Tetrafluorinated versus hydrogenated azobenzene polymers in water: Access to visible-ligh stimulus at the expense of responsiveness

Abstract

Photo-responsive polymers are at the core of numerous applications as actuators in biology but often exhibit the drawback of activation in the UV range. This work describes a methacrylate-based terpolymer presenting tetra-ortho-fluoroazobenzene groups showing activation through visible light illumination. The presence of poly(ethylene oxide) side chains and maleimide functions ensures that it can be easily used for biological applications in aqueous media. The terpolymer was shown to form unimolecular micelles in water. The tetra-ortho-fluoroazobenzene moiety was reversibly switched from trans to cis upon visible light irradiation without significant modification of the unimolecular morphology. The deprotection of the maleimide function resulted in a reactive polymer which was used to obtain hydrogels by reaction with poly(ethylene oxide) tetra-thiols or bioconjugates by reaction with a model peptide or a poly(ethylene oxide) mono-thiol exhibiting a biotin group. Hydrogels incorporating β-cyclodextrin molecules were also obtained. Contrary to polymers bearing hydrogenated azobenzenes, the terpolymer bearing tetra-ortho-fluoroazobenzene moieties did not form a complex with β-cyclodextrin and none of the presented hydrogels exhibited macroscopic changes upon irradiation although the trans/cis isomerization occurred. This was explained by the burial of tetra-ortho-fluoroazobenzene groups towards the polymer backbone, therefore completely hindering its access.