Three-dimensional Sp~2 Carbon-linked Covalent Organic Frameworks:Design Synthesis And Luminescence Characterization

Covalent organic frameworks(COFs)represent a class of crystalline porous materials with periodic structures and ordered pores.It is well known that COFs can be structurally predesigned through topological networks,enabling the development of 2D or 3D structures based on the combination of organic units and specific geometries.Therefore,photofunctional COFs with different topological structures can be prepared by utilizing various chromophores.However,many light-emitting building blocks undergo fluorescence quenching after they are constructed into COFs.This is partly due to the aggregation-induced quenching(ACQ)caused by the strong interaction of π-πstacking between layers of 2D COFs.Another part of the reason is the lack of suitable reactive bond types to construct COFs,because some bond types will produce intramolecular rotation,resulting in the energy of the excited state being consumed by the non-radiative way of intramolecular benzene ring rotation,so that no light is emitted.In addition,with the continuous exploration of the application of COFs,higher requirements are put forward for the stability of materials.However,most of the current COFs that emit light are unstable,and stable COFs do not emit light,which greatly limits their practical applications.Therefore,how to prepare stable photoluminescent COFs is an ideal but challenging task.In view of the above-mentioned problems affecting the light emission of COFs,we chose to build a 3D COFs structure to avoid the influence of two-dimensional "π-πstacking".In principle,three-dimensional COFs are an ideal platform for constructing luminescent COFs,but unfortunately the available building units are very limited.In particular,the three-dimensional light-emitting building units are very few,which greatly limits the development of 3D light-functional COFs.The second is to choose the carbon-carbon double bond reaction bond type to construct COFs,because this bond type has strong electron delocalization,which not only ensures the luminescence performance but also has excellent stability.In principle,the combination of C=C bond and three-dimensional framework will benefit the photoluminescent properties and promote the development of photofunctional COFs.However,studies on the luminescence properties of three-dimensional sp2-carbon-linked COFs have not been explored so far.In response to the above problems,a series of three-dimensional sp2carbon-linked COFs were designed and synthesized in this paper and their luminescent properties were studied.The research content is mainly divided into the following three parts:(1)Three-dimensional sp2-carbon-linked COFs as drug carriers for fluorescence imagingWe chose tetrakis(4-formylphenyl)silane as the tetrahedral unit(Td)and benzonitrile linear units of different lengths to prepare sp2-carbon-linked COFs,and obtained JUC-580 and JUC-581.Given the excellent luminescent stability,good porosity,and excellent chemical stability of 3D sp2-carbon-linked COFs,we investigated the potential application of JUC-581 as a drug carrier and its fluorescence imaging.The results showed that the related drug-loaded material(CIS@JUC-581)was not only a good drug-loaded and sustained-release material,but also a potential cellular fluorescent dye that could provide long-term fluorescent tracer for drug transport.(2)Synthesis and luminescent properties of non-interspersed three-dimensional sp2-carbon-linked COFsIn the previous section,we successfully prepared two cases of three-dimensional sp2-carbon-linked COFs.Although the material has good stability,unfortunately the luminescent properties are not outstanding.We speculated whether the luminescent properties of 3D sp2-carbon-linked COFs are related to the spatial structure,such as the interpenetration of structures.Therefore,in this section,we successfully prepared the first non-interspersed three-dimensional sp2-carbon-linked COFs(JUC-580-nonfold)by screening the reaction conditions of JUC-580-3fold,and explored the effect of interspersed isomerism on its luminescent properties.influences.Fluorescence test results show that the solid quantum yield of JUC-5 80-nonfold is 17%,much higher than the 3%of the same type of JUC-580-3fold.At the same time,we also found that the fluorescence emission wavelength of JUC-5 80-nonfold is hardly affected by the solvent,revealing that the intercalation isomerism is of great significance for improving the luminescent properties.(3)Synthesis of non-interspersed three-dimensional bcu topological sp2-carbonlinked COFs and their luminescent propertiesIn Section Ⅱ,we show that varying the degree of interpenetration of 3D sp2carbon-linked COFs can enhance luminescence.Although the mechanism of luminescence enhancement is still unclear,it shows that the effect of spatial structure on luminescence is crucial.Therefore,in this section,we chose to design two new eightnode monomers:TDFTD and TDFCB,among which TDFCB is a rare high-node lightemitting building unit,and further synthesized them into rare bcu topological COFs,and thus explored the topological structure Effect on luminescence.We also synthesized imine bonded COFs by Schiff base reaction as a comparison to study the luminescent properties of two different bonded COFs.After a series of luminescence tests,we found that the three-dimensional bcu topological imine bonded COFs did not emit light,and only the three-dimensional bcu topological sp2-carbon-linked COFs could emit light whether in solid state or dispersed in various solvents.This result shows that the effect of the reactive bond type on the luminescence is greater than the adjustment of the spatial structure,and once again confirms that the combination of C=C bond and threedimensional framework is beneficial to the preparation of stable photoluminescent COFs and the development of photofunctional COFs.