On the interpretation of the external heavy atom effect on singlet-triplet transitions

Abstract

We have studied the cause for the external heavy atom (EHA) effect on radiative singlet-triplet (S-T) transition in π-electronic molecules by means of the quadratic response calculations accounting for spin-orbit coupling. The model systems employed, X+C 2H 4, X=F −, Cl −, Br −, HCl, Ar, take into account all essential interactions behind the EHA effect and provide an explanation for the principal physical effect, namely how the intermolecular electrostatic interaction can enhance the magnetic spin-orbit coupling responsible for the S-T transitions. The present study, including the first ab initio investigation of this kind, clearly indicates that the EHA effect should be interpreted as an increased spin-orbit coupling due to back-charge-transfer from the heavy atom. The conclusions are also verified by symmetry and molecular orbital arguments and by sum-over-state decompositions of the respons function results. A strong spin-sublevel selectivity of the EHA effect is obtained, with the in-plane sub-level, the most active one in pure hydrocarbon S-T radiative transitions, being greatly enhanced in the complex. The S-T transition moment exhibits a strong intermolecular distance ( r) dependence due to the EHA effect. This effect is found quite significant at an equilibrium distance corresponding to halide anion van der Waals complexes in gas phase. Our model is also relevant for interpreting EHA effects in liquid or rigid alkali halide solutions of dyes in polar solvents, when accounting for an increased distance r due to solvation shells, and for intersystem crossings in supersonic jet molecular beams. For the neutral systems (HCl, Ar) there is no significant EHA effect even for those complexes where a strong enhancement is obtained for the isoelectronic Cl − perturber.