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

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.

Persistent Chirality-Induced Spin-Selectivity Effect in Circular Helix Molecules

The spin-orbit coupling (SOC), the dynamics of the nonequilibrium transport process, and the breaking of time-reversal and space-inversion symmetries have been regarded as key factors for the emergence of chirality-induced spin selectivity (CISS) and chirality-dependent spin currents in helix molecules. In this work, we demonstrated the generation of persistent CISS currents in various circular single-stranded DNAs and 310-helix proteins for the first time, regardless of whether an external magnetic flux is applied or not. This new CISS effect presents only in equilibrium transport processes, distinct from the traditional CISS observed in nonequilibrium transport processes and linear helix molecules; we term it as the PCISS effect. Notably, PCISS manifests irrespective of whether the SOC is chirality-driven or stems from heavy-metal substrates, making it an efficient way to generate chirality-locked pure spin currents. Our research establishes a novel paradigm for examining the underlying physics of the CISS effect.

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.

Nonequilibrium Magneto-Conductance as a Manifestation of Spin Filtering in Chiral Nanojunctions

It is generally accepted that spin-dependent electron transmission may appear in chiral systems, even without magnetic components, as long as significant spin-orbit coupling is present in some of its elements. However, how this chirality-induced spin selectivity (CISS) manifests in experiments, where the system is taken out of equilibrium, is still debated. Aided by group theoretical considerations and nonequilibrium DFT-based quantum transport calculations, here we show that when spatial symmetries that forbid a finite spin polarization in equilibrium are broken, a net spin accumulation appears at finite bias in an arbitrary two-terminal nanojunction. Furthermore, when a suitably magnetized detector is introduced into the system, the net spin accumulation, in turn, translates into a finite magneto-conductance. The symmetry prerequisites are mostly analogous to those for the spin polarization at any bias with the vectorial nature given by the direction of magnetization, hence establishing an interconnection between these quantities.

Sequence-controlled chiral induced spin selectivity effect in ds-DNA

In this research, we explore sequence-dependent chiral-induced spin selectivity (CISS) in double-stranded (ds)-DNA using time-correlated single-photon counting and electrochemical impedance spectroscopy supplemented by tight-binding calculations of the phenomenon for the first time. The average lifetime of the photo-excited electrons in a Quantum Dot-DNA system is influenced by the CISS effect generated by the DNA molecule, and the difference in average time decay of electrons was found to be 345 ps for opposite polarity ("UP" and "DOWN") of spins due to the CISS effect. Moreover, the yield of spin-polarized electrons due to the CISS effect was reduced by more than 35% from perfect DNA to DNA with point mutations. Remarkably, by employing a tight binding method combined with Green's function formalism for transport, simulations of the process support the observed experimental trends. Our results provide a basic understanding of the sequence-specific spin-dependent electron transfer through ds-DNA. These results would help to build spin-based next-generation DNA sensors.

Chiral-induced spin selectivity in biomolecules, hybrid organic-inorganic perovskites and inorganic materials: a comprehensive review on recent progress

The two spin states of electrons are degenerate in nonmagnetic materials. The chiral-induced spin selectivity (CISS) effect provides a new strategy for manipulating electron's spin and a deeper understanding of spin selective processes in organisms. Here, we summarize the important discoveries and recent experiments performed during the development of the CISS effect, analyze the spin polarized transport in various types of materials and discuss the mechanisms, theoretical calculations, experimental techniques and biological significance of the CISS effect. The first part of this review concisely presents a general overview of the discoveries and importance of the CISS effect, laws and underlying mechanisms of which are discussed in the next section, where several classical experimental methods for detecting the CISS effect are also introduced. Based on the organic and inorganic properties of materials, the CISS effect of organic biomolecules, hybrid organic-inorganic perovskites and inorganic materials are reviewed in the third, fourth and fifth sections, especially the chiral transfer mechanism of hybrid materials and the relationship between the CISS effect and life science. In addition, conclusions and prospective future of the CISS effect are outlined at the end, where the development and applications of the CISS effect in spintronics are directly described, which is helpful for designing promising chiral spintronic devices and understanding the natural status of chirality from a new perspective.

