Solving the COF trilemma: towards crystalline, stable and functional covalent organic frameworks

Covalent organic frameworks (COFs) have entered the stage as a new generation of porous polymers which stand out by virtue of their crystallinity, diverse framework topologies and accessible pore systems. An important - but still underdeveloped - feature of COFs is their potentially superior stability in comparison to other porous materials. Achieving COFs which are simultaneously crystalline, stable, and functional is still challenging as reversible bond formation is one of the prime prerequisites for the crystallization of COFs. However, as the COF field matures new strategies have surfaced that bypass this crystallinity - stability dichotomy. Three major approaches for obtaining both stable and crystalline COFs have taken form in recent years: Tweaking the reaction conditions for reversible linkages, separating the order inducing step and the stability inducing step, and controlling the structural degrees of freedom during assembly and in the final COF. This review discusses rational approaches to stability and crystallinity engineering in COFs, which are apt at overcoming current challenges in COF design and open up new avenues to new real-world applications of COFs.