Conformational effects on mesophase stability: Numerical comparison of carborane diester homologous series with their bicyclo[2.2.2]octane, cyclohexane and benzene analogues

Three series of diesters of 4-alkoxyphenols containing 12-vertex p-carborane (1A[n], n=1-22), 10-vertex p-carborane (1B[n], n=1-12) or bicyclo[2.2.2]octane (1C[n], n=1-12) as the central structural element were prepared and investigated by optical and calorimetric methods. All carborane diesters exhibited exclusively nematic behaviour, whereas the carbocyclic analogues 1C[n] and also cyclohexane (1D[n]) and benzene (1E[n]) derivatives, showed early onset of SmA phase and complete disappearance of nematic behaviour. The isotropic transition temperatures, T MI, for the five series of mesogens were analysed numerically using a three-parameter exponential function. The resulting limiting values, T MI(∞), provided a quantitative assessment of the central element ability to support the mesogenic state. They demonstrated that, whereas the T MI(∞) values for the carbocycles, C, D, and E, are around 125°C, for carboranes A and B this value is 70±2°C and 49±19°C, respectively. Two types of comparative analysis of trends in T MI relative to those of the terephthalate series 1E[n] demonstrated abnormal behaviour of both carborane series (1A[n] and 1B[n]) and also the cyclohexane series (1D[n]). The former showed progressive destabilisation of the mesophase, whereas the series 1D[n] exhibited increasing mesophase stability relative to 1E[n] with increasing chain length. Both of these effects were explained using conformational analysis of theoretical models and experimental molecular structures for 1A[3], 1B[4] and 1C[4]. The increasing relative destabilisation of the mesophase in the carborane derivatives was rationalised by the high order rotational axes in A and B and D 4d symmetry for B. The trend of the ΔT MI values for series 1D[n] was explained with the existence of the equatorial-axial conformational equilibrium for the cyclohexane derivatives. The clearing temperatures for the hypothetical pure diequatorial conformers 1D[n]-ee were estimated.