(Digital Presentation) Electrochemical Polymerization of Thiophene in Cholesteric Liquid Crystal with Vitamins
Authors: Miyashita Ryo , Yanagida Kazuki , Goto Hiromasa 
Published: July 2022

Vitamins are natural compounds which is important for biological reactions. Vitamin C (ascorbic acid) is a water-soluble substance, while Vitamin E (tocopherol) is a soluble material in organic solvents. Further, antioxidant action can affect for metabolism in biological system. Vitamin E has three asymmetric centers in the molecular structure. In the liquid crystal technology, nematic liquid crystal is simple and high fluidity similar to isotopic liquid. Addition of small amount of chiral compounds to nematic liquid crystal can induce cholesteric liquid crystal. This is another chiral and helical version of nematic liquid crystal. In this study, we employed vitamin E as a chiral inducer to nematic liquid crystal for formation of helical liquid crystal system. Electrochemical polymerization is a convenient method for preparation of electroactive polymer deposited on an electrode in the form of thin film. Electrochemical polymerization in LC provides some interesting phenomena. First, electrochemical polymerization in liquid crystal allows the resultant to imprint morphology of molecular aggregation structure from molecular level to macroscopic level of liquid crystal. Second, electrochemical polymerization produces electroactive polymer with dynamically color change upon electrochemical redox process, which character can be applied for electrochemical driven display. Third, electrochemical charge and discharge provides voltaic storage function for the resultant polymer. Thus, electrochemical technique and polymer chemistry, such a brilliant combination produces electro-optic character.

Recently, electrochemical polymerization in helical liquid crystal system has been performed. The electrochemical polymerization employs cholesteric liquid crystal as an electrolyte for polymerization. Cholesteric liquid crystal can be prepared by mix of nematic liquid crystal and chiral compounds. We employ synthetic compound as a chiral inducer. Here, we use vitamin E as chiral inducer to nematic liquid crystal preparation of cholesteric liquid crystal electrolyte solution for electrochemical polymerization. Observation of optical texture of the cholesteric liquid crystal electrolyte containing tetrabutylammonium perchlorate (TBAP) as a supporting salt showed fingerprint texture, which is characteristic of cholesteric liquid crystal.

Next, electrochemical polymerization in cholesteric liquid crystal induced by vitamin E was carried out. We employed constant voltage electrochemical polymerization to ITO (indium–tin-oxide) coated glass slides were sandwiched via insulator tetrafluoroethylene as a spacer, and the choleretic liquid crystal electrolyte contained monomer (2,2′-bithiophene) was charged with pipette before application of voltage for electrochemical polymerization. The entire polymerization cell was heated to change the liquid crystal to isotropic phase. Then, gradually cooled to room temperature, and the isotopic phase was changed to cholesteric liquid crystal phase with iridescence. After confirming the electrolyte showed liquid crystal phase, constant voltage of 6 V across the sandwiched cell for 60 min was applied. Then, the polymerization cell was de-assembled and washed wish a large volume of organic solvent to remove the liquid crystal electrolyte solution, oligomer, and residual monomer. The polymer film thus prepared was deposited on cathode. This is quite interesting phenomena because electrochemical polymerization is generally occurred at the anode with oxidation. However, this result indicates that electrochemical polymerization in liquid crystal at cathode is possible. Cathodic electrochemical polymerization has been reported for dibromo aromatic compound or unsaturated aliphatic compounds. However, cathodic electrochemical polymerization of aromatic compounds with no halogen group has been few reported.

The polymer shows electronic optical transition in visible range due to development of p-conjugated system and absorption at long wave lengths due to doping band accompany by generation on polaron (radical cation). The electrochemical character of the resultant polymer was examined using cyclic voltammetry (CV). The CV measurements were achieved in a monomer-free 0.1-M TBAP−acetonitrile solution at the scan rates of 10 mVs−1. The potential was calculated using a reference of silver/silver ion (Ag/Ag+) electrode. The oxidation and reduction signals are observed during the oxidation and reduction processes. The result showed the resultant polymer has good redox property. The resultant polymer shows no CD signal from short to long wavelengths, indicating no optical activity. However, the polymer shows the similar optical texture to that of liquid crystal electrolyte under the polarizing optical microscopy observations, implying the fingerprint texture of the polymer as macroscopic transcription of the liquid crystal structure may not contribute optical active property. Transcription from liquid crystal environment may not always occurs from molecular level to macroscopic level in the polymerization in liquid crystal. The polymer prepared in this study has no hierarchal structure from molecular level to the higher level, indicating macroscopic morphological imprinting in the polymerization is not essential condition for imprinting from the molecular level.