Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)32+ photosensitizers and two peripheral triarylamine (TAA) donors was investigated by transient IR and UV–vis spectroscopies in the presence of 0.2 M p-toluenesulfonic acid (TsOH) in deaerated acetonitrile. In ∼15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH2) via reversible intramolecular electron transfer from the two TAA units (τ = 65 ps), followed by intermolecular proton transfer from TsOH (τ ≈ 3 ns for the first step). Although the light-driven accumulation of reduction equivalents occurs through a sequence of electron and proton transfer steps, the resulting photoproduct decays via concerted PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of 1.4 ± 0.2. Moreover, the reoxidation of AQH2 seems to take place via a double electron transfer step involving both TAA+ units rather than sequential single electron transfer events. Thus, the overall charge-recombination reaction seems to involve a concerted proton-coupled two-electron oxidation of AQH2. The comparison of experimental data obtained in neat acetonitrile with data from acidic solutions suggests that the inverted driving-force effect can play a crucial role for obtaining long-lived photoproducts resulting from multiphoton, multielectron processes. Our pentad provides the first example of light-driven accumulation of reduction equivalents stabilized by PCET in artificial molecular systems without sacrificial reagents. Our study provides fundamental insight into how light-driven multielectron redox chemistry, for example the reduction of CO2 or the oxidation of H2O, can potentially be performed without sacrificial reagents

Photoswitchable Organic Mixed Valence in Dithienylcyclopentene Systems with Tertiary Amine Redox Centers

The electronic structures of the radical cations of two dithienylperfluorocyclopentene molecules with appended tertiary amine units were investigated by electrochemical and optical spectroscopic methods. The through-bond N–N distances in the photocyclized (closed) forms of the two systems are 9.3 and 17.6 Å, respectively, depending on whether the nitrogen atoms are attached directly to the two thienyl units or whether xylyl spacers are in between. In the case of the radical cation with the longer N–N distance, photocyclization of the dithienylperfluorocyclopentene core induces a changeover from class I to class II mixed valence behavior. In the case of the shorter system, the experimental data is consistent with assignment of the photocyclized form to a class III mixed valence species

Proton-Coupled Electron Transfer between 4-Cyanophenol and Photoexcited Rhenium(I) Complexes with Different Protonatable Sites

Two rhenium­(I) tricarbonyl diimine complexes, one of them with a 2,2′-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re­(bpz)­(CO)₃(py)]⁺), and one tricarbonyl complex with a 2,2′-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re­(bpy)­(CO)₃(pz)]⁺) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re­(bpz)­(CO)₃(py)]⁺ complex while in the [Re­(bpy)­(CO)₃(pz)]⁺ complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium­(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton–electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/[Re­(bpz)­(CO)₃(py)]⁺ reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium­(I) complexes

Kinetic Isotope Effects in Reductive Excited-State Quenching of Ru(2,2′-bipyrazine)₃²⁺ by Phenols

Electron transfer (ET) from phenol molecules to a photoexcited ruthenium­(II) complex was investigated as a function of the para-substituent (R = OCH₃, CH₃, H, Cl, Br, CN) attached to the phenols. For phenols with electron-donating substituents (R = OCH₃, CH₃), the rate-determining excited-state deactivation process is ordinary ET. For all other phenols, significant kinetic isotope effects (KIEs) (ranging from 2.91 ± 0.18 for R = Br to 10.18 ± 0.64 for R = CN) are associated with emission quenching, and this is taken as indirect evidence for transfer of a phenolic proton to a peripheral nitrogen atom of a 2,2′-bipyrazine ligand in the course of an overall proton-coupled electron transfer (PCET) reaction. Possible PCET reaction mechanisms for the various phenol/ruthenium couples are discussed. While 4-cyanophenol likely reacts via concerted proton–electron transfer (CPET), a stepwise proton transfer–electron transfer mechanism cannot be excluded in the case of the phenols with R = Br, Cl, and H

