# Plasmon-Assisted Chemistry Using Chiral Gold Helicoids: Toward Asymmetric Organic Catalysis

Polina Bainova, Jean-Patrick Joly, Marie Urbanova, Daria Votkina, Mariia Erzina, Barbora Vokata, Andrii Trelin, Premysl Fitl, Gérard Audran, Nicolas Vanthuyne, et al.

► To cite this version:

Polina Bainova, Jean-Patrick Joly, Marie Urbanova, Daria Votkina, Mariia Erzina, et al., Plasmon-Assisted Chemistry Using Chiral Gold Helicoids: Toward Asymmetric Organic Catalysis. ACS Catalysis, 2023, 13 (19), pp. 12859-12867. 10.1021/acscatal.3c02958. http://dx.doi.org/10.1021/acscatal.3c02958

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# Plasmon-Assisted Chemistry Using Chiral Gold Helicoids: Toward Asymmetric Organic Catalysis

Polina Bainova, Jean-Patrick Joly, Marie Urbanova, Daria Votkina, Mariia Erzina, Barbora Vokata, Andrii Trelin, Premysl Fitl, Gérard Audran, Nicolas Vanthuyne, Jaromír Vinklarek, Václav Svorcik, Pavel Postnikov, Sylvain R. A. Marque, and Oleksiy Lyutakov*

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Cite This: ACS Catal. 2023, 13, 12859–12867

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Supporting Information

**ABSTRACT:** Gold nanoparticles with shape-determined chirality enable the attainment of plasmon-associated optical activities exceeding those exhibited by all previously known natural objects. These nanoparticles, together with their subdiffraction light focusing and excitation of chiral, plasmon-related near-fields, offer a range of very interesting applications. Herein, we propose the use of these chiral plasmon nanoparticles for asymmetric organic catalysis with the implementation of optically active organic probes (alkoxyamines). Plasmon triggering causes the homolysis of the C—ON bond in the structure of the employed organic molecules, forming stable radicals, which can be easily detected using electron paramagnetic resonance spectroscopy. Our investigation delves into the influence of various parameters on the catalytic process, such as the chirality of the nanoparticles, the not-required circular polarization of the incident light, and the optical activity of the probes used. The results

clearly show that the efficiency of a chemical reaction depends on all of these factors but to different extents. The “correct” combination of these parameters facilitates the attainment of the highest chemical reaction rate. To the best of our knowledge, this study pioneers the use of inherently chiral plasmon-based nanoparticles for asymmetric organic transformations. The proposed route of chiral plasmon catalysis can be used in various fields, including polarization-controlled chemistry, asymmetric catalysis, and the enantioselective separation of organic compounds (through the preferential elimination of one enantiomer).

**KEYWORDS:** plasmon chemistry, gold helicoids, asymmetric catalysis, PIRET, spin selectivity

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## INTRODUCTION

Plasmon-assisted photocatalysis is a promising avenue for initiating chemical transformations using small metal nanoparticles or nanostructures.¹ This approach relies on subdiffraction light focusing, achieved by converting photons into plasmonic energy and resulting in a considerable accumulation of light energy in a nanometer space.² Consequently, plasmon(s) can induce diverse chemical processes in closed environments through plasmon heating, the excitation of inner molecular electron(s), and the injection of hot electrons.³ Plasmon catalysis has been implemented in various organic transformations.⁴⁻⁸ Despite the frequent use of metal nanoparticles as (photo)catalysts,⁹ one of the basic trends in organic chemistry, particularly asymmetric catalysis, remains almost unexplored.¹⁰

In an attempt to utilize plasmonics for enantioselective chemical transformations, the creation of templated metal films emerges as a potential approach.¹¹⁻¹³ Such mesoporous surfaces potentially allow for enantio-controlled reactions by capturing molecules (reagents or products) within chiral cavities.¹⁴ Additionally, the use of the intrinsically chiral plasmonic surfaces has demonstrated the potential for

enantioselective photochemistry or light-polarization-dependent photocatalysis.¹⁵,¹⁶ In the latter case, the polarization-dependent difference in the catalytic activity of plasmon nanostructures is attributed to the different absorption of left- or right-rotating incident light, resulting in corresponding differences in plasmon-induced excited-state transfer or hot electron injection.¹⁷⁻²⁰

