Novel Cholesteric Glassy Liquid Crystals Comprising Benzene Functionalized with Hybrid Chiral-Nematic Mesogens

Abstract

With 4′-cyanobiphenyl-4-yl benzoate nematogens chemically bonded to a benzene core via enantiomeric 2-methylpropyl spacers, a new series of cholesteric glassy liquid crystals has been synthesized for an investigation of structure−property relationships. Glass-forming ability, phase-transition temperatures, and stability against crystallization are affected by both the number and the position of substituent groups on the benzene ring with 1,3,5-trisubstituted system possessing the most favorable set of properties, Tg at 73 °C and Tc at 295 °C. With (S)-3-bromo-2-methylpropanol as the chiral precursor, left-handed helical stacking was observed for all the cholesteric GLCs reported herein. Films of the 1,3,5-trisubstituted and meta-disubstituted systems show a selective reflection wavelength, λR, at 413 and 422 nm, respectively, whereas that of the ortho-isomer exhibits a λR at 860 nm. Replacing one of the hybrid chiral-nematic mesogen in the 1,3,5-trisubstituted system by a nematogen loosens the helical pitch to yield a λR at 630 nm, still shorter than that of the ortho-isomer despite the dilution by a nematogen. This observation suggests the importance of regioisomerism to helical twisting. The difference in λR was interpreted in terms of molecular packing involving chiral spacers through computational chemistry. The susceptibility of cholesteric GLCs to photoalignment was tested using the ortho-isomer. The degree of photoalignment improves with an increasing rotational mobility of pendant coumarin monomers to an extent comparable to mechanical alignment on conventional rubbed polyimide films.