Donor-Acceptor-Donor Copolymers with 3,4-Ethylenedioxythiophene Moiety: Electropolymerization and Effect on Optoelectronic and Electrochromic Properties.

Three random copolymers PE- co -M1, PE- co -M2, and PE- co -M3 were obtained by electrochemical polymerization of donor-acceptor-donor monomers M1, M2, and M3 with 3,4-ethylenedioxythiophene moiety, respectively, using a 1:1 molar ratio of the corresponding monomers, to find new properties and a more effective way to control the optoelectronic properties in conjugated system. For comparison purpose, polymers P1, P2, and P3 were prepared from the corresponding monomer units M1-M3, respectively, by electrochemical polymerization. We also present efficient synthesis, characterization, and comparative density functional theory (DFT) calculations of the monomers M1-M3 and polymers P1-P3. Cyclic voltammetry, spectroelectrochemistry, and electrochromic properties of all of the polymers P1-P3 and copolymers PE- co -M1, PE- co -M2, and PE- co -M3 were carried out and a throughout comparison was made. We have shown that electrochemical copolymerization is a powerful strategy to tune the highest occupied molecular orbital energy level, band gap, and color of the copolymer. Thus, this finding clearly indicates that the copolymers show significantly different optoelectronic properties compared to their constituent polymers.

Introduction

Conjugated polymers have gained significant interest due to their potential applications in various fields like organic field-effect transistors,[1] organic light-emitting diodes,[2] electrochromic devices,[3] photovoltaics,[4] electronic displays, supercapacitors, thermoelectric devices,[5] etc. over the past decades.[6,7] Conjugated polymers based on donor–acceptor–donor (DAD) backbone are great exploration as organic semiconductors because they offer tailoring optoelectronic properties by changing the donor and acceptor units. By the appropriate selection of donor and acceptor units, a large variety of electron-rich and electron-deficient polymers can be generated for the stabilization of electron and hole with fine tuning of band gap and energy levels according to requirement.

The optoelectronic properties of DAD polymer can be further effectively tuned by copolymerization of DAD monomer with other building blocks. Recently, copolymers have attracted significant interest due to their tunable physical and chemical properties, namely, band gap, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), optical absorption, morphology, solubility, stability, etc.[8] It is also noted that nowadays a large variety of donor and acceptor units are available for tuning the optoelectronic properties.[9] Among them, thiophene (Th), benzothiadiazole (BTZ), and their derivatives have been the most used chromophores for the synthesis of DAD conjugated polymer. In fact, 3,4-ethylenedioxythiophene (EDOT) and its polymer poly(3,4-ethylenedioxythiophene) (PEDOT) are the most successful building block and conjugate polymer, respectively, used in optoelectronic applications.[10,11]

Development of copolymer not only leads to new materials but also creates interesting new properties. Indeed, the copolymers are significantly different in chemical and physical properties compared to constituting DAD polymer and homopolymer. Generally, DAD conjugated polymer has been prepared from the corresponding DAD monomer by both chemical and electrochemical polymerizations, whereas copolymer was obtained using two or more different comonomers or monomer units. It is noteworthy to mention that electrochemical polymerization has always offered resulting polymers with better and controlled properties like purity, least chemical defects, improved electrical conductivity, etc. It may be mentioned here that the advantage of electrochemical polymerization is that the longer monomer unit requires less number of C–C bond formation to obtain critical polymer length than the shorter monomer unit.[12] Therefore, the resulting polymer from the longer monomer unit has minimum chemical defects due to less number of unnecessary couplings.

