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

Abstract

A series of luminescent Zn(II) and Cd(II) complexes have been prepared based on three different π-conjugated ligands, 2-(2-pyridyl)-imidazole (HL1), 2-(2-pyridyl)-benzimidazole (HL2) and 2,6-bis(benzimidazol-2yl)-pyridine (H 2L3). The PL spectra of the ligands exhibit a bathochromic shift in agreement with the extended π-conjugation of the ligand. The spectra of the complexes with the ligands follow the same rule. The structures of Zn(L2) 2H 2O ( 5), [Cd(L2) 2(H 2O) 2] · 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.