The Construction And Functional Expansion Of Covalent Organic Frameworks

Covalent Organic Frameworks（COFs）are a class of crystalline organic porous polymers composed of light elements（such as C,H,O,N,and B）by strong covalent bonds,which emerge as a new kind of crystalline ordered materials after Metal Organic Frameworks（MOFs）.At present,research on COFs can be roughly divided into two main directions,one is focused on synthetic strategies and the other is devoted to functionalisation and applications.Synthesis is fundamental to the study of COFs,the recent developed"two-in-one"molecular design strategy can reduce the highly dependence on solvent types and meanwhile improving the reproducibility of two-dimensional（2D）COF synthesis.However,to date,only less than ten"two-in-one"monomers were synthetized,and three-dimensional（3D）"two-in-one"monomers have not yet been developed.It is essential to investigate the universality of"two-in-one"molecular design strategies for the synthesis of 2D and 3D COFs.Furthermore,although COFs displayed great potentials in applications such as catalysis,gas separation and storage,and energy storage due to their unique structures,it is also crucial to broaden the range of applications for further development.Based on these considerations,we carried out our research on both synthesis and application of COFs.On the one hand,we prepared new functional monomers via elaborate design,which extend the"two-in-one"molecular design strategy in the synthesis of functional 2D COFs and 3D COFs.For another,via skillful incorpation of functional groups,we have successfully prepared novel functionalized 2D COFs through the bottom-up synthetic strategy,enabling the application of redox COFs materials in aqueous proton battery and of sole COFs photothermal materials in solar steam generation（SSG）,respectively.In particular,the detailed studies are presented as follows:1.Based on the"two-in-one"molecular design strategy,a functional 2D"two-in-one"monomer 4,4’-（2,6-bis（4-aminophenyl）benzo[1,2-d:4,5-d’]bis（oxazole）-4,8-diyl）dibenzaldehyde（BBO）containing benzoxazole was designed and synthesized.2D BBO-COFs were successfully prepared by self-polycondensation of BBO,which realized the extension of the"two-in-one"molecular design strategy in the synthesis of 2D COFs.The experimental results show that the BBO-COFs can be prepared in more than 11 different simplex organic solvent systems,avoiding the tedious screening of solvent combinations.Furthermore,due to the presence of functional benzoxazole,BBO-COF showed prominent photocatalytic activity toward oxidative hydroxylation reaction of arylboronic acids with excellent substrates tolerance and reusability.This work expands the range of functional"two-in-one"monomers and opens up a new avenue for the synthesis of new functional COFs with tailor-made properties.2.Based on the"two-in-one"molecular design strategy,the first"two-in-one"3D-COFs（3D-Flu-COF）were constructed by self-polycondensation of a tetrahedral"two-in-one"monomer 4,4’-（3,6-bis（4-（5,5-dimethyl-1,3-dioxan-2-yl）phenyl）-9H-flu-orene-9,9-diyl）dianiline（A₂B₂-Flu）,which further verified the universality of the"two-in-one"strategy.It shows that 3D-Flu-COF with good crystallinity can be obtained in a variety of simplex organic solvents.Benefitting from the non-conjugated structure and imine linkage,the 3D-Flu-COF showcased excellent acidichromic sensing performance with good sensitivity,reversibility and naked eye visibility.This work demonstrates the feasibility of the"two-in-one"molecular design strategy for the construction of 3D COFs and will provides a new approach to simplify the synthesis of 3D COFs.3.TPAD-COF with multiple reversible redox pairs was prepared by introducing a polyaniline-like quinone unit by the bottom-up synthetic strategy.Due to its reversible redox sites,TPAD-COF was applied as aqueous proton battery electrode,and exhibits excellent specific capacity and long cycle stability.This work demonstrates the use of COFs materials as electrodes in aqueous proton batteries,which expands the range of applications for COFs materials.4.1,4,5,8-tetra（anilino）anthracene-9,10-dione,with hydrophilic and broad light absorption properties,was incorporated into 2D COFs（TPAD-COF,TPAD-Py-COF）by the bottom-up synthetic strategy,enabling the application of sole COFs in solar steam generation（SSG）.Thereinto TPAD-COF inherits the hydrophilic and light-absorbing properties of monomer,TPAD-COF membrane shows an energy conversion efficiency of up to 93.9%and excellent water evaporation(1.42 kg m^-2 h^-1)in 1 sun irradiation.For comparison,we synthesised TPAD-COF-BF₂ and TPAD-Py-COF-BF₂ by post-synthetic modification of chelating the BF₂ group into TPAD-COF and TPAD-Py-COF,transforming the hydrophilic COFs into hydrophobic COFs.Via a series of experimental comparisons,we have investigated the effect of photothermal conversion efficiency and hydrophiicity of photothermal materials on SSG.Notably,TPAD-COF is able to be used for outdoor desalination through a home-made device.This work demonstrates promising application of COFs materials in SSG,which will provide new ideas for subsequent applications of porous organic polymers（POPs）in SSG.