Synthesis of a carbazole substituted unusual cobalt(ii)dioxime complex to design conducting polymers with caged metal centers for enhanced optical and electrical properties

Article abstract of DOI:10.1039/d0nj03931d

Conjugated hybrid structures (conductive metallopolymers) designed by incorporating transition metals into conductive polymers can increase the properties of both materials and eliminate the disadvantages of materials for final applications to achieve the desired optical and electrical properties. Here, S,S′-substituted vicinal dithioglyoxime and its mononuclear cobalt(ii) complex have been synthesized and this pseudo macrocyclic Co(ii) complex has been converted to a full macrocycle by the replacement of the hydrogen atoms with BF2+ moieties which act as strong electron-withdrawing units to enhance the stability of the transition metal and to increase the solubility of the macrocycle in common organic solvents. After electrochemical polymerization, polycarbazole based cross-linked conductive metallopolymers with caged metal centers have been achieved. As a consequence of the optical and electrochemical characterization of the unique structure, the highest optical contrast value was obtained among the polycarbazole derivatives in the literature. Electrochemical polymerization and electrochemical properties of the conducting metallopolymer have also been investigated with EQCM studies.

Full text of DOI:10.1039/d0nj03931d

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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DOI: 10.1039/D0NJ03931D
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Synthesis of carbazole substituted unusual cobalt(II)dioxime
complex to design conducting polymer with caged metal centers
for enhanced optical and electrical properties
Received 00th January 20xx,
Accepted 00th January 20xx
Tugba Sogancia, Yasemin Baygub, Nilgün Kabayc, Gökhan Dumlud, Yaşar Gökd, Metin Aka*
DOI: 10.1039/x0xx00000x
Conjugated hybrid structures (conductive metallopolymers) designed by incorporating transition metals into conductive
polymers can increase the properties of both materials and eliminate the disadvantages of materials for final applications
to achieve the desired optical and electrical properties. Here in, S,S’-substituted vicinal dithioglyoxime and its mononuclear
cobalt(II) complex has been synthesized and this pseudo macrocyclic Co(II) complex has been converted to full macrocycle
+
by replacement of the hydrogen atoms with BF2 moieties which act as strong electron-withdrawing units to enhance
stability of the transition metal and to increase solubility of the macrocycle in common organic solvents. After
electrochemical polymerization, polycarbazole based conductive metallopolymer with caged metal centers have been
achieved. As a consequence of the optical and electrochemical characterization of the unique structure, the highest optical
contrast value was obtained among the polycarbazole derivatives in the literature. Electrochemical polymerization and
electrochemical properties of the conducting metallopolymer have been also investigated with EQCM studies.
conjugated tethers attach the redox active metal centers to
polymerizable monomer units15–19. In this strategy, the metal
centers involve as a side group in the polymer backbone. In
Introduction
Conductive metallopolymers designed by incorporation of
transition metals in conductive polymers have the potential to
greatly expand the function of conjugated systems1,2. These
organic-inorganic hybrid structures obtained with a synergistic
combination of metals and conducting polymers can increase
properties of both materials and eliminate their disadvantages
to obtain desired optical and electrical properties of the
materials for ultimate applications3–5. Therefore, the strong
interaction between polymer and metal components creates
distinct electro-chemical, photo-chemical and photo-physical
properties, which make these conducting polymers potential
materials for many applications such as sensors,
electroluminescence, nonlinear optics, solar energy conversion
and smart devices6–10. It is important to note that the tuning of
optical and electrical properties of conducting polymer is highly
dependent on the nature of the bridging ligand and metal
centers. There exists a diverse group of hybrid systems that
incorporate transition metals into conducting polymer
structures11–14. In the literature, conductive metallopolymers
have been designed with two different strategies3. In the first
strategy emphasized as outer sphere arrangement, non-
another strategy called inner sphere arrangement, the metal
center is located directly on the polymer backbone20–22. A
variety of ligands such as bipyridyl and salen have been used for
incorporation of metals in to conducting polymers23,24
.
