Geometrically isomeric Pt(II)/Fe(II)-based heterometallo-supramolecular polymers with organometallic ligands for electrochromism and the electrochemical switching of Raman scattering

Article abstract of DOI:10.1039/c6tc02929a

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) ions with cis- or trans-conformational organo-Pt(ii) l

Full text of DOI:10.1039/c6tc02929a

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Materials Chemistry C
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Geometrically isomeric Pt(II)/Fe(II)-based
heterometallo-supramolecular polymers with
organometallic ligands for electrochromism and
the electrochemical switching of Raman scattering†
Cite this:DOI: 10.1039/c6tc02929a
ab
a
a
a
Chanchal Chakraborty, Rakesh K. Pandey, Utpal Rana, Miki Kanao,
b
a
Satoshi Moriyama and Masayoshi Higuchi*
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) ions
with cis- or trans-conformational organo-Pt(II) ligands. The conformational difference between cis- and
trans-greatly changed the morphology, crystallinity, ionic conductivity, electrochromic properties, and
redox-triggered fluorescence of the polymers. The cis-polyPtFe exhibited better crystallinity and low
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 the
redox 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 to
cis-polyPtFe over a long time range.
Received 12th July 2016,
Accepted 12th September 2016
DOI: 10.1039/c6tc02929a
www.rsc.org/MaterialsC
covalent counterparts originate from the presence of metallic
centers, providing a wide range of properties, such as optical,
Metallo-supramolecular polymers represent a new dimension electrical, redox, photochemical, magnetic, or catalytic activities.
Introduction
12
that has been growing in interest over the last ten years in the Due to the large range of available organic ligands and metallic
area of polymer and materials science due to their utility in a cations, metallo-supramolecular polymers offer the vast potential to
wide range of applications, including sensors, memory devices, develop smart materials with tunable properties. In this context, if
light emitting diodes, photovoltaic devices, nanolithographic two metal ions are introduced to a metallo-supramolecular polymer
templates, stimuli responsive materials in redox-tunable photo- chain regularly, unique properties due to the neighboring metal–
1
3–15
nic crystals, controlled-drug release, electrochromic displays, and metal interactions are expected to appear in the polymer.
The
1–7
molecular motors. The main reason behind this considerable formation of heteronuclear metal complexes using different coordi-
interest is the great versatility of their fabrication using a very nation environments of ditopic ligands has been widely investi-
Constable et al. used a one-pot synthesis at different
organic ligands. Using such a methodology, many one (1D), temperatures to obtain two metal ion complex structures.
16–23
simple self-assembling process between metal cations and gated.
8
16
two (2D), and three (3D) dimensional structures along with Newkome et al. reported a Sierpinski hexagonal gasket-shaped
2
4
different metal ions have been produced and used as smart supramolecule by connecting Fe, Ru, and tris-terpyridines.
9
–11
materials for various applications.
The main benefits of Licciardello et al. reported the electro-optical properties of
these metallo-supramolecular polymers over their conventional trinuclear complexes with both Ru(II) and Os(II) ions. Again,
Ag–Cu- and Ag–Zn-containing heterometallic coordination poly-
2
5
a
mers were reported by Di Sun et al. to exhibit the interesting
photoluminescence property of heterometallic aggregates.
Electronic Functional Macromolecules Group, National Institute for Materials
Science (NIMS), Tsukuba 305-0044, Japan.
26–28
E-mail: HIGUCHI.Masayoshi@nims.go.jp
However, 1D heterometallo-supramolecular polymers where metal
b
International Center for Materials Nanoarchitectonics (MANA), NIMS, Tsukuba,
Japan
centers are incorporated alternatively are very rarely reported,
13–15
except in our previous reports,
while the reaction steps are
†
Electronic supplementary information (ESI) available: Characterizations of
1
not so precise to incorporate heterometals alternately.
Homometallo-supramolecular polymers were synthesized by the
ligands and cis- and trans-polyPtFe by different H and DOSY NMR techniques,
electrochromic device fabrication for electrochromic measurements, emission
study and Raman spectroscopy. See DOI: 10.1039/c6tc02929a
3
1 : 1 complexation of metal ions and ditopic ligands (Scheme 1a),
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3,7,9,14,15,29–32
to colorless (Fe(III)) in the oxidized state.
The electro-
chromic behaviors of metallo-supramolecular polymers have been
widely investigated, but the correlation between the polymer
structures, especially the structural characteristic, and the electro-
chromic activities has not been elucidated yet.
