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

## TL;DR

This paper introduces a new method to predict enhanced intersystem crossing in chromophore-radical systems using first-order perturbation theory.

## Contribution

A novel theoretical framework is proposed to estimate enhanced intersystem crossing likelihood in organic chromophore-radical molecules.

## Key findings

- The first-order mixing coefficient κ is derived based on exchange interactions between photoexcited chromophore electrons and radicals.
- Exchange coupling constants were calculated using the Heisenberg–Dirac–Van Vleck Hamiltonian and CASSCF/QD-NEVPT2 methods.
- Predictions align well with experimental data from transient absorption spectroscopy.

## Abstract

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.