Although oximes especially vicinal dioximes have been
commonly utilized as ligand for preparation of many metal
complex, any study related to a metallopolymer synthesized by
using metal complexes of these ligands has not been found in
the literature. Vicinal dioximes are amphoteric chelatic ligands
due to the presence of mildly acidic hydroxyl and slightly basic
azomethine groups that form corrin-type square-planar,
square-pyramidal and octahedral complexes with some
transition metal cations 25. The loss of a proton from each
dioxime unit further stabilies due to the subsequent
intramolecular hydrogen bonds to afford a 14-membered
“pseudomacrocycle”. Bis(dioximato) metal (II) complexes are
basically insoluble in a lot of organic solvents and water because
of the formation of strong hydrogen bonds. These kinds of
complexes could be easily converted to full macrocycles by
replacement of the bridging hydrogen atoms with an
electrophilic group such as BF2+ moieties 26. These electrophilic
+
BF2 groups act as a strong electron-withdrawing unit. This
a.
formation increased stability of the lower oxidation states of
transition metals and solubilities in common organic solvents.
BF2-capped dioxime cobalt complexes are able to catalyze the
reduction of strong acids27. Because of the high stability of the
vicinal dioxime complexes have been extensively used for
various purposes, including metal analysis, a non-biological
Graduate School of Sciences, Department of Advanced Technologies, Eskisehir
Technical University, Eskisehir, Turkey
b.Tavas Vocational High School, Pamukkale University, Denizli, Turkey
c. Department of Biomedical Engineering, Technology Faculty, Pamukkale
University, Denizli, Turkey
d.Department of Chemical Engineering, Engineering Faculty, Usak University, Uşak,
Turkey
Electronic Supplementary Information (ESI) available: [1H-NMR, 13C-NMR, FTIR and
MS spectra of the chemicals are provided in ESI]. See DOI: 10.1039/x0xx00000x
mimic of coenzyme B12, hydrogen production etc28–35
.
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In this study S,S’-substituted vicinal dithioglyoxime has been MHz, CDCl3) δ: 9.22 (s, 2H, N-OH, D2O-exchangeabVleie)w, A8r.t0ic2le-O7n.l9in9e
synthesized from 6-(9H-carbazol-9-yl)hexane-1-thiol and (d, 4H, Ar-H), 7.38 (d, 4H, Ar-H), 7.31 (m, 4H,Ar-H), 7.14 m,
DOI: 10.1039/D0NJ03931D
cyanogen di-N-oxide. Mononuclear cobalt(II) complex of 4H,Ar-H), 4.18 (t, 4H,NCH2), 2.68 (t, 4H,SCH2), 1.77 (m, 4H,CH2),
dioxime with a metal:ligand ratio of 1:2 has been isolated and 1.61 (m, 4H,CH2), 1.46 (m, 4H,CH2), 1.25 (m, 4H, CH2) (Figure S1
this pseudo macrocyclic Co(II) complex has been converted to and Figure S2). 13C NMR (100 MHz, CDCl3) δ: 149.47, 140.60,
+
full macrocycle by replacement of the hydrogen atoms with BF2
125.84, 123.01, 120.60, 118.98, 108.87, 43.07, 31.18, 29.80,
moieties which act as strong electron-withdrawing units to 29.01, 28.48, 26.89 (Figure S3). [M]+. Calculated elemental
enhance stability of the transition metal and to increase analysis for C38H42N4S2O2: C,70.12; H,6.50; N,8.91. Found:
solubility of the macrocycle in common organic solvents. After C,70.31; H,6.32; N,8.44 MS (ESI) m/z : 650.639 (Figure S4). FT-IR
electrochemical polymerization the first polycarbazole based (ATR, cm-1): 3221, 3060, 3008, 2971-2855, 1626, 1594, 1483,
conductive metallopolymer with caged metal centers have 1451, 1326, 1215, 969, 885, 747 (Figure S5).
been obtained. As a result of the optical and electrochemical
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Preparation of cobalt(II) complex [Co(HL)2]
characterization of the unique structure, the highest optical
contrast value was obtained among the polycarbazole
derivatives in the literature. Electrochemical polymerization
and electrochemical properties of the conducting
metallopolymer have been also investigated with EQCM
studies.
