Metallosupramolecular polyelectrolytes self-assembled from various pyridine ring-substituted bisterpyridines and metal ions: Photophysical, electrochemical, and electrochromic properties

Article abstract of DOI:10.1021/ja710380a

This work presents several metallosupramolecular coordination polyelectrolytes (MEPEs) self-assembled from rigid, π-conjugated, pyridine ring functionalized bisterpyridines and metal ions. The MEPEs are water-soluble and display different colors spanning the entire visible regions. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. The results show that the properties are profoundly affected by the nature of the substituents at the peripheral pyridine rings. Namely, MEPEs assembled from the electron-rich OMe group modified ligands exhibit high switching reversibility and stability and show a lower switching potential than the unsubstituted and electron-deficient Br-substituted analogues. The response times can be tuned either by the design of the ligands or by the choice of the metal ions to cover a broad time scale from under 1 s to several minutes. The optical memory is enhanced from 30 s to longer than 15 min as a comparison of unsubstituted and substituted MEPEs shows. Thus, the significantly enhanced stability and the ease of tuning the properties render this type of supramolecular assembly attractive as electrochromic materials for applications in a large variety of areas. Most importantly, we presented the structure-property relationships of MEPEs, which lays the groundwork for further design of new bisterpyridine-based metallosupramolecular functional materials.

Full text of DOI:10.1021/ja710380a

Published on Web 01/18/2008
Metallosupramolecular Polyelectrolytes Self-Assembled from
Various Pyridine Ring-Substituted Bisterpyridines and Metal
Ions: Photophysical, Electrochemical, and Electrochromic
Properties
Fu She Han,† Masayoshi Higuchi,*,† and Dirk G. Kurth*,†,‡
Functional Modules Group, Organic Nanomaterials Center, National Institute for Materials
Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Max Planck Institute of Colloids
and Interfaces, Research Campus Golm, D-14424, Germany
Received November 16, 2007; E-mail: higuchi.masayoshi@nims.go.jp; kurth@mpikg.mpg.de
Abstract: This work presents several metallosupramolecular coordination polyelectrolytes (MEPEs) self-
assembled from rigid, π-conjugated, pyridine ring functionalized bisterpyridines and metal ions. The MEPEs
are water-soluble and display different colors spanning the entire visible regions. Optical, electrochemical,
and electrochromic properties of the obtained MEPEs are presented. The results show that the properties
are profoundly affected by the nature of the substituents at the peripheral pyridine rings. Namely, MEPEs
assembled from the electron-rich OMe group modified ligands exhibit high switching reversibility and stability
and show a lower switching potential than the unsubstituted and electron-deficient Br-substituted analogues.
The response times can be tuned either by the design of the ligands or by the choice of the metal ions to
cover a broad time scale from under 1 s to several minutes. The optical memory is enhanced from 30 s to
longer than 15 min as a comparison of unsubstituted and substituted MEPEs shows. Thus, the significantly
enhanced stability and the ease of tuning the properties render this type of supramolecular assembly
attractive as electrochromic materials for applications in a large variety of areas. Most importantly, we
presented the structure-property relationships of MEPEs, which lays the groundwork for further design of
new bisterpyridine-based metallosupramolecular functional materials.
Introduction
and metal oxides, such as the high coloration efficiency, fast
switching rates, ease of processing, and the possibility to tune
Electrochromic materials (ECMs) have received great interest
as molecular switches for optical and electronic applications.1
Commercial applications include a large range of areas, such
as anti-glare mirrors and glasses, indicators and labels, smart
windows, and information storage, where most applications
require electrochromic materials with high contrast ratio, good
coloration efficiency, long-term stability, and write-erase ef-
ficiency. Some parameters such as response rates and optical
memory are application-dependent; for instance, displays need
fast switching rates, whereas smart windows can operate with
longer response times of up to several minutes. For these
purposes, a vast number of materials have been studied,2 and
conducting polymers,2 molecular dyes,3 and metal oxides2,4 are
the most frequently investigated components. Generally, con-
ducting polymers show several advantages over molecular dyes
the properties through chemical modification.5
Although metal coordination complexes have not been
investigated in depth,6 a few reports associated with polypyridyl7
and porphyrin8 complexes indicate that such complexes are
potential candidates for the fabrication of ECMs with high
performance. Transition-metal complexes are characterized by
metal-centered sites with redox-active metal-to-ligand charge
transfer (MLCT), intervalence CT, and intraligand transitions,
giving rise to strong optical contrast, high coloration ability,
(5) (a) Kumar, A.; Welsh, D. M.; Morvant, M. C.; Piroux, F.; Abboud, K. A.;
Reynolds, J. R. Chem. Mater. 1998, 10, 896. (b) Gaupp, C. L.; Welsh, D.
M.; Reynolds, J. R. Macromol. Rapid Commun. 2002, 23, 885. (c) Sonmez,
G.; Meng, H.; Wudl, F. Chem. Mater. 2004, 16, 574.
(6) (a) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (b) Balzani, V.; Scandola,
F. Supramolecular Photochemistry; Ellis Harwood: Chichester, U.K., 1990.
(c) Mortimer, R. J.; Rowley, N. M. In ComprehensiVe Coordination
Chemistry II: From Biology to Nanotechnology; McCleverty, J. A., Meyer,
T. J., Eds.; Elsevier: Oxford, 2004; Vol. 9, pp 581-619. (d) Monk, P. M.
S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism and Electrochromic
DeVices; Cambridge University Press: Cambridge, 2007; p 253.
(7) (a) Chem. Eng. News 1983, 61, 21. (b) Leasure, R. M.; Ou, W.; Moss, J.
A.; Linton, R. W.; Meyer, T. J. Chem. Mater. 1996, 8, 264. (c) Pichot, F.;
Beck, J. H.; Elliott, C. M. J. Phys. Chem. A 1999, 103, 6263. (d) Kimura,
M.; Horai, T.; Muto, T.; Hanabusa, K.; Shirai, H. Chem. Lett. 1999, 1129.
