FromHighly Crystalline to Outer Surface-FunctionalizedCovalent Organic Frameworks—A Modulation Approach

摘要

Crystallinity and porosity are of central importance for many properties of covalent organic frameworks (COFs), including adsorption, diffusion, and electronic transport. We have developed a new method for strongly enhancing both aspects through the introduction of a modulating agent in the synthesis. This modulator competes with one of the building blocks during the solvothermal COF growth, resulting in highly crystalline frameworks with greatly increased domain sizes reaching several hundreds of nanometers. The obtained materials feature fully accessible pores with an internal surface area of over 2000 m² g^–1. Compositional analysis via NMR spectroscopy revealed that the COF-5 structure can form over a wide range of boronic acid-to-catechol ratios, thus producing frameworks with compositions ranging from highly boronic acid-deficient to networks with catechol voids. Visualization of an −SH-functionalized modulating agent via iridium staining revealed that the COF domains are terminated by the modulator. Using functionalized modulators, this synthetic approach thus also provides a new and facile method for the external surface functionalization of COF domains,providing accessible sites for post-synthetic modification reactions.We demonstrate the feasibility of this concept by covalently attachingfluorescent dyes and hydrophilic polymers to the COF surface. We anticipatethat the realization of highly crystalline COFs with the option ofadditional surface functionality will render the modulation conceptbeneficial for a range of applications, including gas separations,catalysis, and optoelectronics.