Novel bright blue emissions IIB group complexes constructed with various polyhedron-induced 2-[2′-(6-methoxy-pyridyl)]-benzimidazole derivatives

Abstract

Five IIB group complexes, [ZnL¹Cl₂] (1), {[CdL¹Cl₂]}₂ (2), [Cd(L¹)₂(NO₃)₂]·H₂O (3), [HgL¹Cl₂] (4), and [ZnL²Cl₂] (5) [L¹ = 1-[2-(6-methoxy-2-pyridylmethyl)]-2-[2-(6-methoxy-pyridyl)]benzimidazole; L² = 2-[2-(6-methoxypyridyl)]benzimidazole] have been synthesized and structurally characterized by elemental analysis, IR spectroscopy, ¹H NMR, ¹³C NMR and X-ray single-crystal analyses. The structural investigations testify that the ionic radius and counterions (Cl⁻ and NO₃⁻) cooperatively affect the coordination mode of central metal. As small and medium radii, four-coordinated Zn²⁺ (1 and 5) and five-coordinated Cd²⁺ (2) possess tetrahedron and trigonal bipyramid geometries, respectively. Though Hg²⁺ (4) has a larger radius, its three-coordinated geometry is trigonal planar in order to eliminate repulsive forces. Further observations illustrate that Cd²⁺ (3) is bound to two ligands L¹ when NO₃⁻ is the counter anion, forming seven-coordinated monocapped trigonal prismatic geometry. Complexes 1–5 display bright blue luminescence with the emission maxima (λ_max) ranging from 399 to 499 nm at 298 K, depending on the N,N′-chelating ligand-centered π* → π transition. Upon cooling to 77 K, the complexes show rich structured emission profiles compared to those at 298 K. The emission lifetimes of L and 1–5 are on the microsecond scale. The emission efficiency of 1–5 is shown by quantum yields ranging from 0.23 to 0.40. Complexes 1–5 offer a good insight into the opportunities in the utilization of blue materials for application and function.