Commonly, the enantioselectivity of organic transformations is influenced by reactant structure, the additions of chiral catalysts, or external “chiral” stimuli.²¹,²² For instance, circularly polarized light can affect the chirality of photochemical reaction products.²³⁻²⁶ Previous attempts to use circularly polarized electromagnetic waves, particularly light, to trigger enantioselective transformations have yielded results

Received: June 29, 2023

Revised: September 6, 2023

Published: September 19, 2023
<figure><img src="image_9.png" /><figcaption>**Figure 1.** (a) Experimental route used for the preparation of L-AuNPs or R-AuNPs and (b) a schematic representation of the proposed experimental concept: utilization of circularly polarized light and chiral plasmonic nanoparticles to enantioselectively initiate organic probe homolysis and the formation of radicals (left), subsequently detected using EPR (middle) to determine the plasmon catalysis rate (i.e., the different reaction rate coefficients *k*₁ and *k*₂, right).</figcaption></figure>

that fell short of ideality, such as enantioselective catalyst utilization.²⁷,²⁸ Notably, plasmons, like photons, can be interpreted as electromagnetic waves.²⁹ This suggests that using chiral plasmons instead of chiral photons could yield similar interesting and useful results.

Similar to common chiral molecules, chiral plasmon-active nanoparticles absorb left- and right-polarized light differently, owing to their asymmetric properties.³⁰ This feature of plasmon-active nanostructures can be associated both with their specific organization into a chiral assembly, often spiral-like, or from the specific morphology of individual nanoparticles.³¹ The former typically manifests as a chiral response in the far-field. However, we used nanoparticles with a specific shape that facilitates the chiral (twisted) distribution of electric and magnetic fields in the near-field.³² In this regard, the plasmonic evanescent wave decay occurs at 1–2 nm from the metal surface, effectively achieving “subdiffraction” light focusing in nanospace.³³ This proximity is comparable to the mean size of a molecule. Therefore, unlike a photon, plasmons exhibit a much greater potential to trigger enantioselective transformations. Herein, we investigate the influence of light and gold nanoparticle chirality on the kinetics of the plasmon-assisted homolysis of the C–ON bond³⁴ within optically active organic probe molecules (alkoxyamines). The proposed approach is applicable in fields such as asymmetric catalysis, where the dominant presence of one enantiomer in the reaction products is required. Similarly, it holds relevance in polarization-controlled chemistry, a domain in which the reaction rate depends on the use of distinct circular light polarization. Finally, the concept can extend to enantioselective purification, wherein selective degradation of an organic

enantiomer facilitates the extraction of the enantio-rich products.

## EXPERIMENTAL SECTION

A detailed description of the materials used, the preparation of the alkoxyamines and chiral AuNPs,³⁵⁻³⁹ and the measurement techniques is given in the *Supporting Information*.

For plasmon-assisted homolysis and radical production, 200 µL of L- or R-AuNPs was dispersed in 2500 µL of water and treated by ultrasonication to obtain a homogeneous dispersion. Then, 300 µL of an organic probe molecule (optically active L-SG₁ or R-SG₁ or optically inactive meso-SG₁) solution in methanol was added and mixed with the NP suspension (the final concentration of the alkoxyamines in the reaction mixture was equal to 0.1 mM in all experiments). The suspension was irradiated (25 mW/cm² at the first glass surface, 630 nm central wavelength) at room temperature with circularly polarized light with the wavelength corresponding to the maximum circular dichroism (CD) band related to the plasmon resonance of the AuNPs (630 nm). After irradiation for a certain amount of time, EPR spectra were recorded, giving information on the nature of the signal and the number of radicals. The total time of irradiation of 3 h corresponds to the reaction proceeding without the apparent depletion of the used organic probes.

