Facile luminescent tuning of ZnII/HgII complexes based on flexible, semi-rigid and rigid polydentate Schiff bases from blue to green to red: structural, photophysics, electrochemistry and theoretical calculations studies

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 (L-1-L-6), 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(L-1)Cl-2] (L-1 = N, N'-bis(2-pyridinylethylidene) phenylene-1,2-diamine, M = Zn, 1a; M = Hg, 1b), [M(L-2)Cl-2] (L-2 = N, N'-bis-( 2-pyridinylethylidene) toluene-3,4-diamine, M = Zn, 2a; M = Hg, 2b), [M-2(L-3)Cl-4]center dot nCH(2)Cl(2) (L-3 = N, N '-bis-( 2-pyridinylmethylene) cyclohexane-1,2-diamine, M = Zn, n = 0, 3a; M = Hg, n = 1, 3b), [M-2(L-4)Cl-4]center dot nCH(3)OH (L-4 = N, N '-bis(2-pyridinylethylidene) cyclohexane-1,2-diamine, M = Zn, n = 1, 4a; M = Hg, n = 0, 4b), [M-2(L-5) Cl-4] (L-5 = N, N'-bis(3-methoxy-2-pyridinylmethylene)-cyclohexane-1,2-diamine, M = Zn, 5a; M = Hg, 5b), [M-2(L-6)Cl-4]center dot nCH(3)CN (L-6 = N, N'-bis(3-methoxy-2-pyridinylmethylene) butane-1,4-diamine, M = Zn, n = 4, 6a; M = Hg, n = 0, 6b). All the ligands and complexes have been characterized by elemental analyses, IR spectra, and H-1 NMR spectra. Twelve structures of L-5, L-6, 1a-6a, 1b, 3b, 5b and 6b 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 6a-1a, 434-627 nm for 6b-1b) in solution by changing the ligand conjugated systems from flexibile (L-6) to semi-rigid (L-3-L-5) to rigid (L-1-L-2). The spectra of the free Schiff bases L-6-L-1 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 1a-6a. However, the emission efficiencies dramatically decline in Hg-II-complexes with quantum yields ranging from 4% to 19%, due to the heavy atom effect.