Internal heavy-atom effect on the radiative deactivation of the triplet state in the dibenzo- p -dioxin chloroderivatives of D 2h and C 2h symmetry

Abstract In this work the heavy-atom effect on the transition dipole moment P i from the i -sublevels of 3 B 1u and 3 B u triplet ππ ∗ electronic stations of dibenzo- p -dioxin and its 2,3,7,8-tetra-, 1,3,6,8-tetra-, and octachlorinated derivatives has been estimated theoretically with the use of the INDO/S and one-center spin–orbit (SO) coupling approximation. The additivity of the contribution from a number of heteroatoms to the P I i and P II i values and also the compensation effect resulting from non-equivalent positions of similar heteroatoms are established, where roman numbers I and II correspond to different mechanisms: the SO perturbations of the ground state and the T 1 triplet state, respectively. The probable causes of the unusual (“inverse”) heavy-atom effect are revealed. The phosphorescence rate constant and lifetime are estimated for some dioxins by using the experimental values of the ratios of the rate constant components governed by SO and vibronic-spin–orbit coupling.