The Interplay between ππ*/nπ* Excited States in Gas-Phase Thymine: A Quantum Dynamical Study

A quantum mechanical study of the interplay between the bright pi pi* (S-pi) and the dark n pi* (S-n) excited states of thymine in the gas phase is reported. TD-PBE0 calculations indicate that within a relevant region of the S-pi surface, connecting the Franck-Condon point with the planar and non-planar S-pi plateau, S-pi and S-n are almost isoenergetic and that a S-pi -> S-n population transfer is therefore likely. This latter process has been studied by two complementary quantum dynamical approaches, a three-dimensional anharmonic (quartic) model, and a full-dimensional harmonic linear vibronic coupling model. Although providing slightly different quantitative indications, both approaches predict a very fast and effective S-pi -> S-n population transfer: already at 50 fs the S-n state is significantly populated (20-40%) and this population persists or even increases on a longer time scale.