Facile luminescent tuning of Zn(II)/Hg(II) complexes based on flexible, semi-rigid and rigid polydentate Schiff bases from blue to green to red: structural, photophysics, electrochemistry and theoretical calculations studies.

Abstract

The photophysical properties of Zn(II)/Hg(II) Schiff base complexes could be fine and predictably tuned over a wide range of wavelengths by changing the ligand structures. A new series of polydentate Schiff base-type ligands, N,N'-bis(2-pyridinylethylidene)R(3)-1,2-diamine (), which contain a flexible, semi-rigid or rigid group (R(3) = butyl, cyclohexane, tolyl and phenylene), has been designed and employed for synthetizing new mononuclear or binuclear trans Zn(II)/Hg(II) complexes with a general formula of [M()Cl2] ( = N,N'-bis(2-pyridinylethylidene)phenylene-1,2-diamine, M = Zn, ; M = Hg, ), [M()Cl2] ( = N,N'-bis(2-pyridinylethylidene)toluene-3,4-diamine, M = Zn, ; M = Hg, ), [M2()Cl4]·nCH2Cl2 ( = N,N'-bis(2-pyridinylmethylene)cyclohexane-1,2-diamine, M = Zn, n = 0, ; M = Hg, n = 1, ), [M2()Cl4]·nCH3OH ( = N,N'-bis(2-pyridinylethylidene)cyclohexane-1,2-diamine, M = Zn, n = 1, ; M = Hg, n = 0, ), [M2()Cl4] ( = N,N'-bis(3-methoxy-2-pyridinylmethylene)-cyclohexane-1,2-diamine, M = Zn, ; M = Hg, ), [M2()Cl4]·nCH3CN ( = N,N'-bis(3-methoxy-2-pyridinylmethylene)butane-1,4-diamine, M = Zn, n = 4, ; M = Hg, n = 0, ). All the ligands and complexes have been characterized by elemental analyses, IR spectra, and (1)H NMR spectra. Twelve structures of , , , , , and crystallized in three different conditions are further determined by single-crystal X-ray diffraction analyses. Their properties are fully characterized by UV-vis and fluorescence spectra both in solution and the solid state at room temperature. The luminescence color of these Zn(II)/Hg(II) Schiff base complexes could be tuned from blue to green to red (429-639 nm for , 434-627 nm for ) in solution by changing the ligand conjugated systems from flexibile () to semi-rigid () to rigid (). The spectra of the free Schiff bases are centered around 402-571 nm, which are perturbed upon the coordination to the Zn(II)/Hg(II) ion. Both the electrochemical data and TD-DFT calculations show that the HOMO-LUMO band gap from the ligand to the complex is reduced by complexation. Meanwhile, the emission efficiencies of Zn(II)-complexes are found to be strongly dependent on the Schiff-base ligands with quantum yields ranging from 14% to 25% for . However, the emission efficiencies dramatically decline in Hg(II)-complexes with quantum yields ranging from 4% to 19%, due to the heavy atom effect.