Charge-Transfer and Thermochromic Phenomena in Solid Polyelectrolytes

Aromatic polyelectrolytes capable of undergoing charge-transfer interactions with their counterions were synthesized and studied in the solid state by several physical techniques. The polyelectrolytes were formed by the condensation of 4,4′-bipyridine with aromatic dihalides yielding polycations with quaternary nitrogens in the repeating unit. The work focused on the polyelectrolyte known as poly(xylylviologen dibromide) (PXV-Br₂) but other polymeric and monomeric chemical analogues were prepared and analyzed as part of the study. Visible light absorption of PXV-Br₂ was characterized by diffuse reflectance measurements as a function of temperature and degree of hydration in the polyelectrolyte. We found a reversible thermochromic effect consisting of a visible bathochromic shift as PXV-Br₂ is heated above 100°C. Through thermogravimetric analysis and other experiments, the shift was attributed to the level of surface hydration, presumably as a result of how water molecules bound to the macroion affect charge-transfer states. A study of polymerization kinetics in PXV-Br₂ by infrared spectroscopy suggests that water molecules are closely associated with the cationic repeating units. Consideration of recent theoretical work and experimental results suggests that the phenomenon of counterion condensation and the presence of amorphous regions contribute to strong charge-transfer interactions in these polyelectrolytic solids.