Monomer blended electrochemical polymerization in chiral liquid crystals to produce electro-optically active copolymer

Abstract Electro-optically active low-bandgap conjugated copolymers were obtained by electrochemical polymerization (EP) in cholesteric liquid crystal (CLC). We prepared CLC electrolyte mixture blends of monomers, and electrochemically polymerized for obtaining conjugated copolymer with optical activity, in which the structure of the CLC is transferred, from achiral monomers on a CLC template. In addition, this copolymer showed a narrow bandgap and a significant red shift of the absorption band compared to the homopolymer. The optical rotatory dispersion (ORD) indicates that the polymer is chiroptically active and that the optical rotation can be tuned via redox. The guest monomers used for the EP in liquid crystal (LC) require solubility in the host LC and a rod-like structure, limiting their structural design. This technique has the advantage that complex synthesis of monomers is no required, and simple composition of monomers can be used to prepare copolymer films in a convenient manner by blending various monomers. This method expands the design range of optically active electrochromic devices based on conjugated polymers with pleochroic properties. Graphical Abstract


Introduction
Conjugated polymers currently receive considerable research interest owing to their characteristic optical, electrochemical, and magnetic properties, all of which can be modulated by rational design and control of their structure. These excellent properties have led to applications in organic thin-film solar cells [1] , electrochromic devices [2] , organic electro-luminescence (EL) devices [3] , and organic transistors. [4] Donor-acceptor (DA) conjugated polymers are of interest for their electrical and optical properties with a low band gap where the charge transfer in the units is spread over the entire main chain via the p-conjugated chain [5,6] Then, low-bandgap conjugated polymers, such as isothianaphthene (ITN) as donor units, have excellent redox properties. However, the synthesis requires an advanced experimental technique for ring closure reactions. [7][8][9] To date, many types of low bandgap monomers have been synthesized. ITN-based polymer exhibited excellent electrochromic properties. [10,11] In addition, among several acceptors, benzo[c] [2,1,3]thiadiazole (BT) is one of the most extensively studied units. BT, which is used to produce high-performance D-A polymers, has contributed significantly to the rapid progress of organic photovoltaics (OPVs) [12] and organic field-effect transistors (OFETs). [13] Liquid crystals (LCs) are one of the most famous self-organizing materials. Electrochemical polymerization (EP) in LC electrolyte solution was developed. [14] It was carried out using several LC phases as nematic liquid crystal (NLC), smectic A (SmA) liquid crystal, and cholesteric liquid crystal (CLC) reaction field. [15][16][17][18][19] CLCs are chiral LCs with one-handed macroscopic helical structures with a twist axis perpendicular to the local director. Our group has energetically developed a method for EP in LCs. EP in CLCs affords various p-conjugated polymer films with CLC-like helical morphologies and optical activity owing to chirality transcription from the CLC structure. These polymer films show fingerprint-like pattern derived from the CLC structure. EP in LCs is a method of synthesizing onehanded helical p-conjugated polymers from achiral monomers.
In this study, we aimed to obtain electro-optically active low-bandgap conjugated copolymers from ITN-BT blend. EP in CLCs requires no chiral modification of the monomer substituents for obtaining optically active polymers. Thus, chiral polymers can be obtained from monomers with simple structures using polymerization in CLCs. First, we prepare two types of simple rod-shaped conjugated monomers, an ITN-based monomer, and a BT-based monomer. Further, the blend of these monomers is prepared for polymerization. Transfer of chirality from the CLC matrix to the resultant polymer is confirmed by surface observation and the circular dichroism (CD) absorption measurements. The redox properties of the polymer films are investigated by the spectroelectrochemical method.
Fourier transform infrared (FT-IR) absorption spectroscopy measurement results of these polymers are shown in Figure 2. The random copolymer P1/P5, consisting of 2T-ITN units and 2T-BT units, showed absorption signals derived from the characteristics of each monomer. Absorption peaks at 457 cm À1 originating from CÀH bending of the four adjacent aromatic H atoms are due to absorption of the 2T-ITN unit, and absorption peaks at 1510 cm À1 derived from C¼N bending of 2,1,3-benzothiazole moiety are due to absorption of the 2T-BT unit. Absorption peaks at 625 cm À1 originating from CÀSÀC bending of the thiophene ring are observed for P1, P5, and P1/P5 as resultant polymers, suggesting P1/P5 consists of 2T-ITN and 2T-BT units. The two monomers are randomly polymerized by mixing two monomers in the CLC electrolyte.
The thickness of polymer films was obtained from the scanning electron microscopy (SEM) images ( Figure S9, SI). The thickness of P1 was 178 nm, the thickness of P5 was 164 nm, and the thickness of P1/P5 was 543 nm. The film of the blend polymer P1/P5 is thicker than the homopolymers P1 and P5 produced by the same condition.     like patterns derived from the helical structure was observed. The black stripes indicate that the conjugated polymer stands perpendicular to the substrate, and the distance between adjacent black stripes is half the helical pitch. The surface exterior of P1/P5 was red color in the reduced state and indigo color in the oxidized state. Similarly, homopolymer P1, P2, P3, P4, P5, and random copolymer P2/P5, P3/P5, P4/P5 also gave polymer films in which the helical structure of the CLC was transferred ( Figure S10, SI). This is because a set of monomers has a rod-like structure and is suitable for transferring the liquid crystal structure.

