Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

Abstract Highly reducing or oxidizing photocatalysts are a fundamental challenge in the field of inorganic and organic photochemistry. Only a few transition metal complexes with earth-abundant metal ions have so far advanced to excited state oxidants, including chromium, iron and cobalt. All these photocatalysts require high energy light for excitation and their oxidizing power has not been fully exploited due to significant energy dissipation before reaching the photoactive state. Herein we demonstrate that the complex [Mn(dgpy)2] 4+ based on earth-abundant manganese can be excited with low-energy NIR light (850 nm, 1.46 eV) to yield a luminescent mixed 2 LMCT/ 2MC excited state (1435 nm, 0.86 eV) with a lifetime of 1.6 ns. The dissipated energy amounts to 0.60 eV. In spite of this energy loss, *[Mn(dgpy)2] 4+ with its excited state redox potential Ered* of 1.80 V vs SCE outcompetes the strongest reported precious metal photooxidant (iridium(III)). *[Mn(dgpy)2] 4+ oxidizes naphthalene (Eox≈1.31 – 1.54 V vs. SCE) to its radical cation giving the manganese(III) complex [Mn(dgpy)2] 3+ in a clean outer-sphere electron transfer process. Unexpectedly, mesitylene, toluene, benzene and nitriles with even extremely high oxidation potentials up to Eox = 2.4 V provoke the [Mn(dgpy)2] 4+/3+ reduction under photolysis. A higher energy short-lived 4 LMCT excited state with a lifetime of 0.78 ps is made responsible for these demanding oxidations, which proceed by static rather than dynamic quenching. This dual excited state reactivity from 2 LMCT/ 2MC and 4 LMCT states is linked to the 4 LMCT→2 LMCT/ 2MC intersystem crossing process. These unique findings demonstrate how the design of manganese complexes (i) expands the absorption cross section to 400 – 850 nm, (ii) increases the 2 LMCT/ 2MC state lifetime to the nanosecond range allowing luminescence and classical dynamic photoredox processes and (iii) enables non-classical static quenching of an extremely oxidizing 4 LMCT excited state by the solvent. This conceptually novel approach of static quenching by the solvent minimizes free energy losses, harnesses the full photooxidizing power and thus allows even oxidation of nitriles and benzene using earth-abundant elements and low-energy light.