Based on appropriate PES data, the Interacting Quantum Atoms method (IQA) allows to quantify the atomic and pair contributions that promote the relaxation of an excited electronic state, thus providing mechanistic information useful for rationalising photophysical processes in molecular systems.

Abstract

A theoretical study of the non-radiative photophysical relaxation mechanisms of the first singlet excited state of benzene, cyclobutadiene and fulvene is presented. For these molecules, the calculation of the Minimum Energy Path (MEP) leading from the Franck–Condon region to the surface crossing with the ground state is carried out. Subsequently, the decomposition of the electronic energies into atomic and pair contributions is performed using the Interacting Quantum Atom (IQA) method. The IQA approach provides the important mechanistic information necessary to rationalise some relevant aspects of the processes, such as the components that explain the appearance of an energy barrier or that favour the crossing between potential energy surfaces (PES); it also allows to quantify the direct effect on the MEP due to the inclusion of a substituent. In particular, it is shown how the IQA energies allow measuring the extent to which the formation of biradicaloid structures affects the crossing of the PES. The analysis of electron density functions suggests that aromaticity is not a driving force on the relaxation processes. Overall, this work shows the potential of the IQA method as a useful tool for the detailed description of photophysical processes.