Competitive intramolecular hydrogen bonding formation and excited-state proton transfer reaction in 1-[(diethylamino)-methyl]-2-hydroxy-3-naphthaldehyde

1-[(diethylamino)-methyl]-2-Hydroxy-3-naphthaldehyde (DMHN) possessing dual hydrogen bonding (HB) sites (O–H⋯NR2 (conformer A) and O–H⋯OC (conformer B)) has been synthesized to study the competitive excited-state intramolecular proton transfer (ESIPT) reaction. Despite near degeneracy between conformers A and B, the associated chromophores are significantly different and can be rationalized by different degrees of hydrogen bond-induced π electrons delocalization. ESIPT takes place in both conformers A and B in cyclohexane, resulting in a zwitterion (λmax∼485 nm) and an extremely weak keto-tautomer (λmax∼730 nm) emission, respectively. The response-limited rise dynamics for both A and B conformers (<150 fs) leads us to conclude that both ESIPT processes are essentially barrierless and the interconversion between two HB conformers is prohibited in the excited state.