Chirality-Induced Spin Selectivity at the Molecular Level: A Different Perspective to Understand and Exploit the Phenomenon

Abstract

Investigating Chirality-Induced Spin Selectivity (CISS) at the molecular level offers a novel perspective, in between Chemistry and Physics, on this still not fully understood phenomenon. Indeed, the molecular approach offers an advantage point for understanding CISS by disentangling the role of chiral molecules from that of the surfaces. Here, we present an overview of experimental observations of CISS in electron transfer on isolated molecules in solution and the current status of theory to model the phenomenon. We discuss what is accomplished and which are the most important questions, and we propose experiments based on electron and nuclear magnetic resonance both to unravel open issues on the CISS effect in electron transfer and to apply it to quantum technologies.

1

(left) Schematic representation of the electron transfer mechanism. The initial state is a singlet on the donor (D), with both electrons in the ground-state orbital. Photoexcitation promotes the system to the D*–A state. Following electron transfer (ET) of the excited electron to the acceptor, a spin-correlated radical pair (SCRP) is formed, which can be a singlet (top) or a triplet (bottom), as discussed in the text. (center, dotted square): SCRP energy levels in the high field limit for an ET starting from either a singlet or a triplet precursor with equal sublevel populations. Enhanced absorptive (a) and emissive (e) microwave-induced transitions are indicated. (right) Simulated TREPR spectra with the following spin Hamiltonian parameters: Δg = 0.004, J = 0 MHz, and D = 4 MHz, θ = 0.

2

Simulated, normalized TREPR spectra of an SCRP with singlet precursor at selected orientations of the electron-transfer direction (θ), using the following spin Hamiltonian parameters: Δg = 0.004, J = 0 MHz, and D = 4 MHz. For each simulation, the SCRP energy levels in the high-field limit and their relative populations are also reported. The numbers indicate EPR transitions in the spectrum from low to high field.

3

Unraveling CISS effect by time-resolved EPR. (a,d) Predicted spectra for samples oriented with liquid crystals (a) with the charge separation axis either parallel (left) or perpendicular (right) to the external field [only two peaks shown due to similar g tensors and line broadening] or in an isotropic solution (d) with different parameters as in refs , , and . (b) Molecular structures of reference achiral PXX-NMI-NDI system (left) and of chiral PXX-NMI₂-NDI. Here PXX = peri-xanthenoxanthene, NMI = naphthalene-1,8-dicarboximides, and NDI = naphthalene-1,8:4,5-bis­(dicarboximide). (c) Corresponding experimental observations on protonated chiral PXX-NMI₂-NDI (averaged on the two enantiomers) and reference achiral PXX-NMI-NDI system. (e) Molecular structures of the reference molecule (left) and of DB_χA enantiomers (right) where D = 2,2,6,6-tetramethyl-[1,3]-dioxolo­[4,5-f ]­[1,3]­benzodioxole, B _χ = (R)- or (S)-2,2′-dimethoxy-4,4′-diphenyl-5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthalene, and A = naphthalene-(1,4:5,8)-bis­(dicarboximide) studied in a randomly oriented solution. (f) Corresponding X-band spectra (black, blue lines) and fit (red) with a CISS contribution of 38%. In the right panel, black and blue lines are the (S) and (R) enantiomer. Reproduced (adapted) with permission from ref . Copyright 2024 American Chemical Society.

4

Modeling for CISS in ET with the chiral bridge as a complex charge-transfer potential. (a) Two-step process proposed by Fay and Limmer, with SOC acting only in the first step and spin–spin exchange interactions only in the CT₁ intermediate charge state. The second step (from CT₁ to CT₂, with one electron on the donor and one on the acceptor) is spin independent. (b) Results for the spin polarization p _DA = (p _D – p _A)/2 dynamics for each charge state for decreasing values of the exchange interaction 2J from top (J/g _e μ _B = 100 mT) to bottom (J/g _e μ _B = 1 mT). Reproduced (adapted) with permission from ref . Copyright 2021 American Chemical Society. (c,d) ET model from ref beyond the Condon approximation. (c) A fast photoexcitation changes the electronic state but not the nuclear coordinates. As a result, the vibrational degrees of freedom are placed in an out of equilibrium state on the excited surface. (d) Population of the charge transfer state without spin–orbit coupling (orange line), and for two opposite signs of the SOC (blue and yellow lines, respectively), with the inclusion of decoherence. A spin filtering effect emerges at short times (see zoom in the inset) and then vanishes. Reproduced (adapted) with permission from ref . Copyright 2022 AIP.

5

(a) Many-body model for CISS in Electron Transfer from ref . The chiral bridge is described by a Hubbard Hamiltonian with nearest neighbor hopping parametrized by t, next-to-nearest neighbors SOC of strength λ and on-site Coulomb repulsion U. Donor (D) and acceptor (A) are coupled via incoherent spin-conserving jump operators, included in the Redfield equation with rates Γ. (b) Population of the different orbitals during the simulated charge transfer dynamics: donor excited orbital (red), acceptor (black) and sum of the sites on the bridge (blue) referred to the equilibrium (N = 4) population. (c) Spin polarization p _A accumulated on the acceptor after ET, as a function of Γ and of the exchange coupling J = 4t ²/U, for λ = 6.25 × 10^–4 U.

6

(a) Sketch of a three-spin system: the acceptor in the chiral D-χ-A SCRP also interacts with a nuclear spin I through isotropic hyperfine interaction. (b,c) NMR spectra as a function of frequency, probing nuclear transitions of a nuclear spin 1/2 (a ¹H, with Larmor frequency ν_L = 42.58 MHz at 1 T) coupled by isotropic hyperfine interaction to the acceptor and prepared in a $(|\uparrow ⟩+|\downarrow ⟩)/\sqrt{2}$ state. Simulations consider samples containing both enantiomers of a D−χ–A dyad in an isotropic solution (b) or oriented with liquid crystals with the static field applied along the chiral/dipolar axis (θ = 0, c). Spectra show distinctive features characterizing the polarized charge-separated state with respect to a singlet state (blue lines), both in the presence of 100% (red lines) or 50% spin polarization (orange lines). Characteristic peaks for θ = 0 and θ = π/2 are labeled in (b). Parameters: D = 6.5 MHz corresponding to r _DA = 2 nm, $\mathcal{A}$ = 3 MHz, g _D = 2.004, g _A = g _D – Δg with Δg = 0.001, B ₀ = 300 mT, T₂ ⁿ = 50 μs.