Thermo and Photoresponsive Behavior of Liquid-Crystalline Helical Structures with the Aid of Dual Molecular Motions

Light- and temperature-driven multistable chiral materials composed of a binaphthyl moiety as a twisting part and an azobenzene moiety as a photoresponsive part in a single component were investigated. It has been found that the materials show reversible change in a dihedral angle of the twisting part at various temperatures. In contrast, no change in the twisting angle of the materials during trans-cis photoisomerization was observed. Furthermore, the chiral compounds were dissolved in nematic (N) liquid crystals (LCs) to produce a chiral N phase with a helical structure. The mixture exhibited a dual molecular response to temperature and light. Helical pitch length of the chiral N LC decreased with increasing temperature because of a molecular twisting motion of the binaphthyl moiety, resulting in a stabilization of the LC helical structure. On the other hand, length of the helical pitch increased upon photoirradiation, and the resulting LC mixture was found to show photoswitching between chiral N and N phases upon trans-cis isomerization of the azobenzene moiety. The photoinduced deformation of the LC helical structure was derived from a bent shape of cis isomer of the azobenzene moiety in the chiral dopant. This dynamic modulation of the self-organizing helical structure was based on dual and selective molecular motions of the guest materials induced by external stimuli.