Chirality enhances oxygen reduction.

Controlled reduction of oxygen is important for developing clean energy technologies, such as fuel cells, and is vital to the existence of aerobic organisms. The process starts with oxygen in a triplet ground state and ends with products that are all in singlet states. Hence, spin constraints in the oxygen reduction must be considered. Here, we show that the electron transfer efficiency from chiral electrodes to oxygen (oxygen reduction reaction) is enhanced over that from achiral electrodes. We demonstrate lower overpotentials and higher current densities for chiral catalysts versus achiral ones. This finding holds even for electrodes composed of heavy metals with large spin-orbit coupling. The effect results from the spin selectivity conferred on the electron current by the chiral assemblies, the chiral-induced spin selectivity effect.

Charge-Transfer Complexes: Halogen-Doped Anthracene as a Case of Study.

Charge transfer (CT) crystals exhibit unique electronic and magnetic properties with interesting applications. We present a rational and easy guide which allows to foresee the effective charge transfer co-crystal production and that is based on the comparison of the frontier molecular orbital (MO) energies of a donor and acceptor couple. For the sake of comparison, theoretical calculations have been carried out by using the cheap and fast PM6 semiempirical Hamiltonian and pure HF/cc-pVTZ level of the theory. The results are then compared with experimental results obtained both by chemical (bromine and iodine were used as the acceptor) and electrochemical doping (exploiting an original experimental set-up by this laboratory: the electrochemical transistor). Infra-red vibrational experimental results and theoretically calculated spectra are compared to assess both the effective donor-acceptor (D/A) charge-transfer and transport mechanism (giant IRAV polaron signature). XPS spectra have been collected (carbon (1 s) and iodine (3d5/2)) signals, yielding further evidence of the effective formation of the CT anthracene:iodine complex.

On the Electrochemical Growth of a Crystalline p-n Junction From Aqueous Solutions.

Our society largely relies on inorganic semiconductor devices which are, so far, fabricated using expensive and complex processes requiring ultra-high vacuum equipment. Here we report on the possibility of growing a p-n junction taking advantage of electrochemical processes based on the use of aqueous solutions. The growth of the junction has been carried out using the Electrochemical Atomic Layer Deposition (E-ALD) technique, which allowed to sequentially deposit two different semiconductors, CdS and Cu2S, on an Ag(111) substrate, in a single procedure. The growth process was monitored in situ by Surface X-Ray Diffraction (SXRD) and resulted in the fabrication of a thin double-layer structure with a high degree of crystallographic order and a well-defined interface. The high-performance electrical characteristics of the device were analysed ex-situ and show the characteristic feature of a diode.

Challenges in the Direct Detection of Chirality-induced Spin Selectivity: Investigation of Foldamer-based Donor-acceptor Dyads.

Over the past two decades, the chirality-induced spin selectivity (CISS) effect was reported in several experiments disclosing a unique connection between chirality and electron spin. Recent theoretical works highlighted time-resolved Electron Paramagnetic Resonance (trEPR) as a powerful tool to directly detect the spin polarization resulting from CISS. Here, we report a first attempt to detect CISS at the molecular level by linking the pyrene electron donor to the fullerene acceptor with chiral peptide bridges of different length and electric dipole moment. The dyads are investigated by an array of techniques, including cyclic voltammetry, steady-state and transient optical spectroscopies, and trEPR. Despite the promising energy alignment of the electronic levels, our multi-technique analysis reveals no evidence of electron transfer (ET), highlighting the challenges of spectroscopic detection of CISS. However, the analysis allows the formulation of guidelines for the design of chiral organic model systems suitable to directly probe CISS-polarized ET.

Growth Dynamics of Ultrathin Films of Benzo[1,2-b:4,5-b']dithiophene Derivatives on Au(111): A Photoelectron Spectroscopy Investigation.

