Jet spectroscopy of buckybowl: Electronic and vibrational structures in the S₀ and S₁ states of triphenylene and sumanene

Sumanene is a typical buckybowl molecule with C_3v symmetry. We observed a fluorescence excitation spectrum and a dispersed fluorescence spectrum of sumanene in a supersonic jet. Bowl effects were clarified by comparing the spectra with those of triphenylene (D_3h symmetry), which is a planar prototype of nonplanar sumanene. The S₁ ¹A₁ ← S₀ ¹A₁ transition is symmetry allowed. We found the 000 band in the fluorescence excitation spectrum at 357.78 nm; this band was missing in the forbidden S ₁A1′1 ← S₀A1′1 transition of triphenylene. The transition moment was shown to be along the oblate symmetric top axis (out of plane) by the observed rotational contour. A large number of vibronic bands were observed, unlike in triphenylene. Some were considered to be out-of-plane vibrational modes, which lead to a bowl-to-bowl inversion reaction assisted by in-plane vibrations. We found that the vibronic bands were markedly weak in the high energy region of triphenylene-d₁₂. This indicates that the fluorescence quantum yield is very low at the high vibrational levels in the S₁ state due to the rapid radiationless transition. The main process is considered to be internal conversion to the S₀ state. The nonplanar structural distortion may also enhance radiationless transitions. We could not, however, observe weakening of the vibronic bands in the fluorescence excitation spectrum of sumanene.