Picosecond Flash Photolysis of Carbonyl Complexes of Ruthenium(II) Porphyrin π Cation Radicals

With picosecond transient absorption techniques we have investigated photolysis of the Ru(II) octaethylporphyrin π cation radicals Ru(OEP⁺·)CO(L): L = EtOH (4a), py (4b), Im (4c), Br^- (4d). Excitation with 35-ps flashes at 532 nm results in the formation and decay of transient states having lifetimes less than the flash duration. The transient lifetimes for ²A_1u ground-state π cation radicals are shorter than those observed after excitation of species with the ²A_2u ground state. The transient behavior is rationalized in terms of both release of CO followed by rapid geminate recombination and of rapid radiationless decay via nondissociative states. The latter process seems more likely, but the former cannot be ruled out. Low-lying (d, π) charge-transfer states of the π cation radicals might provide effective routes for deactivation. Photolysis with 355-nm flashes results in the formation of similar short-lived transients. However, in the case of 4a and 4b, having mainly the ²A_2u ground state, a longer-lived photoproduct is observed. This new transient could be the carbonyl-free π cation radical, but it is more likely the Ru(III) species obtained upon release of the CO followed by internal electron transfer from metal to ring. The quantum yield of the long-lived photoproduct appears to be low (<25%). Decarbonylation with 355-nm pump pulses is ascribed to the population of a dissociative state, such as (d_π d_z²), not accessible with the lower energy 532-nm excitation flashes. Apparently, rapid deactivation to the ground state competes favorably with ligand release even in these cases. The behavior is discussed in terms of the electronic states of the ring, metal, and ligands. The relevance of these findings to the release of ligands by hemoglobin and myoglobin as well as by other transition metal porphyrin complexes is considered.