Syntheses And Properties Of Multifunctional Redoxactive Metal-organic Frameworks

Multifunctional metal-organic frameworks（MOFs）have attracted more and more attentions,and redox activity of MOFs gives new scope of the research.The redox-active MOFs are generally realized by redox-active metals,ligands or exchanged redox-active guests through post-synthesis.In this dissertation,with a view to extending this concept,we have now synthesized nickel bis（dithiolene-dibenzoic acid）,[Ni（C₂S₂（C₆H₄COOH）₂）₂],which can be considered as the inorganic analogue of the organic tetrathiafulvalene-tetrabenzoic acid（H₄TTFTB）.Based on these two redox-active ligands,a series of novel MOFs were successfully obtained solvothermally.The electrical conductivity,photocurrent response,electrochemical sensor,photothermal conversion and electrocatalysis properties of these MOFs were systematically studied.The main achievements of this work are listed as bellow:1.To activate electronic and optical functions of the redox-active metal-organic framework,（Me₂NH₂⁺）[In^III（TTFTB）]·0.7C₂H₅OH·DMF（Me₂NH₂@1,TTFTB=tetrathiafulvalene-tetrabenzoate,DMF=N,N-dimethylformamide）,has been exchanged by tetrathiafulvalenium(TTF^·+)and N,N’-dimethyl-4,4’-bipyridinium(MV²⁺).These cations provide electron carriers and photosensitivity.The exchange retains the crystallinity allowing single-crystal to single-crystal post-synthetic transformation to TTF@1 and MV@1.X-ray diffraction,¹H-NMR,FT-IR,UV-vis/NIR and electron paramagnetic resonance（EPR）spectroscopies provide ample evidence of the successful cation-exchange.Both TTF^·+and MV²⁺enhance the electrical conductivity by a factor of～10² and the visible light induced photocurrent by 4 and 28times respectively.EPR evidences synergetic effect involving charge transfer between the framework redox-active TTFTB bridges and MV²⁺.X-ray powder diffraction（PXRD）analysis revels chemical stabilities in a wide range of solvents and p H range of 2-11,as well as thermal stabilities up to 350°C.The results demonstrate that functionalization of MOF by cation exchange without perturbing the crystallinity extends possibilities to achieve switchable materials.2.Functionalizing the redox-active tetrathiafulvalene（TTF）core with groups capable of coordination to metals enables new perspectives on the modulation of architectures and electronic properties of organic-inorganic hybrid materials.With a view to extending this concept,we have now synthesized nickel bis（dithiolene-dibenzoic acid）,[Ni（C₂S₂（C₆H₄COOH）₂）₂],which can be considered as the inorganic analogue of the organic tetrathiafulvalene-tetrabenzoic acid（H₄TTFTB）.Likewise,[Ni（C₂S₂（C₆H₄COOH）₂）₂]is a redox-active linker for new functional MOFs,as demonstrated here with the synthesis of a series of MOFs,[M₂{Ni（C₂S₂（C₆H₄COO）₂）₂}（H₂O）₂]·2DMF,（M=Mn,Zn,Cd,Co and Ni）.The thermal stabilities,redox activities,UV-vis/NIR,EPR and electrical conductivity of these MOFs were systematically studied.[Mn₂{Ni（C₂S₂（C₆H₄COO）₂）₂}（H₂O）₂]·2DMF（2,DMF=N,N-dimethylformamide）is isomorphic to the reported[Mn₂（TTFTB）（H₂O）₂]（7）,but is a better electrochemical glucose sensor due to the multiple oxidation-reduction states of the[Ni S₄]core which allow glucose to be oxidized to glucolactone by the high oxidation state[Ni S₄]center.As a non-enzymatic glucose sensor,2 on Cu foam（CF）,2-CF was synthesized by a one-step hydrothermal method and exhibits an excellent electrochemical performance.The fabricated 2-CF electrode offers a high sensitivity of27.9 A M^-1 cm^-2,with a wide linear detection range from 2.0?10^-6 to 2.0?10^-3 M,a low detection limit of 1.0?10^-7 M（signal/noise=3）,satisfactory stability and reproducibility.3.Two novel MOFs,[Zn₂{Ni（C₂S₂（C₆H₄COO）₂）₂}（H₂O）₂]·2DMF and[Cd₂{Ni（C₂S₂（C₆H₄COO）₂）₂}（H₂O）₂]·2DMF（3 and 4,DMF=N,N-dimethylformamide）generated by the nickel bis（dithiolene-dibenzoic acid）,have been synthesized.3 and 4 show good stability and excellent photothermal conversion efficiency.Because of the mixing of organic and metal orbitals and defined oxidation states,nickel bis（dithiolene）complexes are often ideal for the electronic and spectroscopic applications.Under the one-sun illumination,the temperature of the MOFs sharply increases 41°C in 300 s,which is due to the charge transfer between bis（dithiolene）ligand and nickel core,as revealed by solid-state UV/vis-NIR diffuse reflectance spectroscopy.This is the first example for photothermal conversion based on bis（dithiolene）,which not only provides insights for the development of novel photothermal materials,but also paves the way of designing functional materials with appealing applications.4.Inspired by interesting physical/chemical properties in metal-organic frameworks（MOFs）with the redox-active tetrathiafulvalene（TTF）ligands,nickel bis（dithiolene-dibenzoic acid）,[Ni（C₂S₂（C₆H₄COOH）₂）₂]has been designed and developed as inorganic analogues of the corresponding TTF-type donors（such as tetrathiafulvalene-tetrabenzoate,TTFTB）,where a metal site（Ni）replaces the central C=C bond.In this work,[Ni（C₂S₂（C₆H₄COOH）₂）₂]and[In（COO）₄]^-have been successfully assembled into a three-dimensional MOF,（Me₂NH₂⁺）{In^III-[Ni（C₂S₂（C₆H₄COO）₂）₂]}·3DMF·1.5H₂O（8）,with satisfying chemical and thermal stabilities.More importantly,the combined advantages of the reversible redox activity and unsaturated sites of[Ni（C₂S₂（C₆H₄COOH）₂）₂],leading to a significantly enhanced performance in electrocatalysis.Two isomorphic MOFs,8 and（Me₂NH₂⁺）[In^III-（TTFTB）]·0.7C₂H₅OH·DMF（1）,on carbon papaer（CP）,8@CP and 1@CP were investigated for the electrocatalytic activities of CO₂ reduction.With the additional chemical and redox characteristic originated from the[Ni（C₂S₂（C₆H₄COOH）₂）₂]ligand,8@CP showed a prominently higher conversion rate and Faradaic effiency（FE,in formate production）compared with 1@CP with pristine TTFTB ligand,with FE for formate increased from 54.7%to 89.2%at-1.3 V(vs.reversible hydrogen electrode,j _HCOO-=36.0 m A cm^-2).Mechanistic investigations further reveal that[Ni S₄]can serve as CO₂ binding sites and efficient catalytic center.The unprecedented effect of redox-active nickel bis（dithiolene-dibenzoic acid）-based MOF catalysts on the electrocatalytic performance of electrocatalytic CO₂ reduction and provides an important system for more stable and efficient crystalline catalysts to reduce CO₂ to high-value carbon products.