A STM Investigation On Two-dimensional Organic Polymers Containing Porphyrin Units Via Surface Reaction

The discovery of graphene has raised great interest for searching for new two dimensional nanomaterials with fascinating properties and scientists have synthesized a series of graphene-like materials such as Mo S2 and WS2, however, these inorganic materials are not feasible in the design of structure, which finally limit their applications. The first report of Covalent Organic Frameworks in 2005 opened the door toward synthesizing two dimensional organic nanomaterials.2D COF is a kind of two-dimensional organic polymers and different from the general polymers in structure. It only has single-atomic-layer thickness, but can be seen infinite on the other two directions, which lead to its high specific surface area, favors its application in gas storage. With the backbone containing extended π-conjugation, 2D COF has exceptional electronic properties and exhibit great application prospect in molecular electronics, sensors and photoelectric devices. Much efforts has been made to synthesize 2D COF with monolayer thickness, currently, the major strategy is the "bottom-up" approach based on surface reaction. According to the characteristics of reaction, surface reactions can be divided into dehalogenation, dehydration and dehydrogenation(or alkyl). Although 2D COFs with various topological structures have been designed and synthesized, to obtain highly uniform 2D COF with high quality still been the major challenge, which is a prerequisite for subsequent application.In this article, we take porphyrin derivatives, meso-Tetra(4-aminophenyl) Porphine(TTP-NH2) and meso-Tetra(4-formldephenyl)Porphine TTP-CHO, as the building blocks, utilize mild surface reaction——Schiff base reaction(a kind of dehydration) to fabricate two dimensional organic polymers containing porphyrin units at the surface of HOPG. The goal is to obtain extended and highly ordered two dimensional polymers with single layer thickness. STM(Scanning Tunneling Microscope)is the major research tools and the results are as follows.we have obtained 2D polymers with anticipated tetragon structure by the on-surface Schiff base reaction between TTP-NH2 and 2,5-Bis(octyloxy)terephthalaldehyde(BOTPA) on the HOPG surface through mild heating under low vacuum. Apart from tetragon,trigon, pentagon and hexagon also exist due to the flexibility of imine network. Statistics on the number of polygons suggests that tetragon pores are the prevailing species, accounting for 78%, followed by trigonal, pentagonal and hexagonal pores whose sites are between domain boundaries. We also discovered that the tunneling bias applied during the scan progress is relevant to the contrast in the STM images. With positive bias and negative bias the STM image reflects the electron density distribution of the conduction band minimum and valence band maximum, respectively. High temperature is beneficial for the formation of more ordered structure, and when the mole ratio of amino to aldehyde group varies from 3:1 to 1:2, the difference between obtained structures is not obvious. Through the comparison of 2D polymers formed by TTP-NH2 with BOTPA and Terephthalaldehyde(TPA), we know that the alkoxy chains of BOTPA play an important role in the formation and stabilization of 2D COF.TTP-CHO and Diamino-p-terphenyl(TPDA) also forms well-ordered 2D polymers with anticipated tetragon structure by the on-surface Schiff base reaction We find that when the concentration of monomers has significant influence on the formation of 2D polymers: with low concentration of 0.002mg/g, polymer possesses low coverage, while when the concentration is between 0.01mg/g and 0.05mg/g, two dimensional polymers with good quality can be obtained. As the mole ratio of the amino to aldehyde group varies between 5:1 to 1:3,the structure of 2D polymer is insensitive to the change. Through the comparison of 2D polymers synthesized by TTP-CHO with different monomers TPDA, p-Phenylenediamine(PPD) and 4-4"-(9,10-Biethynylanthryl) di Aniline(AABA), it indicates that the adsorption and mobility of monomers has great effect on the formation of 2D COF. We have also obtained 2D polymer with tetragon structure from surface reaction of TTP-NH2 and TTP-CHO for the first time.