Large Increase of the Lifetime of a Charge-Separated State in a Molecular Triad Induced by Hydrogen-Bonding Solvent

Life of triad: The lifetime of a charge-separated state (see figure) increases from about 50 to 2000 ns when the solvent is changed from aprotic CH2Cl2 to the strong hydrogen-bond donor hexafluoroisopropanol

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 p-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

Photoinduced Electron Transfer in Linear Triarylamine-Photosensitizer-Anthraquinone Triads with Ruthenium(II), Osmium(II), and Iridium(III)

A rigid rod-like organic molecular ensemble comprised of a triarylamine electron donor, a 2,2′-bipyridine (bpy) ligand, and a 9,10-anthraquinone acceptor was synthesized and reacted with suitable metal precursors to yield triads with Ru(bpy)32+, Os(bpy)32+, and [Ir(2-(p-tolyl)pyridine)2(bpy)]+ photosensitizers. Photoexcitation of these triads leads to long-lived charge-separated states (τ = 80–1300 ns) containing a triarylamine cation and an anthraquinone anion, as observed by transient absorption spectroscopy. From a combined electrochemical and optical spectroscopic study, the thermodynamics and kinetics for the individual photoinduced charge-separation and thermal charge-recombination events were determined; in some cases, measurements on suitable donor–sensitizer or sensitizer–acceptor dyads were necessary. In the case of the ruthenium and iridium triads, the fully charge-separated state is formed in nearly quantitative yield

Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d6 metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ∼750 ns in dichloromethane to ∼3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ∼50 to ∼2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET

Gestion optimisée de l'énergie électrique d'un groupe électrogène hybride à pile à combustible

L'étude porte sur la gestion de la distribution instantanée de la puissance entre une pile à combustible et un élément de stockage afin d'assurer la puissance électrique nécessaire à la traction d'un véhicule électrique hybride. L'objectif visé est la minimisation de la consommation d'hydrogène sur un cycle donné. Le problème est formulé en tant que problème d'optimisation globale sous contraintes. Dans une première approche, le système est décrit sous forme d'une équation dynamique discrétisée et un algorithme de programmation dynamique est appliqué. Une seconde approche consiste à minimiser le hamiltonien après avoir approché le critère coût par une fonction polynomiale. Ces méthodes nécessitent la connaissance a priori du profil de puissance demandée et se classent parmi les méthodes d'optimisation hors ligne. Pour une gestion en ligne de l'énergie, nous avons appliqué un système de décision à base de règles floues. Les fonctions d'appartenance des entrées et sorties du système flou sont optimisés à l'aide d'un algorithme génétique. Afin d'appliquer les stratégies évoquées, un bilan énergétique du groupe électrogène formé de la pile et ses auxiliaires, de l'élément de stockage et des convertisseurs statiques est effectué. ABSTRACT : The study focuses on the power distribution management between a fuel cells stack and a supercapacitor pack in order to ensure the power demand in a hybrid vehicle. The goal is to minimize the hydrogen consumption on a given cycle. The problem is formulated as a global optimization problem under constraints. In a first approach, the system is described with a discretized dynamic equation and a dynamic programming algorithm is applied. A second approach is to minimize the Hamiltonian after having approached the cost criterion by a polynomial function. These methods require a priori knowledge of the power demand profile and are therefore considered as offline optimization methods. For an online energy management, we applied a decision making system based on fuzzy rules. Inputs and output membershipfunctions of the fuzzy system are optimized using a genetic algorithm. In order to implement the outlined strategies, efficiency of the elements of the power train formed by the fuel cells stack and auxiliaries, the storage element and the static converters is characterize

Modulation of the charge transfer and photophysical properties in non-fused tetrathiafulvalene-benzothiadiazole derivatives

Bis(thiomethyl)- and bis(thiohexyl)-tetrathiafulvalene-bromo-benzothiadiazoles, containing electron donor tetrathiafulvalene (TTF) and electron acceptor benzothiadiazole (BTD) units, have been prepared by Stille coupling reactions between the TTF-SnMe3 precursors and BTD-Br2. In another series of experiments, TTF-acetylene-BTD compounds have been synthesized by Sonogashira coupling between either TTF-acetylenes and BTD-Br2 in low yields, or TTF-iodine and BTD-acetylene in moderate yields. In the compound TTF-C [[triple bond, length as m-dash]] C-BTD the TTF and BTD units are coplanar in the solid state, as shown by the single crystal X-ray structure, and there is segregation in the packing between the donor and acceptor units. All the derivatives have good electron donor properties, as determined by cyclic voltammetry measurements, and they can also be reversibly reduced thanks to the presence of the BTD moiety. UV-visible spectroscopy and photophysical investigations show the presence of an intramolecular charge transfer (ICT) band and an emission band originating from the charge transfer. Both the absorption and the emission are modulated by the substitution scheme and the insertion of the acetylenic bridge

Pump-Pump-Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)32+‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)32+ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design

Increasing Electron Transfer Rates with Increasing Donor-Acceptor Distance

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

Machine-enhanced CP-asymmetries in the electroweak sector

The violation of charge conjugation ( C ) and parity ( P ) symmetries are a requirement for the observed dominance of matter over antimatter in the Universe. As an established effect of beyond the Standard Model physics, this could point towards additional C P violation in the Higgs-gauge sector. The phenomenological footprint of the associated anomalous couplings can be small, and designing measurement strategies with the highest sensitivity is therefore of the utmost importance in order to maximize the discovery potential of the Large Hadron Collider. There are, however, very few measurements of C P -sensitive observables in processes that probe the weak-boson self-interactions. In this article, we study the sensitivity to new sources of C P violation for a range of experimentally accessible electroweak processes, including W γ production, W W production via photon fusion, electroweak Z j j production, electroweak Z Z j j production, and electroweak W ± W ± j j production. We study simple angular observables as well C P -sensitive observables constructed using the outputs of machine-learning algorithms. We find that the machine-learning-constructed C P -sensitive observables improve the sensitivity to C P -violating effects by up to a factor of five, depending on the process. We also find that inclusive W γ and electroweak Z j j production have the potential to set the best possible constraints on certain C P -odd operators in the Higgs-gauge sector of dimension-six effective field theories

Measurement of the cross-section and charge asymmetry of WW bosons produced in proton-proton collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

This paper presents measurements of the $W^+ \rightarrow \mu^+\nu$ and $W^- \rightarrow \mu^-\nu$ cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton--proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2~\mbox{fb^{-1}}. The precision of the cross-section measurements varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.Comment: 38 pages in total, author list starting page 22, 5 figures, 4 tables, submitted to EPJC. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-13