Tuning the stacking behaviour of a 2D covalent organic framework through non-covalent interactions†
Two-dimensional covalent organic frameworks (COFs) are crystalline porous materials composed of organic building blocks that are connected via covalent bonds within their layers, but through non-covalent interactions between the layers. The exact stacking sequence of the layers is of paramount importance for the optoelectronic, catalytic and sorption properties of these polymeric materials. The weak interlayer interactions lead to a variety of stacking geometries in COFs, which are both hard to characterize and poorly understood due to the low levels of crystallinity. Therefore, detailed insights into the stacking geometry in COFs is still largely elusive. In this work we show that the geometric and electronic features of the COF building blocks can be used to guide the stacking behavior of two related 2D imine COFs (TBI-COF and TTI-COF), which either adopt an averaged “eclipsed” structure with apparent zero-offset stacking or a unidirectionally slip-stacked structure, respectively. These structural features are confirmed by XRPD and TEM measurements. Based on theoretical calculations, we were able to pinpoint the cause of the uniform slip-stacking geometry and high crystallinity of TTI-COF to the inherent self-complementarity of the building blocks and the resulting donor–acceptor-type stacking of the imine bonds in adjacent layers, which can serve as a more general design principle for the synthesis of highly crystalline COFs.
- This article is part of the themed collections: Celebrating Excellence in Research: 100 Women of Chemistry, Celebrating Excellence in Research: Women at the Frontiers of Chemistry, Materials Chemistry Frontiers HOT articles for 2017 and Functional Open framework materials