Electropolymerization of [2 × 2] grid-type cobalt(II) complex with thiophene substituted dihydrazone ligand

Article abstract of DOI:10.1016/j.electacta.2020.137656

The grid-type complex [Co₄(L1-2H)₄] containing dihydrazone ligand decorated with thiophene rings has been prepared. The complex undergoes oxidative electropolymerization onto ITO electrode to form purple thin film. The morphology of

Full text of DOI:10.1016/j.electacta.2020.137656

Electrochimica Acta 369 (2021) 137656
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Electropolymerization of [2 × 2] grid-type cobalt(II) complex with
thiophene substituted dihydrazone ligand✩
Sergiusz Napierała, Maciej Kubicki, Violetta Patroniak∗, Monika Wałe˛sa-Chorab∗
Faculty of Chemistry, Adam Mickiewicz University in Poznan´, Uniwersytetu Poznan´skiego 8, Poznan´ 61-614, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The grid-type complex [Co4(L1-2H)4] containing dihydrazone ligand decorated with thiophene rings has
been prepared. The complex undergoes oxidative electropolymerization onto ITO electrode to form purple
thin film. The morphology of polymer film on ITO electrode was analyzed by SEM and AFM imagining.
It was found that polymer obtained by electropolymerization method is homogenous and uniform. The
obtained thin film exhibits color transition from purple via violet to blue due to the stepwise reduc-
tion of poly-[CoII4(L1-2H)4] into poly-[CoII2CoI2(L1-2H)4]2− and poly-[CoI4(L1-2H)4]4−. Its electrochromic
properties such as long-term stability coloration efficiency and switching times have been investigated.
Received 4 September 2020
Revised 14 December 2020
Accepted 18 December 2020
Available online 30 December 2020
Keywords:
Grid-type complex
Cobalt(II)
© 2020 Elsevier Ltd. All rights reserved.
Electrochromism
Thiophene
Electropolymerization
1. Introduction
with Schiff base ligands and hydrazones have not been widely in-
vestigated [27].
Grid-type complexes are architectures in which ligands and
metal ions form a rectangular or square array and in which lig-
ands that form nodes are crossed [1]. It has been shown that
grid-type complexes can be deposited on surfaces to give ordered
2D supramolecular arrangements, where single grid units are ad-
dressable within the nanometer regime [2, 3]. Most of the self-
assembled grid-type complexes reported to date contain metal ions
of tetrahedral or octahedral coordination geometry, and examples
of square [n × n] or rectangular [n × m] grids are well docu-
mented [4–9]. They are known to exhibit interesting chemical and
physical properties, for example multielectron stable and reversible
redox potentials in the cyclic voltammogram (CV), [10–12] Fe(II)
and Co(II) grids show multistep spin-crossover (SCO) behavior [13–
16] and lanthanide grid-like entities show single-molecule magnet
(SMM) behavior [17, 18].
Electropolymerization was found to be effective method in for-
mation of thin films of materials [28–30]. Thiophene, [31–34] se-
lenophene [35] and EDOT [26] groups have been used for forma-
tion of thin films of polymeric complexes. These groups are known
to undergo oxidative electropolymerization when positive voltage
is applied to the electrode and obtained in this way thin films have
good adhesion and electrical contact to the electrode surface. Be-
sides electropolymerization, the electrochromic polymers contain-
ing metals can be obtained as a self-assembled coordination poly-
mers and sheets, [36–38] by coordinative layer-by-layer assembly
[39, 40] or thermal polymerization [41]. The color change of elec-
trochromic materials containing metal ions may be caused by a re-
dox reaction of the metal ion and/or the ligand molecule when the
complex contains the electrochromic ligand [42, 43]. Due to the
electronic transitions that occur in polymeric complexes of tran-
sition metal ions which can be classified i.a. as d-d transitions,
metal-to-ligand (MLCT) or ligand-to-metal (LMCT) charge transfers
such materials exhibit interesting electrochromic properties [44–
50]. As it is known that polynuclear character of monomers retains
after polymerization and corresponding polymers exhibit similar
electrochemical properties as the monomers from which they are
built, and such materials can exhibit multielectrochromic proper-
ties, [43] we designed and prepared two grid-type complexes of
cobalt(II) ions with (N2O)2-donor ligands L1 and L2 (Fig. 1).