## RESULTS AND DISCUSSION

The potential of plasmon-assisted asymmetric catalysis was investigated, focusing on the effects of the following factors: (i) the impact of light polarization (designated as L-light and R-light for left-handed and right-handed circular light,
respectively), (ii) the chirality of plasmonic nanostructures (referred to as L- or R-AuNPs based on their morphology and interaction with left- or right-circularly polarized light), and (iii) the optical activity of the used organic probe molecule(s), characterized by chiral centers within its structure. The experimental approach is depicted in Figure 1, and Scheme 1

<figure><img src="image_10.png" /><figcaption>**Scheme 1.** Schematic Representation of the Plasmon-Assisted Model Reaction Used to Evaluate the Impact of Light Chirality, AuNP Chirality, and Organic Probe Molecule Chirality</figcaption></figure>

illustrates the chemical reaction of the used organic probes. We used gold helicoids (L-AuNPs or R-AuNPs) with morphologically determined chirality as chiral plasmon-active nanostructures. These plasmon-active nanoparticles support the excitation of local chiral plasmons in the near-field owing to their specific shape. The L- and R-AuNPs were prepared using amino-acid-assisted synthesis³⁰ using enantiomeric cysteine (Figure 1a), and their preparation steps are confirmed in Figures S1–S3. The prepared gold helicoids were used to

trigger plasmon-assisted chemical transformations of optically active organic probes (alkoxyamines) through C–ON bond homolysis leading to radical formation, and this is further determined using electron paramagnetic resonance (EPR) measurements. The preparation, separation, and characterization of the alkoxyamines, including the optically active and inactive forms (used as the control), are described in the SI (Figures S4–S12 and Table S1, with corresponding discussions). The mechanism of plasmon-induced radical generation via C–ON bond homolysis follows first-order reaction kinetics through the excitation of inner electrons induced by plasmon resonance, a phenomenon previously demonstrated in our previous study.⁴⁰⁻⁴² Plasmon excitation employs both circularly polarized light with opposite rotations and nonpolarized light. Notably, our main goal centers on exploring how different combinations of experimental conditions (alkoxyamine chirality, plasmon-active nanoparticles, and light characteristics) impact the potential of plasmon-assisted enantioselective chemistry in organic transformations.

The SEM analyses depicted in Figures 2a and 2b illustrate the morphology of the prepared L-AuNPs and R-AuNPs. The arrows in these figures highlight the mirror-symmetric characteristics of the AuNPs, as evidenced by the slightly opposite sharpening of their edges, which exhibit slight clockwise or counterclockwise curvature. This shape, as previously discussed,⁴³⁻⁴⁵ determines the optical properties of the nanoparticles. UV–vis measurements indicate similar absorption bands for the prepared L-AuNPs and R-AuNPs, peaking at approximately 600 nm (Figure 2c). This spectral

<figure><img src="image_11.png" /><figcaption>**Figure 2.** (a and b) SEM images of the L-AuNPs and R-AuNPs (the arrows highlight the slight edge sharpening of the AuNPs), (c) UV–vis absorption spectra of the L-AuNPs and R-AuNPs, and (d) CD spectra of the L-AuNPs and R-AuNPs.</figcaption></figure>
region also displays an apparent circular dichroism (CD) signal, indicating the dependence of light absorption on both circular polarization and nanoparticle chirality (Figure 2d). The measured CD signal, surpassing 50 mdeg, exhibits mirror symmetry for the L-AuNPs and R-AuNPs. The g factor at 630 nm was in the region of ±2–3 × 10⁻³ (the 630 nm wavelength was subsequently used for chemical transformation triggering). Remarkably, the “degree of optical activity” (in terms of the CD signal amplitude) of the prepared AuNPs exceeds that of “natural” structures (small organic molecules or macromolecules) by several orders of magnitude.

Subsequently, we proceeded to employ the prepared AuNPs in plasmon catalysis. First, we performed some control experiments, which exclude the potential contribution from traditional photocatalysis or gold-mediated heterocatalysis and confirm the key role of light polarization and nanoparticle shape (Table 1). As previously mentioned, the optically active alkoxyamines were used as the organic probes, which undergo homolysis triggered by plasmons, yielding stable nitroxide radicals. First, experiments aligned all of the potentially important parameters for an efficient reaction progression, including the “optimal” chirality of light and nanoparticles as well as the optical rotation of the organic probes. Subsequently, we replaced one “optimal” parameter with an incorrect one, such as the mismatched chirality of light and nanoparticles, or excluded one parameter, such as substituting circularly polarized light with unpolarized light. The concentrations of the released nitroxide radicals were monitored through conventional continuous-wave EPR spectroscopy (Figure 3a). For all combinations of light polarization, NP chirality, and organic probe structure, consistent first-order kinetic behavior was observed, with a typical kinetic profile shown in Figure 3b. This investigation of the reaction kinetics facilitates the estimation of the reaction rate constant, providing insight into the efficiency of plasmon catalysis.

