Internal conversion in the S_1^1B_[3u] state of pyrene

Abstract

Fluorescence excitation spectra and fluorescence lifetimes at single vibronic levels in the S_1 state have been observed for jet-cooled pyrene. The fluorescence lifetimes at the zero-vibrational levels of the S_1^1B_[3u] states of pyrene-h_10 and pyrene-d10 are 1480 and 1470 ns, respectively, and the relaxation is considered to be dominated by the radiative process. For some vibrational levels, however, the lifetimes are remarkably shorter such as 765 ns at the 22^1 vibronic level of pyrene-h_10 (ν22 (b_[1g]); C–H bending and skeletal deforming mode), indicating that nonradiative transition occurs at a specific vibrational level. In this study, we demonstrate that the main process is internal conversion to the S_0^1Ag state caused by nonadiabatic vibronic interaction via b_[3u] promoting modes.