Singlet oxygen quenching by oxygen in tetraphenyl-porphyrin solutions

Oxygen concentration dependence study of meso-tetra(4-sulphonato)phenylporphyrin (TPPS4) phosphorescence kinetics showed linearity of the dependence of TPPS4 triplet state rate-constant. Corresponding bimolecular quenching constant of (1.5±0.1)×109 l/mol s was obtained. On the other hand, rate constants of singlet oxygen depopulation exhibit nonlinear dependence on oxygen concentration. Comparison of zero oxygen concentration-extrapolated value of singlet oxygen lifetime of (6.5±0.4) μs to (3.7±0.1) μs observed under air-saturated conditions indicates importance of the effect of quenching of singlet oxygen by oxygen. Upward-sloping dependencies of singlet oxygen depopulation rate-constant on excitation intensity evidence that singlet oxygen is predominantly quenched by oxygen in excited singlet state.