Design strategies and redox-dependent applications of insoluble viologen-based covalent organic polymers

Dicationic quaternary salts of 4,4′-bipyridine, also referred to as the viologen family, are well-known for their interesting redox chemistry, whereby they can be reversibly reduced into radical cationic and neutral moieties. Because of this ability to switch between different redox states, viologens have frequently been incorporated into covalent organic polymers (COPs) as molecular switches to construct stimuli-responsive materials. Although many viologen-based COPs have been reported, hyper-conjugated insoluble COPs started to emerge fairly recently and have not been comprehensively reviewed. In this review, we investigate the design strategies employed in the synthesis of insoluble viologen-based COPs, which can be broadly classified as those with viologen in the backbone and those with viologen as pendant groups. Chemical reactions used in the synthesis of each category, including Sonogashira-Hagihara cross-coupling, Menshutkin and Zincke reactions, are highlighted. Diverse applications of these COPs are discussed with particular reference to the redox state of viologen in each material. Uses of these materials for gas adsorption, organic and inorganic pollutant removal, catalysis, sensing and film fabrication are explored.