Excited State Intramolecular Proton Transfer (ESIPT) from -NH₂ to the Carbon Atom of a Naphthyl Ring

Excited state intramolecular proton transfer (ESIPT) has been documented from an amino NH₂ group to a carbon atom of an adjacent aromatic ring. This finding changes the paradigm, as hitherto such processes have not been considered as plausible due to slow protonation of carbon and low (photo)­acidity of the NH₂ group. The ESIPT was studied by irradiation of 2-(2-aminophenyl)­naphthalene in CH₃CN–D₂O, whereupon regiospecific incorporation of deuterium takes place at the naphthalene position 1, with a quantum yield of Φ = 0.11. A synergy of experimental and computational investigations completely unraveled the mechanism of this important photochemical reaction. Upon excitation to the photoreactive S₂(L_a) state, a favorable redistribution of charge sets the stage for ESIPT to the carbon atom in naphthalene position 1. H₂O molecules are needed, as they increase the excitation energy and oscillator strength for the population of the S₂(L_a) state. The gain in energy is used to surmount a small energy barrier on the pathway from the Franck–Condon geometry to the conical intersection with the S₀, delivering aza-quinone methide.