CoCl2. 6H2O (0.059 g, 0.25 mmol) in 10 mL absolute ethanol was
dropwise added to the 25 mL solution of dioxime (0.325 g, 0.5
mmol) in absolute ethanol stirred at 60 °C under argon
atmosphere. A color change and pH decrease of the reaction
mixture was observed (pH = 1.01). pH value of the mixture was
increased with KOH (0.1 M) in ethanol to pH=4.40 which the
complex started to precipitate. After that the mixture was
stirred at 65 °C for a further 4 h, and during this period
precipitation of complex was completed. The brown product
was filtered and washed cold ethanol and then dried under
P4O10 in vacuum. Yield: 0.328 g (96.75%), m.p. 242 °C (dec). FT-
IR (ATR, cm-1): 3048, 3016, 2927-2851, 1768, 1626, 1595, 1483,
1451, 1324, 1213, 1151, 1120, 1020,1000, 920, 883 (Figure S6).
MALDI/TOF (MS) m/z: 648.913 [M-LH2-Co]+. Calculated
elemental analysis for C76H 82N8S4O4Co: C, 67.18; H, 6.08; N,
8.25; Co, 4.34. Found: C, 67.35; H, 5.88; N, 8.47; Co, 4.58 (Figure
S7).
Experimental
Materials and Methods
1H and 13C NMR spectra were recorded on an Agilent-400
spectrometer using TMS as internal reference. FT-IR and UV-vis
spectra were recorded with on
a Perkin-Elmer UATR
spectrometer and on a Shimadzu UV-1601 spectrometer using
1 cm path length cuvettes at room temperature. Mass spectra
were performed on a Bruker MicroTOF ESI/MS or Bruker
microflex LT MALDI-TOF spectrometers. Elemental analysis of
compounds was determined on a Costech ECS 4010 instrument.
Cobalt contents of the complexes have been measured on
Perkin–Elmer Analyst 700 Atomic Absorption Spectrometer.
Melting points were determined on an electro thermal melting
point apparatus in a sealed capillary and are uncorrected. All
reagents and solvents were purchased from commercial
suppliers and solvents were dried over standard drying agents.
(E, E) dichloroglyoxime36, cyanogen-di-N-oxide37 and 6-(9H-
carbazol-9-yl)hexane-1-thiol19 were prepared according to the
reported procedures.
+
Preparation BF2 -capped cobalt(II) complex [Co(LBF2)2]
Under argon atmosphere 30 mL solution of Cobalt(II) complex
(0.271 g, 0.2 mmol) in distilled acetonitrile and
dichloromethane mixture (2:1, v,v) was stirred and heated at 45
°C. Boron trifluoride diethyl etherate complex (0.079 g, 0.5
mmol) was dropwise added to this solution, which immediately
changed color to red. The mixture was heated and stirred at
above temperature for 1h. After removal the solvent under
reduced pressure and the crude product was dissolved in 12 mL
mixture of acetonitrile and dichloromethane (2:1) and the
solvent was removed under reduced pressure. The red crude
product was washed with acetonitrile, cold ethanol and then
dried over P4O10. In order to obtain pure brown solid product,
the dark brown product was purified by column
chromatography [silica gel (THF:hexane)(2:1)]. Yield: 0.21 g
(52.41%), m.p. 121 °C dec.). FT-IR (ATR, cm-1): 3049, 3020, 2927-
2854, 1625, 1595, 1483, 1462, 1451, 1325, 1218, 1152, 1120,
1061, 1020, 925, 883, 802 (Figure S8). MALDI/TOF (MS) m/z:
1492.275 [M+K]+, 1337.277 [M-BO3F3]+. Calculated elemental
analysis for C76H80N8S4O4B2F4Co: C, 62.77; H, 5.54; N, 7.71; Co,
4.05: Found: C, 62.50; H, 5.80; N, 7.99; Co, 3.81 (Figure S9).