It has been reported that platinum acetylide-based organo-
metallic oligomers and polymers are p-conjugated materials with
3
3–36
enhanced photophysical properties
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)- and
Pt(II)-containing metallo-supramolecular polymers by using a
Pt(II)-based organometallic ligand, it must have an effect on the
properties of the particular metal center due to the presence of
hetero-metallic interaction. Thus, trans- and cis-conformational
organo-Pt(II) ligands (trans-PtL and cis-PtL) were prepared by the
1
: 2 organometallic bond formation of Pt(II) and a terpyridyl- and
ethynyl-containing compound. Heterometallo-supramolecular
polymers with Pt(II) and Fe(II) ions to be introduced alternately
(
1
(
trans-polyPtFe and cis-polyPtFe) were precisely prepared via the
: 1 complexation of Fe(II) ions with the organo-Pt(II) ligands
Scheme 2). To the best of our knowledge, this is the first report
to prepare heterometallo-supramolecular polymers using organo-
metallic ligands. We then studied the redox-triggered optical
properties, e.g., electrochromism and solid-state emission, and
their effects due to the heterometallo interaction in the bimetallic
Pt(II)/Fe(II) supramolecular polymers. We also investigated the
Scheme 1 Different synthesis routes of (a) homometallo- and (b–d) geometric aspects of the trans- and cis-bimetallic polymers on
heterometallo-supramolecular polymers.
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, specifically
but the alternate introduction of heterometallic centers precisely the electrochromism, of metallo-supramolecular polymers in
in a polymer chain is very challenging. Scheme 1b exhibits a detail that has not been studied before.
preparation method of a heterometallo-supramolecular polymer
14
using an asymmetrical ligand. The synthetic procedure is smart,
but highly selective complexation between the two metal ion Experimental
species and the two coordination sites of the asymmetrical ligand
are required to obtain the desired polymer structure. In order to
Materials and methods
Unless otherwise noted, all the reagents were reagent grade and
develop divergent syntheses of heterometallo-supramolecular
polymers, in the present study we focused on organometallic
bonds. When ditopic ligands include metal species by organo-
metallic bonds formation, heterometallo-supramolecular poly-
mers are considered to form by the 1 : 1 complexation of the
organometallic 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 through
the organometallic bonds. Alongside this, the other advantage
of organometallic ligands is that it is possible to prepare two
geometrical isomeric organometallic ligands (cis- and trans-) and
their corresponding heterometallo-polymers from one precursor
organic ligand by just changing the metal salts used for preparing
the organometallic ligands (Scheme 1d).
were used without purification. Dehydrated ethanol, dichloro-
methane, 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
H O was used in the experiments where required. 4-Ethynyl
2
benzaldehyde, 2-acetylpyridine, sodium hydroxide, ammonia
solution, cis-dichlorobis(triphenylphosphine)platinum(II), trans-
dichlorobis(triphenylphosphine)platinum(II), trimethylamine,
copper(I) iodide, and iron(II)tetrafluoroborate were purchased
from either Sigma-Aldrich co. Ltd or Tokyo Chemical Industry
Co. Ltd and used as received.
Instrumentation
Recently, Fe(II)-based metallo-supramolecular polymers with
bis(terpyridine) as ditopic ligands were extensively investigated
1
H-NMR spectra were recorded at 300 MHz on a JEOL AL 300/BZ
as fourth generation electrochromic materials due to its redox- instrument using DMSO-d
6 3
and CDCl as the solvent. Chemical
triggered color change from purple (Fe(II)) in the reduced state 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). as the reference electrode. The scan rate for CV analysis was
ꢀ
1
MALDI mass spectra (MALDI-TOF) were measured by using the 10 mV s . For the ionic conductivity measurements, polymer
AXIMA-CFR, Shimadzu/Kratos, TOF mass spectrometer. UV-vis powder was pressed into the form of a pellet using a press. The
spectra were recorded using a Shimadzu UV-2550 UV-visible thicknesses of all the polymer pellets were within 0.2 to 0.4 mm.
