Synthesis, Structure and Photophysical Properties of Dinuclear Zinc(II) Complexes with Schiff Base Bearing p-Phenylenediamine.

Four new dinuclear complexes Zn1-Zn4 were synthesized by the interaction of Zn(NO₃)₂.4H₂O with Schiff bases containing p-phenylenediamine. The data of ESI-MS, IR and ¹H NMR spectroscopy showed that the ligands coordinated with Zn(II) via two sets of O, N_imine atoms. In DMSO, ligands exhibit weak luminescence while the complex solutions emit strongly at maximum wavelength in the range 409-513 nm. Notably, electron-donating groups enhance the quantum yield of the complex solutions. Particularly, Zn2 containing NEt₂ substitutes has outstanding quantum yields of 74%. Besides, the lifetimes of the complexes increase in the order Zn2 < Zn4 < Zn1 < Zn3 in DMSO. Along with the experiment, the structure, absorption and emission spectra of Zn1-Zn4 have been investigated by TD-DFT calculation. The result reproduces well the experimental absorption wavelengths and shows that the absorptions of the complexes are due to the singlet S₀ → S₁^* transition from the π to the π* orbital of the ligand in complexes and the emission occurs from minimum of the S₁ state to the ground singlet state S₀. The calculation results also support for the fact that Zn2 has profoundly quantum yields.