Topology-guided design of the first 3D penta-COFs with superior ultraviolet optical response

Abstract

Covalent organic frameworks (COFs) have gained significant attention in the past two decades due to their porous geometries, high stability and diverse functional applications. Very recently, 2D Cairo pentagon tessellated COFs (penta-COFs) have been successfully synthesized, going beyond conventional 2D COFs with a unique topology and exceptional properties. However, 3D penta-COFs remain unexplored, despite the inherent advantages of 3D frameworks over their 2D counterparts. Here, based on the combination of the topology-guided screening approach with rationally selected building blocks and first-principles calculations, we theoretically design the first 3D penta-COFs. The resulting two distinct 3D COFs, unj-1 and unj-2, exhibit high thermal stability up to 1300 K, intriguing mechanical hyperelasticity, and exceptional ultraviolet optical properties with absorption coefficients in the order of 10⁷ cm⁻¹, far surpassing conventional UV-absorbing materials. Further analysis of their electronic structures uncovers the underlying mechanism, which is attributed to the augmented electron–hole wavefunction overlap and valence bandwidth compression synergistically driven by the pentagonal topology and building blocks of the studied 3D penta-COFs. This study expands penta-COFs from two dimensions to three dimensions, demonstrating the unique role of the pentagonal topology in designing new functional materials.