EPR as a probe for the mechanisms of chirality-induced spin selectivity in photoinduced charge transfer

arXiv:2610.10839 · cond-mat.mes-hall · Submitted 2026-10-07 · Read on arXiv

Listen

Radio episode about this paper

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "EPR as a probe for the mechanisms of chirality-induced spin selectivity in photoinduced charge transfer".

Mira: The gist: This work investigates radical pair states associated with different microscopic models by computing their related electron-paramagnetic resonance spectra to evidence peculiar signatures of different radical pair states,

Kai: First, who's behind it and why it matters.

Title and authors: Mira: We just touched on the core idea, but let's look at how they frame the problem with this paper, "EPR as a probe for the mechanisms of chirality-induced spin selectivity in photoinduced charge transfer." The title itself suggests that EPR isn't just some random measurement; it’s a tool specifically designed to probe these chirality-induced spin selectivity effects.

Kai: It sounds like they are setting up a systematic way to test different physical scenarios for CISS by looking at the resulting EPR spectra. They aren't just doing one calculation; they are investigating a broad class of initial radical pair states that might arise from various microscopic descriptions, which is important because different models predict different spin states for those pairs.

Lev: From an error correction standpoint, this is useful because it’s not just theoretical; it’s about connecting the theory to experimental observables. If we can simulate what the spectra look like for a triplet population versus a singlet population under these conditions, that gives us something concrete to check against real hardware measurements.

Kai: Exactly. And they use this simulation approach to determine exactly how much of the CISS contribution you should expect in your TREPR measurements based on an assumed initial state, which helps narrow down the possibilities for what’s happening at the molecular level.

Mira: The paper highlights a key limitation they address: that obtaining just a spin polarization isn't enough to guarantee agreement with what you measure in TREPR spectra, so they use this complementary approach of treating charge transfer as a state-preparation process and simulating the resulting "CISS state."

Lev: So it’s about moving from just knowing the final spin state to understanding how that final state was prepared, which is where we need to look for those specific spectral signatures.

Kai: And this leads us into what they suggest are improvements, so let's move on to how they think this work can be used better.

The paper's summary: Mira: So, the summary of "EPR as a probe for the mechanisms of chirality-induced spin selectivity in photoinduced charge transfer" boils down to investigating radical pair states arising from different microscopic models by calculating their associated electron paramagnetic resonance spectra. They are trying to find peculiar signatures in those spectra to figure out which radical pair state is physically present.

Kai: It’s like they’re testing several different hypotheses about the molecular spin dynamics, and the TREPR spectra are their litmus test for which hypothesis holds up when compared against what we actually measure experimentally.

Lev: The main point seems to be that just having a certain spin polarization doesn't mean you get the right signal in the resonance spectrum, so they are using simulation to figure out the relationship between those initial states and the observable spectra.

Mira: They also acknowledge that their initial descriptions often rely on simplifying assumptions, like assuming collinearity between the polarization, charge transfer, and radical pair dipolar axes and having no net spin polarization at first, which is denoted as ⟨Sz⟩ = zero <ref:2610.10839#pg2,polarization, charge transfer, and radical pair dipolar axes and>.

Kai: So they are exploring a broader class of initial states than just the simplest one because they think that's where the real physics might hide itself when you look at these spectra.

Lev: And this exploration helps them identify specific experimental conditions that actually make the CISS signature more visible, and also clarifies how those signatures can survive when you average over different molecular orientations.

Mira: The paper suggests that looking at oriented samples, like those in liquid crystals, gives a clearer picture of the effect than just isotropic solutions because it shows how the signal changes with orientation.

The paper's improvements: Kai: Now, let’s talk about the improvements they suggest for this work. They are suggesting ways to make their approach more robust by looking at non-collinear polarization scenarios, where the polarization and dipolar axes aren't necessarily lined up with each other.

Lev: That’s important because microscopic models often predict that these polarization directions can be determined by things like spin-orbit coupling or effective spin-spin interactions, which means they might not align neatly with the charge transfer axis.

Mira: The paper looks at a model based on a vibrationally-mediated Dzyaloshinskii-Moriya interaction between the donor and transferred electron spins. This model predicts a triplet component in the radical pair state that fits what’s seen experimentally, even though it suggests the polarization direction isn't parallel to the charge transfer direction.

Kai: They also consider a static contribution to spin-orbit coupling as a starting point for this study, which predicts a preferred spin-polarization direction that is not parallel to the charge transfer axis; they find that this SOC pseudovector is mostly transverse but has a small longitudinal component.

Lev: From my side, if we were running this on real hardware, the idea of needing to account for these non-collinear interactions means we need much more complex initial state preparation protocols just to keep the simulation realistic.

Mira: They also look at how net spin polarization—a difference between the T L+⟩ and T L−⟩ high-field sublevels—can cause an M-dependent mixing of those triplet states when the magnetic field is perpendicular to the dipolar axis, which could lead to a rise in net spin polarization.