spectrophotometer. The fluorescence was measured by a The pellet was then inserted in between the two electrodes of a
Shimadzu RF-5300PC spectrofluorophotometer. The molecular sample holder. The sample holder used in the conductivity
weights of the polymers were determined by SEC-viscometry- experiments was composed of two electrodes with an attached
RALLS (size exclusion chromatography-viscometry-right angle micrometer on the upper electrode and another fixed electrode
light scattering) on a Viscotek 270 Dual Detector instrument with a guard ring on the electrode, which reduces the effect of
using polyethylene oxide PEO-22K as the standard in methanol stray field lines existing at the edge of the sample. The guard
ꢀ1
solvent (flow rate: 1 mL min ). Flash column chromatographic ring ensures that the electric field lines remain parallel all over
separations were performed on silica gel 60 N (neutral, 40–100 mM), the sample. Besides this, the ac impedance results were also
Kanto Chemical Co. Inc. Wide angle XRD was done by using a normalized using the empty cell measurements by carrying out
RINT ULTIMA III device with Cu Ka radiation (l = 1.54 Å), a the experiment in an air-gap created with the help of the
generator voltage of 40 kV, and a current of 40 mA. Polymer attached micrometer. This procedure eradicates the errors due
powder was placed on a glass holder and scanned in a range of to connection and equipment, etc., thus making the measure-
2y = 2–601 at a scan rate of 1 s per step with a step width of 0.011 ment more trustworthy. A Solartron 1260 impedance gain/phase
at room temperature. Cyclic voltammetry (CV) and ampero- analyzer coupled with a Solartron 1296 dielectric interface was
metric experiments were carried out by drop casting the poly- used for the ac impedance measurements with high sensitivity. A
mer solutions onto a glassy carbon working electrode and then frequency range of 50 Hz to 10 MHz was used to determine the
slowly evaporating the solution. The experiments were executed resistance of the film under humid conditions. For comparison
in nitrogen–saturated anhydrous acetonitrile solution containing with various studies, polyFe was prepared according to the
3
7
0
4
.1 M of lithium perchlorate (LiClO ) as the supporting electrolyte literature. Raman spectroscopy of the compounds was done
by using an electrochemical analyzer, ALS/H CH instruments. with a 532 nm laser as the excitation source using a Raman 11
A platinum wire was used as counter the electrode, and Ag/AgCl instrument (Nanophoton Corporation, Japan). An equimolar
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amount of 4 ,4 -(1,4-phenylene)bis(2,2 :6 ,2 -terpyridine) (Sigma- Synthesis of cis-polyPtFe
Journal of Materials Chemistry C
0
00 0 0
0
0
00
4 2
Aldrich) and Fe(BF ) was refluxed in methanol for 6 h. The
To a 20 mL methanol solution of cis-PtL (50 mg, 0.034 mmol), a
reaction mixture was dried on a petri dish, washed several time
with water and chloroform, and again dried in vacuum overnight
to yield the corresponding polyFe (495%).
methanolic solution (10 mL) of Fe(BF
was added under inert conditions. The mixture was refluxed for
h with continuous stirring. The reaction mixture was dried on
4 2
) (11.48 mg, 0.034 mmol)
6
a petri dish, washed several time with water and chloroform,
0
0
0
00
38
Synthesis of 4 -(4-ethynylphenyl)-2,2 :6 ,2 -terpyridine (L)
and again dried in vacuum overnight to give the corresponding
1
2
-Acetylpyridine (3.63 g, 30 mmol) and 4-ethynylbenzaldehyde cis-polyPtFe as a violet solid (46 mg, 80%). H NMR (300 MHz,
(
1.95 g, 15 mmol) and NaOH (1.2 g, 30 mmol) were taken in a DMSO-d
round bottom flask. Subsequently, a solution of ethanol (200 mL) 8.04 (br, m), 7.92–7.55 (br, m), 7.23 (br, m). M
and concentrated aqueous NH solution (100 mL) was added to
the mixture. The suspension was stirred at room temperature
for 3 days. Soon after, it became a clear solution and after The trans-polyPtFe was synthesized by the same procedure
days it formed a yellow precipitation. The formed precipitate applied for the synthesis of cis-polyPtFe by using trans-PtL
6
): 9.58 (br, s), 9.06 (br, s), 9.06 (br, m), 8.26 (br, m),
4
w
= 2.27 ꢁ 10 Da.
3
Synthesis of trans-polyPtFe
3
was isolated by vacuum filtration and washed with water and ligand in place of cis-PtL. The reaction mixture was dried on a
ethanol. After performing flash column chromatography using petri dish, washed several times with water and chloroform,
chloroform in silica gel and then following recrystallization and again dried in vacuum overnight to yield the corresponding
1
from ethanol, L was obtained as white needles (3.5 g, 70%). trans-polyPtFe as a violet solid (51 mg, 88%). H NMR (300 MHz,
1
H NMR (300 MHz, DMSO-d
6
): 8.77 (d, J = 4.8 Hz, 2H, H6,600), DMSO-d ): 9.55 (br, s), 9.04 (br, s), 8.62 (br, m), 8.22 (br, m), 8.01–
6
4
8
.72 (s, 2H, H
3
0
,5
0
), 8.69 (d, J = 9.0 Hz, 2H, H3,300), 8.04 (t, 2H, 7.59 (br, m), 7.20 (br, m). M = 2.22 ꢁ 10 Da.
