Influence of Building Block Symmetry on the Band Structure of Stacked 2D Polyimide Covalent Organic Frameworks

Covalent organic frameworks (COFs) are a class of crystalline porous materials distinctively built solely from organic elements, carbon, oxygen, hydrogen, and often nitrogen or boron. They form light, mechanically rigid, and chemically stable networks that have many advantages, but their low solubility and poor processability create issues with developing large-scale films or membranes. Two-dimensional (2D) COFs possess periodic porous crystallinity, functionality, modularity, and layered one-dimensional (1D) transport channels. All of these traits, along with the semiconducting properties of selected COFs, make them interesting candidates for integration in optoelectronic devices. Therefore, it is still a challenge to explore computationally and structurally the semiconductivity of COFs and to determine their final potential. Herein, we report on the possible semiconducting properties and results of polyimide-COF materials using density functional theory calculations. Our analysis includes monolayers and multilayers (AA- and AB-stacked modes) of mellitic triimide frameworks designed from mellitic trianhydride (MTA) as the main building knot, including MTI-TAPB-COF, which was previously synthesized from the condensation reaction of MTA and 1,3,5-tris(4-aminophenyl)benzene (TAPB), and other previously unreported structures based on MTA. Respective frameworks have been selected due to the difference in building block symmetry (C₃ + C₂ and C₃ + C₃) and different chemical linkages, either by benzene or by pyridine rings. We find the polyimide multilayers to be stable and with varying electronic properties. The finite band gap exhibited by every structure (monolayer and stacked) was sensitive to atomic arrangement. Stacking introduces dispersion to an otherwise flat band structure of the materials, which appeared to be highly sensitive to stacking direction. The effect of stacking was similar for each COF, but the magnitude of band structure change was different and dependent on the symmetry of the building blocks.