The effect of carborane, bicyclo[2.2.2]octane and benzene on mesogenic and dielectric properties of laterally fluorinated three-ring mesogens

Abstract

Six series of structurally similar compounds containing 12- and 10-vertex p-carborane (A and B), bicyclo[2.2.2]octane (C), and benzene (D) were prepared and their mesogenic and dielectric properties investigated. Comparative analysis showed that all carborane derivatives form significantly less stable mesophases than their carbocyclic analogs, however they exhibit a relatively high shielding ability for lateral fluorination. Depression of the clearing temperature upon fluorination of series 1, 3, and 5 is approximately constant for each series A–D and correlates with the diameter of the ring (the slope = 14.8 °C Å⁻¹ and R² = 0.997). Compounds in series 2 (X = F) were used as low concentration additives to a nematic host, 6-CHBT. Dielectric parameters were extrapolated to pure additives and analyzed using the Maier–Meier equation. The Kirkwood factors g and apparent order parameters S_app that are required to reproduce the extrapolated dielectric values follow the trend in the size of ring . The smallest g (0.47) and the largest S_app (6.3) are obtained for carborane 2A, and the largest g (0.69) and the smallest S_app (0.7) are obtained for the terphenyl derivative 2D. The increase of S_app in the series D→A corresponds to the increasing disorder of the nematic solution with increasing size of ring .