Reversible pH-responsive supramolecular aggregates from viologen based amphiphiles – a molecular design perspective†

Abstract

pH responsive self-assembled supramolecular systems in water hold tremendous promise spanning across the various realms of science and technology. Herein, we report the design and synthesis of benzyl viologen (BV) based amphiphiles and their ability to form pH responsive aggregates with a water soluble anionic dye (electron donor), a polyelectrolyte (PE), and a surfactant. To counter the low solubility of viologen derivatives, β-cyclodextrin (β-CD) was employed as a solubility promoter and the host–guest complexes were characterized by NMR spectroscopy. The impacts of increasing the number of benzyl units on (i) the water solubility of viologens, (ii) the response of the aggregates of viologens with pyranine, PE, and surfactants towards pH, and (iii) the influence of β-CD on the pH-responsive nature of BV–pyranine, BV–PE, BV–surfactant, etc. were investigated. Apart from improving the solubility of viologens, β-CD also imparted pH-responsive dissolution/aggregation behavior to the viologen–anionic polyelectrolyte and viologen–anionic surfactant complexes. The pH switchable behaviour of the soft supramolecular aggregates in water was rationalized in light of a delicate balance prevailing between multiple non-covalent interactions. Based on the results, we propose an elegant molecular design principle to generate pH responsive colloidal aggregates from amphiphiles and oppositely charged molecular systems.