Photoreversible defect complexation dynamically modulates guest adsorption in metal-organic frameworks

Abstract Reversible light-driven adsorption in framework materials offers a route to energy-efficient separations, yet current mechanisms deliver modest working capacities that constrain practical utility. Here we introduce photo-reversible defect complexation as a design principle for enhancing light-induced working capacities, coupling photoswitch isomerisation to active binding site modulation rather than pore geometry changes alone. We exploit coordinatively unsaturated Zr sites in defective UiO-67 as reversible binding sites, where a grafted spiropyran (SP) photoswitch undergoes UV-induced isomerisation to the zwitterionic merocyanine (MC) form, whose phenolate oxygen coordinates an additional open Zr site, suppressing guest binding availability. Visible light fully reverses this process. This mechanism delivers an 83% enhancement in photo-modulated CO 2 working capacity, reaching 1.1 mmol g −1 , nearly double that of the structural analog of P-UiO that does not enable photo-reversible MC-defect switching. Light-controlled metal cation and CO 2 uptake confirms that binding site availability, rather than pore geometry, drives this switching response. Density functional theory (DFT) simulations and multimodal X-ray photoelectron spectroscopy (XPS) confirm reversible MC-Zr site complexation as the operative mechanism with performance retained across multiple photocycles. These findings establish photo-reversible defect engineering as a robust and broadly applicable principle for light-responsive frameworks with modulable guest adsorption.