Hybrid mesoporous electrodes evidence CISS effect on water oxidation.

Controlling product selectivity is essential for improving the efficiency of multi-product reactions. Electrochemical water oxidation is a reaction of main importance in different applications, e.g., renewable energy schemes and environmental protection, where H2O2 and O2 are the two principal products. In this Communication, the product selectivity of electrochemical water oxidation was controlled by making use of the chiral induced spin selectivity (CISS) effect at mesoporous-TiO2 on the molecule-modified Au substrate. Our results show a decrease in H2O2 formation when using chiral hetero-helicene molecules adsorbed on the Au substrate. We propose a mechanism for this kinetic effect based on the onset of CISS-induced spin polarization on the Au-helicene chiral interface. We also present a new tunable substrate to investigate the CISS mechanism.

Probing chiral discrimination in biological systems using atomic force microscopy: The role of van der Waals and exchange interactions.

We analyze from a theoretical perspective recent experiments where chiral discrimination in biological systems was established using Atomic Force Microscopy (AFM). Even though intermolecular forces involved in AFM measurements have different origins, i.e., electrostatic, bonding, exchange, and multipole interactions, the key molecular forces involved in enantiospecific biorecognition are electronic spin exchange and van der Waals (vdW) dispersion forces, which are sensitive to spin-orbit interaction (SOI) and space-inversion symmetry breaking in chiral molecules. The vdW contribution to chiral discrimination emerges from the inclusion of SOI and spin fluctuations due to the chiral-induced selectivity effect, a result we have recently demonstrated theoretically. Considering these two enantiospecific contributions, we show that the AFM results regarding chiral recognition can be understood in terms of a simple physical model that describes the different adhesion forces associated with different electron spin polarization generated in the (DD), (LL), and (DL) enantiomeric pairs, as arising from the spin part of the exchange and vdW contributions. The model can successfully produce physically reasonable parameters accounting for the vdW and exchange interaction strength, accounting for the chiral discrimination effect. This fact has profound implications in biorecognition where the relevant intermolecular interactions in the intermediate-distance regime are clearly connected to vdW forces.

Spin-phonon coupling in a double-stranded model of DNA.

We address the electron-spin-phonon coupling in an effective model Hamiltonian for DNA to assess its role in spin transfer involved in the Chiral-Induced Spin Selectivity (CISS) effect. The envelope function approach is used to describe semiclassical electron transfer in a tight-binding model of DNA at half filling in the presence of intrinsic spin-orbit coupling. Spin-phonon coupling arises from the orbital-configuration dependence of the spin-orbit interaction. We find spin-phonon coupling only for the acoustic modes, while the optical modes exhibit electron-phonon interaction without coupling to spin. We derive an effective Hamiltonian whose eigenstates carry spin currents that are protected by spin-inactive stretching optical modes. As optical phonons interact more strongly than acoustic phonons, side buckling and tilting optical base modes will be more strongly associated with decoherence, which allows for the two terminal spin filtering effects found in CISS.

Chiral Single-Molecule Potentiometers Based on Stapled ortho- Oligo(phenylene)ethynylenes.

We report on the chemical design of chiral molecular junctions with stress-dependent conductance, whose helicity is maintained during the stretching of a single molecule junction due to the stapling of both ends of the inner helix. In the reported compounds, different conductive pathways are observed, with clearly different conductance values and plateau-length distributions, attributed to different conformations of the helical structures. The large chiro-optical responses and the potential use of these molecules as unimolecular spin filters have been theoretically proved using state-of-the-art Density Functional Theory (DFT) calculations, including a fully ab-initio estimation of the CISS-originating spin polarization which is done, for the first time, for a realistic molecular system.

Chiral-Induced Spin Selectivity and Non-equilibrium Spin Accumulation in Molecules and Interfaces: A First-Principles Study.

Electrons moving through chiral molecules are selected according to their spin orientation and the helicity of the molecule, an effect known as chiral-induced spin selectivity (CISS). The underlying physical mechanism is not yet completely understood. To help elucidate this mechanism, a non-equilibrium Green's function method, combined with a Landauer approach and density functional theory, is applied to carbon helices contacted by gold electrodes, resulting in spin polarization of transmitted electrons. Spin polarization is also observed in the non-equilibrium electronic structure of the junctions. While this spin polarization is small, its sign changes with the direction of the current and with the handedness of the molecule. While these calculations were performed with a pure exchange-correlation functional, previous studies suggest that computationally more expensive hybrid functionals may lead to considerably larger spin polarization in the electronic structure. Thus, non-equilibrium spin polarization could be a key component in understanding the CISS mechanism.

A Chirality-Based Quantum Leap.

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Theory of Chirality Induced Spin Selectivity: Progress and Challenges.

