Chiral molecules-ferromagnetic interfaces, an approach towards spin controlled interactions

Chirality is a fundamental manifestation of symmetry in nature and is found in almost all aspects of living systems.¹ DNA, peptides, proteins, sugars, and many other molecules are chiral. In living systems, chiral biomolecules exist predominantly in only one of their possible enantiomeric forms,² e.g., amino acids in the L-form and sugars in the D-form. It is known that enantioselectivity in biological processes is highly important. Opposite enantiomers of a chiral drug molecule can have extremely different biological effects. One infamous example is the drug thalidomide that was marketed as a mixture of its two enantiomers; one of them had the desired therapeutic effect, while the other enantiomer caused severe birth defects.³ Enantiospecificity in molecular recognition is commonly believed to manifest through the three-dimensional spatial arrangement of a chiral molecule's surface and intermolecular interactions with other molecules in solution, the “lock and key” model.⁴ In fact, the conventional wisdom in chemistry is that an enantioselective chemical process requires a chiral bias. Commonly, this bias is expressed as one enantiomer of a chiral reactant, a chiral catalyst, or a chiral solvent.^5,6 The Chiral Induced Spin Selectivity (CISS) effect,⁷  vide infra, implies that electron spin control can be used to realize enantioselectivity in chemical reactions and molecular separations.

The CISS effect refers to the correlation found between the electron transmission through chiral molecules and the electron spin.⁸ Depending on the handedness of the molecule, electrons of a certain spin can traverse the molecule more easily in one direction than in the other.⁹ These directions are reversed for electrons of opposite spin. Thus, charge displacement and charge transfer in chiral molecules generate a spin-polarized electron distribution. This effect is general and has been measured for a number of different chiral organic molecules, nucleic acids,^10,11 helicene,¹² and peptides.¹³ Since its discovery, the effect has been observed experimentally and verified in different systems.^14–19 These observations are surprising as spin filtering is commonly associated with magnetic materials or with substances that possess large spin–orbit coupling, rather than organic molecules that typically are neither magnetic nor have large spin–orbit coupling. Often, the theoretical descriptions of CISS^20–29 couple the electron spin direction and the velocity to the helical geometry of a molecule, using scattering or tight binding mechanisms. While the spin–orbit coupling alone is too weak to account for the CISS effect, its combination with a dipole electric field³⁰ and/or spin exchange interaction with a substrate can lead to strong spin-dependent transmission at room temperature. The CISS effect provides a dissymmetric physical force that could explain (or help explain) homochirality in biology.³¹ Experiments show that CISS contributes to long distance dissipationless electron transport,³² local charge separation,³³ and biorecognition between chiral molecules³⁴—all processes that are important in biology.

Attempts to identify dissymmetric physical forces that could explain biological homochirality have a long history.³⁵ Most attempts failed, and only vortex mechanical forces, such as stirring of solutions³⁶ or inducing vortex flow,³⁷ maybe the “weak force”³⁸ have proved to be significant enough to provide some differentiation between the forces operating on opposite enantiomers. Using a magnetic field in conjunction with photoinduced processes can initiate an enantioselective process, e.g., magnetochiral anisotropy.³⁹ Recently, it was demonstrated that the CISS effect gives rise to an enantioselective interaction of chiral molecules with a substrate magnetized perpendicular to its surface that is large enough to perform enantioseparation of a racemic mixture.⁴⁰ As a corollary, charge redistribution in chiral molecules is also accompanied by enantiospecific spin polarization, indicating that chiral molecules will interact by spin exchange with perpendicularly magnetized surfaces. This interaction should be spin sensitive by virtue of short-range spin-exchange interactions. The study was followed by additional studies on enantiospecific crystallization onto magnetized surfaces from racemic solutions.⁴¹ 

