Photo-Chromium: Sensitizer for Visible-Light-Induced Oxidative C-H Bond FunctionalizationElectron or Energy Transfer?

The chromium(III) sensitizer [Cr(ddpd)(2)](3+) (ddpd=N,N-dimethyl-N,N-dipyridine-2-ylpyridine-2,6-diamine), based on an earth-abundant metal centre, possesses a unique excited-state potential-energy landscape. The very large energy gap between the redox-active and substitutionally labile T-4(2) state and the long-lived low-energy E-2 spin-flip state enables a selective, efficient sensitization of triplet dioxygen to give singlet dioxygen. Ultrafast intersystem crossing after the Franck Condon point from the T-4(2) to the E-2 excited state within 3.5ps precludes intermolecular electron-transfer pathways from the ultrashort-lived excited T-4(2) state. This specific excited-state reactivity enables a selective visible-light-induced C-H bond activation of tertiary amines with O-1(2) and subsequent trapping with cyanide to yield -aminonitriles in good to excellent yields.