Essential aspects of the chiral induced spin selectivity (CISS) effect and their implications for spin-controlled chemistry and asymmetric electrochemical reactions are described. The generation of oxygen through electrolysis is discussed as an example in which chirality-based spin-filtering and spin selection rules can be used to improve the reaction's efficiency and selectivity. Next the discussion shifts to illustrate how the spin selectivity of chiral molecules (CISS properties) allows one to use the electron spin as a chiral bias for inducing asymmetric reactions and promoting enantiospecific processes. Two enantioselective electrochemical reactions that have used polarized electron spins as a chiral reagent are described; enantioselective electroreduction to resolve an enantiomer from a racemic mixture and an oxidative electropolymerization to generate a chiral polymer from achiral monomers. A complementary approach that has used spin-polarized, but otherwise achiral, molecular films to enantiospecifically associate with one enantiomer from a racemic mixture is also discussed. Each of these reaction types use magnetized films to generate the spin polarized electrons and the enantiospecificity can be selected by choice of the magnetization direction, North pole versus South pole. Possible paths for future research in this area and its compatibility with existing methods based on chiral electrodes are discussed.
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Chiral Induced Spin Selectivity and Its Implications for Biological Functions
Chirality in life has been preserved throughout evolution. It has been assumed
that the main function of chirality is its contribution to structural
properties. In the past two decades, however, it has been established that
chiral molecules possess unique electronic properties. Electrons that pass
through chiral molecules, or even charge displacements within a chiral
molecule, do so in a manner that depends on the electron’s spin and the
molecule’s enantiomeric form. This effect, referred to as chiral induced spin
selectivity (CISS), has several important implications for the properties of
biosystems. Among these implications, CISS facilitates long-range electron
transfer, enhances bio-affinities and enantioselectivity, and enables efficient
and selective multi-electron redox processes. In this article, we review the
CISS effect and some of its manifestations in biological systems.We argue
that chirality is preserved so persistently in biology not only because of its
structural effect, but also because of its important function in spin polarizing
electrons.
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- Award ID(s):
- 1900078
- NSF-PAR ID:
- 10331851
- Date Published:
- Journal Name:
- Annual review of biophysics
- Volume:
- 51
- ISSN:
- 1936-122X
- Page Range / eLocation ID:
- 99-114
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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. -
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Abstract Monolayers of chiral molecules can preferentially transmit electrons with a specific spin orientation, introducing chiral molecules as efficient spin filters. This phenomenon is established as chirality‐induced spin selectivity (CISS) and was demonstrated directly for the first time in self‐assembled monolayers (SAMs) of double‐stranded DNA (dsDNA)1. Here, we discuss SAMs of double‐stranded peptide nucleic acid (dsPNA) as a system which allows for systematic investigations of the influence of various molecular properties on CISS. In photoemission studies, SAMs of chiral, γ‐modified PNA show significant spin filtering of up to
P = (24.4 ± 4.3)% spin polarization. The polarization values found in PNA lacking chiral monomers are considerably lower at aboutP = 12%. The results confirm that the preferred spin orientation is directly linked to the molecular handedness and indicate that the spin filtering capacity of the dsPNA helices might be enhanced by introduction of chiral centers in the constituting peptide monomers. -
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Abstract 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.
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Abstract In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals. Comparison with ab initio theory reveals the large circular dichroism arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenon driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands, resulting in a Cotton effect. Our study suggests that previous data for spin-coated films largely underestimate the optical chirality and provides quantitative insights into the intrinsic optical properties of chiral perovskites for chiroptical and spintronic applications.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1146/annurev-biophys-083021-070400
