Coexistence of ground and excited state intramolecular proton transfer of the Schiff base 2-((E)-(naphthalene-3-ylimino)-methyl)phenol: A combined experimental and computational study

Abstract

Prototropic behavior of the Schiff base compound, 2-((E)-(naphthalene-3-ylimino)-methyl)phenol (NMP), has been explored by photophysical/photochemical means and reported in this paper in a comprehensive manner. The study, comprising of absorption, steady state and time resolved fluorescence, 1H NMR, together with quantum chemical calculations, establishes that the probe undergoes intramolecular proton transfer (IPT) process in both ground and lowest excited singlet state. Resolution of the low energy absorption band of NMP in different solvents and observation of two different emission bands upon excitation at two different positions within the low energy absorption band indicates co-existence of two distinct species of the probe in the ground state, implying occurrence of the ground state intramolecular proton transfer (GSIPT) process. Large downshifted 1H NMR peak at δ > 13 ppm suggests a strong intramolecular H-bonding in the ground state and goes in favor of the GSIPT process. Excited state intramolecular proton transfer (ESIPT) of the probe has been confirmed by the existence of the large Stokes shifted emission band of the tautomer along with the normal (enol) emission band, especially in non-polar media. Observation of the single isoemissive point in the time resolved area normalized emission spectra (TRANES) in nonpolar solvents confirms existence of two distinct species (enol (N*) and tautomer (T*)) in the photoexcited state. Quantum chemical calculations using DFT/TDDFT have been performed to provide support to the experimental findings.