Showcasing research from Masayoshi Higuchi’s laboratory,
National Institute for Materials Science, Tsukuba, Japan.

As featured in:Geometrically isomeric Pt(II)/Fe(II)-based
heterometallo-supramolecular polymers withorganometallic ligands for electrochromism and theelectrochemical switching of Raman scattering
Heterometallo-supramolecular polymers with Pt(II) and Fe(II)ions introduced alternately were prepared by the 1 : 1complexation of Fe(II) ions with cis- or trans-conformationalorgano-Pt(II) ligands.

See Masayoshi Higuchi et al.,J. Mater. Chem. C, 2016, 4, 9428.

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Materials Chemistry C
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Cite this: J. Mater. Chem. C, 2016,
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Geometrically isomeric Pt(II)/Fe(II)-based
heterometallo-supramolecular polymers withorganometallic ligands for electrochromism andthe electrochemical switching of Raman scattering†
Chanchal Chakraborty,ab Rakesh K. Pandey,a Utpal Rana,a Miki Kanao,aSatoshi Moriyamab and Masayoshi Higuchi*a
Heterometallo-supramolecular polymers with Pt(II) and Fe(II) ions introduced alternately (cis-polyPtFe
and trans-polyPtFe) in a precise way were prepared successfully by the 1 : 1 complexation of Fe(II) ionswith cis- or trans-conformational organo-Pt(II) ligands. The conformational diﬀerence between cis- andtrans-greatly changed the morphology, crystallinity, ionic conductivity, electrochromic properties, andredox-triggered fluorescence of the polymers. The cis-polyPtFe exhibited better crystallinity and low

Received 12th July 2016,
Accepted 12th September 2016
DOI: 10.1039/c6tc02929a

ionic conductivity, whereas trans-polyPtFe showed an amorphous nature with high ionic conductivity.
Both the polymers exhibited reversible electrochromism between purple and yellow colors due to theredox of Fe(II)/(III) upon applying a potential of 0 V or +3 V. The trans-polyPtFe showed better
electrochromic stability and response times compared to cis-polyPtFe. In addition, the trans-polyPtFe
also showed an improved response in redox-triggered Raman scattering switching compared towww.rsc.org/MaterialsC

cis-polyPtFe over a long time range.

Introduction
Metallo-supramolecular polymers represent a new dimensionthat has been growing in interest over the last ten years in thearea of polymer and materials science due to their utility in awide range of applications, including sensors, memory devices,light emitting diodes, photovoltaic devices, nanolithographic
templates, stimuli responsive materials in redox-tunable photonic crystals, controlled-drug release, electrochromic displays, andmolecular motors.1–7 The main reason behind this considerable
interest is the great versatility of their fabrication using a very
simple self-assembling process between metal cations andorganic ligands.8 Using such a methodology, many one (1D),
two (2D), and three (3D) dimensional structures along withdifferent metal ions have been produced and used as smart
materials for various applications.9–11 The main benefits ofthese metallo-supramolecular polymers over their conventionalaElectronic Functional Macromolecules Group, National Institute for Materials
Science (NIMS), Tsukuba 305-0044, Japan.
E-mail: HIGUCHI.Masayoshi@nims.go.jpbInternational Center for Materials Nanoarchitectonics (MANA), NIMS, Tsukuba,
Japan† Electronic supplementary information (ESI) available: Characterizations ofligands and cis- and trans-polyPtFe by diﬀerent 1H and DOSY NMR techniques,
electrochromic device fabrication for electrochromic measurements, emission
study and Raman spectroscopy. See DOI: 10.1039/c6tc02929a

9428 | J. Mater. Chem. C, 2016, 4, 9428--9437

covalent counterparts originate from the presence of metallic
centers, providing a wide range of properties, such as optical,
electrical, redox, photochemical, magnetic, or catalytic activities.12
Due to the large range of available organic ligands and metallic
cations, metallo-supramolecular polymers offer the vast potential todevelop smart materials with tunable properties. In this context, if
two metal ions are introduced to a metallo-supramolecular polymer
chain regularly, unique properties due to the neighboring metal–
metal interactions are expected to appear in the polymer.13–15 The
formation of heteronuclear metal complexes using different coordination environments of ditopic ligands has been widely investigated.16–23 Constable et al. used a one-pot synthesis at different
temperatures to obtain two metal ion complex structures.16
Newkome et al. reported a Sierpinski hexagonal gasket-shapedsupramolecule by connecting Fe, Ru, and tris-terpyridines.24
Licciardello et al. reported the electro-optical properties oftrinuclear complexes with both Ru(II) and Os(II) ions.25 Again,
Ag–Cu- and Ag–Zn-containing heterometallic coordination polymers were reported by Di Sun et al. to exhibit the interesting
photoluminescence property of heterometallic aggregates.26–28
However, 1D heterometallo-supramolecular polymers where metal
centers are incorporated alternatively are very rarely reported,
except in our previous reports,13–15 while the reaction steps are
not so precise to incorporate heterometals alternately.
Homometallo-supramolecular polymers were synthesized by the1 : 1 complexation of metal ions and ditopic ligands (Scheme 1a),3This journal is © The Royal Society of Chemistry 2016

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Scheme 1 Diﬀerent synthesis routes of (a) homometallo- and (b–d)
heterometallo-supramolecular polymers.

