Mode-selective internal conversion of perylene

We observed fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation of jet-cooled perylene-h ₁₂ and perylene-d ₁₂, and carefully examined the vibrational structures of the S₀ ¹ A _g and S₁ ¹ B _2u states. We performed vibronic assignments on the basis of the results of ab initio calculation, and found that the vibrational energies in the S₁ state are very similar to those in the S₀ state, indicating that the potential energy curves are not changed much upon electronic excitation. We conclude that the small structural change is the main cause of its slow radiationless transition and high fluorescence quantum yield at the zero-vibrational level in the S₁ state. It has been already reported that the lifetime of perylene is remarkably short at specific vibrational levels in the S₁ state. Here, we show that the mode-selective nonradiative process is internal conversion (IC) to the S₀ state, and the ν₁₆(a _g ) in-plane ring deforming vibration is the promoting (doorway) mode in the S₁ state which enhances vibronic coupling with the high-vibrational level (b _2u ) of the S₀ state.