Mechanistic Investigations on the role of Excited state Aromaticity during Photochemical Ring Opening Reaction

Photochemical activation enables selective control over specific bond dissociation pathways by tuning the excitation wavelength. Here, we investigate the photochemical activation of 2-aryl-5-carboxytetrazole (ACT), a highly efficient genetically encoded photo-cross-linker, whose reactivity arises from a coupled interplay of charge transfer, bond cleavage, and excited-state aromaticity modulation. Using state-of-the-art multireference electronic structure theory combined with semiclassical surface-hopping dynamics, we elucidate the complete reaction mechanism. Nonadiabatic dynamics at the CASSCF level, performed with both [2,2] and [4,4] active spaces, reveal that upon S₁ photoexcitation, electron density migrates from the pyrrole π-system to the tetrazole π*, initiating N–N bond activation and N₂ release via a conical intersection. Nucleus Independent Chemical Shift (NICS) calculations along the reaction coordinate confirm an intramolecular electron-catalyzed process, in which the pyrrole ring switches from aromatic to antiaromatic in the excited state and regains aromaticity after bond cleavage. Time-dependent DFT (B3LYP) calculations were employed solely for initial spectral benchmarking and bright-state identification, yielding good agreement with experiment. This integrated approach demonstrates that excited-state aromaticity can serve as a powerful driving force for photochemical transformations, offering a mechanistic blueprint for the rational design of next-generation photoactive molecular systems.