Structural Correlations between Luminescent Properties and Excited State Internal Proton Transfer in Some Zinc(II) N,N′‑Bis(salicylidenes)

In this study, two salicylidene ligands, N,N′-bis­(salicylidene)-1,2-phenylenediamine and N,N′-bis­(salicylidene)-4,5-diaminopyrimidine, and their respective aquo-zinc­(II) coordination compounds were synthesized. Their characterization was performed by FTIR, proton and carbon NMR, elemental analysis, mass spectroscopy, and cyclic voltammetry. Crystal structures of the ligands were determined by monocrystal X-ray diffraction. The photoluminescent properties under photostationary conditions indicate that the ligand emission predominates in both the pristine materials and their zinc­(II)­complexes. For both ligands, the coordination of a metal atom leads to a redshift of their emission bands in both solvent and solid state. Molecular structures and excitation energies of ligands and complexes were evaluated at the DFT level using PBE0/aug-cc-pVDZ. Their ligand and complex electronic transitions can be assigned mainly to the intraligand π→π* type, mainly involving frontier molecular orbitals, with a small participation of the metal. According to our calculations, there is an increase in the planarity of the ligand structure in the complex, which could explain the redshifting observed in the absorption and emission spectra. The dynamic photoluminescence suggests the occurrence of excited state intramolecular proton transfer from the oxygen to the nitrogen atoms in the coordination site of the sal-4,5-pym. Moreover, they are able to predict the occurrence of the excited state internal proton transfer for the sal-4,5-pym. The dynamic of this proton transfer is demonstrated by both time-resolved emission spectra (TRES) and studies in protic solvent (ethanol).