The Preparation And Functionalization Of Schiff-Base Two Dimensional Covalent Organic Frameworks

Surface covalent organic frameworks(surface COFs or 2D polymers), is a novel class of graphene-like two dimensional(2D) materials, whose structure can be designed following traditional organic chemistry principles because of the mild preparation conditions. A key challenge in the design of surface COFs is the “bottom-up” construction of covalently bonded molecular architectures in a well-defined arrangement. The research on its structure and properties and its combination with other materials(such as graphene) can promote their applications in different fields. Moreover, surface COF functionalized with active groups will have important applications in the field of nanosensing and catalysis. Additionally, the band gap of π-conjugated frameworks could be tuned by introducing side groups or alternating the π-conjugated backbone of precursors, which is highly relevant for the application in the field of nano-electronics. In this thesis, we have investigated the construction of surface COFs via Schiff-base coupling on highly oriented pyrolytic graphite(HOPG) or graphene substrate. We have synthesized surface COFs with tunable periodicity and side-functionalization with nearly full surface coverage. Efforts were also made to tune the band gap by altering the backbone structure of precursors. Scanning tunneling microscopy(STM), atomic force microscopy(AFM) and density function theroy(DFT) calculation were used to characterize the chemical and electronic structures of the obtained surface COFs. The main results are as follows:We performed Schiff-base coupling between a triangular aromatic aldehyde and linear aromatic diamine monomers on HOPG surface either at a solid/liquid interface at room temperature or in low vacuum with moderate heating. With this simple and moderate methodology, we have obtained surface-confined 2D COFs with few defects and almost entire surface coverage. The domains size of surface COFBTA-TDA can extend to more than 1 μm2. By varying the backbone length of aromatic diamines the pore size of surface COF is tunable from ~1.7 to 3.5 nm. Formation of small portions of bilayers was observed by both STM and AFM, which clearly reveals an “eclipsed” stacking manner.A surface confined covalent Schiff-base network was prepared on single layer graphene grown on copper foil. A stable, long-range ordered Schiff-base surface COF with almost full surface coverage was obtained at the solid/liquid interface. The dynamic self-healing process was investigated with STM. DFT simulations provide understanding of the electronic structures and the interactions between the surface COF and graphene. Strong coupling between the surface COF and graphene was confirmed by the dispersive bands of surface COF after interacting with graphene, and also by the experimental observation of tunneling condition dependent contrast of the surface COF.Surface COFs functionalized with chemical active or passive groups was obtained on HOPG surface. Functionalization with chemical active-OH groups results in side reactions and leads to distored surface COF at higher temperature. However, regular-OH functionalized surface COF can be obtained by optimizing the reaction temperature and time. Two inert groups functionalized surface COFs were also obtained by replacing the –OH group with methyl or methoxy group, respectively. Both surface COFs show high quality with few defects and entire surface coverage. By this means, the pore size of the surface COF can be accurately adjusted without changing the repeating period. The pore diameter of unfunctionalized, methyl and methoxy functionalized surface COFs are 2.6±0.1 nm, 2.4±0.1 nm and 2.3±0.1 nm respectively.We have tried to tune the band gap of surface COFs by changing the backbone of aromatic diamines. Two different surface COFs were obtained by replacing p-phenylenediamine and benzidine with(4-amino-1-naphthyl) amine and 3,8-diamino-6-phenylphenanthridine respectively. STM characterizations reveal that the regularity of the surface COFs can be affected by the existence of lateral bulky groups. DFT simulation revealed significant narrowing of the band gap of surface COF formed by benzene-1,3,5-tricarbaldehyde and(4-amino-1-naphthyl) amine in compare with that with p-phenylenediamine, while the one formed by benzene-1,3,5-tricarbaldehyde and 3,8-diamino-6-phenylphenanthridine show nearly identical band gap as that of benzene-1,3,5-tricarbaldehyde and benzidine. The DFT calculation suggests that the planarity of the surface COF has more important influence on the band gap than the lateral dimension of monomer conjugation.