The polarized charge displacement of chiral molecules generates transient spin polarization; for a chiral molecule interacting with a magnetized substrate, or another chiral molecule, spin exchange interactions are critical at short range. For example, when two helical chiral molecules of the same handedness interact, the charge polarization is accompanied by a spin polarization acting in the same direction, e.g., pointing outward along the helical axis. In this case, the exchange interaction between the molecules is characterized by two opposite spin polarization directions, analogous to a singlet state. In contrast, the spin polarizations of two interacting molecules of opposite chirality have opposite directions. Here, the exchange interaction between the molecules corresponds to two parallel spins, analogous to a triplet state.³³ Consequently, spin polarization makes the overall interaction between chiral molecules enantiospecific. Most force fields that are used in (bio)molecular simulations do not include these effects.

The spin exchange interactions are critical for understanding the interaction of chiral molecules with magnetic substrates. When chiral molecules are adsorbed on a soft ferromagnet, which is commonly magnetized by either external magnetic fields or spin polarized currents, the magnetization can be switched.⁴² Magnetization switching depends on the handedness of the adsorbed molecules. In the transient state, less than 10¹³ electrons per cm² are transferred between the substrate and the molecules. These are sufficient to induce magnetization reversal in thin ferromagnetic films. These results led us to consider the possible enantiospecific interaction between chiral molecules and perpendicularly magnetized surfaces and to use it to facilitate the separation of a racemic mixture into its two enantiomeric components simply by allowing the mixture to interact with a magnetized substrate.

Figure 1 summarizes experimental results for the enantiospecific interaction of L– and D–polyalanine (PAL)-based peptides with a thin magnetic film. A thiolated α-helix polyalanine SH-CAAAAKAAAAKAAAAKAAAAKAAAAKAAAAKAAAAK (C, A, and K represent cysteine, alanine, and lysine, respectively) was exposed to a ferromagnetic (FM) cobalt film covered with 5 nm of gold for 2 s. SiO₂ nanoparticles (NPs) were attached to the adsorbed PAL to act as a marker in the scanning electron microscopy (SEM) micrographs for the monolayer adsorption density. Figures 1(a) and 1(b) show SEM images of the L-enantiomer adsorbed on the FM substrate, with the latter magnetized up (panel a) or down (panel b). The concentration of adsorbed NPs in (panel a) is 40 (± 4) × 10⁹ NPs/cm², whereas the concentration in panel b is 6 (± 1) × 10⁹ NPs/cm². The results of the complementary experiment with D-PAL are shown in Figs. 1(c) and 1(d), with concentrations of 10 (± 2) × 10⁹ and 40 (± 5) × 10⁹ NPs/cm² found for the up- and down-magnetized substrates. As a control experiment, L- and D-enantiomers were also adsorbed on a nonmagnetic, pure gold substrate, with an external magnetic field applied either normal or antinormal to the substrate. In this case, no enantioselectivity was observed. Figure 1(e) shows a comparison for all the measurements in a bar graph form. The L enantiomer with the natural chirality is purer than the D enantiomer, and as a result, the enantiomer selectivity is improved.

Based on the interaction of chiral molecules with a magnetic substrate, magnetized perpendicular its surface, enantiopure crystallization (and separation) that is controlled by the magnetization direction was demonstrated.⁴¹ This approach can be used to separate enantiomers spatially by using spin exchange interactions and magnetized surfaces. Enantioselective crystallization is induced from a racemic solution. The enantioselectivity is manipulated by the magnetization direction of the magnetized surface. Asymmetric induction was demonstrated for three amino acids: Asparagine (Asn), glutamic acid hydrochloride (Glu•HCl), and threonine (Thr). The L enantiomer was crystallized preferentially at one pole of the magnet, and the D enantiomer was found to crystallize preferentially at the other pole. Figure 2(a) shows the results for the enantiospecific crystallization of Glu•HCl. The red line is a guide to the eye, schematically separating between the North and South magnetic part of the magnetic substrate. Figure 2(b) presents the amount of crystals formed in each half of the magnetic substrate. The enantiopurity of the crystallization reached a value of 80%.

