Synthesises,Structure,and Stimuli-Responsive Of The Naphthalimide-Based MOFs

As a fascinating class of functional materials,stimuli-responsive materials have received increasing attention in recent years due to their potential technological impact in various applications.The current research on these materials mostly concentrate on polymers,carbon materials,gels and liquid crystals,which have lacked structural order（i.e.,the responsive groups/moieties are disorderly in the structures）,resulting a relatively low response speeds,energy transformation ineffeciencies,and unclear structure-property relationships.Therefore,crystalline materials with well-defined and regular molecular arrays can provide new opportunities for the research of novel smart stimuli-responsive materials.On other hand,Metal-Organic Framworks（MOFs）with the well-defined structural order,structural modifiability and facile functionalization have paved a new way for the synthesis of stimuli-responsive materials.Combining the studies of naphthalimide（NDI）and MOFs,which the formers have stimuli-responsive behavior,and using NDI as an organic ligand to build naphthalimide-based MOFs（NDI-based MOFs）,as a result,it is possible to both preserve the stimuli-responsive behavior of NDI and to impart more advanced properties to the materials（i.e.,photoconductive and guest-responsive properties）.In this work,three novel NDI-based MOFs were synthesized by using N,N’-bis（5-isophthalate）-naphthalene diimide（H₄ip NDI）as the organic ligand.Material design is based on the following considerations:（1）Grafting NDI with isophthalic acid groups makes it easier to coordinate with metal irons,resulting in more diverse structures.（2）NDI-based MOFs formed in this way not only retain the characteristics of NDI,but also possess the long-range ordered crystalline structure of MOFs,allowing us to use single crystal-X-ray diffraction technology to deeply explore the relationship between structure and function.（3）The electron-deficient and largeπplane of NDI can bring a strong electron-pulling effect,which can produce a strong recognition effect on electron-rich molecules,and can be designed as a sensing material for molecular recognition.The specific results are present as follows:（1）Solvothermal method with using H₄ip NDI and Pr（NO₃）₃·6H₂O as raw materials,a novel 2D-MOF（FJU-210）was synthesized,which combine with dual-core[Pr₂O₁₆]clusters as nodes and connected by ip NDI.FJU-210 exhibit reversible photochromic and electrochromic.Interestingly,the electrical conductivity of the material increases from 5.20×10^-12 S·cm^-1 to 2.54×10^-10S·cm^-1 with photochromic,change nearly 100 times.Combining spectroscopic and XPS analysis,the mechanism of FJU-212 photochromism is to generate stable free radicals,thereby changing the carrier concentration and significantly improving the electrical conductivity.（2）Under the guidance of crystal engineering,an NDI-based MOF（FJU-211）with a similar structure to FJU-210 was synthesized by using phenanthroline for structural modification.FJU-211 is a two-dimensional layered structure based on a new dual-nuclear[Pr₂N₄O₁₂]cluster as a node and connected by ip NDI.Layer-to-layer infiniteπ-πinteractions consisting of ortho-phenanthroline provides additional electron transport pathways and improve thermal stability.In addition,conductivity changes up to 100,000times before and after UV illumination（nearly 5 orders of magnitude）.Combined with single crystal structure analysis and related tests,it was found that the lattice molecule（DMF）of FJU-211 plays an important role in stabilizing photo-induced radicals.（3）FJU-212 was synthesized by solvothermal method using H₄ip NDI and calcium nitrate tetrahydrate,which reveals a 3D framework with Z-type{Ca₄（DMF）₄（OAc）₄（Et O）₂（CO₂）₈}clusters as nodes,connected by ip NDI,and can be simplified to{4¹².6¹⁶}{4⁴.6²}{4⁵.6}topology.FJU-212 exhibits reversible photochromic behavior and can adapt to alkaline environment（p H=7～11）.On the other hand,the material exhibits an obvious and fast stimulus-response behavior to hydrazine hydrate.Combined with UV spectroscopy and single electron spin spectroscopy（ESR）analysis,FJU-212 can be reduced to a state with stable radicals by hydrazine hydrate.In addition,the hydrazine hydrate steam fumigation experiment also proved that the material has the potential to be used as a practical detection of hydrazine hydrate.