w
H4,400), 7.98 (d, J = 9.0, 2H, H ), 7.70 (d, J = 9.0, 2H, H ), 7.54 (m,
2
3
b
a
+
H, H5,500), 4.38 (s, 1H). MALDI-TOF: [M ] calcd. for C H N :
33.13, [(M + H) ] found 334.92, [(M + Na) ] found 356.79 and
Preparation of a polymer film on ITO
23
15 3
+
+
A polyPtFe film was prepared on an ITO-coated glass by spin-
coating. polyPtFe was dissolved in methanol (concentration:
+
[(M + K) ] found 372.74.
ꢀ
1
2
2
mg mL ). The solution (200 mL) was spin-coated (40 rpm for
50 s and 100 rpm for 600 s) onto an ITO glass (2 ꢁ 1 cm),
Synthesis of ligand cis-PtL
0
0
0
00
and the prepared film was then dried at room temperature for
another 20 min.
4
-(4-Ethynylphenyl)-2,2 :6 ,2 -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 the
presence of dry triethylamine (10 mL) and anhydrous dichloro-
(
Preparation of the semi-gel electrolyte
methane (20 mL). Copper(I) iodide (6 mg, 0.075 mmol) was The semi-gel electrolyte was prepared by mixing 0.9 g LiClO
4
added to the solution and the resulting solution was stirred at and 2.1 g poly(methylmethacrylate) (PMMA) in 6 mL propylene
room temperature for 7 days under an argon atmosphere. The carbonate. The mixture was added to 5 mL of acetonitrile to make
yellow precipitation was filtered and successively washed several a semi-gel material. The mixture was stirred at room temperature
times with water and dichloromethane, respectively. The pro- to make a well-blended semi-gel electrolyte.
duct was dried in vacuum at 50 1C, affording a greenish yellow
1
Preparation of the electrochromic (EC) devices and the
measurements
solid (135 mg, 65%). H NMR (300 MHz, CDCl
3
): 8.71–8.59
(
m, 12H), 7.86–7.77 (m, 24H), 7.53–7.40 (m, 22H). MALDI-TOF:
+
58
[m/z] calcd for C82H N
6
P
2
Pt: 1383.38, [(M + H) ] found 1384.27. The semi-gel electrolyte consisting of poly(methylmethacrylate)
PMMA), propylene carbonate, and lithium perchlorate in aceto-
(
Synthesis of ligand trans-PtL
nitrile was put on a polymer-coated ITO. The ITO was kept for
15 min at room temperature. Finally, another ITO was put over
it 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 window
of +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.
0
0
0
00
4
-(4-Ethynylphenyl)-2,2 :6 ,2 -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 the
(
presence of dry triethylamine (10 mL) and anhydrous dichloro-
methane (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 washed
several times with water and dichloromethane, respectively.
The product was dried in vacuum at 50 1C, affording a yellow
Result and discussions
1
solid (186 mg, 90%). H NMR (300 MHz, CDCl ): 8.71–8.62
3
(m, 12H), 8.02 (br, 4H), 7.86–7.77 (m, 20H), 7.53 (m, 4H), 7.42– A terpyridyl- and ethynyl-containing asymmetric ligand (L) was
7
1
.40 (m, 18H). MALDI-TOF: [m/z] calcd for C82
383.38, [(M + H) ] found 1384.24.
H
58
N
6
P
2
Pt: prepared by the reaction between 2-acetylpyridine and 4-ethynyl-
benzaldehyde in the presence of NaOH and ammonia (Scheme 2).
