The effect of the linking group on mesogenic properties of three-ring derivatives of p-carborane and biphenyl

Four series of mesogenic derivatives of p-carborane (series A[0] and A[1]) and their benzene analogues (series B[0] and B[1]) with variable linking groups were prepared and investigated for phase behaviour. The data allowed a comparison of the effect of the group on the mesophase stability as a function of the adjacent ring (carborane or benzene), the variable central ring (carborane or benzene), and the presence of an oxygen atom in the terminal chain. The results showed that substitution of carborane for a benzene ring in B[m] depresses the clearing point by 50K to 208 K and eliminates all smectic behaviour in A[m]. The carborane derivatives A[m] are weakly dependent (effectiveness of L: -CH=CHCOO-similar to-COO-similar to-CH=CH->-CH=N-similar to-CH2CH2->-CONH-), whereas the benzene analogues B[m] are strongly dependent (effectiveness of L: -CONH->-CH=CH->-CH=N-similar to -CH=CHCOO->-COO->-CH2CH2-) on the structure of the linking group. The difference in the effectiveness of the amide group on mesophase stability (Delta T-I=208 K) has been attributed to the stabilizing intermolecular H-bonding in B[m], which is prevented in A[m] by steric and electronic effects of the carborane cage on the carbonyl group.