Molecular engineering of head-tail terpyridine-Fe(II) coordination polymers employing alkyl chain linkers toward enhanced electrochromic performance

Article abstract of DOI:10.1016/j.dyepig.2021.109233

For the first time, we demonstrated that employing long alkyl chains as a nonconjugated linker in ditopic ligands is a promising strategy to construct Fe(II) coordination polymers for efficient electrochromics. Three new Fe(II) coordination polymers with ditopic organic ligands, in which two terpyridine moieties were connected by nonconjugated alkylchain linkers were synthesized. The morphology and hydrophobicity of the thin film made from the Fe(II) coordination polymers can be adjusted by changing the length of alkylchain linker. Benefiting from the longer length of the linker, the ditopic terpyridine-Fe coordination polymer FeL2 performed superior electrochromic performance as compared to FeL1 in the same condition, including shorter switch time, lower operation potential, higher optical contrast and coloration efficiency. Remarkably, the elaborate coordination polymer FeL2 exhibited the highest optical contrast up to 76% among the reported electrochromic materials based on Fe-complexes up to date. Due to the hydrophobic property of FeL3 probably benefit from its longer alkyl chain linkers, we carried out the electrochromic switch based on ionic Fe(II) coordination polymers using aqueous electrolyte for the first time.

Full text of DOI:10.1016/j.dyepig.2021.109233

Dyes and Pigments 189 (2021) 109233
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Molecular engineering of head-tail terpyridine-Fe(II) coordination
polymers employing alkyl chain linkers toward enhanced
electrochromic performance
a
b
a,
c
a,c,*
Jieni Xing , Youfeng Yue , Rui Zhang , Jian Liu
a
College of Chemical Engineering, Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals, Nanjing Forestry University, Nanjing, 210037, China
Research Institute for Advanced Electronics and Photonics, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, 305-8565,
b
Tsukuba, Japan
c
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing, 210037, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
For the first time, we demonstrated that employing long alkyl chains as a nonconjugated linker in ditopic ligands
is a promising strategy to construct Fe(II) coordination polymers for efficient electrochromics. Three new Fe(II)
coordination polymers with ditopic organic ligands, in which two terpyridine moieties were connected by
nonconjugated alkylchain linkers were synthesized. The morphology and hydrophobicity of the thin film made
from the Fe(II) coordination polymers can be adjusted by changing the length of alkylchain linker. Benefiting
from the longer length of the linker, the ditopic terpyridine-Fe coordination polymer FeL2 performed superior
electrochromic performance as compared to FeL1 in the same condition, including shorter switch time, lower
operation potential, higher optical contrast and coloration efficiency. Remarkably, the elaborate coordination
polymer FeL2 exhibited the highest optical contrast up to 76% among the reported electrochromic materials
based on Fe-complexes up to date. Due to the hydrophobic property of FeL3 probably benefit from its longer
alkyl chain linkers, we carried out the electrochromic switch based on ionic Fe(II) coordination polymers using
aqueous electrolyte for the first time.
Electrochromic
Fe(II)-Complex
Terpyridine
Alkyl chain linkers
1
. Introduction
Electrochromic materials that exhibit color change under external
ions and/or organic ligands [27–32]. To avoid the intrinsic low per-
formance of electrochromic materials based on small molecular com-
plexes, transition metal coordination polymers that are easy to obtain
stable layers have been developed and intensively investigated [33–37].