but the alternate introduction of heterometallic centers precisely
in a polymer chain is very challenging. Scheme 1b exhibits a
preparation method of a heterometallo-supramolecular polymer
using an asymmetrical ligand.14 The synthetic procedure is smart,
but highly selective complexation between the two metal ion
species and the two coordination sites of the asymmetrical ligand
are required to obtain the desired polymer structure. In order todevelop divergent syntheses of heterometallo-supramolecularpolymers, in the present study we focused on organometallic
bonds. When ditopic ligands include metal species by organometallic bonds formation, heterometallo-supramolecular polymers are considered to form by the 1 : 1 complexation of theorganometallic ligand and another metal ion (Scheme 1c). The
obtained polymers are also expected to show unique electronic/
optical properties based on the metal–metal interactions throughthe organometallic bonds. Alongside this, the other advantage
of organometallic ligands is that it is possible to prepare twogeometrical isomeric organometallic ligands (cis- and trans-) andtheir corresponding heterometallo-polymers from one precursor
organic ligand by just changing the metal salts used for preparing
the organometallic ligands (Scheme 1d).
Recently, Fe(II)-based metallo-supramolecular polymers withbis(terpyridine) as ditopic ligands were extensively investigatedas fourth generation electrochromic materials due to its redoxtriggered color change from purple (Fe(II)) in the reduced stateThis journal is © The Royal Society of Chemistry 2016

Journal of Materials Chemistry C

to colorless (Fe(III)) in the oxidized state.3,7,9,14,15,29–32 The electrochromic behaviors of metallo-supramolecular polymers have been
widely investigated, but the correlation between the polymer
structures, especially the structural characteristic, and the electrochromic activities has not been elucidated yet.
It has been reported that platinum acetylide-based organometallic oligomers and polymers are p-conjugated materials withenhanced photophysical properties33–36 and that one can tune
the geometry of the Pt-complex by the proper choice of precursor
Pt salt used in the complexation. It is anticipated that, if we
could effectively synthesize alternatively introduced Fe(II)- andPt(II)-containing metallo-supramolecular polymers by using aPt(II)-based organometallic ligand, it must have an effect on theproperties of the particular metal center due to the presence ofhetero-metallic interaction. Thus, trans- and cis-conformationalorgano-Pt(II) ligands (trans-PtL and cis-PtL) were prepared by the1 : 2 organometallic bond formation of Pt(II) and a terpyridyl- andethynyl-containing compound. Heterometallo-supramolecularpolymers with Pt(II) and Fe(II) ions to be introduced alternately(trans-polyPtFe and cis-polyPtFe) were precisely prepared via the1 : 1 complexation of Fe(II) ions with the organo-Pt(II) ligands(Scheme 2). To the best of our knowledge, this is the first reportto prepare heterometallo-supramolecular polymers using organometallic ligands. We then studied the redox-triggered opticalproperties, e.g., electrochromism and solid-state emission, andtheir effects due to the heterometallo interaction in the bimetallic
Pt(II)/Fe(II) supramolecular polymers. We also investigated thegeometric aspects of the trans- and cis-bimetallic polymers on
their material properties, such as the electrochromism and ionic
conductivity. These studies can provide a correlation between the
structural aspects and the electro-optical properties, specificallythe electrochromism, of metallo-supramolecular polymers indetail that has not been studied before.

Experimental
Materials and methods
Unless otherwise noted, all the reagents were reagent grade andwere used without purification. Dehydrated ethanol, dichloromethane, and methanol were used as the reaction solvents.
Spectroscopic grade methanol was used for the spectral analysis.
These solvents were purchased from either Aldrich or Wako or
Kanto Chemical Co. Inc. and were used as received. Deionized
H2O was used in the experiments where required. 4-Ethynyl
benzaldehyde, 2-acetylpyridine, sodium hydroxide, ammonia
solution, cis-dichlorobis(triphenylphosphine)platinum(II), transdichlorobis(triphenylphosphine)platinum(II), trimethylamine,
copper(I) iodide, and iron(II)tetrafluoroborate were purchasedfrom either Sigma-Aldrich co. Ltd or Tokyo Chemical Industry
Co. Ltd and used as received.
Instrumentation
1

H-NMR spectra were recorded at 300 MHz on a JEOL AL 300/BZ
instrument using DMSO-d6 and CDCl3 as the solvent. Chemical
shifts are given herein relative to TMS. DOSY NMR spectra were

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Scheme 2 Synthesis of an unsymmetrical ligand (L) and the heterometallo-supramolecular polymers with Pt(II) and Fe(II) ions introduced alternately(cis- and trans-polyPtFe).

recorded on a JNM-ECA400 spectrometer (JEOL Resonance).
MALDI mass spectra (MALDI-TOF) were measured by using the
AXIMA-CFR, Shimadzu/Kratos, TOF mass spectrometer. UV-vis
spectra were recorded using a Shimadzu UV-2550 UV-visible
spectrophotometer. The fluorescence was measured by a
Shimadzu RF-5300PC spectrofluorophotometer. The molecularweights of the polymers were determined by SEC-viscometryRALLS (size exclusion chromatography-viscometry-right angle
light scattering) on a Viscotek 270 Dual Detector instrument
using polyethylene oxide PEO-22K as the standard in methanol
solvent (flow rate: 1 mL min1). Flash column chromatographic
separations were performed on silica gel 60 N (neutral, 40–100 mM),
Kanto Chemical Co. Inc. Wide angle XRD was done by using a
RINT ULTIMA III device with Cu Ka radiation (l = 1.54 Å), a
generator voltage of 40 kV, and a current of 40 mA. Polymer
powder was placed on a glass holder and scanned in a range of
2y = 2–601 at a scan rate of 1 s per step with a step width of 0.011
at room temperature. Cyclic voltammetry (CV) and amperometric experiments were carried out by drop casting the polymer solutions onto a glassy carbon working electrode and thenslowly evaporating the solution. The experiments were executedin nitrogen–saturated anhydrous acetonitrile solution containing
0.1 M of lithium perchlorate (LiClO4) as the supporting electrolyte
by using an electrochemical analyzer, ALS/H CH instruments.
A platinum wire was used as counter the electrode, and Ag/AgCl