Conclusion: Kai: So, wrapping up this discussion on "EPR as a probe for the mechanisms of chirality-induced spin selectivity in photoinduced charge transfer," the clearest signature they find is usually the triplet population itself. But for that to be useful, you also need a net magnetization, which means an imbalance between those T L+⟩ and T L−⟩ sublevels.

Mira: That imbalance should then yield a non-zero integral in the spectrum, although that effect can get masked by some overlapping background noise. They also point out that real singlet-triplet coherences can be seen in perfectly aligned samples or at very short times before they fade away due to electronic spin decoherence.

Lev: For us running this on hardware, the main thing is that if we see an imbalance between those triplet states, it should definitely show up as a non-zero integral in the measured spectrum, which is a strong signal to hunt for.

Kai: So, this work helps constrain which initial states are physically admissible by comparing their simulated spectra with actual experimental data from both oriented and isotropic ensembles. It gives us better tools to test the models we have about how chirality influences spin transfer.

Mira: Ultimately, it’s a tool to help validate those microscopic models that predict different radical pair states, especially when you look at the conditions under which these signatures are most visible.

Lev: It sets a clear path for what experimental conditions we should be looking for when we try to verify those specific theoretical predictions.

Kai: That's all on this paper, "EPR as a probe for the mechanisms of chirality-induced spin selectivity in photoinduced charge transfer." We’ll be back next time to look at how these concepts connect to other quantum sensing applications.

Dipartimento di Scienze Matematiche, Fisiche e Informatiche, Universita di Parma · Gruppo Collegato di Parma, INFN-Sezione Milano-Bicocca · UdR Parma · Department of Chemistry, Institute for Quantum Information Research and Engineering and Center for Molecular Quantum Transduction, Northwestern University

cond-mat.mes-hall

Submitted: 2026-10-07

Updated: 2026-10-07

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 77/100

The gist: The gist: This work investigates radical pair states associated with different microscopic models by computing their related electron-paramagnetic resonance spectra to evidence peculiar signatures of

Key concepts

Chirality-Induced Spin Selectivity (CISS)
CISS is a phenomenon where molecular chirality influences the spin of electrons, leading to spin selectivity. This effect arises from the interaction between an electron's spin and the chiral structure of a molecule, which can be observed in charge transfer processes.
Radical Pair Hamiltonian
This mathematical description models the spin dynamics of two electrons that have formed after a charge transfer event. It describes how the spins of these two electrons interact with external magnetic fields and molecular interactions, determining the resulting spin state.
TREPR Spectrum
The TREPR spectrum is a calculated resonance spectrum derived from rotating the initial spin state in relation to the Hamiltonian's eigenstates. By analyzing this spectrum, researchers can observe characteristic features that reveal the specific nature of the radical pair states and their CISS efficiency.

Terminology

Summary

The gist: This work investigates radical pair states associated with different microscopic models by computing their related electron-paramagnetic resonance spectra to evidence peculiar signatures of different radical pair states, to study their experimental accessibility and identify conditions to verify proposed microscopic models.

Introduction and Motivation

The chirality-induced spin selectivity (CISS) effect encompasses a broad range of phenomena arising from the interplay between electron spin and molecular chirality The first experimental manifestation of CISS was reported by Ray et al., who observed asymmetric scattering of photoelectrons transmitted through ordered monolayers of chiral organic molecules A large fraction of CISS experiments involves electron transport through a chiral material Photoinduced charge-transfer (PCT) offers several advantages over conventional transport experiments, such as allowing the investigation of spinselective charge transfer without metallic electrodes. The observation of CISS in PCT demonstrates that metal contacts are not necessary for the effect, which arises in organic molecules characterized by a weak intrinsic spin–orbit coupling (SOC).

Modeling Radical Pair States and TREPR Spectra

The spin dynamics of the charge-separated state is described by the radicalpair Hamiltonian Hˆ = µB h B0 ·gD ·SˆD + µB h B0 ·gA ·SˆA +SˆD ·dDA ·SˆA (1). The density matrix describing the spin state prepared upon formation of the radical pair, ρRP(0), encodes the outcome of the preceding CT process, and different microscopic descriptions may produce radical pairs with different spin states. The TREPR spectrum is then computed by rotating the prepared state ρRP(0) in the basis of the Hamiltonian (1) eigenstates and computing transition intensities between the eigenstates in linear response theory.

Initial State Descriptions for CISS States

The work relaxes commonly adopted assumptions, examining a broader class of initial radical-pair states compatible with spin-selective PCT. One phenomenological description used is a coherent superposition of the singlet S⟩ and triplet T0⟩ states: ψ χ∥ E = cos χ 2S⟩ + e iγ sin χ 2T0⟩ (2). The CISS efficiency assessed by TREPR is related to the triplet population by p = (1 − cos χ). This state ψ χ∥⟩ is restricted to the ⟨Sz⟩ = 0 subspace and assumes the CISS axis coincides with the dipolar and CT axes. The molecular orientation relative to the external magnetic field may vary, but polarization, dipolar, and charge-transfer axes remain mutually collinear.