+
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1
The ligand L was characterized by MALDI-TOF and H NMR The red-shift of the MLCT transition in trans-PtL compared to
spectroscopy, as shown in Fig. S1 in the ESI.† L (2 equiv.) was cis-PtL is due to the enhancement of the MLCT energy gap in
treated with cis- or trans- dichlorobis(triphenylphosphine)- cis-PtL rather than in trans-PtL as the p-orbitals of cis-PtL are
3
9
platinum(II) in dichloromethane solution and catalyzed by not aligned to extend the conjugation. The successive addition
CuI in the presence of NEt to prepare the corresponding cis- of a methanol solution of Fe(BF ) to trans-PtL solution up to
3
4 2
or trans-PtL ligands with a good yield. Both the ligands (cis- and a molar ratio of [Fe(BF
4
)
2
]/[trans-PtL] of 2 also revealed the
trans-PtL) were sparsely soluble in common solvents and could generation of a new absorption peak at 574 nm based on the
be readily purified by washing with water and dichloromethane MLCT band of Fe(II) complexed with the terpyridine moieties. Here
to remove the small impurities. The ligands were characterized also, the MLCT absorption was linearly increased and saturated at
1
by H NMR and MALDI-TOF mass spectroscopy (ESI,† Fig. S2 the ratio of [Fe(BF
4
)
2
]/[trans-PtL] of 1 (Fig. 1c and d). These spectral
1
and S3). A comparison of the H NMR spectra of L and cis- and changes suggest the formation of a heterometallo-supramolecular
trans-PtL in Fig. S6a and b (ESI†) reveals that the characteristic polymer with Pt(II) and Fe(II) ions introduced alternately to prepare
singlet H signal of the ethynyl proton of L at 4.38 ppm com- 1D linear cis- and trans-polyPtFe, according to a stepwise com-
1
pletely disappeared after complexation with Pt(II). This desertion plexation behavior with complexation between cis- or trans-PtL
of the ethynyl proton along with no change in the terpyridine and Fe(II) with a 1 : 1 molar ratio, as shown in Schemes 1 and 2.
proton position of cis- and trans-PtL clearly confirmed the Furthermore, the MLCT absorption of Fe(II) did not change upon
attachment of Pt(II) with the ethynyl binding site of L to prepare the further addition of Fe(II), indicating that the polymers were
cis- and trans-PtL.
stable in solution.
The complexation behavior of cis- and trans-PtL with Fe(II)
The cis- and trans-polyPtFe were obtained by refluxing the
was investigated in detail by UV-vis spectroscopic measurement. corresponding Pt(II)-containing ligand with Fe(BF ) in methanol
4
2
The spectrum of a methanol solution of cis-PtL (6 mM) exhibited as a violet solid in a satisfactorily high yield (480%), as shown in
a metal-to-ligand charge transfer (MLCT) band for the complexa- Scheme 2. The polymers were soluble in alcohols, such as
tion of Pt(II) ions with the ethynyl moiety of L at 383 nm (Fig. 1a). methanol, ethanol, and ethylene glycol, and polar solvents, such
A methanol solution of Fe(BF
4 2
cis-PtL solution up to a molar ratio of [Fe(BF )
4
)
2
was successively added to the as DMSO and DMF, but sparsely soluble in the common organic
]/[cis-PtL] of 2. A solvents, such as n-hexane, chloroform, dichloromethane, and
new absorption at 574 nm based on the MLCT band of Fe(II) tetrahydrofuran. The molecular weights of the polymers were
complexed with the terpyridine moieties of cis-PtL appeared determined by the SEC-Viscometry/RALLS method using PEO
4
during the titration. The MLCT absorption was increased linearly as the standard (for cis-polyPtFe: Mw = 2.27 ꢁ 10 Da; and for
4
and was clearly saturated at a ratio of [Fe(BF ) ]/[cis-PtL] of 1 trans-polyPtFe: Mw = 2.22 ꢁ 10 Da), which strongly indicated
4
2
(
Fig. 1a and b). The spectrum of a methanol solution of trans-PtL that both cis- and trans-polyPtFe form a polymer structure with
1
(
6 mM) also demonstrated a MLCT band for the complexation moderate polydispersity. The H NMR spectrum of the poly-
of Pt(II) ions with the ethynyl moiety of L at 398 nm (Fig. 1c). mers are shown in ESI,† Fig. S4 and S5. The comparison of the
1
H NMR spectra of L and the cis- and trans-PtL ligands and the
corresponding 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 polymerization
of the cis- or trans-PtL ligands by Fe(BF ) . To gain a clearer idea
4
2
about the formation of cis- and trans-metallo-supramolecular
polymers, we performed two dimensional Diffusion-Ordered
1
NMR Spectroscopy (DOSY) analysis. Generally, H DOSY NMR
can distinguish between cis- and trans-polymers as their diffu-
sion is different, and hence their diffusion coefficients are also
1
different. The H DOSY NMR study of the cis- and trans-polyPtFe
(
ESI,† Fig. S7 and S8) revealed average diffusion coefficients of
ꢀ
11
2
ꢀ1
ꢀ10
2
ꢀ1
about 9.14 ꢁ 10
m s and 1.7 ꢁ 10
m s , respectively,
for the two polymers. The diffusion coefficient in the trans-
polymer is quite a bit higher due to the better mobility in the
trans-polymer chains than in the cis-configuration. The diffusion
coefficient of cis-polyPtFe is one order lower due to its aggre-
gated structure (which was further confirmed by the XRD study
also), such that the diffusion is slower.