Transition metal coordination polymers are typically constructed by
complexation of ditopic organic ligands with d-block metal ions, in
which the metal-to-ligand charge transfer (MLCT) plays a crucial role for
electrochromic behaviour, as the colour responsible for the MLCT can be
regulated in the change of the redox states of the metal ions [38–42]. For
instance, coordination polymers consisted of terpyridine-based ligands
and Fe(II) ion generally proceeded broad absorption in visible region
originated from MLCT transition, and exhibited obvious change to
colorless state under oxidation, which guaranteed potential applications
in smart windows [43–48]. To construct ditopic terpyridine ligands,
conjugated spacers have been frequently introduced between the head-
and tail-terpyridine moieties via C–C coupling reactions [49–52]. Due to
driving voltage, have been received great attention due to their practical
applications in various photoelectronic fields, including colorful dis-
plays, smart windows, information displays, antiglare automotive mir-
rors and electronic paper [1–8]. During the past decades, tremendous
research efforts have been devoted to developing efficient electro-
chromic materials that play a crucial role in electrochromic devices
[
9–15]. So far, there are mainly three kinds of electrochromic materials
categorized by elemental composition, involving inorganic
transition-metal oxides, organic electrochromic materials and transition
metal-complexes [16–26]. Among them, transition metal-complexes
have been considered as promising electrochromic candidates with
combined merits of both organic and inorganic parts, of which the
electrochromic properties can be easily tuned by changing the metal
*
Corresponding author. College of Chemical Engineering, Jiangsu Key Lab of Biomass-based Green Fuels and Chemicals, Nanjing Forestry University, Nanjing,
2
10037, China.
E-mail address: liu.jian@njfu.edu.cn (J. Liu).
https://doi.org/10.1016/j.dyepig.2021.109233
Received 13 November 2020; Received in revised form 30 December 2020; Accepted 8 February 2021
Available online 26 February 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

J. Xing et al.
Dyes and Pigments 189 (2021) 109233
Scheme 1. Molecular structures of the head-tail terpyridine ligands and the formation diagram of the corresponding Fe (II) coordination polymers.
the conjugation throughout the backbone of ditopic ligand, the electron
cloud distribution of the coordination moieties was highly dependent on
the conjugated spacer [53,54]. Consequently, it was difficult to predict
the electrochromic behavior during the molecular engineering of
organic ligands toward enhanced device performance. In addition, the
synthesis of conjugated ditopic ligands was often carried out by C–C
coupling reaction under noble-metal catalysis, and is very difficult when
introducing a spacer with long conjugation length [55]. Therefore, it is
urgent to develop new strategies for the molecular engineering of
ditopic organic ligands for coordination polymer-based electrochromics.
Herein, we proposed an alternative strategy for the design of ditopic
ligands with alkylchain linkers instead of the conventional conjugated
backbone. We speculated that it maybe have the following advantages:
150 mL of 95% ethanol solution, following by the addition of 2-acetyl-
pyridine (6.05 g, 50 mmol). After stirring for 1 h, ammonium hydrox-
ide (60 mL, 25%) was slowly added to the solution and the reaction
◦
mixture was stirred at 50 C overnight under an argon atmosphere.
Evaporation of ethanol under vacuum, the precipitate was collected and
purified by chromatography using dichloromethane/methanol as an
1
eluent to get compound 3 as a white solid (3.09 g, 38%). H NMR (600
MHz, DMSO) δ. 9.92 (s, 1H), 8.76 (d, J = 4.2 Hz, 2H), 8.66 (m, 4H), 8.03
1
3
(m, 2H), 7.79 (d, J = 8.4 Hz, 2H), 7.52 (m, 2H), 6.97 (m, 2H); C NMR
(150 MHz, DMSO) δ 159.42, 155.97, 155.60, 149.77, 137.89, 128.67,
128.38, 124.90, 121.36, 117.51, 116.69.
Preparation of ligand L1. To a suspension of compound 3 (650 mg, 2
mmol) and KOH (336 mg, 6 mmol) in 20 mL of DMF was added 1,4-
dibrombutane (0.12 mL, 1 mmol) dropwise. The resulting mixture was
1
) the microstructure of the assemble thin film can be easy tuned by
◦
adjusting the length of alkylchain linker without increased synthetic
difficulty; 2) the introduction of long alkylchain between head- and tail-
coordination moieties will lead to the increase of free volume of the
polymer, which will facilitate the diffusion of electrolyte and ion for fast
switch; 3) the electron cloud distribution of the coordination ligand is in
regardless with the length of alkylchain spacer, thus it is easy to keep the
color change during the variation of alkylchain spacer, and electro-
chromism can be anticipated; 4) the hydrophobicity of long alkylchain
in ionic coordination polymer may enable its eco-friendly aqueous
electrolyte compatibility. Based on these assumptions, three new
terpyridine-Fe complexes (Scheme 1) have been designed for electro-
chromics, and investigated the relationship between alkylchain linkers
and electrochromic performance.
