Light-driven anisotropy of 2D metal-organic framework single crystal for repeatable optical modulation

Structural transformations of metal-organic frameworks (MOFs) go through a complex energy landscape with multiple intermediate states. Although the transformations allow controlling the functional properties of the MOFs, an imbalance between MOF flexibility and rigidity sets a fundamental barrier to achieving fast and multiple transformations. Here, we study the stimuli-responsive structural transformation in a 2D MOF assembled from paddle-wheel secondary building units joined by a semi-flexible organic ligand with 1,2,4-triazole and carboxylate groups with a rigid adamantane cage between them. The structure results in a distinctive combination of MOF flexibility and rigidity, thus, facilitating a continuous transformation driven by laser light. We reveal the laser-induced anisotropic thermal expansion nature of such transformation, initiating optical changes of the 2D MOF. The latter is utilized for fast and highly repeatable optical modulation of over 10,000 cycles. The endurance of such a 2D MOF-based optical modulator during 1 year of storage at ambient conditions paves the way to design tunable and robust MOFs for diverse applications. Structural transformations offer a route to control functional properties but it is difficult to design metal-organic frameworks with multiple and fast transformations. Here, a 2D metal-organic framework was designed with continuous structural transformations driven by light and used for optical modulation.