Photoinduced Phase Transitions of Imine-Based Liquid Crystal Dimers with Twist-Bend Nematic Phases.

Photoisomerizable molecules in liquid crystals (LCs) allow for photoinduced phase transitions, facilitating applications in a wide variety of photoresponsive materials. In contrast to the widely investigated azobenzene structure, research on the photoinduced phase-transition behavior of imine-based LCs is considerably limited. We herein report the thermal and photoinduced phase-transition behaviors of photoisomerizable imine-based LC dimers with twist-bend nematic (N TB ) phases. We synthesize two homologous series of ester- and thioether-linked N -(4-cyanobenzylidene)aniline-based bent-shaped LC dimers with an even number of carbon atoms ( n = 2, 4, 6, 8, and 10) in the central alkylene spacers, namely, CBCOO n SBA(CN) and CBOCO n SBA(CN), possessing oppositely directed ester linkages, C=OO and OC=O, respectively. Their thermal phase-transition behavior is examined using polarizing optical microscopy and differential scanning calorimetry. All dimers form a monotropic N TB phase below the temperature of the conventional nematic (N) phase upon cooling. Remarkably, the N TB phases of CBCOO n SBA(CN) ( n = 2, 4, 6, and 8) and CBOCO n SBA(CN) ( n = 6 and 8) supercool to room temperature and vitrify without crystallization. In addition, the phase-transition temperatures and entropy changes of CBCOO n SBA(CN) are lower than those of CBOCO n SBA(CN) at the same n . Under UV light irradiation, the N TB and N phases transition to the N and isotropic phases, respectively, and reversibly return to their initial LC phases when the UV light is turned off.