Transition metal complexes are attracting much attention due
to their application as active materials in organic electronics. [19–
22] Well defined redox properties and intense charge transfer ab-
sorption of transition metal complexes make them promising ma-
terials for their use as electrochromic materials. The major part of
transition metal complexes examined as electrochromic materials
are complexes with oligopyridines [23, 24] or phtalocyjanines, [25,
26] while electrochromic properties of transition metal complexes
One of the complexes is decorated with thiophene rings which
are able to electropolymerization onto electrode surface. It was
found the complex containing ligand L1 undergoes the oxidative
✩
Dedicated to Professor Janusz Jurczak on his 80th birthday.
∗
Corresponding authors.
E-mail address: mchorab@amu.edu.pl (M. Wałe˛sa-Chorab).
https://doi.org/10.1016/j.electacta.2020.137656
0013-4686/© 2020 Elsevier Ltd. All rights reserved.

S. Napierała, M. Kubicki, V. Patroniak et al.
Electrochimica Acta 369 (2021) 137656
using CasaXPS software and the binding energies were standard-
ized using C 1s peak at 285.0 eV.
Electrochemical measurements were done using
a multi-
channel BioLogic VSP potentiostat using anhydrous 0.1M solution
of tetrabutylammonium perchlorate (TBAClO4) in propylene car-
bonate (PC) as a supporting electrolyte. The investigated com-
pounds were dissolved in electrolyte and the solutions were
purged with argon for 20 min to remove the dissolved oxygen. A
platinum electrode was used as a working electrode, platinum wire
as a as the auxiliary electrode and the non-aqueous Ag/Ag+ elec-
trode as a reference electrode. Spectroelectrochemical measure-
ments were done using a multi-channel BioLogic VSP potentio-
stat connected to a Jasco V-770 UV-vis-NIR spectrometer. ITO plate
coated with the layer of poly-1 was used as the working elec-
trode with a platinum wire as the auxiliary electrode and the sil-
ver wire as a pseudoreference electrode. Before polymerization, ITO
plates were cleaned by the sonication in water for 15 min, followed
by the sonication in 2-propanol for 15 min, dried and cleaned by
ozone generation using the Ossila UV-ozone cleaner.
3. Results and discussion
Fig. 1. Structures of ligands L1 and L2.
The ligands L1 containing thiophene substituents has been syn-
thesized in multistep synthesis as outlined in Fig. 2.
electropolymerization during multi-scan cyclic voltammetry and it
is the first example of electropolymerized grid-type complex. The
electrochromic properties of the thin film have been investigated.
5-(thiophen-2-yl)salicylaldehyde
A
has been obtained in
Suzuki-Miyaura coupling reaction between 5-bromosalicylaldehyde
and 2-thienylboronic acid. The reaction was carried out
in degassed THF/water mixture (3:1 v/v) under an ar-
2. Experimantal section
gon atmosphere using potassium carbonate as
a
base,
tetrakis(triphenylphosphine)palladium(0) as a catalyst and tetra-
butylammonium bromide as a phase-transfer catalyst. In such con-
ditions, the compound A has been obtained in high yield of 83%. In
comparison, while using toluene/water mixture as a solvent, no re-
action has been observed. It was probably due to the low solubility
of 2-thienylboronic acid in toluene. 4,6-bis(1-methylhydrazinyl)-
2-phenylpyrimidine D has been obtained according the method
reported previously [54] starting from benzamidine hydrochlo-
ride and diethyl malonate (Figure S1), which in the malonic
ester synthesis in the presence of sodium ethoxide gave 2-
Diffraction data were collected by the ω-scan technique at
130(1) K on Agilent Technologies SuperNova four-circle diffrac-
tometer with Atlas CCD detector and mirror-monochromated CuK
˚
α
radiation (λ=1.54178 A. The data were corrected for Lorentz-
polarization as well as for absorption effects [51]. Precise unit-cell
parameters were determined by a least-squares fit of 3307 reflec-
tions of the highest intensity, chosen from the whole experiment.
