Unconventional multi-arm azobenzene liquid crystal with different lengths of mesogenic arm based on cholic acid: synthesis, mesophase properties and photo-induced transition

Two series of novel four-arm azobenzene liquid crystal molecules were designed and synthesized, and their mesophase properties, photo-induced transition were studied. Two series of mesogenic arms were screened with different length of flexible chain (the carbon number n of flexible chain is 4, 6, 8, 10) using methoxy and nitro as terminal substituents R. The four-arm azobenzene LCs were synthesized by introducing the mesogenic arms into cholic acid. FACAE-O-n, FACAE-N-n all belonged to photosensitive thermotropic enantiotropicnematic LCs. For methoxyl series, melting transitions (Tm), liquid crystal-isotropic transitions (Ti) and mesogenic ranges decreased with the increase of carbon numbers in flexible chain both on heating and cooling. The mesophase performance of FACAE-O-4 was the most excellent. For nitro series, Tm and Ti on heating and cooling decreased with the increase of carbon numbers. Nevertheless, mesogenic ranges increased with the increase of carbon numbers only on heating. The cooling mesogenic ranges showed a trend of rising firstly and then falling, reaching the maximum 103.1oC (n=8). Under ultraviolet irradiation, there was no red shift. The intensity of maximum absorption has changed with the increase of ultraviolet irradiation time. After photo-induced, Tm and Ti on heating and cooling of the four-arm LCs decreased, and the mesogenic ranges became shorter.