Ultrathin films of a stereoisomeric mixture of benzo[1,2-b:4,5-b']dithiophene derivatives were grown by thermal evaporation in vacuum on Au(111), and they were studied in situ by photoelectron spectroscopy. X-ray photons from a non-monochromatic Mg Kα conventional X-ray source and UV photons from a He I discharge lamp equipped with a linear polarizer were used. He I photoemission results were compared with density functional theory (DFT) calculations: density of states (DOS) and 3D molecular orbital density distribution. Au 4f, C 1s, O 1s, and S 2p core-level components suggest a surface rearrangement as a function of film nominal thickness, with the variation of the molecular orientation, from flat-laying at the initial deposition to tilted toward the surface normal at coverages exceeding 2 nm. Eventually, the DFT results were exploited in assigning of the valence band experimental structures. Moreover, polarization-dependent photoemission confirmed the tilted arrangement of the molecules, starting at 2 nm. A variation of the work function of 1.4 eV with respect to the clean substrate was measured, together with a valence band offset of 1.3 eV between the organic layer and gold.

Spin-dependent electrochemistry and electrochemical enantioselective recognition with chiral methylated bis(ethylenedithio)-tetrathiafulvalenes.

Enantio-discrimination and spin-dependent electrochemistry (SDE), as a manifestation of the chirality-induced spin selectivity (CISS) effect, are important phenomena that can be probed by "chiral" electrochemistry. Here, we prepared chiralized surfaces of gold and nickel, to serve as working electrodes, through effective chemisorption of enantiopure dimethyl-bis(ethylenedithio)-tetrathiafulvalene (DM-BEDT-TTF) 1, tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) 2, and their capped silver nanoparticle (AgNPs) aggregate by simple incubation of the metallic substrates. The effective chemisorption was checked by means of ultrahigh vacuum x-ray photoelectron spectroscopy (XPS) and by electro-desorption experiments, i.e., cyclic voltammetry (CV) scans showing a first electro-desorption peak at about -1.0 V. The Au|1 and Au|2 chiral electrodes were successfully used in CV experiments exploiting chiral redox probes. Finally, the hybrid interfaces Ni|enantiopure 1 or 2|AgNPs served as working electrodes in SDE experiments. In particular, the hybrid chiral interfaces Ni|(R)-2|AgNPs and Ni|(S)-2|AgNPs exhibited a significant spin-filtering ability, as a manifestation of the CISS effect, with average spin polarization values of 15%.

Spin-dependent charge transmission through chiral 2T3N self-assembled monolayer on Au.

A gold surface is functionalized by chemisorption of the enantiopure N,N'-bis-[2,2';5',2″]tert-thiophene-5-yl methylcyclohexane-1,2-diamine (2T3N), a chiral oligothiophene derivative, via overnight incubation in a 2T3N ethanol solution. The Au|2T3N interface is characterized by x-ray photoelectron circular dichroism and comparing x-ray photoemission spectroscopy and electro-desorption results. Charge transmission at the Au|2T3N| solution interface is characterized by recording the cyclic voltammetry of the Fe(III)/Fe(II) reversible redox couple, finding a charge transfer rate constant, k°, variation from 1 × 10-1 to 3.3 × 10-2 cm s-1, when comparing the bare Au and the Au|2T3N interfaces, respectively. The "anomalous" high value of k° found for the chiral Au|2T3N interface can be rationalized on the basis of the chiral-induced spin selectivity effect, as further proved by magnetic-conductive atomic force microscopy measurements at room temperature. A spin polarization of about 30% is found.

Ethanol electro-oxidation reaction on the Pd(111) surface in alkaline media: insights from quantum and molecular mechanics.

The ethanol electro-oxidation catalyzed by Pd in an alkaline environment involves several intermediate reaction steps promoted by the hydroxyl radical, OH. In this work, we report on the dynamical paths of the first step of this oxidation reaction, namely the hydrogen atom abstraction CH3CH2OH + OH → CH3CHOH + H2O, occurring at the Pd(111) surface and address the thermodynamic stability of the adsorbed reactants by means of quantum and molecular mechanics calculations, with special focus on the effect of the solvent. We have found that the impact of the solvent is significant for both ethanol and OH, contributing to a decrease in their adsorption free energies by a few dozen kcal mol-1 with respect to the adsorption energy under vacuum. Furthermore, we observe that hydrogen atom abstraction is enhanced for those simulation paths featuring large surface-reactant distances, namely, when the reactants weakly interact with the catalyst. The picture emerging from our study is therefore that of a catalyst whose coverage in an aqueous environment is largely dominated by OH with respect to ethanol. Nevertheless, only a small amount of them, specifically those weakly bound to the catalyst, is really active in the ethanol electro-oxidation reaction. These results open the idea of a rational design of co-catalysts based on the tuning of surface chemical properties to eventually enhance exchange current density.

