Syntheses, Structures And Fluorescence Properties Of The Novel C3-symmetric Schiff Base Ligands And Their Complexes

The crystal engineering of novel coordination polymers is nowdays one of the vital research science that attracts increasing interst. Some researchers use a variety of organic ligands and the noncovalent interaction between different molecules or ions with the aim to acquire such metal-organic materials with unique structural and functional properties by controlling the building-up process. Recently, in order to design and synthesize some special functional compounds which can used in many particular field (such as catalysis, gas adsorption, magnetism, molecular recognition, molecular materials, optical device and so on), the crystal engineering has been combined gradually and obviously with coordination chemistry, supramolecular chemistry and material chemistry so that it is becoming one of the most active research field. Among the wide variety of organic ligands, the type of ligands with C3-symmetric structure has been pursed with a large number of researchers due to their special structure and properties.In this paper, we designed and synthesized the two C3-symmetric Schiff base ligands:tris(4-pyridinecarboxaldehyde)triaminoguanidinium chloride(L1) and tris(2-pyridinecarboxaldehyde)triaminoguanidinium chloride(L2). Although the structures of the two ligands are very silimar, their’s coordination model with metal ions are completely different, which reflects the phenomenon that the position of the N atom of pyridine also has a great influence on the structure of the final coordination polymer. Six novel coordination polymers, namely{[Zn(Ll)(H2O)2](N03)2·(H20)2}n (1),{[Zn(Ll)(H2O)2]·(NO3)2·(H2O)3}n (2),{[Zn(L1)2/3(bdc)(H2O)]·(H2O)3}n (3), {Zn2(L1)2(5-OH-bdc)2}n (4),{[Cd2(L1)(bdc)1.5(H2O)2]·(H2O)3}n (5) and [Zn6(L2)2(Cl)9(H2O)3] (6) have been obtained by hydro(solvo)thermal methold with quite lower temperature (90℃) or the method of solution slow volatility through the reaction of the two ligands with divalent transition metal salts in the absence or presence of ancillary carboxylic acid ligand(1,4-benzenedicarboxylate(H2bdc), 5-hydroxyisophthalic acid (5-OH-H2bdc)). They were all charactered with X-ray single crystal diffraction, elemental analysis, IR etc. We also studied the solid-state fluorescence of polymeric compounds which synthesized from ligand L1.Fluorescent chemosensors for metal ions have attracted much attention due to their convenient use and high sensitivity, and they have been employed to clarify the real-time dynamics and various biological functions of targeted metal cations in living cells. Thus far, a number of fluorescent chemosensors for their targeted metal cations-selective detection were reported and have been used with some success in biological applications. Nevertheless, most of the reported fluorescent as well as colorimetric sensors function in either pure organic or organic-aqueous solutions. This limits their practical applications in physiological environment. Moreover, their preparations require laborious multi-step organic synthesis, which renders their discovery processes slow and causes prohibitively high cost. In this context, the design and synthesis of simple, easy-to-prepare, facile and efficient fluorescent probes remains of great interest, particularly for those with high solubility in complete water system.To our surprise, we found the solution of compound L2 alone has almost no fluorescence. A Zn2+ titration experiment led to a prominent fluorescence enhancement, accompanied with a perceived colour change from colourless to dark yellow. Compared to that of free L2 in aqueous buffer a～21-fold significant fluorescence enhancement could be observed when 3.0 equiv of Zn2+ was present. Subsequently, we evaluated the emission response of L2 against different transition metal ions in buffer solutions, and found that only Zn2+ caused a significant ratio signal output, indicating a Zn2+-selective OFF-ON fluorescent signaling behavior. The study of these findings and results has to be very heavy meanings.