Nanoscrolls Formed from Two-Dimensional Covalent Organic\nFrameworks

Abstract

Two-dimensional (2D)\ncovalent organic frameworks (COFs) represent\nan emerging class of nanomaterials with building blocks precisely\nconnected in-plane through covalent bonds. Gaining insights into their\nstructure and stabilities is critical to both their preparation and\napplications. Here, via atomistic molecular mechanics simulations\nand free-energy calculations, we investigate 2D COFs both under vacuum\nconditions and in solution, taking representative boronate ester-based\nand imine-based COFs as examples. Rather than remaining flat, single-layer\n2D COF sheets with at least their length larger than ∼15–∼20\nnm are found to preferably form nanoscrolls. These nanoscrolls display\na finite number of configurations and represent open structures due\nto the large pores present in the 2D sheets; this feature distinguishes\nthem from nanoscrolls formed by dense 2D materials such as graphene.\nDensity functional theory calculations indicate that the intrasheet\ninteractions in the nanoscrolls make their optical and electrical\nproperties different from those of stand-alone sheets. The formation\nof such scroll-like structures can pave the way to extended spiral\ngrowth of 2D polymer networks and porous nanotubes.