Nanoscrolls Formed from Two-Dimensional Covalent Organic Frameworks

Two-dimensional (2D) covalent organic frameworks (COFs) represent an emerging class of nanomaterials with building blocks precisely connected in-plane through covalent bonds. Gaining insights into their structure and stabilities is critical to both their preparation and applications. Here, via atomistic molecular mechanics simulations and free-energy calculations, we investigate 2D COFs both under vacuum conditions and in solution, taking representative boronate ester-based and imine-based COFs as examples. Rather than remaining flat, single-layer 2D COF sheets with at least their length larger than similar to 15-similar to 20 nm are found to preferably form nanoscrolls. These nanoscrolls display a finite number of configurations and represent open structures due to the large pores present in the 2D sheets; this feature distinguishes them from nanoscrolls formed by dense 2D materials such as graphene. Density functional theory calculations indicate that the intrasheet interactions in the nanoscrolls make their optical and electrical properties different from those of stand-alone sheets. The formation of such scroll-like structures can pave the way to extended spiral growth of 2D polymer networks and porous nanotubes.