stirred at 80 C for 24 h. After being cooled to room temperature, the
reaction mixture was poured into water (100 mL), the precipitate was
collected and purified by chromatography using dichloromethane/
methanol as an eluent to get ligand L1 as a white powder (521 mg,
1
74.0%). H NMR (600 MHz, CDCl
3
) δ 8.72 (m, 8H), 8.66(d, J = 7.8 Hz,
4H), 7.87 (m, 8H), 7.34(t, J = 5.4 Hz, 4H), 7.04(d, J = 8.4 Hz, 4H), 4.14
(s, 4H), 2.06 (s, 4H); 13C NMR (150 MHz, CDCl
) δ 159.97, 156.43,
155.85, 149.80, 149.12, 136.85, 130.71, 128.56, 123.75, 121.38,
3
36 6 2
118.29, 114.88, 67.59, 26.04. ESI (m/z): Calculated for C46H N O ,
704.29; Found, 704.30.
2
. Experimental
All chemicals and reagents were used as received from chemical
companies without further purification. Column chromatography was
performed using silica gel as a stationary phase. Cyclic voltammetry
(
CV) was performed on a CHI 760E potentiostat/galvanostat system.
Platinum electrode was used as a working electrode, Ag/AgCl in satu-
rated KNO (aq.) as a reference electrode, and a platinum wire as a
3
counter electrode. UV–Vis spectra were measured in DMF solution or
thin film on ITO glass using UV-3600 Spectrophotometer (SHIMADZU).
Mass spectra were measured on a Shimadzu Biotech matrix-assisted
1
laser desorption ionization (MALDI) mass spectrometer. The H and
1
3
C NMR measurements were performed by a DRX-400 or DRX-600
spectrometer (Bruker BioSpin). SEM images were obtained on a Hita-
chi S-4800 field-emission microscope operated at 5 kV. FT-IR spectra
were recorded using KBr pellets by a PerkinElmer Spectrum Two FT-IR
spectrometer. Contact angels of the coordination polymer thin films
were measured by a Contact angle meter (JC2000D1).
Preparation of compound 3. In a dual-neck flask, 4-hydroxybenzalde-
hyde (3.05 g, 25 mmol) and KOH (4.20 g, 75 mmol) was dissolved in
Scheme. 2. Synthesis routes of three ditopic ligands and the corresponding Fe
II) coordination polymers.
(
2

