Excited state quenching kinetics of zinc meso-tetrakis (N-methylpyridinium-4-yl) porphyrin by methyl viologen in AOT reverse micelles

The singlet and triplet excited state quenching of Zn (II) meso-tetrakis (N-methylpyridinum-4-yl) porphyrin (ZnTMPyP4+) by methyl viologen (Mv(2+)) in AOT reverse micelles has been studied using steady state fluorescence and laser ash photolysis techniques. The water content in the reverse micelle was varied by using different values of W-0 = [H2O]/[AOT] at fixed AOT (0.1 M) concentration and the quenching process efficiency was found to decrease with increasing W-0. The ZnTMPyP4+ triplet state is strongly quenched by Mv(2+) and the probe decay profiles were nonexponential. Data analysis with an active sites quenching model gave aggregation numbers which compare well with the literature values reported for the AOT reverse micelles. The rate constants for the quenching of singlet and triplet excited states correlate with the micelle size and were found to decrease with increasing micellar radii giving evidence of a geometrical confinement effect. The magnitudes of the rate constants are of the order of diffusion controlled reactions affected by electrostatic interactions between charged species in the interfacial micellar region.