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 2LMCT/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 4LMCT 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 2LMCT/2MC and 4LMCT states is linked to the 4LMCT 2LMCT/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 2LMCT/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 4LMCT 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.

Stable Molybdenum(0) Carbonyl Complex for Upconversion and Photoredox Catalysis

“…The development of photoactive metal complexes with earth-abundant metals holds great promise to enable sustainable photocatalysis, (solar) energy conversion, and light-emitting diodes (LEDs). − Hence, considerable efforts have been undertaken to explore this compound class in the recent years , with creative designs utilizing excited states of different characters like metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), ligand-to-ligand charge transfer (LL’CT), and metal-centered (MC) states, , covering zirconium, − vanadium, − chromium, − iron, − manganese, − cobalt, , copper, − molybdenum, − and zinc as metal centers , among others.…”

Solvent Dependency of Catalyst‐Substrate Aggregation Through π‐π Stacking in Photoredox Catalysis

“…Such a finding reveals the critical role that solvents play in influencing outcomes of PRC reactions even in homogeneous solutions and not only micellular dispersions [25][26][27] and demands that solvent molecules are treated in experiments and calculations as an additional variable capable of tuning and not just as a spectator to the PRC reaction. Finally, our study highlights the added value of MD calculations in unravelling the mechanisms of PRC reactions that complements other theoretical approaches (QC) [28] and experimental approaches like isolation of authentic redox states. [29][30][31]…”

Electronic Structure and Excited-State Dynamics of the NIR-II Emissive Molybdenum(III) Analogue to the Molecular Ruby

“…Research in the field of photophysics and photochemistry of transition-metal complexes in the past few decades primarily focused on materials based on precious metals due to their favorable excited-state properties. − In the past 10 years, scientific curiosity has driven researchers to explore earth-abundant metal complexes in more detail. − These efforts uncovered photoactive complexes with intriguing excited-state properties that can show unique reactivity following novel mechanisms. − …”

“…Generally, spin-flip emission is not limited to either chromium­(III) or the d 3 electron configuration . Other metal centers such as vanadium­(III) or manganese­(IV) have been explored as well and show dramatically different excited-state energies due to their different M–L bond covalencies. ,− …”