Two Competing Excited-State Intramolecular Proton Transfer Pathways in AHMD

Excited-state intramolecular proton transfer (ESIPT) represents a fundamental process governing the photophysical behavior of hydrogen-bonded chromophores. In this study, we present a comprehensive theoretical investigation of two competitive ESIPT mechanisms in 4-amino-7-hydroxy-2-methylisoindoline-1,3-dione (AHMD), a newly synthesized fluorescent dye exhibiting high quantum yield and environmental stability. Using multiconfigurational electronic structure calculations (CASSCF/MS-CASPT2) combined with nonadiabatic surface-hopping dynamics simulations, we unravel the competitive proton transfer pathways and their coupling to nonradiative decay channels. Two distinct ESIPT routes are identified: a higher-barrier N2-H1 → O6 transfer (ESIPT-1, 7.26 kcal·mol⁻¹) and a near-barrierless O8-H7 → O12 transfer (ESIPT-2, 4.25 kcal·mol⁻¹), with the latter dominating ultrafast excited-state relaxation. The nonradiative deactivation predominantly occurs through a conical intersection (S₁S₀-C) associated with the ESIPT-2 channel, while the ESIPT-1 pathway is less favored both energetically and dynamically. Statistical analysis of surface-hopping trajectories shows that 33.8% of photoexcited molecules undergo nonradiative decay within 779 fs, whereas the majority persist in the excited state, rationalizing the high fluorescence efficiency observed experimentally. This study not only provides an atomistic resolution of proton transfer in a compact fluorophore but also offers guiding principles for the rational design of photostable ESIPT-active materials.