Anion-Coordination-Induced Turn-On Fluorescence

With development of more than 100 years,coordination chemistry has become a big family consisting of an ever-widening range of branches,from classical Werner transition-metal complexes,clusters and organometallics,to host–guest complexes and supramolecular complexes?alkali metal and alkaline-earth metal complexes of macrocyclic ligands,crown ethers and cryptands?.More recently,a new branch has grown up in this big‘‘tree”,which is called Anion Coordination Chemistry,a concept first proposed by Lehn in1978.In the past few decades,a large number of anion ligands and their binding properties have been synthesized and reported.And the corresponding applications in sensing,crystal engineering,transmembrane transport and anion-based catalysis etc.have been attracted a lot of interests.However,there is still a challenge to design the excellent ligands for the coordination of phosphate ions due to their complicated protonation states and large size.Based on the previous literatures,urea unit is the most used one to incorporate in the anion ligands.In this dissertation,we designed a series of anion ligands based on tetraphenylethene?TPE?,which is a a typical“aggregation-induced emission”?AIE?chromophore,and have explored the coordination properties with phosphate anions,as well as the phosphate anions induced fluorescence enhancement.The specific contents as following:The first chapter of this dissertation summarized the development of anion coordination chemistry,the typical anion ligands based on hydrogen bonding and various urea-containing anion ligands.At the same time,we also simply summarized the main working mechanisms of AIE,and the typical examples of structural motifs of chromophores and their applications.Based on the two research areas,we have the ideas for this thesis and set the aim and significance for this topic.The second chapter of this dissertation described the TPE-decorated tripodal tris?urea?ligand L¹ and its binding abilities with different anions with different valence state.Ligand L¹tends to form the 1:1 complex with the mono-or divalent anions,while in the case of trivalent PO₄^3-,the much stronger binding ability may overcome the steric repulsion?bulky TPE substituents?and allow for formation of the full capsule.Different binding mode would strongly influence the fluorescence property.Ligand L¹ displays significant emission enhancement upon binding with an orthophosphate anion.This is possibly because the trivalent PO₄^3-ion has a strong binding ability to form the fully encapsulated structure,which causes entanglement of the TPE moieties of two ligands and thus turns on the fluorescence.The third chapter of this dissertation described the tetrakis?bisurea?-decorated tetraphenylethene ligands L² and L³.Ligands L² showed very weak emission even at high concentrations,possibly owing to the electron withdrawing effect of the nitro group.Ligand L³ displayed almost no emission in dilute solution,but the intensity of fluorescence was significantly enhanced by increasing the concentration or adding a poor solvent,which is typical of AIE activity.Ligand L³ upon coordination with phosphate ions,displays fluorescence“turn-on”over a wide concentration range,from dilute to concentrated solutions and to the solid state.The fluorescence enhancement can be attributed to the restriction of the intramolecular rotation of TPE by anion coordination.This unique“anion-coordination-induced emission”may find application in various areas such as fluorescent sensors.The fourth chapter of this dissertation described bis-?bisurea?-decorated tetraphenylethene ligands L⁴.Ligands L⁴ displays fluorescence“turn on”upon phosphate anion coordination.A new complex was achieved with further addition of methyl viologen?MV?,accompanied by the enhancement of fluorescence intensity and a 44 nm red-shift.