Ultrafast vibrational relaxation and ligand photodissociation/photoassociation processes of nickel(II) porphyrins in the condensed phase

We have carried out a femtosecond transient absorption spectroscopic study on nickel(II) porphyrins in various solvents in order to obtain detailed information on vibrational relaxation processes occurring in the initial stage after photoexcitation to the highly excited states. We found the decay process of time constant of approximately 1 ps corresponding to the intramolecular vibrational relaxation process for Ni(II)TPP and Ni-(II)OEP in toluene. In addition to this process, the intermolecular vibrational relaxation process with 10-20 ps lifetime was also observed for Ni(II)OEP in toluene, although its contribution to the overall decay process is relatively weak probably due to the weak solute/solvent interaction. In coordinating solvents such as pyridine and piperidine, we observed the intramolecular vibrational relaxation processes before complete population of the bottleneck excited metal ¹|0,d_z2〉 or ³|0,³(d,d)〉 state. In this case, it is likely that the intermolecular vibrational relaxation process associated with photodissociation/photoassociation processes depending on the selective excitation of four- and six-coordinate species is accompanied by the intramolecular vibrational relaxation due to the strong solute/solvent interaction. These processes are also believed to be responsible for the excess energy dissipation of highly excited nickel(II) porphyrins into the surrounding solvent molecules.