Lyotropic Liquid Crystalline Main-Chain Viologen Polymers: Homopolymer of 4,4‘-Bipyridyl with the Ditosylate of trans-1,4-Cyclohexanedimethanol and Its Copolymers with the Ditosylate of 1,8-Octanediol

Abstract

A series of viologen polymers based on 4,4'-bipyridyl and the ditosylates of trans-1,4-cyclohexanedimethanol and 1,8-octanediol was prepared by the quaternization reaction in acetonitrile. Their polyelectrolyte behavior in a common organic solvent was studied by viscosity measurements, and their chemical structures were determined by FTIR and 1D and 2D NMR spectroscopies. They were characterized for their lyotropic properties in a number of polar organic solvents by polarizing light microscopy and differential scanning calorimetry and for their photochromic properties by UV-vis spectroscopy. As expected, their inherent viscosity in methanol decreased dramatically with the increase in concentration in the absence of an added electrolyte. Although each of these polymers contained the ionic group along the backbone of the polymer chain, there was an array of weak ion-pair dipoles between the tosylate ion and the 4,4'-bipyridinium ion in each of them, which was indicated by the appearance of symmetric stretching band of S=O at a lower frequency (1034 cm -1 ) when compared with that of -SO 3 Na (1064 cm -1 ) in its IR spectrum. The aromatic protons of 4,4'-bipyridinium ion in each of these polymers exhibited three pairs of signals as opposed to a pair of doublets that generally occurs in small molecules containing this ion. These three pairs of signals were related to the structure and dynamics of these ionic polymers in a polar solvent such as methanol, which were studied by DEPT, 1 H- 1 H COSY, and NOESY spectroscopies. Each of them showed a lyotropic lamellar phase in methanol, ethanol, ethylene glycol, diethylene glycol, glycerine, and benzyl alcohol. Their critical concentrations (C * values) were as low as 5 wt % and as high as 20-40 wt %, and their concentrations for the development of fully grown lyotropic phase were as low as 10-20 wt % and as high as 40-50 wt % in various alcohols depending on the polarity of the solvents, the hydrophilicity and hydrophobicity of the solvents, and the microstructures of these polymers. Additionally, their lyotropic solutions in either glycerine or benzyl alcohol, on irradiation with UV light, produced a deep blue color in the presence of air, but in absence of both a photosensitizer and a sacrificial donor. This deep blue solution had two absorption bands at 390 and 608 nm, which are the characteristic absorption bands of the viologen cation radical. The generation of this blue color occurred due to the photoreduction of the 4,4'-bipyridinium moiety via the electron transfer from the tosylate counterion.