Synthesis and fluorescent property of metal-organic frameworks [Zn(BDC)(H2O)2-2x · (8-Hq)x] with tunable fluorescence

Nanoporous metal-organic frameworks (MOFs) materials have been developed rapidly in recent years. The materials were constructed of organic ligands and metal ions. Multi-functional MOFs materials were designed by different organic ligands and metal ions, i.e. Zn2+ and 1, 4-benzenedicarboxylic acid (H2BDC) can assembled to 1-D, 2-D, 3-D MOFs materials. They can be applied in gas storage, chiral separation, catalysis, molecular magnet and photoelectric material. However, only a few examples of porous MOFs with open metal sites have been illustrated for their functional properties, mainly because of the difficulty of constructing robust MOFs that hold open metal sites. In this paper, H2BDC and Zn2+ are chosen to synthesize 1-D MOF [Zn(BDC)(H2O)2] by using precipitation method, which is a facile and environmentally friend method. A series of coordination polymers, i.e. [Zn(BDC)(H2O)2-2x · (8-Hq)x], were obtained by substitution and adsorption in the reaction of [Zn(BDC)(H2O)2] with different content of 8-hydroxyquinoline. Their structures were characterized by XRD and FT-IR. The results of XRD show no structure change in low content of 8-hydroxyquinoline. How-ever, when n(8-Hq): n([Zn(BDC)(H2O)2])=1:20, the peaks of XRD have obviously difference to standard chart. It may be attribute to 8-Hq substituting coordination water. When n(8-Hq): n([Zn(BDC)-(H2O)2]) > 1:1, coordination water was substituted whole, and the adsorption effects is dominant. TEM images showed nanocrystals of [Zn(BDC)(H2O)2] are grown with increase of 8-Hq concentration. Nanocrystals of [Zn(BDC)(H2O)2] and [Zn(BDC)(H2O)2-2x · (8-Hq)x] exhibit strong solid state fluorescence properties. Fluorescence emission spectra revealed that the wavelength of maximum emission (λem) redshifts regularly with increasing the amounts of 8-hydroxyquinoline, λem redshifts from 376 nm to 428 nm with n(8-Hq): n([Zn(BDC)(H2O)2])=1:200~1:80. Changing the ratio of n(8-Hq): n[Zn-(BDC)(H2O)2]=4:1, λem redshifts to 528 nm. These phenomena were explained rationally by fluorescence area integral. The results suggest that emission spectrum can be tuned by varying ratio of [Zn(BDC)-(H2O)2] to 8-hydroxyquinoline. These kind of metal-organic frameworks have potential applications for display devices, fluorescence imaging, and fluorescence label, and so on.