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

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

A modified Wald interval for the area under the ROC curve (AUC) in diagnostic case-control studies

Sociocultural and economic conditions interact with biological processes throughout the course of life determining vulnerability or resilience in old age. The scope of this study was to investigate relationships between social vulnerability (gender, age and income); individual vulnerability (comorbidities, signs and symptoms, functional ability, perceived social support and perceived health), and programmatic vulnerability (indices of dependence on the public health system, social vulnerability and access to health services) in a sample of individuals aged 65 and more. 688 elderly people were interviewed in a single data gathering session in their homes in 88 selected urban census sectors in Campinas. 470 of the interviewees were women, with more comorbidities and more signals and symptoms, though more socially engaged in AADL and IADL than men. Mean age was 72.28 ± 5.41; mean family income = 4.72 ± 5.28 minimum wages. The variables with most explanatory power over the joint variation of the data were access and use of health services, levels of social vulnerability and dependence on public healthcare services, and family income. Social conditions as well as family income coexist with individual vulnerability in old age

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

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

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

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