All obtained reaction constants $k_d$ were normalized with respect to the reaction constant under the “optimal” experimental conditions. Figures 3c and 3d illustrate the normalized $k_{d\text{NPs/lights/alkoxyamines}}$ through bar plots for different combinations of light circular polarization, nanoparticle chirality, and optical activity of the organic probes (absolute values of the reaction coefficients are presented in Table S2). Analysis of the established trends reveals that the use of any mismatched parameter immediately leads to a drop in the $k_d$ of the plasmon-assisted reaction, signifying an apparent decrease in the reaction rate. However, the influence of the individual factors differs. Replacing chiral AuNPs with their “opposite” counterparts or using nonchiral light yields a less pronounced “negative” impact (~30% decrease in the reaction rate). Discrepancies arising from the measured CD spectra (Figure 2d) indicate that these differences cannot be solely attributed to a simple difference in L- or R-light absorption. Rather, they should be attributed to other plasmon-chemistry-related phenomena, possibly involving light or plasmon-chirality-determined hot electron transfer or resonant energy transfer.46,47 Much more important is the use of opposing optical activity within the organic probes or the alteration of light polarization. In the latter case, the reaction rate decreases to 40% of its original value, indicating a 2.5-fold reduction in the reaction rate. Like in the previous case, such a decrease cannot be explained by simple differences in the absorption of L- or R-polarized light and should be attributed to the interplay

<table><caption>Table 1. List of Control Experiments: An Estimation of the Impact of Modified Experimental Conditions⁴</caption><thead><tr><th>no.</th><th>conditions</th><th>description</th><th>results and conclusion</th></tr></thead><tbody><tr><td>1</td><td>illumination of optically active organic probes without AuNPs</td><td>exclude the potential photocatalysis</td><td>nitroxide radicals were not detected; reaction proceeds solely under plasmon triggering</td></tr><tr><td>2</td><td>absence of illumination</td><td>exclude the reaction catalysis by the gold surface (i.e., in absence of a plasmon)</td><td>nitroxide radicals were not detected; reaction proceeds solely under plasmon triggering</td></tr><tr><td>3</td><td>nonchiral AuNPs, optically inactive organic probes, and circularly polarized light</td><td>estimate the impact of light polarization in isolation</td><td>nitroxide radicals were formed with the same reaction rate for both L- and R-polarized light</td></tr><tr><td>4</td><td>nonchiral AuNPs, optically active organic probes, and circularly polarized light</td><td>estimate the impact of plasmon excitation on “common” spherical AuNPs using circularly polarized light on the reaction rate</td><td>nitroxide radicals were formed at the same reaction rate for both probes, irrespective of light polarization; simple changes in light polarization lack importance in plasmon-assisted asymmetric catalysis</td></tr><tr><td>5</td><td>illumination of L-AuNPs and R-AuNPs with L- or R-circularly polarized light</td><td>estimate the impact of plasmon heating</td><td>the observed difference between “correct” and “incorrect” chirality combinations was negligible</td></tr><tr><td>6</td><td>SERS measurements of L-AuNP arrays before and after interaction with optically active organic probes</td><td>estimate the possibility of organic probe adsorption on the AuNP surface and explore the related mechanism of plasmon catalysis</td><td>the SERS peaks were observed for both organic probes with a similar averaged intensity; plasmon catalysis cannot be related to a specific surface sorption and should be attributed to “direct” or “indirect” mechanisms</td></tr><tr><td>7</td><td>illumination of the L-AuNP and -SG and R-SG racemic mixture with left-circularly polarized light</td><td>confirm the role of probe molecule chirality</td><td>the “averaged” value of the reaction rate constant was observed (corresponding to an intermediate value between the utilization of L- or R-optically active probes)</td></tr></tbody><tfoot><tr><td colspan="4"><sup>a</sup>Such as the absence of AuNPs, the absence of illumination, the absence of AuNP chirality, an estimation of the thermal contribution, and the illumination of the racemic mixture.</td></tr></tfoot></table>
<figure><img src="image_12.png" /><figcaption>**Figure 3.** (a) Example of an EPR signal from the nitroxide radicals, generated through the plasmon-assisted homolysis of the organic probe molecules (before and after reaction mixture illumination); (b) representative nitroxide concentration dependency on the plasmon-assisted alkoxyamine homolysis time (dashed line indicates the time predominantly used for the reaction rate estimation); and (c and d) normalized reaction rate constants ($k_d$), obtained via utilization of L- or R-AuNPs, L- or R-light polarization (or nonpolarized light, designated as “0”), and different organic probes (including opposite or negligible optical activity, the latter designated as “0”).</figcaption></figure>

