SALT EFFECTS ON STEADY-STATE QUANTUM YIELDS OF ULTRAFAST, DIFFUSION-INFLUENCED, REVERSIBLE PHOTOACID DISSOCIATION REACTIONS

The steady-state fluorescence quantum yields of the protonated and deprotonated forms of 8-hydroxyprene-1,3,6-trisulfonate are determined in three solvents as a function of NaNO3 concentration. The measured quantum yields, and therefore also the overall dissociation rate coefficients, decrease with increasing salt concentration, in contrast to Bronsted's classical model for primary salt effects. This is a consequence of the reversibility and the nearly diffusion-controlled recombination rate of the proton in the reaction studied. A quantitative analysis is based on newly derived results for reversible diffusion-influenced reactions. Comparison with experiment is made for two screened potentials. Numerical integration with the Debye-Huckel potential indicates that it overestimates the salt effect. A second model (the "naive approximation") is based on a simple screened potential form which analytical expressions for reaction yields and rates are derived. It shows better agreement with experiment in the range 20-200 mM salt.