Kinetics of triplet decay of water soluble porphyrins. Analysis of ionic strength effects

Abstract

The triplet decay of zinc tetrakis(N-methyl-4-pyridiniumyl)porphyrin(4+) (ZnTMPyP/sup 4 +/) and zinc tetrakis(N-(propylsulfonato)-4-pyridiniumyl)porphyrin(0) (ZnTPSPyP) in deoxygenated aqueous solution has been studied by laser flash photolysis. Kinetic analysis using a model comprising unimolecular decay, triplet-triplet annihilation, and ground-state quenching yielded rate constants (extrapolated to zero ionic strength) of k/sub uni/ = 350 s/sup -1/, K/sub T-T/ = 1.9 x 10/sup 7/M/sup -1/s/sup -1/, and k/sub GS/ = 5 x 10/sup 6/M/sup -1/s/sup -1/ for ZnTMPyP/sup 4 +/, and k/sub uni/ = 1990 s/sup -1/, k/sub T-T/ = 1.5 x 10/sup 9/ M/sup -1/s/sup -1/, and k/sub GS/ < 10/sup 6/M/sup -1/s/sup -1/ for ZnTPSPyP. Ionic strength dependence of triplet decay was zero for zwitterionic ZnTPSPyP but very strong for ZnTMPyP/sup 4 +/. Results are analyzed in terms of simple Debye-Hueckel theory, and applications to solar energy storage reactions are discussed. 4 figures, 1 table.