(e) Bernhard, S.; Goldsmith, J. I.; Takada, K.; Abrun˜a, H. D. Inorg. Chem.
2003, 42, 4389. (f) Kurth, D. G.; Lo´pez, J. P.; Dong, W. F. Chem. Commun.
2005, 2119.
† National Institute for Materials Science.
‡ Max-Planck-Institute.
(1) (a) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism:
Fundamentals and Applications; Wiley-VCH: Weinheim, Germany, 1995.
(b) Mortimer, R. J. Chem. Soc. ReV. 1997, 26, 147.
(2) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27, 2 and
references therein.
(3) Sapp, S. A.; Sotzing, G. A.; Reynolds, J. R. Chem. Mater. 1998, 10, 2101
and references therein.
(4) (a) Granqvist, C. G. Handbook of Inorganic Electrochromic Materials;
Elsevier: Amsterdam, 1995. (b) Liu, S.; Kurth, D. G.; Mo¨hwald, H.;
Volkmer, D. AdV. Mater. 2002, 14, 225 and references therein.
(8) Trombach, N.; Hild, O.; Schlettwein, D.; Wo¨hrle, D. J. Mater. Chem. 2002,
12, 879.
9
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J. AM. CHEM. SOC. 2008, 130, 2073-2081
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A R T I C L E S
Han et al.
Figure 2. Structures of ligands L1-L5 and the corresponding MEPEs
ML1-MEPE-ML5-MEPE, where M indicates the metal ion Fe(II), Ru-
(II), and Co(II), L represents the ligands, and MEPE stands for metallo-
supramolecular coordination polyelectrolytes.
Thus far, terpyridines, bipyridines, and porphyrins have been
investigated,2 and of the available ligands, terpyridines are
particularly interesting. This type of ligand has a rich coordina-
tion chemistry and generally displays a high binding affinity to
many metal ions,10 offering the thermodynamic driving force
for stable metallosupramolecular formation.10,15 Furthermore,
use of terpyridine ligands affords a well-defined coordination
geometry, preventing diastereoisomeric formation as appeared
with their sister bipyridines. This is an important issue in
particular if multinuclear polymer, which is advantageous for
processing, is under consideration.10a
During the development of new metallosupramolecule-based
ECMs, we paid attention to rigid, π-conjugated bisterpyridines
having substituents at the 6-position of the pyridine ring as
contrasted with the unsubstituted analogues reported in previous
reports.7c-e We reasoned that a functional group at the 6-position
of the pyridine periphery will influence through steric or
electronic effects the ligand field stabilization energy and, thus,
the properties of the metallosupramolecular assemblies. On the
other hand, to investigate in systematic ways the structure-
property relationships, which are virtually unknown but are
highly desirable for the design of new functional materials, we
focus on the synthesis of a range of different metallosupramo-
lecular coordination polyelectrolytes (MEPEs). Thus, a series
of bisterpyridines having electron-rich or -deficient function-
alities was synthesized, from which several MEPEs were
assembled using Fe(II), Ru(II), and Co(II) as metal ions.
Electrochromic studies of the resulting MEPEs reveal that the
relevant properties of these materials, such as optical absorption,
color, switching potential, response time, stability, reversibility,
and optical memory, can be readily tuned either by the proper
design of the ligands or by the choice of the metal ions, making
this class of metallosupramolecular assemblies highly attractive
for electrochromic applications.
Figure 1. X-ray crystal structure of L2. (A) Top view. (B) Side view. (C)
Molecular stacking in the crystal lattice. Hydrogen atoms in B and C are
omitted for clarity. Crystal data at 110 K: C42H28N6 (MW: 616.71), 0.20
× 0.05 × 0.04 mm3, colorless crystalline needles, monoclinic space group
C2/c, a ) 28.0200(12) Å, b ) 10.6337(3) Å, c ) 10.9221(4) Å, V ) 3201.6-
(2) Å3, â ) 100.330(3)°, Z ) 4; final R1 ) 0.0445 for all 15 519 reflections
of I > 2σ(I), wR2 ) 0.1404 for all 15 519 reflections.
and low switching potential.9 Furthermore, the properties of the
complexes can be modified by changing the ligands or metal
ions.10 Finally, processing preferentially from aqueous media
in various device configurations such as nanostructures,11 thin
films,12 mesophases,13 and liquid crystalline and gel14 represents
an added advantage of using coordination complexes for
practical applications as ECMs.
(9) (a) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes;
Plenum Press: New York, 1994. (b) Juris, A.; Balzani, V.; Barigelletti, F.;
Campagana, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988,
84, 85.
(10) For reviews, see: (a) Constable, E. C. Chem. Soc. ReV. 2007, 36, 246. (b)
Kurth, D. G.; Higuchi, M. Soft Matter 2006, 2, 915. (c) Andres, P. R.;
Schubert, U. S. AdV. Mater. 2004, 16, 1043.
Results and Discussion
Synthesis of Ligands and Metallosupramolecular Poly-
(11) (a) Kurth, D. G.; Lehmann, P.; Schu¨tte, M. Proc. Natl. Acad. Sci. U.S.A.
2000, 97, 5704. (b) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem.,
Int. Ed. 2002, 41, 3681; Angew. Chem. 2002, 114, 3833.
(12) (a) Schu¨tte, M.; Kurth, D. G.; Linford, M. R.; Co¨lfen, H.; Mo¨hwald, H.
Angew. Chem., Int. Ed. 1998, 37, 2891; Angew. Chem. 1998, 110, 3058.
(b) Lehmann, P.; Kurth, D. G.; Brezesinski, G.; Symietz, C. Chem.sEur.
