Internal conversion rates for single vibronic levels of S2 in azulene

Simultaneous measurements of absorption and fluorescence in a pulsed planar supersonic jet are used to determine fluorescence quantum yields of nine single vibronic levels (SVL’s) in the S2 electronic manifold of azulene. The quantum yield at the spectroscopic origin is 0.042±0.004, somewhat larger than that obtained in earlier room-temperature measurements in solution. Most SVL’s have quantum yields within experimental error of this value, except for two SVL’s characterized by large-amplitude motion at carbons 1 and 3, whose quantum yields are about 0.06. Our quantum yield data are combined with recent lifetime measurements by other workers to obtain fluorescence and internal conversion decay rates for each SVL. The radiative rates vary by a factor of 2, as expected for the S2↔S0 transition, whose absorption strength is strongly enhanced by vibronic coupling between S2 and S4. The internal conversion rates increase with excess vibrational energy in a way that parallels the extent of intramolecular vibrational redistribution (IVR), as determined by other workers using SVL emission and time-and-energy-resolved emission. Thus, the internal conversion rates in this electronic state appear to exhibit behavior typical of large aromatic molecules.