Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with alpha-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)(3) groups attached to the CN- ligands stabilize the metal-based t(2g)-like orbitals by similar to 0.8 eV, leading to high (MLCT)-M-3 energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their alpha-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion

Sensitized Triplet-Triplet Annihilation Upconversion in Water and its Application to Photochemical Transformations

Sensitized triplet–triplet annihilation (TTA) is a promising mechanism for solar energy conversion, but so far its application has been practically completely limited to organic solvents and self-assembled or solid state systems. Combining water-soluble ruthenium complex–pyrene dyads with particularly long excited-state lifetimes as sensitizers and highly fluorescent commercial anthracenes as acceptors/annihilators, we were able to achieve green-to-violet upconversion with unprecedented quantum yields in pure water. Compared to the only known system exploiting sensitized TTA in homogeneous aqueous solution, we improve the overall photon upconversion efficiency by a full order of magnitude and present the very first example for a chemical transformation on a laboratory scale via upconversion in water. Specifically, we found that a thermodynamically challenging carbon–chlorine bond activation can be driven by green photons from an inexpensive continuous wave light source in the presence of dissolved oxygen. Our study is thus potentially relevant in the context of cleaning water from halogenated (toxic) contaminants and for sustainable photochemistry in the most environmentally friendly solvent

Aryl dechlorination and defluorination with an organic super-photoreductant

Direct excitation of the commercially available super-electron donor tetrakis(dimethylamino)ethylene (TDAE) with light-emitting diodes at 440 or 390 nm provides a stoichiometric reductant that is able to reduce aryl chlorides and fluorides. The method is very simple and requires only TDAE, substrate, and solvent at room temperature. The photoactive excited state of TDAE has a lifetime of 17.3 ns in cyclohexane at room temperature and an oxidation potential of ca. −3.4 V vs. SCE. This makes TDAE one of the strongest photoreductants able to operate on the basis of single excitation with visible photons. Direct substrate activation occurs in benzene, but acetone is reduced by photoexcited TDAE and substrate reduction takes place by a previously unexplored solvent radical anion mechanism. Our work shows that solvent can have a leveling effect on the photochemically available redox power, reminiscent of the pH-leveling effect that solvent has in acid–base chemistry

Quantitative Insights into Charge-Separated States from One- and Two-Pulse Laser Experiments Relevant for Artificial Photosynthesis

Charge-separated states (CSSs) are key intermediates in photosynthesis and solar energy conversion. However, the factors governing the formation efficiencies of CSSs are still poorly understood, and light-induced electron–hole recombinations as deactivation pathways competing with desired charge accumulations are largely unexplored. This greatly limits the possibility to perform efficient multi-electron transfer, which is essential for artificial photosynthesis. We present a systematic investigation of two donor–sensitizer–acceptor triads (with different donor–acceptor distances) capable of storing as much as 2.0 eV in their CSSs upon the absorption of a visible photon. Using quantitative one- and two-pulse laser flash photolysis, we provide deep insights into both the CSS formation quantum yield, which can reach up to 80%, and the fate of the CSS upon further (secondary) excitation with green photons. The triad with shorter intramolecular distances shows a remarkable excitation wavelength dependence of the CSS formation quantum yield, and the CSS of this triad undergoes more efficient light-induced charge recombination than the longer equivalent by about one order of magnitude, whilst thermal charge recombination shows the exact opposite behavior. The unexpected results of our detailed photophysical study can be rationalized by detrimental singlet charge transfer states or structural considerations, and could significantly contribute to the future design of CSS precursors for accumulative multi-electron transfer and artificial photosynthesis

Modulation of Acridinium Organophotoredox Catalysts Guided by Photophysical Studies

