A Qualitative Approach for Predicting Enhanced Intersystem Crossing in Chromophore-Radical Systems.

Enhanced Intersystem Crossing (EISC) is an important mechanism that allows for formally forbidden population transfer from the singlet to triplet manifold in chromophore-radical (C-R) systems. We use first order perturbation theory to estimate the likelihood of EISC in various organic C-R molecules. The first order mixing coefficient κ between the states involved in EISC depends on the difference in pairwise exchange interactions between photoexcited chromophore electrons and the radical. Exchange coupling constants were calculated with the Heisenberg-Dirac-Van Vleck Hamiltonian using excited state wave functions and energies obtained from the CASSCF/QD-NEVPT2 calculations. The predictions derived using this framework are in a good agreement with the available experimental data on EISC observed with transient absorption spectroscopy.