Steric effects of substituents in mesogens and phase transition temperatures in uniaxial liquid crystals

Abstract

The influence of steric effects of lateral substituents on the thermal stability of smectic A, nematic and cholesteric mesophases in mesogens with one (biphenyl derivatives) and two (stilbene derivatives, cinnamic acids and their cyanophenyl esters, cholesteryl cinnamates) actual conformational degrees of freedom is considered. The compounds studied differ in the following factors: (i) the length of the aromatic core and the flexible aliphatic chains, (ii) the central or off-central position of the fragment in the core structure and (iii) the presence or absence of 71-electronic conjugation of the fragment with other core fragments. A linear dependence of the smectic A—nematic (cholesteric)—isotropic transition temperatures on the generalized coordinate (Q = cos² oS) for the conformational degree of freedom has been confirmed for the compounds studied over the complete range Q = 0–1; here oS is the dihedral angle between the planes of two ω-conjugated fragments, joined by a single chemical bond.