Time-resolved fluorescence and absorption spectroscopies of porphyrin J-aggregates

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 S-2-exciton state to the S-1-exciton state is observed as an S-2-fluorescence and a recovery from a bleaching of the S-2-exciton state. The S-2-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 S-2-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 S-1-exciton following S-2-->S1IC is also studied and several relaxation processes such as an intra-aggregate vibrational energy redistribution, vibrational and phase-space coolings are investigated. (C) 2002 American Institute of Physics.