Excitation relaxation of zinc and free-base porphyrin probed by femtosecond fluorescence spectroscopy

Fluorescence rise and decay times of zinc porphyrin (ZnP) and free-base porphyrin (H2P) were measured using a fluorescence up-conversion method. The observed fluorescence signals were analyzed to be a sum of three exponentials: 150 fs, 600 fs, and 1.5 ns for ZnP; and 90 fs, 1.5 ps, and 12 ns for H2P. The two fastest components of 150 and 600 fs for ZnP are associated with the decay processes of B→Q internal conversion and the vibrational relaxation in the Q state, respectively. The short-lived components of 90 fs and 1.5 ps for H2P are correlated with the decay processes of Qy→Qx internal conversion and the vibrational relaxation in the Qx state, respectively.