Band engineering in two-dimensional porphyrin- and phthalocyanine-based covalent organic frameworks: insight from molecular design

Abstract

Two-dimensional covalent organic frameworks (2D COFs) represent an emerging class of crystalline polymeric networks, characterized by their tunable architectures and porosity, synthetic adaptability, and interesting optical, magnetic, and electrical properties. The incorporation of porphyrin (Por) or phthalocyanine (Pc) core units into 2D COFs provides an ideal platform for exploring the relationship between the COF geometric structure and its electronic properties in the case of tetragonal symmetry. In this work, on the basis of tight-binding models and density functional theory calculations, we describe the generic types of electronic band structures that can arise in tetragonal COFs. Three tetragonal lattice symmetries are examined: the basic square lattice, the Lieb lattice, and the checkerboard lattice. The potential topological characteristics of each lattice are explored. The Por-/Pc-based COFs exhibit characteristic band dispersions that are directly linked to their lattice symmetries and the nature of the frontier molecular orbitals of their building units. We show that the band dispersions in these COFs can be tailored by choosing specific symmetries of the molecular building units and/or by modulating the relative energies of the core and linker units. These strategies can be extended to a wide array of COFs, offering an effective approach to engineering their electronic properties.