J. 2001, 7, 1646.
mers. L1 (Figure 2) is commercially available. Ligands L2 and
(14) (a) Aamer, K. A.; Tew, G. N. Macromolecules 2007, 40, 2737. (b) Camerel,
F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon, D.; Schmutz, M.;
Iacovita, C.; Bucher, J. P. Angew. Chem., Int. Ed. 2007, 46, 2659.
(15) (a) Iyer, P. K.; Beck, J. B.; Weder, C.; Rowan, S. J. Chem. Commun. 2005,
319. (b) Yount, W. C.; Juwarker, H.; Craig, S. L. J. Am. Chem. Soc. 2003,
125, 15302.
(13) Kurth, D. G.; Meister, A.; Thunemann, A. F.; Forster, G. Langmuir 2003,
19, 4055.
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2074 J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008

MEPEs Self-Assembled from Bisterpyridines
A R T I C L E S
Figure 3. UV-vis spectra of free ligands L1-L5 in 10 µM CH2Cl2 solution (A), FeL1-MEPE-FeL5-MEPE in 70 µM MeOH solution (B), RuL1-
MEPE-RuL5-MEPE in 50 µM MeOH/H2O (v/v ) 4:1) solution (C), and CoL1-MEPE-CoL5-MEPE in 70 µM MeOH solution (D).
L4 were synthesized according to a one-pot Suzuki-type
dimerization16 as shown in Scheme S1. Syntheses of ligands
L3 and L5 were carried out employing a two-step Kro¨hnke
approach17 (Scheme S2). X-ray analysis18 of L2 indicates that
this ligand takes a straight rigid conformation along the central
axis of ring A1-B1-B2-A2 (Figure 1A). The dihedral angles
between the rings A1 and B1, B1 and B2, and A1 and A2 are
27.976 (6)°, 27.302 (3)°, and 83.243 (2)°, respectively. A side
view reveals that the two terpyridine planes are almost
perpendicular with a calculated dihedral angle of 83.243 (2)°
between rings A1 and A2 (Figure 1B). In the crystal lattice,
the molecules displace from each other by about 4.33 Å along
the A1-B1-B2-A2 central axis, a distance approximately
corresponding to the length of a C-C bond and the diagonal
distance of a benzene ring (Figure 1C). The dihedral angles
between the rings B1 and A1′, B2 and B1′, and A2 and B2′ are
calculated to be 55.269 (3)°, 0°, and 55.269 (3)°, respectively.
The calculated centroid-to-centroid distances between the rings
B1 and A1′, B2 and B1′, and A2 and B2′ are 4.451(1), 4.345-
(1), and 4.442 Å, respectively, indicating only weak interlayer
π-π interaction. Owing to the rigid structure and linear
connectivity of the metal ion receptor, it is anticipated that well-
defined rodlike MEPEs are formed from L2 and its analogue
ligands, L119 (Figure 2) and L3-L5, avoiding ring formation
as detected with less rigid ligands.20
ion in a suitable solvent (Figure 2). While self-assembly with
kinetically labile metal ions such as Fe(II) or Co(II) occurs at
room temperature even in water, refluxing in solvent signifi-
cantly accelerates product formation. The Co(II)-based MEPEs,
CoL1-MEPE-CoL5-MEPE, were assembled by refluxing
ligands and Co(OAc)2 in MeOH. The Ru(II)-based MEPEs,
RuL1-MEPE-RuL5-MEPE, were prepared by stirring a
mixture of ligands and RuCl2(DMSO)4 in ethylene glycol at
150 °C. The Fe(II)-based MEPEs, FeL1-MEPE-FeL5-MEPE,
were prepared according to the previously published procedure.21
All MEPEs exhibit moderate to good solubility in H2O, MeOH,
EtOH, and HOAc, and some of them are also soluble in THF
and CHCl3, making these materials processable from various
solvent systems, including environmentally friendly aqueous
media. As a comparison, the solubilities of their PF6- and ClO4
-
forms are much poorer. The formation of coordination com-
plexes was confirmed by using UV-vis spectroscopy and ESI-
TOF high-resolution mass analysis.
Optical Properties. The optical properties of the ligands L1-
L5 and their corresponding Fe(II)-, Ru(II)-, and Co(II)-MEPEs
are investigated by UV-vis spectroscopy. The spectra are shown
in Figure 3, and the spectroscopic data are collected in Table
1. For comparison, the absorption data of Fe(II)-based MEPEs
published previously is also included.21 For the five ligands
(Figure 3A and Table 1), an increase in the spacer length from
one phenylene to two phenylene units and the introduction of
either electron-donating OMe, L3 and L4, or electron-withdraw-
ing Br groups, L5, result in a slight shift of the π-π* transition
bands toward longer wavelength, giving rise to a red shift of
Coordination polymerization of various MEPEs was per-
formed by heating an equimolar amount of ligand and metal
(16) Han, F. S.; Higuchi, M.; Kurth, D. G. Org. Lett. 2007, 9, 559.
(17) Eryazici, I.; Moorefield, C. N.; Durmus, S.; Newkome, G. R. J. Org. Chem.
2006, 71, 1009 and references therein.
(18) See also the Supporting Information of ref 16 for the CIF file of L2.
(19) Kolb, U.; Buscher, K.; Helm, C. A.; Lindner, A.; Thunemann, A. F.; Menzel,
M.; Higuchi, M.; Kurth, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
10202.
(20) El-Ghayoury, A.; Schenning, A. P. H. J.; Meijer, E. W. J. Polym. Sci.,
Part A: Polym. Chem. 2002, 40, 4020.
(21) Han, F. S.; Higuchi, M.; Kurth, D. G. AdV. Mater. 2007, 19, 3928.
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A R T I C L E S
Han et al.
