Internal conversion and vibronic relaxation from higher excited electronic state 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 vibronic relaxation in porphyrin derivatives, we have studied the fluorescence dynamics of Zn-tetraphenylporphyrin (ZnTPP) and Zn-diphenylporphyrin derivatives (ZnDPP) in fs-ps time regimes by means of fluorescence up-conversion technique. Detailed measurements on ZnTPP in ethanol have demonstrated fluorescence dynamics over the whole spectral range from 430 to 620 nm when excited to the S₂ state. The time constant (∼2.3 ps) of the single-exponential decay of S₂ fluorescence around 430 nm agreed with that of the single-exponential rise of S₁ fluorescence around 600 nm (wavelength of 0-0 transition in the stationary spectrum), indicating that the relaxation by the ultrafast vibronic redistribution immediately after S₂ → S₁ internal conversion mainly gives lower vibronic states near the bottom of the S₁ state. However, we have observed the dynamics of weak hot fluorescence probably from the nonrelaxed vibronic state immediately after internal conversion and also higher vibronic states in S₁ formed in competition with the main product of the vibronic redistribution, all over the wavelength region between S₂ and S₁. Preliminary results of our studies on ZnDPP were very similar to those of ZnTPP.