Fluorescent zinc(II) thione and selone complexes for light-emitting applications

Three 1-(anthracene-9-ylmethyl)-3-isopropyl-imidazol-2-thione Zn(II) halide complexes (1-3) and one 1-(anthracene-9-ylmethyl)-3-isopropyl-imidazol-2-selone Zn(II) dichloride complex (4) were synthesized and characterized. Complexes 2, 3, and 4 exhibited distorted tetrahedral geometries, while complex 1 adopted a regular tetrahedral geometry. All these complexes displayed emission in the crystalline state, with complex 3 emitting in the yellow region and complex 1 and 4 in the blue region, while complex 2 gave a bluish-green emission. The ligands L1 and L2, however, showed no emission in the solution and crystalline state. The photophysical properties of these four complexes were studied, and their quantum yields in the crystalline states were determined. Complex 1 exhibited the highest quantum yield of 7.72%, and complexes 2 and 3 demonstrated 5.95% and 5.07% yield, respectively. Complex 4 exhibited a relatively lower quantum yield of 3.87%. The crystalline state quantum yields for the complexes were found to vary with the variation of the halide ion coordinated to the metal center, following the trend Cl^- > Br^- > I^-. The quantum yield for the thione complexes 1-3 was found to be superior to that of the selone complex 4. Density functional theory calculations were performed to study their structural properties and emissive nature. TD-DFT natural transition orbital calculations revealed that the observed emission behaviour is primarily driven by intra-ligand charge transfer (¹ILCT) mediated through the metal center.