9430 | J. Mater. Chem. C, 2016, 4, 9428--9437

as the reference electrode. The scan rate for CV analysis was
10 mV s1. For the ionic conductivity measurements, polymer
powder was pressed into the form of a pellet using a press. The
thicknesses of all the polymer pellets were within 0.2 to 0.4 mm.
The pellet was then inserted in between the two electrodes of asample holder. The sample holder used in the conductivity
experiments was composed of two electrodes with an attachedmicrometer on the upper electrode and another fixed electrodewith a guard ring on the electrode, which reduces the effect ofstray field lines existing at the edge of the sample. The guard
ring ensures that the electric field lines remain parallel all overthe sample. Besides this, the ac impedance results were also
normalized using the empty cell measurements by carrying out
the experiment in an air-gap created with the help of theattached micrometer. This procedure eradicates the errors due
to connection and equipment, etc., thus making the measurement more trustworthy. A Solartron 1260 impedance gain/phaseanalyzer coupled with a Solartron 1296 dielectric interface wasused for the ac impedance measurements with high sensitivity. A
frequency range of 50 Hz to 10 MHz was used to determine theresistance of the film under humid conditions. For comparison
with various studies, polyFe was prepared according to theliterature.37 Raman spectroscopy of the compounds was donewith a 532 nm laser as the excitation source using a Raman 11
instrument (Nanophoton Corporation, Japan). An equimolarThis journal is © The Royal Society of Chemistry 2016

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amount of 4 0 ,4 0 0 0 0 -(1,4-phenylene)bis(2,20 :60 ,200 -terpyridine) (SigmaAldrich) and Fe(BF4)2 was refluxed in methanol for 6 h. The
reaction mixture was dried on a petri dish, washed several timewith water and chloroform, and again dried in vacuum overnightto yield the corresponding polyFe (495%).
Synthesis of 4 0 -(4-ethynylphenyl)-2,2 0 :6 0 ,200 -terpyridine (L)38
2-Acetylpyridine (3.63 g, 30 mmol) and 4-ethynylbenzaldehyde(1.95 g, 15 mmol) and NaOH (1.2 g, 30 mmol) were taken in around bottom flask. Subsequently, a solution of ethanol (200 mL)
and concentrated aqueous NH3 solution (100 mL) was added tothe mixture. The suspension was stirred at room temperaturefor 3 days. Soon after, it became a clear solution and after
3 days it formed a yellow precipitation. The formed precipitate
was isolated by vacuum filtration and washed with water andethanol. After performing flash column chromatography using
chloroform in silica gel and then following recrystallizationfrom ethanol, L was obtained as white needles (3.5 g, 70%).
1
H NMR (300 MHz, DMSO-d6): 8.77 (d, J = 4.8 Hz, 2H, H6,600 ),8.72 (s, 2H, H3 0 ,5 0 ), 8.69 (d, J = 9.0 Hz, 2H, H3,300 ), 8.04 (t, 2H,
H4,400 ), 7.98 (d, J = 9.0, 2H, Hb), 7.70 (d, J = 9.0, 2H, Ha), 7.54 (m,2H, H5,500 ), 4.38 (s, 1H). MALDI-TOF: [M+] calcd. for C23H15N3:
333.13, [(M + H)+] found 334.92, [(M + Na)+] found 356.79 and[(M + K)+] found 372.74.
Synthesis of ligand cis-PtL
4 0 -(4-Ethynylphenyl)-2,20 :6 0 ,200 -terpyridine (L) (100 mg, 0.33 mmol)
was added with cis-dichlorobis(triphenylphosphine)platinum(II)(118.5 mg, 0.15 mmol) under an argon environment in thepresence of dry triethylamine (10 mL) and anhydrous dichloromethane (20 mL). Copper(I) iodide (6 mg, 0.075 mmol) was
added to the solution and the resulting solution was stirred atroom temperature for 7 days under an argon atmosphere. The
yellow precipitation was filtered and successively washed several
times with water and dichloromethane, respectively. The product was dried in vacuum at 50 1C, aﬀording a greenish yellow
solid (135 mg, 65%). 1H NMR (300 MHz, CDCl3): 8.71–8.59(m, 12H), 7.86–7.77 (m, 24H), 7.53–7.40 (m, 22H). MALDI-TOF:[m/z] calcd for C82H58N6P2Pt: 1383.38, [(M + H)+] found 1384.27.
Synthesis of ligand trans-PtL
4 0 -(4-Ethynylphenyl)-2,20 :60 ,200 -terpyridine (L) (100 mg, 0.33 mmol)
was added with trans-dichlorobis(triphenylphosphine)platinum(II)(118.5 mg, 0.15 mmol) under an argon environment in thepresence of dry triethylamine (10 mL) and anhydrous dichloromethane (20 mL). Copper(I) iodide (6 mg, 0.075 mmol) was
added to the solution and the resulting solution was stirred
at room temperature for 7 days under an argon atmosphere.
The yellow precipitation was filtered and successively washedseveral times with water and dichloromethane, respectively.
The product was dried in vacuum at 50 1C, aﬀording a yellow
solid (186 mg, 90%). 1H NMR (300 MHz, CDCl3): 8.71–8.62(m, 12H), 8.02 (br, 4H), 7.86–7.77 (m, 20H), 7.53 (m, 4H), 7.42–
7.40 (m, 18H). MALDI-TOF: [m/z] calcd for C82H58N6P2Pt:
1383.38, [(M + H)+] found 1384.24.