between the light chirality and the probability of electron or energy transfer. The use of nonpolarized light similarly decelerates the reaction by approximately 2 times. Notably, the activation of an inactive organic probe (last column in the graph) induces a notably smaller decrease in the reaction rate compared to the case of an organic probe with “incorrect” chirality. In addition, the $k_d$ values measured for the L- and R-AuNPs differ, manifesting to some extent (up to 15%; absolute reaction rate constants are given in Table S2) within the absolute values of the reaction kinetic coefficient(s). This disparity is possibly attributed to chiral nanoparticle synthesis given the challenges in achieving entirely identical solutions in terms of their CD signal absolute values and symmetry. Despite this minor discrepancy, the results agree qualitatively, highlighting that eliminating one of the asymmetric parameters or using its “wrong” counterpart adversely impacts the C–ON bond homolysis in alkoxyamines. Finally, we observed a somewhat unexpected effect on the optical activity of the organic probe. In cases involving “non-optimal” molecules, the reaction rate slows by more than half, introducing an unexpected dimension to the findings. Despite the uneven influence of various parameters on the rate of the proposed plasmon-triggered enantioselective chemical transformation, a pivotal observation remains constant: the highest reaction rate is consistently attained when all experimental factors coincide,

i.e., when all experimental factors are optimal. This underscores the importance of every parameter: light polarization, nanoparticle chirality, and the optical activity of the organic probe molecules. Thus, it is evident that the interplay between the chirality of the light and the nanoparticles and the substance participating in the reaction stands as a key factor in plasmon-assisted asymmetric transformation.

Initially, our anticipation rested on the pivotal role of the chirality of the light- and plasmon-active NPs. However, the results show a great contribution from the optical activity of the organic probes involved in the reaction. Notably, this fact cannot be explained by the different surface sorption of the probe molecules onto the Au helicoid surface, confirmed through control experiments (Table 1, control no. 6). At this point, offering a definitive explanation for these results proves challenging. Considering the potential mechanisms of plasmon catalysis$^{48–50}$ involving hot electron transfer or excited-state excitation, we can assume that these effects depend on the configuration (optical activity) of the organic probe used. In the case of hot electrons, a similar dependency has already been observed in the phenomenon known as chiral-induced spin selectivity.$^{51,52}$ It can be assumed that the transfer of an excited state also exhibits selectivity toward the configuration of the probe molecule. Thus, we can assume that the probability of internal electron excitation within an organic
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Proposed reaction mechanism: the hybridization of the probe molecules with the metal surface; the shift of the HOMO and LUMO positions of the probe molecules; and the plasmon-induced resonant energy transfer, which is a function of the chirality of the probe molecules, the AuNPs, and the light polarization.</figcaption></figure>

sample depends on the chiral polarization of the plasmon and the structural configuration of the organic molecule.

We also paid special attention to plasmon heating, a concept frequently addressed in the literature.⁵³,⁵⁴ However, this aspect does not pertain to our specific case. Although L- and R-AuNPs can indeed generate different amounts of heat,⁵⁵ this factor cannot explain the different kinetics observed in probe molecule homolysis. A thermal mechanism minimally necessitates an identical homolysis dependence without the asymmetric factor. Experimental studies conducted under illumination with circularly polarized light (Table 1, control no. 5) are presented in Figure S13. Generally, the plasmon-based heating did not exceed 2 °C, and the differences between the right- and left-handed light polarizations (illumination of the L-AuNP suspension) were within the measurement error. This fact is confirmed via theoretical calculations, indicating that under our experimental conditions, the temperature difference between the “correct” and “incorrect” combinations of light-nanoparticle chirality cannot exceed 100 °C (at the used nanoparticle concentration). However, the local (surface) heating achieved before nanoparticle thermalization through the scattering of hot electrons on lattice atoms can substantially exceed the “average” measured value for the suspension.⁵⁶ In any case, the thermal heating effect should be correlated with the number of absorbed photons and the related probability of circular photon absorption. In this case, the observed kinetic difference between the L- and R-AuNPs (or L- and R-polarized light) cannot be attributed to the difference in the number of absorbed photons (and thus plasmon heating). This is evident from the CD value (i.e., the difference between the right- and left-polarized photon absorption) measured as ~60 mdeg, considerably lower than the 30% (or more) difference in reaction constants caused by nanoparticle symmetry and light polarization.