Apart from chemical and other copolymerization methods,[13] several studies have been reported for electrochemical copolymerization of EDOT moiety with various units such as thiophene,[14,15] EDOT–methanol,[16] bithiophene,[17] pyrrole,[18−20] aniline,[21] indole,[22,23] carbazole,[24] thieno[3,4-b]thiophene,[25] thieno[3,4-b]pyrazine,[26] etc.[27] For example, Ludwigs and co-workers reported a three-dimensional randomly branched copolymer from EDOT and thiophene units by electrochemical polymerization, followed by optoelectronic properties discussion.[14] Saavedra et al. reported electrochemical copolymerization of EDOT with an EDOT–methanol monomer.[16] Estrany and co-workers synthesized the copolymers obtained from N-methyl pyrrole and EDOT using various concentration ratios and finally studied the electric and electronic properties.[18] Sotzing and co-workers reported electrochemical copolymerization of EDOT and thieno[3,4-b]thiophene in different conditions and studied the stability and doping level.[25] Recently, Zhang and co-workers have reported a cross-linked copolymer by electrochemical polymerization of dithienyl pyrrole derivative with EDOT moiety.[28]

Given the many advantages of copolymers, over the last few decades, significant progress has been made in this area. However, it is surprising that practically nothing is known about the copolymer of EDOT with the DAD monomer unit.[28,29] Although, the DAD polymer has interesting optoelectronic properties for different possible applications.[8,30] According to the recent investigation, regio-irregular polymer synthesized from the unsymmetrical monomer unit enhances the charge-transport properties.[31] Thus, these results created our interest to consider the three unsymmetrical DAD monomer units M1–M3 for electrochemical copolymerization with EDOT moiety (Scheme ).

Scheme 1

Chemical Structures of Monomers M1–M3 and EDOT Used for Electrochemical Polymerization

Furthermore, various conjugated polymers such as polythiophene, polyaniline, polypyrrole, polyselenophene, polycarbazole, DAD polymer, and so forth have been successfully reported for electrochromic applications.[3] Among these PEDOT, DAD polymers and their derivatives have attracted more attention due to their fast switching time, high optical contrast, and persistent reversible optical response.[3] According to the past study performed on conjugated polymers, most of the electrochromic polymers have either red or blue color in their neutral state, whereas very few conjugated systems[32] have been reported in the literature to be a green or other color polymer in the neutral state.

Considering many promising properties of PEDOT and DAD polymers and their derivatives, we have decided to synthesize random copolymers by electrochemical polymerization using a mixture of EDOT and DAD monomers to find new properties and a more effective way to control the optoelectronic properties in conjugated polymers. In this work, we have prepared three random copolymers PE--M1, PE--M2, and PE--M3 by electrochemical polymerization of DAD monomers M1–M3 with EDOT moiety, respectively, using a 1:1 molar ratio of the corresponding monomers. For comparison purpose, polymers P1–P3 have been obtained from the corresponding monomer units by electrochemical polymerization. We also present efficient synthesis, characterization, and comparative density functional theory (DFT) calculations of the monomers M1–M3 and polymers P1–P3. Furthermore, we also report the cyclic voltammetry (CV), spectroelectrochemistry, and electrochromic properties of all polymers P1–P3 and copolymers PE--M1, PE--M2, and PE--M3. These results clearly indicate that the copolymers show significantly different optoelectronic properties compared to their constituent polymers. This investigation presents a general method for electrochemical copolymerization by using a mixture of DAD and EDOT units for making random copolymers.

Results and Discussion

Synthesis of DAD Monomers

The known DAD monomers M1,[33]M2,[34] and M3(34) were synthesized according to the adopted modified literature procedure from commercially available compounds. The fluorine-substituted monomer M1 was obtained by Stille coupling reaction between 2-tributylstannylthiophene and 5-fluoro-4,7-dibromo-2,1,3-benzothiadiazole in the presence of a catalytic amount of tetrakis(triphenylphosphine)palladium(0) [(Ph3P)4Pd] in refluxing toluene in 66% yield as an orange solid (see Scheme S1 in the Supporting Information (SI)). 5-Fluoro-4,7-dibromo-2,1,3-benzothiadiazole was prepared from 1,2-diamino-4-fluorobenzene in two steps.[35] Similarly, the Stille coupling reaction of 4,7-dibromo-thiadiazolo[3,4-c]pyridine with 2-tributylstannylthiophene in the presence of (Ph3P)4Pd in toluene gave unsymmetrical monomer M2 in 60% yield as a red solid (Scheme S2). The unsymmetrical selenium analogue M3 was prepared from 2-tributylstannylthiophene and 4,7-dibromo-selenadiazolo[3,4-c]pyridine by the Stille coupling reaction in 52% yield as a red solid (Scheme S3). 4,7-Dibromo-selenadiazolo[3,4-c]pyridine was synthesized from 2,5-dibromopyridine-3,4-diamine, as shown in Scheme S3.[36]