Ruthenium-Phenothiazine Electron Transfer Dyad with a Photoswitchable Dithienylethene Bridge: Flash-Quench Studies with Methylviologen

A molecular ensemble composed of a phenothiazine (PTZ) electron donor, a photoisomerizable dithienyl­ethene (DTE) bridge, and a Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) electron acceptor was synthesized and investigated by optical spectroscopic and electrochemical means. Our initial intention was to perform flash-quench transient absorption studies in which the Ru­(bpy)₃²⁺ unit is excited selectively (“flash”) and its ³MLCT excited state is quenched oxidatively (“quench”) by excess methylviologen prior to intramolecular electron transfer from phenothiazine to Ru­(III) across the dithienylethene bridge. However, after selective Ru­(bpy)₃²⁺¹MLCT excitation of the dyad with the DTE bridge in its open form, ¹MLCT → ³MLCT intersystem crossing on the metal complex is followed by triplet–triplet energy transfer to a ³π–π* state localized on the DTE unit. This energy transfer process is faster than bimolecular oxidative quenching with methylviologen at the ruthenium site (Ru­(III) is not observed); only the triplet-excited DTE then undergoes rapid (10 ns, instrumentally limited) bimolecular electron transfer with methylviologen. Subsequently, there is intramolecular electron transfer with PTZ. The time constant for formation of the phenothiazine radical cation via intramolecular electron transfer occurring over two <i>p</i>-xylene units is 41 ns. When the DTE bridge is photoisomerized to the closed form, PTZ⁺ cannot be observed any more. Irrespective of the wavelength at which the closed isomer is irradiated, most of the excitation energy appears to be funneled rapidly into a DTE-localized singlet excited state from which photoisomerization to the open form occurs within picoseconds

Photophysics and Photoredox Catalysis of a Homoleptic Rhenium(I) Tris(diisocyanide) Complex

Herein a homoleptic rhenium­(I) complex bearing three chelating diisocyanide ligands and its photophysical properties are communicated. The complex emits weakly from a high-energy triplet metal-to-ligand charge-transfer excited state with an 8 ns lifetime in deaerated CH₃CN at 22 °C and is shown to act as an efficient photoredox catalyst comparable to [Ir­(ppy)₃] (ppy = 2-phenylpyridine) in representative test reactions

Mechanistic Diversity in Proton-Coupled Electron Transfer between Thiophenols and Photoexcited [Ru(2,2′-Bipyrazine)₃]²⁺

Proton-coupled electron transfer (PCET) with phenols has been investigated in considerable detail in recent years while at the same time analogous mechanistic studies of PCET with thiophenols have remained scarce. We report on PCET between a series of thiophenols and a photoexcited Ru­(II) complex, which acts as a combined electron/proton acceptor. Depending on the exact nature of the thiophenol, PCET occurs through different reaction mechanisms. The results are discussed in the context of recent studies of PCET between phenols and photoexcited d⁶ metal complexes

Photoacid Behavior versus Proton-Coupled Electron Transfer in Phenol–Ru(bpy)₃²⁺ Dyads

