Crystal Surface/Liquid Crystal Interfacial Polymerisation: Preparation of Helical  -Conjugated Polymer on Mineral Crystal

. Crystal surface/liquid crystal interfacial polymerisation was developed. Poly(3,4-ethylenedioxythiophene)


Introduction
Electro-active conjugated polymers show excellent performances, such as electrochromism with beautiful colours [1]. We have developed electrochemical polymerisation in liquid crystal. The electrochemical polymerisation has been carried out between sandwiched indium-tin-oxide (ITO) coated glass electrodes in liquid crystal electrolyte solution containing monomer by applying voltage to give polymers on the ITO, which we originally developed previously [2]. Polymers prepared in liquid crystal shows liquid crystal like order because of transcription of liquid crystal order during the polymerisation. The polymer thus prepared shows fingerprint surface structure and chiroptical activity, which is derived from chiral aggregation of cholesteric liquid crystal. Furthermore, circular dichroism and optical rotation of the resultant polymers can be controlled with electrochemical doping-dedoping. This function is referred to as "electro-chiroptic effect".
In this research, we conducted electrochemical polymerisation on pyrite in place of ITO. Pyrite shows beautiful steps structure on the surface. Pyrite can be used as a functional electrode due to semiconducting property [3].
In the present study, we synthesise poly(3,4-thylenedioxythiophene) (PEDOT) from terEDOT (prepared by Grignard reaction) [5], and poly(EDOT-fluorene-EDOT, EFE) [6,7] in cholesteric liquid crystal on pyrite as a mineral crystal electrode. This is a first report of preparation of helical -conjugated polymers on mineral crystal. Combination of chiroptical-electroactivity and p-n junction between the organic conjugated polymer (p-type semiconductor) and pyrite (n-type semiconductor) has possibility for realization of organic/metal semi-conductor devices having optical activity. This preparation method can be referred to as "crystal surface/liquid crystal interfacial polymerisation".
, 2,7-dibromofluorene (0.70 g, 2.2 mmol) in toluene (7 mL) was stirred for 5 min at rt. Then, Pd(PPh 3 ) 4 (0.050 g, 0.043 mmol) was added to the solution and stirred for 10 h at 80 C. The crude product was purified with column chromatography (CHCl 3 /hexane =2/1). Recrystallisation from CHCl 3 /hexane followed by evaporation, and dry under reduce pressure afforded pale green product. Y = 48% (0.46g, 1.0 mmol). 1 Table 1. 6CB is a matrix liquid crystal showing nematic phase. Addition of small amount of chiral inducer C-Pel to the nematic liquid crystal produces formation of cholesteric liquid crystal having helical structure. This is not phase transition. Fundamentally, cholesteric and nematic liquid crystals are the same category in liquid crystal. Distance between two stripes of cholesteric fingerprint structure corresponds to helical half-pitch (half-turn).

ILCPA Volume 69
Electrochemical oxidative polymerisation was carried out. We employed terEDOT (abbreviated as EEE), and EDOT-fluorene-EDOT (EFE) as monomers. Direct current (dc) voltage was applied across the cell (Fig. 1d). Scheme 2 shows polymerisation reaction. Electrochemical polymerisation occurs at the pyrite surface as an anode. Then, the surface was washed with large volume of methanol, and dried.

Surface structure
Surface observation with circular polarised differential interference contrast microscopy (C-DIM) was carried out. Fig. 4 shows the C-DIM image of PEEE deposited on the pyrite. Fingerprint structure on the surface steps of the pyrite was confirmed, although low magnification image can not indicate it clearly (Fig. 4a). Fig. 5 shows scanning electron microscopy (SEM) image of PEEE. Magnification image evaluates that PEEE shows fine rope-like structure drawing fingerprint structure.

Mechanism
The rope like texture and fingerprint texture are formed through transcription of molecular aggregation of cholesteric liquid crystals during the polymerisation process.
International Letters of Chemistry, Physics and Astronomy Vol. 69 The surface steps structure of the substrate [10] provides mechanical steps of the polymers. Plausible structure for PEEE is shown in Fig. 7. Polymerisation was occurred in the cholesteric liquid crystal matrix with helical structure from anode side of the pyrite. The polymer was propagated with transcription of molecular aggregation form of the helical structure in the cholesteric liquid crystal. Resultant polymer shows helical structure derived from the liquid crystal matrix, and steps structure derived from pyrite surface. Although PEFE shows no rope like structure, fingerprint structure is observable with the C-DIM. Difference of the morphologies between PEEE and PEFE can be derived from difference of crystallinity and molecular packing form.

Conclusions
Electrochemical polymerisation in cholesteric liquid crystal electrolyte solution was successfully carried out on pyrite. The resultant polymers show fingerprint structure on the steps of pyrite in micro-level. This method is combination of liquid crystal transcription polymerisation and crystal surface polymerisation. Note that this report can be a first example of preparation of helical -conjugated polymer with chiroptical activity on a mineral crystal. A p-n junction between the helical conjugated polymer (p-type) and pyrite (n-type) can be expected for applications as organic/inorganic semiconductor devices.