Visible-Light-Induced Reversible Photochemical Crystal–Liquid Transitions of Azo-Switches for Smart and Robust Adhesives

Photochemical crystal ↔ liquid transitions (PCLTs) are interesting phenomena that couple reversible photochemical transformations with thermophysical phase transitions. A potential application of PCLTs is the development of photoresponsive smart materials capable of exerting reversible adhesion capacities on specific surfaces at a desired timing, which are unattainable for conventional adhesives. However, PCLT-based adhesives generally use UV light as the stimulus, which could lead to degradation of materials and health problems. Here, visible-light-controlled smart and robust adhesives are developed using small-molecule azo photoswitches. These azo molecules can undergo very efficient trans-crystal → cis-liquid and cis-liquid → trans-crystal transitions under 405 and 532 nm light irradiations, respectively. Their trans-crystal state displays strong adhesion strengths on various substrates, e.g., 1.13 MPa on quartz/quartz and 1.58 MPa on wood/wood, and very fast light-induced separation of glued substrates can be accomplished within 1 s along with the loss of adhesion strength in the cis-liquid state. Robust switching of the adhesion strength is demonstrated in multiple cycles, and these adhesives can also work well in underwater environments. Visible-light-controlled reversible PCLTs can be a very promising strategy in the pursuit of high-performance photoresponsive adhesives.