The structures were solved with SHELXT-2013 [52] and refined
with the full-matrix least-squares procedure on F2 by SHELXL-2013
[53]. All non-hydrogen atoms were refined anisotropically, hydro-
gen atoms were placed in idealized positions and refined as ‘riding
model’ with isotropic displacement parameters set at 1.2 (1.5 for
methyl or hydroxyl groups) times Ueq of appropriate carrier atoms.
phenylpyrimidine-4,6-diol B. 4,6-Dichloro-2-phenylpyrimidine
C
has been obtained in the reaction of B with phosphoryl chloride
followed by the reaction with methylhydrazine. Finally, the con-
densation of 4,6-bis(1-methylhydrazinyl)-2-phenylpyrimidine
D
Crystal data 2: C104H88Co4N24O8, Mr= 2037.70, tetragonal, I41a,
with twofold molar amount of thiophene-substituted salicylalde-
hyde A carried out in absolute ethanol at 60°C for 24 h under an
argon atmosphere gave ligand L1 as a yellowish powder, which
has been characterized by spectroscopic methods. Ligand L1 has
been further subjected to complexation reactions with cobalt(II)
perchlorate. The reaction was carried out in 1:1 molar ratio in
dichloromethane/acetonitrile (1:1 v/v) mixture for 48 h at room
temperature. During the reaction, the precipitation of complex of
Co(II) 1 occurred and it was isolated by centrifugation followed
by washing with dichloromethane and diethyl ether. The complex
has been characterized by spectroscopic methods and elemental
analysis that confirmed 1:1 metal:ligand stoichiometry in obtained
complexes. Due to the variety of supramolecular structures that
can be formed in complexation reactions of transition metal ions
with pyrimidine-dihydrazone based ligands [55, 56] unequivocal
evidence of the structure of obtained compounds can be sought
by X-ray crystallography. Due to the low solubility of obtained
complex in common organic solvents it was impossible to obtain
single crystal of 1 that could be suitable for the X-ray measure-
ments. Due to this, we attempt to prepare ligand L2 (Fig. S2),
[57] being analogue of L1, which possess exactly the same struc-
ture and coordination donor subunits as L1, but its complexes are
better soluble in organic solvents, such as acetonitrile, what makes
3
˚
˚
˚
a = 20.7736(3) A, c = 23.3695(3) A, V = 10084.9(3) A , Z = 4,
dx= 1.342 gꢀcm-3, μ = 5.616 mm-1, F(000) = 4208. 16087 reflec-
tions collected up to 2ꢀ = 151.2°, 5119 symmetry independent
(Rint= 2.29%), 4517 with I>2σ(I). Final R [I>2σ(I)] = 11.213.74%,
wR2 [I>2σ(I)]
refl.] = 11.45%, S = 1.01, max/min ꢁρ = 0.22/-0.36 eꢀAꢀ
=
10.98%,
R
[all refl.]
=
4.37%, wR2 [all
-3
˚
.
General: Reagents were used without further purification as
supplied from Aldrich or Fluorochem. NMR spectra were run on a
Brucker Ultra 300 MHz or Varian 400MHz spectrometer and were
calibrated against the residual protonated solvent signals (CDCl3, δ
7.24, DMSO 2.50 ppm) and shifts are given in parts per million.
HRMS spectra were recorded on a QTOF mass spectrometer (Im-
pact HD, Brucker) in positive ion mode. ESI mass spectra for ace-
tonitrile solutions ~10−4 M of complexes were measured using a
Waters Micromass ZQ spectrometer. Microanalyses were obtained
using a Vario EL III CHN element analyzer. SEM and AFM images
were done on the scanning electron microscope Quanta 250 FEG,
FEI. XPS spectra were measured using a Specs UHV/XPS/SPM sys-
tem. Al Kα was used as X-ray source. The sample of complex 1 was
measured after deposition on a piece of a conducting carbon tape,
while polymer poly-1 was measured as prepared by electropoly-
merization on ITO-coated glass slide. The spectra were analysed
2

S. Napierała, M. Kubicki, V. Patroniak et al.
Electrochimica Acta 369 (2021) 137656
Fig. 2. Synthetic scheme for the preparation of L1.
Table 1
Relevant geometrical data; only three largest angles around Co1
i
are listed. Symmetry code denotes operation 5/4-y,-3/4+x,1/4-z.
