Reversible Self-assembly of a Viologen-based Supramolecular Gel Network under Electrochemical Control.

Redox-responsive supramolecular gels involving highly ordered assemblies of small molecules that can exist in different stable oxidation states are very promising soft materials for many applications ranging from catalysis to electronics. Stimulation of such materials has however so far relied mainly on the addition of chemical fuels (oxidizing or reducing agents) which makes the reversibility of the induced phenomena intrinsically limited. In this context, electrochemical stimulation is particularly interesting but unfortunately remains largely unexplored due to major scientific and technical obstacles. Here we present the electrochemical response of a supramolecular gel obtained by self-assembly of a low molecular weight gelator derived from 4,4'-bipyridinium salts into chiral hollow fibers. The reduction of this viologen-based supramolecular gel was carried out under both photochemical and electrochemical stimulation. The effects of reduction on sample composition and microstructure have been extensively studied using in operando optical microscopy and absorption spectroscopy measurements. We found that the reduction-triggered macroscopic changes result from the dissociation of charge-transfer complexes formed between the viologen acceptor and iodide counter-anions to form π-dimers. We have also developed different strategies to stimulate the viologen subunits that are embedded in the supramolecular network of the gel. The dissolution and reformation of the viologen-based supramolecular network could thus be achieved using different electrochemical stimuli. We therefore report here one of the very first low molecular weight gels capable of undergoing a reversible electrochemically-induced phase transition.