Ring-alkyl connecting group effect on mesogenic properties of p-carborane derivatives and their hydrocarbon analogues

The effect of the phenyl-alkyl connecting group on mesogenic properties of several series of isostructural compounds containing p-carborane (A and B), bicyclo[2.2.2] octane (C), and benzene (D) was investigated using thermal and optical methods. Results demonstrated that mesophase stability in the series containing A-D follows the order (Alk)CH2CH2-< (Alk)OOC-< (Alk)CH2O-< (Alk)COO-. Surprisingly, the connecting groups (Alk)CH2CH2- and (Alk)OOC- destabilize the mesophase significantly stronger for carboranes (A and B) than for carbocyclic derivatives (C and D). Analysis indicates that this effect may have quadrupolar and conformational origin.