Metal-directed assembly of two one-dimensional coordination polymers: Structural, theoretical, fluorescent and non-linear optical studies

Abstract

The reaction of Zn(II)/Cd(II) salts with azo-containing bipyridyl ligands afforded two coordination polymers (CPs), [Zn(μ2-4,4′-azpy)(NO2)2]n (1) and [Cd2(μ2-4,4′-azpy)3(NO2)4]n (2) (4,4′-azpy=4,4′-azopyridine). The identities of the CPs 1 and 2 have been unambiguously established by single-crystal X-ray diffraction, and further characterized by elemental analysis, powder X-ray diffraction, thermogravimetric analyses, electrospray ionization-mass spectrometry (ESI-MS), fluorescence analysis, Fourier-transform infrared and ultraviolet/visible spectroscopies. CP 1 exhibits one-dimensional (1-D) zigzag chains containing an octahedral [ZnO4N2] unit bridged by N-donor ligands in the crystal lattice. CP 2 presents a 1-D ladder chain constructed by T-shape units of [Cd(NO2)2(μ2-4,4′-azpy)3] and further forms a 1-D→three-dimensional (3-D) interpenetrated structure. In addition, density functional theory and time-dependent density functional theory calculations at the B3LYP/LanL2DZf+6-31G∗ level were performed on 4,4′-azpy and CP 1 to rationalize their experimental absorption spectra. The fluorescent properties of the free 4,4′-azpy ligand, 1 and 2 have been studied at room temperature. The results reveal that CPs 1 and 2 exhibit enhanced fluorescent emissions at about 410 and 427nm compared with the 4,4′-azpy ligand, respectively. Third-order non-linear optical (NLO) properties of 1 and 2 were also investigated.