Viologen-Based Conjugated Covalent Organic Networks via Zincke Reaction

Morphology influences the functionality of covalent organic networks and determines potential applications. Here, we report for the first time the use of Zincke reaction to fabricate, under either solvothermal or microwave conditions, a viologen-linked covalent organic network in the form of hollow particles or nanosheets. The synthesized materials are stable in acidic, neutral, and basic aqueous solutions. Under basic conditions, the neutral network assumes radical cationic character without decomposing or changing structure. Solvent polarity and heating method determine product morphology. Depending upon solvent polarity, the resulting polymeric network forms either uniform self-templated hollow spheres (HS) or hollow tubes (HT). The spheres develop via an inside-out Ostwald ripening mechanism. Interestingly, microwave conditions and certain solvent polarities result in the formation of a robust covalent organic gel framework (COGF) that is organized in nanosheets stacked several layers thick. In the gel phase, the nanosheets are crystalline and form honeycomb lattices. The use of the Zincke reaction has previously been limited to the synthesis of small viologen molecules and conjugated viologen oligomers. Its application here expands the repertoire of tools for the fabrication of covalent organic networks (which are usually prepared by dynamic covalent chemistry) and for the synthesis of viologen-based materials. All three materials-HT, HS, and COGF-serve as efficient adsorbents of iodine due to the presence of the cationic viologen linker and, in the cases of HT and HS, permanent porosity.