Excited electronic states and effect of vibronic-spin–orbit coupling on the radiative deactivation of the lowest triplet states of dioxins

Abstract Semiempirical, ab initio, and density functional (DFT) calculations of vibrational frequencies and excited electronic states have been performed for 1,3,6,8-tetrachlorodibenzo- p -dioxin. The DFT calculations allow to assign the fine structure phosphorescence spectrum at 4.2 K. The phosphorescence rate constant K ph is found to be determined by the vibronic-spin–orbit coupling ( k ph = k VSO ). The symmetry and energy of the lowest triplet state ( 3 A g , 24 689 cm −1 ) as well as frequencies of A g and B u (low energy) fundamental vibrations have been obtained experimentally. The k VSO value is estimated. In series of dioxins the k VSO values are compared.