Synthesis and Mesomorphism of an Homologous Series of Ionic Polycatenar Liquid Crystals based on the N- Phenylpyridinium Moiety

An homologous series of ionic polycatenar liquid crystals based on the N-phenylpyridinium motif was prepared with triflate, triflimide, octylsulfate and dodecylsulfate anions. Triflate and the alkylsulfate salts gave rise to a SmA phase in short-chain homologues due to an increase in the core volume by the associated anion; observation of a SmA phase is extremely rare in polycatenar mesogens. It was also found that the columnar mesophases of longer-chain homologues was stabilised significantly compared to the SmA phase (70 °C with a triflate anion) and that this observation is characteristic of all ionic polycatenar materials studied within this thesis. The triflimide salts behaved differently and all homologues formed a columnar rectangular mesophase.

The effect of added solvent on the mesomorphism of the N-phenylpyridinium ions was investigated and it was found that a range of mesophases were induced. It was postulated that small, polar, aprotic solvents such as DMSO associated at the polar core of the cation, increasing the effective core volume and leading to the induction of a lamellar phase when the dry material was columnar. In contrast, long-chain alcohols and linear alkanes were regarded as concentrating in the apolar terminal chains to increase the effective chain volume, so promoting mesophases with curvature (cubic and columnar). Preferential solvent location can readily be rationalised on the basis of the amphiphilic nature of the N-phenylpyridinium salts.

Finally, a series of phenylpyridine complexes of silver(I) were prepared and their mesomorphism was compared both with the N-phenylpyridinium salts and with the stilbazole complexes of silver(I). Once the difference in size and shape of the ligands was accounted parallels were observed between the two families of complexes, but comparison with the N-phenylpyridinium salts revealed both the strong influence in the way in which the anion is accommodated and also the formally ionic nature of the materials.