Hydrogen Bonded Liquid Crystals from Fluorophenols and Alkoxystilbazoles

Hydrogen-bonded complexes between 4-alkoxy-4’-stilbazoles and phenols with different degrees of fluorine substitution were prepared. In contrast to the components, the complexes were observed to exhibit nematic and smectic A phases by polarised optical microscopy, and nematic tendency was found to decrease with increasing alkoxy chain length. A correlation between mesophase stability and the fluorine substitution positions for a given number of fluorine substituents in the phenol was also observed. The mesomorphism of the fluorophenol complexes prepared are compared with similar complexes formed between stilbazoles and 2-, 3- and 4-cyanophenol, as well as between stilbazoles and 3- and 4-nitrophenol. Nine X-ray crystallographic structures of the 4-octyoxy-4’-stilbazole complexed with fluorophenols were also obtained. Eight of the complexes crystallised in the space group, with a back-to-back dimer motif, while the complex of 4-fluorophenol crystallised in the Pbca space group and formed head-to-tail, zig-zagged chains. A linear relationship between the N•••H separation and the pKa value of the corresponding fluorophenol was discovered. Comparisons are made with the analogous halogen-bonded 4-(N,N-dimethylamino)pyridine and iodofluorobenzene complexes, in addition to with the crystal packing of the pure fluorophenols.