Electronic Structure of Covalent Organic Frameworks COF66 and COF366: from Monomers to Two-Dimensional Framework

Two-dimensional covalent organic frameworks (COFs) are an emerging important class of porous materials with periodically ordered structures consisting of core and linker units held together by strong covalent bonds. Their well-designed architecture, tunable functional properties and good thermal stabilities provide great potential for applications in photoenergy conversion and semiconductors. For the aim to explore the relationship between the structure and properties of COFs, which should be very useful for the design of new type of such attractive photo functional materials, the energy character of organic building units, fragment molecules and periodic two-dimensional planar structures are analyzed for COF66 and COF366. The results show that the structure of periodic planar frameworks will be influenced by the geometry of organic building units. Moreover, the relative energy level of the frontier orbitals of the organic building blocks will affect the ionization potential and electron affinity. And the electronic properties of the COFs are also in the same case. Therefore, the electronic property of organic building units is a key factor for determining the electronic structure of periodic structure of COFs. In addition, the electronic coupling of COFs will be larger if the conjugation of the organic building blocks is better. Then, the valence band maximum and conduction band minimum will be more dispersive, and the charge carrier mobility will get larger.