Chromic And Fluorescence-responsive Metal-organic Frameworks Afforded By N-quaternization Modification

Sensing materials that respond to specific gases or vapors with color or fluorescence change have important applications in many fields.Metal-organic framework(MOFs)are highly porous crystalline materials that are self-assembled by coordination bonds between metal nodes and organic linkers.The tunable porous framework can be made or modified to introduce various functional components,while the open pores allow the entry of target analytes and the access to functional sites.Introducing highly electron-deficient bipyridinium units into MOFs is a new strategy to construct multi-responsive sensing materials.It is difficult to obtain stable and porous MOFs by direct synthesis from viologen derived ligands because the positively charged bipyridinium easily affects the coordination between metals and ligands.The author's research group have introduced 2,2'-bipyridinium units by N-alkylation post-modification.The synthesized MOFs showed obvious chromic response to electron-rich alkylamines/amines.However,the fluorescence is weak,which limits its sensing performance.In this paper,two kinds of electron-deficient MOFs with fluorescence response were designed and synthesized,and their color and fluorescence response to some gaseous analytes were studied.1.Chromic and fluorescence-responsive MOFs afforded by N?Amination modificationAn electron-deficient MOF with reactive amino groups(Zr-bpy-A)was synthesized by post-synthetic N-amination modification of the Zr-MOF bearing free2,2'-bipyridine groups(Zr-bpy).The MOF was characterized by IR,PXRD,NMR,TGA and SEM.Zr-bpy-A exhibits an obvious and fast chromic response to alkylamine and ammonia vapors,and the MOF shows a unique purple-red response to formaldehyde vapor.The reversible response to alkylamines/ammonia is based on the charge transfer(CT)noncovalent interactions between electron-deficient pyridinium with electron-rich amine molecules,and the irreversible response to formaldehyde is based on the covalent chemical reaction of amino groups with aldehyde groups.Zr-bpy-A can be used to prepare portable colorimetric test papers for the detection of alkylamines/ammonia and formaldehyde with a detection limit of 3 ppm.Schiff-base condensation of N-amino groups was used to graft different PHAs,which endow the MOFs(Zr-bpy-APh,Zr-bpy-ANa,Zr-bpy-AAn,and Zr-bpy-APy)with tunable luminescent wavelengths.Of these MOFs,Zr-bpy-ANa shows the strongest fluorescence emisson,so its fluorescence response was further studied.Zr-bpy-ANa shows not only chromic but also fluorescent response to formaldehyde and amine molecules,which can be applied for dual-responsive sensing.Based on the protonation of unquaternized pyridine sites in the structure,Zr-bpy-ANa also exhibits a sensitive fluorescence response to acids.The multi-responsive MOFs can be used to fabricate portable dual-response sensors for naked-eye on-site identification and detection of analytes,with potential applications in switching and anti-counterfeiting.2.N-alkylated MOFs and their fluorescence responsesN-Alkylation postsynthetic modification of Zr-bpy was performed to simultineously introduce electron-deficient pyridinium groups and the naphthalene fluorophore.The MOF thus obtained,Zr-bpy-MNap,was characterized by PXRD,IR,UV-vis,XPS and TGA.The MOF in the pristine state is nonluminous but becomes luminescent in some solvents,and exhibits even stronger luminescence on exposure to alkylamines/ammonia.The fluorescence turn-on response is proposed to arise from complex host-guest interactions and solvation effects.Zr-bpy-MNap is non-fluorescent due to the quenching by host-guest interaction between the bromide ion and the framework.However,the quenching mechanism is weakened or destroyed by solvation of bromide and the framework or host-guest interactions between amines/ammonia and framework components,leading to fluorescence turn-on.The turned-on fluorescence originates from the charge-transfer transition between the naphthyl group and the electron-deficient framework.A simple test paper can be prepared using Zr-bpy-MNap,and its luminescence response can be used to identify solvents and detect alkylamines/ammonia.