Rapid Photoswitching of Low Molecular Weight Arylazoisoxazole Adhesives

Abstract

Adhesion is one of the most ubiquitous practical applications at surfaces. With today’s society calling increasingly for more reusable and “green” alternatives, the demand for readily reversible adhesives has triggered many studies into this field, in particular by incorporating molecular photoswitches into composite materials. Responsive polymers can act as reversible adhesives, but their employment brings about synthetic drawbacks and challenges in reproducibility and reusability. Here, our results demonstrate that even a low molecular weight photoswitch can serve as an on-demand adhesive when the intermolecular interactions are sufficiently strong. We show that readily accessible arylazoisoxazoles display a fast photoreversible solid-to-liquid phase transition and perform as excellent photoreversible adhesives, with a remarkable durability over 10 immediate reuse cycles without a loss in adhesive strength or an increase in the unprecedented response time. Furthermore, the versatility of photoreversible adhesion is shown at various surfaces ranging from polymeric materials to metals, demonstrating a wide field of potential application.

Acid-catalysed liquid-to-solid transitioning of arylazoisoxazole photoswitches

“…The larger thermodynamic ratio of the Z -to E -azonium in presence of CF 3 SO 3 H in 1 H NMR spectroscopy (6% Z -form as opposed to ca. 1% in absence of a proton source 44 ) is in line with the change in energetic barriers observed by DFT ( Fig. 2A ).…”

“…S9 and S10 † ). DFT calculations of activation free energies confirm that the Z -to E -barrier is substantially reduced in the azonium species from 1.29 eV ( versus 1.11 eV experimentally 44 ) to 0.86 eV ( Fig. 2A ).…”

“…Owing to mesomerism, 46 the uncatalysed half-life of the 4-alkoxy-substituted Z -AIZs 2 and 3 in solution was determined to be 7 d (ref. 44 ) rather than the 45 d for Z -AIZ 1 . 43 Although we observe no reactivity of Z -AIZ 2 and 3 to a large excess of acetic acid (CH 3 COOH, 5000 equivalents in acetonitrile, Fig.…”

“…Previously, we have utilised this remarkable retention of photophysics in the solid state to develop photoreversible low molecular weight (LMW) adhesives, cohering glass surfaces at 30.8 N cm −2 with rapid debonding upon UV-irradiation. 44 Although the control over the bonding/debonding process is remarkably swift and reversible even upon physical breaking of the cohesion, the photochemical approach requires the use of at least one transparent surface. In this contribution, we seek to expand on the promising reusable photophysical features of these low molecular weight azoisoxazoles by exploring the possibility to kinetically control their solidification with protons.…”

Advances in Application of Azobenzene as a Trigger in Biomedicine: Molecular Design and Spontaneous Assembly

“…Azobenzene‐driven morphological switching has also been applied to photoresponsive phase changes. [ 288 ] Solid‐liquid reversible transitions have potential applications in the field of light‐induced reversible adhesives for biological tissues or cells. Yin et al.…”