The 1A1(ππ∗)←X1A1 transition in jet-cooled 4H-pyran-4-thione: evidence for a dark 1B1(nπ2∗) state below 1A1(ππ∗)

The 3A2(nπ∗)→X1A1 phosphorescence excitation spectrum of jet-cooled 4H-pyran-4-thione (PT) was investigated in the region of the 1A1(ππ∗)←X1A1 absorption band between 27 300 and 28 800 cm−1. The spectrum exhibits complex features, which are typical for the (strong) vibronic coupling case of two adjacent electronic states. The observed intermediate level structure was attributed to the coupling with a lower lying dark electronic state 1B1(nπ2∗), whose origin was estimated to be ∼825–1025 cm−1 below the origin 1A1(ππ∗)0, using calculations of vibronic state densities in combination with symmetry considerations. Consequences of the vibronic coupling on the decay dynamics of 1A1(ππ∗) as well as tentative assignments of vibronic transitions 1A1(ππ∗)ν←X1A1 are also briefly discussed.