Metalloporphyrin Photochemistry with Matrix Isolation

The photochemistry of a number of metalloporphyrin oxoanion complexes has been examined by matrix isolation techniques, using both frozen solvent glasses and polymer films. After an extensive search for a noncoordinating, unreactive, glassing solvent, a 3:1 mixture of 2,2-dimethylbutane and tert-butylbenzene was found to work well at temperatures below 70 K. Alternatively, the photochemistry of metalloporphyrins was monitored in polymer films by the evaporation on a sapphire window of metalloporphyrin solutions in toluene containing either poly (methyl methacrylate) or poly(α-methylstyrene). The polymer films have the added advantage of a greatly increased temperature range, providing diffusional isolation even at room temperature. The photoreduction of the metal by homolytic α-bond cleavage and loss of the axial ligand appears to be a general mechanism for all metalloporphyrin complexes examined. The formation of metal-oxo species from photolysis of metalloporphyrin oxoanion complexes in solution derives from secondary, thermal reactions.