DFT study on the ground state and excited state intramolecular proton transfer of propargyl arm containing Schiff bases in solution and gas phases

Electronic structure calculations on 6,6′-(1E,1′E)-1,1′-(propane-1,3-diylbis(azan-1-yl-1-ylidene))bis(ethan-1-yl-1-ylidene)bis(3-(prop-2-ynyloxy)phenol) (L1) and (E)-2-(1-(2-hydroxyethylimino)ethyl)-5-(prop-2-ynyloxy)phenol (L2) compounds are carried out at B3LYP/6-311 + G(d,p) level of theory. The enol forms are found to be more stable than the corresponding keto forms in gas phase, whereas in solvent phase the reverse is true. The computed vibrational frequencies of L1 and L2 are compared with the available experimental data. Major orbital contributions for each electronic transition are assigned with the help of time-dependent density functional theory (TD-DFT). The UV–Visible spectral data of L1 and L2 coincide with the theoretical data of keto forms, which reveal that the compounds L1 and L2 exist mostly in keto forms rather than in enol forms in solution. Potential energy curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states are generated in gas and solution (solvent is dimethyl sulfoxide) phases. GSIPT for both L1 and L2 goes through a low activation barrier, whereas in case of ESIPT, barrierless proton transfer occurs.