Table 1. UV-Vis Properties of Free Ligands L1-L5 and Their
nm) the MLCT band when RuL5-MEPE and RuL1-MEPE
are compared. In addition, an increase in the spacer length results
in a smaller ꢀmax value (RuL1-MEPE vs RuL2-MEPE and
RuL3-MEPE vs RuL4-MEPE). These behaviors are reversed
when Fe(II) instead of Ru(II) is used as metal ion.21 Depending
on the metal ion, the increase in spacer length of the ligand
associated with an increased or a decreased ꢀmax is also observed
for the mono- or dinuclear complexes prepared from L1 and
L2.22,24 Finally, it should be mentioned that as an important
parameter for electrochromic application, ꢀmax of the MLCT
bands of Ru(II)-based MEPEs is generally much higher than
that of the corresponding Fe(II)-based analogues.
Corresponding Fe(II)-, Ru(II)-, and Co(II)-Based MEPEsa
λ
max (nm)b
[M-1 cm-1])
λ
max (nm)b
(ꢀ
[M-1 cm-1])
entries
(
ꢀ
entries
L1
L2
L3
L4
L5
292c
(61 300)
310
(59 800)
298
(68 500)
316
(81 400)
299
(70 600)
586
(18500)
579
(25700)
585 (616)
(14300)
578 (614)
(16700)
612 (639)
(7700)
RuL1-MEPE
513
(40 960)
502
(35 520)
536
(25 420)
524
(22 520)
507
(32 460)
522
(2514)
521
(2300)
521
RuL2-MEPE
RuL3-MEPE
RuL4-MEPE
RuL5-MEPE
CoL1-MEPE
CoL2-MEPE
CoL3-MEPE
CoL4-MEPE
CoL5-MEPE
FeL1-MEPEd
FeL2-MEPE
FeL3-MEPE
FeL4-MEPE
FeL5-MEPE
Figure 3D shows the UV-vis spectra of Co(II)-MEPEs; the
spectroscopic data are listed in Table 1. No MLCT is observed
for this series; instead, a weak d-d* transition band of Co(II)
around 520 nm appears for all five polymers.7f Similar to that
of the two series of MEPEs based on Fe(II) and Ru(II) ions,
the ligand-dependent changes in d-d* transitions of Co(II)-
MEPEs clearly show that ꢀmax is reduced largely by attaching
either electron-donating OMe groups or -withdrawing Br groups,
although only a weak effect on λmax is observed.
The significant effect of the ligand structure and the metal
ion on the optical properties of the resulting assemblies is easily
recognized by the different colors displayed by MEPEs.
Accordingly, the Fe(II)-MEPEs FeL1-MEPE-FeL5-MEPE
are dark-blue, purple, blue, sky-blue, and grass-green, respec-
tively (Figure 4A). The Ru(II)-MEPEs RuL1-MEPE-RuL5-
MEPE show magenta, deep-orange, purple-red, violet, and red,
respectively (Figure 4B). The Co(II)-MEPEs CoL1-MEPE-
CoL5-MEPE are orange, red, yellow, and pale-yellow, respec-
tively (Figure 4C). Intermediate colors can be realized by mixing
different MEPEs in appropriate ratios. Tailoring the color of
electrochromic materials remains a particularly important chal-
lenge. We show here that the color of bisterpyridine-based
coordination polymers is readily tuned to span the entire visible
region either by the design of the ligand or by the choice of the
metal ion.
(214)
519
(457)
526
(429)
a At room temperature, in CH2Cl2 (10 µM) for ligands L1-L5; in MeOH
(70 µM) for FeL1-MEPE-FeL5-MEPE; in MeOH/H2O (4/1, 50 µM) for
RuL1-MEPE-RuL5-MEPE; and in MeOH (70 µM) for CoL1-MEPE-
CoL5-MEPE. b The maximum absorption (λmax) and molar absorptivity
(ꢀmax) are given as π-π* transition for ligands L1-L5; MLCT for FeL1-
MEPE-FeL5-MEPE and RuL1-MEPE-RuL5-MEPE; and d-d* transi-
tion for CoL1-MEPE-CoL5-MEPE. c See also ref 22 for absorption data
of ligands L1 and L2. d Data for Fe(II)-based MEPEs taken from ref 21.
up to 15 nm when L4 and L1 are compared. On the other hand,
the absorption intensity remains unchanged by varying the
spacer length from one to two phenylene units;22 however,
incorporation of OMe or Br substituents at the 6-position of
the pyridine ring results in an increased absorbance (L3-L5).
A profound influence of the ligands on the optical properties
of the corresponding metallosupramolecular assemblies is
observed. Panels B and C of Figure 3 depict the UV-vis spectra
of Fe(II)- and Ru(II)-MEPEs, respectively, and the spectroscopic
data are listed in Table 1. For the Fe(II)-MEPEs, a strong
absorption band occurs around 580 nm, which is characteristic
of MLCT of the bisterpyridine-Fe(II) complex with a quasi-
octahedral coordination geometry.23 For the Ru(II)-MEPEs, a
strong band around 520 nm is present, which can be assigned
to the MLCT of complexes between terpyridine and Ru(II).24
Similar to the Fe(II)-based polymers, the MLCT bands of Ru-
(II)-MEPEs are blue-shifted by approximately 10 nm when the
spacer length is increase from one to two phenylene groups
(RuL1-MEPE vs RuL2-MEPE), and the introduction of
electron-rich OMe or -deficient Br groups at the pyridine
periphery leads to a smaller ꢀmax as a comparison of RuL3-
MEPE-RuL5-MEPE vs unsubstituted RuL1-MEPE and
RuL2-MEPE shows. However, somewhat different from the
behavior exhibited by the Fe(II)-MEPEs, electron-rich OMe
groups cause a large red shift of the MLCT of more than 20
nm in the Ru(II)-MEPEs as a comparison of RuL3-MEPE vs
RuL1-MEPE and RuL4-MEPE vs RuL2-MEPE shows,
whereas electron-deficient Br groups slightly blue shift (ca. 6
Electrochemical Properties. The electrochemical properties
of ligands L1-L5 and their corresponding metallosupramo-
lecular assemblies are investigated by cyclic voltammetry or
differential pulse voltammetry (DPV). No redox peaks are
observed for free ligands. The cyclic voltammograms and the
redox potentials (E1/2) of the MEPEs are summarized in Figure
5 and Table 2, respectively. The Fe(II)-MEPEs (Figure 5A and
Table 2) exhibit a reversible redox potential of the Fe(II)/Fe-
(III) couple (the potential for FeL5-MEPE was determined by
DPV). FeL3-MEPE and FeL4-MEPE, whose ligands are
modified by electron-donating OMe groups, display a redox
potential at ca. 0.70 V, about 70 mV more negative than the
unsubstituted MEPEs FeL1-MEPE and FeL2-MEPE. In
contrast, FeL5-MEPE, whose ligand is modified by electron-
withdrawing Br groups, shows a potential at ca. 0.93 V, about
160 mV more positive than its unsubstituted analogue FeL1-
MEPE. However, no significant effect is observed by varying
the spacer, as a comparison of FeL1-MEPE vs FeL2-MEPE
and FeL3-MEPE vs FeL4-MEPE shows.
