Z-azobenzenes compatible with liquid crystals: phase stability and guest-host interaction

Photoresponsive azobenzene/nematic liquid crystal (LC) systems were developed which exhibit no phase separation upon E → Z photoisomerization, even at azobenzene loading as high as 20 wt %. It was shown that Z-3,3′-dialkanoyloxy azobenzene 1a and 1b correlated with the LC host as well as the corresponding E-isomers. The compatibility increased when methyl groups were embedded to the 2 and 2′ position of 3,3′-dialkanoyloxy azobenzene (1b), proving the effectiveness of a steric hindrance to the formation of stable rod-like conformations. After UV exposure, system with 1b exhibited a very sharp T_NI at 35.7°C. In contrast, the Z-4,4′-disubstituted analog 2 disorganized the LC orientation because of the bent structure. Experimental results were in agreement with simulation results.