Chiroptical properties of cholesteric liquid crystals of chitosan phenylcarbamate in ionic liquids

Abstract

Chitosan phenylcarbamate (CtsPC) samples were synthesized with different degrees of substitution (DS) ranging from 2.3 to 3.5, and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the concentrated lyotropic system formed a cholesteric liquid-crystalline phase and mostly exhibited vivid colorations owing to selective reflection of visible light. Ultraviolet-visible-near-infrared spectroscopy and wide-angle X-ray diffractometry were used to evaluate the maximum wavelength of selective light reflection and the distance and rotational difference between adjacent nematic thin layers in the cholesteric mesophase, respectively. The cholesteric helical pitch increased with decreasing polymer concentration, increasing temperature or increasing DS. The helical sense remained right-handed under the adopted measurement conditions, although CtsPCs are known to form a left-handed cholesteric structure in polar organic solvents, such as N,N-dimethylformamide (DMF). Accordingly, an inversion of the cholesteric sense was achieved by combination of [C2Mim][N(CN)2] and DMF as solvent; circular dichroism and optical rotatory dispersion spectroscopy verified that the sense of CtsPC cholesterics changed from left-handed to right-handed with increasing [C2Mim][N(CN)2] content in the mixed solvent.