Banana-calamitic dimers: unexpected mesophase behaviour by variation of the direction of ester linking groups in the bent-core unit

A five-ring bent-core unit and a phenyl benzoate moiety are linked together by means of a flexible spacer. Starting from dimers having different length of the aliphatic parts, that is the terminal alkyloxy chains and the spacer, the direction of each of the four ester connecting groups in the bent-core unit is reversed step by step. In this way thermodynamically stable polar and non-polar mesophases in unusual sequences could be obtained. For instance, a SmCsPAF-SmCPAF transition and the existence of two nematic phases (N, NX) have been observed. The nematic phases of the banana-calamitic dimers exhibit unusual properties comparable to those of the nematic phases reported for some monomeric bent-core mesogens. The electric field-induced enhancement of the SmCP-SmA transition temperature of one compound proves the existence of polar clusters already in the non-polar SmA phase.