Cu-Enabled Photoinduced Charge Transfer in Green-Processed Cu-BPDC Metal-Organic Framework Films on FTO

Photosensitive metal-organic frameworks (MOFs) provide chemically tunable platforms for studying light-induced charge transfer at solid/electrolyte interfaces, yet many reported systems rely on energy-intensive synthesis, organic solvents, or rare and/or toxic metal ions. Here, building on our previously reported ambient, water-assisted eco-LAGent synthesis of Cu-BPDC and Zn-BPDC MOFs, we formulate these materials as Nafion-bound films on fluorine-doped tin oxide (FTO) electrodes and evaluate their photoelectrochemical behavior. The resulting electrode platform enables a controlled comparison of the two MOFs under matched coating, electrolyte, and illumination conditions. The combination of earth-abundant Cu and Zn with the carboxylate ligand biphenyl-4,4'-dicarboxylate (BPDC) provides a chemically matched platform for comparing a redox-active Cu-BPDC MOF with a closed-shell Zn analogue under the same coating, electrolyte, and illumination geometry. Comparable scan-rate cyclic voltammetry (CV) showed a broad Cu-associated redox feature for Cu-BPDC, whereas Zn-BPDC remained comparatively featureless over the same potential window. Under chopped-light chronoamperometry (CA) at +0.449 V versus RHE, Cu-BPDC generated a sustained cathodic response of +0.448 ± 0.009 μA cm^-2 when plotted as -Δj, while Zn-BPDC, bare FTO, and Nafion/FTO controls showed only weak responses under identical conditions. Optical measurements showed similar apparent direct-transition band gaps for Zn-BPDC and Cu-BPDC (∼4.00 eV), whereas distinct solid-state photoluminescence (PL) profiles indicated that optical absorption alone does not account for the different photoelectrochemical behaviors. These results support a Cu-centered photoinduced charge-transfer response in Cu-BPDC films and establish a controlled platform for interrogating metal-node-dependent photoelectrochemistry in MOF-based electrode coatings.