SPring-8. The authors are
grateful to Mr. Tatsuo Hikage (Nagoya Univ.) for X-ray powder
diﬀraction measurements.

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