Thermally Facilitated Visible-Light-Induced Crystal Melting Stimulated by Photoisomerization for Rapid and Reversible Adhesion

The recently emerging photoinduced direct crystal melting, based on photoresponsive dynamic molecular crystals, is gaining attention due to its potential applications in photoswitches, actuators, optical storage, micropatterning, and adhesives. Here, we report the photoactive molecular crystals ((E)-NT and (E)-NTB) composed of a naphthalene framework linked with a heteroatom cyclic unit, that exhibit photoinduced E-to-Z isomerization upon visible light illumination and undergo direct melting when the environmental temperature is elevated. Their bulk polycrystals exhibit rapid crystal-to-liquid transitions within several seconds (≤5 s) upon light irradiation at an external temperature of around 40 °C, significantly below the melting temperatures of pristine crystals. As the temperature increases, fractions of resulting Z-photoisomers (×∼2.7) and photochemical E-to-Z isomerization kinetic rates (×∼4.5) would vastly improve. It is revealed that the formed Z-isomers adopt more distorted molecular configurations due to the steric hindrance from heterocyclic structures, which may cause drastic changes in the crystal lattice, thereby depressing the eutectic temperature during the photochemical reaction. Through the synergistic effects of physical anchoring and chemical intermolecular interactions, the resulting liquid exhibits formidable adhesive strength. When glued to two glass substrates, it can bear a weight up to 105 times its mass. Fast adhesion (<10 s) with various substrates, ranging from plastics and rubber to metal and glass, can be achieved with cyclic adhesion recovery upon heating, allowing for at least 50 adhesion-detach cycles. Moreover, two bonded glass slides can remain undamaged after 1 week of storage in seawater and can be reutilized after solvent dissolution and recovery. Our work shows that these photoresponsive molecular crystals are promising candidates for a versatile platform of next-generation photon-controlled adhesives.