Photoswitchable Liquid‐to‐Solid Transition of Azobenzene‐Decorated Polysiloxanes

Abstract Having external control over fundamental properties of polymers, such as their physical state, is a crucial yet challenging design criterion for smart materials. Liquifying polymers through photochemical events has significantly advanced various research lines. However, the opposite process of solidifying a polymer that is intrinsically in a liquid state reversibly with light is unattained. Herein, the light‐controlled liquid‐to‐solid transition of polysiloxanes is reported, which are decorated with a small number of azobenzene‐functionalized ureidopyrimidinone (Azo‐UPy) pendants. The UPy moieties toggle between intra‐ and intermolecular hydrogen bonding via trans→cis photoisomerization of the azobenzene. This transformation on the molecular level leads to the formation of strong supramolecular cross‐links, which, in turn, results in the macroscopic solidification of the material. The photoswitching event enables the post‐synthetic tailoring of the polymers’ mechanical properties, thus providing an alternative to the addition of plasticizers or hardeners. Moreover, the adhesion strength of the photochromic material increases by a factor of 6 upon exposure to UV light. In situ illumination during rheological measurements reveals the delicate interplay between wavelength dependent penetration depth and photoswitching efficiency. This conceptually new (de)bonding on demand strategy paves the way for creating light‐responsive materials with exciting applications in temporal adhesion, recycling, lithography, and material processing.