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Journal of Materials Chemistry C

Synthesis of cis-polyPtFe
To a 20 mL methanol solution of cis-PtL (50 mg, 0.034 mmol), a
methanolic solution (10 mL) of Fe(BF4)2 (11.48 mg, 0.034 mmol)
was added under inert conditions. The mixture was refluxed for
6 h with continuous stirring. The reaction mixture was dried ona petri dish, washed several time with water and chloroform,
and again dried in vacuum overnight to give the corresponding
cis-polyPtFe as a violet solid (46 mg, 80%). 1H NMR (300 MHz,DMSO-d6): 9.58 (br, s), 9.06 (br, s), 9.06 (br, m), 8.26 (br, m),
8.04 (br, m), 7.92–7.55 (br, m), 7.23 (br, m). Mw = 2.27  104 Da.
Synthesis of trans-polyPtFe
The trans-polyPtFe was synthesized by the same procedure
applied for the synthesis of cis-polyPtFe by using trans-PtL
ligand in place of cis-PtL. The reaction mixture was dried on apetri dish, washed several times with water and chloroform,
and again dried in vacuum overnight to yield the correspondingtrans-polyPtFe as a violet solid (51 mg, 88%). 1H NMR (300 MHz,DMSO-d6): 9.55 (br, s), 9.04 (br, s), 8.62 (br, m), 8.22 (br, m), 8.01–
7.59 (br, m), 7.20 (br, m). Mw = 2.22  104 Da.
Preparation of a polymer film on ITO
A polyPtFe film was prepared on an ITO-coated glass by spincoating. polyPtFe was dissolved in methanol (concentration:
2 mg mL1). The solution (200 mL) was spin-coated (40 rpm for
250 s and 100 rpm for 600 s) onto an ITO glass (2  1 cm),and the prepared film was then dried at room temperature foranother 20 min.
Preparation of the semi-gel electrolyte
The semi-gel electrolyte was prepared by mixing 0.9 g LiClO4
and 2.1 g poly(methylmethacrylate) (PMMA) in 6 mL propylene
carbonate. The mixture was added to 5 mL of acetonitrile to makea semi-gel material. The mixture was stirred at room temperature
to make a well-blended semi-gel electrolyte.
Preparation of the electrochromic (EC) devices and themeasurements
The semi-gel electrolyte consisting of poly(methylmethacrylate)(PMMA), propylene carbonate, and lithium perchlorate in acetonitrile was put on a polymer-coated ITO. The ITO was kept for
15 min at room temperature. Finally, another ITO was put overit and it was kept at 60 1C for another 1 h for thermal curing.
These devices were used for the electrochromism studies. The
EC devices were oxidized or reduced by applying a voltage
between the two ITO electrodes in between a potential windowof +3 to 0 V by chronoamperometric studies with a 5 s interval
time using an electrochemical analyzer, ALS/H CH instruments,
attached to a UV-vis and fluorescence spectrophotometer.

Result and discussions
A terpyridyl- and ethynyl-containing asymmetric ligand (L) was
prepared by the reaction between 2-acetylpyridine and 4-ethynylbenzaldehyde in the presence of NaOH and ammonia (Scheme 2).

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The ligand L was characterized by MALDI-TOF and 1H NMR
spectroscopy, as shown in Fig. S1 in the ESI.† L (2 equiv.) was
treated with cis- or trans- dichlorobis(triphenylphosphine)platinum(II) in dichloromethane solution and catalyzed by
CuI in the presence of NEt3 to prepare the corresponding cisor trans-PtL ligands with a good yield. Both the ligands (cis- andtrans-PtL) were sparsely soluble in common solvents and could
be readily purified by washing with water and dichloromethane
to remove the small impurities. The ligands were characterizedby 1H NMR and MALDI-TOF mass spectroscopy (ESI,† Fig. S2and S3). A comparison of the 1H NMR spectra of L and cis- andtrans-PtL in Fig. S6a and b (ESI†) reveals that the characteristic
singlet 1H signal of the ethynyl proton of L at 4.38 ppm completely disappeared after complexation with Pt(II). This desertionof the ethynyl proton along with no change in the terpyridine
proton position of cis- and trans-PtL clearly confirmed theattachment of Pt(II) with the ethynyl binding site of L to preparecis- and trans-PtL.
The complexation behavior of cis- and trans-PtL with Fe(II)was investigated in detail by UV-vis spectroscopic measurement.
The spectrum of a methanol solution of cis-PtL (6 mM) exhibiteda metal-to-ligand charge transfer (MLCT) band for the complexation of Pt(II) ions with the ethynyl moiety of L at 383 nm (Fig. 1a).
A methanol solution of Fe(BF4)2 was successively added to thecis-PtL solution up to a molar ratio of [Fe(BF4)2]/[cis-PtL] of 2. A
new absorption at 574 nm based on the MLCT band of Fe(II)complexed with the terpyridine moieties of cis-PtL appeared
during the titration. The MLCT absorption was increased linearly
and was clearly saturated at a ratio of [Fe(BF4)2]/[cis-PtL] of 1
(Fig. 1a and b). The spectrum of a methanol solution of trans-PtL(6 mM) also demonstrated a MLCT band for the complexationof Pt(II) ions with the ethynyl moiety of L at 398 nm (Fig. 1c).

