Chirality-Dependent Electron Spin Filtering by Molecular Monolayers of Helicenes

Chirality-Induced Spin Selectivity: Analysis of Density Functional Theory Calculations

Chirality-induced spin selectivity (CISS), which was demonstrated in several molecular and material systems, has drawn much interest recently. The phenomenon, described in electron transport by the difference in the transport rate of electrons of opposite spins through a chiral system, is however not fully understood. Herein, we employed density functional theory in conjunction with spin-orbit coupling to evaluate the percent spin-polarization in a device setup with finite electrodes at zero bias, using an electron transport program developed in-house. To study the interface effects and the level of theory considered, we investigated a helical oligopeptide chain, an intrinsically chiral gold cluster, and a helicene model system that was previously studied (Zöllner et al. J. Chem. Theory Comput. 2020, 16, 7357-7371). We find that the magnitude of the spin-polarization depends on the chiral system-electrode interface that is modeled by varying the interface boundary between the system's regions, on the method of calculating spin-orbit coupling, and on the exchange-correlation functional, e.g., the amount of exact exchange in the hybrid functionals. In addition, to assess the effects of bias, we employ the nonequilibrium Green's function formalism in the Quantum Atomistix Toolkit program, showing that the spin-flip terms could be important in calculating the CISS effect. Although understanding CISS in comparison to experiment is still not resolved, our study provides intrinsic responses from first-principles calculations.

Insights into the Mechanism of Chiral-Induced Spin Selectivity: The Effect of Magnetic Field Direction and Temperature

Chiral oligopeptide monolayers are adsorbed on a ferromagnetic surface and their magnetoresistance is measured as a function of the angle between the magnetization of the ferromagnet and the surface normal. These measurements are conducted as a function of temperature for both enantiomers. The angle dependence is found to follow a changing trend with a period of 360°. Quantum simulations reveal that the angular distribution can be obtained only if the monolayer has significant effective spin orbit coupling (SOC), that includes contribution from the vibrations. The model shows that SOC only in the leads cannot reproduce the observed angular dependence. The simulation can reproduce the experiments if it included electron-phonon interactions and dissipation.

Synthesis of Aza[n]helicenes up to n = 19: Hydrogen-Bond-Assisted Solubility and Benzannulation Strategy

Synthetic challenges toward anomalous structures and electronic states often involve handling problems such as insolubility in common organic solvents and oxidative degradation under aerobic conditions. We designed benzo-annulated aza[n]helicenes, which benefit from both the suppressed elevation of highest occupied molecular orbital (HOMO) energies and high solubility due to hydrogen bonding with solvent molecules to overcome these challenges. This strategy enabled the synthesis of six new aza[n]helicenes ([n]AHs) of different lengths (n = 9-19) from acyclic precursors via one-pot intramolecular oxidative fusion reactions. The structures of all of the synthesized aza[n]helicenes were determined by X-ray diffraction (XRD) analysis, and their electrochemical potentials were measured by cyclic voltammetry. Among the synthesized aza[n]helicenes, [17]AH and [19]AH are the first heterohelicenes with a triple-layered helix. The noncovalent interaction (NCI) plots confirm the existence of an effective π-π interaction between the layers. The absorption and fluorescence spectra red-shifted as the helical lengths increased, without any distinct saturation points. The optical resolutions of N-butylated [9]AH, [11]AH, [13]AH, and [15]AH were accomplished, and their circular dichroism (CD) and circularly polarized luminescence (CPL) were measured. Thus, the structural, (chir)optical, and electrochemical properties of the aza[n]helicenes were comprehensively analyzed.

Helical interfacial modulation for perovskite photovoltaics

Hybrid metal halide perovskites have demonstrated remarkable performances in modern photovoltaics, although their stabilities remain limited. We assess the capacity to advance their properties by relying on interfacial modulators featuring helical chirality based on P,M-(1-methylene-3-methyl-imidazolium)[6]helicene iodides. We investigate their characteristics, demonstrating comparable charge injection for enantiomers and the racemic mixture. Overall, they maintain the resulting photovoltaic performance while improving operational stability, challenging the role of helical chirality in the interfacial modulation of perovskite solar cells.

Spin-Selective Electron Transport Through Single Chiral Molecules

The interplay between chirality and magnetism is a source of fascination among scientists for over a century. In recent years, chirality-induced spin selectivity (CISS) has attracted renewed interest. It is observed that electron transport through layers of homochiral molecules leads to a significant spin polarization of several tens of percent. Despite the abundant experimental evidence gathered through mesoscopic transport measurements, the exact mechanism behind CISS remains elusive. This study reports spin-selective electron transport through single helical aromatic hydrocarbons that are sublimed in vacuo onto ferromagnetic cobalt surfaces and examined with spin-polarized scanning tunneling microscopy (SP-STM) at a temperature of 5 K. Direct comparison of two enantiomers under otherwise identical conditions revealed magnetochiral conductance asymmetries of up to 50% when either the molecular handedness is exchanged or the magnetization direction of the STM tip or Co substrate is reversed. Importantly, the results rule out electron-phonon coupling and ensemble effects as primary mechanisms responsible for CISS.

Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State

We report a neutral high-spin diradical of chiral C2-symmetric bis[5]diazahelicene with ΔEST ≈ 0.4 kcal mol-1, as determined by EPR spectroscopy/SQUID magnetometry. The diradical is the most persistent among all high-spin aminyl radicals reported to date by a factor of 20, with a half-life of up to 6 days in 2-MeTHF at room temperature. Its triplet ground state and excellent persistence may be associated with the unique spin density distribution within the dihydrophenazine moiety, which characterizes two effective 3-electron C-N bonds analogous to the N-O bond of a nitroxide radical. The enantiomerically enriched (ee ≥ 94%) (MM)- and (PP)-enantiomers of the precursors to the diradicals are obtained by either preparative chiral supercritical fluid chromatography or resolution via functionalization with the chiral auxiliary of the C2-symmetric racemic tetraamine. The barrier for the racemization of the solid tetraamine is ΔG‡ = 43 ± 0.01 kcal mol-1 in the 483-523 K range. The experimentally estimated lower limit of the barrier for the racemization of a diradical, ΔG‡ ≥ 26 kcal mol-1 in 2-MeTHF at 293 K, is comparable to the DFT-determined barrier of ΔG‡ = 31 kcal mol-1 in the gas phase at 298 K. While the enantiomerically pure tetraamine displays strong chiroptical properties, with anisotropy factor |g| = |Δε|/ε = 0.036 at 376 nm, |g| ≈ 0.005 at 548 nm of the high-spin diradical is comparable to that recently reported triplet ground-state diradical dication. Notably, the radical anion intermediate in the generation of diradical exhibits a large SOMO-HOMO inversion, SHI = 35 kcal mol-1.

Enantioselective Adsorption on Magnetic Surfaces

From the beginning of molecular theory, the interplay of chirality and magnetism has intrigued scientists. There is still the question if enantiospecific adsorption of chiral molecules occurs on magnetic surfaces. Enantiomer discrimination was conjectured to arise from chirality-induced spin separation within the molecules and exchange interaction with the substrate's magnetization. Here, it is shown that single helical aromatic hydrocarbons undergo enantioselective adsorption on ferromagnetic cobalt surfaces. Spin and chirality sensitive scanning tunneling microscopy reveals that molecules of opposite handedness prefer adsorption onto cobalt islands with opposite out-of-plane magnetization. As mobility ceases in the final chemisorbed state, it is concluded that enantioselection must occur in a physisorbed transient precursor state. State-of-the-art spin-resolved ab initio simulations support this scenario by refuting enantio-dependent chemisorption energies. These findings demonstrate that van der Waals interaction should also include spin-fluctuations which are crucial for molecular magnetochiral processes.

Electron Spin Selective Iridium Electrocatalysts for the Oxygen Evolution Reaction

Highly efficient electrocatalysts for water electrolysis are crucial to the widespread commercialization of the technology and an important step forward toward a sustainable energy future. In this study, an alternative method for boosting the electrocatalytic activity toward the oxygen evolution reaction (OER) of a well-known electrocatalyst (iridium) is presented. Iridium nanoparticles (2.1 ± 0.2 nm in diameter) functionalized with chiral molecules were found to markedly enhance the activity of the OER when compared to unfunctionalized and achiral functionalized iridium nanoparticles. At a potential of 1.55 V vs Reference Hydrogen Electrode (RHE), chiral functionalized iridium nanoparticles exhibited an average 85% enhancement in activity with respect to unfunctionalized iridium nanoparticles compared to an average 13% enhancement for the achiral functionalized iridium nanoparticle. This activity enhancement is attributed to a spin-selective electron transfer mechanism taking place on the chiral functionalized catalysts, a characteristic induced by the chirality of the ligand. This alternative path for the OER drastically reduces the production of hydrogen peroxide, which was confirmed via a colorimetric method.

Chiral Induced Spin Selectivity

Since the initial landmark study on the chiral induced spin selectivity (CISS) effect in 1999, considerable experimental and theoretical efforts have been made to understand the physical underpinnings and mechanistic features of this interesting phenomenon. As first formulated, the CISS effect refers to the innate ability of chiral materials to act as spin filters for electron transport; however, more recent experiments demonstrate that displacement currents arising from charge polarization of chiral molecules lead to spin polarization without the need for net charge flow. With its identification of a fundamental connection between chiral symmetry and electron spin in molecules and materials, CISS promises profound and ubiquitous implications for existing technologies and new approaches to answering age old questions, such as the homochiral nature of life. This review begins with a discussion of the different methods for measuring CISS and then provides a comprehensive overview of molecules and materials known to exhibit CISS-based phenomena before proceeding to identify structure-property relations and to delineate the leading theoretical models for the CISS effect. Next, it identifies some implications of CISS in physics, chemistry, and biology. The discussion ends with a critical assessment of the CISS field and some comments on its future outlook.

