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

Controlling spin-correlated radical pairs with donor-acceptor dyads : a new concept to generate reduced metal complexes for more efficient photocatalysis

One-electron reduced metal complexes derived from photoactive ruthenium or iridium complexes are important intermediates for substrate activation steps in photoredox catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic efficiency problems resulting from rapid geminate recombination of radical pairs within the so-called solvent cage. In this study, we prepared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relying on a sequence of energy and electron transfer processes between a Ru complex and a covalently connected anthracene moiety. Our test reaction to establish the proof-of-concept is the photochemical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemical key step in the Ru-anthracene dyads is the reduction of a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spin-correlated radical pair whose (unproductive) recombination is strongly spin-forbidden. By carrying out detailed laser flash photolysis investigations, we provide clear evidence for the indirect reduced metal complex generation mechanism and show that this pathway can outperform the conventional direct metal complex photoreduction. The further optimization of our approach involving relatively simple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields

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

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

Triplet-sensitized cyclobutane pyrimidine dimer damage and crosslinks in DNA: filling the triplet energy gap between xanthone and thioxanthone

Thioxanthones are tunable photosensitizers used for studying the formation of triplet-induced cyclobutane pyrimidine dimer (CPD) damage in DNA. To probe the gap between the triplet energy of xanthone (310 kJ mol−1) and of thioxanthone (265 kJ mol−1), we synthesized two new C-nucleosides with two differently modified thioxanthones. Ternary and photoactive DNA architectures were prepared with these C-nucleosides to analyze the CPD formation quantitatively. The dimethoxy-substituted thioxanthone as a photosensitizer has a high triplet energy (288 kJ mol−1). We observed the CPD formation over up to 6 A–T pairs in DNA, and the distance dependence is characterized by a low β-value of 0.02 Å−1, indicating energy hopping over the A–T pairs. The triplet energy of the chloro-and methoxy-modified photosensitizer is low (273 kJ mol−1) and only slightly above the threshold for DNA photosensitization set by the triplet energy of T in DNA (267 kJ mol−1). Here, only low amounts of CPDs were obtained because the energy difference compared to T is very small. These results show clearly that the triplet energy of the photosensitizer incorporated into the DNA is decisive for not only whether CPDs can be induced at all but also how much CPDs are formed; the higher the triplet energy of the photosensitizer, the more CPDs are formed

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

Direct Observation of Triplet–Triplet Energy Transfer in DNA between Energy Donor and Acceptor C-Nucleotides

Investigating the migration of excited-state energy in DNA is crucial for a deep understanding of protection mechanisms and light-induced DNA damage. While numerous reports focused on single electron transfer and Förster-type energy transfer in DNA, studies on the Dexter-type triplet–triplet energy transfer are scarce, in particular, those with direct detection of photoexcited triplet states. Herein, we present direct measurements of the distance-dependent triplet–triplet energy transfer rates through DNA by using transient absorption spectroscopy. This was achieved through the synthetic incorporation of thioxanthone as an energy donor and naphthalene as an energy acceptor into a DNA double strand at defined positions. The energy transfer rates strongly depend on the number of A-T base pairs (up to four) separating the energy donor from the energy acceptor. We observed a fast energy transfer rate with a time constant of 17 ns for the DNA sample in which the donor and acceptor are directly adjacent in the DNA. By analyzing two additional donor–acceptor distances, a steep exponential distance dependence with an attenuation factor of 1.15 Å–1 could be obtained. Our results demonstrate that DNA acts as a poor conductor of triplet energy when energy donors with triplet energies below 2.7 eV are used, complementing more indirect studies on sensitized DNA damage

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