Fig. 1 (a) UV-vis spectral change of a methanol solution of cis-PtL during
titration with Fe(BF4)2 at room temperature. (b) The absorption change at
574 nm as a function of the [Fe(BF4)2]/[cis-PtL] ratio. (c) UV-vis spectralchange of a methanol solution of trans-PtL during titration with Fe(BF4)2 atroom temperature. (d) The absorption change at 574 nm as a function of
the [Fe(BF4)2]/[trans-PtL] ratio.

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The red-shift of the MLCT transition in trans-PtL compared tocis-PtL is due to the enhancement of the MLCT energy gap in
cis-PtL rather than in trans-PtL as the p-orbitals of cis-PtL are
not aligned to extend the conjugation.39 The successive additionof a methanol solution of Fe(BF4)2 to trans-PtL solution up toa molar ratio of [Fe(BF4)2]/[trans-PtL] of 2 also revealed thegeneration of a new absorption peak at 574 nm based on the
MLCT band of Fe(II) complexed with the terpyridine moieties. Herealso, the MLCT absorption was linearly increased and saturated at
the ratio of [Fe(BF4)2]/[trans-PtL] of 1 (Fig. 1c and d). These spectralchanges suggest the formation of a heterometallo-supramolecularpolymer with Pt(II) and Fe(II) ions introduced alternately to prepare
1D linear cis- and trans-polyPtFe, according to a stepwise complexation behavior with complexation between cis- or trans-PtL
and Fe(II) with a 1 : 1 molar ratio, as shown in Schemes 1 and 2.
Furthermore, the MLCT absorption of Fe(II) did not change upon
the further addition of Fe(II), indicating that the polymers werestable in solution.
The cis- and trans-polyPtFe were obtained by refluxing thecorresponding Pt(II)-containing ligand with Fe(BF4)2 in methanol
as a violet solid in a satisfactorily high yield (480%), as shown in
Scheme 2. The polymers were soluble in alcohols, such as
methanol, ethanol, and ethylene glycol, and polar solvents, suchas DMSO and DMF, but sparsely soluble in the common organic
solvents, such as n-hexane, chloroform, dichloromethane, andtetrahydrofuran. The molecular weights of the polymers were
determined by the SEC-Viscometry/RALLS method using PEO
as the standard (for cis-polyPtFe: Mw = 2.27  104 Da; and fortrans-polyPtFe: Mw = 2.22  104 Da), which strongly indicatedthat both cis- and trans-polyPtFe form a polymer structure withmoderate polydispersity. The 1H NMR spectrum of the polymers are shown in ESI,† Fig. S4 and S5. The comparison of the1
H NMR spectra of L and the cis- and trans-PtL ligands and thecorresponding cis- and trans-polyPtFe in the ESI† Fig. S6a and b
also revealed a broadening of the proton signals along with some
downfield shifting of the terpyridyl protons after polymerizationof the cis- or trans-PtL ligands by Fe(BF4)2. To gain a clearer idea
about the formation of cis- and trans-metallo-supramolecularpolymers, we performed two dimensional Diﬀusion-Ordered
NMR Spectroscopy (DOSY) analysis. Generally, 1H DOSY NMR
can distinguish between cis- and trans-polymers as their diﬀusion is diﬀerent, and hence their diﬀusion coeﬃcients are also
diﬀerent. The 1H DOSY NMR study of the cis- and trans-polyPtFe(ESI,† Fig. S7 and S8) revealed average diﬀusion coeﬃcients ofabout 9.14  1011 m2 s1 and 1.7  1010 m2 s1, respectively,
for the two polymers. The diﬀusion coeﬃcient in the transpolymer is quite a bit higher due to the better mobility in thetrans-polymer chains than in the cis-configuration. The diﬀusion
coeﬃcient of cis-polyPtFe is one order lower due to its aggregated structure (which was further confirmed by the XRD studyalso), such that the diﬀusion is slower.
The UV-vis spectra of both cis- and trans-polyPtFe exhibitedstrong absorptions based on the MLCT (Fig. 2a). After polymerization with Fe(II) metal ions, the MLCT band for complexationof the Pt(II) ions with the ethynyl moiety of the two organometallic ligands were further red-shifted to 407 nm (Fig. 2a).

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Fig. 2 (a) UV-vis spectra of polyFe and the cis- and trans-polyPtFe
polymers in methanol solution and (b) cyclic voltammograms for polyFe
and the cis- and trans-polyPtFe films coated on a glassy carbon electrode innitrogen–saturated anhydrous acetonitrile (electrolyte: 0.1 M LiClO4; counterelectrode: Pt wire; reference electrode: Ag/Ag+; scan rate: 10 mV s1).
(c) Powder XRD of cis- and trans-polyPtFe.

