Resonance effects of diabatic surface crossing within the torsional spectrum of 9-(N-carbazolyl) anthracene observed by supersonic jet fluorescence spectroscopy

Using the supersonic jet technique and laser-induced fluorescence spectroscopy, the ground and excited state surface of isolated 9-(N-carbazolyl) anthracene (C9A) is investigated. Ground and excited state torsional potentials of high accuracy are deduced from excitation and fluorescence spectra, considering characteristic patterns of Franck–Condon factors within the dispersed fluorescence. S0 exhibits a very flat double minimum potential (equilibrium twist angle 77.5°, barrier 17 cm−1); the barrier for perpendicularity in S1 is approximately 1050 cm−1 and the equilibrium angle is shifted towards coplanarity (64°). An unusual intensity profile of the long progression found in the fluorescence excitation spectrum is ascribed to a resonant nonradiative decay channel within the excited state surface. State selective fluorescence decay rates vs excess vibrational energy confirm this resonant relaxation process. This uncommon observation leads to a model of diabatic surface crossing along the torsional coordinate where the crossing ‘‘dark’’ state is discussed as a predicted charge transfer state or a higher lying triplet state, mediating further electronic relaxation. Although extended intermolecular vibrational redistribution (IVR) is present in the fluorescence spectra from high vibrational levels, this process is of secondary importance for the resonant nonradiative relaxation.