Unveiling Linker-Born Electron Spin Centers in UiO-66-NH ₂ MOF

Metal–organic frameworks (MOFs) offer a tunable platform for integrating functional spin centers into porous architectures with potential applications in catalysis and quantum sensing. Here, we identify a stable radical intrinsic to the aminoterephthalic acid linker in UiO-66-NH2. Combining multifrequency EPR and DFT, we unambiguously assign the long-misinterpreted EPR spectrum of UiO-66-NH2 to this linker-centered NH• spin center. The radical can pre-exist in the linker precursor and remains stabilized within the MOF. At the same time, we explore the possibility of introducing this spin center postsynthetically. We examine its electronic structure and coupling to the nuclear spin environment in as-synthesized UiO-66-NH2, underlining a new layer of quantum functionality in this widely used MOF. As a linker-based, chemically accessible species, this spin center could be introduced in other MOFs, offering a new platform for spin-defect engineering in catalysis, photophysics, and quantum sensing.