Synthesis
Preparation of S,S’-bis [(6-(9H-carbazol-9-yl)hexane-1]thioglyoxime
(H2L)
A solution of cyanogen di-N-oxide which was prepared by
treating a suspension of (E, E)-dichloroglyoxime (0.785 g, 5
mmol) in dichloromethane (50 mL) with aqueous solution of
Na2CO3 (25 mL, 0.5 M) at -15 °C was added to a solution of 6-
(9H-carbazol-9-yl)hexane-1-thiol (1.132 g,
4
mmol) in
dichloromethane under argon atmosphere at the same
temperature. The solution was stirred at -15 °C for 24 h, then
allowed to warm up to room temperature. The end of this
period, the solution was evaporated to 20 mL under reduced
pressure and then cooled in a refrigerator at -18 °C overnight,
whereupon the product crystallized from the solution. The
colorless product was filtered off and dried in vacuum over
1
P4O10. Yield: 0.48 g (44.61%), m.p: 178 °C (dec.). H NMR (400
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Electrochemical, Spectroelectrochemical and EQCM Studies
ophosphat, TBP). A three-electrode containing electrochemical
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cell was used. In the system a silver wireD(cOaI:li1b0r.1a0t3e9d/Dfe0NrrJo0c3e9n31eD-
ferrocenium couple) and a platinum wire was used as reference
electrode and counter electrode, respectively. Indium-tin-oxide
coated glass slide (ITO, 0.047 cm2) cleaned by using ultrasonic
bath with water and isopropanol was used as working
electrode. The optimum potential range was determined as -
0.7/1.6 V in cyclic voltammetry studies. The polymer (pCoOxim)
film obtained by electropolymerization method was washed
with DCM to remove any impurities. The scanrate,
spectroelectrochemistry and electrochromic studies were
accomplished with the obtained polymer film.
For spectroelectrochemical and electrochromic studies,
pCoOxim film was accumulated onto ITO surface
potentiostatically at different number of cycles (2,4,6 and 8
cycle) in DCM-TBP solvent-electrolyte couple. Iviumstat
potentiostat– galvanostat and Agilent 8453 model UV–vis
spectrophotometer was combined to analyze opto-
electrochemical properties of the polymer film.
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All electrochemical processes (electropolymerization and
electrochemical characterizations) were performed in a 3 mL
electrolysis cell containing three electrodes (Silver wire as
reference, Platinum wire as counter and ITO coated glass
electrode as working electrode) using Ivium CompactStat
Potentiostat. The pseudo reference electrode (Ag wire) was
externally
calibrated
by
Fc/Fc+
redox
couple.
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Spectroelectrochemical characterizations of the polymer-
coated electrode have been performed by using the
potentiostat and UV-vis Spectrophotometer (Agilent HP8356)
together. EQCM experiment were collected using a Gamry
eQCM 10M instrument coupled with a Gamry Reference 600
potentiostat. Working electrode in EQCM analyses was an Au
deposited on 10 MHz quartz crystals.
Results and Discussion
Spectral analyses
The synthetic route of novel thiohexyl-linked carbazole
substituted vicinal dioxime (H2L) and its cobalt(II) complexes are
given at Scheme 1. Novel vic-dioxime was prepared by the
reaction of 6-(9H-carbazol-9-yl)hexane-1-thiol and cyanogen di-
N-oxide, which was prepared from (E, E)-dichloro glyoxime at -
15 °C via the coupling reaction. Novel vic-dioxime (H2L) was
formed in ca. 44.61% yield as a crystalline product. The
1
structure of H2L was confirmed with a combination of H, 13C
NMR, FT-IR, MS spectral and elemental analysis data (Figure S1,
1
S2, S3, S4). In the H NMR spectrum of compound, there is a
triplet concerning –CH2SH group at δ = 1.32 ppm38. After the
formation of H2L, this resonance disappears and a new signal at
δ = 2.68 ppm which can be assigned to –CH2S- moieties belong
to the carbazole linked hexyl groups. The novel chemical shifts
observed at δ = 9.22 ppm as a singlet after the formation of
dioxime is obvious hydroxyimino protons which can be easily
identified by deuterium exchange39. This singlet indicates an (E,
E) isomeric and S-trans structure for the vicinal dioxime40. This