Upon the formation of the polymer by cis- and trans-PtL withFe(II) through terpyridyl chelation, apart from the occurrence ofa new intense absorption at ca. 574 nm from the MLCT for
terpyridine–Fe(II) complexation, the absorption bands due tothe Pt(II)–metal-perturbed p - p* (CRC) and/or MLCT transitions were red-shifted ca. 24 nm for the cis-PtL and 9 nm for thetrans-PtL organometallic ligands. The terpyridyl chelating toFe(II) lowers the p* orbital energy in the CRCtpy and reduces
the energy gap between the HOMO (d) and LUMO (p*), thus
inducing an obvious red-shift of the Pt–metal-perturbed p - p*
(CRC) and/or MLCT absorption.39 The red-shift was quite a bit
lesser in trans-polyPtFe as the MLCT energy gap was already
reduced in the trans-PtL ligand compared to its cis-counterpart.
Again, the MLCT absorption due to Fe(II) complexation withterpyridine in the polyPtFe polymers is meagerly blue-shiftedcompared to that of the bisterpyridine ligand containing theFe(II) metallo-supramolecular polymer polyFe (MLCT band at
583 nm). This small but significant blue-shift indicates the lowest
unoccupied molecular orbital (LUMO) potential of the Pt(II)containing ligand in the heterometallo polymer was increased
rather than in the normal 4 0 ,4 0 0 0 0 -(1,4-phenylene)bis(2,20 :60 ,200 terpyridine) ligand in polyFe due to the incorporation of Pt(II)in the ligand structure. The cyclic voltammogram of polyFe in
Fig. 2b exhibits a reversible redox wave of the Fe(II)/Fe(III) couple(E1/2 = 0.77 V). The redox potential of the reversible Fe(II)/Fe(III)couple in both trans- and cis-polyPtFe showed similar values of
E1/2 (0.73 V and 0.74 V for trans- and cis-polyPtFe, respectively)
with a smaller negative shift of the E1/2 of the Fe(II)/Fe(III) couple
than that of polyFe. This negative shift of the E1/2 of the
Fe(II)/Fe(III) couple clearly supports the intramolecular metal–
metal interactions between the neighboring Pt and Fe ions.13
Again, the DE of the Fe(II)/Fe(III) couple in polyFe (0.13 V)
is slightly higher than the DE of the Fe(II)/Fe(III) couple inthe cis- and trans-polyPtFe (0.09 V and 0.08 V for trans- andThis journal is © The Royal Society of Chemistry 2016

Journal of Materials Chemistry C

cis-polyPtFe, respectively) indicating the good electronic communication in the film of Fe/Pt-containing polymers.
The comparison of the powder XRD study of cis- and transpolyPtFe revealed very sharp peaks from the cis-polyPtFe compared to from the trans-polyPtFe (Fig. 2c). From this study, we
can assume that cis-polyPtFe possessed a better crystallinity,
whereas trans-polyPtFe was amorphous in nature. In the cispolymer structure, we can assume a polarization occurred as theindividual Pt-complex centers have some dipole moment due toits angular shape (Fig. S9, ESI†); whereas the trans-polymer islinear in shape and the individual Pt-complex centers do notpossess any dipole moment as it is cancelled out due to theirlinear shape. As the polarization and dipole moment are higher
in individual Pt centers in the cis-polymer, we can assume there isa better crystallinity in the cis-polymer.40,41
The morphological characteristics of the two polymers werestudied by a SEM image study (Fig. 3a–d). The SEM images
revealed an agglomerated polymer network in cis-polyPtFe(Fig. 3a and b) whereas, in trans-polyPtFe, the polymer network
was very well defined to produce high-aspect-ratio fibers (Fig. 3cand d). In the trans-polymer, self-assembly between the polymer
chains is reasonably easier due to its linear structure, which
gives more structural preference rather than in the cis-polymer
to produce a well-defined polymer network.
Electrochromic materials (ECMs) have received increasing
interest for their use in optoelectronic applications, such as smart
windows and electronic papers.3,42,43 Recently, it has been revealed
that metallo-supramolecular polymers synthesized by the 1 : 1complexation of transition metals, such as Fe(II)/Ru(III) andFe(II)/Cu(I) ions with some ditopic ligands, are an excellent
ECM with both high durability and ample color variation.13,15
These polymers display a specific color based on MLCT absorption
and can show electrochromic behavior based on the disappearance/
reappearance of MLCT absorption, which is triggered by theelectrochemical redox reaction of the metal ions. A solidstate device (2  1 cm, Fig. 4a) was fabricated using a cis- ortrans-polyPtFe film on ITO and a semi-gel electrolyte layer
including a lithium perchlorate salt was sandwiched between

Fig. 3 SEM images of cis-polyPtFe (a and b) and trans-polyPtFe (c and d)film drop-casted over glass from methanol solution..

