Synthesis of new chiral inducer and preparation of semi-conducting polymer films showing fingerprint structure

We synthesised a rod like shaped new chiral inducer to construct chiral liquid crystal electrolyte solution. Next, electrochemical polymerisation was carried out in the chiral liquid crystal electrolyte solution. The surface morphology of the polymers were observed with polarising optical microscopy. Synthesis of the new chiral inducer and preparation of semi-conducting thin film in the cholesteric liquid crystal electrolyte solution were performed.


INTRODUCTION
Optically active semi-conducting polymers have been prepared by electropolymerisation of achiral monomers in a cholesteric liquid crystal electrolyte solution [1][2][3][4][5][6]. The polymer films thus synthesised show optical activities. This method can be referred to as "asymmetric electrochemical polymerization". Here, an addition of smal amout of chiral compound to achiral nematic LC induces cholesteric liquid crystal as a chiral form of nematic liquid crystal. Individual molecules of cholesteric liquid crystals agregate in helical manner. Cholesteric liquid crystals thus prepared can be used for electrolyte solution for electrochemical polymerisation. The resultant polymer films synthesised in cholesteric liquid crystals show fingerprint structure under polarising optical microscopy, which is very resemble to cholesteric liquid crystal.
In the present study, we synthesise a new type chiral inducer to prepare cholesteric liquid crystal electrolyte solution. Next, electrochemical polymerisation of bis(3,4-ethylene dioxythiophene) are carried out to obtain poly(3,4-ethylenedioxythiophene) (PEDOT), known as a semi-conducting polymer, showing vortex structure. Electrochemical polymerisation with conventional method can not obtain such an ordered structure.

1. Synthesis of chiral inducer
Firstly, 4-hydroxybiphenyl-4'-carboxylic acid was coupled with an opticallyt active octanol ((R)-2-octanol) with the Mitsunobu reaction. The resultant material was dimerisation via Williamson etherification reaction to obtain compound 2S as shown Scheme 1. 1 H NMR (CDCl 3 ,  from tetramethyl silane (TMS) as an internal stadard) measurements evaluated the the proton attached at the stereo genic centres (sextet signal), as shown in Figure 1.
The reaction mixture was extracted with ethyl acetate 3 times. The organic layer was dried over magnesium sulfate and purified by silica gel chromatography (eluent: nhexane/ethyl acetate = 9/1). The solvent was removed under vacuum to afford yellow solid (8.8 mg, 1.4 ).
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2,2-Bis(3,4-ethylnenedioxythiophene) (bis-EDOT).
To a solution of 3,4-ethylenedioxythiophene (2.84 g, 20 mmol) in dried THF (70 mL) was added n-butyllithium (1.6 M in hexane) (12.5 mL, 20 mmol) dropwise over 5 min at -78 °C. After stirring for 1 h at -78 °C, CuCl 2 (2.69 g, 20 mmol) was added in one portion, and the mixture was gradually warmed up to room temperature over 2 h. Water (30 mL) and triethylamine (10 mL) was added.  The mixture was extracted with chloroform and the organic layer was dried over magnesium sulfate. The crude compound was passed through a short plug of silica (neutralized with triethylamine) by using chloroform as an eluent. The chloroform solution

3. Preparation of cholesteric electrolyte solution
n-Hexylcyanobiphenyl (6CB) as an achiral nematic liquid crystal shows Schlieren texture at room temperature, as shown in Figure 1.

μm
International Letters of Chemistry, Physics and Astronomy Vol. 27

Structural colour
Shadow (dark blue)

Polymerisation in cholesteric liquid crystal
Electochemical polymerisation in cholesteric liquid crystals was carried out by applying voltage of 4V to a sandwitch cell charged with the cholesteric electrolyte solution containing the monomer (bis-EDOT) with 0.2 mm Tefron spacer (Scheme 3). After application of voltage for 10 min, the resultant polymer film deposited on anode side was washed and dried [8]. Figure 3 shows polarising optical microscopy image of the resultant polymer. The vortrex structure can be produdced by structural form imprinting from the cholesteric matrix during the polymerisation. The molecular aggergation form transcription process can be referred to as "liquid crystal asymmetric electrochemical polymerisation". Polymerisation in the nematic phase is also possible. However, the polymer prepared in the nematic liquid crystal show no vortex structure. The resultant polymer thus synthesised exhibited selective reflection, according to the periodic structure of the polymer film upon irradiation of white light, as shown in Figure 4. While, the shadow colour is dark blue (transmission colour). This is an evidence that the polymer shows butterflies like structural colour.

CONCLUSIONS
We prepared a new chiral inducer having two stereogenic centers. This molecular form is very good affinity with nematic liquid crystals. Electrochemical polymerisation in the cholesteric liquid crystal electrolyte solution containing the monomer produced a thin polymer film showing vortex structure.
The chirality of the inducer (2S) produced helical structure of cholesteric phase from nematic phase. The resultant polymer transcribed the helical aggregation form of the cholesteric liquid crystal. This is a structural form imprinting from liquid crystals. Molecular imprinting (MIP, molecular level imprinting) may be also occurred in the polymerisation.