UV–Vis Absorption Spectroscopy

UV–vis absorption spectra of monomers M1–M3 were measured in dichloromethane. All of the three monomers show two well-defined peaks, as shown in Figure . The peak at around 270–350 nm originated from π–π* transition derived from the main conjugation of backbone, while the lower-energy peak corresponds to the charge transfer from thiophene to acceptor moiety. It may be noted that parent thiophene absorbs at λmax 232 nm and EDOT absorbs at λmax 256 nm. The lower-energy absorption bands of M2 and M3 (λonset of M2 and M3 are 560 and 595 nm, respectively) show bathochromic shift (red shift) compared to M1 (λonset = 515 nm), which may be due to more electron-deficient nature of pyridine ring in M2 and M3 compared to benzene ring in M1. Furthermore, the selenium analogue M3 exhibits red-shifted absorption compared to monomer M2. Previously, it is reported that selenium analogue has a lower band gap compared to sulfur analogue.[37]

Figure 1

Normalized UV–vis absorption spectra of monomers M1–M3 in dichloromethane.

Electropolymerization

The electrochemical polymerization of monomers M1–M3 was performed in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dichloromethane containing 0.01 M monomer concentration by repetitive cycles over the active range of anodic redox potential of the monomers. The oxidation peaks of M1–M3 were initiated to be around ∼1.11, 1.23, and 1.14 V, respectively (Figure a for M1 and Figure S1 for M2 and M3 in the SI).[34] Under repeated cycles, monomers were polymerized to produce insoluble and stable polymer films P1–P3 (Scheme ) on the working electrodes.

Figure 2

Multisweep electropolymerization of monomers: (a) M1, (b) mixture of EDOT and M1 (1:1), (c) mixture of EDOT and M2 (1:1), and (d) mixture of EDOT and M3 (1:1) on glassy carbon electrode using 0.1 M TBAPF6 in dichloromethane.

Scheme 2

Polymers P1–P3 Obtained via Electrochemical Polymerization of M1–M3, Respectively

Following the electrochemical polymerization of the monomers M1–M3, we focused our attention on electrochemical copolymerization using DAD monomers with EDOT moiety. Copolymers PE--M1, PE--M2, and PE--M3 (Scheme ) were prepared by electrochemical polymerization using a 1:1 molar ratio of DAD monomers M1–M3 with EDOT, respectively, having a total monomer concentration of 0.01 M and 0.1 M TBAPF6 as supporting electrolyte in dichloromethane (Figure b–d). In all of the cases under anodic polymerization condition, insoluble and stable polymer films were produced on electrodes. It should be mentioned here that small difference in oxidation potentials between DAD monomers M1–M3 and EDOT is quite feasible to produce equimolecular copolymer.

Scheme 3

Copolymers PE-, PE-, and PE- Obtained via Electrochemical Copolymerization of EDOT with M1–M3, Respectively, Using a 1:1 Molar Ratio

