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 π-conjugated ligands, 2-(2-pyridyl)-imidazole (HL1), 2-(2-pyridyl)-benzimidazole (HL2) and 2,6-bis(benzimidazol-2yl)-pyridine (H2L3). 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)2H2O (5), [Cd(L2)2(H2O)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.

Graphical abstractA series of luminescent Zn(II) and Cd(II) complexes have been prepared based on three differently π-conjugated ligands, 2-(2-pyridyl)-imidazole (HL1), 2-(2-pyridyl)-benzimidazole (HL2) and 2,6-bis(benzimidazol-2yl)-pyridine (H2L3). The spectra of the complexes with the ligands show the same rule. The structures of Zn(L2)2 · H2O, [Cd(L2)2 · (H2O)2] · 2DMSO, Zn(HL2)2 and Cd(HL3)2 have been established by X-ray crystallography. They display a blue luminescence and have high luminescence quantum efficiency. A theoretical study is consistent with the experimental results.Figure optionsDownload full-size imageDownload as PowerPoint slide