Suneesh C. Karunakaran, Brian J. Cafferty, Kyan S. Jain, Gary B. Schuster, and Nicholas V. Hud, Reversible transformation of a supramolecular hydrogel by redox switching of methylene blue - A noncovalent chain stopper, ACS Omega, 5, 344-349 (2020).
The simple and reversible control of the degree of polymerization, and thereby the bulk material properties, of a supramolecular polymer is reported. Noncovalent capping agents (chain stoppers) modulate the length of supramolecular polymers by stacking on the surfaces of the polymer’s ends. Methylene blue (MB) is a positively charged, planar polycyclic dye that acts as a chain stopper. It can be reversibly switched between its colored, planar, cationic state and a colorless, nonplanar, neutral state (leucomethylene blue, LMB) by reduction with ascorbic acid and then reoxidized to MB by O2. LMB does not act as a chain stopper. This behavior was utilized to reversibly trigger the gel to sol transformation of supramolecular polymers formed by the self-assembly of hexameric rosettes comprising 2,4,6-triaminopyrimidine and a hexanoic acid-substituted cyanuric acid (CyCo6) in aqueous media. The results of our experiments highlight the ability of this approach to reversibly switch between the gel and solution states of materials formed from supramolecular polymers and thereby control their bulk properties.