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 p-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

Exceptionally Long-Lived Photodriven Multi-Electron Storage without Sacrificial Reagents

Photoexcitation of a molecular pentad in the presence of Sc3+ in de-aerated CH3CN leads to a quinone dianion that is stable on the millisecond timescale. Light-driven electron accumulation on the quinone unit is sensitized by two Ru(bpy)32+ complexes in an intramolecular process, which relies on covalently attached triarylamine donors rather than on sacrificial reagents. Lewis acid–Lewis base interactions between Sc3+ and quinone dianion are responsible for the exceptionally long lifetime of this photoproduct. Our study of photoinduced multi-electron transfer is relevant in the greater context of solar energy conversion

Reaction Rate Maxima at Large Distances between Reactants

One commonly thinks that two reactants need to come very close to one another in order for a chemical reaction to occur. This is true for most reaction types, but electron transfer is an exception in this regard. It is a well-documented fact that electron transfers can occur over long distances (≥15 Å), but it is much less well-known that theory predicts a regime in which electron transfer rates in crease with increasing distance between reactants. This contribution explains the physical origin of this counter-intuitive behavior, and it identifies a set of conditions that might facilitate its experimental observation

Unusual Distance Dependences of Electron Transfer Rates

Usually the rates for electron transfer (kET) decrease with increasing donor–acceptor distance, but Marcus theory predicts a regime in which kET is expected to increase when the transfer distance gets longer. Until recently, experimental evidence for such counter-intuitive behavior had been very limited, and consequently this effect is much less well-known than the Gaussian free energy dependence of electron transfer rates leading to the so-called inverted driving-force effect. This article presents the theoretical concepts that lead to the prediction of electron transfer rate maxima at large donor–acceptor distances, and it discusses conditions that are expected to favor experimental observations of such behavior. It continues with a consideration of specific recent examples in which electron transfer rates were observed to increase with increasing donor–acceptor distance, and it closes with a discussion of the importance of this effect in the context of 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 p-xylene spacers (n = 0–3) was synthesized and investigated. The species with a single p-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (n = 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 (n = 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 n = 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 (n = 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

Light-Driven Electron Accumulation in a Molecular Pentad

Accumulation and temporary storage of redox equivalents with visible light as an energy input is of pivotal importance for artificial photosynthesis because key reactions, such as CO2 reduction or water oxidation, require the transfer of multiple redox equivalents. We report on the first purely molecular system, in which a long-lived charge-separated state (τ≈870 ns) with two electrons accumulated on a suitable acceptor unit can be observed after excitation with visible light. Importantly, no sacrificial reagents were employed

Homoleptic complexes of a porphyrinatozinc(II)-2,2’:6’,2’’-terpyridine ligand

Three homoleptic complexes containing the metalloligand 7-(4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)-5,10,15,20-tetraphenylporphyrinatozinc(II), 1, have been prepared. [Zn(1)2][PF6]2, [Fe(1)2][PF6]2 and [Ru(1)2][PF6]2 were characterized by 1H and 13C NMR spectroscopy and mass spectrometry, and the electrochemical and photophysical properties of the complexes have been investigated. In solution, each complex undergoes two reversible porphyrin-centred oxidation processes, with an additional reversible metal-centred oxidation for [Fe(1)2][PF6]2 and [Ru(1)2][PF6]2. Solution absorption spectra are dominated by the Soret and Q bands of the metalloligand 1. Spectroelectrochemical data for the complexes are presented. The results of a nanosecond transient absorption spectroscopic investigation of [Zn(1)2][PF6]2, [Fe(1)2][PF6]2 and [Ru(1)2][PF6]2 are presented. For [Zn(1)2][PF6]2, S1 excitation leads to an efficient intersystem-crossing to the T1 state, whilst for [Fe(1)2][PF6]2, excitation of the 1MLCT transition is followed by fast deactivation to the 3MC state followed by thermal decay to the ground state. Excitation of the 1MLCT transition of [Ru(1)2][PF6]2 results in an intersystem crossing to 3MLCT; triplet-to-triplet energy transfer occurs giving the [Zn(TPP)] T1 state which regenerates the ground state of the complex

Stimuli‐Responsive Resorcin[4]arene Cavitands: Toward Visible‐Light‐Activated Molecular Grippers

Resorcin[4]arene cavitands, equipped with diverse quinone (Q) and [Ru(bpy)(2)dppz](2+)(bpy=2,2 '-bipyridine, dppz=dipyrido[3,2-a:2 ',3 '-c]phenazine) photosensitizing walls in different configurations, were synthesized. Upon visible-light irradiation at 420 nm, electron transfer from the [Ru(bpy)(2)dppz](2+)to theQgenerates the semiquinone (SQ) radical anion, triggering a large conformational switching from a flatkiteto avasewith a cavity for the encapsulation of small guests, such as cyclohexane and heteroalicyclic derivatives, in CD3CN. Depending on the molecular design, theSQradical anion can live for several minutes (approximate to 10 min) and thevasecan be generated in a secondary process without need for addition of a sacrificial electron donor to accumulate theSQstate. Switching can also be triggered by other stimuli, such as changes in solvent, host-guest complexation, and chemical and electrochemical processes. This comprehensive investigation benefits the development of stimuli-responsive nanodevices, such as light-activated molecular grippers

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

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 p-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