Preparation And Properties Of Covalent Organic Frameworks (COFs) Electrochromic Materials Based On Triamines And Dialdehydes Condensation

Electrochromism refers to the reversible color conversion of electroactive materials under alternating voltage.Electrochromic materials are usually used in smart windows and military fields because of their excellent modulation ability of UV-vis and near-infrared light.Organic electrochromic materials have been widely studied because of their high contrast,strong light modulation ability in near-infrared band and multi-color conversion.Covalent organic frameworks(COFs)are a kind of new functional materials with porous,light weight and strong structural unit selectivity,which are formed by light elements such as carbon,hydrogen,oxygen,nitrogen and boron through covalent bonds.Two-dimensional COFs are mostly used in the energy storage field.As a kind of electroactive material,COFs can be applied in electrochromic field.In this paper,we chose tris(4-aminophenyl)amine(TAPA)as the triamine structural unit,p-benzaldehyde(BDA),4,4-benzaldehyde(BPDA)and 1,4-bis(4-aldehyde phenyl)benzene(TPDA)as the dialdehyde structural unit,and successfully synthesized three kinds of pure COF films which were TABD-COF,TABP-COF and TATP-COF on the ITO glass substrate by solvothermal synthesis method.TAPA was used as a structural unit of COFs can provide electrochromic properties to COFs due to the reversible redox properties of TAPA.We chose dialdehyde units with different lengths as the dialdehyde monomers,in order to explore the color change of COF films with different structural unit sizes.Moreover,in order to make up for the poor conductivity of COF materials,we selected the reduced graphene oxide as the composite unit,prepared TABP-COF/rGO composite films by one pot method,and explored the synergistic effect of GO and COF.The good conductivity and charge transfer efficiency of rGO could bring better electrochromic properties and stability to the composites.Details as follows:(1)The mixed solvent system of o-dichlorobenzene and ethanol and the reaction temperature of 90℃ were determined by preliminary experiment.TABP-COFs with different reaction time were synthesized by uniform drop addition method.SEM test showed that growth process of TABP-COF was nanorod array self-assembly into sphere.FTIR,Raman,XRD and TGA test showed that the reaction of TABP-COF was more complete and better thermal stability when the reaction time was 72 h;CV,EIS,UV-vis and kinetic tests show that TABP-COF has a reversible electrochromic process from yellow to dark brown,and TABP-COF-3 has a greater contrast of 0.45 and a shorter responsing time of 7.3 s/13.6 s.(2)TABD-COF and TATP-COF films were prepared by uniform drop addition method and circulating freezing method respectively.TABD-COF showed a reversible transition from orange to dark brown and TATP-COF showed a reversible transition from yellow to dark brown.They all showed spherical morphology and good thermal stability,and the contrast of the two films were 0.32 and 0.34,respectively.The responsing time of TABD-COF and TATP-COF are 7.1 s/12.4 s adn 13.7 s/11.7 s.(3)rGO and TABP-COF were compounded by one pot method to preparing TABPCOF/rGO composite film,which showed reversible conversion between yellow and dark brown.It was observed by SEM that the monomer was polymerized in-situ on rGO sheet to obtaining spherical COF material.FTIR,Raman and XRD all proved the successful composite of TABPCOF and rGO,and thermal stability of TABP-COF/rGO was little weaker than TABP-COF;CV and EIS tests showed that the electrochemical properties of the composite films were significantly enhanced.UV-vis,kinetic test and stability test show that the composite film has higher contrast of 0.56 and faster responsing time of 5.2 s/8.2 s.After 5000 s of cycling,it still has an initial contrast of 80.1%,which is significantly enhanced compared with TABP-COF.