Helical twisting power and circular dichroism in nematic liquid crystals doped with chiral molecules

Explicit expressions for the helical twisting power of a chiral dopant in t he nematic phase have been obtained in the mean field approximation taking into account chiral dispersion intermolecular interactions. The results of the theory enable one to explain a correlation between the signs of helical twisting power and circular dichroism of selected electronic transitions w hich have recently been established experimentally for some mono- and bis-a minoantroquinones. Helical twisting power is proportional to the pseudoscal ar parameter that specifies the chirality of the dopant molecule in terms o f its dipole and quadrupole matrix elements. This expression is simplified for a special class of molecules in which chirality is induced by a perturb ing achiral group into an achiral skeleton. In this case both helical twist ing power and circular dichroism are approximately proportional to some sim ple pseudoscalar functions that specify the location of the achiral perturb ing group with respect to the symmetry planes of the unperturbed achiral sk eleton. Simple sector rules have been proposed to determine the sign change of the helical twisting power associated with the change of location of th e perturbing group.