Zinc(II) complexes with 1,8-naphthyridine-based ligand: Crystal structures and luminescent properties

The reactions of 2,4-dimethyl-7-(2-pyridylamino)-1,8-naphthyridine (L1) with Zn(ClO4)2 · 6H2O, and bis(5,7-dimethyl-1,8-naphthyrid-2-yl)amine ligand (L2) with Zn(OAc)2 · 2H2O, ZnCl2 or Zn(ClO4)2 · 6H2O afforded four blue luminescent zinc(II) complexes, [Zn(L1)2](ClO4)2 · 2CH2Cl2 (1), [Zn(L2)(OAc)2] · CH2Cl2 (2), [Zn(L2)2][ZnCl4] · 3.5CH2Cl2 (3) and [Zn(L2)2](ClO4)2 (4), respectively. Crystal structures of complexes 1–3 have been determined by X-ray structural analyses as mononuclear complexes with pseudo-tetrahedral geometry. The crystal packing of 1 reveals the coordination cation which is self-assembled to stair chains through aromatic π–π interactions. The intermolecular N–H⋯O hydrogen bond in 2 generates a centrosymmetric H-bonded dimer. However, the crystal lattice of 3 shows that the molecules are linked by extensive intermolecular hydrogen bonds between the amino groups and the ZnCl 4 2 - anions, resulting in a one-dimensional zigzag chain. Furthermore, these molecular pairs or chains were self-assembled to two-dimensional sheets or three-dimensional networks through aromatic π–π interactions. All the zinc(II) complexes display intense intraligand 1(π–π∗) fluorescence with λmax at 380 and 393 nm for 1, 385 and 404 nm for 2–4 in methanol at room temperature, respectively. Emission quantum yields of these complexes are in the range from 0.41 to 0.57. The broad emission bands in their solid-state emission spectra are attributed to intraligand 1(π–π∗) transition and aromatic π–π interactions as well.