Viscosity effects on 9,10-dicyanoanthracene-sensitized photooxidation of N,N-dibenzylhydroxylamine and its derivatives in protic polar solvents

An analysis of viscosity effects on the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by the title hydroxylamines (1) and oxygen in protic polar solvents revealed the occurrence of a diffusion-limited emission quenching for both the quenchers. It was found that the viscosity dependence of the former quenching rate constant is a major factor that controls the magnitude of dependence of the sensitized oxidation efficiency at a given hydroxylamine concentration on the solvent viscosity. The interesting finding that an increase in solvent viscosity exerts only a minor effect on the limiting quantum yield for the reaction led us to conclude that the increased viscosity lowers the rates for both processes of the return electron transfer and the diffusive separation of radical ions to almost the same extent. This finding substantiates the intervention of a solvent-separated radical ion pair as a key intermediate. Based on molecular size effects on the limiting quantum yield in methanol, it was suggested that an unpaired electron generated on the hydroxylamino nitrogen is not appreciably delocalized into the aromatic rings, thus permitting strong electrostatic and hydrogen bonding interactions between the caged 1 cation radical–DCA anion radical pair and protic polar solvent molecules.