Ultrafast dynamics of exciton localization in photochemically reactive molecular crystals

Ultrafast exciton localization in molecular crystals plays an essential role in the solid-state photochemical reactions. Studying exciton dynamics that convert separate electrons and holes into a molecular excited state is challenging because it requires a computational approach that bridges condensed-phase and molecular electronic structure theories. In this work, we take advantage of the periodic time-dependent density functional theory to investigate the exciton localization dynamics in three molecular crystals, 1,2-bis(4-(anthracen-9-yl)phenyl)diazene (1), 1-methylimidazoyl enone (2), and 3-azido-1-(4-chlorophenyl)propenone (3), undergoing typical photochemical isomerization, [2 + 2]-cycloaddition, and dissociation. Our calculations showed delocalized ππ*-type electron and hole distributions during vertical excitation of ground-state molecular crystals. The S1-state geometry optimization revealed a partially localized exciton across multiple molecules in the crystals. Comprehensive S1 dynamics simulations uncovered the system-dependent role of exciton localization: (1) the exciton in molecular crystal 1 showed an intermolecular charge-transfer from the neighboring anthracene and azobenzene groups, where the hole on the anthracene groups moved around the electron at central azobenzene; (2) the exciton dynamics in molecular crystal 2 showed competing exciton transportation and localization, where electron and hole separate or combine via the ππ-stacking; and (3) the exciton in molecular crystal 3 showed dominant intramolecular CT, while the close-packing still generates a competing intermolecular CT between a dimer. These findings provide a fundamental understanding of exciton dynamics in photochemically reactive molecular crystals, paving the way for improving simulations of solid-state photochemical reactions.