Nickel porphyrin photophysics and photochemistry. A picosecond investigation of ligand binding and release in the excited state

The photophysical behavior and some photochemical processes for nickel (II) porphyrins have been examined with picosecond transient absorption techniques. Detailed results are reported for Ni-octaethylporphyrin (NiOEP) and Ni-protoporphyrin IX dimethylester (NiPPDME) in toluene, pyridine and piperidine. Excitation flashes at six wavelengths between 355 and 532 nm have been employed. In toluene, rapid (≤ 15 ps) radiationless decay occurs via several pathways to the low-lying ³B_1g excited state. In the basic solvents pyridine and piperidine, excited states with nickel ¹A_1g (d_z2) character have a tendency to release ligands bound to the metal. Excited states with nickel ¹B_1g or ³B_1g (d_z2, d_x²-y²) character on the other hand, have an affinity for basic ligands, which rapidly bind to the metal. The competition between radiationless decay, ligand binding, and ligand release depends on the nickel porphyrin, solvent, and excitation wavelength. A set of "rules" has been developed that gives a consistent view of all of our results and those of previous investigators. These results may be helpful in understanding photoprocesses in other transition-metal porphyrins, including hemes, which have particular biological significance.