Chiral Induced Spin Selectivity Gives a New Twist on Spin-Control in Chemistry

Abstract

The electron's spin, its intrinsic angular momentum, is a quantum property that plays a critical role in determining the electronic structure of molecules. Despite its importance, it is not used often for controlling chemical processes, photochemistry excluded. The reason is that many organic molecules have a total spin zero, namely, all the electrons are paired. Even for molecules with high spin multiplicity, the spin orientation is usually only weakly coupled to the molecular frame of nuclei and hence to the molecular orientation. Therefore, controlling the spin orientation usually does not provide a handle on controlling the geometry of the molecular species during a reaction. About two decades ago, however, a new phenomenon was discovered that relates the electron's spin to the handedness of chiral molecules and is now known as the chiral induced spin selectivity (CISS) effect. It was established that the efficiency of electron transport through chiral molecules depends on the electron spin and that it changes with the enantiomeric form of a molecule and the direction of the electron's linear momentum. This property means that, for chiral molecules, the electron spin is strongly coupled to the molecular frame. Over the past few years, we and others have shown that this feature can be used to provide spin-control over chemical reactions and to perform enantioseparations with magnetic surfaces.In this Account, we describe the CISS effect and demonstrate spin polarization effects on chemical reactions. Explicitly, we describe a number of processes that can be controlled by the electron's spin, among them the interaction of chiral molecules with ferromagnetic surfaces, the multielectron oxidation of water, and enantiospecific electrochemistry. Interestingly, it has been shown that the effect also takes place in inorganic chiral oxides like copper oxide, aluminum oxide, and cobalt oxide. The CISS effect results from the coupling between the electron linear momentum and its spin in a chiral system. Understanding the implications of this interaction promises to reveal a previously unappreciated role for chirality in biology, where chiral molecules are ubiquitous, and opens a new avenue into spin-controlled processes in chemistry.

Figure 1

Spin dependent charge reorganization (SDCR) effect. (A) Two chiral reactant molecules are represented by helices. (B) As the chiral molecules approach each other, dispersion forces generate induced dipoles on each molecule. Because of the SDCR effect, the charge polarization is accompanied by a spin polarization. In the case of homochiral reactants, the spins on the opposite electric poles are antiparallel (singlet configuration), while if the molecules are of opposite handedness, the spins on the opposite poles will be parallel (triplet configuration). (C) The two chiral molecules react to give a product. If the spin polarizations on the molecules are antiparallel, then a singlet state is formed, and if the spin polarizations are parallel, a triplet state is formed. Since commonly the singlet state is more energetically favorable, the reaction between homochiral molecules is more favorable as compared to reaction between heterochiral molecules.

Figure 2

Schematic presentation of the experimental set of Hall measurement in the polarization (i) and electrochemical mode (ii). Hall potential recorded in polarization mode as a function of time for (A) poly-1L and (B) poly-1D for various gate pulses. (C) The Hall voltage as a function of the gate voltage for monolayers of poly-1L (blue) or poly-1D (red). (D) Hall potential recorded in the electrochemical mode as a function of the voltage when the working electrode (the Hall device) is coated with monolayers of poly-1L (blue) or poly-1D (red). (E) The voltammograms are shown for a working electrode that is coated with monolayers of poly-1L (blue) or poly-1D (red). Note that all electrochemical measurements were performed using ferrocene as a redox probe in water. A Pt wire was used as the counter electrode, the drain electrode of the Hall device was used as the working electrode, and a silver wire was used as a reference electrode. Reprinted with permission from ref (21). Copyright 2020 Wiley-VCH.

Figure 3

FM/chiral molecule interactions. (top) Direct measurement of the exchange interaction between the FM and the chiral molecules. (A) α-Helix polyalanine (AHPA) is adsorbed on a gold AFM cantilever. The system is immersed in ethanol to reduce capillary forces. The sample under study is an MBE grown Co-based nanostructure with an out-of-plane easy axis. When the tip is close to the sample, reorganization of the electric charges in the molecule (1) results in spin filtering due to the CISS effect (2), which is followed by an exchange interaction between the molecular wave function and the wave function of the substrate (3). This interaction is sensed by the deflection of the AFM cantilever (4). (B) (left) Schematic of the tip with the adsorbed molecules; (right) typical force dependence on the tip–surface distance; the pulling point of the molecule and the integrated area represent the pulling energy of the molecule. Inset shows the mean pulling energy for the up and down direction of perpendicular sample magnetization, showing a difference of 150 meV. The standard error of the mean is shown. Reprinted from ref (4) with permission. Copyright 2019 Wiley-VCH. (bottom) Contact potential difference for chiral and achiral self-assembled monolayers on magnetized surfaces. (C) Histograms, obtained from Kelvin probe measurements of the contact potential difference (CPD) for chiral and achiral self-assembled monolayers on Ni/Au magnetized surfaces, reveal an enantiospecific response for chiral molecules and no magnetization response for achiral molecules. (D) Change in the CPD as a function of the Au layer thickness for Co magnetized films with adsorbed L-A5 (SH-(CH₂)₂-NH-(Ala-Aib)₅-COOH) SAMs. The top diagram represents the Au wedge, and the color of each plot corresponds to the region indicated on the gradient bar by the same shade. Reprinted with permission from ref (18). Copyrights 2020 American Chemical Society.

Figure 4

Spin controlled water splitting. The results obtained using 20 nm Fe₃O₄ nanoparticles and nanoparticles coated with achiral or chiral molecules, which are supported on an FTO substrate to make the OER anode. (A) Absorption spectra of a 1 mM L-A3 solution (blue line) and Fe₃O₄@L-A3 (red line); (B) CD spectra of a 1 mM solution of the molecules (blue line) and Fe₃O₄ particles to which the chiral molecules were attached (red line). (C) Current density of Fe₃O₄ NPs linked with L-A3 (chiral) and pure Fe₃O₄ NPs. (D) Visible absorption spectra from the titration of the electrolyte used (0.1 M Na₂SO₄) with o-tolidine of bare Fe₃O₄, Fe₃O₄@L-A11 (chiral), Fe₃O₄@L-A3 (chiral), Fe₃O₄@MPA (achiral), and Fe₃O₄@AIB₁₀ (NH-(CH₂)₂-SH-(Aib)₁₀-NH₂, achiral). The absorption scale in (A) and (D) is arbitrary. Reprinted with permission from ref (37). Copyright 2018 American Chemical Society.