A near-infrared light-driven composite for smart and robust adhesion based on dynamic photochemistry

In recent years, liquid-crystalline polymers containing azobenzene moieties that show phototunable glass transition temperatures have been successfully applied in adhesion, reversible mechanical actuation, athermal nano-imprinting, self-healing, energy storage, and other applications. Unfortunately, the physical principles and properties underlying the dynamic equilibrium behaviors when both <i>trans</i> and <i>cis</i> isomers are triggered simultaneously remain elusive, and experiments are challenging due to the difference in penetration depth between ultraviolet and visible light sources. Here, we develop a novel environmentally friendly adhesive with near-infrared (NIR) light-driven photoresponsiveness and reusability through the integration of highly efficient UV/blue-emitting upconversion nanoparticles (UCNPs) and siloxane main-chain azobenzene molecular photoswitches, and achieve simultaneously bicolor emissions inside the composite by facilely modulating the external excitation power, triggering both the <i>trans</i> and <i>cis</i> isomers. We numerically and experimentally show that these azobenzene molecules inside the composite exhibited the dynamic equilibrium of partial <i>trans</i>-isomer → <i>cis</i>-isomer and <i>cis</i>-isomer → <i>trans</i>-isomer transitions under NIR irradiation of high power and low power, respectively. Under high-power NIR light excitation, the high-intensity dynamic isomerization reaction of the complex not only caused "free volume expansion" but also led to structural changes at the condensed matter scale, thereby significantly decreasing its elasticity and viscosity. In addition, robust switching of its adhesion strength was demonstrated over six cycles. This study shows that the combination of azobenzene polymers and upconversion luminescent materials has great application potential as NIR photoresponsive materials.