Novel planar chiral dopants with high helical twisting power and structure-dependent functions

Abstract

Three kinds of novel planar chiral azobenzenophane derivatives for controlling the helical structures of cholesteric liquid crystals were synthesized and found to exhibit excellent chiroptical properties in both chloroform and liquid crystal media. The results of doping experiments showed that all the chiral switches possess high helical twisting power (HTP) values in nematic liquid crystal hosts (>80 μm⁻¹) so that they can effectively transfer the chirality to the host LC medium resulting in a helical supramolecular structure even at very low doping concentrations (<1.0 wt%). In addition, the dependency of function-structure on various cyclic azobenzenophane scaffolds and flexible chains was also discussed in detail. The results show that the introduction of ordered segments and flexible long alkyloxy chains in the dopants can increase the cholesteric induction ability of dopants effectively. And the screw direction of the formed chiral nematic liquid crystal was dependent upon the planar chiral configuration of the molecular switch. Moreover, in this mixed system, reversible tuning between orange and green reflection colors were achieved upon irradiation with alternating UV/Vis light.