deuterium exchangeable singlet that correspond to equivalent
–OH protons also indicate the (E, E)- geometric isomer of
dioxime. The desire for coordination with transition metal
catios is the (E, E)-isomer since the hydroxyl moieties are
directed “outwards” thus enabling the donor atoms lone pair
electrons to point “inwards” and coordinate the appropriate
metal cations. The other chemical shifts for the, the aromatic
protons of the carbazole moieties appeared as singlet at δ =
8.22, dublets at δ = 8.02-7.99, 7.40-7.38 and multiplets at δ =
7.31 and 7.14 ppm in 1H NMR spectrum of the H2L. The
expectation aliphatic chemical shifts for CH2 NCH2, SCH2 and
groups were observed at δ = 1.74, 1.61, 1.46, 1.25 as multiplet
and at δ = 4.18, 2.68 ppm as triplet. In the 13C NMR spectrum of
dioxime, the new resonances of the azomethine groups was
observed at δ = 149.47 ppm and this unique chemical shifts also
Scheme 1 Synthesis scheme of the CoOxim [Co(LBF2)2]
Preparation of Cobalt Cage Conducting Polymer
Electrochemical studies were performed with potentiodynamic
scan (cyclic voltammetry) that were recorded by an Ivium
Compactstat potentiostat/galvanostat. The 0.001 mM CoOxim
was dissolved in dichloromethane (DCM) contanining 0.05 M
supporting electrolyte (tetrabutyl ammonium hexafluor
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confirmed (E, E) dioxime form. The appearance of novel
resonances at 3221 and 1626 cm-1 in the FT-IR spectrum of H2L,
concerning with O-H and C=N moieties along with the
disappearance of the –SH stretching vibration are in agreement
with the proposed structure. The ESI mass spectrum of dioxime
exhibits an intense peak at m/z = 650.639 because molecular
ion is in accord with the proposed structure. The Co(II) complex
of the vic-dioxime was prepared in ca. 96.75% yield by treating
cobalt(II) chloride hexahydrate in absolute ethanol with
equivalent amount of H2L in the presence of base (0.10 M KOH
in absolute ethanol) at 60 °C under inert atmosphere. A distinct
lowering in the pH of the reaction mixture was observed during
complexation caused by the deprotonation of the ligand 29,41. In
the FT-IR spectrum of Co(HL)2 complex (Figure S6), the
deformation of O-H stretching and weak bending vibrations due
to the intra-molecular O-H··O bridge at 1768 cm-1 indicate the
formation of pseudo macrocycle complexation. The ESI mass
spectrum of this complex was the intense peak at m/z = 648.913
confirms loss of the ligand and cobalt moieties (Figure S7). The
synthesis of 14-membered macrocyclic BF2- capped cobalt(II)
complex [Co(LBF2)2] was performed by adding boron trifluoride
diethyl ether and a mixture of dry acetonitrile:dichloromethane The charge density of the polymer was calculated as 1.203
solution of the pseudo macrocyclic cobalt(II) complex under mC/cm2 by integrating the CV graph of the obtained p(Cooxim)
argon atmosphere at 45 °C, giving desired macrocyclic complex polymer (Fig 1b). It was observed that the cv plot of the ligand
in ca.72.41% yield. The FT-IR spectrum of this complex (Figure (H2L) was completely different from that of the Cooxim. While
S8) resembles that of the precursor pseudo macrocyclic the onset potential of H2L was observed as 0.89 V, the oxidation
cobalt(II) complex except for the occurrence of novel potential of its polymer was determined as 0.48 V (Fig 1c). It was
characteristic resonances of the BF2+ moieties. The presence of determined that the charge density of p(H2L) calculated from
the new absorptions at 802 and 1061 cm-1 are ascribed to the the CV graph (Fig 1d) is smaller than that of pCoOxim.
B-O and B-F vibrations, respectively. The MALDI-TOF mass
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DOI: 10.1039/D0NJ03931D
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Figure 1 a) Cyclic voltammogram of CoOxim and b) pCoOxim c) Ligand (H2L) and P(H2L)
in 0.05 M TBP/DCM solution at 250 mV/s.
spectrum of this compound did not exhibit a molecular ion Cyclic voltammogram of the conductive polymer film (Figure 2a)
peak; however, peaks at m/z = 1492.275 and 1337.277 were at different scan rates shows that the pCoOxim is electroactive.
