Liquid-crystalline compounds containing both a strong push–pull azo chromophore and a cholesteryl unit as photoresponsive molecular glass materials

Abstract

A series of liquid-crystalline (LC) compounds (R-Chol, R: CA, CN and NT), containing both a strong push–pull azo chromophore and a cholesteryl unit, was synthesized as novel molecular glass materials. The molecular structures and phases of the condensed state were characterized by spectral analyses, DSC, XRD and POM. The azo compounds existed as glassy solids at room temperature and entered the smectic LC phase above the glass transition temperature. Optical-quality solid films of R-Chol were obtained by spin-coating. The spin-coated films of CA-Chol and CN-Chol existed in the amorphous state and only the NT-Chol film showed weak birefringence and polydomain texture. Upon irradiation with a linearly polarized Ar⁺ laser (488 nm) beam, photoinduced orientation rapidly developed for CA-Chol and CN-Chol films, which showed significant photoinduced birefringence and dichroism. When the irradiating light was switched off, the birefringence did not show decay relaxation as is the usual case, but showed a small increase in the relaxation process. Photoinduced surface-relief-grating (SRG) formation was investigated by irradiating the films with two p-polarized interfering Ar⁺ laser beams. SRGs formed on the CA-Chol films showed the highest inscription rate in the series. The efficiencies of both photoinduced orientation and SRG formation were enhanced for the azo chromophore bearing the carboxylic group as the electron-withdrawing substituent. Two-dimensional (2D) quasi-crystal structures with 10- and 12-fold rotation symmetries were successfully fabricated on the CN-Chol films by using the dual-beam multiple exposure technique. The capability of the materials as photo-storage media was demonstrated by holographic recording.