Claudio Fontanesi

Spin-dependent electron transmission through bacteriorhodopsin embedded in purple membrane

Significance The role of the electron spin in chemistry and biology has been receiving much attention because of a plausible relation to electromagnetic field effects on living organisms. Part of the difficulty in studying the subject arises from the lack of a physical model that can rationalize these phenomena. Recently the chiral-induced spin selectivity effect was observed in electron transmission through organic molecules. The question is to what extent the effect takes place in proteins. In the present study, we probed bacteriorhodopsin embedded in its native membrane environment. We observed clear evidence for spin-dependent electron transmission through this system. The results point to the possibility that the effect may play a role in electron transfer in biological systems. Spin-dependent photoelectron transmission and spin-dependent electrochemical studies were conducted on purple membrane containing bacteriorhodopsin (bR) deposited on gold, aluminum/aluminum-oxide, and nickel substrates. The result indicates spin selectivity in electron transmission through the membrane. Although the chiral bR occupies only about 10% of the volume of the membrane, the spin polarization found is on the order of 15%. The electrochemical studies indicate a strong dependence of the conduction on the protein’s structure. Denaturation of the protein causes a sharp drop in the conduction through the membrane.

How intermolecular geometrical disorder affects the molecular doping of donor-acceptor copolymers

Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

Electrochemistry and spectroelectrochemistry of ruthenium(II)-bipyridine building blocks. Different behaviour of the 2,3- and 2,5-bis(2-pyridyl)pyrazine bridging ligands

We report the results of an investigation, using electrochemical and spectroelectrochemical techniques, into redox properties of uncoordinated free bis-chelating 2,3- and 2,5-bis(2-pyridyl)pyrazine ligands (2,3- and 2,5-dpp) and of the complexes of the [Ru(2,3-dpp)n(bpy)3−n]2+ and [Ru(2,5-dpp)n(bpy)3−n]2+ families (bpy=2,2′-bipyridine), which are used as building blocks for obtaining polynuclear complexes. For comparison purposes, the electrochemical behaviour of the [Ru(2,3-dpp)(DCE-bpy)2]2+complex, where DCE-bpy is 5,5′-dicarboxyethyl-2,2′-bipyridine, has also been investigated. Correlations of the E1/2 values observed for the compounds examined (genetic diagrams) have allowed us to assign all the ligand-based reduction processes as well as to discuss electronic interactions. The localisation of the first three reduction processes for each complex has also been established on the basis of the spectroelectrochemical results. Theoretical calculations (AM1 semiempirical and ab-initio level) carried out for the 2,5-dpp and 2,3-dpp ligands show that, in the uncoordinated state, the former ligand does not exhibit any substantial conformation arrangement, whereas the latter has a stable conformation for a large (56°) dihedral angle between the pyridyl and pyrazine rings. The changes in conformation upon mono- and bis-coordination of 2,3-dpp can account for its peculiar electrochemical behaviour consisting in a change of the number of redox processes with varying coordination state.

Chiral Conductive Polymers as Spin Filters

DOI: 10.1002/adma.201405249 electrode. By controlling the direction of the magnetization of nickel, it is possible to inject electrons that have mainly one spin orientation and to verify their transport through the PCT by monitoring the cyclic voltammetry (CV) curve in the electrochemical cell in which the redox couple is not chiral. The CV curves refl ect a steady state in which a double layer is formed near the working electrode. Another method to monitor the spin selectivity is by chronoamperometry, in which one monitors time-dependent current at a fi xed potential. At the beginning, when the potential is turned on, the current is high, but it is reduced with time due to the formation of the double layer. [ 18,19 ]

An unforeseen polymorph of coronene by the application of magnetic fields during crystal growth

The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a β-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The β-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal.Coronene, a polyaromatic hydrocarbon, has been crystallized for the first time in a different polymorph using a crystal growth method that utilizes magnetic fields to access a unit cell configuration that was hitherto unknown. Crystals grown in magnetic field of 1 T are larger, have a different appearance to those grown in zero field and retain their structure in ambient conditions. We identify the new form, beta-coronene, as the most stable at low temperatures. As a result of the new supramolecular configuration we report significantly altered electronic, optical and mechanical properties.

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

Conspectus 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.

Role of the Electron Spin Polarization in Water Splitting

We show that in an electrochemical cell, in which the photoanode is coated with chiral molecules, the overpotential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. The hydrogen evolution efficiency is studied comparing seven different organic molecules, three chiral and four achiral. We propose that the spin specificity of electrons transferred through chiral molecules is the origin of a more efficient oxidation process in which oxygen is formed in its triplet ground state. The new observations are consistent with recent theoretical works pointing to the importance of spin alignment in the water-splitting process.

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.

Computational electrochemistry. Ab initio calculation of solvent effect in the multiple electroreduction of polypyridinic compounds

Abstract The electrochemical multiple reduction of a series of polypyridinic compounds, yielding mono, di and trianionic species is theoretically studied. Calculated electron affinity values were used to obtain molecular-structure/reactivity relationships, the latter reflected by the experimental half-wave electroreduction potentials. Gas-phase electron affinity data vs. half-wave potentials produced satisfactory linear correlations, but separated for each successive electron transfer step (i.e. a linear relationship for the first electron transfer, yield of the monoanion, neatly separated from the one concerning the second electron transfer, yield of the dianionic species). The theoretical approach was pushed further, also including solvent effects. This was done by means of two methods based on the continuum solvation model: the Onsager cavity (SCRF) and the more sophisticated SM5.42R solvation models. In particular, the latter is able to group in a single correlation the potentials referring to the first, second and third electron transfer.

A theoretical and topological study on the electroreduction of chlorobenzene derivatives

The electrochemical behaviour of 12 polychlorobenzene derivatives is discussed on the basis of calculated quantum-mechanical indexes (CNDO/2, MNDO, AM1 methods) and of pure structural parameters reckoned by means of graph theory (Wierner, Balaban, Randic, PID and Wrms numbers). Even the existence of good relationships between reduction potentials and energies of virtual molecular orbitals, π* or σ*(closed-shell calculations), does not account for the details of the electron uptake at the molecular level. Open-shell calculations (MNDO) regarding unrelaxed radical anions are much more promising in this respect, the occurrence of the so-called ‘ortho effect’ can be so justified. The reductive process (at least in the experimental conditions considered here) is shown to be scarcely affected by solvent-shell or internal molecular rearrangements; this view is supported by the good description of the process, strictly on structural grounds regarding the whole of the molecule, provided by the graph-theoretical indexes.