Co1-O1
2.0386(13)
Co1-N8
2.1089(16)
Co1-N11
Co1-N23i
N13i-Co1-O26i
N8-Co1-N23i
2.2346(15)
2.1094(15)
157.02(6)
170.58(6)
Co1-O26i
Co1-N13i
2.0161(14)
2.2346(15)
158.23(5)
O1-Co1-N11
[62, 63] The analysis of the crystal structure of complex 2 showed
that no perchlorate anions exist in the crystal lattice of 2 (Figure
S3). Cobalt(II) ions coordinate with two pyrimidine nitrogen atoms,
two nitrogen atoms from imine group and two oxygen atoms from
salicylaldehyde group. All OH groups of salicylaldehyde moieties
are deprotonated, what leads to formation of neutral Co4(L2-2H)4
supramolecular entities.
The electrochemical properties of grid-type complexes 1 and 2
were measured in three electrode system in degassed and anhy-
drous 0.1M tetrabutylammonium perchlorate (TBAClO4) in propy-
lene carbonate as a supporting electrolyte. The electrochemical
data are summarized in Table 2 and the cyclic voltammograms are
presented in Fig. 5A.
Fig. 3. Anisotropic ellipsoid representation of the complex 2; ellipsoids are drawn
at the 50% probability level, hydrogen atoms are shown as spheres of arbitrary radii.
It was found that complex 1 exhibits two oxidation/reduction
processes at cathodic peak potentials Epc
=
-550 mV and
possible to obtain single crystals suitable for X-ray analysis. As it
is known that the structure of complexes is dependent on many
factors such as type of anion, [58] nature of solvent, [59] presence
of oxygen [60] or metal:ligand stoichiometry, [61] ligand L2 has
been used in complexation reaction with the same transition
metal salt under exactly the same conditions as L1. Single crystal
of complex of L2 with Co(II) 2 suitable for X-ray diffraction anal-
ysis has been obtained by the slow diffusion of diethyl ether into
the acetonitrile solution of the compound. It was found that ligand
L2 in reactions with Co(ClO4)2 forms 4:4 metal:ligand complex
of grid structure. Fig. 3 shows perspective view of the tetrameric
complex 2, as observed in its crystal structure. The complex
molecule is S4-symmetrical, with the four-fold rotoinversion -4
axis running through the middle of the complex. The Co ions are
six-coordinated in a distorted octahedral fashion (Table 1 lists the
relevant bond lengths and angles).
-1270 mV, and associated with them anodic peak potentials at
Epa = -440 mV and -1180 mV respectively. Both of these re-
dox peaks were assigned to the Co(II)/Co(I) reductions of metal-
lic centers in the grid complex. The Co(II)/Co(I) reduction waves
of cobalt(II) complex with ligand L2 were found to be at Epc
=
-390 mV and -1180 mV, which are 160 mV and 90 mV respectively
more positive in comparison to those observed for cobalt(II) com-
plex with ligand L1. This is due to the strong electron-donating
effect of thiophene ring, [64] which displaces the Co(II)/Co(I) po-
tential waves to more negative values in contrast to electron-
withdrawing substituents [65, 66]. The first reduction wave is
associated with the oxidation wave at -290 mV and the pro-
cess was found to be quasi-reversible, while the second reduc-
tion is irreversible. No Co(II)/Co(III) oxidation/reduction waves has
been observed what can be due to the different self-electron-
exchange ability, [67] which has been previously observed for
cobalt complexes with terpyridine ligands [24, 68]. The shift of the
first reduction potential to less negative value in comparison to
Co(II)/Co(I) waves observed for another Co(II) complexes [24, 69]
Cobalt(II) perchlorate has been chosen for complexation reac-
tions due to the fact that perchlorate anion has been found to
act as a template in formation of grid-type complexes and cages.
3

S. Napierała, M. Kubicki, V. Patroniak et al.
Electrochimica Acta 369 (2021) 137656
Table 2
Electrochemical data for Co(II) grids 1 and 2.
