The quenching of the fluorescence decay of electronically excited 5,10,15,20-tetraphenyl- 21H, 23H-porphinetetrasulfonate (TPPS (4-)*) in the presence of methylviologen cations (MV (2+)) was measured at various ionic strengths in methanol. Analysis of the fluorescence decay curves revealed strong evidence for the presence of a second fluorescent species over the entire range of ionic strength used in this work, which is attributed to solvent-separated ion pairs (TPPS (4-)-S-MV (2+)). Transient effects of the fluorescence decays were analyzed, and values for the effective reaction distance, R NH, and the diffusion coefficients, D, were obtained. Diffusion coefficients were independently measured for TPPS (4-) and MV (2+) using the Taylor dispersion method. The values for D obtained by the analysis of the transient effect were found to be smaller than those for the sum of the diffusion coefficients of TPPS (4-) and MV (2+) obtained by the Taylor dispersion method and a possible explanation for this result is given.
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