Carborane-containing liquid crystals:: a comparison of 4-octyloxy-4′-(12-pentyl-1,12-dicarbadodecaboran-1-yl)biphenyl with its hydrocarbon analogues

4-Octyloxy-4'-(12-pentyl-1,12-dicarbadodecaboran-1-yl)biphenyl (1BC) has been synthesized along with three hydrocarbon analogues in which the 1,12-dicarbadodecaborane is replaced by a phenyl (IPH), trans-cyclohexyl (1CH) or bicycle [2.2.2]octyl (1BO) ring. The mesogenic properties of these materials have been compared and contrasted in both their pure states and as binary mixtures. The binary phase diagrams for the liquid crystal 1BC, with its hydrocarbon analogues 1CH and 1BO exhibit excellent miscibility of the smectic A phase while the more highly ordered smectic phases (SmB and SmE) for the hydrocarbons are suppressed by 1BC. In contrast the binary mixture of 1BC with the terphenyl analog (1PH) exhibits complex behaviour in which the thermal stability of the smectic E phase is enhanced. X-ray diffraction data for the 1PH-1BC binary mixture suggest a strong in-plane molecular ordering which might be attributed to intermolecular associations stabilizing the smectic E phase in preference to other smectic modifications.