Thermal Crystal-to-Crystal Transformation Unlocks TiO(OOC)-Cluster-Based Metal-Organic Frameworks.

Titanium-based metal-organic frameworks (Ti-MOFs) are promising photocatalysts, yet their development has been constrained by the limited diversity of Ti-oxo clusters successfully incorporated into MOFs under solvothermal conditions. Herein, we demonstrate a thermal solid-state synthetic strategy to access new Ti-MOFs (MIL-125-HT series) featuring TiO(OOC) clusters the crystal-to-crystal transformation of MIL-125 and its functionalized derivatives. Controlled thermal treatment of MIL-125 induces dehydration and rearrangement of the original TiO(OH)(OOC) clusters while preserving crystallinity and the framework topology. The atomic-resolution structure of MIL-125-HT was determined through three-dimensional electron diffraction and synchrotron powder X-ray diffraction. The phase transition temperature and crystallinity of the resulting MIL-125-HT are governed by the functional groups on the linkers, which affect both the rotational flexibility required for cluster rearrangement and the thermal stability necessary to avoid framework collapse. Naphthalene-based linkers provide an optimal combination of flexibility and robustness, yielding highly crystalline high-temperature phases. Compared to MIL-125, MIL-125-HT exhibits a reduced band gap and enhanced photocatalytic activity in hydrogen peroxide production. This work establishes a solid-state approach for uncovering previously inaccessible Ti-MOFs, broadening their synthetic landscape and enabling new opportunities in photocatalysis.