Excited state intramolecular proton transfer in 3-hydroxy-2-naphthaldehyde: A combined study by absorption and emission spectroscopy and quantum chemical calculation

The photophysical behavior of 3-hydroxy-2-napthaldehyde (HN32) has been investigated on the basis of steady state absorption and fluorescence spectroscopy, time-resolved emission spectroscopy and quantum chemical calculations. In the ground state, depending on the nature of solvents, HN32 exists as closed (intramolecular hydrogen bonded), open and intermolecular hydrogen bonded conformers. The closed conformer present in the hydrocarbon as well as other solvents, on excitation, undergoes intramolecular proton transfer across the pre-existing intramolecular hydrogen bond to form the keto tautomer having red shifted emission. In case of hydroxylic polar solvents, an additional low energy fluorescence band is attributed to the intermolecular hydrogen bonded solvated complex of HN32 molecule. Structural calculation and the theoretical potential energy curves along the proton transfer coordinate performed by ab initio and Density Functional Theory (DFT) account well the observed spectral characteristics of HN32 for the ground and excited states.