Spin polarized electron dynamics enhance water splitting efficiency by yttrium iron garnet photoanodes: a new platform for spin selective photocatalysis

This work presents a spectroscopic and photocatalytic comparison of water splitting using yttrium iron garnet (Y3Fe5O12, YIG) and hematite (α-Fe2O3) photoanodes. Despite similar electronic structures, YIG significantly outperforms widely studied hematite, displaying more than an order of magnitude increase in photocurrent density. Probing the charge and spin dynamics by ultrafast, surface-sensitive XUV spectroscopy reveals that the enhanced performance arises from (1) reduced polaron formation in YIG compared to hematite and (2) an intrinsic spin polarization of catalytic photocurrents in YIG. Ultrafast XUV measurements show a reduction in the formation of surface electron polarons compared to hematite due to site-dependent electron-phonon coupling. This leads to spin polarized photocurrents in YIG where efficient charge separation occurs on the Td sub-lattice compared to fast trapping and electron/hole pair recombination on the Oh sub-lattice. These lattice-dependent dynamics result in a long-lived spin aligned hole population at the YIG surface, which is directly observed using XUV magnetic circular dichroism. Comparison of the Fe M2,3 and O L1-edges show that spin aligned holes are hybridized between O 2p and Fe 3d valence band states, and these holes are responsible for highly efficient, spin selective water oxidation by YIG. Together, these results point to YIG as a new platform for highly efficient, spin selective photocatalysis.

Switching the on-surface orientation of oxygen-functionalized helicene

Helicenes represent an important class of chiral organic material with promising optoelectronic properties. Hence, functionalization of surfaces with helicenes is a key step toward new organic materials devices. The deposition of a heterohelicene containing two furano groups and two hydroxyl groups onto copper(111) surface in ultrahigh vacuum leads to different adsorbate modifications. At low coverage and low temperature, the molecules tend to lie on the surface in order to maximize van der Waals contact with the substrate. Thermal treatment leads to deprotonation of the hydroxyl groups and in part into a reorientation from lying into a standing adsorbate mode.

Highly Conductive Topologically Chiral Molecular Knots as Efficient Spin Filters

Knot-like structures were found to have interesting magnetic properties in condensed matter physics. Herein, we report on topologically chiral molecular knots as efficient spintronic chiral material. The discovery of the chiral-induced spin selectivity (CISS) effect opens the possibility of manipulating the spin orientation with soft materials at room temperature and eliminating the need for a ferromagnetic electrode. In the chiral molecular trefoil knot, there are no stereogenic carbon atoms, and chirality results from the spatial arrangements of crossings in the trefoil knot structures. The molecules show a very high spin polarization of nearly 90%, a conductivity that is higher by about 2 orders of magnitude compared with that of other chiral small molecules, and enhanced thermal stability. A plausible explanation for these special properties is provided, combined with model calculations, that supports the role of electron-electron interaction in these systems.

Spin-Dependent Destructive and Constructive Quantum Interference Associated with Chirality-Induced Spin Selectivity in Single Circular Helix Molecules

Chirality-induced spin selectivity (CISS) effect in straight helical molecules has received intense studies in past decade; however, the CISS effect in circular helical molecules (CHMs) has still rarely been explored. Here, we have constructed single CHMs having chirality-induced spin-orbit coupling (SOC) and connected by two nonmagnetic leads and successfully gained the required conditions for CISS effect occurring in CHMs for the first time. Our results uncover that only when the CHMs form a closed loop and when the lattice positions are coupled asymmetrically with both leads does the CISS effect occur. More importantly, the CISS-associated spin-dependent destructive and constructive quantum interference (QI) together with their phase transition appears in CHMs. The combination of CISS effect and spin-dependent QI phenomena opens up a new door to understand the underlying physics of the CISS effect in helical molecules.

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%.

Weakly Self-Assembled [6]Helicenes: Circularly Polarized Light and Spin Filtering Properties

Self-assembling features, chiroptical activity, and spin filtering properties are reported for 2,15- and 4,13-disubstituted [6]helicenes decorated in their periphery with 3,4,5-tris(dodecyloxy)-N-(4-ethynylphenyl)benzamide moieties. The weak non-covalent interaction between these units conditions the corresponding circularly polarized luminescence and spin polarization. The self-assembly is overall weak for these [6]helicene derivatives that, despite the formation of H-bonding interactions between the amide groups present in the peripheral moieties, shows very similar chiroptical properties both in the monomeric or aggregated states. This effect could be explained by considering the steric effect that these groups could generate in the growing of the corresponding aggregate formed. Importantly, the self-assembling features also condition chiral induced spin selectivity (CISS effect), with experimental spin polarization (SP) values found between 35-40 % for both systems, as measured by magnetic-conducting atomic force microscopy (AFM) technique.