Charge transfer modulation in charge transfer co-crystals driven by crystal structure morphology.

The electronic properties of a charge-transfer (donor-acceptor) semiconducting organic co-crystal, Perylene:F4-TCNQ (PE:F4) (the donor, D, is PE and the acceptor, A, is 2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane (F4)) in its 3 : 2 stoichiometry, are experimentally and theoretically studied. This is performed by means of electron paramagnetic resonance (EPR) and solid state electrochemical techniques, such as cyclic voltammetry (CV) measurements on single crystals. In particular, solid state electrochemistry proves to be an effective tool to probe, on a macroscopic scale, the electronic characteristics of the co-crystal. However, EPR highlights the presence of spin ½ radicals localized on F4 molecules, possibly linked to defects. The experimental findings are discussed on the basis of density functional theory (DFT) based calculations, carried out using both the projector augmented wave (PAW), with "periodic boundary conditions" (pbc), method and the localized orbitals, molecular cluster, approach. In particular, a satisfying agreement is found between the experimental, 0.336 eV (electrochemical), and theoretical, 0.303 eV (PAW), band gaps. Differences with the reported optical bandgap are discussed considering excitonic effects.

Low-Cost Potentiometric Sensor for Chloride Measurement in Continuous Industrial Process Control.

Recently, the new updates in legislation about drinking water control and human health have increased the demand for novel electrochemical low-cost sensors, such as potentiometric ones. Nowadays, the determination of chloride ion in aqueous solutions has attracted great attention in several fields, from industrial processes to drinking water control. Indeed, chloride plays a crucial role in corrosion, also influencing the final taste of beverages, especially coffee. The main goal is to obtain devices suitable for continuous and real-time analysis. For these reasons, we investigated the possibility to develop an easy, low-cost potentiometric chloride sensor, able to perform analysis in aqueous mediums for long immersion time and reducing the need of periodic calibration. We realized a chloride ion selective electrode made of Ag/AgCl sintered pellet and we tested its response in model solutions compatible with drinking water. The sensor was able to produce a stable, reproducible, and accurate quantification of chloride in 900 s, without the need for a preliminary calibration test. This opens the route to potential applications of this sensor in continuous, in situ, and real time measurement of chloride ions in industrial processes, with a reduced need for periodic maintenance.

On the Dynamics of the Carbon-Bromine Bond Dissociation in the 1-Bromo-2-Methylnaphthalene Radical Anion.

This paper studies the mechanism of electrochemically induced carbon-bromine dissociation in 1-Br-2-methylnaphalene in the reduction regime. In particular, the bond dissociation of the relevant radical anion is disassembled at a molecular level, exploiting quantum mechanical calculations including steady-state, equilibrium and dissociation dynamics via dynamic reaction coordinate (DRC) calculations. DRC is a molecular-dynamic-based calculation relying on an ab initio potential surface. This is to achieve a detailed picture of the dissociation process in an elementary molecular detail. From a thermodynamic point of view, all the reaction paths examined are energetically feasible. The obtained results suggest that the carbon halogen bond dissociates following the first electron uptake follow a stepwise mechanism. Indeed, the formation of the bromide anion and an organic radical occurs. The latter reacts to form a binaphthalene intrinsically chiral dimer. This paper is respectfully dedicated to Professors Anny Jutand and Christian Amatore for their outstanding contribution in the field of electrochemical catalysis and electrosynthesis.

The Fundamental and Underrated Role of the Base Electrolyte in the Polymerization Mechanism. The Resorcinol Case Study.

