Sunlight-driven azobenzene-based thin films for smart coating applications

Nature provides extensive inspiration for the design of materials which move in response to external stimuli. Among these, molecular photoswitches enable reversible external control of biological systems, nanomachines, and smart materials. However, to enhance the effectiveness of these smart materials, photoswitching under visible light (green, white, or red) is highly desirable, as it allows for non-invasive operation. Here, we present a novel thin-film coating achieved by synthesizing a copolymer that combines red-light-shifted tetra-ortho-chloroazobenzene with zwitterionic sulfobetaine methacrylate (SBMA). Following synthesis, we systematically determined photochemical conversion efficiency and photokinetic parameters using NMR stoichiometry analysis and UV–vis kinetics under various visible lights. Computational calculations and molecular orbital (MO) theories were applied to the newly synthesized tetra-ortho-chloroazobenzene monomer, confirming the key n → π* separation responsible for the observed red-light-shifted transition between its two isomers. Atomic force microscopy (AFM) studies of the thin film revealed excellent reversibility in surface morphology and mechanics under direct irradiation of green/blue light and a lab-scale solar simulator. The observed surface topography and transition kinetics suggest a reversible photoinduced elastic deformation of the thin film. Building on our previously reported UV/visible light-activated copolymer coatings, we now introduce a semi-crosslinked variant that achieves surface reversibility using only safe, visible light activation. These light-responsive materials hold significant potential for future development in photopharmacology and self-cleaning surfaces powered by low-energy visible lights or non-invasive sunlight.