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

Five IIB group complexes, [(ZnLCl2)-Cl-1] (1), {[(CdLCl2)-Cl-1]}(2) (2), [Cd(L-1)(2)(NO3)(2)]center dot H2O (3), [(HgLCl2)-Cl-1] (4), and [(ZnLCl2)-Cl-2] (5) [L-1 = 1-[2-(6-methoxy-2-pyridylmethyl)]-2-[2-(6-methoxy-pyridyl)]benzimidazole; L-2 = 2-[2-(6-methoxypyridyl)]benzimidazole] have been synthesized and structurally characterized by elemental analysis, IR spectroscopy, H-1 NMR, C-13 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 L-1 when NO3- is the counter anion, forming seven-coordinated monocapped trigonal prismatic geometry. Complexes 1-5 display bright blue luminescence with the emission maxima (lambda(max)) ranging from 399 to 499 nm at 298 K, depending on the N,N'-chelating ligand-centered pi* -> pi 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.