Spin dynamics and chirality induced spin selectivity

By now, it is well known that chiral molecules can affect the spin of electrons passing through. In addition, the magnetization of, e.g., nanomagnets covered by chiral molecules can be affected by the presence of molecules. We are studying the mechanisms that explain various observations involving combinations of magnets and chiral molecules. We find that there exists a molecule induced contribution to the magnetic anisotropy of the magnets. Out of equilibrium, when electrons are actually being transported through a nano-magnet covered with chiral molecules, a molecule induced torque acting on the magnetization is emerging. It is of the spin-transfer-torque kind, already discussed in other parts of spintronics. This current induced torque can help explain the observed breaking of the Onsager reciprocity principle in experiments involving magnets and chiral molecules.

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.

Temperature activated chiral induced spin selectivity

Recent experiments performed on chiral molecules, comprising transition metal or rare earth elements, indicate temperature reinforced chiral induced spin selectivity. In these compounds, spin selectivity is suppressed in the low temperature regime but grows by one to several orders of magnitude as the temperature is increased to room temperature. By relating temperature to nuclear motion, it is proposed that nuclear displacements acting on the local spin moments, through indirect exchange interactions, generate an anisotropic magnetic environment that is enhanced with temperature. The induced local anisotropy field serves as the origin of a strongly increased spin selectivity at elevated temperature.

Interplay of structural chirality, electron spin and topological orbital in chiral molecular spin valves

Chirality has been a property of central importance in physics, chemistry and biology for more than a century. Recently, electrons were found to become spin polarized after transmitting through chiral molecules, crystals, and their hybrids. This phenomenon, called chirality-induced spin selectivity (CISS), presents broad application potentials and far-reaching fundamental implications involving intricate interplays among structural chirality, topological states, and electronic spin and orbitals. However, the microscopic picture of how chiral geometry influences electronic spin remains elusive, given the negligible spin-orbit coupling (SOC) in organic molecules. In this work, we address this issue via a direct comparison of magnetoconductance (MC) measurements on magnetic semiconductor-based chiral molecular spin valves with normal metal electrodes of contrasting SOC strengths. The experiment reveals that a heavy-metal electrode provides SOC to convert the orbital polarization induced by the chiral molecular structure to spin polarization. Our results illustrate the essential role of SOC in the metal electrode for the CISS spin valve effect. A tunneling model with a magnetochiral modulation of the potential barrier is shown to quantitatively account for the unusual transport behavior.

Current State of Computational Modeling of Nanohelicenes

This review considers the works that focus on various aspects of the theoretical description of nanohelicenes (other equivalent names are graphene spirals, graphene helicoid, helical graphene nanoribbon, or helical graphene)-a promising class of one-dimensional nanostructures. The intrinsic helical topology and continuous π-system lead to the manifestation of unique optical, electronic, and magnetic properties that are also highly dependent on axial and torsion strains. In this paper, it was shown that the properties of nanohelicenes are mainly associated with the peripheral modification of the nanohelicene ribbon. We have proposed a nomenclature that enables the classification of all nanohelicenes as modifications of some prototype classes.

Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

Highly Durable Spin Filter Switching Based on Self-Assembled Chiral Molecular Motor

Chiral molecules have recently received renewed interest as highly efficient sources of spin-selective charge emission known as chiral-induced spin selectivity (CISS), which potentially offers a fascinating utilization of organic chiral materials in novel solid-state spintronic devices. However, a practical use of CISS remains far from completion, and rather fundamental obstacles such as (i) external controllability of spin, (ii) function durability, and (iii) improvement of spin-polarization efficiency have not been surmounted to date. In this study, these issues are addressed by developing a self-assembled monolayer (SAM) of overcrowded alkene (OCA)-based molecular motor. With this system, it is successfully demonstrated that the direction of spin polarization can be externally and repeatedly manipulated in an extremely stable manner by switching the molecular chirality, which is achieved by a formation of the covalent bonds between the molecules and electrode. In addition, it is found that a higher stereo-ordering architecture of the SAM of OCAs tailored by mixing them with simple alkanethiols considerably enhances the efficiency of spin polarization per a single OCA molecule. All these findings provide the creditable feasibility study for strongly boosting development of CISS-based spintronic devices that can simultaneously fulfill the controllability, durability, and high spin-polarization efficiency.

Spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on Cu(111) surfaces

It has been demonstrated previously that electrons interact differently with chiral molecules depending on their polarization. For enantiomeric pure monolayers of heptahelicene, opposite asymmetries in spin polarization were reported and attributed to the so-called chirality-induced spin selectivity effect. However, these promising proof-of-concept photoemission experiments lack the angular and energy resolution that could provide the necessary insights into the mechanism of this phenomenon. In order to fill in the missing gaps, we provide a detailed spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on a Cu(111) substrate. Throughout the large accessible energy and angle range, no chirality induced spin asymmetry in photoemission could be observed. Possible reasons for the absence of signatures of the spin-dependent electron transmission through the chiral molecular layer are briefly discussed.

