Tuning mesomorphic properties and handedness of chiral calamitic liquid crystals by minimal modification of the effective core

Two pairs of calamitic liquid crystalline molecules, (+)-2-4'-(S)-sec-butoxyphenyl]-5-(4'-hexoxyphenyl)toluene ((+)-S-1) and (+)-2-(4'-hexoxyphenyl)-5-4'-(S)-sec-butoxyphenyl]toluene ((+)-S-2), (-)-2-4'-(R)-sec-butoxyphenyl]-5-(4'-hexoxyphenyl)toluene ((-)-R-1) and (-)-2-(4'-hexoxyphenyl)-5-4'-(R)-sec-butoxyphenyl]toluene ((-)-R-2), have been designed and synthesized. Each of the molecules consists of a p-terphenyl core substituted with a methyl group on the middle ring, a chiral sec-butoxy tail, and an achiral n-hexoxy tail. The geometrical difference between (+)-S-1 ((-)-R-1) and (+)-S-2 ((-)-R-2) lies only in the location of the methyl group on the effective mesogenic core. Yet, such a small change in the structure gives rise to remarkable differences in mesogenic properties and handedness. Both (+)-S-1 and (-)-R-1 have an enantiotropic cholesteric phase (N*) and a monotropic twist grain boundary C* phase (TGBC*), whereas (+)-S-2 and (-)-R-2 exhibit only a monotropic N* phase. Moreover, (+)-S-1 ((-)-R-1) and (+)-S-2 ((-)-R-2) have opposite handedness in the N* phase, and (+)-S-1 and (-)-R-1 even have a helical inversion from N* to TGBC* phase through a non-helical chiral mesophase.