Structural studies of a novel tautomeric Schiff base derived from 4-(N,N’-diethylamino)salicylaldehyde and 2-amino-4-methyl phenol: An experimental and theoretical study

Abstract

A novel Schiff base, LMAP synthesized from 4-(N,N’-diethylamino)salicylaldehyde and 2-amino-4-methyl phenol has been characterized by single crystal X-ray diffraction and spectroscopic methods. DFT methods is used to investigate the enol-imine (EI) ⇄ keto-amine (KA) tautomeric behavior with respect to phase and in solvent media. The synthesized Schiff base crystallizes in the keto-amine (KA) form in the orthorhombic lattice and Pna21space group with four atoms per unit cell (Z = 4). Bond length analysis indicates that the KA form possesses significant zwitter ionic character. Study of electronic absorption and emission properties was conducted in the solid state as well as in different solvents. The UV–Vis result reveals the varied structural preference of the compound depending on the solvent media: polar protic solvents stabilize the KA form, while nonpolar and polar aprotic solvents favor the EI form. Quantum mechanical computations using DFT and TD-DFT also confirm these findings. Fluorescence emission spectra show multiple emission bands which indicate both excited state intramolecular proton transfer (ESIPT) and charge transfer (CT) phenomena occur simultaneously within the molecule. DFT predicted NLO responses of KA and EI were compared and found that the NLO response of KA is superior to that of EI.