observed (Figure S9). The peaks at m/z = 1492.275 [M+K]+ and The nature of the anions in the electrolyte and their diffusion
the other 1337.277 [M-BO3F3]+ could be attributed the into the polymer film as the counter ions are determine the
proposed structure. It is well known that, the solubility of shape of the redox wave and the kinetics of the electron
dioximes and their transition metal complexes in common transfer process. It is also observed from Figure 2a that the
organic solvents such as chloroform, dichloromethane, diethyl oxidation peaks are displaced towards the more positive
ether are very limited 42–47. In this study, synthesized dioxime potential when the scan rate is increased; on the contrary, the
and its BF2-capped Co(II) complex solubility in organic solvents reduction peaks are displaced towards the more negative
such as, chloroform, dichloromethane, diethyl ether are potential. Characteristic of the diffusion-controlled current
exciting features. This result could be attributed the bulky processes has been observed in redox waves of the CV as it
carbazole-linked hexyl chains due to the increase of the follows linear plot of Ip versus square root of scan rate. (Figure
lipophilicity of the synthesized compounds 48
.
2 b, c). The reason for the increase in peak separation with
increasing scan rate is due to decreasing diffusion layer formed
on the working electrode as the scan rate increases. Besides,
the Ipa / Ipc current rate for this polymer film close to 1 showed
that the redox processes were reversible.
Electrochemical Studies
The Fig. 1 shows cyclic voltammograms of the CoOxim,
pCoOxim, H2L and p(H2L) at scan rate of 250 mV/s. Onset
potential of the CoOxim was observed as 0.59 V during the first
cycle of the cyclic voltammetry (Fig 1a). It was observed that the
polymer film formed after the onset potential value was
reduced at 0.02 and 0.5 V peak potential values during the
reverse scan of the first cycle. When analysis of based on the
increasing number of cycles, the increase current peak values
observed at approximately 0.54 V in each cycle was showed that
polymerization actualized on the working electrode surface.
Apart from the electrochemical characterizations of the films,
FTIR characterizations were carried out. The FTIR spectra of the
conductive films obtained as
a
result of the
electropolymerization of the ligand (H2L) and the complex
(CoOxim) are given in the ESI as Figure S10. The observation of
both peaks at 802 and 1063 cm-1 which ascribed to the B-O and
B-F vibrations in the FTIR spectrum of the pCoOxim polymer film
shows caged metal centers remains intact after
electropolymerization of the CoOxim.
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Spectroelectrochemical Studies
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In all practical applications using tDhOinI: 10f.i1l0m3s9,/D0tNhJic0k3n93e1sDs
optimization is very important as the film thickness affects
properties of the material such as optical and electrical
properties. Since the film thickness can easily be adjusted in
electrochemical techniques, film thickness optimization was
carried out by changing the number of scan in cyclic
voltammetry technique. For this purpose, conductive polymer
films with different thickness were obtained on the ITO
electrode by applying a different number of cycles (2, 4, 6, 8, 10
cycles) in cyclic voltammetry and their spectrochemical
properties were examined. After the analysis of the
spectroelectrochemical curves, it has been observed that
pCoOxim film at reduced state showed an absorption bands
with maximum at 305, 315, 308, 305 and 309 nm assigned to
the π-π* electronic transition respectively for each study (2, 4,
6, 8, 10 cycles) (Figure 4). The absorption between 300-315 nm
is thought to belong to the π-π* transitions of polycarbazole
derivatives. Noteworthy, as the number of cycles changed, the
energetic state of the polymer has changed with the arising of a
broad absorption band at the low energy side of the spectra.
This band is thought to be due to the corresponds to the
appearance of new energy levels inside the band gap during the
doping process depending on the amount of polymer that
differs. When the applied potential value reached 1.4 V, a
significant change was observed in the region where bipolaron
bands were formed.
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Figure 2 a) Cyclic voltammetry graphs of the pCoOxim polymer film at different scan
rates, b) scan rate-peak current density graphs of the pCoOxim polymer film, c) the redox
peak current vs. square root of scan rates of the pCoOxim polymer film
Redox stability of conductive polymers is one of the most
important requirements for their ultimate applications. Due to
environmental conditions and repetitive potential applications,
there may be failures in the redox stability of conductive
polymers. Usually, different potentials are applied to the
polymers for a long time with repetitive cyclic voltammetry
technique at determined scanning rate to examine the redox
stability of conductive polymers in monomer free solution.
