Hydrogen-Bonded Complexes between 4-Alkoxystilbazoles and Fluorophenols: Solid-State Structures and Liquid Crystallinity



48 new hydrogen-bonded complexes have been prepared by combining 16 fluorophenols of general formula C6FnH5−nOH with three different alkoxystilbazoles (butyloxy-, octyloxy- and dodecyloxy-). Single-crystal X-ray structures were obtained for 10 of the 16 complexes of octyloxystilbazole from which it was found that most of the structures could be collected into one of two groups according to both the motif shown by the complex and by the solid-state packing. Because all but one crystallised in the Pequation image space group, meaningful comparisons could be drawn and it was observed that six structures were extremely close in nature so that significant molecular overlap was found. On this basis, doubt is cast on the significance of some of the weaker intermolecular contacts found in the solid state. 40 of the new complexes showed liquid-crystal properties and it was found that although complexes of butyloxystilbazole were all nematic, almost all of those with dodecyloxystilbazole showed a smectic A (SmA) phase. Complexes of octyloxystilbazole showed a mixture of both. Structure/property correlations showed that clearing points were independent of the pKa of the phenol. The most stable mesophases were found when the fluorophenol contained a fluorine at the 2-position, which was interpreted in terms of the formation of an intramolecular H⋅⋅⋅F hydrogen bond to give a six-membered ring linking the two components into a stable, coplanar conformation. The least stable mesophases were found when no such ring formation was possible and the phenol was relatively free to move.