Luminescent and Redox Probes of Structure and Dynamics in Quaternized Poly( 4-vinylpyridine) Films on Electrodes

Abstract

The properties of partially quaternized poly(4-vinylpyridine) films have been studied by coordinatively attaching luminescent and redox probes to free pyridine units within the polymer. Samples were examined as thin films spin coated onto glass or electrodes. The luminescent moiety was Re(CO),(phen) (phen = 1 ,lo-phenanthroline). It showed properties that depended strongly on the identity of the anion in the supporting electrolyte adjacent to the film. The emission maximum shifted to the red, and the quantum yield decreased in the order dry film > 0.1 M NaC104 > 0.1 M potassium p-toluenesulfonate > 0.1 M KNO,. The isotope effect caused by changing the solvent from H20 to D20 fell in the reverse order. The results show that the anion has a large impact on film structure. In nitrate the films are strongly hydrated, but in perchlorate they are dry and compact. The redox moiety was Ru(bpy),Cl2'/'+. The activation energies for electron diffusion in the network of redox centers were assessed by temperature-resolved chronocoulometry. The activation energies were independent of the concentration of redox centers in the film but were ordered according to electrolyte as given above. They were roughly linearly dependent on the degree of chemical cross-linking. The permeabilities of the films to redox-active ions in solution were larger than the electron diffusion coefficients by an order of magnitude but were strongly dependent on the anion of the supporting electrolyte, inversely in the order given above. The results suggest that electron diffusion is controlled by segmental motions within the polymer and that the anion dependence is a structural effect.