Long-term redox stability of pCoOxim film in monomer free
solution at scan-rate of 250 mV/s has been shown in Figure 3.
As a result of the redox stability test performed for pCoOxim,
only 16.1 percent loss was observed from the initial charge
density value. This study revealed that synthesized polymer film
with caged metal centers has excellent electrochemical and
optical stability.49–55
Figure 3 . Long term stability of pCoOxim film in monomer free solution at scan-rate of
250 mV/s.
Besides, the absorptions between 658-665 nm belongs the
bipolaron band absorptions of the polymeric film56. While the
location of these bands are maintained with the increasing
potential its intensity its density varied depending on the
number of cycles. The highest absorption value was observed in
polymer film coated with 4 cycles. Also, the electrochromic
colors and corresponding CIELAB color space values of PCoOxim
film have been shown in Figure 5. As the applied potential
increases, the pCoOxim film showed a gradual color changing
starting from bleached (undoped) to green (doped). For
comparison, the spectroelectrochemistry plot of p (H2L) is given
in ESI as Figure S11)
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semi-conductivity depending on the coating amount of the
polymer. Also, the Figure 8, represent the transmittance at the
maximum wavelength for colored and bleached states. The data
of the spectroelectrochemical and electrochromic study are
summarized in Table 1.
View Article Online
DOI: 10.1039/D0NJ03931D
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Figure 4 Spectroelectrochemical investigations of the pCoOxim polymer films coated
with a) 2 cycles, b) 6 cycles, c) 4 cycles, d) 8 cycles and e) 10 cycles.
Figure 6 Long-term chronoabsormetry investigation for polymer synthesized in
different cycle numbers in cyclic voltammetry at 250 mV/s.
Optical contrast values in the reduction and oxidation states of
conductive polymer films obtained different cycle numbers
during CV studies were plotted to determine highest optical
contrast value for specific thickness. It was seen that in Fig 7, as
the film thickness increases, the optical contrast values of the
colored and bleached states of the conductive polymer tend to
decrease. It was determined form Fig 7 that the highest optical
contrast value belongs to the conductive polymer obtained by
cycle number of 4.
Figure 5 Electrochromic colors of the pCoOxim film when applied different potentials
Electrochromism is the ability to switch between the reduced
and oxidized states of polymeric films, which allows the
conductive polymer to be used in many applications. The
square-wave potential step is usually applied to evaluate the
When the literature analyzed within the scope of this study, it
has been determined that studies on oximes are very limited. It
electrochromic switching properties of CPs.
The most
important factors affecting the electrochromic switching time
of conductive polymers are film morphology, electrolyte type,
and the penetration of ions into electroactive centers. For this
reason, in order to evaluate the reversibility process, the
polymer film was switched between its' reduction and oxidation
potential in monomer free solution (Figure 6). When the optical
contrast values of the materials were compared, it was found
that the polymer obtained by coating with 4 cycles had the
highest value. This is thought to be related to the change in
is therefore very difficult to make
a comparison. For
comparison, metal and carbazole containing polymers were
researched and the comparison table (Table 2) is given below.
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synthesis and electrochemical behavior of conductVivieewpAortilcylemOenlrinse.
Recently, EQCM technique has made great contributions to the
DOI: 10.1039/D0NJ03931D
analysis of the growth mechanism in synthesis, doping-
dedoping process and charge transfer properties of conductive
polymers. For this purpose, the electropolymerization of the
CoOxim monomer and the redox behavior of its conductive
polymer (pCoOxim) have been examined with the EQCM
technique.
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Figure 7 Graph of transmittance at coloured and bleached states
Table 1 Electrochemical and optical properties of the pCoOxim
Switching
Time
pCoOxim
Cycle Number
Band Gap
(eV)
%T
Optical Contrast
(s)
2 cycle
4 cycle
6 cycle
8 cycle
10 cycle
3.68
2.86
3.66
3.68
3.87
81.6
86.8
80.0
44.8
56.8
2.5
5.5
4.5
6.0
5.0
Table 2 Comparison of electrochromicl performance of metallopolymer.