Optical and electrical properties of polymers
Optical and electrochemical properties of polymers thus prepared in CLC as thin film are summarized in Table 2. The absorption maximum (k max film ) in the reduced state is due to the p-p Ã transition of p-conjugated backbone. The absorption maximum (k max film ) at long wavelengths in the oxidized state is due to the generation of polarons (radical cations) by doping to the polymer. The optical band gap (E g opt ) was estimated from the absorption onset of the thin film in the reduced state. First, the absorption peak derived from the p-p Ã transition of the main chain was 552 nm for P1 and 602 nm for P5, while P1/P5 was 574 nm, indicating intermediate physical properties between P1 and P5 in the optical absorption. In contrast, the absorption of the polaron band of P1/P5 was 827 nm, which was red-shifted compared to P1 and P5. The formation of an ITN and BT structure induced an increase in the degree of delocalization of polaron due to the expansion of the p-conjugated system. In addition, the optical band gap (E g opt ) was estimated by onset absorption wavelength on the low energy side (k onset ), E g opt ¼ 1240/k onset . Then, E g opt of P1/P5 was reduced to 1.41 eV compared to P1 and P5 as single-block polymers ( Figure 4a). Although P1/P5 was introduced to an ITN backbone, no significant red shift in absorption wavelength due to the low-bandgap character of ITN unit was observed. This is because the main chain has a twist structure due to the  transcription of the helical structure by the EP in CLC, resulting small reduction of effective conjugation length. UV-vis absorption spectra of each polymer are shown in Figure S11 (SI). All the random copolymers showed red-shifted k max film and decreased E g opt compared to homopolymers due to the introduction of BT units. The maximum absorption wavelength of P4 containing a thiazole ring was shifted to a shorter wavelength than that of P3. The thiazole ring site of  P4 has an increased dihedral angle with respect to the CPDT plane compared to the thiophene ring site of P3, according to the optimal structure by DFT calculation. This can be due to the decrease in planarity when the 2-positions of the thiazole rings are propagated.
CV analyses with an Ag/Ag þ reference electrode calibrated with the ferrocene/ferrocenium (Fc/Fc þ ) oxidation potential were performed at a scan rate of 50 mV/s. The CV results of P1, P5, and P1/P5 are shown in Figure 4b. The first oxidation wave indicates that the polymer film was electrochemically doped, and polarons (radical and cation) were generated (Figure 4c). At the reduction process, the reduced wave was confirmed, indicating that the films were electrochemically dedoped. The films exhibited repeatable redox behavior in the applied potential range from À0.1 to 1.5 V (vs. Ag/Ag þ ). Then, the highest occupied molecular orbital (HOMO) energy levels of the polymers were calculated using the equation, E HOMO ¼ -(E onset OX1 þ 4.8) eV. The lowest unoccupied molecular orbital (LUMO) energy levels are estimated using the equation, E LUMO ¼ E HOMO þ E g opt . CV result of P1/P5 shows that the HOMO is located at À5.04 eV and that the LUMO is at À3.63 eV, which was estimated as an optical bandgap (E g opt ) of 1.41 eV. Random copolymers with 2T-BT containing benzothiadiazole moieties have shallower HOMO and deeper LUMO compared to their homopolymers. This means that 2T-ITN, 2T-EDOT, 2T-CPDT, and 2Tz-CPDT each form random copolymer with 2T-BT.
DFT calculation results of each monomer unit and comparison of energy levels estimated from experimental data of the polymer film synthesized by EP in CLCs were shown in Figure 5. The energy gap of the experimental data for the polymers is narrow compared to the DFT calculation result for the monomer. An increase in the sequence of the monomer unit for forming the polymer reduces the bandgap. The HOMO-LUMO of 2T-ITN (monomer) is lower than that of 2T-BT. This tendency was observable in P1 and P5, and the HOMO-LUMO energy level of P1 was lower than that of P5. The P1/P5 HOMO was even lower than that of P1. 2T-EDOT, 2T-CPDT, and 2Tz-CPDT also show similar tendency in the HOMO-LUMO values obtained by the DFT calculation and actual measurement.
The redox properties of P1, P5, and P1/P5 films were investigated by the CV measurement at scan rates of 50-300 mV/s in a monomer-free 0.1 M TBAP-acetonitrile solution ( Figure 6). During the oxidation (doping) process, especially for the P1/P5 film, an oxidation signal at 0.575 V (scan rate: 50 mV/s) was observed, and the potential of the oxidation signal gradually increases with scan rate. The oxidation signal is shifted to 0.694 V at a scan rate of 300 mV/s. During the reduction (dedoping) process for the P1/P5 film,    a reduction signal at 0.255 V (scan rate: 50 mV/s) is observed, and the potential of the reduction signal gradually decreased with scan rate. The reduction signal is shifted to 0.00367 V at a scan rate of 300 mV/s. Signal intensities as a function of scan rates in the reduction process were plotted, as shown in Figure 6d. The linearity of intensity change indicates that these polymers show good redox properties.