A critical overview of the theory of the chirality-induced spin selectivity (CISS) effect, that is, phenomena in which the chirality of molecular species imparts significant spin selectivity to various electron processes, is provided. Based on discussions in a recently held workshop, and further work published since, the status of CISS effects-in electron transmission, electron transport, and chemical reactions-is reviewed. For each, a detailed discussion of the state-of-the-art in theoretical understanding is provided and remaining challenges and research opportunities are identified.

Spin Fano Resonances in Chiral Molecules: An Alternative Mechanism for the CISS Effect and Experimental Implications.

Experiments on spin transport through a chiral molecule demonstrated the attainment of significant spin polarization, demanding a theoretical explanation. We report the emergence of spin Fano resonances as a mechanism in the chiral-induced spin-selectivity (CISS) effect associated with transport through a chiral polyacetylene molecule. Initializing electrons through optical excitation, we derive the Fano resonance formula for the spin polarization. Computations reveal that quasidegeneracy is common in this complex molecular system. A remarkable phenomenon is the generation of pronounced spin Fano resonances due to the contributions of two near-degeneracy states. We also find that the Fano resonance width increases linearly with the coupling strength between the molecule and the lead. Our findings provide another mechanism to explain the experimental observations and lead to new insights into the role of the CISS effect in complex molecules from the perspective of transport and spin polarization resonance, paving the way for chiral molecule-based spintronics applications.

Influence of Electronic Structure Modeling and Junction Structure on First-Principles Chiral Induced Spin Selectivity.

We have carried out a comprehensive study of the influence of electronic structure modeling and junction structure description on the first-principles calculation of the spin polarization in molecular junctions caused by the chiral induced spin selectivity (CISS) effect. We explore the limits and the sensitivity to modeling decisions of a Landauer/Green's function/two-component density functional theory approach to CISS. We find that although the CISS effect is entirely attributed in the literature to molecular spin filtering, spin-orbit coupling being partially inherited from the metal electrodes plays an important role in our calculations on ideal carbon helices, even though this effect cannot explain the experimental conductance results. Its magnitude depends considerably on the shape, size, and material of the metal clusters modeling the electrodes. Also, a pronounced dependence on the specific description of exchange interaction and spin-orbit coupling is manifest in our approach. This is important because the interplay between exchange effects and spin-orbit coupling may play an important role in the description of the junction magnetic response. Our calculations are relevant for the whole field of spin-polarized electron transport and electron transfer, because there is still an open discussion in the literature about the detailed underlying mechanism and the magnitude of physical parameters that need to be included to achieve a consistent description of the CISS effect: seemingly good quantitative agreement between simulation and the experiment can be caused by error compensation, because spin polarization as contained in a Landauer/Green's function/two-component density functional theory approach depends strongly on computational and structural parameters.

An Ideal Spin Filter: Long-Range, High-Spin Selectivity in Chiral Helicoidal 3-Dimensional Metal Organic Frameworks.

An enantiopure, conductive, and paramagnetic crystalline 3-D metal-organic framework (MOF), based on Dy(III) and the l-tartrate chiral ligand, is proved to behave as an almost ideal electron spin filtering material at room temperature, transmitting one spin component only, leading to a spin polarization (SP) power close to 100% in the ±2 V range, which is conserved over a long spatial range, larger than 1 µm in some cases. This impressive spin polarization capacity of this class of nanostructured materials is measured by means of magnetically polarized conductive atomic force microscopy and is attributed to the Chirality-Induced Spin Selectivity (CISS) effect of the material arising from a multidimensional helicity pattern, the inherited chirality of the organic motive, and the enhancing influence of Dy(III) ions on the CISS effect, with large spin-orbit coupling values. Our results represent the first example of a MOF-based and CISS-effect-mediated spin filtering material that shows a nearly perfect SP. These striking results obtained with our robust and easy-to-synthesize chiral MOFs constitute an important step forward in to improve the performance of spin filtering materials for spintronic device fabrication.

Coherence preservation and electron-phonon interaction in electron transfer in DNA.

We analyze the influence of electron-phonon (e-ph) interaction in a model for electron transfer (ET) processes in DNA in terms of the envelope function approach for spinless electrons. We are specifically concerned with the effect of e-ph interaction on the coherence of the ET process and how to model the interaction of DNA with phonon reservoirs of biological relevance. We assume that the electron bearing orbitals are half filled and derive the physics of e-ph coupling in the vicinity in reciprocal space. We find that at half filling, the acoustical modes are decoupled to ET at first order, while optical modes are predominant. The latter are associated with inter-strand vibrational modes in consistency with previous studies involving polaron models of ET. Coupling to acoustic modes depends on electron doping of DNA, while optical modes are always coupled within our model. Our results yield e-ph coupling consistent with estimates in the literature, and we conclude that large polarons are the main result of such e-ph interactions. This scenario will have strong consequences on decoherence of ET under physiological conditions due to relative isolation from thermal equilibration of the ET mechanism.

