Electron Density Redistribution as a Descriptor for Excited-State Reactivity.

This study introduces the electronic redistribution function, Δρij(r), as a descriptor for analyzing excited-state reactivity in photochemical processes. Building upon the Quantum Theory of Atoms in Molecules (QTAIM) and state-specific density partitioning, Δρij(r) quantifies electron density changes between two electronic states (i and j) and identifies reactive sites by integrating charge redistribution over atomic basins. This approach includes state weights (ci2, cj2) to ensure physically meaningful interpretations of density differences. Applied to diverse systems─including the [2 + 2] cycloaddition of ethylene, Paternò-Büchi reactions of carbonyl/thiocarbonyl compounds, and photocyclizations of 1,3-butadiene─the descriptor reveals distinct reactivity patterns. For instance, Δρij(r) delineates charge depletion (δ+) and accumulation (δ-) sites, predicting interactions that drive photoproduct formation. The framework is further extended to analyze conical intersection regions, offering insights into bond-forming mechanisms for processes such as benzene isomerization and ethylene cycloaddition.