Radiative deactivation of the lowest triplet state of dibenzo-p-dioxine

A theoretical evaluation of the dipole moments of vibronic transitions (which are governed by vibronic spin-orbit interactions) was carried out for dibenzo-p-dioxin molecules. On the basis of the comparison of the obtained results with the data on the distribution of line intensities in a fine-structure phosphorescence spectrum of dibenzo-p-dioxin at 4.2 K, it is inferred that radiative deactivation of the lowest triplet state T ₁ (ππ*) occurs from one of the triplet sublevels. For the molecules of dibenzo-p-dioxin and one of the isomers of tetrachlorodibenzo-p-dioxin, the effect of the form of vibrational modes and individual constants of the spin-orbit coupling in O and Cl atoms on vibronic spin-orbit interactions is discussed.