Reversible Photoswitchable Axially Chiral Dopants with High Helical Twisting Power

摘要

The development of reversible optical memory and light-driven chiral molecular switches is attracting tremendous interest since they hold a great promise in applications as well as fundamental science. A promising solution lies in the photoresponsive chiral nematic (cholesteric) phase whose unique helical structures and physical properties can be tuned upon light irradiation. Such a phase can be achieved by doping both azo molecules and chiral molecules into a nematic liquid crystal host. trans-Azobenzene is thermo-dynamically more stable than as-azobenzene by ca. 56 kJ/mol; however, irradiation with ultraviolet light at a wavelength of about 360 nm leads to its trans—cis isomerization. The reverse process from cis to trans isomer can occur thermally or photochemically with visible light. Since the physical and chemical properties of the azobenzene configurational isomers are different, the reversible optically induced switching effect has been the basis for many functional molecules and materials with applications in photonics and liquid crystal displays. It is also well-known, that when a chiral molecule is dissolved in an achiral nematic liquid crystal, its molecular chirality is transferred to the nematic solvent which then organizes into a macroscopic helical architecture that reflects light according to Bragg's law. The wavelength λ of the selective reflection is defined by λ = np, where p is the pitch length of the helical structure and n is the average index of refraction of the liquid crystal material. The ability of a chiral dopant to twist the nematic liquid crystal phase is defined as helical twisting power (HTP)_β according to the equation: β = (pc)~(-1), where c is either weight concentration or molar fraction of the chiral dopant. The isomerization upon photo irradiation can control the HTP and the helical pitch of the cholesteric phases, providing the tool for optical display addressing.