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

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)R³-1,2-diamine (L¹–L⁶), which contain a flexible, semi-rigid or rigid group (R³ = 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(L¹)Cl₂] (L¹ = N,N′-bis(2-pyridinylethylidene)phenylene-1,2-diamine, M = Zn, 1a; M = Hg, 1b), [M(L²)Cl₂] (L² = N,N′-bis(2-pyridinylethylidene)toluene-3,4-diamine, M = Zn, 2a; M = Hg, 2b), [M₂(L³)Cl₄]·nCH₂Cl₂ (L³ = N,N′-bis(2-pyridinylmethylene)cyclohexane-1,2-diamine, M = Zn, n = 0, 3a; M = Hg, n = 1, 3b), [M₂(L⁴)Cl₄]·nCH₃OH (L⁴ = N,N′-bis(2-pyridinylethylidene)cyclohexane-1,2-diamine, M = Zn, n = 1, 4a; M = Hg, n = 0, 4b), [M₂(L⁵)Cl₄] (L⁵ = N,N′-bis(3-methoxy-2-pyridinylmethylene)-cyclohexane-1,2-diamine, M = Zn, 5a; M = Hg, 5b), [M₂(L⁶)Cl₄]·nCH₃CN (L⁶ = 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 ¹H NMR spectra. Twelve structures of L⁵, L⁶, 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 (L⁶) to semi-rigid (L³–L⁵) to rigid (L¹–L²). The spectra of the free Schiff bases L⁶–L¹ 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.