Enantiospecific Response in Cross-Polarization Solid-State Nuclear Magnetic Resonance of Optically Active Metal Organic Frameworks.

We report herein on a NMR-based enantiospecific response for a family of optically active metal-organic frameworks. Cross-polarization of the 1H-13C couple was performed, and the intensities of the 13C nuclei NMR signals were measured to be different for the two enantiomers. In a direct-pulse experiment, which prevents cross-polarization, the intensity difference of the 13C NMR signals of the two nanostructured enantiomers vanished. This result is due to changes of the nuclear spin relaxation times due to the electron spin spatial asymmetry induced by chemical bond polarization involving a chiral center. These experiments put forward on firm ground that the chiral-induced spin selectivity effect, which induces chemical bond polarization in the J-coupling, is the mechanism responsible for the enantiospecific response. The implications of this finding for the theory of this molecular electron spin polarization effect and the development of quantum biosensing and quantum storage devices are discussed.

Role of Exchange Interactions in the Magnetic Response and Intermolecular Recognition of Chiral Molecules.

The physical origin of the so-called chirality-induced spin selectivity (CISS) effect has puzzled experimental and theoretical researchers over the past few years. Early experiments were interpreted in terms of unconventional spin-orbit interactions mediated by the helical geometry. However, more recent experimental studies have clearly revealed that electronic exchange interactions also play a key role in the magnetic response of chiral molecules in singlet states. In this investigation, we use spin-polarized closed-shell density functional theory calculations to address the influence of exchange contributions to the interaction between helical molecules as well as of helical molecules with magnetized substrates. We show that exchange effects result in differences in the interaction properties with magnetized surfaces, shedding light into the possible origin of two recent important experimental results: enantiomer separation and magnetic exchange force microscopy with AFM tips functionalized with helical peptides.

Insight into the Origin of Chiral-Induced Spin Selectivity from a Symmetry Analysis of Electronic Transmission.

The chiral-induced spin selectivity (CISS) effect, which describes the spin-filtering ability of diamagnetic structures like DNA or peptides having chiral symmetry, has emerged in the past years as the central mechanism behind a number of important phenomena, like long-range biological electron transfer, enantiospecific electrocatalysis, and molecular recognition. Also, CISS-induced spin polarization has a considerable promise for new spintronic devices and the design of quantum materials. The CISS effect is attributed to spin-orbit coupling, but a sound theoretical understanding of the surprising magnitude of this effect in molecules without heavy atoms is currently lacking. We are taking an essential step into this direction by analyzing the importance of imaginary terms in the Hamiltonian as a necessary condition for nonvanishing spin polarization in helical structures. On the basis of first-principles calculations and analytical considerations, we perform a symmetry analysis of the key quantities determining transport probabilities of electrons of different spin orientations. These imaginary terms originate from the spin-orbit coupling, and they preserve the Hermitian nature of the Hamiltonian. Hence, they are not related to the breaking of time-reversal symmetry resulting from the fact that molecules are open systems in a junction. Our symmetry analysis helps to identify essential constraints in the theoretical description of the CISS effect. We further draw an analogy with the appearance of imaginary terms in simple models of barrier scattering, which may help understanding the unusually effective long-range electron transfer in biological systems.

Carrier Transport Engineering in Carbon Nanotubes by Chirality-Induced Spin Polarization.

Carbon nanotubes (CNTs), helically wrapped with single-stranded DNA, have recently emerged as a spin-filtering material. The inversion asymmetric helical potential of DNA creates a spin-filtering effect (commonly known as "chirality-induced spin selectivity" or CISS), which polarizes carrier spins in the nanotube. Thus, tuning of the DNA-CNT interaction is expected to affect carrier spins in nanotubes. The CISS effect induces spin polarization, which is coupled with the carrier's momentum direction, and therefore, in one-dimensional systems, such as nanotubes, momentum flip must be accompanied by a simultaneous spin flip. This spin momentum locking can have a profound impact on charge transport in nanotubes as backscattering due to phonons and disorder will be suppressed as these mechanisms are spin-independent. Here, we report DNA-CNT spin filters in which CNTs have been functionalized with two different classes of sequences, exhibiting different degrees of interaction with the CNT. They induce different degrees of spin polarization in the channel, with significant impact on temperature-dependent charge transport and interference phenomena arising from carrier backscattering. This work raises the intriguing possibility of engineering charge transport in nanotubes via CISS-induced spin polarization by tailor-made DNA sequences.