Compound
Redox process
Epa
Epc
E1/2
ꢁE
Complex
[Co(II)4]/ [Co(II)2(Co(I))2]
[Co(II)2(Co(I))2]/ [Co(I)4]
[Co(II)4]/ [Co(II)2(Co(I))2]
[Co(II)2(Co(I))2]/ [Co(I)4]
-440 mV
-1180 mV
-290 mV
-
-550 mV
-1270 mV
-390 mV
-1180 mV
-495 mV
-1225 mV
-340 mV
-
110 mV
90 mV
100 mV
-
1
Complex
2
Fig. 4. A) Cyclic voltammetry of cobalt(II) complexes 1 (black) and 2 (red) measured in anhydrous and deaerated 0.1 M TBAClO4 in propylene carbonate as a supporting
electrolyte at scan rate 100 mVꢀs−1. B) Crystal structure of complex 2 showing diagonal distance. C) The scheme showing two-step oxidation/reduction process of grid-type
complexes.
was probably caused by the higher number of metallic centers in
the complex. Such dependence on oxidation/reduction potential on
nuclearity was previously observed for trinuclear Ru(II) complexes
[43]. Although it is possible to stepwisely oxidize grid-type com-
plexes with the separation of four oxidation/reduction waves for all
metallic centers, [13] in the case of Co(II) grid-type complexes with
ligands L1 and L2 only two oxidation/reductions peaks have been
observed. It is due to the cobalt(II) ions lying diagonally are too
far apart to communicate electronically [70]. The distance between
˚
two diagonal metal ions was found to be 9.0952(5)A (Fig. 4B), what
is much larger that the distance measured for e.g. iron(II) grid
complex with pyrazole-bridged ligand or 2,6-bis(8-quinolylamino)-
˚
4-(tert-butyl)phenol (6.291A) [11, 13]. As a result of this, as shown
in Fig. 4C, at first reduction potential two cobalt(II) ions lying di-
agonally are reduced and at the second reduction potential the re-
duction of the other two metal ions has been observed. In the con-
sequence these are two-electron processes and the peak separa-
tions of 90–110 mV indicate the quasi-reversible character of the
processes [71]. When complex 1 was subjected to multiple oxi-
dation/reduction cycles the gradual increase of redox currents at
E1/2 = -495 mV and -1225 mV (Fig. 5) was observed indicating
that the electropolymerization of 1 on the electrode surface occurs
and the amount of deposited polymer was increased with succes-
sive CV scans. As expected, due to the absence the thiophene rings
in complex 2 no electropolymerization was observed.
Fig. 5. Cyclic voltammograms of complex 1 in degassed and anhydrous 0.1M solu-
tion of TBAClO4 in PC at scan speed 100 mVꢀs−1. Insert: the picture of ITO glass
slide coated with thin film of poly-1.
tial, producing the corresponding monomer radical cation (1+•).
The second step involves the coupling of two radical cations to
produce a dihydrodimer dication, which after the loss of two pro-
tons and rearomatization gives a dimer. Formation of insoluble
thin layer of poly-1 proceeds through successive electrochemi-
cal and chemical steps according to a general E(EC)n mechanism
[75].
The mechanism of electropolymerization is similar to those ob-
served for thiophene and its derivatives [72–74]. The monomer is
first oxidized by the application of an appropriate oxidation poten-
4

S. Napierała, M. Kubicki, V. Patroniak et al.
Electrochimica Acta 369 (2021) 137656
Fig. 6. A) CV profiles of poly-1 at a different scan rates; B) Linear dependence of the peak currents on the scan rates of the polymeric film of poly-1.
Fig. 7. A) SEM image of thin film of poly-1 with magnification 3 μm; B) 3D AFM micrograph of poly-1 deposited on ITO electrode; C) AFM cross-section profiles measured
at three different places; D) High-resolution XPS spectra of Co 2p core for poly-1 sample.
The two-step reduction process of metallic centers has been
observed for poly-1, what indicates that the basic electrochemi-
cal properties of the polynuclear monomer are retained after poly-
merization [76]. The half-wave potentials of Co(II)/Co(I) waves ob-
served for poly-1 shifts to more negative values (-525 mV and
-1285 mV respectively) in comparison to monomer 1, what can
be due to higher electron donating character of bithiophene sub-
stituent [77]. To investigate whether the redox process are con-
trolled by diffusion or adsorption the cyclic voltammograms at a
fixed scan rates have been recorded. As seen in Fig. 6 the good lin-
ear relationship of ipa and ipc versus the scan speed has been ob-
served for poly-1 (R2 = 0.9956 and 0.9823 respectively) indicating
that the redox processes are confined on an electrode surface.
