Fast switching near a smectic-to-nematic phase transition

We have designed, synthesised and evaluated the physical properties of high-birefringence (Delta n) isothiocyanato phenyl-tolane and terphenyl liquid crystals. These compounds exhibit a Delta n similar to 0.36-0.52 at room temperature and wavelength lambda=633nm. A different mesomorphic sequence was observed depending on the terminal alkyl chain length. Compounds with a longer chain show smectic and nematic phases, whereas short-chain homologues only exhibit a nematic phase. In the vicinity of the nematic-to-smectic transition, the viscoelastic coefficient decreases, instead of increases, as the temperature decreases. The observed change is as large as 75% for the 4\u27-(3,5-difluoro-4-isothiocyanatophenylethynyl)-4-pentylbiphenyl compound studied. This behaviour is useful for improving the response time of a liquid crystal device, if the operating temperature can be controlled accurately