Stimulus Responsiveness Of Pyridine And Coordination Cage Functionalized Metal-Organic Frameworks

Metal-organic frameworks(MOFs)have great potential in stimulus responsiveness because of their diverse composition,highly adjustable structure,porosity,large specific surface area and easy functionalization.The stimulus response of MOFs can come from the interaction between metal ions and organic ligands,the change of electronic structure of any component,host-guest interaction,guest-guest interaction,and the confinement effect of framework.Moreover,MOFs can be functionalized by ligand modification,metal cluster modification,guest introduction and modification and so on,which allows great versatility in stimulus response.Therefore,MOFs provide excellent platforms for developing new stimulus-responsive materials and exploring the underlying science.The dissertation includes three parts:1.Acid response and cooperative protonation of MOFsMOFs can have designable and orderly distributed multiple binding sites.The cooperative bonding of some MOFs to gas molecules and its significance for gas adsorption and separation have been studied.However,cooperative bonding of MOFs and the and effects in other aspects have not been reported or have been ignored.In this dissertation,positive cooperativity protonation of MOF and its effects on fluorescence and proton conduction were demonstrated for the first time.A bipyridyl-based tetracarboxylic acid ligand(H₄tcpbp)with sterically hindered basic sites was designed and used to construct two MOFs,Zr-TCPBP and Zn-TCPBP.Zr-TCPBP has microporous structure with high thermal stability and acid resistance.The pyridine sites with steric hindrance can selectively bind protons,and the binding event is accompanied by a unique fluorescence response to pH:synchronous but opposite dual-emission transitions in the range of pH 3.3-1.8.Reducing pH leads to adrupt fluorescence trun-off and turn-on of the blue emission at 435 nm and the green emission at 510 nm,respectively.Fluorescence transitions greatly improve the response sensitivity and can be used to detect small pH fluctuations,while dual-wavelength changes can be used to realize visual color detection and self-calibration ratiometric detection.Based on Hill equation analysis,the fluorescence transitions are attributed to the positive cooperative protonation of the multiple pyridine sites(Hill coefficient 1.6,microscopic ionization constant p K_1/2=2.6).The cooperativity comes from the change of pore properties after preliminary protonation.The conversion between bistable fluorescent states can be attributed to the switching between intraligand(n,?*)and(?,?*)charge-transfer luminescence before and after protonation.Different from the usual dual-emission systems with two emissive component,the unique dual-emission luminescence based on a single emissive component can improve the reliability of self-calibration detection.In addition,the blue emission before protonation is not affected by anions,while the green emission after protonation depends on anions,which is due to photoinduced host-guest electron transfer and ground-state host-guest charge transfer in the protonated MOF.Proton conduction of MOFs containing acid-base sites should be closely related to protonation,but its pH dependence is rarely reported.The proton conductivity of Zr-TCPBP shows a unique change trend when treated with hydrochloric acid at different pH:it began to increase rapidly below pH 3,increased by more than two orders of magnitude at pH 1,and decreased rapidly at lower pH.We believe that the rapid increase of conductivity is due to cooperative protonation,and the rapid decrease at high acid concentration is because the introduction of too many proton-free anions is not conducive to the formation of continuous hydrogen bond network of water molecules.Zn-TCPBP is composed of 4,4'-bipyridine,H₄tcpbp and Zn²⁺and shows 4-fold interpenetrated diamond frameworks.The MOF also showed a fluorescence transition related to cooperative protonation,but unlike Zr-TCPBP,the fluorescence transitions of Zn-TCPBP appeared in a higher and narrower pH range(6.2-5.4).This indicates that the pyridine sites of Zn-TCPBP show stonger basicity(p K_1/2=5.8)and a greater degree of protonation cooperativity(Hill coefficient is 2.8).Based on the crystal structure data,it can be assumed that the differences arise from the different steric hindrances around basic sites and the different distribution of these sites.The two MOFs can be used as fluorescent switches and probes in different pH ranges.By virtue of the acid responsive fluorescence,Zn-TCPBP shows appealing sensing performance for the detection of 3-nitropropionic acid,a major mycotoxin in moldy sugar cane,with a detection limit of 1?M.2.Alkali response and stability of zirconium-based organic frameworksIn this section,the fluoresce response of Zr-TCPBP to alkali is studied.The emission of Zr-TCPBP aqueous dispersion remains unchanged in the range of pH 6.7-7.5.However,with the increase of alkali concentration,the emission is enhanced and blue shifted,and the maximum emission wavelength?_Maxshowed different linear relationships with pH in the ranges of pH 7.5-9.6 and 9.6-11.5.The emission intensity shows an abrupt transition in the range of pH 11.0-11.8(critical pH was11.4).This unique dual response(emission shift and enhancement)make the MOF a good candidate of pH probe in alkaline media.The mechanism study shows that the alkali-induced fluorescence response is due to ligand leaching caused by partial or even complete decomposition of Zr-TCPBP under alkaline conditions.Based on the fluorescence response induced by alkaline hydrolysis of Zr-TCPBP,a fluorometric method is proposed for characterizing the alkaline stability of MOFs.Based on comparative analysis using other characterization methods such as PXRD,SEM and TGA,the feasibility and reliability of fluorescence analysis method are verified.The research is extended to Ui O-66-NH₂,which preliminarily proves the generalizability of this method for studying the alkaline stability of luminescent or luminogenic MOFs.This method has the advantages of simple operation and high sensitivity.It can give stability information that is difficult to obtain by conventional solid-phase characterization methods,such as detecting a small amount of decomposition and ligand exudation,monitoring the whole hydrolysis process from small ligand leaching to obvious hydrolysis and to complete hydrolysis,and giving the accurate pH range of rapid decomposition.3.Spin crossover responsiveness and spin state regulation of MOF encapsulated coordination cageIn the solid state,the aggregation and interaction of MOCs often lead to inaccessible pores,poor stability and limited function.Some Fe(II)-MOCs exhibit spin crossover properties in solution or solid state.Considering that the spin crossover behavior is greatly affected by the molecular environment,Fe-MOC is encapsulated in the cage cavity of mesoporous MOF for the first time to study its spin crossover properties and responsiveness."Cage in Cage"and subcomponent self-assembly strategy were adopted to encapsulate a hydrophilic anionic coordination cage(Fe-dabpds)into the mesopore of MIL-101,where the cage is smaller than the pore but larger than the windows of the pore.Solid-state ¹³C NMR,FTIR,PXRD,~1H NMR and ICP analyses confirm the successful synthesis of Fe-dabpds@MIL-101 composite with the coordination cages being encapsulated as counteranions.Gas adsorption shows that Fe-dabpds@MIL-101remains highly porous with enhanced hydrophilicity.Water induced spin crossover response of Fe-dabpds@MIL-101 was confirmed by magnetic measurements,Mossbauer and UV-vis spectroscopy.Dehydration of the composite leads to spin crossover of the coordination cage from low-spin to high-spin,while water absorption leads to the opposite conversion.The occurrence of spin conversion is due to the isolation and confinement of the cage by the pores of the MOF.In particular,the appropriate size of the pore relative to the cage,which affords a certain degree of freedom of structural expansion for the cage in dehydrated materials.Based on the chromic response induced by water adsorption,a humidity sensor with specific selectivity for water was developed.By an in-situ encapsulation-assembly approach,a hierarchical encapsulation composite,benzene@Fe-dabpds@MIL-101,was prepared,It is demonstrated that encapsulation of benzene stabilize the low-spin state of the coordination cage,So the host-guest chemistry of the cage can be utilized to control the spin state of the cage encapsulated within solid phase.