We also tried an alternative approach by using a racemic mixture of the L- and R-alkoxyamines (instead of meso-SG) under left-polarized light illumination with the addition of L-AuNPs. In this case, the observed reaction rate constant aligned with the averaged value obtained from the individual utilization of pristine L- and R-alkoxyamines (Table 1, control no. 7). This observation additionally confirms the important role of the organic probe chirality in our experiments.

The two remaining mechanisms of plasmon catalysis involve the “indirect” and “direct” pathways. The former implies an “indirect” electron transfer mechanism, where an external hot electron is injected from the plasmon-excited nanostructure

into the organic probe. The “direct” pathways proceed through a plasmon-assisted excitation from the highest occupied molecular orbital (HOMO) to the least unoccupied molecular orbital (LUMO) within the probe molecule.⁵⁷,⁵⁸ Despite its feasibility, the “indirect” mechanism is considered less plausible.⁴¹,⁴² However, it cannot be completely excluded, as evidenced by the observed surface adsorption of probe molecules on nanoparticles, which enable electron transfer (Figure S14, Tables S3 and S4, and related discussions).

Conversely, our previous experiments and quantum mechanics calculations revealed the great possibility of “direct” plasmon catalysis in the case of C-ON bond homolysis in alkoxyamines.⁴¹,⁴² In particular, our previous work involved plasmon-triggered homolysis in various alkoxyamines with distinct chemical structures, revealing a strong correlation between the reaction kinetics and the position of the HOMO relative to the Au Fermi level. Notably, the reaction kinetics practically did not depend on the position of LUMO, indicating that plasmon catalysis proceeds through a HOMO-LUMO excitation of the inner electron rather than the injection of a hot electron from AuNPs to the LUMO. From this point of view, we can assume that the probability of excited-state transfer (termed “direct plasmon catalysis”, involving the excitation of the inner electron in the alkoxyamine molecule) is closely related to both the chirality of the plasmon and the enantiomer (as well as light chirality). This proposition offers a fundamental explanation for our observed results, to some extent.

A clear dependence of the reaction kinetics on the chirality of the plasmon and alkoxyamine was observed. Temperature effects and the resulting difference in the absorption of L- and R-polarized light cannot explain such a clear dependence (the difference in absorption does not exceed 5%, based on the CD spectra). Moreover, the difference in absorption cannot explain the disparate kinetics observed in the homolysis of the optically active samples. However, the chiral plasmon-related chemistry can be related to spin effects occurring within the surrounding optically active dielectric medium (considering recent studies).⁵⁹ A schematic representation (based on our assumptions and previous investigations) of this observed phenomenon is presented in Figure 4. The hybridization of organic molecules with a noble metal surface leads to a shift in the HOMO and LUMO positions. Subsequent plasmon-triggered events lead to inner electron excitation (with the wavelength aligning with the plasmon-induced resonant energy transfer (PIRET) mechanism⁶⁰), inducing the destabilization
of the alkoxyamine probe and ensuring its subsequent homolysis, resulting in radical creation. Here, the previous observation that the dielectric environment can support a spin-oriented electron transition (referred to as chiral-induced spin selectivity) should be noted.⁵⁹,⁶¹ Concurrently, chiral plasmons can also support the selective spin excitation of organic molecules. Although a predominant spin-polarized electron transition would rather correlate with the direct plasmon catalysis mechanism, we are instead dealing with PIRET (considering the wavelength of the light used for the plasmon excitation). Thus, it can be assumed that the PIRET probability (denoting the transfer of an excited state from a chiral plasmon-active nanoparticle to an organic molecule through the excitation of an internal electron) is also a function of both plasmon chirality and the configuration of the optically active molecule.