In contrast, in case of complex 2 the linear dependence of ipa
and ipc versus the square root of scan speed (v1/2) (R2 = 0.9768
and 0.9904, respectively) has been found, which is characteristic of
the diffusion-controlled electrochemical events (Figure S4).
lated to be 51.66 nm. To measure the average thickness of the film
it was cutted with a blade and the difference between bare ITO
and polymer surface in three different places (Fig. S7A) was mea-
sured using AFM. The measured average film thickness was found
to be ~150 nm.
X-ray photoelectron spectroscopy (XPS) was carried out to in-
vestigate the composition and chemical oxidation states of cobalt
ion in complex 1 and polymer poly-1. XPS survey spectra are
shown in Figure S8 and they contain peaks of the S 2p, C 1s, N
1s, O 1s and Co 2p core levels. Additionally, the absence of Cl 2p
core level confirms that no anions are present in both complex 1
and polymer poly-1. The surface molar ratio of Co:N:S in the poly-
mer sample has been calculated to be 1:2.82:1.08 based on the
XPS measurement, which is consistent with the composition ob-
tained for complex 1, where the Co:N:S molar ratio was found to
be 1:3.14:1.04. The Co 2p spectrum in Fig. 7D can be deconvoluted
into two spin-orbit peaks at the binding energies of 781.48 eV (Co
2p3/2) and 797.03 eV (Co 2p1/2), and the corresponding satellite
peaks at 787.84 eV and 801.94 eV which are consistent with the
presence of Co2+ [78].
The quality of the prepared film was investigated using atomic
force microscopy (AFM) and scanning electron microscopy (SEM).
These measurements were done to investigate the surface struc-
ture of the film and to confirm polymer coverage of the substrate
with the electropolymerization method. The AFM topography im-
age was obtained by the scanning of an area of 3 μm x 3μm. The
SEM and AFM images are shown in Figs. 7 and S5.
The ligands and their corresponding Co(II) complexes were in-
vestigated using UV-Vis spectroscopy in propylene carbonate so-
lution (Figure S9). The absorption bands in the range 300 nm –
400 nm for ligands L1 and L2 and their Co(II) complexes have
been assigned to π-π∗ transitions, [54] while the shoulders in the
ranges 390 nm – 450 nm for complex 2 and 400 nm – 490 nm
for complex 1 are MLCT bands [79]. The bathochromic shift of
MLCT absorption of 1 in comparison to the MLCT band observed
A homogeneous and continuous electrodeposited film of poly-1
was formed on the substrate. The film is uniform throughout the
substrate, with grain sizes below 0.1 μm (Figs. S6 and S7B). The
root mean square (RMS) value of the polymer surface was calcu-
5

S. Napierała, M. Kubicki, V. Patroniak et al.
Electrochimica Acta 369 (2021) 137656
result of reduction of poly-[CoII4(L2-2H)4] into poly-[CoII2CoI2(L2-
2H)4]2−. When potential of -1.4V was applied the reduction into
poly-[CoI4(L2-2H)4]4− occurred what resulted in further increase
of absorption band at ~600 nm and decrease MLCT band at 442
nm. The film exhibited the color changes from purple via violet to
blue based on Co(II)/Co(I) electrochemical process which was sim-
ilar to this observed for another cobalt(II) complexes. [24, 25, 81]
The obtained thin film of poly-1 was further examined in terms
of its electrochromic properties such as long term stability, switch-
ing time, color contrast and coloration efficiency. As it is known
that the stability of the thin films increases with the degree of
cross-linking, [82, 83] due to the complex contains four thiophene
rings it was expected to form highly cross-linked film of good elec-
trochromic stability. The switching characteristics of the film was
investigated by chronoamperometry and the corresponding in situ
transmittance at 600 nm. The stability of the film was investigated
by multiple switching between purple and blue colors by applying
potentials of -1.4V and +0.1V in 30 s intervals and the changes in
transmittance were monitored at 600 nm (Fig. 9A).
Fig. 8. UV-Vis spectra of poly-1 with applied potential of 0V (black), -1.0V (blue)
and -1.4V (red). Insert: Photographs of poly-1 with applied voltage of 0V (left) -1.0V
(middle) and -1.4V (right).
