The mechanism of the excited-state proton transfer of Salicylaldehyde azine and 2,2'-[1,4-Phenylenebis{(E)- nitrilomethylidyne}] bisphenol: Via single or double proton transfer.

Abstract

The Salicylaldehyde azine (H2SA) and 2,2'-[1,4-Phenylenebis{(E)-nitrilomethylidyne}] bisphenol (H2SPA) with double proton transfer characteristics were synthesized recently (Phys. Chem. Chem. Phys., 2018, 20, 23,762). However, the detailed theoretical interpretation of proton transfer (PT) mechanism is inadequate. In the present work, density functional theory (DFT) and time-density functional theory (TDDFT) are employed to study the proton transfer mechanism of H2SA and H2SPA in detail. Bond parameters, infrared (IR) spectra and frontier molecular orbitals (FMOs) calculated by PBE0/TZVP method indicate the strength of hydrogen bond is enhanced in S1 state, which can be visualized by the reduced density gradient (RDG) analysis. The potential energy surfaces (PESs) of H2SA and H2SPA are also constructed. The small barriers indicate that both the single proton transfer and double proton transfer of H2SA and H2SPA are more likely to occur in the S1 state. In addition, the properties of H2SA and H2SPA after chelation with Li+ have also been theoretically characterized. According to the calculated fluorescence spectra of compounds (H2SA-Li+ and H2SPA-Li+), it was found that only the planar structure of H2SA-Li+ can form metallogel, which verified the experimental results.