# Non-resonant plasmon energy transfer processes for catalysis

Arthur Andreis, Jorge Herrera, Virginie Mouriès-Mansuy, Cyril Ollivier, Salem Ba Sowid, François Ribot, Louis Fensterbank, Jacinto Sá

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Arthur Andreis, Jorge Herrera, Virginie Mouriès-Mansuy, Cyril Ollivier, Salem Ba Sowid, et al. Non-resonant plasmon energy transfer processes for catalysis. Communications Materials, 2026, 7, pp.68. 10.1038/s43246-026-01077-1 . hal-05525293

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# Non-resonant plasmon energy transfer processes for catalysis

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Arthur Andreis<fnref n="1" />✉, Jorge Herrera✉², Virginie Mouriès-Mansuy✉², Cyril Ollivier✉², Salem Ba Sowid<fnref n="3" />, François Ribot✉⁴, Louis Fensterbank✉²,<fnref n="3" />✉ & Jacinto Sá✉<fnref n="1" />,⁵✉

How can energy transfer catalysis move beyond fragile molecular light absorbers toward more robust and tunable systems? Here we show that tiny gold nanostructures can act as universal energy donors to activate otherwise inactive gold-based catalysts using light. By introducing a molecular mediator, we demonstrate that the localized energy within the nanoparticles can be passed along through a two-step mechanism, ultimately creating a reactive excited state in the gold complex, even when the light does not directly match its energy levels. Spectroscopic measurements confirm the formation and lifetime of this state and provide clear evidence of successful energy transfer. These results establish how plasmonic materials can drive catalytic reactions through controlled energy flow, opening new opportunities for designing durable and versatile systems for light-driven chemistry.

For over a century, chemists have pursued the vision of using light to drive chemical transformations, an idea inspired by natural photosynthesis, one of the most selective and efficient photochemical processes known<fnref n="1" />. Light provides a concentrated² and traceless<fnref n="3" /> form of energy, capable of promoting molecular activation through pathways inaccessible to thermal excitation⁴. At the core of this field lies the exploitation of excited-state reactivity⁵,⁶, which has enabled a broad array of synthetically valuable transformations⁷,⁸.

Among the emerging strategies, energy transfer (EnT) catalysis has garnered particular attention for its ability to activate substrates via indirect excitation. This modality unlocks reaction pathways such as cycloadditions, rearrangements, fragmentations, and challenging organometallic steps that are otherwise difficult to achieve under conventional conditions⁹. Its integration into dual catalytic systems has markedly expanded the reactivity landscape of metal complexes under mild, visible-light irradiation, enhancing both synthetic utility and sustainability⁹. Unlike traditional photoredox catalysis, which relies on single electron transfer (SET) and redox-active partners, EnT photocatalysis enables the direct population of electronically excited states in metal complexes and/or substrates, granting access to otherwise thermally inaccessible reaction pathways¹⁰,¹¹. This decoupling of light absorption from redox reactivity allows for greater flexibility in catalyst and substrate design, overcoming many of the intrinsic limitations of redox-matched systems¹².

As highlighted by Strieth-Kalthoff and Glorius perspective, energy transfer catalysis represents a “next-level” approach in photocatalysis, offering unprecedented opportunities to modulate reactivity, spin states,

and selectivity through sensitized excitation⁹. It enables mechanistically distinct transformations beyond classic single-electron transfer photoredox catalysis, such as excited-state activations. This is particularly valuable for metals like gold(I) that are redox-inert under conventional conditions.¹³,¹⁴ Moreover, EnT offers compatibility with a wide range of metals and organic substrates, precise temporal control, and a lower energy footprint, factors that are critically important for developing sustainable and modular synthetic technologies⁹,¹⁰,¹².

