The effect of molecular polarity on nem/catic phase stability in 12-vertex carboranes

Weakly polar-polar isosteric pairs of 12-vertex p-carborane [closo-1,12-C2B10H12] (1[12]) and monocarbaborate [closo-1-CB11H12](-) (2[12]) nematic liquid crystals, in which the difference in the calculated molecular dipole moment is 11.3 D, were synthesised, and the effect of the dipole moment on nematic phase stability was investigated. The trend observed for the 12-vertex series ([12]) was identical to that of the previously investigated 10-vertex series ([10]) containing [closo-1,10-C2B8H10] (1[10]) and [closo-1-CB9H10](-) (2[10]): the uniform increase in the molecular dipole moment in the pairs of mesogens does not correspond to a uniform change in the clearing temperature, T-NI. This demonstrates the role of a remote substituent in modulating the intermolecular dipole-dipole interactions. The magnitude of such interactions was calculated (using density functional theory methods) for a pair of polar (2[12] d-2[12] d) and an analogous pair of weakly polar (1[12] d-1[12] d) molecules. All results for the 12-vertex series ([12]) were analysed relative to the 10-vertex analogues ([10]).