Internal conversion and vibronic relaxation from higher excited electronicstate of porphyrins: Femtosecond fluorescence dynamics studies

To elucidate the dynamics and mechanisms of radiationless transitions from higher excited electronic states as well as the ultrafast intramolecular vi bronic relaxation in porphyrin derivatives, we have studied the fluorescenc e dynamics of Zn-tetraphenylporphyrin (ZnTPP) and Zn-diphenylporphyrin deri vatives (ZnDPP) in fs-ps Lime regimes by means of fluorescence up-conversio n technique. Detailed measurements on ZnTPP in ethanol have demonstrated fl uorescence dynamics over the whole spectral range from 430 to 620 nm when e xcited to the S-2 state. The time constant (similar to 2.3 ps) of the singl e-exponential decay of S-2 fluorescence around 430 nm agreed with that of t he single-exponential rise of S-1 fluorescence around 600 nm (wavelength of 0-0 transition in the stationary spectrum), indicating that the relaxation by the ultrafast vibronic redistribution immediately after S-2 --> S-1 int ernal conversion mainly gives lower vibronic states near the bottom of the S-1 stale. However, we have observed the dynamics of weak hot fluorescence probably from the nonrelaxed vibronic state immediately after internal conv ersion and also higher vibronic states in S-1 formed in competition with th e main product of the vibronic redistribution, all over the wavelength regi on between S-2 and S-1. Preliminary results of our studies on ZnDPP were ve ry similar to those of ZnTPP.