In previous work, our group demonstrated the feasibility of promoting oxidative addition to gold(I) complexes via photosensitized triplet energy transfer, enabling alkynylative cyclization through visible-light activation¹⁵. The concept was subsequently corroborated by Martynova and coworkers¹⁶ and extended to include the synthesis of alkynylated indoles through Csp²-Csp cross-coupling¹⁵, as well as alkynylative cyclization of *ortho*-alkynylbenzoic acids¹⁷. These studies provided mechanistic support for an energy-transfer-driven pathway that bypasses the need for stoichiometric oxidants or strongly electrophilic reagents, thereby highlighting the broader potential of EnT mechanisms in gold catalysis. However, such strategies typically rely on organic or organometallic photosensitizers, whose performance can be constrained by photobleaching, redox incompatibility, limited triplet energies, or complex quenching dynamics. These challenges call for alternative, more robust light-harvesting systems that can deliver excitation energy efficiently and universally across different catalytic platforms.

Here, we expand the framework of energy-transfer-mediated activation by investigating the potential of plasmonic nanostructures as localized

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e-mail: andreisarthur@gmail.com; louis.fensterbank@college-de-france.fr; jacinto.sa@kemi.uu.se

<footnote n="1">¹Department of Chemistry-Ångström, Uppsala University, Uppsala, Sweden. ²Institut Parisien de Chimie Moléculaire, Sorbonne Université, CNRS, Paris, France.</footnote>

<footnote n="3">³Collège de France, CNRS, Sorbonne Université, Laboratoire d'Activation Moléculaire, Paris, France. ⁴Laboratoire de Chimie de la Matie re Condensée de Paris (LCMCP), Sorbonne Université, CNRS, Paris Cedex 05, France. ⁵Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland.</footnote>
energy transducers capable of activating molecular systems, including catalytic metal complexes and substrates, via non-resonant mechanisms, i.e., using photons of insufficient energy to create a triplet state on a mediator, catalyst, or substrate. Whereas conventional plasmon-mediated energy transfer typically relies on resonant processes requiring significant spectral overlap between the plasmon and the molecular acceptor¹⁸, such overlap fundamentally constrains the scope of compatible systems. By exploring a non-resonant regime, where spectral overlap is not required, we uncover an alternative activation pathway that could substantially broaden the range of molecules amenable to plasmon-assisted excitation. This approach opens up new possibilities for designing light-driven processes and catalytic systems that transcend the limitations of traditional resonance-based strategies.

## Results and discussion

To investigate this alternative activation mechanism, we employed a two-pronged approach, combining (i) spectroscopic analysis to support energy transfer by monitoring the activation of an inert gold(I) complex, and (ii) photocatalytic triplet [2 + 2] cycloaddition of styrene.

For the spectroscopic validation we employed nanosecond transient absorption spectroscopy (ns-TAS) to probe the underlying energy transfer dynamics. As a foundational step, we established the spectroscopic fingerprint of the target gold complex, (2-(p-Tolyl)benzofuranyl)((tri(p-trifluomethyl)phenylphosphine)gold (Au-CF₃)) (nuclear magnetic resonant (NMR) spectra of the complex are shown in Figs. S1-3)¹⁵. Using the well-characterized iridium photosensitizer Ir[df(CF₃)ppy₁₂(dtbbpy)PF₆ (Ir-F)] (NMR of the photosensitizer, Figs. S4-5); absorption and emission spectra (Fig. S6), λ<sub>abs</sub> = 380 nm; λ<sub>em</sub> = 470 nm; Φ<sub>em</sub> = 0.68 and τ = 2.3 µs)¹⁹, previously shown to activate Au-CF₃ via energy transfer, we recorded the characteristic excited-state features of the complex under conventional photoexcitation. This reference allowed us to unambiguously track the photophysical response of Au-CF₃ under varying excitation conditions (i.e. direct excitation vs indirect sensitization with a light absorber). With this benchmark in hand, we next investigated whether comparable activation could be achieved using plasmonic nanostructures. This directly tested our hypothesis that non-resonant plasmonic energy transfer can effectively drive the excitation of catalytic metal complexes or substrates. In other words, we sought to determine whether the plasmonic hot carrier process, excited by photons with energies below the conventional requirement, could still photogenerate the desired triplet state via EnT.

