Evidence of vibrational relaxation from non-Boltzmann to Boltzmann distribution over v′=0,1,2 levels in S1 perylene in solution: Franck–Condon analysis of time-resolved fluorescence

Thermal relaxation process of a vibrational excess energy of ∼2800 cm−1 deposited on v′=2 level of ring-breathing mode has been investigated on S1 perylene and S1 12-(3-perylenyl)dodecanoic acid in 2-methyltetrahydrofuran at room temperature. The Franck–Condon analysis of femtosecond time-resolved fluorescence spectra indicated that noticeable vibrational transients exist at v′=2 level even a few picoseconds after photoexcitation. It was confirmed that the population distribution over v′=0,1,2 levels is non-Boltzmann type at early time stage and converges into Boltzmann type with a few picoseconds time constant.