Electron spin resonance
ESR spectra of the polymers prepared by EP in CLC are shown in Figure 7. As-prepared polymer films were in an oxidized state (doped state) having polarons (radical cations) as the charge carriers. The ESR data for the polymers, such as DH pp , g-value, and spin concentration, are summarized in Figure 9. In-situ circular dichroism (CD) absorption spectra of (a) P3/P5 and (b) P4/P5 films, and in-situ UV-vis absorption spectra of (c) P3/P5 and (d) P4/P5 films in 0.1 M TBAP/acetonitrile solution at different voltages. TBAP: tetrabutylammonium perchlorate. Table 3. From the ESR spectra, the g-values are 2.0041 for P1, 2.00403 for P5, and 2.00415 for P1/P5. The g-value of each polymer is around 2.004, confirming the radical species are derived from polarons. The spin concentrations were 6.61 Â 10 19 spins/g for P1, 3.50 Â 10 19 spins/g for P5, and 9.85 Â 10 19 spins/g for P1/P5. The P1/P5, a blend of two monomers, had the highest spin concentration, suggesting the copolymer has good susceptibility for dopant and stability of polarons in the main chain.

Circular dichroism
CD absorption spectroscopy measurements were carried out for as-prepared films and reduced films ( Figure S13, SI). The reduced film was prepared by treatment with hydrazine vapor for 30 min. In the reduction state, the inflection point of the CD is located at the maximum wavelength of the UV-vis absorption spectrum as p-p Ã transition of the main chain, as the Davydov split pattern, indicating the formation of intra-molecular twisting of the chiral chromosphere. The consistent CD and the UV-vis results indicate that the main chain forms a helical p-stacking structure.
The CD spectrum of the reduced state of the P1 film shows a negative signal at 741 nm and a positive signal at 436 nm (Figure 8). The inflection point coincides with the maximum wavelength of the UV-vis absorption spectrum, which is ca. 550 nm. In the P1 film of oxidation state, a positive signal was observed at 497 nm. The inflection point was ca. 760 nm, coinciding with the maximum wavelength of the UV-vis absorption spectrum derived from the polarons. Therefore, the main chain and the polaron of P1 were found to form a helical p-stacking structure. The polaron is in chiral environment, referred to as chiral charge carrier chiralions. In the same way, the CD spectrum of the reduced state of the P5 film shows a negative signal at 716 nm and a positive signal at 469 nm. In the P5 film of oxidation state, a positive signal was observed at 487 nm. No inflection point was observed in the polaron state due to the wavelength range of the measurement, while the inflection point at around 550 nm was obtained in the reduced state. Furthermore, the CD spectrum of the reduced state of the P1/P5 film shows a negative signal at 818 nm and a positive signal at 487 nm. P1/P5 film in the oxidation state, a positive signal was observed at 628 nm. The copolymer P1/P5, prepared from a blend of equimolar amounts of the ITN-based monomer 2T-ITN and the BT-based monomer 2T-BT, showed different CD spectra from the CD of P1 and P5. This can be due to the fact that occurrence of red shift in the CD spectrum of the reduced state. The reduced-state CD and UVÀvis absorption spectra of homopolymers ( Figure S14(a), SI) and blended copolymers ( Figure S14(b), SI) were summarized. The films of random copolymers exhibited a characteristic CD signals, which are no observable in the homopolymers. In addition, P2 did not show a clear CD signal, while P2/P5 showed a clear Davydov splittype Cotton effect.
The optical rotatory dispersion (ORD) spectroscopy measurement was carried out for the as-prepared films and the reduced films of P1/P5 (Figure 8). The ORD spectrum of the reduced state of the P1/P5 film shows a positive signal at 564 nm. A broad positive band was observed at 730 nm with an isosbestic point at 680 nm in the oxidized state ( Figure S13(j), SI). This result confirms that the copolymer is chiroptically active and that the optical rotation can be tuned via the redox process.

