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

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 Cu1 sites and clustered Cux species (x = 2–8), we decode optical fingerprints tied to nuclearity, oxidation states (Cu0, Cu(I), Cu(II)), and ligand coordination shells. Quantitative analysis reveals that the Cu1/UiO-66 catalyst maintains a single-site-dominated landscape (Cu(I): 52–77%), with minor Cu2 (12–18%) and Cu3 (12–27%) contributions. In contrast, the Cux/UiO-66 catalyst exhibits a dynamic multisite environment, balancing Cu(I) (46–63%) with Cu3 clusters (36–48%) and variable Cu2 (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.