Reducing Exciton Binding Energy in 2D Covalent Organic Frameworks by Decreasing Layer Planarity

Abstract

2D covalent organic frameworks (2D COFs) exhibit pronounced excitonic effects, which severely limit the yield of free charge carriers and photocatalysis performance. While attempts to mitigate this limitation are scarce, and convincing relationship between molecular structure and excitonic effects remains unclear. A straightforward design principle is presented for optimizing excitonic effects by reducing layer planarity, using pyrene-derived COFs as exemplary cases. Three pyrene-derived COFs are constructed from 4,4′,4″,4‴-(pyrene-1,3,6,8-tetrayl) tetraaniline (PyTTA) and terephthalaldehyde building blocks incorporating additional functional groups (─OH, ─H, and ─OCH₃). Compared to the parent case (Py-H-COF), the ─OH groups introduce a layer locking effect through the presence of hydrogen bonds, whereas the ─OCH₃ substituents facilitate local rotation out of the layer due to their large steric hindrance. Decreased layer planarity upon going from Py-OH-COF to Py-H-COF and to Py-OCH₃-COF hampers local conjugation and, as shown by experimental and theoretical results, is accompanied by a marked decrease in exciton binding energy. This distortion, in turn, accelerates exciton dissociation and suppresses free carrier recombination. Accordingly, Py- OCH₃-COF, the case with the lowest exciton binding energy (43.2 meV), demonstrates superior photocatalytic degradation of organic pollutants. These findings provide valuable insights for improved design of 2D COFs as high-performance photocatalysts.