Chirality in Mixed Nematic and Cholesteric Liquid Crystals

Abstract

A mechanism is discussed explaining the different types of pitch-concentration dependences encountered in chiral mesophases. Addition of a low concentration of a chiral compound to a nematic produces a mesophase in which pitch is initially inversely proportional to concentration, but as the concentration increases, three different types of behavior occur: (a) the pitch slowly saturates as it approaches that of the pure chiral (cholesteric) compound; (b) the pitch decreases to a minimum pitch value (MPV) and then increases again to that of the pure cholesteric; (c) the pitch decreases to a MPV, increases to infinity and then decreases again to that of the pure cholesteric. This type of “compensation” which usually occurs only when a right-handed and left-handed cholesteric are mixed, is reported here for a mixture of choles-teryl-2(2-ethoxyethoxy) ethyl carbonate and the nematic N-p-methoxy-benzylidene-p-n-butylaniline. Thus, starting from a pure cholesteric, pitch may either increase or decrease upon adding a non-chiral solute. The common mechanism involves the induced circular dichroism (ICD) in the non-chiral compound caused by the helical arrangement of the chiral mesophase, which may be of the same or opposite sign as the ICD. The pitch of the pure cholesteric compound, the structure of the non-chiral compound and its affinity to ICD are important in determining which type of spectral behavior is observed.