To find more information about the redox behavior of the polymer films, cyclic voltammetry (CV) was studied in a monomer-free solution using TBAPF6/acetonitrile as an electrolyte and solvent system. All of the polymer films P1, P2, P3, PE--M1, PE--M2, and PE--M3 were also investigated under different scan rates in CVs, as presented in Figures and S2. It is observed that the CVs of copolymers PE--M1, PE--M2, and PE--M3 are quite different from those of polymers P1–P3, respectively (Table ). A linear dependence between peak current intensity and scan rate for all of the polymers has been observed (Figure S3 in the SI). This linear relationship suggests that the electrochemical behaviors of the films are non-diffusion-controlled. The ratio of ipa to ipc suggests the pseudo behavior of polymers. This might be due to slow interfacial transfer of electrons and slow transition between oxidized and neutral states. Moreover, all of the polymers films are stable under different scan rates and show good adherence behavior on the electrode surface. Further, the stability during cycling and switching is also a very important characteristic of such type of polymers for possible commercial applications. The electrochemical stabilities of the polymer films P1, P2, P3, PE--M1, PE--M2, and PE--M3 were investigated by repeated cycles between neutral and oxidized states at a scan rate of 200 mV s–1, and the films are found to be reasonably stable (Figure S4). The introduction of the EDOT unit in the copolymers improved the stabilities of PE--M1, PE--M2, and PE--M3 compared to the polymers P1–P3. It may be mentioned here that most of the charge losses occurred during the initial cycles (i.e., between 1st and 50th cycles), whereas no significant charge loss has been observed between 51th and 100th cycles. Thus, the stability of the films is comparable to the previously reported stability of the other DAD polymers.[38]

Figure 3

CV of (a) P1, (b) PE-, (c) PE-, and (d) PE- films in monomer-free solution using 0.1 M TBAPF6 in acetonitrile at different scan rates.

Table 1

Electrochemical and Optical Properties of Polymers from CV and Spectroelectrochemical Measurements

polymers	Eonset (V)	EHOMO (eV)a	ELUMO (eV)b	λonset (nm)	Eg,opt. (eV)c
P1	0.52	–4.92	–3.28	756	1.64
P2	0.78	–5.18	–3.55	761	1.63
P3	0.69	–5.09	–3.52	792	1.57
PE-co-M1	–0.55	–3.85	–2.30	798	1.55
PE-co-M2	–0.39	–4.01	–2.42	782	1.59
PE-co-M3	–0.57	–3.83	–2.34	835	1.49

Experimental HOMO energy levels were calculated from the onset of the oxidation peaks in CV of the polymer in monomer-free solution.

LUMO energy levels were obtained from HOMO (from the onset of CV) and optical band gap using LUMO = HOMO + Eg,opt (eV).

Optical band gap was calculated from the onset of the absorption spectra in the neutral state.

Spectroelectrochemistry

Spectroelectrochemical studies of the electrochemically prepared polymer films P1, P2, P3, PE--M1, PE--M2, and PE--M3 were carried out to investigate the electronic properties, band gap, doping level, and the effect of copolymerization.[34] All of the spectroelectrochemical measurements were recorded in acetonitrile using 0.1 M TBAPF6 as electrolyte by applying different potentials from initially neutral to oxidized state. The spectroelectrochemical studies of P1, PE--M1, PE--M2, and PE--M3 obtained on indium tin oxide (ITO)-coated glass are shown in Figure , while P2 and P3 films are presented in Figure S5.

Figure 4

Spectroelectrochemistry of (a) P1, (b) PE-, (c) PE-, and (d) PE- on ITO-coated glass in monomer-free solution using 0.1 M TBAPF6 in acetonitrile at different applied potentials.

The optical band gaps of the polymers calculated from the onset of the absorption spectra are shown in Table . The optical band gaps of electron-deficient pyridine ring-based polymers P2 and P3 are 1.63 and 1.57 eV, respectively, which are comparable to the reported values (slight difference might be due to the point of comparison being from the onset of absorption spectra).[34] The optical band gap of P3 is slightly lower than that of P2 because of the presence of Se atom in the peripheral ring in P3. It should be noted that previously the Se-containing polymers have a slightly lower band gap than the S-containing polymers.[37] The optical band gap of benzene ring-based polymer P1 (1.64 eV) is comparable to that of P2. The optical band gaps of all of the copolymers PE--M1, PE--M2, and PE--M3 are slightly lower compared to the band gaps of constituting polymers P1–P3, respectively. As shown in spectroelectrochemistry of all of the polymers and copolymers (Figures and S5), a series of spectra were collected at various potentials ranging from neutral state to oxidized state. All of the films in the neutral state exhibit a wide range of colors, as shown in Figure S6. As electrochemical doping of the polymers increases by applying potential, the strong absorption peak at about ∼550 nm gradually decreases and generates polarons and bipolarons at the near-IR region. Importantly, few polymers change their color significantly from neutral state to oxidized state.