(22) Auffrant, A.; Barbieri, A.; Barigelletti, F.; Collin, J. P.; Flamigni, L.;
Sabatini, C.; Sauvage, J. P. Inorg. Chem. 2006, 45, 10990.
(23) Se´ne´chal-David, K.; Leonard, J. P.; Plush, S. E.; Gunnlaugsson, T. Org.
Lett. 2006, 8, 2727.
(24) Haga, M.; Takasugi, T.; Tomie, A.; Ishizuya, M.; Yamada, T.; Hossain,
M. D.; Inoue, M. Dalton Trans. 2003, 2069 and references therein.
A similar effect of ligand on the electrochemical property is
observed for the MEPEs based on Ru(II) (Figure 5B and Table
2; the voltammogram of RuL1-MEPE is not shown for clarity).
The OMe group functionalized RuL3-MEPE and RuL4-MEPE
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2076 J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008

MEPEs Self-Assembled from Bisterpyridines
A R T I C L E S
Figure 4. Colors of FeL1-MEPE-FeL5-MEPE (0.25 mM, MeOH) (A), RuL1-MEPE-RuL5-MEPE (0.05 mM, MeOH/H2O (v/v ) 4/1)) (B), and
CoL1-MEPE-CoL5-MEPE (0.5 mM, MeOH) (C). Photo of Fe(II)-MEPEs taken from ref 21.
The length of spacer does not show an effect on the redox
potential. It is thus evident that the electron-donating or
-withdrawing nature of the substituents at the peripheral
pyridines affects markedly the redox behavior of the MEPEs.25
In general, the electron-donating group leads to a smaller redox
potential while the electron-withdrawing substituent results in
a higher one as compared to the unsubstituted analogues.
Substitution effect at the spacer region (4′-position of terpyri-
dine) has been extensively studied.24,26-30
Of interest is that both the Fe(II) and Ru(II)-MEPEs as-
sembled from the OMe- or Br-substituted ligands, FeL3-
MEPE-FeL5-MEPE and RuL3-MEPE-RuL5-MEPE, show
an additional weak reversible redox waves at a lower potential
(see zoomed cyclic voltammograms of the Fe(II)-based MEPEs
in Figure S1 for a clear view), while the corresponding
unsubstituted MEPEs show only a single wave. The reason is
not clear, although extensive control experiments have been
performed for clarification. Apparently, the substitution at the
6-position of the pyridine rings with OMe or Br group in ligands
L3-L5 is expected to perturb the bond distance and coordina-
tion geometry through the electronic and steric effects in
particular if the unsymmetric structures of substitution pattern
are compared to the symmetrical, unsubstituted ligands L1 and
L2. For instance, substitution at this position may affect the
equilibrium of high- and low-spin Fe(II) species in MEPE
depending on the nature of substitutents,25 thus presenting two
redox waves corresponding to each species. Substitution effect
has also been observed in the optical properties of these MEPEs
(vide supra).21
Figure 5. Cyclic voltammograms of Fe(II)- (A) and Ru(II)-MEPEs (B)
recorded from thin films coated on a glassy carbon electrode in an electrolyte
solution of n-Bu4NClO4 (0.10 M) in argon-saturated acetonitrile using a
platinum wire as counter electrode, and Ag/AgCl electrode as referenced
electrode at room temperature with a scan rate of 100 mV s-1
.
Table 2. Electrochemical Properties of Fe(II)-, Ru(II)-, and
Co(II)-MEPEsa
Investigating the redox behavior of Co(II)-based MEPEs
reveals weak redox peaks of the Co(II)/Co(III) couple at 0.02
and 0.04 V for CoL1-MEPE and CoL2-MEPE, respectively
(Table 2), while redox peaks for OMe- and Br-modified
analogues CoL3-MEPE-CoL5-MEPE are not observed.
Electrochromic Properties. The electrochromic nature of
MEPEs is examined by probing their reversibility, switching
potential, long-term stability, response time, and optical memory.
E1
E1
E1
/2
/
2
/
2
MEPEs
(V)
MEPEs
(V)
MEPEs
(V)
FeL1-MEPEb 0.77 RuL1-MEPE 0.95 CoL1-MEPE 0.02
FeL2-MEPE 0.78 RuL2-MEPE 0.95 CoL2-MEPE 0.04c
FeL3-MEPE 0.70 RuL3-MEPE 0.84 CoL3-MEPE
FeL4-MEPE 0.70 RuL4-MEPE 0.85 CoL4-MEPE
FeL5-MEPE 0.93 RuL5-MEPE 1.16 CoL5-MEPE
d
a Cyclic voltammograms recorded from thin films coated on a glassy
carbon electrode in an electrolyte solution of n-Bu4NClO4 (0.10 M) using
a platinum wire as counter electrode, and Ag/AgCl electrode as referenced
electrode in argon-saturated acetonitrile at room temperature, with a scan
(25) Constable, E. C.; Baum, G.; Bill, E.; Dyson, R.; Eldik, R.; van Fenske, D.;
Kaderli, S.; Morris, D.; Neubrand A.; Neuburger, M.; Smith, D. R.;
Wieghardt, K.; Zehnder, M.; Zuberbu¨hler, A. D. Chem.sEur. J. 1999, 5,
498.