Control over redox states and spin multiplicity of photo catalysts throughout a catalytic cycle is crucial for selective and efficient photocatalytic processes. However, the rational design of photocatalysts is often hampered by the mechanistic complexity and low modularity of the catalyst structure. Herein, we demonstrate a photophysical study of diverging photocatalytic pathways that guides the design of organic acridinium catalysts to complement polypyridyl transition metal systems. A combined halogen metal exchange/directed ortho-metalation provides reagents for a broad range of modular acridinium catalysts with fine-tuned photophysical and photochemical properties such as excited-state lifetimes, redox potentials, and photostabilities poised to refine organocatalytic photoredox methodology

Stepwise Photoinduced Electron Transfer in a Tetrathiafulvalene-Phenothiazine-Ruthenium Triad

A molecular triad comprising a [Ru(bpy)3]2+ (bpy = 2,2′‐bipyridine) photosensitizer, a primary phenothiazine (PTZ) donor and a secondary (extended) tetrathiafulvalene (exTTF) donor was synthesized and explored by UV/Vis transient absorption spectroscopy. Initial photoinduced electron transfer from PTZ to the 3MLCT‐excited [Ru(bpy)3]2+ occurs within less than 60 ps, and subsequently PTZ is regenerated by electron transfer from exTTF with a time constant of 300 ps. The resulting photoproduct comprising exTTF·+ and [Ru(bpy)3]+ has a lifetime of 6100 ps in de‐aerated CH3CN at room temperature. Additional one‐ and two‐pulse laser flash photolysis studies of the triad were performed in the presence of excess methyl viologen (MV2+), to explore the possibility of light‐driven charge accumulation on exTTF. MV2+ clearly oxidized [Ru(bpy)3]+ and thereby re‐instated ground‐state [Ru(bpy)3]2+ in triads in which exTTF had been oxidized to exTTF·+, but further excitation of the solution containing the exTTF·+‐PTZ‐[Ru(bpy)3]2+ photoproduct did not provide evidence for exTTF2+. Nevertheless, it seems that the design principle of a covalent donor‐donor‐sensitizer triad (as opposed to simpler donor‐sensitizer dyads) is beneficial for light‐driven accumulation of oxidation equivalents. These investigations are relevant in the greater context of multi‐electron photoredox chemistry and artificial photosynthesis

Improved Photostability of a CuI Complex by Macrocyclization of the Phenanthroline Ligands

The development of molecular materials for conversion of solar energy into electricity and fuels is one of the most active research areas, in which the light absorber plays a key role. While copper(I)-bis(diimine) complexes [Cu-I(L)(2)](+) are considered as potent substitutes for [Ru-II(bpy)(3)](2+), they exhibit limited structural integrity as ligand loss by substitution can occur. In this article, we present a new concept to stabilize copper bis(phenanthroline) complexes by macrocyclization of the ligands which are preorganized around the Cu-I ion. Using oxidative Hay acetylene homocoupling conditions, several Cu-I complexes with varying bridge length were prepared and analyzed. Absorption and emission properties are assessed; rewardingly, the envisioned approach was successful since the flexible 1,4-butadiyl-bridged complex does show enhanced MLCT absorption and emission, as well as improved photostability upon irradiation with a blue LED compared to a reference complex

Improved Photostability of a Cu-I Complex by Macrocyclization of the Phenanthroline Ligands

The development of molecular materials for conversion of solar energy into electricity and fuels is one of the most active research areas, in which the light absorber plays a key role. While copper(I)-bis(diimine) complexes [Cu$^{l}$ (L) $_{2}$] $^{+}$ are considered as potent substitutes for [Ru$^{ll}$ (bpy) $_{3}$] $^{2+}$, they exhibit limited structural integrity as ligand loss by substitution can occur. In this article, we present a new concept to stabilize copper bis(phenanthroline) complexes by macrocyclization of the ligands which are preorganized around the Cu$^{l}$ ion. Using oxidative Hay acetylene homocoupling conditions, several Cu$^{l}$ complexes with varying bridge length were prepared and analyzed. Absorption and emission properties are assessed; rewardingly, the envisioned approach was successful since the flexible 1,4-butadiyl-bridged complex does show enhanced MLCT absorption and emission, as well as improved photostability upon irradiation with a blue LED compared to a reference complex