Infrared-activated dynamic crystals: remote motion in a metal-organic framework

Dynamic crystals are smart materials that respond to external stimuli with structural transformations that translate into shape and size changes, producing motion at the micrometer scale. They are promising candidates for soft microrobotics and memory devices and can be built from organic, biological or inorganic components. Metal-Organic Frameworks (MOFs) are porous crystalline materials that have demonstrated structural and morphological flexibility. Here we demonstrate the actuation of MOFs triggered by low-energy infrared (IR) radiation from a low-power laser. The stimulus is user-tunable, spatially addressable, and potentially tissue- penetrating compared with commonly employed triggers. IR absorption arises from lanthanide ions forming the coordination nodes, producing a temperature increase that drives the phase transition. Notably, the introduction of additional lanthanide dopants enables upconversion energy transfer that activates non- radiative relaxation pathways, generating further localized heating and allowing the phase transition to occur at lower IR power densities. The phase transitions induce microscopic motions, including crystal contraction, morphology changes, and jumping. These responses are reversible and repeatable over multiple cycles. The structural transformations underlying the dynamics were elucidated using complementary spectroscopic techniques, including in situ single-crystal X-ray diffraction under irradiation. This strategy can be integrated into several MOF architectures, opening new opportunities for remotely controlled, low-energy crystal actuators.