The Kane-Maguire polymerization mechanism is disassembled at a molecular level by using DFT-based quantum mechanical calculations. Resorcinol electropolymerization is selected as a case study. Stationary points (transition states and intermediate species) leading to the formation of the dimer are found on the potential energy surface (PES), and elementary reactions involved in the dimer formation are characterized. The latter allow to further propagate the polymerization chain reaction, when applied recursively. In this paper, the fundamental role of the sulfate anion (a typical base electrolyte) is addressed. Investigation of the PES in terms of both stationary-state properties and of ab initio molecular dynamics results (dynamic reaction coordinate) allows the appreciation in detail of the critical role of the base electrolyte anion in making the proton dissociation from the initial radical ion, a feasible (downhill in energy) process.

Asymmetric reactions induced by electron spin polarization.

Essential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed.

Influence of Chiral Compounds on the Oxygen Evolution Reaction (OER) in the Water Splitting Process.

Results are presented concerning the influence on the water splitting process of enantiopure tartaric acid present in bulk solution. Stainless steel and electrodeposited nickel are used as working electrode (WE) surface. The latter is obtained by electrodeposition on the two poles of a magnet. The influence and role played by the chiral compound in solution has been assessed by comparing the current values, in cyclic voltammetry (CV) experiments, recorded in the potential range at which oxygen evolution reaction (OER) occurs. In the case of tartaric acid and nickel WE a spin polarization of about 4% is found. The use of the chiral environment (bulk solution) and ferromagnetic chiral Ni electrode allows for observing the OER at a more favorable potential: About 50 mV (i.e., a cathodic, less positive, shift of the potential at which the oxygen evolution is observed).

The Electron Spin as a Chiral Reagent.

We show that enantioselective reactions can be induced by the electron spin itself and that it is possible to replace a conventional enantiopure chemical reagent by spin-polarized electrons that provide the chiral bias for enantioselective reactions. Three examples of enantioselective chemistry resulting from electron-spin polarization are presented. One demonstrates the enantioselective association of a chiral molecule with an achiral self-assembled monolayer film that is spin-polarized, while the other two show that the chiral bias provided by the electron helicity can drive both reduction and oxidation in enantiospecific electrochemical reactions. In each case, the enantioselectivity does not result from enantiospecific interactions of the molecule with the ferromagnetic electrode but from the polarized spin that crosses the interface between the substrate and the molecule. Furthermore, the direction of the electron-spin polarization defines the handedness of the enantioselectivity. This work demonstrates a new mechanism for realizing enantioselective chemistry.

Effect of Oxidative Damage on Charge and Spin Transport in DNA.

A Hall device was used for measuring spin polarization on electrons that are either reorganized within the molecules or transmitted through the self-assembled monolayers of DNA adsorbed on the device surface. We were able to observe spin-dependent charge polarization and charge transport through double-stranded DNA of various lengths and through double-stranded DNA containing oxidative damage. We found enhancement in the spin-dependent transport through oxidatively damaged DNA. This phenomenon can be rationalized either by assuming that the damaged DNA is characterized by a higher barrier for conduction or by charge transfer through the DNA being conducted through at least two channels, one involves the bases and is highly conductive but less spin selective, while the other pathway is mainly through the ribophosphate backbone and it is the minor one in terms of charge transmission efficiency, but it is highly spin selective.

Electrochemistry of Metalloproteins Attached through Functional Self-Assembled Monolayers on Gold and Ferromagnetic Electrodes.

We report the experimental results of a study of the electron-transfer processes of redox-active metalloproteins bound to mixed self-assembled monolayers (SAMs) on magnetic (nickel or ultrathin gold-coated nickel) or nonmagnetic (gold) electrodes. Metalloproteins, such as hemoglobin (Hb), Cytochrome C (Cyt C), and Cyt C oxidase, are attached through electrostatic interactions to the free carboxylate or imidazole groups present in the mixed SAMs. The formation of both mixed SAMs and SAM/metalloprotein heterostructures were confirmed by using advanced surface analysis techniques, such as polarization modulation infrared reflection absorption spectroscopy and aqueous contact angle measurements. Electrochemical measurements indicated a stronger electronic coupling between Hb and Cyt C oxidase and the mixed-SAM-coated gold or gold-coated-nickel electrodes, whereas a weaker coupling was found between the protein and the pure nickel electrode. Surface coverage and the electron-transfer rate constant were estimated from the cyclic voltammetry data.