Electrochromic Properties

The electrochromic behaviors of the polymer films P1, P2, P3, PE--M1, PE--M2, and PE--M3 were explored using the chronoamperometric technique to investigate the switching time, optical contrasts, and transmittance change in both visible and near-IR regions. The polymers show good optical contrasts in both visible and near-IR regions. The electrochromic properties for the polymers are summarized in Table S1. For example, the optical contrasts for polymer P1 were calculated as 11.4% at 382 nm, 15.3% at 582 nm, and 24.8% at 1100 nm, whereas P2 shows slightly lower optical contrasts (Figure a,b). However, we could not find relevant data for P3 as the transmittance was very poor and no redox-stable species was found after few cycles. Similarly, optical contrasts of copolymers PE--M1, PE--M2, and PE--M3 are shown in Figure c–e. Interestingly, PE- and PE--M3 show significant transmittance changes between their redox states in both visible and IR regions. The coloration efficiency was also calculated for all of the polymers, which show maximum values for PE--M2 (99.92 cm2 C–1 at 548 nm and 69.23 cm2 C–1 at 1100 nm). The electropolymerized polymers exhibit a short response time (<1 s) in several reduction and oxidation processes. We found that in general, reduction process shows comparatively less response time than their oxidation (Table S1). In particular, P1 obtained its 95% of optical contrast in 0.83 s at 382 nm during the reduction process. P2 also shows less response times of 1.11 and 0.31 s in the visible region and 0.57 and 0.26 s in the IR region during its oxidation and reduction processes, respectively (for 95% of the optical contrasts). Likely, copolymers also show good response time for both the reduction and oxidation processes. The Coulomb efficiency for all of the polymers (except P3, no suitable data could be generated) was found to be close to 100% for many cycles. This predicts that the charge consumed in the oxidation process is almost the same as its ejection during reduction. These data indicate that the redox process and the color variation occur completely and reversibly. The electrochromic properties analyzed in this study are promising, and all of the polymers and copolymers may be suitable candidates for the energy-saving window applications.

Figure 5

Transmittance–time profiles of (a) P1, (b) P2, (c) PE-, (d) PE-, and (e) PE- using chronoamperometry (switching time, 5 s).

Computational Study

Chemical structures of monomers M1–M3 were optimized at the B3LYP/6-31G(d) level of theory. The calculated dihedral angle, HOMO, LUMO, and HOMO–LUMO gap for the monomers are presented in Table S2. The optimized structures are highly planar, and the calculated HOMO–LUMO gaps of M1–M3 are in excellent agreement with the experimental optical band gap (Figure S7 and Table S2). Polymers P1–P3 were calculated at the PBC/B3LYP/6-31G(d) level of theory by using the polymer unit cell by attaching two monomer units  anti to each other. Interestingly, the optimized geometries of P1–P3 are planar as shown in Figures S8–S10. The calculated band gaps of P1 (Eg = 1.66 eV), P2 (Eg = 1.52 eV), and P3 (Eg = 1.44 eV) are in excellent agreement with the experimental band gap from the spectroelectrochemistry (the calculated band gap is consistently overestimated by 0.1 eV) (Table ). It is observed that the replacement of S atom with Se atom of the heterocyclic ring in polymer P3 causes slight decrease in the band gap (∼0.1 eV).

