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.