Electron Density Redistribution as a Descriptor for Excited-state Reactivity

The present work introduces a novel framework to ana- lyze excited-state reactivity by leveraging the redistribution of electron density between two states. This methodology builds upon and extends the Quantum Theory of Atoms in Molecules (QTAIM) and state-specific density partition- ing to characterize reactive sites in excited molecules. By employing the electronic redistribution function ∆ρij (r), we demonstrate its ability to delineate reactive sites, de- scribe bond formation dynamics, and predict photoprod- ucts across diverse molecular systems. Examples include the [2+2] cycloaddition of ethylene, the Paternò–Büchi re- action, and thiocarbonyl-initiated cycloadditions, each ex- hibiting distinct electronic redistribution patterns that gov- ern their excited-state behavior. Furthermore, ∆ρij (r) ef- fectively captures reactivity near conical intersections, offer- ing new insights into degenerate regions of potential energy surfaces. This approach provides a unifying descriptor for excited-state reactivity, bridging classical chemical intuition with a quantum mechanical approach, and sets the stage for developing predictive models for complex photochemi- cal systems.