Structure, aggregation and spectroscopic properties of self-assembled Zn(II) Schiff base complexes

The structures and photophysical properties of Zn(II) complexes with the tridentate Schiff base ligand HL: 1-((E)-((pyridin-2-yl)methylimino)methyl)naphthalen-2-ol, have been synthesized and fully characterized. The X-ray structures of the complexes showed unusual and inequivalent dimeric assemblies of Zn(II) that were tuned by the counter ions used. The complex [ZnL(HL)]2(ClO4)2, 1, proves the self-assembly of a box-like structure that stabilized by OH⋯OR contacts within the complex. The complex [Zn2(μ-L)2(NO3)2], 2, contains two symmetrical Zn(II) centers that are directly linked by two oxo bridges from two ligands. The 1H NMR spectra in non-polar solvents such as CDCl3 confirmed the stability of the dimeric assemblies and aggregation in solution. By controlling the counter ions used, e.g. ClO4 or NO3 and the polarity of solution, the Zn(II) complexes would form molecular aggregates at different extents with different emission properties in the solid and solution states. The solid-state emission spectra exhibited a spectral red-shift on the ligand-based emission besides a growth of a low-energy feature in 1 due to face-to-face π⋯π contacts between the aromatic rings in the box-like assembly. Therefore, complex 1 emits greenish-blue light and complex 2 emits blue light.