(26) Fang, Y.; Taylor, N. J.; Laverdie`re, F.; Hanan, G. S.; Loiseau, F.; Nastasi,
F.; Campagna, S.; Nierengarten, H.; Leize-Wagner, E.; Dorsselaer, A. V.
Inorg. Chem. 2007, 46, 2854.
(27) Laine´, P. P.; Bedioui, F.; Loiseau, F.; Chiorboli, C.; Campagna, S. J. Am.
Chem. Soc. 2006, 128, 7510.
(28) Carlson, C. N.; Kuehl, C. J.; Da Re, R. E.; Veauthier, J. M.; Schelter, E.
J.; Milligan, A. E.; Scott, B. L.; Bauer, E. D.; Thompson, J. D.; Morris, D.
E.; John, K. D. J. Am. Chem. Soc. 2006, 128, 7230.
(29) Flamigni, L.; Baranoff, E.; Collin, J. P.; Sauvage, J. P. Chem.sEur. J.
2006, 12, 6592.
(30) Vaduvescu, S.; Potvin, P. Eur. J. Inorg. Chem. 2004, 1763.
rate of 100 mV s-1 b Data for Fe(II)-based MEPEs taken from ref 21.
.
c Measured by DPV. d Cannot be determined.
display a redox potential at ca. 0.85 V, about 100 mV lower
than the unsubstituted MEPEs RuL1-MEPE and RuL2-MEPE
(ca. 0.95 V), respectively. In contrast, the Br group function-
alized RuL5-MEPE shows a redox potential about 1.16 V,
about 210 mV higher than that of unsubstituted RuL1-MEPE.
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J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008 2077

A R T I C L E S
Han et al.
Figure 6. Absorbance changes of RuL4-MEPE (A), RuL2-MEPE (B), RuL5-MEPE (C), and CoL2-MEPE (D) films coated on an ITO electrode as
representative examples at different potentials (vs Ag/AgCl) in an argon-saturated acetonitrile solution of n-Bu4NClO4 (0.1 M).
These parameters are important when practical application of
an ECM is concerned, but the available ECMs based on metal
coordination complexes have not been investigated suffi-
ciently.7,24
ligands. As examples, the MLCT band of Ru(II)-MEPEs having
electron-rich OMe groups recovers completely upon reversing
the potential (Figure 6A). The unsubstituted analogues RuL1-
MEPE and RuL2-MEPE (Figure 6B) show a slight loss of
their absorbance, although the unsubstituted Fe(II)-MEPEs,
FeL1-MEPE and FeL2-MEPE, exhibit equally good revers-
ibility as the OMe-modified MEPEs, FeL3-MEPE and FeL4-
MEPE. The loss becomes noticeable for FeL5-MEPE and
RuL5-MEPE, whose ligands are modified by electron-deficient
Br groups (Figure 6C). The data also reveal that the switching
potential of the MEPEs are structure-dependent, which is
reduced by introducing OMe groups, but is increased by
Switching Reversibility and Potential. To examine the
reversibility, the spectral changes of coated ITO slides are
recorded as a function of the applied potential. We observed
that, for Fe(II)- and Ru(II)-MEPEs, the MLCT absorbance
decreases gradually upon incrementally increasing the potential
and finally is lost completely to leave the film colorless.
However, the recovery of the MLCT band is affected by the
nature of the substituents in the peripheral pyridines of the
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2078 J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008

MEPEs Self-Assembled from Bisterpyridines
A R T I C L E S
Table 3. Optical Intensity Loss of Various MEPEs after Multiple
Steps of Redox Cyclesa
loss
(%)
loss
(%)
loss
(%)
MEPEs
MEPEs
MEPEs
FeL1-MEPE
FeL2-MEPE
FeL3-MEPE
FeL4-MEPE
FeL5-MEPE
21
18
22
17
48b
RuL1-MEPE
RuL2-MEPE
RuL3-MEPE
RuL4-MEPE
RuL5-MEPE
29
CoL1-MEPE
CoL2-MEPE
CoL3-MEPE
CoL4-MEPE
CoL5-MEPE
<2e
<2e
f
41c
19
22
49d
a Unless otherwise noted, the long-term redox processes were performed
for up to 4000 steps using a coated ITO electrode by stepping a double
potential between 0.0 and +1.5 V for the Fe(II)- and Ru(II)-MEPEs, and
between -0.5 and +1.0 V for the Co(II)-based series. All the experiments
were carried out in an argon-saturated acetonitrile solution of n-Bu4NClO4
(0.1 M) using a platinum wire as counter electrode, and Ag/AgCl as
reference electrode. b Percent loss after 36 steps. c After 2000 steps. d After
600 steps. e After 300 steps. f Cannot be determined because of the weak
d-d* transitions.
Figure 7. MLCT absorbance change of a thin film of RuL4-MEPE at
potentials of +1.5 and 0.0 V vs Ag/AgCl with a 4 s delay between switching.
First 40 (blue), middle 40 (black), and last 40 (red) of 4000 redox cycles
are shown.
solution of supporting electrolyte rather than the redox-induced
deterioration or dissociation.
To inspect the effect of oxygen on the electrochromic
property, multiple redox switching experiments were performed
on several selected MEPEs in an oxygen-saturated acetonitrile
solution of n-Bu4NClO4 (0.1 M). The results show that the loss
of optical intensity is increased only slightly after 4000 double-
potential cycles under oxygen. As a typical example, 25 vs 22%
of the optical intensity is lost for RuL4-MEPE in the presence
or absence of oxygen. The stability was also inspected by a
long-term standing of the MEPE solutions at ambient environ-
ment; no changes are observed after longer than 6 months
(Figure S3). These results indicate that the materials are highly
stable under various applied conditions.
