Tuning the Electronic Structure of Two-Dimensional Covalent Organic Frameworks (2D-COFs) by an Asymmetrical Bonding Scheme*

Recently, there has been a growing interest in two-dimensional covalent organic frameworks (2D COFs) and its bottom-up synthesis from molecular precursors provides an important new methodology for designing and fabricating such 2D materials. The electronic structure of 2D-COFs is highly dependent on the chemical bonds within these networks. Here we report the synthesis and characterization of a porphyrin-based single-layer 2D COF with a square lattice. The rational design of molecular precursors leads to an asymmetrical bonding scheme in which each porphyrin core is in a different chemical bonding environment than its four nearest neighboring porphyrin cores. This is achieved via an in situ UHV condensation coupling reaction between two different molecular precursors on a Au(111) surface. The chemical structure and local electronic properties of the resulting COF were explored using scanning tunneling microscopy (STM) and spectroscopy (STS). The different bonding environment of the porphyrin cores within this asymmetrically bonded COF results in the formation of a type II heterojunction within a COF network.