ANALYSIS OF THE TRANSIENT EFFECT FOR A BIMOLECULAR FLUORESCENCE-QUENCHING REACTION BETWEEN IONS IN AQUEOUS-SOLUTION .2. TEMPERATURE-DEPENDENCE OF KINETIC-PARAMETERS

The diffusion-influenced electron-transfer reaction between electronically excited 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine (H-2TPPS4-*) and methylviologen (MV2+) in aqueous solution was investigated over the temperature range 10-35-degrees-C using nanosecond time-resolved fluorescence decay measurements. The ionic strength of these solutions was maintained at 0.004 mol/kg by the addition of NaCl as an inert electrolyte in order to account for the increase in screening of reactant charges that occurs as the MV2+ concentration increases. The decay of fluorescence from H-2TPPS4-* is highly nonexponential for all temperatures and MV2+ concentrations used in this work. These nonexponential fluorescence decay curves were analyzed according to the model for the kinetics of diffusion-influenced ionic reactions that was developed by Hong and Noolandi. This has enabled values for the optimized electron-transfer distance, R, the sum of the diffusion coefficients of the reactants, D, and the intrinsic reaction rate coefficient, k(act), to be calculated at each temperature. This reaction was found to obey the Stokes-Einstein relation. The values for R at each temperature were slightly larger than the estimated contact distance, and an activation energy of 16.2 +/- 1.2 kJ/mol was calculated from an Arrhenius-type treatment of the temperature dependence of k(act).