Preparation of Polythiophene Films Showing Optical Activity by Electrochemical Polymerisation in Cholesteric Liquid Crystal Containing Coumarine

We carried out electrochemical polymerisation in a cholesteric liquid crystal electrolyte solution. The polymer film prepared in the cholesteric liquid crystal showed chiropticality even though its monomer is an achiral. The surface morphology of the polymer was observed with polarising optical microscopy. Optical and electric properties were examined by UV-vis optical absorption, circular dichroism spectroscopy, and cyclic voltammetry


INTRODUCTION
Electrochemical polymerisation is one of the effective methods for preparation of conjugated polymer films. Mechanism of the polymerisation is considered to be oxidative reaction.
In general, common solvents such as acetonitrile, sulfuric acid aqueous solution, and propylene carbonate have been employed for electrochemical polymerisation [1][2][3]. Although these isotropic liquids are suitable for electrochemical polymerisation, control of the conformation and morphology of the polymer chains cannot be performed.
Recently, electrochemical polymerisation in liquid crystal has been developed, and the resultant polymer films have the similar form to that of the matrix liquid crystal [4][5].
In this study, we carried out electrochemical polymerisation of an achiral monomer in cholesteric liquid crystal medium containing coumarine. The polymer film shows optical activity, although the monomer is achiral. This indicates cholesteric liquid crystal as chiral solvent for electro-polymerisation can induce chirality of the polymer film.

Preparation of cholesteric electrolyte solution and polymerisation in cholesteric liquid crystal
Cholesteric liquid crystal materials can be obtained by adding chiral inducer to nematic liquid crystal. n-Hexylcyanobiphenyl (6CB, nematic liquid crystal) and an addition of small amount of chiral inducer 2S induces cholesteric liquid crystal. Figure 1 (left) shows polarising optical microscopy image of the cholesteric electrolyte solution containing 6CB, the monomers, and n-tetrabutyl ammonium perchlorate (TBAP). Quantity used; 2S (chiral inducer, 2.96 mg), TBAP (supporting salt, 0.511 mg), terthiophene (monomer, 1.67 mg), coumarine (additive, 1.43 mg) 6CB (nematic liquid crystal, 89.04 mg). This is so called fingerprint texture, which is a typical of cholesteric liquid crystal.
Electochemical polymerisation in cholesteric liquid crystals was carried out by applying voltage of 4 V across indium-tin-oxide (ITO)coated glass cell charged with the cholesteric electrolyte solution containing the monomers (terthiophene) with 0.2 mm Tefron spacer (Scheme 2). After application of voltage for 30 min, the resultant polymer film was washed with hexane and tetrahydrofuran to remove the liquid crytal electrolyte solution. Figure 1 International Letters of Chemistry, Physics and Astronomy Vol. 46 43 (right) shows polarising optical microscopy image of the polymer film. The texture is not similar to that of cholesteric liquid crystal but some morphology may be transcribed from the liquid crystalline medium.

3. Infrared absorption spectroscopy
The polymer, coumarine and chiral inducer (2S) were characterized by infrared absorption spectroscopy (Figure 2). The polymer does not show the signal at around 1710 cm 1 , which is characteristic of C=O vibration.
This suggests that the coumarine molecules and chiral inducer (2S) are nocaptured into the polymer.

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ILCPA Volume 46  Figure 3 shows absorption spectrum of the polymer film in the reduced state. The maximum absorption wavelength is at around 450 nm due to -* transition of the main chain.      Figure 5 shows the cyclic voltammetry of the polymer film on an ITO working electrode versus Ag/Ag + reference electrode with a Pt wire counter electrode immersed in 0.1 M TBAP in acetnitrile at a rate of 100 mVs -1 . This redox behavior is a typical of conducting polymers. On the oxidation process, the peaks around 0.75 and 1.4 V are attributed to the generation of polarons and bipolarons, respectively.

CONCLUSIONS
Electrochemical polymerisation in cholesteric liquid crystal produced the polymer film. The polymer film shows optical activity even though the surface morphology shows no fingerprint texture. This indicates that cholesteric liquid crystal medium induces chirality to the polymer, and cholesteric liquid crystal like fingerprint texture is not essentially required for optical activities. Moreover, if coumarine molecules are captured into polymers, this composite would exhibit fluorescence with electro-chiroptical activity.