J. Xing et al.
Dyes and Pigments 189 (2021) 109233
1
7
4
8
2.0%). H NMR (600 MHz, CDCl
3
) δ 8.72 (m, 8H), 8.66(d, J = 7.8 Hz,
H), 7.85 (m, 8H), 7.33 (m, 4H), 7.01 (d, J = 8.4 Hz, 4H), 4.01 (t, J =
.4 Hz, 4H), 1.82 (m, 4H), 1.49 (m, 4H), 1.38 (m, 4H); 1 C NMR (150
3
MHz, CDCl
3
) δ 160.15, 156.43, 155.82, 149.82, 149.11, 136.85, 130.46,
28.49, 123.75, 121.38, 118.25, 114.87, 68.15, 29.59, 29.43, 29.29,
6.07. ESI (m/z): Calculated for C50 , 760.35; Found, 760.35.
Preparation of ligand L3. To a suspension of compound 3 (650 mg, 2
1
2
44 6 2
H N O
mmol) and KOH (336 mg, 6 mmol) in 20 mL of DMF was added 1,12-
dibromododecane (328 mg, 1 mmol) dropwise. The resulting mixture
◦
was stirred at 80 C for 24 h. After being cooled to room temperature,
the reaction mixture was poured into water (100 mL), the precipitate
was collected and purified by chromatography using dichloromethane/
methanol as an eluent to get ligand L3 as a white powder (556 mg,
1
6
4
7
8.0%). H NMR (400 MHz, CDCl
3
) δ 8.71 (m, 8H), 8.66 (d, J = 8.4 Hz,
H), 7.86 (m, 8H), 7.33 (m, 4H), 7.01 (d, J = 9.0 Hz, 4H), 4.02 (t, J =
.2 Hz, 4H), 1.82 (m, 4H), 1.49 (m, 4H), 1.32 (m, 12H); 13C NMR (100
Fig. 1. Absorption spectra of FeL1, FeL2 and FeL3 in dichloromethane and
MHz, CDCl
3
) δ 160.14, 156.43, 155.82, 149.82, 149.10, 136.84, 130.46,
28.49, 123.74, 121.37, 118.24, 114.87, 68.15, 29.59, 29.43, 29.28,
6.07. ESI (m/z): Calculated for C54 , 816.42; Found, 816.41.
Preparation of complex FeL1, FeL2, FeL3. An equimolar amount of
organic ligand L1 and Fe(BF ) ⋅6H O were refluxed in the mixture of 1:1
thin film state.
1
2
52 6 2
H N O
Preparation of ligand L2. To a suspension of compound 3 (650 mg, 2
mmol) and KOH (336 mg, 6 mmol) in 20 mL of DMF was added 1,8-
dibromooctane (0.18 mL, 1 mmol) dropwise. The resulting mixture
4
2
2
dichloromethane and methanol for 24 h. The reaction mixture was
cooled to room temperature and filtered to give a violet solid, which was
further washed successively by ethanol, water, aceton and tetrahydro-
furan to obtain the corresponding complex FeL1 as a tetrafluoroborate.
The same procedure was employed to prepare the coordination
◦
was stirred at 80 C for 24 h. After being cooled to room temperature,
the reaction mixture was poured into water (100 mL), the precipitate
was collected and purified by chromatography using dichloromethane/
methanol as an eluent to get ligand L2 as a white powder (547 mg,
Fig. 2. SEM images (left) and contact angles (right) of FeL1, FeL2 and FeL3 thin films on ITO, respectively.
3

J. Xing et al.
Dyes and Pigments 189 (2021) 109233
4

J. Xing et al.
Dyes and Pigments 189 (2021) 109233
Table 1
a
Optical and electrochromic properties of FeL1, FeL2, FeL3 films. .
Polymers
Operation voltage
V)
Transmittance change (ΔT,
%)
Bleaching time (t
s)
b
,
Coloration time (t
s)
c
,
Charge/discharge amount (Q
d
,
Coloration efficiency (CE,
mC⋅cmꢀ
2
)
cm ⋅C
2
ꢀ 1
)
(
FeL1
FeL2
FeL3
0–1.4 V
0–1.3 V
0–1.2 V
71
76
66
4.1
2.8
1.6
1.0
0.7
0.4
2.47/1.76
2.28/1.51
2.35/1.24
458
481
440
a
All the measurements were performed in three-electrode systems.
polymers FeL2, FeL3 by using L2 and L3 as organic ligands,
investigate the water solubility of these three coordination polymers, we
put powder samples of them in the water, respectively. As shown in
Fig. S17, after few minutes, the color of the water in samples based on
FeL1 and FeL2 become obvious purple, which determined their certain
water solubility, respectively. In contrast, FeL3 exhibited superior
durability in water, which may enable its aqueous electrolyte
compatibility.
respectively.