Table 2

Calculated (at the PBC/B3LYP/6-31G(d) Level of Theory) HOMO, LUMO, and Band Gaps of the Polymers P1–P3, Together with the Experimental Band Gaps from the Absorption Spectra

polymer	HOMO (eV)	LUMO (eV)	Eg(calc.) (eV)a	Eg(exp.) (eV)b
P1	–4.82	–3.16	1.66	1.64
P2	–4.90	–3.38	1.52	1.63
P3	–4.82	–3.38	1.44	1.57

Band gap calculated from Eg(calc.) = (LUMO – HOMO) eV.

Experimental band gaps from the onset of the absorption spectra in the neutral state.

We have shown that electrochemical copolymerization is a potential strategy to tune the HOMO energy level, band gap, and color of the copolymer. This observation is supported by CV, spectroelectrochemistry, and electrochromic measurement. We can summarize that the copolymer with EDOT moiety has a higher-lying HOMO level and a lower band gap compared to its constituent polymers.

Experimental Section

General Information

Reagents of reagent grade were purchased from either Sigma-Aldrich or Alfa Aesar and used without purification unless noted. In most of the column chromatographic separations, hexane and dichloromethane were used as eluents unless mentioned. Columns were prepared with silica gel (60–230 mesh). 1H NMR spectra were recorded using CDCl3 solvent with tetramethylsilane as the external standard. Mass spectra were taken using a Agilent 6530 Accurate-Mass Q-TOF LC/MS spectrometer. UV–vis–NIR spectra were obtained on a UV-1800 Shimadzu spectrophotometer.

Electrochemistry and Spectroelectrochemistry Measurements

CV spectra were carried out with Nova 1.1 Metrohm AUTOLAB-PGSTAT. TBAPF6 was dried under vacuum. CV measurements were performed with a three-electrode cell using 0.1 M TBAPF6 as a supporting electrolyte in dichloromethane or acetonitrile solvent. Polymer films were obtained on electrodes using dichloromethane as the solvent, and the CV analysis of the polymer films was done in acetonitrile solvent. All of the measurements were performed under a N2 atmosphere, and before each measurement, N2 was purged through the solution for 10 min to deoxygenate the system. A Au wire was used as the counter electrode, glassy carbon electrode (diameter, 2.0 mm) was used as the working electrode, and Ag/Ag+ was used as the reference electrode. Ferrocene/ferrocenium couple in this system was used for external standard. For spectroelectrochemical measurements, polymer films were prepared on ITO-coated glass electrodes (dimension 7 mm × 50 mm × 1.1 mm (T), Rs < 10 Ω sq–1) as the working electrode, the counter electrode was a gold wire, and Ag/Ag+ was used as the reference electrode. The 0.1 M TBAPF6 as an electrolyte was used in acetonitrile. Before examining the optical properties, the polymer films were rinsed with acetonitrile to remove the monomers.

Conclusions

In summary, three random copolymers PE--M1, PE--M2, and PE--M3 based on DAD monomers M1–M3 with EDOT moiety were obtained by electrochemical polymerization, respectively. For comparison purpose, the monomers M1–M3 were electrochemically polymerized to obtain the polymers P1–P3, respectively. We have also presented efficient synthesis, characterization, and comparative DFT calculations of the monomers M1–M3 and polymers P1–P3. A comparative study on CV, spectroelectrochemistry, and the electrochromic properties of all polymers P1–P3 and copolymers PE--M1, PE--M2, and PE--M3 was carried out and it was found that electrochemical copolymerization is a powerful strategy to tune the HOMO level, band gap, and color of the copolymer. It was summarized that the copolymer with EDOT moiety has a higher-lying HOMO level and a lower band gap. Furthermore, copolymers PE--M2 and PE--M3 show better electrochromic behavior than their corresponding polymers P2 and P3; however, in the case of PE--M1, no significant improvement has been found. Further work to extend this copolymerization using other types of donor and acceptor units for optoelectronic applications is in progress in our laboratory.