Polymer
Switching Time
References
T%
(s)
6.8
5.0
1.96
0.9
1.0
2.9
7.0
5.5
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P(ZnPc)
P(n-ZnPc)
CuPc-PANI
CoPc
Mn(Cl)Pc
Fe(Cl)Pc
Sn(Cl)2Pc
pCoOxim
53
70.5
58
1.5
4.0
7.0
16.0
86.8
60
Our work
EQCM Studies
Piezoelectric
micro-gravimetry
using
quartz
crystal
microbalance (QCM) to monitor mass change with nano-gram
sensitivity is a very useful technique that can be applied to many
areas such as sensor applications, reaction mechanisms and
material structure change investigation. QCM technique is
based on the mechanical oscillation of the piezoelectric material
when exposed to an AC potential and the frequency change of
the oscillation of the piezoelectric material is associated with
mass change on the quartz crystal surface. By coupling
electrochemistry
(EQCM),
electrochemical
reaction
mechanisms can be revealed by comparing the mass changes
on the crystal surface with the increased or consumed charge
change from the redox transformations. Detailed information
about the relative contribution of different anions and cations
in electrochemical processes and the rate of the mass transport
processes can be obtained with the EQCM technique61,62. This
technique is of particular importance in the investigation of the
Figure 8 Cyclic voltammetric deposition of pCoOxim from a solution of 0.001 M CoOxim
and 0.05 M TBP/DCM and b) Simultaneously recorded mass-potential plot. Working
electrode: 10 MHz quartz crystals c) Cyclic voltammogram of 0.001 M pCoOxim and d)
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The mass-time graph of pCoOxim polymer in monomer free solution at 250 mV/s scan
rate.
been obtained. As a result of the optical and electrochemical
characterization of the unique structure, the highest optical
View Article Online
DOI: 10.1039/D0NJ03931D
contrast value was obtained among the polycarbazole
derivatives in the literature. Electrochemical polymerization
and electrochemical properties of the conducting
metallopolymer have been also investigated with EQCM
studies. Thanks to the superior properties of this unique
organic-inorganic hybrid structure obtained with simple organic
reactions, it is obvious that it will have the great potential to be
used in numerous applications such as smart windows, sensor
applications, OLED etc.
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Figure 8 shows the cyclic voltammogram and EQCM responses
to the cyclic voltammetric deposition of pCoOxim at a scan rate
of 250 mV/s in dichloromethane (DCM) solution consist of 0.001
M CoOxim monomer and 0.05 M Tetrabutylammonium
hexafluorophosphate (TBP). Cooxim monomer is electro-
oxidized at potentials positive of about 1.0 V in the first positive
sweep and the simultaneous mass increase at the EQCM
demonstrates the onset potential of the pCoOxim deposition on
the electrode. Since monomer oxidation continues, mass
increase proceeds until the onset potential value is reached in
reverse scan of the CV. Whereas the mass is remains constant
over a certain potential range after the onset potential value, a
decrease is observed after the reduction potential value of the
polymer formed. The reason for the mass decrease can be
considered as the supporting electrolyte and solvent that
removed from reduced polymer structure. When each cycle was
analyzed separately, it was revealed that the amount of
pCoOxim coated on the electrode decreased as the number of
cycles increased. This result can be explained by the fact that
the working electrode has higher conductivity than the coated
polymer. The polymer mass coated on the electrode in each
cycle has been calculated and written on the Fig 8b graph. After
the EQCM characterization of the electrochemical
polymerization process, redox properties of conductive
polymer film obtained on the EQCM electrode has been
investigated in the monomer free electrolyte solution. When
mass changes in repetitive cyclic voltammetry in the electrolyte
solution of the polymer film have been examined by the EQCM
technique, it has been seen that the mass changes remain
approximately constant (Fig.9). This mass change,
approximately 1.14 µg, corresponds to dopant anions taken into
and removed from the polymer structure as a result of the
redox process of the conductive polymer. These studies
revealed that synthesized polymer film with caged metal
centers has excellent electrochemical and optical stability.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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centers was synthesized with a synergistic combination of metal
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Polycarbazole based conductive metallopolymer with caged metal centers have been obtained. As a
result of the optical and electrochemical characterization of the unique structure, the highest optical
contrast value was obtained among the polycarbazole derivatives.