Synthesis, Assembly And Fluorescence Property Of C₃-symmetric Schiff Base

The Schiff base complexes have attracted considerable interest and become one of the subjects of intensive research due to their excellent properties as potential drugs and functional materials such as strong coordination ability, good biological activities and so on. In the last decades, considerable efforts have been made to design and synthesize the Schiff base and their complexes. In this paper, a series of well-designed organic Schiff base ligands with C₃ symmetry have been designed and synthesized. The high levels of functionality that integrated into a molecular system through the incorporation of a specific metal centre suggests a promising role of the Werner-type capsules for the potential application in sensing special guest with selective singnal responding.1. By considered as a class well-controlled coordinate structure tradintional ligands pyridine imine Schiff base derivatives have been used to build a wide variety of functional complexes, macrocyclic metal system, coordination polymer and so on. Our preparation strategy is based on using a disk-type C₃-symmetric Schiff base ligand L¹ which comprises three bidentate coordinating sites, forces various metal ions to form metal-variable isostructural tetrahedral nanocages with considerable stabilities. Crystal structure shows that the cationic cage has an ideal tetrahedral symmetry which is achieved by arranging the four planar ligands onto the four triangle face of the tetrahedron defined by six coordination transitional metal ions such as Co²⁺, Zn²⁺ and Cd²⁺. ESI-TOF mass spectrometry of the compound showed that the cage exhibits high pronounced thermodynamic stability in solution.2. By combining three quinoline groups into the disk-shape molecule we developed a new strategy for the preparation of tetrahedral nanocages, which can be used as artificial chemosensors. Each quinoline group acts as metal-tunable functional site and signal output group, while the amide groups act as fascinating guest-accessible functional sites to achieve efficient guest interactions and a consequently good signal response. We reported here that the amide groups interact with uridine molecule and convert the recognition information into the chromogenic and fluorogenic singnals through the quinoline. Since the formation of donor-type hydrogen bonds with the amide groups may perturb the electronic distribution on the conjugated backbone of the ligand, such host-guest interactions can affect the charge transfer associated with the quinoline units and lead to significant changes in the optical properties. The addition of uridine molecule causes a significant enhancement of the luminescence. It is clear that the cage is an efficient fluorescent sensor with highly selectivity. ESI-TOF mass spectrometry of the compound showed that the cage exhibits high pronounced thermodynamic stability in solution.3. 8-hydroxyquinoline as the second most important chelating agent with metal ions after the EDTA, are widely used in building a high-sensitivity fluorescence molecular probe of metal ions. Derived from the basis of lihand L¹, a large conjugated system as multi-functional metal ions probe H₃L⁵ has been synthesized. The compound has three tridentate chelate sites, could work as a fluorescent "on-off" probe for the detection of the Co²⁺ and Hg²⁺. Furthermore, by utilizing the 2-mercaptoethanol or Na₂EDTA as suitable shielding agents, H₃L⁵could selectively detect Co²⁺ or Hg²⁺ in DMF solution, respectively. Such a strategy by using suitable shielding agents to enhance the selectivity should be a promising method in the filed of the multi-channel fluorescent probe application.