Two dyads composed of a Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) photosensitizer and a covalently attached phenol were synthesized and investigated. In the shorter dyad (Ru–PhOH) the ruthenium complex and the phenol are attached directly to each other whereas in the longer dyad there is a <i>p</i>-xylene (xy) spacer in between (Ru–xy–PhOH). Electrochemical investigations indicate that intramolecular electron transfer (ET) from phenol to the photoexcited metal complex is endergonic by more than 0.3 eV in both dyads, explaining the absence of any ³MLCT (metal-to-ligand charge transfer) excited-state quenching by the phenols in pure CH₃CN and CH₂Cl₂. When pyridine is added to a CH₂Cl₂ solution, significant excited-state quenching can be observed for both dyads, but the bimolecular quenching rate constants differ by 2 orders of magnitude between Ru–PhOH and Ru–xy–PhOH. Transient absorption spectroscopy shows that in the presence of pyridine both dyads react to photoproducts containing Ru­(II) and phenolate. The activation energies associated with the photoreactions in the two dyads differ by 1 order of magnitude, and this might suggest that the formation of identical photoproducts proceeds through fundamentally different reaction pathways in Ru–PhOH and Ru–xy–PhOH. For Ru–PhOH direct proton release from the photoexcited dyad is a plausible reaction pathway. For Ru–xy–PhOH a sequence of a photoinduced proton-coupled electron transfer (PCET) followed by an intramolecular (thermal) electron transfer in the reverse direction is a plausible reaction pathway; this two-step process involves a reaction intermediate containing Ru­(I) and phenoxyl radical that reacts very rapidly to Ru­(II) and phenolate. Thermal back-reactions to restore the initial starting materials occur on a 30–50 μs time scale in both dyads; i.e., due to proton release the photoproducts are very long-lived. These back-reactions exhibit inverse H/D kinetic isotope effects of 0.7 ± 0.1 (Ru–PhOH) and 0.6 ± 0.1 (Ru–xy–PhOH) at room temperature

Hydrogen-Bond Strengthening upon Photoinduced Electron Transfer in Ruthenium–Anthraquinone Dyads Interacting with Hexafluoroisopropanol or Water

Quinones play a key role as primary electron acceptors in natural photosynthesis, and their reduction is known to be facilitated by hydrogen-bond donors or protonation. In this study, the influence of hydrogen-bond donating solvents on the thermodynamics and kinetics of intramolecular electron transfer between Ru­(bpy)₃²⁺ (bpy = 2,2′-bipyridine) and 9,10-anthraquinone redox partners linked together via one up to three <i>p</i>-xylene units was investigated. Addition of relatively small amounts of hexafluoroisopropanol to dichloromethane solutions of these rigid rodlike donor–bridge–acceptor molecules is found to accelerate intramolecular Ru­(bpy)₃²⁺-to-anthraquinone electron transfer substantially because anthraquinone reduction occurs more easily in the presence of the strong hydrogen-bond donor. Similarly, the rates for intramolecular electron transfer are significantly higher in acetonitrile/water mixtures than in dry acetonitrile. In dichloromethane, an increase in the association constant between hexafluoroisopropanol and anthraquinone by more than 1 order of magnitude following quinone reduction points to a significant strengthening of the hydrogen bonds between the hydroxyl group of hexafluoroisopropanol and the anthraquinone carbonyl functions. The photoinduced intramolecular long-range electron transfer process thus appears to be followed by proton motion; hence the overall photoinduced reaction may be considered a variant of stepwise proton-coupled electron transfer (PCET) in which substantial proton density (rather than a full proton) is transferred after the electron transfer has occurred

Electron Accumulation on Naphthalene Diimide Photosensitized by [Ru(2,2′-Bipyridine)₃]²⁺

In a molecular triad comprised of a central naphthalene diimide (NDI) unit flanked by two [Ru­(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) sensitizers, NDI^2– is formed after irradiation with visible light in deaerated CH₃CN in the presence of excess triethylamine. The mechanism for this electron accumulation involves a combination of photoinduced and thermal elementary steps. In a structurally related molecular pentad with two peripheral triarylamine (TAA) electron donors attached covalently to a central [Ru­(bpy)₃]²⁺-NDI-[Ru­(bpy)₃]²⁺ core but no sacrificial reagents present, photoexcitation only leads to NDI^– (and TAA⁺), whereas NDI^2– is unattainable due to rapid electron transfer events counteracting charge accumulation. For solar energy conversion, this finding means that fully integrated systems with covalently linked photosensitizers and catalysts are not necessarily superior to multicomponent systems, because the fully integrated systems can suffer from rapid undesired electron transfer events that impede multielectron reactions on the catalyst