Photoelectrochemical C−H Activation Through a Quinacridone Dye Enabling Proton‐Coupled Electron Transfer

This work reports on dye sensitized photoanodes for C-H activation in organic substrates, assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto Fluorine doped tin oxide (FTO), TiO2 and SnO2 slides. The photoanodes display extended absorption in the visible (450-600 nm) and ultrafast photoinduced electron injection (1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton-coupled electron-transfer reactivity of QNC is exploited for generating a nitrogen-based radical as its oxidized form, which is competent in C-H bond activation. In particular, the key reactivity parameter is the bond dissociation free energy (BDFE) associated with the N•/N-H couple in QNC of 80.5±2.3 kcalmol-1, which enables hydrogen atom abstraction from allylic or benzylic C-H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C-H bonds with BDFE below the 80.5 kcalmol-1 threshold, such as γ-terpinene, xanthene, dihydroanthracene. This work opens a rationale and mechanistically oriented design of dye-sensitized photoelectrodes for selective organic transformations.