Laser Photolysis Studies on Photodissociation of Axial Ligands from Isocyanide Complexes of Cobalt(III) and Rhodium(III) Porphyrins in Toluene Solutions. A Comparison with the Photochemistry of Carbonylrhodium(III) Porphyrin

Chlorocobalt(III) and iodorhodium(III) tetraphenylporphyrins, (Cl)CoIIITPP and (I)RhIIITPP, react with 2,6dimethylphenyl isocyanide (R-NC) in toluene solutions to give the isocyanide complexes, (Cl)(R-NC)CoIIITPP and (I)(R-NC)RhIIITPP, respectively. The 355-nm laser photolysis studies have shown that these complexes photodissociate the axial ligand, R-NC. The quantum yield, φ, for the photodissociation of R-NC from (Cl)(R-NC)CoIIITPP is determined as 0.091 in both degassed and oxygen-saturated toluene. On the other hand, (I)(R-NC)RhIIITPP gives φ ) 0.19 in degassed toluene and φ ) 0.07 in oxygen-saturated toluene. The dissociation of R-NC from (I)(R-NC)RhIIITPP was confirmed to occur from both the excited singlet and triplet states by transient spectroscopic measurements. The small quantum yield obtained with the oxygensaturated solution is explained by the quenching of the triplet state, 3(π,π*) of (I)(R-NC)RhIIITPP. The triplet state decays with a rate constant 9.5 × 104 s-1 in degassed toluene and is effectively quenched by oxygen with a bimolecular rate constant 6.7 × 108 M-1 s-1. In comparison with (I)(R-NC)RhIIITPP, the lifetime of the 3(π,π*) state of (Cl)(R-NC)CoIIITPP is too short to be detected by the present laser photolysis system, probably because of the very fast radiationless process, 3(π,π*) f (π,dz) f (dπ,dz). The dissociation of R-NC from (Cl)(R-NC)CoIIITPP is assumed to occur from the (dπ,dz) state. The long lifetime of the 3(π,π*) state of (I)(R-NC)RhIIITPP is interpreted on the assumption that the (dπ,dz) is located higher in energy than the 3(π,π*) state. The dissociation of R-NC from (I)(R-NC)RhIIITPP at the triplet state is suggested to occur from the thermally populated (dπ,dz) state. The laser photolysis studies of carbonylrhodium(III) tetraphenylporphyrin are also described to elucidate the ligand dissociation mechanism of rhodium(III) porphyrins.