Our investigation reveals that the unsubstituted MEPEs are
slightly less stable than the OMe-functionalized analogues as a
comparison of RuL1-MEPE vs RuL3-MEPE and RuL2-
MEPE vs RuL4-MEPE shows, although the Fe(II)-based
unsubstituted MEPEs display an equally good stability as the
OMe-functionalized analogues. In sharp contrast to the MEPEs
with OMe-modified and unsubstituted ligands, MEPEs with the
electron-withdrawing Br-substituted ligands, FeL5-MEPE and
RuL5-MEPE, are unstable with an absorbance loss of ap-
proximately 50% within only 600 cycles.
appending Br groups, with a scale of 100 mV at least as
compared to the unsubstituted polymers.
Finally, the reversibility of Co(II)-MEPEs was probed by
inspecting the spectral change of d-d* transition as a function
of the applied potential. Although the spectral changes of the
pyridine ring functionalized MEPEs, CoL3-MEPE-CoL5-
MEPE, are difficult to monitor because of the weak absorbance,
their unsubstituted analogues, CoL1-MEPE and CoL2-MEPE,
operate reversibly at the applied potentials between -0.5 to +0.5
V, as shown by bleaching and coloration behavior of CoL2-
MEPE (Figure 6D).
Long-Term Stability. The stability of the bleached and/or
colored states toward multiple redox steps represents one of
the most important parameters for the practical utilization of
ECMs. To inspect the structure-related stability of the obtained
supramolecular polymer materials here, we performed a long-
term redox switching study by continuously stepping the
voltages between two given values, while the optical changes
of coated ITO electrodes were monitored as a function of time
up to 4000 double-potential cycles.
Thus, the stability of MEPEs is mainly ligand-dependent,
which can be rationalized on the basis of the electronic nature
of the substituents at the peripheral pyridines. Apparently, the
interactions between the ligands and metal ions are strengthened
by electron-rich OMe groups and, consequently, lead to a high
resistance against fatigue of the resulting MEPEs. In contrast,
an electron-deficient substituent weakens the coordination
between the ligands and metal ions, thereby inducing deteriora-
tion during the redox processes.
The stability of the Co(II)-MEPEs is probed by monitoring
the changes of the d-d* transition as a function of time.
Although the OMe- and Br-substituted materials (CoL3-
MEPE-CoL5-MEPE) are not examined owing to their weak
absorbance, the unsubstituted MEPEs, CoL1-MEPE and CoL2-
MEPE, display good stability, showing a loss in absorbance of
the d-d* transition within 2% after 300 steps.
Figure 7 depicts the results of RuL4-MEPE as a representa-
tive example over 4000 switching cycles (first 40 (blue), middle
40 (black), and last 40 (red) of 4000 redox cycles are shown).
The % loss of the optical intensity of this and the other MEPEs
is listed in Table 3. The results suggest that the stabilities are
correlated mainly with the structure of the ligands, although an
effect of metal ions can also be recognized. We observe a ca.
20% loss of the MLCT intensity for Fe(II)- and Ru(II)-MEPEs
assembled from the electron-donating OMe-modified ligands
after 4000 cycles (FeL3-MEPE, FeL4-MEPE, RuL3-MEPE,
and RuL4-MEPE). Interestingly, the main part of the loss is
observed in the first several hundreds of redox cycles, after
which the absorbance of the device stabilizes. As illustrated with
RuL4-MEPE (Figure 7), about 12% loss in absorbance intensity
is observed in the first 1000 cycles whereas only ca. 10% loss
is detected in the following 3000 cycles (ca. 4, 3, and 3% in
each thousand of cycles, respectively). After 4000 cycles, the
UV-vis spectra of the films are essentially identical to those
of original materials in solution (Figure S2). Thus, the loss in
absorbance is most likely the desorption of the materials in the
Response Times. The response times of the MEPEs are
examined by stepping the potentials between 0.0 and +1.5 V
for the Fe(II)- and Ru(II)-MEPEs, and between -0.5 and +1.0
V for the Co(II)-MEPEs. For a comparison, MEPE films are
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A R T I C L E S
Han et al.
Table 4. Response Times of Fe(II)- and Ru(II)-MEPEsa
MEPEs
Tb (s)
Tc (s)
MEPEs
Tb (s)
Tc (s)
FeL1-MEPEb
FeL2-MEPE
FeL3-MEPE
FeL4-MEPE
FeL5-MEPE
2.12
2.08
0.65
0.89
>30
2.06
2.02
0.69
0.96
>30
RuL1-MEPE
RuL2-MEPE
RuL3-MEPE
RuL4-MEPE
RuL5-MEPE
2.25
2.58
1.01
0.99
2.97c
1.41
1.44
1.17
1.01
1.36
a Unless otherwise noted, response times for bleaching (Tb) and coloration
(Tc) are given as an average of 150 redox cycles at applied potentials of
+1.5 and 0.0 V. b Data for Fe(II)-based MEPEs taken from ref 21. c Because
of the quick loss of optical intensity, response times are given as an average
of 25 redox cycles.
prepared by casting equimolar amounts of material on an ITO
slide with an area of ca. 8 mm × 25 mm, except for FeL5-
MEPE (4.0 equiv) and Co(II)-MEPEs (6.0 equiv), where a
larger amount is used because of the weak ꢀmax. The response
times for CoL3-MEPE-CoL5-MEPE could not be determined
reliably because of the weak d-d* transitions.
Figure 8. Optical memory times for FeL3-MEPE-FeL5-MEPE. Inset:
FeL1-MEPE and FeL2-MEPE determined at λmax of the MLCT band.
