Construction And Luminescence Properties Of Terpyridine-liganded Supramolecular Cages For Fluorescence Sensing

Enzyme sensors are widely used for environmental pollutant detection because of their rapidity,sensitivity,high selectivity and specificity.However,enzyme sensors have high cost,short lifetime,poor stability,few modifiable groups and single variation,which are hindered in practical applications.In recent years,scientists in the field of biosynthesis and chemical synthesis have explored how to mimic enzyme structure and function using supramolecular molecules.Due to the highly directional and designable nature of metal-ligand bonds,a large number of three-dimensional metal-ligand supramolecules of the terpyridine class have been reported as a driving force,but it is still a great challenge to exploit their large cavities in combination with sensing elements to form artificial mimetic enzyme structures that can be monitored for contaminants.Accordingly,we designed a supramolecular cage with ultra-large domain-limited cavities and further modified two aggregation-induced emission（AIE）groups with different luminescence colors to construct two supramolecular cages with high intensity luminescence.With their high luminescence properties,these supramolecular cages can be used for fluorescence imaging and molecular sensing.Therefore,they can be used for environmental detection.The specific work is:（1）A typical blue-emitting AIE group,tetraphenylethylene（TPE）,was chosen to synthesize five triclinic organic ligands with modified AIE luminescent groups by linking TPE groups and intermediates with hydroxyl modification centers through five different lengths of alkyl chains.The ligands were self-assembled with Zn²⁺to form five cuboctahedral shaped metal-organic supramolecular cages with 12 TPE groups internal modification.Nuclear magnetic resonance（NMR）,mass spectrometry（MS）and transmission electron microscopy（TEM）were used to confirm the supramolecular cages consisted of 12 ligands,24 Zn²⁺and 72 anions.As a result,the fluorescence emission intensity of the cuboctahedral supramolecular cage can be precisely controlled by the length of alkyl chains,which determine how much TPE molecules aggregate inside its restricted cavity:the alkyl chains lengthen,preventing TPE molecules from moving inside the cage cavity,which results in an increase in fluorescence quantum yield from 3.23%to 81.37%.The research work in this chapter provides a new way to achieve high-intensity fluorescence-emitting materials by increasing the chain length to compress the supramolecular restricted cavity to increase the degree of aggregation of AIE molecules（TPE）in a dilute state,and the strategy of enclosing fluorescent molecules in supramolecular cages by internal modification is expected to have promising applications in the fields of bionanotechnology,environmental sensors,and fluorescence imaging.（2）Secondly,TPE derivatives were used to design and synthesize metal-organic supramolecular cages with long wavelength fluorescence emission.The ligand N1 with fluorescence emission at 560 nm was synthesized by linking the TPE derivative and the intermediate L0 through a 12-carbon alkyl chain.The ligand N1 self-assembled with Zn²⁺to form supramolecular cage Y1 with an internally modified yellow-emitting AIE group and was characterized by NMR,MS,and TEM.Yellow-emitting AIE groups were heavily aggregated in the restricted cavity during the construction of supramolecular cage Y1.This resulted in a much larger fluorescence emission from supramolecular cage Y1 than ligand N1,as well as an increase in fluorescence quantum yield from 0.96%to 17.21%.High-intensity fluorescence-emitting materials can be formed by aggregating TPE derivatives with long-wavelength fluorescence emission within supramolecular cavities.This provides the opportunity for subsequent specific modifications（attached to electron-giving or co-electron groups）to the luminescent clusters to extend their fluorescence emission wavelengths,promising the formation of metal-organic molecular cages with high-intensity near-infrared light emission,which are expected to be used as fluorescent sensors or contrast agents for fluorescence imaging in future work.