Chirality-Induced Spin Selectivity (CISS) Effect: Magnetocurrent-Voltage Characteristics with Coulomb Interactions I

One of the manifestations of chirality-induced spin selectivity (CISS) is the appearance of a magnetocurrent. Magnetocurrent is the observation that the charge currents at finite bias in a two terminal device for opposite magnetizations of one of the leads differ. Magnetocurrents can only occur in the presence of interactions of the electrons either with vibrational modes or among themselves through the Coulomb interaction. In experiments on chiral molecules assembled in monolayers, the magnetocurrent seems to be dominantly cubic (odd) in bias voltage while theory finds a dominantly even bias voltage dependence. Thus far, theoretical work has predicted a magnetocurrent which is even bias. Here we analyze the bias voltage dependence of the magnetocurrent numerically and analytically involving the spin-orbit and Coulomb interactions (through the Hartree-Fock and Hubbard One approximations). For both approximations it is found that for strong Coulomb interactions the magnetocurrent is dominantly odd in bias voltage, confirming the symmetry observed in experiment.

Spin Polarization of Polyalanine Molecules in 2D and Dimer-Row Assemblies Adsorbed on Magnetic Substrates: The Role of Coupling, Chirality, and Coordination

Propagation of electrons along helical molecules adsorbed on surfaces comes along with a robust spin polarization effect called chirality induced spin selectivity CISS. However, experiments on the molecular scale that allow a true correlation of spin effects with the molecular structure are quite rare. Here we have studied the structure of self-assembled chiral molecules and the electronic transmission and spin polarization of the current through the system by means of ambient scanning tunneling microscopy and spectroscopy in heterostructures of various α-helix polyalanine-based molecules (PA) adsorbed on Al₂O₃/Pt/Au/Co/Au substrates with perpendicular magnetic anisotropy. We have found a phase separation of the molecules into well-ordered enantiopure 2D hexagonal phases and quasi-1D heterochiral-dimer structures, which allows for the analysis of the spin polarization with almost atomic precision of PA in different phases. The spin polarization reaches up to 75% for chemisorbed molecules arranged in a hexagonal phase. On the contrary, for weakly coupled PA molecules without cysteine anchoring groups in a quasi-1D phase, a spin polarization of around 50% was found. Our results show that both the intermolecular interaction as well as the coupling to the substrate are important and point out that collective effects within the molecules and at the interfaces are required to achieve a high chiral induced spin selectivity.

Chiral-Induced Spin Selectivity in Photon-Coupled Achiral Matters

Chiral-induced spin selectivity is a phenomenon in which electron spins are polarized as they are transported through chiral molecules, and the spin polarization depends on the handedness of the chiral molecule. In this study, we show that spin selectivity can be realized in achiral materials by strongly coupling electrons to a circularly polarized mode of an optical cavity or waveguide. Through the investigation of spin-dependent electron transport in a two-terminal setup using the nonequilibrium Green's function approach, it is found that a large spin polarization can be obtained if the rate of dephasing is sufficiently small and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the optical mode. To obtain a wider range of energies for a large spin polarization, chiral molecules can be combined with light-matter interactions. To demonstrate this, the spin polarization of electrons transported through a helical molecule strongly coupled to a circularly polarized optical mode is evaluated.

Spin-Dependent Momentum Conservation of Electron-Phonon Scattering in Chirality-Induced Spin Selectivity

The elucidation of the mechanisms underpinning chirality-induced spin selectivity remains an outstanding scientific challenge. Here we consider the role of delocalized phonon modes in electron transport in chiral structures and demonstrate that spin selectivity can originate from spin-dependent energy and momentum conservation in electron-phonon scattering events. While this mechanism is robust to the specific nature of the vibrational modes, the degree of spin polarization depends on environmental factors, such as the specific temperature and phonon relaxation rates, as well as the presence of external driving fields. This parametric dependence is used to present experimentally testable predictions of our model.

Easily processable spin filters: exploring the chiral induced spin selectivity of bowl-shaped chiral subphthalocyanines

Herein a new class of spin filters based on subphthalocyanines is reported. We measure the CISS effect by means of magnetic conductive probe atomic force microscopy (mc-AFM). Remarkably, the resulting devices show spin polarizations (SPs) as high as ca. 50%.

