Ring effect on helical twisting power of optically active mesogenic esters derived from benzene, bicyclo[2.2.2]octane and p-carborane carboxylic acids

Three structurally similar esters containing carborane (1A), bicyclo[2.2.2]octane (1B), and benzene (1C) were prepared and their mesogenic properties investigated. All esters exhibited chiral nematic phases, and only 1B showed rich smectic behavior. The esters were used as additives to three structurally different nematic hosts. The resulting cholesteric pitch p was measured as a function of concentration and temperature. The calculated helical twisting power βM followed the order 1A < 1B ≤ 1C, and the temperature dependence of βM was ≥0 for all mixtures except for 1C in host I. These results are discussed in terms of size and conformational properties of each ring system A–C and the role of their “biaxiality” in chirality transfer. The observed trends in βM are consistent with non-specific solute–solvent interactions in which the chiral additive breaks the uniaxial symmetry imposed by the phase and differentiates energy of the chiral conformers of the host.