Iron(III) Carbene Complexes with Tunable Excited State Energies for Photoredox and Upconversion

Abstract

Substituting precious elements in luminophores and photocatalysts by abundant first-row transition metals remains a significant challenge, and iron continues to be particularly attractive owing to its high natural abundance and low cost. Most iron complexes known to date face severe limitations due to undesirably efficient deactivation of luminescent and photoredox-active excited states. Two new iron(III) complexes with structurally simple chelate ligands enable straightforward tuning of ground and excited state properties, contrasting recent examples, in which chemical modification had a minor impact. Crude samples feature two luminescence bands strongly reminiscent of a recent iron(III) complex, in which this observation was attributed to dual luminescence, but in our case, there is clear-cut evidence that the higher-energy luminescence stems from an impurity and only the red photoluminescence from a doublet ligand-to-metal charge transfer ( 2 LMCT) excited state is genuine. Photoinduced oxidative and reductive electron transfer reactions with methyl viologen and 10-methylphenothiazine occur with nearly diffusion-limited kinetics. Photocatalytic reactions not previously reported for this compound class, in particular the C−H arylation of diazonium salts and the aerobic hydroxylation of boronic acids, were achieved with low-energy red light excitation. Doublet−triplet energy transfer (DTET) from the luminescent 2 LMCT state to an anthracene annihilator permits the proof of principle for triplet−triplet annihilation upconversion based on a molecular iron photosensitizer. These findings are relevant for the development of iron complexes featuring photophysical and photochemical properties competitive with noble-metal-based compounds.

Manganese Complexes with Consecutive Mn(IV) → Mn(III) Excitation for Versatile Photoredox Catalysis

“…Recent advancements have highlighted the potential of NHC carbenes as valuable ligands in designing photocatalysts incorporating 3d transition metals. − Following a systematic selection process, we pinpointed a boron-containing tridentate NHC ligand ( L1 ) as the foundational structure, known for stabilizing complexes of elements like Co, Fe, Mn, etc. Strategic introduction of bromo and phenyl substituents onto this backbone further enables adjustment of its electronic and conjugate effects, yielding derivatives L2 and L3 .…”

“…Our group dedicated to developing cost-effective photochemical synthetic methods, propelled by the use of transition metal catalysts. − Drawing from our research experiences and inspired by relevant literature, − we postulated that manganese complexes with promising visible light response and elongated excited-state lifetimes could be attained through careful selection of complex frameworks and tailored modification of binding ligands. Central to this approach is the integration of robust σ-donor ligands to amplify ligand field strength, as evidence by increased ligand field parameters (10 Dq). − Fundamentally, the nature of excited-state electronic configurations is governed by the intricate balance between the 10 Dq parameter and the Racah B factor, reflecting the degree of covalency in metal–ligand bonds and playing a pivotal role in modulating nephelauxetic effects. − By meticulous adjustment of metal–ligand covalency to diminish the Racah B value, enhanced delocalization of d-electrons around the manganese nucleus becomes attainable, a strategy that reduces electron–electron repulsion and is instrumental in achieving the desired prolongation of excited-state lifetimes (Figure B).…”