3
. Results and discussion
As shown in Scheme 2, the synthesis of three terpyridine ligands with
different alkylchain linkers was achieved via alkylation reaction from a
key intermediate 3, which was prepared by the reported method pre-
viously [56,57]. Three Fe(II) complexes in their tetrafluoroborate form
were prepared by coordination reaction between the ditopic terpyridine
Cyclic voltammograms (CVs) of the above-obtained thin films
deposited on ITO glass by spin coating method was carried out in a
three-electrode electrochemical system, in which the polymer films on
ITO as working electrode, Pt wire as the counter electrode, Ag/AgCl as
the reference electrode, respectively. Due to the different water solubi-
lity of these three new coordination polymers, the CV of thin films of
ligands and FeBF
4
⋅6H
2
O in high yield, which were denoted as FeL1,
FeL2, FeL3, respectively. It is note that the target complexes were
washed by a variety of solvents to remove the species with low molec-
ular weight. The structures of desired organic ligands were confirmed by
FeL1 and FeL2 were tested in CH containing 0.1 M tetrabuty-
4
lammonium tetrafluoroborate (Bu NBF ) as a supporting electrolyte,
2
Cl
2
1
13
H and C NMR spectroscopy (Figs. S1–S8) and mass spectrometry. The
4
target Fe(II) coordination polymers were characterized by Fourier
transform infrared spectroscopy (FT-IR). As shown in Figs. S9–S11, all
these three samples exhibited similar signals among the region of
while that of FeL3 was tested in an aqueous electrolyte of 0.1 M lithium
tetrafluoroborate (LiBF ). As shown in Fig. 3, one pair redox waves were
4
ꢀ
1
ꢀ 1
observed under different scan rate from 10 mV s to 100 mV s , which
revealed a reversible redox characteristic of Fe(II)/Fe(III) couple
[58–60]. The anodic (Epa) and cathodic peak potential (Epc) values of
FeL1 and FeL2 are quite same, around +1.2 V and +1.0 V, respectively,
suggesting their same electric cloudy environment of metal ion. The Epa
and Epc values of FeL3 are +0.9 V and +0.7 V, respectively. Further-
more, the values of Epa and Epc are dependent on the scan rate and
increased in a linear and proportional fashion (Fig. S18). The minimum
ꢀ 1
1
000–1600 cm , which were attributed from their similar coordination
ꢀ
moities (terpyridine) and anions (BF
4
). X-ray photoelectron spectros-
copy (XPS) analysis of these three samples were shown in Figs. S12–S13.
The peaks at 193, 285, 400, 685, 708–721 eV indicated that the presence
of B, C, N, F and Fe elements in these complexes, respectively. Moreover,
the thermogravimetric analysis (TGA) results shown in Fig. S14 indi-
cated good thermostability of these complexes.
2
With three desired Fe(II) coordination polymers in hand, we firstly
characterized their absorption spectra in dichloromethane solutions,
respectively. As shown in Fig. 1, FeL1, FeL2 and FeL3 exhibited two
distinct absorption bands: the longer wavelength absorption band in the
visible region (500–650 nm) that can be assigned to an MLCT transition
from the Fe2 ion to terpyridine moiety; the shorter wavelength ab-
mean square errors (R ) was quite close to 1.0, which indicated that the
electrochemical process was controlled by faradic reaction on the sur-
face, implying pseudocapacitive behavior of these materials [61–63].
We then focused on the investigation of the electrochromic proper-
ties of the present coordination polymer FeL1, FeL2 and FeL3. The
UV–Vis absorption spectra and the colour change were monitored dur-
ing the regulation of applied voltage at room temperature. As shown in
Fig. 4, with increase of the working potential gradually, the absorption
intensity around 574 nm corresponding to the typical MLCT band in Fe
(II)-bis-terpyridine complexes decreased clearly and even disappeared at
1.4 V, 1.3 V and 1.2 V for FeL1, FeL2 and FeL3, respectively. Mean-
while, a new absorption peak around 410 nm was observed accompa-
nying the increase of the absorption intensity for each sample, resulting
in the color change from purple to yellow. The electrochromic phe-
nomenon was associated with the oxidation process of metal center in
these new coordination polymers and thus lead to the change of electron
transition. Moreover, FeL1, FeL2, FeL3, even possessing different
linkers, showed nearly the same color change from purple to yellow,
which further demonstrated that the present strategy for molecular
engineering of Fe(II) coordination polymers by employing nonconju-
gated linkers can be easy to design the customized electrochromics.
We further investigated the optical electrochromism by chro-
noamperometry with putting on square-wave potential between 0 and
1.4 V for FeL1, 1.3 V for FeL2, 1.2 V for FeL3, respectively. The trans-
mittance changes around 574 nm were monitored, and the detailed data
were summarized in Table 1. As shown in Fig. 5, the sample FeL1
+
sorption band in the UV region (350–400 nm) is originated from the
π-π*
electron transitions of the ditopic organic ligand. There was no obvious
difference between the absorption spectra of FeL1, FeL2 and FeL3,
which implied that the configuration and electronic structure of their
coordination center should not be affected by the length of non-
conjugated alkylchain linkers. Moreover, thin films of these three new
Fe(II) coordination polymers deposited on ITO via spin-coating process
performed fine absorption peaks similar to those in solution, suggesting
that the negligible intermolecular energy transfer existed in their thin
film states, respectively.