Electrochromic properties
The electrochromic properties of the copolymer films were investigated by in-situ UV-vis absorption spectroscopy at various voltages using a monomer-free 0.1 M TBAPacetonitrile solution. The electrochromic properties of these random copolymers were investigated. Both copolymers exhibited good electrochromic properties. The CD signal of P3/P5 had an inflection point at 472 nm and was greatly modulated in the visible region during the redox process ( Figure 9a). This is a characteristic behavior different from P3 and P5 homopolymers. Moreover, the maximum absorption wavelength in the reduced state is 474 nm. These results indicate that the main chain has a helical structure ( Figure  9c). The absorption band at 800 nm increased with the formation of polarons. At the same time, the CD signal reversed from negative to positive. Positive CD signals were observed at 371 nm and 523 nm for P4/P5 (Figure 9b). The maximum absorption wavelengths at 389 nm and 555 nm corresponding to p-p Ã transitions were observed ( Figure  9d), indicating 2Tz-CPDT backbone and the 2T-BT backbone are copolymerized, forming a film. P2/P5 also showed typical electrochromic behavior. The 482 nm inflection point of the CD signal corresponds to the maximum absorption wavelength of the UV absorption spectrum ( Figure S15, SI). P2 showed no clear CD signal, while P2/P5 showed a clear signal. In addition, by including the EDOT backbone, the contrast ratio is higher than those of other polymer films.
Kinetic tests including double potential step chronoamperometry were performed for P1, P5, and P1/P5. First, long-term continuous cycling by switching the applied potential of P1/P5 showed similar electrochromic stability to P5 with optical contrast (DT) decay of more than 30% after 30 cycles for the polaron bands at 950 nm (Figure 10a). While P1 had an optical contrast (DT) decay of around 10%. Next, the transmittance profile during the redox process recorded by dual-potential step chronoamperometry was shown by applying voltages from 0 to 1.5 V with a switching time of 10 s (Figure 10b). Completion time of color change with the oxidation (t ox ) and reduction (t red ) (90% change of DT) of P1/P5 was required for 2.0 s and 1.7 s, respectively. Then, the response time of P1 was 2.4 s, 3.9 s, and P5 was 3.8 s, 3.0 s. The response time of P1/P5 was faster than those of P1 and P5. Therefore, the copolymer films P1/P5 consisting of the two monomer units had values similar to the lower cycling stability of the homopolymers, and P1/P5 had faster response times than those of the homopolymers. Electrochromic parameters for P1, P5, and P1/P5 are summerized in Table 4.

Conclusion
In this study, EP in CLC was performed to synthesize ITN-BT-based, and several types of conjugated polymers. These films synthesized with CLC electrolyte containing blends of monomers were imprinted with helical structures from the CLC fingerprint-like patterns with unique optical properties. Stabilization of the polaronic state by the extension of the p-conjugated system caused a red-shift in optical absorption wavelength. The bandgap of copolymer P1/P5 decreased compared to those of the homopolymers. ESR spectra confirmed that the radical species in the polymer were polarons. This technique has the advantage of tuning electro-optical activity by the simple blending of monomers for polymerization in LCs without the need for complex synthesis. Applications to a tunable optical rotator based on this system are possible.