Photodegradation of free base and zinc porphyrins in the presence and absence of oxygen.

Comparison of photostability in degassed and aerated toluene solutions is reported for 5,10,15,20-tetraphenylporphyrin, 5,10,15-tri(p-tolyl)porphyrin, and their zinc analogues. After degassing, quantum yields of photodegradation are higher, but the photodecomposition rates decrease. Lower stability in deoxygenated solutions is due to much longer triplet lifetimes: 200-300 microseconds, compared to 200-360 ns in non-degassed toluene. For the zinc porphyrins, the LC-MS results show that the initial photoproduct contains two oxygen atoms. Based on electronic absorption and calculations, it is assigned to dehydrated zinc biladienone structure, relatively stable in toluene, but readily demetallated in dichloromethane. A similar species is formed also in the case of free bases, but it then undergoes hydration due to traces of water present in the solvent. Zinc derivatives were found to form biladienones even in degassed solutions. To explain this observation, we postulate formation of a complex with remaining oxygen or oxygen-containing species which is not removed by freeze-thaw procedure. This hypothesis is confirmed by MS results and by the analysis of photodegradation products obtained when zinc porphyrin is complexed with dimethylsulfoxide (DMSO). Under these circumstances, changes in absorption are the same as in the absence of DMSO when non-degassed toluene is used, but irradiation of deoxygenated solutions leads to a different photoproduct. For both degassed and non-degassed solvents, complexation with DMSO results in the enhancement of photostability.