Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C−N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the here reported hybrid photocatalysts feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state oxidation potential of our best-performing catalyst (+2.5 V vs. SCE) and they provide evidence for mechanistic key steps and intermediates.