Figure S7 presents the ns-TAS of Ir-F in argon-purged acetonitrile following excitation at 410 nm, recorded at various delay times. The excitation wavelength was optimized to avoid significant spectral overlap with Au-CF₃ signals, thereby ensuring efficient and selective excitation of the Ir-F photosensitizer. The spectra are dominated by a strong negative signal spanning 450–600 nm, with a pronounced minimum centered around 470 nm. This feature is attributed to the emission of the first triplet metal-to-ligand charge-transfer state (³MLCT₁), formed via rapid intersystem crossing from the initially excited singlet state (¹MLCT₁)²⁰,²¹. The lifetime of the Ir-F in argon was estimated to be 1.5 ± 0.1 µs, consistent with the 2.3 µs, reported elsewhere²¹.

This observation was further supported by the pronounced acceleration of the decay kinetics of the emissive state monitored at 490 nm (Fig. S8) upon exposure to molecular oxygen, indicating dynamic quenching of the triplet state and subsequent generation of singlet oxygen (¹O₂)²². In the presence of oxygen, the lifetime of Ir-F decreased to 235.6 ± 0.5 ns, corresponding to a quenching efficiency of approximately 84.3%. The high quenching efficiency of the excited state by molecular oxygen strongly supports the involvement of a triplet excited state; thus, O₂ quenching can be used as additional evidence for the presence of triplet character and its quantification.

To establish a spectroscopic fingerprint for the ³MLCT₁ state of Au-CF₃, the complex was photoexcited at 355 nm, and the resulting transient absorption was monitored by ns-TAS. Figure S9 shows the temporal evolution of the spectra, which are characterized by a distinct positive absorption band centered at 400 nm that decays completely within 1000 ns

(Figs. S9 and S10). Since the temporal resolution of ns-TAS is insufficient to resolve the singlet excited state, this feature is attributed to the absorption of the ³MLCT₁ state of Au-CF₃. The lifetime of the Au-CF₃ in argon after excitation at 355 nm was estimated to be 218.8 ± 1.8 ns.

This assignment is further supported by the accelerated decay observed in the presence of molecular oxygen (Fig. S10), consistent with triplet-state quenching and the formation of ¹O₂. The lifetime of the ³MLCT₁ state of Au-CF₃ was reduced to 87.3 ± 0.3 ns, equating to 60.1% quenching efficiency. The reduced quenching efficiency relative to the Ir complex arises from the bimolecular, diffusion-controlled nature of O₂ quenching. Shorter excited-state lifetimes limit the frequency of productive collisions with molecular oxygen, thereby decreasing the overall quenching efficiency as internal relaxation processes begin to dominate.

Theoretical calculations place the lowest-lying ³MLCT₁ state of Au-CF₃ at 504 nm (2.46 eV). The absence of emission in the visible range under these conditions indicates that this state is a non-emissive, an interpretation further supported by the relatively low intensity of its absorption feature¹⁵. Importantly, the ³MLCT₁ absorption lies within a spectral region where the Ir-F photosensitizer exhibits negligible contribution, allowing this band to serve as a direct and selective marker of ³MLCT₁ state formation in Au-CF₃.

Having established the presence of a distinct transient feature corresponding to the ³MLCT₁ state of Au-CF₃, we next explored whether this state could be populated through energy transfer from the photogenerated ³MLCT₁ state of Ir-F. Upon 410 nm excitation, which selectively excites I Ir-F, the ns-TAS spectra of the Ir-F/Au-CF₃ mixture (Fig. 1a) reveal the characteristic negative emission signal of Ir-F centered at 470 nm, accompanied by the emergence of a positive absorption band at 400 nm attributable to the Au-CF₃ ³MLCT₁ state. This spectral signature provides clear evidence for intermolecular triplet-triplet energy transfer. Further confirmation is provided by the spectral comparison at 100 ns (Fig. 1b), where the Ir-F/Au-CF₃ mixture exhibits enhanced absorption at 400 nm and a concomitant decrease in Ir-F emission relative to Ir-F alone, consistent with efficient energy transfer and dynamic quenching of the Ir-F excited state.