Fig. 1 (a) UV-vis spectral change of a methanol solution of cis-PtL during
titration with Fe(BF at room temperature. (b) The absorption change at
74 nm as a function of the [Fe(BF ]/[cis-PtL] ratio. (c) UV-vis spectral
change of a methanol solution of trans-PtL during titration with Fe(BF at
room temperature. (d) The absorption change at 574 nm as a function of
the [Fe(BF ]/[trans-PtL] ratio.
The UV-vis spectra of both cis- and trans-polyPtFe exhibited
strong absorptions based on the MLCT (Fig. 2a). After polymer-
ization with Fe(II) metal ions, the MLCT band for complexation
of the Pt(II) ions with the ethynyl moiety of the two organo-
metallic ligands were further red-shifted to 407 nm (Fig. 2a).
4 2
)
5
4 2
)
4 2
)
4 2
)
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cis-polyPtFe, respectively) indicating the good electronic com-
munication in the film of Fe/Pt-containing polymers.
The comparison of the powder XRD study of cis- and trans-
polyPtFe revealed very sharp peaks from the cis-polyPtFe com-
pared 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 cis-
polymer structure, we can assume a polarization occurred as the
individual Pt-complex centers have some dipole moment due to
its angular shape (Fig. S9, ESI†); whereas the trans-polymer is
linear in shape and the individual Pt-complex centers do not
possess any dipole moment as it is cancelled out due to their
linear shape. As the polarization and dipole moment are higher
in individual Pt centers in the cis-polymer, we can assume there is
4
0,41
a better crystallinity in the cis-polymer.
The morphological characteristics of the two polymers were
studied by a SEM image study (Fig. 3a–d). The SEM images
revealed an agglomerated polymer network in cis-polyPtFe
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 in
nitrogen–saturated anhydrous acetonitrile (electrolyte: 0.1 M LiClO
4
; counter (Fig. 3a and b) whereas, in trans-polyPtFe, the polymer network
+
ꢀ1
electrode: Pt wire; reference electrode: Ag/Ag ; scan rate: 10 mV s ).
was very well defined to produce high-aspect-ratio fibers (Fig. 3c
and d). In the trans-polymer, self-assembly between the polymer
chains is reasonably easier due to its linear structure, which
(
c) Powder XRD of cis- and trans-polyPtFe.
Upon the formation of the polymer by cis- and trans-PtL with gives more structural preference rather than in the cis-polymer
Fe(II) through terpyridyl chelation, apart from the occurrence of to produce a well-defined polymer network.
a new intense absorption at ca. 574 nm from the MLCT for
Electrochromic materials (ECMs) have received increasing
terpyridine–Fe(II) complexation, the absorption bands due to interest for their use in optoelectronic applications, such as smart
3,42,43
the Pt(II)–metal-perturbed p - p* (CRC) and/or MLCT transi- windows and electronic papers.
Recently, it has been revealed
tions were red-shifted ca. 24 nm for the cis-PtL and 9 nm for the that metallo-supramolecular polymers synthesized by the 1 : 1
trans-PtL organometallic ligands. The terpyridyl chelating to complexation of transition metals, such as Fe(II)/Ru(III) and
Fe(II) lowers the p* orbital energy in the CRCtpy and reduces Fe(II)/Cu(I) ions with some ditopic ligands, are an excellent
1
3,15
the energy gap between the HOMO (d) and LUMO (p*), thus ECM with both high durability and ample color variation.
inducing an obvious red-shift of the Pt–metal-perturbed p - p* These polymers display a specific color based on MLCT absorption
3
9
(
CRC) and/or MLCT absorption. The red-shift was quite a bit and can show electrochromic behavior based on the disappearance/
lesser in trans-polyPtFe as the MLCT energy gap was already reappearance of MLCT absorption, which is triggered by the
reduced in the trans-PtL ligand compared to its cis-counterpart. electrochemical redox reaction of the metal ions. A solid-
Again, the MLCT absorption due to Fe(II) complexation with state device (2 ꢁ 1 cm, Fig. 4a) was fabricated using a cis- or
terpyridine in the polyPtFe polymers is meagerly blue-shifted trans-polyPtFe film on ITO and a semi-gel electrolyte layer
compared to that of the bisterpyridine ligand containing the including a lithium perchlorate salt was sandwiched between
Fe(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
0
0 00 0
0
0
00
rather than in the normal 4 ,4 -(1,4-phenylene)bis(2,2 :6 ,2 -
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
(
E
1/2 = 0.77 V). The redox potential of the reversible Fe(II)/Fe(III)
couple in both trans- and cis-polyPtFe showed similar values of
1/2 (0.73 V and 0.74 V for trans- and cis-polyPtFe, respectively)
E
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–
1
3
metal interactions between the neighboring Pt and Fe ions.
