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This perspective focuses on strategies to manipulate and optimize three key determinants of metal-based molecular photosensitizers – the absorption profile, the excited-state redox potentials, and the excited-state lifetime.

 

Photoredox catalysis has been prominent in many applications, including solar fuels, organic synthesis, and polymer chemistry. Photocatalytic activity directly depends on the photophysical and electrochemical properties of photocatalysts in both the ground state and excited state. Controlling those properties, therefore, is imperative to achieve the desired photocatalytic activity. Redox potential is one important factor that impacts both the thermodynamic and kinetic aspects of key elementary steps in photoredox catalysis. In many challenging reactions in organic synthesis, high redox potentials of the substrates hamper the reaction, leading to slow conversion. Thus, the development of photocatalysts with extreme redox potentials, accompanied by potent reducing or oxidizing power, is required to execute high-yielding thermodynamically demanding reactions. In this review, we will introduce strategies for accessing extreme redox potentials in photocatalytic transformations. These include molecular design strategies for preparing photosensitizers that are exceptionally strong ground-state or excited-state reductants or oxidants, highlighting both organic and metal-based photosensitizers. We also outline methodological approaches for accessing extreme redox potentials, using two-photon activation, or combined electrochemical/photochemical strategies to generate potent redox reagents from precursors that have milder potentials. 

Abstract Photoredox catalysis has proven to be a powerful tool in synthetic organic chemistry. The rational design of photosensitizers with improved photocatalytic performance constitutes a major advancement in photoredox organic transformations. This review summarizes the fundamental ground-state and excited-state photophysical and electrochemical attributes of molecular photosensitizers, which are important determinants of their photocatalytic reactivity. 

A series of bis-cyclometalated iridium complexes were prepared which combine triazole or NHC-based cyclometalating ligands with substituted β-diketiminate (NacNac) ancillary ligands. The HOMO is localized on the NacNac ligand and its energy and associated redox potential are determined by the NacNac substitution pattern. The effect of the cyclometalating ligand, relative to the more common 2-phenylpyridine derivatives, is to destabilize the LUMO and increase the triplet excited-state energy ( E T1 ). These results are supported by DFT calculations, which show HOMOs and LUMOs that are respectively localized on the NacNac and cyclometalating ligands. With this new design, we observe more negative excited-state reduction potentials, E (Ir IV /*Ir III ), with two members of the series standing out as the most potent visible-light iridium photoreductants ever reported. Stern–Volmer quenching experiments with ketone acceptors (benzophenone and acetophenone) show that the increased thermodynamic driving force for photoinduced electron-transfer correlates with faster rates relative to fac -Ir(ppy) 3 and previous generations of NacNac-supported iridium complexes. A small selection of photoredox transformations is shown, demonstrating that these new photoreductants are capable of activating challenging organohalide substrates, albeit with modest conversion. 

Photoredox catalysis has emerged as a powerful strategy in synthetic organic chemistry, but substrates that are difficult to reduce either require complex reaction conditions or are not amenable at all to photoredox transformations. In this work, we show that strong bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands enable high-yielding photoredox transformations of challenging substrates with very simple reaction conditions that require only a single sacrificial reagent. Using blue or green visible-light activation we demonstrate a variety of reactions, which include hydrodehalogenation, cyclization, intramolecular radical addition, and prenylation via radical-mediated pathways, with optimized conditions that only require the photocatalyst and a sacrificial reductant/hydrogen atom donor. Many of these reactions involve organobromide and organochloride substrates which in the past have had limited utility in photoredox catalysis. This work paves the way for the continued expansion of the substrate scope in photoredox catalysis.