Kinetic analysis (Fig. S11) reveals that quenching of the ¹Ir-F ³MLCT₁ state by Au-CF₃ is highly efficient, as evidenced by the reduction in the Ir-F ³MLCT₁ lifetime from 1.5 ± 0.1 µs to 102.5 ± 0.6 ns, corresponding to a quenching efficiency of 93.2%. This efficiency is comparable to that observed with molecular oxygen, and consequently, only a minor additional change in the Ir-F ³MLCT₁ state was detected upon oxygen addition to the Ir-F/Au-CF₃ mixture (Figs. S12 and 1c).

Oxygen-quenching experiments on the Ir-F/Au-CF₃ mixture following excitation of the Ir-F complex (Fig. 1c) revealed a pronounced decrease in the absorption band at 400 nm. This reduction is indicative of triplet-state quenching by molecular oxygen, and since this spectral region is exclusively associated with the Au-CF₃ complex, it confirms that the observed signal indeed originates from the Au-CF₃ ³MLCT₁ state.

Analysis of the rising edge of the kinetic trace recorded under argon (Fig. 1d) allows determination of the formation kinetics of the Au-CF₃ ³MLCT₁ state generated via energy transfer from the Ir-F complex, yielding a formation time of 70.8 ± 1.0 ns. The resulting Au-CF₃ ³MLCT₁ state exhibits a lifetime of 243.1 ± 2.7 ns, comparable to that observed upon direct excitation of Au-CF₃ and sufficiently long to drive photochemical transformations¹⁵. Upon oxygen addition, the lifetime decreases to 97.2 ± 0.3 ns, corresponding to a quenching efficiency of approximately 60.0%, consistent with efficient triplet-state quenching by molecular oxygen as shown with quenching experiments upon direct excitation of Au-CF₃ complex.

Having established the spectroscopic fingerprint and excited-state dynamics of the Au-CF₃ ³MLCT₁ state via energy transfer from a well-defined molecular photosensitizer (Ir-F), we next sought to determine whether a comparable mechanism could be initiated through plasmonic non-resonant excitation, thereby replacing the molecular sensitizer with a nanostructured alternative. Gold nanoparticles (AuNP) were synthesized using a modified version of the Turkevich method, having citrate as the capping agent²³.
<figure><img src="image_5.png" /><figcaption>**Fig. 1 |** Dynamics of Ir-F in the presence of Au-CF₃ following excitation at 410 nm. a ns-TAS spectra of the Ir-F/Au-CF₃ mixture recorded at various time delays under an argon atmosphere; b Comparison of ns-TAS spectra at 100 ns for Ir-F in the absence and presence of Au-CF₃ under argon, highlighting the impact of Au-CF₃ on Ir-F excited-state dynamics; c Effect of oxygen on the ns-TAS spectrum of the</figcaption></figure>

Ir-F/Au-CF₃ mixture at 100 ns, showing quenching of triplet features; and d Kinetic traces extracted at 400 nm for the Ir-F/Au-CF₃ mixture under argon and oxygen atmospheres, indicating the formation and decay dynamics of the Au-CF₃
³MLCT₁ state.

The resulting AuNPs exhibited a characteristic localized surface plasmon resonance (LSPR) peak centered at approximately 545 nm (Fig. S13). In addition, the AuNP absorption spectrum displayed a broad increase below ~520 nm, attributed to interband transitions in gold between the 5 d and 6sp bands. The observed optical absorption is consistent with an average particle diameter of 15 ± 2.0 nm, as determined by dynamic light scattering (DLS, Fig. S14). Transmission electron microscopy (TEM) measurements (Fig. S15) supported the DLS analysis by revealing particles with an average core size of 9.5 ± 4.0 nm. The slight discrepancy in size arises from the difference in what each technique measures: TEM captures the metal core dimensions, whereas DLS determines the hydrodynamic diameter, which includes the ligand shell and solvent layer, water, in this case.