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 in
Fig. 3 SEM images of cis-polyPtFe (a and b) and trans-polyPtFe (c and d)
the cis- and trans-polyPtFe (0.09 V and 0.08 V for trans- and film drop-casted over glass from methanol solution..
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in the solid-state color change of the devices from purple to
yellow in Fig. 4b. The yellow color is prominent due to only the
MLCT of the Pt(II)–ethynyl complex. The spectral change was
reversible; the original spectrum could be re-obtained by applying
the opposite voltage (0 V) to the device (Fig. 4b). As Fe(III) centers
were 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 trans-
polyPtFe (Fig. 4d and e). It also showed same reversible electro-
chromic 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 to
yellow, 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 s
for 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 absorp-
tions from 0 to 200 s for cis- and trans-polyPtFe, respectively.
Fig. 4 (a) Electrochromic device structure with a heterometallo-supra-
molecular polymer film and a semi-gel electrolyte layer. (b) Color change Both the polymer devices revealed good switching stability with
of the cis-polyPtFe polymer device during the electrochemical studies and
the applied potential; however, the device with the cis-polyPtFe
(c) that for the cis-polyPtFe polymer device during the electrochemical
polymer film showed some decrement of the absorbance differ-
ence of the Fe–terpyridine MLCT between the reduced and
oxidized states with time (Fig. 5a). We calculated the response
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 the
EC device of the trans-polyPtFe polymer by applying a potential (from
0
to +3 V).
times for the disappearance (bleaching time t ) and reappear-
b
ance (coloring time t ) of the MLCT absorption at 580 nm, and
c
for the cis-polyPtFe polymer device the values were 4.23 and
the ITO-containing polymer film and another ITO (ESI,† Fig. S10). 2.17 s, respectively (Fig. 5c); whereas, the response times for
) and reappearance (t ) of the MLCT
Both devices showed reversible electrochromic behavior (from the disappearance (t
purple 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 compared
to 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 they
give rise to a strong change in the molecular dipole moment
in the excited state. Using a polar solvent, like methanol, can
enhance the energy gap between the HOMO (d) and LUMO (p*),
thus inducing the obvious blue-shift to 407 nm in methanol
b
c
solution in Fig. 2a from 467 nm in the solid film in EC devices Fig. 5 Absorbance change in the absorption at 580 nm during the
electrochromic switching of the EC device (applied voltages: 0 and +3 V;
in Fig. 4c and e.
interval time for each step: 5 s) for up to 200 s of (a) cis-polyPtFe and
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
(
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 (t
c
b
and t ) were defined as the time taken for 95%
oxidation of Fe(II) to Fe(III). This phenomenon was also reflected of the D absorbance to change.
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ꢀ
1
ꢀ1
absorption at 580 nm were 0.93 and 1.08 s, respectively, for the emission peak (19 880 cm ) is also about B1533 cm . So, we
trans-polyPtFe device (Fig. 5d).
can assume that the narrow emission peak at 503 nm is solely due
Again, in the fluorescence spectroscopy both polymers revealed to Raman scattering of the ligand L. To gain further insight,
ꢀ
1
a narrow bandwidth peak around 503 nm (19 880 cm ) when we studied the Raman scattering of ligand L and both the
ꢀ
1
excited at 467 nm (21 413 cm ), which is characteristic of the polymers, which revealed a strong peak with a Raman shift of
ꢀ
1
MLCT absorption of the Pt–ethynyl unit (Fig. 6a and b). However, B1530–1580 cm for typical CQC stretching (ESI,† Fig. S14).
