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Dynamics of excited states in porphyrin J-aggregates has been investigated using femtosecond time-resolved fluorescence and absorption spectroscopies. An ultrafast relaxation process due to internal conversion (IC) from the S2-exciton state to the S1-exciton state is observed as an S2-fluorescence and a recovery from a bleaching of the S2-exciton state. The S2-fluorescence shows a sharp spectrum with almost no Stokes shift with a decay-time constant of 360±70 fs. In the transient absorption spectrum, the bleaching of the S2-exciton state disappears with a time constant of about 300 fs, which is in agreement with the result of the time-resolved fluorescence data. Relaxation dynamics of the S1-exciton following S2→S1IC is also studied and several relaxation processes such as an intra-aggregate vibrational energy redistribution, vibrational and phase-space coolings are investigated.
Kano et al. (Tue,) studied this question.