On-Surface Synthesis of 2D Porphyrin-Based Covalent Organic Frameworks Using Terminal Alkynes

The surface chemistry of large molecules such as tetraphenylporphyrin molecules functionalized with terminal alkyne groups has not yet been studied, as these molecules are too reactive for standard deposition by thermal sublimation. In this scanning tunneling microscopy study, this problem is elegantly circumvented in the in situ deposition of 5,10,15,20-tetra(4-ethynylphenyl)porphyrin molecules in ultrahigh vacuum using the “direct contact transfer” (DCT) method. The metal substrate has been found to play a key role in determining the reaction pathway of the terminal alkynes after annealing, i.e., alkyne–alkyne cyclodimerization is the predominant reaction pathway on Au(111), while Glaser coupling and nondehydrogenative head-to-head coupling are dominant pathways on Ag(111). Our work not only demonstrates the feasibility of using highly reactive large molecules for on-surface synthesis through the DCT method but also establishes that the bonding in porphyrin-based covalent organic frameworks can be controlled by varying the metal substrate.