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Fig. 4 (a) Electrochromic device structure with a heterometallo-supramolecular polymer film and a semi-gel electrolyte layer. (b) Color changeof the cis-polyPtFe polymer device during the electrochemical studies and(c) that for the cis-polyPtFe polymer device during the electrochemical
studies. (d) UV-Vis spectral change of the EC device of the trans-polyPtFe
polymer by applying a potential (from 0 to +3 V) and (e) that for theEC device of the trans-polyPtFe polymer by applying a potential (from0 to +3 V).

the ITO-containing polymer film and another ITO (ESI,† Fig. S10).
Both devices showed reversible electrochromic behavior (frompurple to blue to yellow) when the applied voltage was changed
from 0 to +3 V (Fig. 4b and d). This electrochromic color change
could be clearly monitored by UV-vis spectroscopy (Fig. 4c and e).
For the device with cis-polyPtFe, the UV-vis spectrum showed
two absorptions around 467 and 580 nm based on the MLCT
absorptions of the Pt(II)–ethynyl and Fe(II)–terpyridyl complex
moieties, respectively (Fig. 4c). Generally the lmax of the polymer
in the solid state is bathochromically shifted when comparedto the solution spectra, due to the major conformational order,
resulting in a different energy levels distribution. So, the MLCT
peak for the Pt(II)–ethynyl complex at 407 nm in methanol
solution in Fig. 2a is red-shifted to 467 nm in the solid film in
EC devices in Fig. 4c and e. Again, the MLCT transitions are
typically extremely sensitive to the solvent polarity because theygive rise to a strong change in the molecular dipole momentin the excited state. Using a polar solvent, like methanol, canenhance the energy gap between the HOMO (d) and LUMO (p*),
thus inducing the obvious blue-shift to 407 nm in methanol
solution in Fig. 2a from 467 nm in the solid film in EC devices
in Fig. 4c and e.
Interestingly, the absorption around 580 nm for the MLCT
absorption of the Fe(II)–terpyridyl complex disappeared when
the device was exposed at +3 V, because of the electrochemical
oxidation of Fe(II) to Fe(III). This phenomenon was also reflected

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Paperin the solid-state color change of the devices from purple toyellow in Fig. 4b. The yellow color is prominent due to only the
MLCT of the Pt(II)–ethynyl complex. The spectral change wasreversible; the original spectrum could be re-obtained by applying
the opposite voltage (0 V) to the device (Fig. 4b). As Fe(III) centerswere reduced to Fe(II) at 0 V, the prominent purple color of the
MLCT absorption of the Fe(II)–terpyridyl complex reappeared. We
observed the same phenomenon with the device made of transpolyPtFe (Fig. 4d and e). It also showed same reversible electrochromic behavior by the electrochemical redox of Fe ions,whereby the MLCT absorption at 580 nm of the Fe(II)–terpyridine
complex disappeared to give a color change from purple toyellow, upon the oxidation of Fe(II) to Fe(III) by exposing the film
at +3 V and then reappeared upon the reduction of Fe(III) to
Fe(II) at 0 V.
In order to confirm the device durability in these EC changes,
a long-term redox-switching repeatability study was done by
applying two voltages (+3 and 0 V) alternately (interval time: 5 sfor each step) to both the devices for up to 200 s, and the MLCT
absorption at 580 nm was monitored as a function of time.
Fig. 5a and b show the switching behavior of the MLCT absorptions from 0 to 200 s for cis- and trans-polyPtFe, respectively.
Both the polymer devices revealed good switching stability withthe applied potential; however, the device with the cis-polyPtFe
polymer film showed some decrement of the absorbance diﬀerence of the Fe–terpyridine MLCT between the reduced andoxidized states with time (Fig. 5a). We calculated the response
times for the disappearance (bleaching time tb) and reappearance (coloring time tc) of the MLCT absorption at 580 nm, andfor the cis-polyPtFe polymer device the values were 4.23 and2.17 s, respectively (Fig. 5c); whereas, the response times forthe disappearance (tb) and reappearance (tc) of the MLCT

Fig. 5 Absorbance change in the absorption at 580 nm during theelectrochromic switching of the EC device (applied voltages: 0 and +3 V;
interval time for each step: 5 s) for up to 200 s of (a) cis-polyPtFe and(b) trans-polyPtFe. (c and d) Are the absorbance change of cis-polyPtFe
and trans-polyPtFe, respectively, during two cycles (150–170 s). The times
for coloring and bleaching (tc and tb) were defined as the time taken for 95%
of the D absorbance to change.

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absorption at 580 nm were 0.93 and 1.08 s, respectively, for thetrans-polyPtFe device (Fig. 5d).
Again, in the fluorescence spectroscopy both polymers revealed
a narrow bandwidth peak around 503 nm (19 880 cm1) when
excited at 467 nm (21 413 cm1), which is characteristic of the
MLCT absorption of the Pt–ethynyl unit (Fig. 6a and b). However,this narrow bandwidth peak at 503 nm cannot be the emission
peak for the organo-Pt moiety as the bandwidth is too small andthe peak position is far more blue-shifted than usual for organo-Pt
lumophores.44 To evaluate the origin of this narrow bandwidthpeak, such as whether it comes from an emission from theorgano-Pt moiety or terpyridine–ethynyl ligand itself, we also
performed an emission study of the pristine ligand L in thesolid film state. The ligand L has only one absorption peak for
p–p* transition at 323 nm (ESI,† Fig. S11). The emission studyof L with 323 nm excitation exhibited only one emission band,with its maxima at 384 nm (ESI,† Fig. S12). However, theemission study of L with excitation at 467 nm (where no absorption is present from L) revealed an identical fluorescence pattern(lmax at 504 nm, 19 841 cm1) as for polyPtFe polymers (ESI,†
Fig. S13). This experiment clearly indicated that the narrow peakaround 503 nm in both the polymers is not from the emission oforgano-Pt moieties but may be due to Raman scattering from theligand. To further investigate the peak and whether it came fromthe Raman scattering or not, we studied the fluorescence spectraof L at two other arbitrary excitations, such as 480 and 488 nm,
and in both cases we obtained the same pattern of spectra with
only a change in the peak positions (519 and 529 nm, respectively) (ESI,† Fig. S13). However, if the peak is due to theemission, then it should not change with changing the excitation wavelengths. The energy gap between the excitation wavenumber and the emission peak was about B1570 cm1, which
is a typical value for the first Stokes line of Raman scattering by
a CQC double bond and so on. In polymers, the energy gap
between the excitation wavenumber (21 413 cm1) and theFig. 6 The fluorescence spectra of EC devices of (a) cis-polyPtFe and(b) trans-polyPtFe at potential of as cast, +3.0 and 0 V. Corresponding
intensity change of the devices of (c) cis-polyPtFe and (d) trans-polyPtFe
at +3.0 and 0 V exited at 467 nm.