this narrow bandwidth peak at 503 nm cannot be the emission We assume that when the emission was measured, this peak
peak for the organo-Pt moiety as the bandwidth is too small and interfered and exhibited a narrow bandwidth peak at 503 nm in
the peak position is far more blue-shifted than usual for organo-Pt both the polymers. Again, an enhancement of this Raman
44
lumophores. To evaluate the origin of this narrow bandwidth scattering band was observed upon the electrochemical oxida-
peak, such as whether it comes from an emission from the tion of Fe(II) ions to Fe(III) at +3 V (Fig. 6a and b). The Raman
organo-Pt moiety or terpyridine–ethynyl ligand itself, we also scattering intensity was reversibly weakened again by the
performed an emission study of the pristine ligand L in the electrochemical reduction of Fe(III) to Fe(II). We also performed
solid film state. The ligand L has only one absorption peak for a repeatability study of this redox-triggered-scattering switching
p–p* transition at 323 nm (ESI,† Fig. S11). The emission study behavior of these two devices within a potential range of +3 and
of L with 323 nm excitation exhibited only one emission band, 0 V (Fig. 6c and d). Initially, both polymers showed very nice
with its maxima at 384 nm (ESI,† Fig. S12). However, the redox-triggered-scattering switching; however, in cis-polyPtFe,
emission study of L with excitation at 467 nm (where no absorp- the intensity difference between the two states decreased with
tion is present from L) revealed an identical fluorescence pattern time (Fig. 6c). The Raman scattering intensity enhancement
ꢀ
1
(
lmax at 504 nm, 19 841 cm ) as for polyPtFe polymers (ESI,† after electrochemical oxidation to Fe(II) to Fe(III) in both polymers
Fig. S13). This experiment clearly indicated that the narrow peak is due to electrochromic bleaching, which can increase the
4
5
around 503 nm in both the polymers is not from the emission of optical transmittance of the polymer film. When the polymer
organo-Pt moieties but may be due to Raman scattering from the film was reduced, the optical transmittance of the film again was
ligand. To further investigate the peak and whether it came from reduced to exhibit an overall lower intensity of Raman scattering.
the Raman scattering or not, we studied the fluorescence spectra
To gain an insight into the reason for the better electrochromic-
of L at two other arbitrary excitations, such as 480 and 488 nm, and redox-triggered Raman scattering switching behavior in the
and in both cases we obtained the same pattern of spectra with trans-polyPtFe compared to in the cis-polyPtFe device, we measured
only a change in the peak positions (519 and 529 nm, respec- the ionic conductivity of the polymers by impedance measurement.
tively) (ESI,† Fig. S13). However, if the peak is due to the Fig. 7 shows the impedance response for the trans- and cis-polyPtFe.
ꢀ
5
ꢀ1
emission, then it should not change with changing the excita- The conductivities of the polymers are 3.0 ꢁ 10 mS cm and
ꢀ
5
ꢀ1
tion wavelengths. The energy gap between the excitation wave- 0.9 ꢁ 10 mS cm , respectively. The more than three times
ꢀ
1
number and the emission peak was about B1570 cm , which higher conductivity in the trans-polyPtFe compared to that of
is a typical value for the first Stokes line of Raman scattering by the cis-polyPtFe could be assigned to the structural preference in
a CQC double bond and so on. In polymers, the energy gap the trans-polyPtFe as it contains a straight chain type structure,
ꢀ1
between the excitation wavenumber (21 413 cm ) and the 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 well-
defined nanofiber structure, which can produce a channel to
transport 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 is
evident that the cis-polyPtFe possessed higher crystallinity in the
Fig. 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.
Fig. 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 amor-
phous content. Owing to its higher crystalline nature in film, the
cis-polyPtFe showed poor electrochromic performance, as electro-
chemical 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 a
weaker and slow response rather than its trans-counterpart.
5 T. Sato and M. Higuchi, Chem. Commun., 2012, 48, 4947.
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1
1
1
0 S. B. Clendenning, S. Fournier-Bidoz, A. Pietrangelo,
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Conclusions
cis- and trans-conformational heterometallo-supramolecular poly-
mers with Pt(II) and Fe(II) ions introduced alternately (cis- and
trans-polyPtFe) were synthesized by the 1 : 1 complexation of Fe(II)
ions with organo-Pt(II) ligands. Both polymers exhibited similar
molecular weights, but their crystallinity and the ionic con-
ductivity of their solid films were different due to their different
spatial arrangements. An electronic interaction between the
two hetero-metal ions through the p-conjugated spacer unit of
the ligand was observed in the cyclic voltammetry measure-
ments. Both the polymers exhibited reversible electrochromism
between purple and yellow based on the electrochemical redox of
Fe(II) to Fe(III). The trans-polyPtFe showed better electrochromic
stability 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 in
the solid thin film. The trans-polyPtFe also showed improved
response in redox-triggered Raman scattering switching com-
pared 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 and
the electrochromic behavior of metallo-supramolecular polymers
with different geometries. As we successfully prepared two pre-
cise heterometallo-supramolecular polymers with Pt(II) and Fe(II)
ions introduced alternately via stepwise complexation, these
polymers can also be used as a precursor to fabricate patterning
of FePt nanoparticles.
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