Controlling anisotropic properties by manipulating the orientation of chiral small molecules

Chiral π-conjugated molecules bring new functionality to technological applications and represent an exciting, rapidly expanding area of research. Their functional properties, such as the absorption and emission of circularly polarized light or the transport of spin-polarized electrons, are highly anisotropic. As a result, the orientation of chiral molecules critically determines the functionality and efficiency of chiral devices. Here we present a strategy to control the orientation of a small chiral molecule (2,2'-dicyano[6]helicene) by the use of organic and inorganic templating layers. Such templating layers can either force 2,2'-dicyano[6]helicene to adopt a face-on orientation and self-assemble into upright supramolecular columns oriented with their helical axis perpendicular to the substrate, or an edge-on orientation with parallel-lying supramolecular columns. Through such control, we show that low- and high-energy chiroptical responses can be independently 'turned on' or 'turned off'. The templating methodologies described here provide a simple way to engineer orientational control and, by association, anisotropic functional properties of chiral molecular systems for a range of emerging technologies.

[5]-Helistatins: Tubulin-Binding Helicenes with Antimitotic Activity

Helicenes are high interest synthetic targets with unique conjugated helical structures that have found important technological applications. Despite this interest, helicenes have had limited impact in chemical biology. Herein, we disclose a first-in-class antimitotic helicene, helistatin 1 (HA-1), where the helicene scaffold acts as a structural mimic of colchicine, a known antimitotic drug. The synthesis proceeds via sequential Pd-catalyzed coupling reactions and a π-Lewis acid cycloisomerization mediated by PtCl₂. HA-1 was found to block microtubule polymerization in both cell-free and live cell assays. Not only does this demonstrate the feasibility of using helicenes as bioactive scaffolds against protein targets, but also suggests wider potential for the use of helicenes as isosteres of biaryls or cis-stilbenes-themselves common drug and natural product scaffolds. Overall, this study further supports future opportunities for helicenes for a range of chemical biological applications.

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen-vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level.

Deposition of Chiral Heptahelicene Molecules on Ferromagnetic Co and Fe Thin-Film Substrates

The discovery of chirality-induced spin selectivity (CISS), resulting from an interaction between the electron spin and handedness of chiral molecules, has sparked interest in surface-adsorbed chiral molecules due to potential applications in spintronics, enantioseparation, and enantioselective chemical or biological processes. We study the deposition of chiral heptahelicene by sublimation under ultra-high vacuum onto bare Cu(111), Co bilayer nanoislands on Cu(111), and Fe bilayers on W(110) by low-temperature spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS). In all cases, the molecules remain intact and adsorb with the proximal phenanthrene group aligned parallel to the surface. Three degenerate in-plane orientations on Cu(111) and Co(111), reflecting substrate symmetry, and only two on Fe(110), i.e., fewer than symmetry permits, indicate a specific adsorption site for each substrate. Heptahelicene physisorbs on Cu(111) but chemisorbs on Co(111) and Fe(110) bilayers, which nevertheless remain for the sub-monolayer coverage ferromagnetic and magnetized out-of-plane. We are able to determine the handedness of individual molecules chemisorbed on Fe(110) and Co(111), as previously reported for less reactive Cu(111). The demonstrated deposition control and STM/STS imaging capabilities for heptahelicene on Co/Cu(111) and Fe/W(110) substrate systems lay the foundation for studying CISS in ultra-high vacuum and on the microscopic level of single molecules in controlled atomic configurations.

Spin-Polarized Photoemission from Chiral CuO Catalyst Thin Films

The chirality-induced spin selectivity (CISS) effect facilitates a paradigm shift for controlling the outcome and efficiency of spin-dependent chemical reactions, for example, photoinduced water splitting. While the phenomenon is established in organic chiral molecules, its emergence in chiral but inorganic, nonmolecular materials is not yet understood. Nevertheless, inorganic spin-filtering materials offer favorable characteristics, such as thermal and chemical stability, over organic, molecular spin filters. Chiral cupric oxide (CuO) thin films can spin polarize (photo)electron currents, and this capability is linked to the occurrence of the CISS effect. In the present work, chiral CuO films, electrochemically deposited on partially UV-transparent polycrystalline gold substrates, were subjected to deep-UV laser pulses, and the average spin polarization of photoelectrons was measured in a Mott scattering apparatus. By energy resolving the photoelectrons and changing the photoexcitation geometry, the energy distribution and spin polarization of the photoelectrons originating from the Au substrate could be distinguished from those arising from the CuO film. The findings reveal that the spin polarization is energy dependent and, furthermore, indicate that the measured polarization values can be rationalized as a sum of an intrinsic spin polarization in the chiral oxide layer and a contribution via CISS-related spin filtering of electrons from the Au substrate. The results support efforts toward a rational design of further spin-selective catalytic oxide materials.

