Elucidating charge-transfer mechanisms and their effect on the light-induced reactivity of metastable MIL-125(Ti)

Alcohol and water photooxidation reactions are employed in concert with optical spectroscopy analyses to demonstrate the occurrence of multiple and distinctive charge-transfer (CT) mechanisms in the environmental photocatalyst MIL-125(Ti). The contribution of ligand-to-metal CT (LMCT) mechanisms increases at wavelengths lower than 320 nm while that of node oxygen-to-metal CT (OMCT) mechanisms increases at longer wavelengths. The localization of photogenerated holes on different atoms leads to a selective reactivity of the framework depending on the mechanism and, during hydroxylation processes, to its spontaneous transition to the isostructural MIL-125-OH(Ti) and the development of an additional LMCT mechanism with a long-lived emission. Furthermore, a previously unidentified and extrinsic CT mechanism is spectroscopically related to the formation of terephthalate-based oligomers. The coexistence of distinctive CT mechanisms in MIL-125(Ti) implies their critical role in catalyst efficiency, and mastering them proves to be a powerful and simple strategy to produce the valuable MIL-125-OH(Ti).