Design and syntheses of blue luminescent zinc(II) and cadmium(II) complexes with bidentate or tridentate pyridyl-imidazole ligands

A series of luminescent Zn(II) and Cd(II) complexes have been prepared based on three different pi-conjugated ligands, 2-(2-pyridyl)-imidazole (HL1), 2-(2-pyridyl)-benzimidazole (HL2) and 2.6-bis(benzimidazol-2yl)-pyridine (H(2)L3). The PL spectra of the ligands exhibit a bathochromic shift in agreement with the extended pi-conjugation of the ligand. The spectra of the complexes with the ligands follow the same rule. The structures of Zn(L2)(2)H2O (5), [Cd(L-2)(2)(H2O)(2)] center dot 2DMSO (6), Zn(HL3)(2) (9) and Cd(HL3)(2) (10) have been established by X-ray crystallography. The structure of 5 can be best described as a square pyramid. The coordination geometries of 6, 9 and 10 can be described as distorted octahedrons. In the solid state and in DMSO, these complexes show emission maxima in the blue region. The complexes have a bathochromic shift of the emission energy in sharp contrast to the free ligands. They display blue luminescence and have high luminescence quantum efficiency. A theoretical study reveals that the emissions are assigned to intraligand transitions of the deprotonated L2 and HL3 ligands, and the metal ions in the complexes play a key role in stabilizing the ligand and promoting the luminescence, which is consistent with the experimental results. (c) 2005 Elsevier Ltd. All rights reserved.