The transmittance difference was 20.7% at the beginning. After
100 oxidation/reduction cycles it decreased to 20.1%, after ~300 ox-
idation/reduction cycles it dropped down to 16.0% and after ~600
cycles it was found to be 10.7%. Another important criterion for
identifying the electrochemical performance of ECDs is the col-
oration efficiency (CE), which represents the change in the optical
density (ꢁOD) per unit charge density, which is the change of the
charge (Q) consumed per unit electrode area during switching and
it can be estimated using following equations:
for complex 2 was due to the increase of conjugation of the ligand
molecule [80]. Because the electropolymerization of 1 onto the ITO
substrate is efficient enough to observe a purple colored film by
the naked eye, the absorption profile of the poly-1 film was an-
alyzed (Figure S10). It was found that the MLCT peak of poly-1
was observed at ~442 nm, red-shifted from the MLCT transition
of the monomer due to extended conjugation after polymerization
[74]. When negative potential was applied to the film the change
of the color of the thin film was observed what was spectroscopi-
cally tracked using UV-Vis spectroscopy (Fig. 8).
CE = ꢁOD/Q
The poly-1 in its neutral state exhibits MLCT band at ~442 nm.
When the negative potential of -1.0V was applied to the film the
decrease in the intensity of the MLCT band was observed and si-
multaneously new absorption band at ~600 nm was formed sim-
ilarly to those observed for other Co(I) species [24]. This was the
ꢁOD = log(Tb/Tc)
where ꢁOD is the optical density of the film, Q is the intercalated
charge per unit electrode area (Cꢀcm−2), Tb and Tc are the values
Fig. 9. A) Electrochromic stability of the film of sample 1 of poly-1 immobilized onto ITO-coated glass slide electrode measured in anhydrous and deaerated 0.1M solution
of TBAClO4 in PC by switching between -1.4V and +0.1V in 30 s intervals and monitored at 600 nm. Insert: Photographs of thin film of poly-1 in its neutral (left) and fully
reduced (right) states; B) Coloration and bleaching times of sample 1 of poly-1; C) Double potential chronoamperometric study of the switching between -1.4V and +0.1V
in 30-s intervals; D) optical density of the poly-1 at 600 nm versus charge density plot. The CE value was evaluated from the slope of the line fitted to the linear region of
the curve (red line) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
6

S. Napierała, M. Kubicki, V. Patroniak et al.
Electrochimica Acta 369 (2021) 137656
Table 3
Declaration of Competing Interest
Electrochromic data of samples of poly-1.
ꢁT% at 600 nm
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
ꢁT% at 600 nm
after 100 cycles
Tc (sec.)
Tb (sec.)
Sample 1
Sample 2
Sample 3
Sample 4
20.7
20.0
20.8
20.45
20.1
17.8
20.0
19.7
19.9 s
20.1 s
19.2 s
19.9 s
7.7 s
7.9 s
7.6 s
7.7 s
Acknowledgments
We thank Ms. Beata Cichowicz for assistance in synthesis of
complex 2. The financial support received from the National Sci-
ence Centre, Poland. Grant No 2016/21/D/ST5/01631 are gratefully
acknowledged.
of transmittance of the film in its bleached and colored states, re-
spectively. Fig. 9D shows the plot of the in situ optical density at
a wavelength of 600 nm versus inserted charge density at a col-
oration potential of -1.4 V. The CE value is extracted as the slope
of the line fitted to the linear region of the curve and it was cal-
culated to be 119.2 cm2/C.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.electacta.2020.137656.
The coloration time (Tc,90) and bleaching time (Tb,90), defined
as the time to reach 90% of the final change in transmittance be-
tween the steady bleached and colored states [84] were found to
be 19.9 s for the coloring and 7.7 s for bleaching processes for
sample 1 (Fig. 9B). The long switching times are probably due to
slow migration of bulk tetrabutylammonium cations through the
polymer layer. To investigate the reproducibility of obtained data
the color contrast over 100 switching cycles and switching times
were measured for three more samples (Samples 2–4 in Table 3) of
poly-1. The samples were prepared potentiodynamic method using
the same concentration of 1 in the supporting electrolyte and the
same parameters of electropolymerization process that were used
for preparation of sample 1, what allowed to obtain samples with
similar amount of deposited electrochromic material (Figure S11).
The electrochromic data of four samples of poly-1 shown in the
Table 3 and Figs. 9 and S12 indicate that the differences in trans-
mittance between two redox states as well as coloring and bleach-
ing times are in case of all prepared samples are similar. This fur-
ther confirm the reproducibility of obtained results.
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