We also investigated the surface morphologies of the as-prepared
coordination polymer films by scanning electron microscopy (SEM)
analysis. As shown in Fig. 2 and Fig. S15, all these samples showed
uniform morphologies with high surface coverage. Different from the
compact thin film of FeL1, FeL2 and FeL3 thin films exhibited obvious
poroid morphologies. The result indicated the microstructures of thin
films of these new type coordination polymers can be tuned by changing
the nonconjugated linkers, while their absorption properties remained
the same (Fig. 1). X-ray diffraction (XRD) characterization of these three
thin films suggested their lower crystallinities (Fig. S16). Moreover, we
studied the hydrophobic properties of thin films of these three Fe(II)
coordination polymers by contact angle measurement. As shown in
Fig. 2, water contact angles of these three samples increased in the order
c
exhibited an optical contrast of 71%, with coloration (t ) and bleaching
b
(t ) time of 4.1 and 1.0 s, respectively. Interestingly, the sample FeL2
performed a high optical contrast of 76%, that is the highest value
among the reported electrochromic materials based on Fe(II) complexes
up to date (Table S1). Moreover, shorter response times in both
◦
◦
◦
of FeL1 (68 ) < FeL2 (79 ) < FeL3 (90 ), associated with the length of
their corresponding alkylchain linkers, respectively. To further
5

J. Xing et al.
Dyes and Pigments 189 (2021) 109233
6

J. Xing et al.
Dyes and Pigments 189 (2021) 109233
coloration and bleach processes were obtained in FeL2 than FeL1 under
the same condition. It suggested that the increase of the length of the
nonconjugated linkers between two coordination moieties may facilitate
the diffusion of electrolyte and/or ion migration. These results indicated
that the present strategy for molecular engineering of Fe(II) coordina-
tion polymers can modulate the microstructure of the thin film and then
improve the electrochromic performance by adjusting the nonconju-
gated alkylchain linkers. On the other hand, due to the superior hy-
drophobicity and excellent durability in water, FeL3 possessed good
compatibility of aqueous electrolyte, and exhibited an optical contrast of
CRediT authorship contribution statement
Jieni Xing: Investigation, Formal analysis, Writing – original draft.
Youfeng Yue: Formal analysis, Writing – review & editing. Rui Zhang:
Project administration, Writing – review & editing. Jian Liu: Concep-
tualization, Writing
acquisition.
–
review & editing, Supervision, Funding
Declaration of competing interest
6
6% under an impulse voltage of 1.2 V, with bleaching time of 1.6 s and
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
coloration time of 0.4 s. It is note that this is the first example of elec-
trochromic material based on Fe(II) complex using an aqueous
electrolyte.
To evaluate the coloration efficiencies of these three new Fe(II) co-
ordination polymers, we employed chrono-absorptometric and chro-
Acknowledgements
d
noamperometric techniques. The charge/discharge amount (Q ) was
This work was supported in part by the Natural Science Foundation
of Jiangsu Province (BK20191385), the National Natural Science
Foundation of China (21631006), and the High Level Talent Project of
Nanjing Forestry University (GXL2018003).
tested by monitoring the change in current with time (Figs. S19–21). The
coloration efficiency (CE) can be calculated by using the following
equations [64–68].
CE = [log (T
b
/ T
c
)] / Q
d
Appendix A. Supplementary data
where T and T
c
b
are the percent transmittance of coloring and bleaching
state at 574 nm, respectively. As summarized in Table 1, CE of these
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109233.
2
three new electrochromic materials were recorded as 458 cm /C for
2
2
FeL1, 481 cm /C for FeL2, 440 cm /C for FeL3, respectively. Finally,
the switch stability of these three Fe(II) coordination polymers in the
corresponding electrolytes were investigated. As shown in Fig. 6, FeL1
and FeL2 films exhibited a little decrease of optical contrast about 5%
after 1100 s, respectively. There is almost no loss of transmittance
change (ΔT) observed in the sample of FeL3 film switching in aqueous
electrolyte, which further demonstrated its excellent durability in water
mainly due to its good hydrophobic property. Moreover, electrochromic
devices based on the present electrochromic materials FeL1, FeL2 and
FeL3 were fabricated, in which an organic gel electrolyte (PMMA:
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