Cyclodextrin ‘chaperones’ enable quasi-ideal supramolecular network formation and enhanced photodimerization of hydrophobic, red-shifted photoswitches in water

Abstract

Precision-engineered light-triggered hydrogels are important for a diversity of applications. However, fields such as biomaterials require wavelength outside the harsh UV regime to prevent photodamage, typically requiring chromophores with extended π-conjugation that suffer from poor water solubility. Herein, we demonstrate how cyclodextrins can be used as auxiliary agents to not only solubilize such chromophores, but even to preorganize them in a 2 : 2 host-guest inclusion complex to facilitate photodimerization. We apply our concept to styrylpyrene-end-functionalized star-shaped polyethylene glycols (sPEGs). We initially unravel details of the host-guest inclusion complex using spectroscopy and mass spectrometry to give clear evidence of a 2 : 2 complex formation. Subsequently, we show that the resultant supramolecularly linked hydrogels conform to theories of supramolecular quasi-ideal model networks, and derive details on their association dynamics using in-depth rheological measurements and kinetic models. By comparing sPEGs of different arm length, we further elucidate the model network topology and the accessible mechanical property space. The photo-mediated dimerization proceeds smoothly, allowing to transform the supramolecular model networks into covalent ones. We submit that our strategy opens avenues for executing hydrophobic photochemistry in aqueous environments with enhanced control over reactivity, hydrogel topology or programmable mechanical properties.