Simulations of Non-Collinear Polarization

The investigation extends the conventional initial state description to radical pairs in which the polarization and dipolar axes are independently oriented using the form ψ χα,β⟩ = R(α, β)ψ χ∥⟩ ≡ Rz(β)Ry(α)ψ χ∥⟩ (3). Simulations were performed on a prototypical molecule with specific g tensors and an anisotropic dipole-dipole interaction.

** In isotropic solution, the main spectral features come from the polarization component parallel to the dipolar axis. A triplet component orthogonal to the dipolar axis could be present without yielding clear spectral features unless with high resolution. 5**

** In oriented samples aligned with liquid crystals (LC), a clear inversion of the signal is observed for 100% CISS contribution when the magnetic field is orthogonal to the dipolar axis and the polarization direction is parallel to it. The two configurations of Fig. 2c and Fig. 2d are directly comparable to Fig. 1d. A clear CISS signature is observed when a magnetic field is parallel to the dipolar axis and orthogonal to the polarization direction. The spherical average configurations with the dipolar axis orthogonal to the field dominate and make the visibility of the effect much weaker. 6**

Comparison with Microscopic Models

Microscopic models suggest that polarization directions can be non-collinear with the CT axis, determined by SOC or effective spin–spin interactions.

** A model based on a vibrationally-mediated Dzyaloshinskii-Moriya interaction between donor and transferred electron spin predicts a triplet component in the radical pair state compatible with experimental observations. The origin of these spin–spin interactions can be mediated by virtual excitations via hopping and SOC processes, with the most important contribution being mediated by molecular vibrations. A static contribution to SOC as a first case study is considered, and it predicts a preferred spin-polarization direction that is not parallel to the CT direction. The resulting SOC pseudovector for the BDX-B-NDI molecules is predominantly transverse to the CT direction, with a small but nonnegligible longitudinal component. 7**

Net Spin Polarization Effects

The spectroscopic effect of a net spin polarization (a different population of the T L+⟩ and T L−⟩ high-field sublevels) when the magnetic field is perpendicular to the dipolar axis is most susceptible to CISS effect in LC measurements.

** When a significant magnetic field is applied, it could induce an M-dependent mixing of the different triplet sublevels with the consequent rise of a net spin polarization. For ferromagnetic isotropic exchange, triplet states are energetically favored in zero field and hence, for a generic orientation of the molecule and value of the applied field, we expect a larger mixing with T L+⟩ (maximized close to the avoided level crossing). A similar mechanism could also arise from models where the traveling electron is also subject to isotropic exchange and DMI. 8**

Conclusion

The clearest signature is usually (i) the triplet population, and a net magnetization, associated to an imbalance between T L+⟩ and T L−⟩ (ii), should yield a non-zero integral of the spectrum. This effect could be masked by an overlapping non-trivial background. Real (iii) and imaginary (iv) singlet-triplet coherences can be evidenced by measurements in perfectly aligned samples and/or at short times, before losing the electronic spin coherences. 9

Acknowledgments

The work was funded by the Horizon Europe Programme within the ERC-Synergy project CASTLE (proj. n. 101071533) The research benefits from the High-Performance Computing facility of the University of Parma, Italy. 4

Appendix A

The electronic DMI contribution vanishes for this C2v system within the mechanism considered here because no pair of orbitals can be simultaneously coupled by hopping and SOC. An electronic DMI contribution is symmetry-allowed here, oriented normal to the mirror plane and hence perpendicular to the CT axis. 5

Supporting Information

The simulations for the benchmark system studied in the previous section are reported in Fig. S1 and Fig. S2 of the Supplementary Information. The results indicate that experimentally observed behaviours (explained so far with w = 0) can be compatible with significant non-zero values of w, possibly by adjusting the value of the CISS efficiency.

Improvements for AI systems

  1. This system can perform ab initio calculations to determine if a proposed spin-polarization direction is non-collinear with the charge transfer axis, specifically by examining the resulting SOC pseudovector for this orbital pair and its orientation relative to the CT direction. This allows it to test microscopic models where the polarization direction (or more generally the quantization axis of the triplet component) is determined by the SOC.

  2. The AI can simulate time-resolved electron paramagnetic resonance (TREPR) spectra using a broad class of initial radical-pair states, including those with unequal populations of the T+⟩ and T−⟩ triplet sublevels, to connect them with experimental observables. This capability allows it to determine the amount CISS contribution by simulating TREPR spectra based on an assumed CISS state.

  3. The system can discriminate between spectral features in isotropic solutions and oriented samples by analyzing the dependence of the spectrum on molecular orientation, as shown in Figure 1 and Figure 2. It can identify that the clearest signature is usually (i) the triplet population and that a net magnetization, associated to an imbalance between T L+⟩ and T L−⟩ (ii), should yield a non-zero integral of the spectrum.

Abstract

Several models for chirality-induced spin selectivity (CISS) in charge transfer have been proposed, but rarely compared directly with experiments. This step is essential for their validation and future progress of the field. Here we investigate a broad class of radical pair states associated with different microscopic models by computing the related electron-paramagnetic resonance spectra. This allows us to evidence peculiar signatures of different radical pair states, to study their experimental accessibility and identify conditions to verify proposed microscopic models.

Sources

Related papers