Control of Electrons' Spin Eliminates Hydrogen Peroxide Formation During Water Splitting.

The production of hydrogen through water splitting in a photoelectrochemical cell suffers from an overpotential that limits the efficiencies. In addition, hydrogen-peroxide formation is identified as a competing process affecting the oxidative stability of photoelectrodes. We impose spin-selectivity by coating the anode with chiral organic semiconductors from helically aggregated dyes as sensitizers; Zn-porphyrins and triarylamines. Hydrogen peroxide formation is dramatically suppressed, while the overall current through the cell, correlating with the water splitting process, is enhanced. Evidence for a strong spin-selection in the chiral semiconductors is presented by magnetic conducting (mc-)AFM measurements, in which chiral and achiral Zn-porphyrins are compared. These findings contribute to our understanding of the underlying mechanism of spin selectivity in multiple electron-transfer reactions and pave the way toward better chiral dye-sensitized photoelectrochemical cells.

Spin-Dependent Transport through Chiral Molecules Studied by Spin-Dependent Electrochemistry.

Molecular spintronics (spin + electronics), which aims to exploit both the spin degree of freedom and the electron charge in molecular devices, has recently received massive attention. Our recent experiments on molecular spintronics employ chiral molecules which have the unexpected property of acting as spin filters, by way of an effect we call "chiral-induced spin selectivity" (CISS). In this Account, we discuss new types of spin-dependent electrochemistry measurements and their use to probe the spin-dependent charge transport properties of nonmagnetic chiral conductive polymers and biomolecules, such as oligopeptides, L/D cysteine, cytochrome c, bacteriorhodopsin (bR), and oligopeptide-CdSe nanoparticles (NPs) hybrid structures. Spin-dependent electrochemical measurements were carried out by employing ferromagnetic electrodes modified with chiral molecules used as the working electrode. Redox probes were used either in solution or when directly attached to the ferromagnetic electrodes. During the electrochemical measurements, the ferromagnetic electrode was magnetized either with its magnetic moment pointing "UP" or "DOWN" using a permanent magnet (H = 0.5 T), placed underneath the chemically modified ferromagnetic electrodes. The spin polarization of the current was found to be in the range of 5-30%, even in the case of small chiral molecules. Chiral films of the l- and d-cysteine tethered with a redox-active dye, toludin blue O, show spin polarizarion that depends on the chirality. Because the nickel electrodes are susceptible to corrosion, we explored the effect of coating them with a thin gold overlayer. The effect of the gold layer on the spin polarization of the electrons ejected from the electrode was investigated. In addition, the role of the structure of the protein on the spin selective transport was also studied as a function of bias voltage and the effect of protein denaturation was revealed. In addition to "dark" measurements, we also describe photoelectrochemical measurements in which light is used to affect the spin selective electron transport through the chiral molecules. We describe how the excitation of a chromophore (such as CdSe nanoparticles), which is attached to a chiral working electrode, can flip the preferred spin orientation of the photocurrent, when measured under the identical conditions. Thus, chirality-induced spin polarization, when combined with light and magnetic field effects, opens new avenues for the study of the spin transport properties of chiral molecules and biomolecules and for creating new types of spintronic devices in which light and molecular chirality provide new functions and properties.

Magnetless Device for Conducting Three-Dimensional Spin-Specific Electrochemistry.

Electron spin states play an important role in many chemical processes. Most spin-state studies require the application of a magnetic field. Recently it was found that the transport of electrons through chiral molecules also depends on their spin states and may also play a role in enantiorecognition. Electrochemistry is an important tool for studying spin-specific processes and enantioseparation of chiral molecules. A new device is presented, which serves as the working electrode in electrochemical cells and is capable of providing information on the correlation of spin selectivity and the electrochemical process. The device is based on the Hall effect and it eliminates the need to apply an external magnetic field. Spin-selective electron transfer through chiral molecules can be monitored and the relationship between the enantiorecognition process and the spin of electrons elucidated.