## ■ CONCLUSION

Herein, we demonstrate the utilization of chiral plasmonic nanoparticles for the asymmetric homolysis of organic probe molecules. As chiral nanoparticles, we used gold helicoids, which are able to support the excitation of the chiral plasmon near-field. We investigated the reaction rate constants and the corresponding plasmon catalysis efficiency through EPR measurements of the radical concentrations resulting from organic probe molecules via plasmon triggering. Different circular polarizations of light were used in order to investigate their effects on plasmon-assisted asymmetric catalysis. The results convincingly confirmed that the reaction rate depends on the chirality of light, the chirality of the nanoparticles, and the optical activity of the probe molecules. Effective plasmon catalysis occurred when the chirality of the light and the nanoparticles coincided with the “suitable” optical activity of the probe molecules. Replacing at least one of these parameters with the “wrong” one did not stop the reaction completely but led to a noticeable deceleration in the reaction rate (up to 2.5 times slower). Based on our present and previous results, we can also propose that the most probable mechanism of plasmon catalysis is the “direct” one: through the excitation of internal electrons in the organic probes. At this stage, we state that the probability of plasmon-assisted inner electron excitation is a function of the chirality of the plasmon and the optical activity of the organic probe molecules (along with the chirality of light, correlating with the probability of plasmon excitation). Our work is one of the first examples of using chiral plasmonics to drive asymmetric organic transformations, complementing recent studies on enantioselective catalytic activity activation in semiconductors.¹⁸ With further investigation, our proposed approach may find applications in enantioselective chemical technology or advanced photochemistry, leveraging polarization control of the reaction kinetics and products.

## ■ ASSOCIATED CONTENT

### ■ Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.3c02958.

Experimental details, including a description of the materials, the sample preparation, the synthesis of the chiral nanoparticles, the synthesis and separation of the alkoxyamine enantiomers, plasmon-assisted homolysis and radical creation, and the measurement techniques;

characterization of the particular steps of chiral AuNP synthesis, including TEM and SEM images of the Au nanocubes and DLS measurements of the Au seeds and Au nanocubes; details of the preparation and separation of the alkoxyamine enantiomer(s), including ¹H and ¹³C NMR spectra, mass spectra, preparative separation chromatograms, and optical rotations; absolute values of the reaction rate constants; and control experiments, including time-resolved plasmon heating, a calculation of plasmon heating, SERS spectra, and band affiliations (PDF)

## ■ AUTHOR INFORMATION

### Corresponding Author

Oleksiy Lyutakov – Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic; orcid.org/0000-0001-8781-9796; Email: lyutakoo@vscht.cz

### Authors

Polina Bainova – Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic

Jean-Patrick Joly – Aix-Marseille Univ, CNRS, ICR case 551, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France

Marie Urbanova – Department of Physics and Measurements, University of Chemistry and Technology, 16628 Prague, Czech Republic

Daria Votkina – Research School of Chemistry and Applied Biomedical Sciences, Tomsk Polytechnic University, 634034 Tomsk, Russia

Mariia Erzina – Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic

Barbora Vokata – Department of Biochemistry and Microbiology, University of Chemistry and Technology Prague, 16628 Prague 6, Czech Republic

Andrii Trelin – Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic

Premysl Fitl – Department of Physics and Measurements, University of Chemistry and Technology, 16628 Prague, Czech Republic

Gérard Audran – Aix-Marseille Univ, CNRS, ICR case 551, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France

Nicolas Vanthuyne – Aix-Marseille Univ, CNRS, ISM2, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France; orcid.org/0000-0003-2598-7940

Jaromír Vinklarek – Department of General and Inorganic Chemistry, University of Pardubice, 53210 Pardubice, Czech Republic

Václav Svorcik – Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic

Pavel Postnikov – Department of Solid State Engineering, University of Chemistry and Technology, 16628 Prague, Czech Republic; Research School of Chemistry and Applied Biomedical Sciences, Tomsk Polytechnic University, 634034 Tomsk, Russia; orcid.org/0000-0001-9713-1290
Sylvain R. A. Marque - Aix-Marseille Univ, CNRS, ICR case 551, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France

Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.3c02958

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work was supported by the GACR under Project 21-02550S.

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