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Journal of Materials Chemistry C

emission peak (19 880 cm1) is also about B1533 cm1. So, we
can assume that the narrow emission peak at 503 nm is solely due
to Raman scattering of the ligand L. To gain further insight,
we studied the Raman scattering of ligand L and both thepolymers, which revealed a strong peak with a Raman shift of
B1530–1580 cm1 for typical CQC stretching (ESI,† Fig. S14).
We assume that when the emission was measured, this peakinterfered and exhibited a narrow bandwidth peak at 503 nm in
both the polymers. Again, an enhancement of this Raman
scattering band was observed upon the electrochemical oxidation of Fe(II) ions to Fe(III) at +3 V (Fig. 6a and b). The Raman
scattering intensity was reversibly weakened again by theelectrochemical reduction of Fe(III) to Fe(II). We also performed
a repeatability study of this redox-triggered-scattering switching
behavior of these two devices within a potential range of +3 and
0 V (Fig. 6c and d). Initially, both polymers showed very niceredox-triggered-scattering switching; however, in cis-polyPtFe,
the intensity difference between the two states decreased withtime (Fig. 6c). The Raman scattering intensity enhancement
after electrochemical oxidation to Fe(II) to Fe(III) in both polymers
is due to electrochromic bleaching, which can increase theoptical transmittance of the polymer film.45 When the polymer
film was reduced, the optical transmittance of the film again was
reduced to exhibit an overall lower intensity of Raman scattering.
To gain an insight into the reason for the better electrochromicand redox-triggered Raman scattering switching behavior in thetrans-polyPtFe compared to in the cis-polyPtFe device, we measured
the ionic conductivity of the polymers by impedance measurement.
Fig. 7 shows the impedance response for the trans- and cis-polyPtFe.
The conductivities of the polymers are 3.0  105 mS cm1 and0.9  105 mS cm1, respectively. The more than three times
higher conductivity in the trans-polyPtFe compared to that of
the cis-polyPtFe could be assigned to the structural preference inthe trans-polyPtFe as it contains a straight chain type structure,
which facilitates a swift ion transfer process in the polymer.
From the SEM images (Fig. 3), it also evident that the counter
anion mobility is easier in trans-polyPtFe as it contains a welldefined nanofiber structure, which can produce a channel totransport the counter anions. As the ionic conductivity is higher
in the trans-polyPtFe, it showed better electrochromic behavior
over its cis-counterpart. Again, from the XRD study in Fig. 2c, it isevident that the cis-polyPtFe possessed higher crystallinity in theFig. 7 The Nyquist plots for cis- and trans-polyPtFe at 25 1C and 98% RH.

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film rather than the trans-polyPtFe, which has a higher amorphous content. Owing to its higher crystalline nature in film, thecis-polyPtFe showed poor electrochromic performance, as electrochemical reactions are harder in a rigid structure with higher
crystallinity. In the cis-polymer, the interchange and diﬀusion of
the counter anion is diﬃcult than in the trans-polymer due to its
more crystalline nature. As a result, the cis-polymer showed aweaker and slow response rather than its trans-counterpart.

Conclusions
cis- and trans-conformational heterometallo-supramolecular polymers with Pt(II) and Fe(II) ions introduced alternately (cis- andtrans-polyPtFe) were synthesized by the 1 : 1 complexation of Fe(II)ions with organo-Pt(II) ligands. Both polymers exhibited similarmolecular weights, but their crystallinity and the ionic conductivity of their solid films were diﬀerent due to their diﬀerent
spatial arrangements. An electronic interaction between thetwo hetero-metal ions through the p-conjugated spacer unit of
the ligand was observed in the cyclic voltammetry measurements. Both the polymers exhibited reversible electrochromism
between purple and yellow based on the electrochemical redox ofFe(II) to Fe(III). The trans-polyPtFe showed better electrochromicstability and response times compared to the cis-polyPtFe owing
to its better ionic conductivity and higher amorphous nature of
the film. The bimetallic polymers also revealed reversible Raman
scattering switching by triggering the redox of the Fe(II) ions inthe solid thin film. The trans-polyPtFe also showed improved
response in redox-triggered Raman scattering switching compared to the cis-polyPtFe over the long-time range. The studies
have tremendous impact for understanding the correlation
between the structural characteristic and ionic conductivity andthe electrochromic behavior of metallo-supramolecular polymers
with diﬀerent geometries. As we successfully prepared two precise heterometallo-supramolecular polymers with Pt(II) and Fe(II)ions introduced alternately via stepwise complexation, thesepolymers can also be used as a precursor to fabricate patterningof FePt nanoparticles.

Acknowledgements
The authors thank the CREST Project, Japan Science and Technology Agency (JST) for financial support. Authors also acknowledge Dr Miharu Eguchi for her help regarding photophysicaldiscussion.

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