Structural Adaptability Driven by Nonuniform Ligand Torsion and Relocation Enables Microregulated Photothermal...

Beyond the structural precision and design versatility of metal-organic frameworks (MOFs), dynamic MOFs responsive to external stimuli are emerging as programmable crystalline systems. Herein, two isostructural cobalt-based MOFs, [Co₂(L1)₂(L2)]_n (NBU-X4) and [Co₂(L1)₂(L3)]_n (NBU-X5), constructed from the ligands 3,3'-(9,9-diethyl-9H-fluorene-2,7-diyl)dibenzoic acid (H₂L1), 1,4-bis(4-pyridyl)naphthalene (L2), and 9,10-di(4-pyridyl)anthracene (L3), were synthesized to elucidate the interplay between ligand flexibility and framework rigidity. Single-crystal X-ray diffraction analyses reveal that NBU-X4 exhibits structural adaptability mediated by nonuniform ligand torsion, enabling a reversible single-crystal-to-single-crystal transformation between NBU-X4-1 and NBU-X4-2 that modulates its photothermal response. Steric hindrance in NBU-X5 suppresses torsional motion, resulting in a rigid framework. NBU-X5 achieves a maximum surface temperature of 316 °C within 10 s under 808 nm laser irradiation (1.6 W cm^-2) on the glass substrate. NBU-X4-1 and NBU-X4-2 exhibit durable photothermal cycling over 50 cycles at 0.2 W cm^-2 and maintain a consistent temperature gap, enabling memory-type photothermal sensing on an alumina substrate. These findings establish a clear structure-function correlation among ligand torsion, framework adaptability, and photothermal efficiency, offering a rational design strategy for dynamic MOF-based temperature-responsive materials.