Quenching of MgTPP and ZnTPP fluorescence by molecular oxygen

The rate constants for the quenching by oxygen of the lowest excited singlet states of two model metalloporphyrins, MgTPP and ZnTPP, have been measured in acetonitrile, ethanol and toluene and the data used to determine the quenching mechanism. The quenching rates are close to, but significantly slower than, diffusion-limited and do not scale with the solvent viscosity. Quenching via charge transfer in the encounter complex is a minor process even in polar solvents. The major quenching process involves spin-allowed S1–T2 radiationless relaxation of the metalloporphyrin in the 3(M⋯O2) encounter complex, a process that has a small apparent potential energy barrier.