Azobenzene based multistimuli responsive supramolecular hydrogels

Abstract

Multistimuli responsive supramolecular aqueous gelators (C₄-Azo-C₅-D₂₃₀, C₄-Azo-C₅-D₄₀₀, C₄-Azo-C₅-ED₉₀₀), composed of alkyl chains, an azobenzene unit, and an amine terminated polyether were prepared. We studied their reversible hydrogelation into three-dimensional entangled supramolecular gels upon changes in temperature, light exposure, pH, and shear. Upon irradiation with UV light, the trans isomer of the C₄-Azo-C₅-D₄₀₀ photoisomerized to the cis isomer, which goes to a new steady state between both isomers, resulting in disruption of the gel. Rheological measurements of the hydrogel of C₄-Azo-C₅-D₄₀₀ suggested that the non-covalent interactions were disrupted. Likewise, high temperature also caused a reversible disruption to the gel. While the binary mixture of C₄-Azo-C₅-D₄₀₀ and water formed gels from a solution under neutral and basic conditions, under the acidic conditions the molecules aggregated and precipitated. After intense shaking of the hydrogel, a solution separated from the gel, resulting in a rapid drop in both modulus and complex viscosity. This photoresponsive gelator can also form lyotropic liquid crystal (LLC) mesophases above 70 °C. Through rational design, multistimuli responsive hydrogelators were successful devised, potentially providing an impetus to the ‘design’ of new gelators through the incorporation of other stimuli responsive features.