Vectorial Electron Spin Filtering by an All-Chiral Metal-Molecule Heterostructure

The discovery of the electrons' chiral induced spin selective transmission (CISS) through chiral molecules has opened the pathway for manipulating spin transport in nonmagnetic structures on the nanoscale. CISS has predominantly been explored in structurally helical molecules on surfaces, where the spin selectivity affects only the spin polarization of the electrons along their direction of propagation. Here, we demonstrate a spin selective electron transmission for the point-chiral molecule 3-methylcyclohexanone (3-MCHO) adsorbed on the chiral Cu(643)^R surface. Using spin- and momentum-resolved photoelectron spectroscopy, we detect a spin-dependent electron transmission through a single layer of 3-MCHO molecules that depends on all three components of the electrons' spin. Crucially, exchanging the enantiomers alters the electrons' spin component oriented parallel to the terraces of the Cu(643)^R surface. The findings are attributed to the enantiomer-specific adsorption configuration on the surface. This opens the intriguing opportunity to selectively tune CISS by the enantiospecific molecule-surface interaction in all-chiral heterostructures.

The contribution of intermolecular spin interactions to the London dispersion forces between chiral molecules

Dispersion interactions are one of the components of van der Waals (vdW) forces that play a key role in the understanding of intermolecular interactions in many physical, chemical, and biological processes. The theory of dispersion forces was developed by London in the early years of quantum mechanics. However, it was only in the 1960s that it was recognized that for molecules lacking an inversion center, such as chiral and helical molecules, there are chirality-sensitive corrections to the dispersion forces proportional to the rotatory power known from the theory of circular dichroism and with the same distance scaling law R^-6 as the London energy. The discovery of the chirality-induced spin selectivity effect in recent years has led to an additional twist in the study of chiral molecular systems, showing a close relation between spin and molecular geometry. Motivated by it, we propose in this investigation to describe the mutual induction of charge and spin-density fluctuations in a pair A-B of chiral molecules by a simple physical model. The model assumes that the same fluctuating electric fields responsible for vdW forces can induce a magnetic response via a Rashba-like term so that a spin-orbit field acting on molecule B is generated by the electric field arising from charge density fluctuations in molecule A (and vice versa). Within a second-order perturbative approach, these contributions manifest as an effective intermolecular exchange interaction. Although expected to be weaker than the standard London forces, these interactions display the same R^-6 distance scaling.

Enhancement of electrocatalytic oxygen evolution by chiral molecular functionalization of hybrid 2D electrodes

A sustainable future requires highly efficient energy conversion and storage processes, where electrocatalysis plays a crucial role. The activity of an electrocatalyst is governed by the binding energy towards the reaction intermediates, while the scaling relationships prevent the improvement of a catalytic system over its volcano-plot limits. To overcome these limitations, unconventional methods that are not fully determined by the surface binding energy can be helpful. Here, we use organic chiral molecules, i.e., hetero-helicenes such as thiadiazole-[7]helicene and bis(thiadiazole)-[8]helicene, to boost the oxygen evolution reaction (OER) by up to ca. 130 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D Ni- and NiFe-based catalysts via a spin-polarization mechanism. Our results show that chiral molecule-functionalization is able to increase the OER activity of catalysts beyond the volcano limits. A guideline for optimizing the catalytic activity via chiral molecular functionalization of hybrid 2D electrodes is given.

Chiral molecular intercalation superlattices

The discovery of chiral-induced spin selectivity (CISS) opens up the possibility to manipulate spin orientation without external magnetic fields and enables new spintronic device designs^1-4. Although many approaches have been explored for introducing CISS into solid-state materials and devices, the resulting systems so far are often plagued by high inhomogeneity, low spin selectivity or limited stability, and have difficulties in forming robust spintronic devices^5-8. Here we report a new class of chiral molecular intercalation superlattices (CMIS) as a robust solid-state chiral material platform for exploring CISS. The CMIS were prepared by intercalating layered two-dimensional atomic crystals (2DACs) (such as TaS₂ and TiS₂) with selected chiral molecules (such as R-α-methylbenzylamine and S-α-methylbenzylamine). The X-ray diffraction and transmission electron microscopy studies demonstrate highly ordered superlattice structures with alternating crystalline atomic layers and self-assembled chiral molecular layers. Circular dichroism studies show clear chirality-dependent signals between right-handed (R-) and left-handed (S-) CMIS. Furthermore, by using the resulting CMIS as the spin-filtering layer, we create spin-selective tunnelling junctions with a distinct chirality-dependent tunnelling current, achieving a tunnelling magnetoresistance ratio of more than 300 per cent and a spin polarization ratio of more than 60 per cent. With a large family of 2DACs of widely tunable electronic properties and a vast selection of chiral molecules of designable structural motifs, the CMIS define a rich family of artificial chiral materials for investigating the CISS effect and capturing its potential for new spintronic devices.

Chiral-Induced Spin Selectivity in Capacitively Coupled Molecules

The chiral-induced spin selectivity (CISS) effect is usually associated with electrons passing through chiral molecules. There are a number of experiments where external electrical fields rearrange electrons inside otherwise isolated molecules, allegedly also generating spin polarization of the molecule. There are theoretical suggestions of pairs of molecules generating spin polarizations in each other due to fluctuating electrical dipoles. This paper critically theoretically evaluates such spin-dependent charge reorganization (SDCR) effects. We find that such effects are not ruled out in principle, but they are very small and probably not detectable.

