Constructions And Properties Of Functional MOFs Targeting Catalysis And Multiple Fluorescence Responses

As a novel category of inorganic–organic hybrid porous crystalline materials,metal–organic frameworks(MOFs)assume great potential in gas storage,gas separation,heterogeneous catalysis,chemical sensing,and many other fields,benefiting from their large surface areas and tailored pore structures.Over the past few years,as the industry evolves,function-oriented synthesis(FOS)of MOF materials has emerged as a current research hotspot.Function-oriented design and synthesis of MOFs generally require introductions of corresponding functional sites to exert the advantages of MOF materials in specific fields.From the perspective of inorganic secondary building units(SBUs),metal-based functional sites can be introduced by modulating metal species or regulating the coordination modes of metals or metal clusters.On the other hand,the introduction of functional groups can also be achieved by functionalizing organic ligands.On this basis,in this work,focusing on the functional requirements of MOFs in catalysis and fluorescence detection,a series of MOF materials containing functional sites were constructed in a targeted manner through the above two approaches.Targeting the problem that the catalytic performance of MOFs is far from ideal due to the lacks and limited numbers of active sites in Knoevenagel condensation and carbon dioxide cycloaddition,functional groups and open metal sites that can function as active catalytic sites are introduced into MOF materials to achieve high catalytic efficiencies of both reactions through regulating the types and numbers of active catalytic sites in MOF materials.In addition,in view of some major drawbacks of fluorescencequenching-based chemical detections of MOFs,such as loss of signal,poor recyclability,and vulnerability to stimulus from the peripheral environment,multiple fluorescence responses of a MOF material were achieved through structural control and the introduction of a chromophore,and the mechanisms were studied by theoretical calculations.The results of our research are as follows:(1)Targeting high catalytic performance for Knoevenagel condensation,compounds 1 ～ 5 were synthesized by choosing functional group-based organic ligands and multiple inorganic SBUs.High-efficiency Knoevenagel condensation reactions were achieved through increasing the types and numbers of active sites in MOF materials step by step.The effects of the active sites on the catalytic performances were elucidated.Compounds 1 and 2 were constructed from copper-based paddle-wheels and ureabased ligands.Benefiting from the catalytic activity of urea Br(?)nsted basic sites,compound 1 exhibits good catalytic performance for Knoevenagel condensation.To further increase the numbers of Br(?)nsted basic sites and introduce open metal sites to MOF materials,compound 3 was successfully synthesized by amide-based tricarboxylate ligands and yttrium–oxygen-based chains.Under the synergistic catalysis of Lewis acid sites(LASs)and amide Br(?)nsted basic sites,compound 3exhibits a higher catalytic efficiency for Knoevenagel condensation than compound 1.To further increase the numbers of LASs in the framework,6-connected indium-based trinuclear clusters were introduced to construct compounds 4 and 5 with two amidebased tricarboxylate ligands.Benefiting from the ultra-high densities of acid and basic active sites and suitable pore environments,both two materials exhibit high catalytic activities for the Knoevenagel condensation reactions.In addition,compounds 4 and 5,with more types,higher quantities,and better distributions of active sites,also exhibit high catalytic performances for carbon dioxide cycloaddition under mild conditions and a simulated flue gas environment.(2)In order to increase the types and densities of open metal sites in MOF materials to achieve high catalytic efficiency for carbon dioxide cycloaddition,as well as expand the structural diversity of MOFs materials and improve their chemical stability,bimetallic and bi-ligand-based compounds 6 ～ 9 were synthesized based on hard-softacid-base theory and molecular building block approach.Compound 6 is constructed from trinuclear indium-based clusters and six-membered copper-based rings,where all indium ions and copper ions can function as LASs.Compound 7 is constructed from hexanuclear zirconium-based clusters nested by six-membered copper-based crowns through formic acids,where copper ions can function as LASs.Both materials assume ultra-high densities of LASs,and exhibit excellent catalytic activities for carbon dioxide cycloaddition.The catalytic efficiency of compound 6 is higher than compound 7 due to more LASs in compound 6.(3)Aiming at the fluorescence responses of luminescent MOF materials,a bent ligand containing a carbazole chromophore was designed and synthesized.A luminescent MOF material—compound 10,was constructed by the ligands and 12-linked hexanuclear yttrium-based clusters.Compound 10 exhibits ultra-sensitive fluorescence-quenching-based chemical detections of nitroaromatic compounds due to the combined effects of excited-state electron transfer,resonance energy transfer,and dynamic quenching mechanism.In addition,compound 10 exhibits unique turn-on and unprecedented turn-off-on fluorescent responses towards acids.Density functional theory and time-dependent density functional theory-based calculations confirmed that the rotation of the benzene ring of the ligand was the main source of energy consumption through non-radiative decay.The calculations of the changes of the dihedral angles between the benzene ring and the carbazole of the ligand,the binding energies of the acid-ligand complexes,as well as the analyses of the excited-state electron transfer,Frank-Condon integrals and the interactions between acids and ligands,clarified the turn-on and turn-off-on mechanisms and their universality to multiple acid compounds.