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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
(<b>HS</b>) or hollow tubes (<b>HT</b>). 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 (<b>COGF</b>) 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<b>HT</b>, <b>HS</b>, and <b>COGF</b>serve as efficient adsorbents of iodine due to
the presence of the cationic viologen linker and, in the cases of <b>HT</b> and <b>HS</b>, permanent porosity