The ability to create a current of spin polarized electrons without a ferromagnetic material, or magnetize a ferromagnetic material locally, offers a strategy for overcoming some of the difficulties one faces when attempting to utilize spin in information technologies. For device applications, inorganic films are valued for supplying the robustness that is needed for integrated circuit technology. Indeed, some evidence for the CISS effect in chiral inorganic films was shown recently.⁴³ One advantage of CISS is the ability to use even a single chiral molecule as a spin injector, thereby being able to reduce the size of a spin injector much below the size dictated by the ferromagnetic-superparamagnetic transition, which is typically a few tens of nanometers. Preliminary results show that ferromagnetism can be imprinted on superparamagnetic 10 nm nanoparticles by asymmetric adsorption of polyalanine chiral molecules.⁴⁴ Another advantage of the CISS effect is the high spin polarization achieved, unlike what one faces with a high Schottky barrier, which tends to reduce the spin polarization. Moreover, the sensitivity of the charge distribution in adsorbed chiral molecules to the change in a ferromagnetic substrate's magnetization, the spinterface property, may prove to be useful for sensing and for various spintronics applications.

As indicated above, surfaces will play a major role in developing CISS-based spinterface devices. Controlling and engineering the spinterface will require understanding how to relate the transient spin polarization in the chiral molecules/material, which arises from spin–orbit coupling, to the spin exchange interaction between the spin-polarized chiral layer and the substrate. The spin exchange energy is much larger than the spin–orbit coupling in chiral molecules, and it will be important to perform systematic studies that examine how the spin-exchange interactions between a chiral layer and substrate change with the type of magnetic material, its coercivity, and its magnetization direction. Such studies should help to develop design principles for optimizing the spin selectivity and performance of spinterface devices.

In terms of chemistry, the CISS effect provides a way to introduce spin-polarized electrons into a “chemical reactor.” Some recent experiments are pointing the way to applications of the CISS effect for enhancing selectivity in reactions. It has been shown that when electrochemical water splitting occurs with an anode that accepts preferentially one spin, the process is enhanced and the formation of the by-product, hydrogen peroxide, is diminished. The spin specificity of the electrode is achieved either by coating the electrode with a chiral overlayer of molecules^45,46 or oxide^41,43 or by using a magnetized electrode.⁴⁷ These studies indicate that reaction selectivity can be affected by spin-polarized electrons, and CISS implies that spin-polarized electrons will interact in an enantiospecific manner with chiral molecules. Thus, it may be possible to replace a conventional enantiopure chemical reagent by spin-polarized electrons that provide the chiral bias for enantioselective reactions. One can even envision that spin-polarized electrons from magnetic electrodes will be able to induce enantiospecific electrochemical redox reactions.

The charge distribution in chiral molecules adsorbed on a ferromagnet responds to the change in the direction of the spin in the substrate. This effect can be applied as a simple and robust method for locally resolved magnetic imaging—based on the short-range spin exchange interactions that can be achieved using a conventional AFM tip functionalized with a chiral molecule.⁴⁸ 

Although the applications of the CISS effect are many, it is important to realize that a more basic understanding of the phenomenon is required. While current theories can account for the qualitative aspects of CISS, theories that predict the magnitude of the spin polarization effect (experiments give effects that are ten, or more, times larger than theoretical models) from the molecular structure are not yet available. Schemes for enhancing spin polarization⁴⁹ require a better understanding and may enable one to increase the effect by combining the chiral molecules with other elements, for example, metal ions, or large dipole moments. To support the theoretical effort, systematic studies that compare between different substrates and different types of chiral molecules (and chiral materials) should be done. The experiments done so far indicate that CISS is a robust effect, with the polarization value varying somewhat depending on the specific molecule. Therefore, one major theoretical challenge is to explain this robustness from general principles, without resorting to detailed features of the molecular structure.

The CISS effect is a recent comer to spin-chemistry and spin-physics with a bright future.