For comparison, equal amounts of MEPEs (0.25 mM × 25 µL in MeOH)
are coated on the ITO slides (8 × 30 mm2). The experiments are carried
out in an argon-saturated acetonitrile solution of n-Bu4NClO4 (0.1 M) using
a platinum wire as counter electrode, and Ag/AgCl as reference electrode.
We note that MEPEs assembled from Co(II) exhibit a slow
response time longer than 3 min under the applied conditions.
The response times of Fe(II)- and Ru(II)-MEPEs are listed in
Table 4. As a general observation, the unsubstituted MEPEs
(FeL1-MEPE and FeL2-MEPE, and RuL1-MEPE and RuL2-
MEPE) show response times of more than 2 s in most cases.
Interestingly, this time is reduced to about 1 s or less in the
assemblies whose ligands are modified by electron-donating
OMe groups as exhibited by FeL3-MEPE and FeL4-MEPE,
and RuL3-MEPE and RuL4-MEPE. On the contrary, substitu-
tion with the electron-withdrawing Br groups leads to prolonged
response times in FeL5-MEPE and RuL5-MEPE, although the
coloration time (Tc) of RuL5-MEPE is less affected. More
interestingly, comparison of the bleaching (Tb) and coloration
times (Tc) of each MEPE, especially the Ru(II)-based analogues,
reveals that the bleaching process is relatively slower than the
coloration process (Tb > Tc) for the unsubstituted MEPEs. This
difference is further enlarged by attaching an electron-deficient
Br group (RuL5-MEPE). However, incorporation of OMe
groups into the ligands results in not only a reduced response
time but also almost identical bleaching and coloration times.
At this point, no definite mechanism can be proposed to explain
this adequately because the charge transport kinetic, which
governs the response times, is affected by several factors such
as electron and ion mobility in the film. But one of the factors,
here the electronic nature of the substituents, should be
considered. The electron-donating or -withdrawing substituents
may change the charge density in the polymer backbone,
ultimately affecting the electron-transfer rates as well as the
interaction strength of ligand and metal ion of both the
oxidized (M3+) and reduced (M2+) states. On the other hand,
the structure of the ligands may also affect the morphology of
the films, thereby changing the diffusion of ions and the response
times.3
OMe or by electron-withdrawing Br groups show an enhanced
memory time of up to 15 min for the FeL3-MEPE-FeL5-
MEPE, and 8 min for the RuL3-MEPE-RuL5-MEPE. We
propose that, through steric effects, the substituents (OMe or
Br) at the 6-position of pyridine ring protect the metal center
from being reduced by chemical reductants from the environ-
ment. The relatively shorter memory time exhibited by Ru(II)-
MEPEs is mainly attributed to the higher redox potential of
Ru(II)/(III) compared to Fe(II)/(III) couple (Table 2).31 The
optical memory effect is further controlled through the device
design (e.g., carefully encapsulating the coating).
Conclusions
To search new electrochromic materials as well as to
investigate the structure-property relationships of metal-ligand
directed coordination complexes, three types of MEPEs (as
differentiated by different metal ions) were assembled from
various newly synthesized bisterpyridines and investigated in
terms of their electrochromic properties. The presented MEPEs
are soluble in a broad range of organic solvents and water. A
key feature of these materials is that the colors are readily
modified, covering the whole visible region through the design
of the ligands or the choice of the metal ions, demonstrating
the ease for color tuning, which is a topic of great concern in
electrochromic applications. Study of electrochromic property
reveals that introduction of the electron-donating OMe sub-
stituent to the ligand enhances markedly the switching stability
and reversibility of the resulting MEPEs with an observed
absorption loss of ca. 20% after 4000 double-redox steps. This
substituent also reduces the switching potential of the derived
MEPEs. The other properties such as the color, the response
times, and the optical memory can be tuned either by the proper
design of ligands or by the choice of metal centers. As such,
the electrochromic MEPE materials described here have a high
potential for technological applications for a broad range of
purposes. Equally or even more importantly, the basic structure-
property relationships established through this systematic study
Electrochromic Memory. Memory time (defined here as the
ability of an ECM to remain transparent with the potential turned
off) is also an important parameter if particular applications such
as e-paper or displays are under consideration, but has not been
investigated concerning the metallosupramolecular polymers.
Our investigation shows that, for Fe(II)- and Ru(II)-MEPEs,
the unsubstituted assemblies recover the colored state quite fast
within a time scale of shorter than 30 s (Figure 8, inset). In
sharp contrast, MEPEs functionalized either by electron-donating
(31) Storrier, G. D.; Colbran, S. B.; Craig, D. C. J. Chem. Soc., Dalton Trans.
1997, 3011.
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2080 J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008

MEPEs Self-Assembled from Bisterpyridines
A R T I C L E S
should lay the groundwork to serve for the de novo design of
new ligands for new functional materials.
Acknowledgment. We thank the Ministry of Education,
Culture, Sports, Sciences, and Technology, Japan, for financial
support.
Experimental Section
Supporting Information Available: Detailed synthetic pro-
cedures and characterization for ligands and MEPEs, methods
for optical, electrochemical, and electrochromic analyses, and
X-ray crystallographic information. This material is available
free of charge via the Internet at http://pubs.acs.org.
Coated ITO electrodes for amperometric experiments were prepared
by casting MEPE solutions in MeOH (or 4:1 MeOH/H2O for Ru(II)-
MEPEs) on an ITO slide with an area of ca. 25 mm × 8 mm, followed
by slowly evaporating the solvent and drying at 50 °C for 10 min
(70 °C for Ru(II)-based MEPEs). The amount of MEPEs is ca. 6.3
µmol for FeL1-MEPE-FeL4-MEPE and Ru(II)-MEPEs, 37.5 µmol
for Co(II)-MEPEs, and 25 µmol for FeL4-MEPE.
JA710380A
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J. AM. CHEM. SOC. VOL. 130, NO. 6, 2008 2081