Beyond composition: Local metal mixing controls the electronic structure of metal–organic frameworks

Understanding how heterometallic incorporation affects the electronic structure of metal–organic frameworks (MOFs) is essential for their rational design. However, the role of metal distribution within secondary building units (SBUs) is often overlooked. Here, we investigate a series of isostructural Co-, Ni-, and Co/Ni-based MOFs to elucidate the impact of well-mixed heterometallic SBUs on their electronic properties. X-ray photoelectron spectroscopy reveals binding energy shifts in the heterometallic materials, confirming the coexistence of Co and Ni within the same SBUs rather than domain segregation. Optical measurements show that heterometallic incorporation leads to moderate changes in optical band gap values, with deviations from simple compositional trends. These variations are consistent with modifications in the local electronic environment of the metal centers induced by mixing. Overall, this work demonstrates that local metal mixing within structurally equivalent MOFs provides a powerful strategy to modulate electronic structure and functional behavior beyond simple compositional averaging.