A Group-Theoretic Approach to the Origin of Chirality-Induced Spin-Selectivity in Nonmagnetic Molecular Junctions

Spin-orbit coupling gives rise to a range of spin-charge interconversion phenomena in nonmagnetic systems where certain spatial symmetries are reduced or absent. Chirality-induced spin-selectivity (CISS), a term that generically refers to a spin-dependent electron transfer in nonmagnetic chiral systems, is one such case, appearing in a variety of seemingly unrelated situations ranging from inorganic materials to molecular devices. In particular, the origin of CISS in molecular junctions is a matter of an intense current debate. Here, we derive a set of geometrical conditions for this effect to appear, hinting at the fundamental role of symmetries beyond otherwise relevant quantitative issues. Our approach, which draws on the use of point-group symmetries within the scattering formalism for transport, shows that electrode symmetries are as important as those of the molecule when it comes to the emergence of a spin-polarization and, by extension, to the possible appearance of CISS. It turns out that standalone metallic nanocontacts can exhibit spin-polarization when relative rotations which reduce the symmetry are introduced. As a corollary, molecular junctions with achiral molecules can also exhibit spin-polarization along the direction of transport, provided that the whole junction is chiral in a specific way. This formalism also allows the prediction of qualitative changes of the spin-polarization upon substitution of a chiral molecule in the junction with its enantiomeric partner. Quantum transport calculations based on density functional theory corroborate all of our predictions and provide further quantitative insight within the single-particle framework.

Symmetric Post-Transition State Bifurcation Reactions with Berry Pseudomagnetic Fields

We investigate how the Berry force (i.e., the pseudomagnetic force operating on nuclei as induced by electronic degeneracy and spin-orbit coupling (SOC)) might modify a post-transition state bifurcation (PTSB) reaction path and affect product selectivity for situations when multiple products share the same transition state. To estimate the magnitude of this effect, Langevin dynamics are performed on a model system with a valley-ridge inflection (VRI) point in the presence of a magnetic field (that mimics the Berry curvature). We also develop an analytic model for such selectivity that depends on key parameters such as the surface topology, the magnitude of the Berry force, and the nuclear friction. Within this dynamical model, static electronic structure calculations (at the level of generalized Hartree-Fock with spin-orbit coupling (GHF+SOC) theory) suggest that electronic spin induced Berry force effects may indeed lead to noticeable changes in methoxy radical isomerization.

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.

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.

ON/OFF Spiroconjugation through Peripheral Functionalization: Impact on the Reactivity and Chiroptical Properties of Spirobifluorenes

Spirobifluorenes are an important class of spiro compounds frequently used in the field of organic electronics. However, harnessing spiroconjugation to obtain high-performance in such structural motifs remains unexplored. We herein propose that peripheral functionalization may serve as a useful tool to control spiroconjugation in an ON/OFF manner on both chemical reactivity and photophysical properties. In particular, the ratio of mono- and di-functionalized spirobifluorenes found experimentally during their synthesis were found to be 3/2, 7/2, and 12/2 for phenyl, nitro-phenyl and amino-phenyl analogues, respectively. These remarkable reactivity differences correlate with the spiroconjugation character evaluated theoretically at the CAM-B3LYP/6-31G(d,p) level of theory. Additionally, comparison of experimental and predicted optical and chiroptical responses shows that spiroconjugated molecular orbitals have a significant or negligible involvement on the main electronic transitions depending on the peripheral functionality of the spirobifluorene.

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).

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.