Electronic Structure and Triplet-Triplet Energy Transfer in Artificial Photosynthetic Antennas.

Three Pd(II) phthalocyanine-carotenoid dyads featuring chromophores linked by amide bonds were prepared in order to investigate the rate of triplet-triplet (T-T) energy transfer from the tetrapyrrole to the covalently attached carotenoid as a function of the number of conjugated double bonds in the carotenoid. Carotenoids having 9, 10 and 11 conjugated double bonds were studied. Transient absorption measurements show that intersystem crossing in the Pd(II) phthalocyanine takes place in 10 ps in each case and that T-T energy transfer occurs in 126, 81 and 132 ps in the dyads bearing 9, 10 and 11 double bond carotenoids, respectively. To identify the origin of this variation in T-T energy transfer rates, density functional theory (DFT) was used to calculate the T-T electronic coupling in the three dyads. According to the calculations, the primary reason for the observed T-T energy transfer trend is larger T-T electronic coupling between the tetrapyrrole and the 10-double bond carotenoid. A methyl group adjacent to the amide linker that connects the Pd(II) phthalocyanine and the carotenoid in the 9 and 11-double bond carotenoids is absent in the 10-double bond carotenoid, and this difference alters its electronic structure to increase the coupling.

Chirality-Induced Electron Spin Polarization and Enantiospecific Response in Solid-State Cross-Polarization Nuclear Magnetic Resonance.

NMR-based techniques are supposed to be incapable of distinguishing pure crystalline chemical enantiomers. However, through systematic studies of cross-polarization magic angle spinning (CP-MAS) NMR in a series of amino acids, we have found a rather unexpected behavior in the intensity pattern of optical isomers in hydrogen/nitrogen nuclear polarization transfer that would allow the use of CP NMR as a nondestructive enantioselective detection technique. In all molecules considered, the d isomer yields higher intensity than the l form, while the chemical shift for all nuclei involved remains unchanged. We attribute this striking result to the onset of electron spin polarization, accompanying bond charge polarization through a chiral center, a secondary mechanism for polarization transfer that is triggered only in the CP experimental setup. Electron spin polarization is due to the chiral-induced spin selectivity effect (CISS), which creates an enantioselective response, analogous to the one involved in molecular recognition and enantiospecific separation with achiral magnetic substrates. This polarization influences the molecular magnetic environment, modifying the longitudinal relaxation time T1 of 1H, and ultimately provoking the observed asymmetry in the enantiomeric response.

Thermal Decoherence and Disorder Effects on Chiral-Induced Spin Selectivity.

We use a nonlinear master equation formalism to account for thermal and disorder effects on spin-dependent electron transport in helical organic molecules coupled to two ideal leads. The inclusion of these two effects has important consequences in understanding the observed length and temperature dependence of spin polarization in experiments, which cannot be accounted for in a purely coherent tunneling model. Our approach considers a tight-binding helical Hamiltonian with disordered onsite energies to describe the resulting electronic states when low-frequency interacting modes break the electron coherence. The high-frequency fluctuating counterpart of these interactions, typical of intramolecular modes, is included by means of temperature-dependent thermally activated transfer probabilities in the master equation, which lead to hopping between localized states. We focus on the spin-dependent conductance and the spin-polarization in the linear regime (low voltage), which are analyzed as a function of the molecular length and the temperature of the system. Our results at room temperature agree well with experiments because our model predicts that the degree of spin-polarization increases for longer molecules. Also, this effect is temperature-dependent because thermal excitation competes with disorder-induced Anderson localization. We conclude that a transport mechanism based on thermally activated hopping in a disordered system can account for the unexpected behavior of the spin polarization.

Enhanced Magnetoresistance in Chiral Molecular Junctions.

Chirality-induced spin selectivity (CISS) is a recently discovered effect, whose precise microscopic origin has not yet been fully elucidated; it seems, however, clear that spin-orbit interaction plays a pivotal role. Various model Hamiltonian approaches have been proposed, suggesting a close connection between spin selectivity and filtering and helical symmetry. However, first-principles studies revealing the influence of chirality on the spin polarization are missing. To clearly demonstrate the influence of the helical conformation on the spin polarization properties, we have carried out spin-dependent Density-Functional Theory (DFT) based transport calculations for a model molecular system. It consists of α-helix and ß-strand conformations of an oligo-glycine peptide, which is bonded to a nickel electrode and to a gold electrode in a two-terminal setup, similar to a molecular junction or a local probe, for example, in STM or AFM configurations. We have found that the α-helix conformation displays a spin polarization, calculated through the intrinsic magneto-resistance of the junction, about 100-1000 times larger than the linear ß-strand, clearly demonstrating the crucial role played by the molecular helical geometry on the enhancement of spin polarization associated with the CISS effect.