Microwave-assisted synthesis of a viologen-based covalent organic polymer with redox-tunable polarity for dye adsorption

We report the efficient synthesis of a viologen-based covalent organic polymer by a microwave-assisted Menshutkin reaction. The polymer's viologen sub-units were synthesized in the 4,4′-bipyridinium dicationic state but could be reduced to radical-cationic and neutral forms. Regardless of the redox state of its viologens, the polymer was found to be thermally stable and capable of removing three dyes of differing polarities—fluorescein (FL), rhodamine B (RhB), and Nile red (NR)—from aqueous solutions. With its viologens in the dicationic state, the polymer was most efficient at removing FL, which is hydrophilic. With its viologens in the neutral state, it was most efficient at removing NR, which is hydrophobic. This strategy of using a single redox-tunable material for adsorbing multiple contaminants of different polarities represents a unique approach to water purification.