Electron Transfer Rate Maxima at Large Donor–Acceptor Distances

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor–acceptor distance. We report here on electron transfer rate maxima at donor–acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor–photosensitizer–acceptor triads with <i>p</i>-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor–acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion

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

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

Dependence of Reaction Rates for Bidirectional PCET on the Electron Donor–Electron Acceptor Distance in Phenol–Ru(2,2′-Bipyridine)₃²⁺ Dyads

A homologous series of three donor–bridge–acceptor molecules in which a phenolic unit is attached covalently to a Ru­(bpy)₃²⁺ (bpy =2,2′-bipyridine) complex via rigid rod-like <i>p</i>-xylene spacers was investigated. Photoexcitation at 532 nm in the presence of a large excess of methyl viologen leads to rapid (<10 ns) formation of Ru­(bpy)₃³⁺. When imidazole base is present in CH₃CN solution, intramolecular electron transfer from the phenol to Ru­(bpy)₃³⁺ occurs, and this is coupled to proton transfer from the phenol to imidazole. All mechanistic possibilities for this proton-coupled electron transfer (PCET) process are considered, and based on a combination of kinetic and thermodynamic data, one arrives at the conclusion that electron and proton release by the phenol occur in concert. By varying the number of <i>p</i>-xylene bridging units, it then becomes possible to investigate the dependence of the reaction rates for concerted proton–electron transfer (CPET) on the phenol–Ru­(bpy)₃³⁺ distance. A distance decay constant of 0.87 ± 0.09 Å^–1 is obtained. This is one of the largest β values reported for electron transfer across oligo-<i>p</i>-phenylene-based molecular bridges, but it is still relatively close to what was determined for “simple” (i. e., not proton-coupled) electron transfer across oligo-<i>p</i>-xylenes. Bidirectional CPET plays a key role in photosystem II. Understanding the distance dependence of such reactions is of interest, for example, in the context of separating protons and electrons across artificial membranes in order to build up charge gradients for light-to-chemical energy conversion

Influence of Donor–Acceptor Distance Variation on Photoinduced Electron and Proton Transfer in Rhenium(I)–Phenol Dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium­(I) tricarbonyl diimine photooxidant via a variable number of <i>p</i>-xylene spacers (<i>n</i> = 0–3) was synthesized and investigated. The species with a single <i>p</i>-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (<i>n</i> = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (<i>n</i> = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with <i>n</i> = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (<i>n</i> = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor–acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor–bridge–acceptor molecules

Influence of Donor–Acceptor Distance Variation on Photoinduced Electron and Proton Transfer in Rhenium(I)–Phenol Dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium­(I) tricarbonyl diimine photooxidant via a variable number of <i>p</i>-xylene spacers (<i>n</i> = 0–3) was synthesized and investigated. The species with a single <i>p</i>-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (<i>n</i> = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (<i>n</i> = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with <i>n</i> = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (<i>n</i> = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor–acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor–bridge–acceptor molecules

Directing Energy Transfer in Panchromatic Platinum Complexes for Dual Vis–Near-IR or Dual Visible Emission from σ‑Bonded BODIPY Dyes

We report on the platinum complexes <i>trans</i>-Pt­(BODIPY)­(8-ethynyl-BODIPY)­(PEt₃)₂ (<b>EtBPtB</b>) and <i>trans</i>-Pt­(BODIPY)­(4-ethynyl-1,8-naphthalimide)­(PR₃)₂ (R = Et, <b>EtNIPtB-1</b>; R = Ph, <b>EtNIPtB-2</b>), which all contain two different dye ligands that are connected to the platinum atom by a direct σ bond. The molecular structures of all complexes were established by X-ray crystallography and show that the different dye ligands are in either a coplanar or an orthogonal arrangement. π-stacking and several CH···F and short CH···π interactions involving protons at the phosphine substituents lead to interesting packing motifs in the crystal. The complexes feature several strong absorptions (ε = 3.2 × 10⁵–5.5 × 10⁵ M^–1 cm^–1) that cover the regime from 350 to 480 nm (<b>EtNIPtB-1</b> and <b>EtNIPtB-2</b>) or from 350 to 580 nm (<b>EtBPtB</b>). Besides the typical absorption bands of both kinds of attached dyes, they also feature an intense band near 400–420 nm, which is assigned by time-dependent density functional theory calculations to a higher-energy transition within the ethynyl-BODIPY (EtB) ligand or to charge transfer between the BODIPY (B) and naphthalimide (NI) chromophores. All complexes show dual fluorescence and phosphorescence emission from either the B (<b>EtNIPtB-1</b> and <b>EtNIPtB-2</b>) or EtB (<b>EtBPtB</b>) ligand with a maximum phosphorescence quantum yield of 41% for <b>EtNIPtB-1</b>. The latter seems to be the highest reported value for room temperature phosphorescence from a BODIPY dye. The complete quenching of the emission from the chromophore absorbing at the higher energy and the appearance of the corresponding absorption bands in the fluorescence and phosphorescence excitation spectra indicate complete and rapid energy transfer to the chromophore with the lower-energy excited state, i.e., EtNI → B in <b>EtNIPtB-1</b> and <b>EtNIPtB-2</b> and B → EtB in <b>EtBPtB</b>. The latter process was further investigated by transient absorption spectroscopy, indicating that energy transfer is complete within 0.6 ns. <b>EtNIPtB-1</b> catalyzes the photooxidation of 1,5-dihydroxynaphthalene with photogenerated ¹O₂ to Juglone at a much faster rate than methylene blue but with only modest quantum yields of 37% and with the onset of photodegradation after 60 min