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.

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.

Electron transfer can readily occur over long (≥15 Å) distances. Usually reaction rates decrease with increasing distance between donors and acceptors, but theory predicts a regime in which electron-transfer rates increase with increasing donor-acceptor separation. This counter-intuitive behavior can result from the interplay of reorganization energy and electronic coupling, but until now experimental studies have failed to provide unambiguous evidence for this effect. We report here on a homologous series of rigid rodlike donor-bridge-acceptor compounds in which the electron-transfer rate increases by a factor of 8 when the donor-acceptor distance is extended from 22.0 to 30.6 Å, and then it decreases by a factor of 188 when the distance is increased further to 39.2 Å. This effect has important implications for solar energy conversion.

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy) 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 (AQH) 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 AQH 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 AQH. 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 CO or the oxidation of HO, can potentially be performed without sacrificial reagents.

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.