Relationship between the temperature dependence of the induced helical pitch and the anisometry of molecules of chiral dopants

The regularities of changes in the temperature dependence of the helical pitch P and the parameter dP/dt are analyzed for induced cholesteric liquid-crystal systems based on nematic 4-pentyl-4′-cyanobiphenyl, including chiral arylidene derivatives of (S)-1-phenylethylamine and (1R),(4R)-isomenthone. In the molecules of these compounds, the length of the π-electron arylidene fragment, the nature of the bridge group between its benzene rings and of the lateral substitution in the cyclohexanone ring, and the length of the terminal alkyl or oxyalkyl substituent are varied. The statistical molecular anisometry of the chiral dopants is calculated using the density functional method and the semiempirical AM1 method. It is shown that the temperature dependence of the induced helical pitch and, correspondingly, its quantitative characteristic dP/dt and the parameter dt t /dC, characterizing the effect of a chiral dopant on the thermal stability of the mesophase, are determined by the molecular anisometry of the chiral component.