Ultrafast photodissociation of a metalloporphyrin in the condensed phase

Abstract

The photodissociation of axial ligands from six-coordinate nickel(II) porphyrins in solution has been studied by transient absorption spectroscopy with subpicosecond resolution. Our observations indicate that the deligation process takes place during the first several picoseconds following optical excitation of the macrocycle as the initially observed 3(π,π*) state relaxes to a dissociative ligand field excited state. No evidence is found for geminate recombination of the two ejected ligands with the four-coordinate photoproduct. A small fraction of the complexes appear to loose only one ligand. The absorption bands of the initially observed excited state and of the deligated species are also found to undergo a spectral evolution within the first 10 ps similar to that uncovered recently in the photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents [J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. Several possibilities are considered for these time-dependent spectral shifts, including vibrational dynamics.