Long-Lived Photoinduced Charge Separation in a Trinuclear Iron-μ3-oxo-based Metal–Organic Framework

The presence of long-lived charge-separated excited states in metal–organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these charge-separated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100(Fe), composed of Fe3-μ3-oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ3-oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to further support this assignment and corresponding decay time. The MOF is prepared as a colloidal nanoparticle suspension for these measurements, so both its fabrication and particle size analysis are presented as well.