The [formula omitted] transition in jet-cooled 4H-pyran-4-thione: evidence for a dark [formula omitted] state below [formula omitted

Abstract

The 3 A 2(n π ∗)→ X 1 A 1 phosphorescence excitation spectrum of jet-cooled 4H-pyran-4-thione (PT) was investigated in the region of the 1 A 1( ππ ∗)← X 1 A 1 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 1 B 1(n π 2 ∗) , whose origin was estimated to be ∼825–1025 cm −1 below the origin 1 A 1( ππ ∗) 0 , using calculations of vibronic state densities in combination with symmetry considerations. Consequences of the vibronic coupling on the decay dynamics of 1 A 1( ππ ∗) as well as tentative assignments of vibronic transitions 1 A 1( ππ ∗) ν← X 1 A 1 are also briefly discussed.