Electron transfer and spin-orbit coupling: Can nuclear motion lead to spin selective rates?

We investigate a spin-boson inspired model of electron transfer, where the diabatic coupling is given by a position-dependent phase, e^iWx. We consider both equilibrium and nonequilibrium initial conditions. We show that, for this model, all equilibrium results are completely invariant to the sign of W (to infinite order). However, the nonequilibrium results do depend on the sign of W, suggesting that photo-induced electron transfer dynamics with spin-orbit coupling can exhibit electronic spin polarization (at least for some time).

Mutual Monomer Orientation To Bias the Supramolecular Polymerization of [6]Helicenes and the Resulting Circularly Polarized Light and Spin Filtering Properties

We report on the synthesis and self-assembly of 2,15- and 4,13-disubstituted carbo[6]helicenes 1 and 2 bearing 3,4,5-tridodecyloxybenzamide groups. The self-assembly of these [6]helicenes is strongly influenced by the substitution pattern in the helicene core that affects the mutual orientation of the monomeric units in the aggregated form. Thus, the 2,15-substituted derivative 1 undergoes an isodesmic supramolecular polymerization forming globular nanoparticles that maintain circularly polarized light (CPL) with g_lum values as high as 2 × 10^–2. Unlike carbo[6]helicene 1, the 4,13-substituted derivative 2 follows a cooperative mechanism generating helical one-dimensional fibers. As a result of this helical organization, [6]helicene 2 exhibits a unique modification in its ECD spectral pattern showing sign inversion at low energy, accompanied by a sign change of the CPL with g_lum values of 1.2 × 10^–3, thus unveiling an example of CPL inversion upon supramolecular polymerization. These helical supramolecular structures with high chiroptical activity, when deposited on conductive surfaces, revealed highly efficient electron-spin filtering abilities, with electron spin polarizations up to 80% for 1 and 60% for 2, as measured by magnetic conducting atomic force microscopy.

Spontaneous chiral resolution of pentahelicene molecules on Cd(0001)

Chiral resolution is of fundamental importance to conglomerate or racemate crystallization. Here we demonstrate that the spontaneous chiral resolution of pentahelicene racemates occurred in the monolayer domains. When deposited on a Cd(0001) surface, pentahelicene molecules crystallize into a commensurate (6 × 6)R0° structure built mainly from homochiral trimers. Spontaneous chirality separation takes place in the form of opposite mirror domains, where 2D enantiomorphism is not expressed by the oblique adlattice, but by the supramolecular chirality of the pentahelicene trimers. Furthermore, annealing the sample or extreme close-packing lead to the presence of lattice handedness through the formation of a porous network structure or an edge-on phase. These results provide valuable insight for 2D conglomerate crystallization and stereochemical recognition.

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.

Chirality-Induced Magnetoresistance Due to Thermally Driven Spin Polarization

Chirality-induced current-perpendicular-to-plane magnetoresistance (CPP-MR) originates from current-induced spin polarization in molecules. The current-induced spin polarization is widely recognized as a fundamental principle of chiral-induced spin selectivity (CISS). In this study, we investigate chirality-induced current-in-plane magnetoresistance (CIP-MR) in a chiral molecule/ferromagnetic metal bilayer at room temperature. In contrast to CPP-MR, CIP-MR observed in the present study requires no bias charge current through the molecule. The temperature dependence of CIP-MR suggests that thermally driven spontaneous spin polarization in chiral molecules is the key to the observed MR. The novel MR is consistent with recent CISS-related studies, that is, chiral molecules in contact with a metallic surface possess a finite spin polarization.

Doubly linked chiral phenanthrene oligomers for homogeneously π-extended helicenes with large effective conjugation length

Helically twisted conductive nanocarbon materials are applicable to optoelectronic and electromagnetic molecular devices working on the nanometer scale. Herein, we report the synthesis of per-peri-perbenzo[5]- and [9]helicenes in addition to previously reported π-extended [7]helicene. The homogeneously π-extended helicenes can be regarded as helically fused oligo-phenanthrenes. The HOMO−LUMO gap decreased significantly from 2.14 to 1.15 eV with increasing helical length, suggesting the large effective conjugation length (ECL) of the π-extended helical framework. The large ECL of π-extended helicenes is attributed to the large orbital interactions between the phenanthrene subunits at the 9- and 10-positions, which form a polyene-like electronic structure. Based on the experimental results and DFT calculations, the ultrafast decay dynamics on the sub-picosecond timescale were attributed to the low-lying conical intersection.

Helically twisted conductive nanocarbon materials are applicable to optoelectronic and electromagnetic molecular devices but the design of nanocarbons with an absorption edge in the low energy region is challenging. Here, the authors report the synthesis of a helically fused oligophenanthrenes and demonstrate an increased effective conjugation length leading to an absorption edge in the NIR region.