Fingerprinting Single and Clustered Cu Sites in Metalated MOF Catalysts via TD-DFT and In Situ Diffuse Reflectance UV–Vis Spectroscopy

High Resolution Image Download MS PowerPoint Slide Metal–organic frameworks (MOFs) are transformative platforms for heterogeneous catalysis, but distinguishing atomically dispersed metal sites from subnanometric clusters remains a major challenge. This often demands the integration of multiple characterization techniques, many of which either lack the resolving power to distinguish active sites from their surrounding environments (e.g., low Z-contrast for light elements in electron microscopy) or are costly and require tedious data processing (e.g., X-ray absorption spectroscopy). Here, we introduce an integrated diffuse reflectance UV–vis spectroscopy and time-dependent DFT approach to overcome these limitations, enabling in situ discrimination of Cu centers installed into Zr-based UiO-66. By systematically modeling isolated Cu 1 sites and clustered Cu x species ( x = 2–8), we decode optical fingerprints tied to nuclearity, oxidation states (Cu 0, Cu(I), Cu(II)), and ligand coordination shells. Quantitative analysis reveals that the Cu 1 /UiO-66 catalyst maintains a single-site-dominated landscape (Cu(I): 52–77%), with minor Cu 2 (12–18%) and Cu 3 (12–27%) contributions. In contrast, the Cu x /UiO-66 catalyst exhibits a dynamic multisite environment, balancing Cu(I) (46–63%) with Cu 3 clusters (36–48%) and variable Cu 2 (0–8%). This approach resolves the spatial and temporal gaps of conventional and high-resolution techniques, offering a cost-effective and atomically precise strategy to correlate spectral signatures with active site architectures. This contribution establishes a broadly applicable pathway to characterize and modulate MOF-based catalysts with tunable optical and catalytic properties for sustainable energy, chemical transformations, and optoelectronics.