Spin-orbit coupling and the fine optical structure of chiral helical polymers

With the discovery of the chiral-induced spin selectivity (CISS) effect, it has been recognized that spin and structural spin-orbit coupling (SOC) play important roles in the electro-optical properties of chiral materials. We redefine the spin-dependent current and magnetic moment operators to include chiral-induced SOC in a helical polymer and deduce optical absorption and circular dichroism (CD) formulae. The fine structure in the optical spectra is calculated for a helical polymer described with the tight-binding model. The effects of both the electron orbit and spin on the optical absorption and CD are discussed. Our investigations demonstrate that the synergy between the electron orbit and spin will contribute to higher-sensitivity circularly polarized light (CPL) detection.

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.

Spinterface chirality-induced spin selectivity effect in bio-molecules

The chirality-induced spin selectivity (CISS) effect, namely the dependence of current through a chiral molecule on spin of the electron, was discovered over two decades ago, and has been suggested for various spin- and chirality-related applications. Yet, quite surprisingly, its physical origin remains elusive, and no theoretical description can quantitatively describe it. Here, we propose a theory for the CISS effect in bio-molecular junctions, based on the interplay between spin–orbit coupling in the electrodes, molecular chirality and spin-transfer torque across the electrode-molecule interface. This theory leads to the first ever quantitative analysis of experimental data, and provides insights into the origin of the CISS effect. The theory presented here can be used to analyze past experiments and to design new experiments, which may lead to deeper understanding of what is considered one of the outstanding problems in molecular electronics and nano-scale transport.

A “spinterface” model for the chirality-induced spin selectivity effect, adjusted for bio-molecules, can be used to obtain remarkable fits to experimental data.

Spin selectivity in chiral metal-halide semiconductors

Controlling the spin states of freedom represents a significant challenge for the next-generation optoelectronic and spintronic devices. Chiral metal-halide semiconductors (MHS) have recently emerged as an important class of materials for spin-dependent photonic and electronic applications. In this Minireview, we first discussed the chemical and structural diversity of chiral MHS, highlighting the chirality formation mechanism. We then provided our current understanding on the spin-sensitive photophysical and transport process with a focus on how chirality enables the spin selectivity in chiral MHS. We summarized recent progress on the experimental demonstration of spin control in various photonic and spintronic devices. Finally, we discussed ongoing challenges and opportunities associated with chiral MHS.

Semiclassical description of nuclear dynamics moving through complex-valued single avoided crossings of two electronic states

The standard fewest-switches surface hopping (FSSH) approach fails to model nonadiabatic dynamics when the electronic Hamiltonian is complex-valued and there are multiple nuclear dimensions; FSSH does not include geometric magnetic effects and does not have access to a gauge independent direction for momentum rescaling. In this paper, for the case of a Hamiltonian with two electronic states, we propose an extension of Tully's FSSH algorithm, which includes geometric magnetic forces and, through diabatization, establishes a well-defined rescaling direction. When combined with a decoherence correction, our new algorithm shows satisfying results for a model set of two-dimensional single avoided crossings.

Electronic spin separation induced by nuclear motion near conical intersections

Though the concept of Berry force was proposed thirty years ago, little is known about the practical consequences of this force as far as chemical dynamics are concerned. Here, we report that when molecular dynamics pass near a conical intersection, a massive Berry force can appear as a result of even a small amount of spin-orbit coupling (<10⁻³ eV), and this Berry force can in turn dramatically change pathway selection. In particular, for a simple radical reaction with two outgoing reaction channels, an exact quantum scattering solution in two dimensions shows that the presence of a significant Berry force can sometimes lead to spin selectivity as large as 100%. Thus, this article opens the door for organic chemists to start designing spintronic devices that use nuclear motion and conical intersections (combined with standard spin-orbit coupling) in order to achieve spin selection. Vice versa, for physical chemists, this article also emphasizes that future semiclassical simulations of intersystem crossing (which have heretofore ignored Berry force) should be corrected to account for the spin polarization that inevitably arises when dynamics pass near conical intersections.

Spin polarization is at the basis of quantum information and underlies some natural processes, but many aspects still need to be explored. Here, the authors, by quantum mechanical computations, show that even a weak spin-orbit coupling near a conical intersection can induce large spin selection, with consequences for spin manipulation in photochemical or electrochemical reactions.