[en] The ground and excited state properties of 1-hydroxy-2-naphthaldehyde (HN12) towards proton transfer reaction have been elaborately investigated on the basis of steady state absorption and emission, time-resolved fluorescence spectroscopy and quantum chemical calculations by ab initio (Hartree-Fock) and density functional theory (DFT) methods. The molecule HN12 exists as closed (intramolecular hydrogen bonded) and open conformer and their anions, and the hydrogen bonded solvated cluster in the ground state. The closed conformer on excitation undergoes photo-induced keto-enol tautomerism across the pre-existing intramolecular hydrogen bond (IMHB) and shows a red shifted emission band for the keto tautomer. Theoretically a significant change of some structural parameters such as O_d-H₁ bond length, O_d...H₁...O_a bond angle and charge distribution at the proton translocation site in the excited state favours intramolecular proton transfer process. Calculated potential energy curves along the proton transfer coordinate at the pre-existing hydrogen bonding site by DFT level account well for the feasibility of a barrierless ultrafast excited state intramolecular proton transfer reaction in HN12 molecule