The DLS measurements yielded a polydispersity index (PDI) of 0.14, indicating a reasonably narrow particle size distribution and good colloidal stability. TEM analysis revealed a size variation of approximately 15%, which is acceptable for gold nanoparticles of this size range. More importantly, it is essential to assess whether such size dispersion significantly influences the optical absorption properties, as this could affect the quality and interpretation of spectroscopic data. To evaluate this effect, the dielectric function of gold was modeled using a Drude free-electron response incorporating a size-dependent damping term to account for enhanced electron-

surface scattering at small particle sizes, within the framework of the quasi-static dipole approximation of Mie theory.

Figure S16 shows the calculated LSPR peak for AuNP with different diameters. The simulated spectra display a single dominant feature corresponding to the LSPR around ~505 nm, which narrows in width with increase in size but not shift in position. Note that in this simplified Drude-only model, only the free-electron (i.e. intraband) contribution is included. While incorporating experimental dielectric data or Lorentz oscillators could reproduce the interband absorption features, these were intentionally omitted to maintain a simplified model.

Upon transferring the AuNP from water to acetonitrile, as required for the energy transfer experiments, a shift in the DLS peak was observed. This behavior is expected due to the lower viscosity of acetonitrile (0.343 mPa·s at 25 °C) compared to water (0.890 mPa·s), which affects the hydrodynamic size measurement. Additionally, the change in solvent refractive index from 1.33 (water) to 1.344 (acetonitrile)²⁴ resulted in a slight red-shift of the simulated LSPR peak to approximately ~510 nm (Fig. S17). This shift was consistently observed across all simulated particle sizes, further confirming that the particle size distribution in this study does not significantly influence the optical absorption properties of the AuNP.
Before discussing the results obtained with the AuNP, it is important to clarify that these particles can be excited through both interband and intraband (plasmonic) transitions, and both excitation modes were employed in this study. For clarity, interband excitation refers to electronic transitions between the 5 *d* and 6*p* bands of gold, achieved here using an excitation wavelength of 490 nm (corresponding to 2.53 eV, or ~244.1 kJ/mol). In contrast, intraband excitation “plasmonic excitation” was performed at 600 nm (corresponding to 2.07 eV, or ~199.0 kJ/mol), a wavelength chosen to minimize contributions from interband transitions and selectively excite the localized surface plasmon resonance. For reference, the center of the Au 5*d*-band lies approximately 2.50–2.58 eV (~496–480 nm) below the Fermi level (E<sub>F</sub>(Au) ≈ −5.1 eV vs vacuum)<sup>25</sup>.

Direct excitation of the AuNPs at both the interband and plasmonic transitions in the presence of the Au-CF₃ complex in acetonitrile proved ineffective; no signal corresponding to the Au-CF₃ ³MLCT₁ state was detected. The most likely explanation for this absence is insufficient interaction between the Au-CF₃ complex and the AuNP surface, as evidenced by the lack of significant changes in the AuNP absorption spectrum upon addition of the complex. This behavior can be attributed to the presence of the citrate capping layer, which hinders the Au-CF₃ complex from approaching the nanoparticle surface closely enough to enable efficient energy transfer. Recent studies show that proximity and directionality are key factors in energy transfer processes from nanoparticles to molecules<sup>26</sup>.

To overcome this limitation, we introduced a molecular mediator between the AuNP and the Au-CF₃ complex, following a strategy commonly employed in photon upconversion via triplet-triplet annihilation systems.<sup>27,28</sup> The mediator was selected based on two key criteria: (i) the presence of a carboxylic acid group to facilitate interaction with the AuNP surface, mimicking the binding mode of the native citrate capping agents, and (ii) a triplet energy level sufficiently high to enable energy transfer to the Au-CF₃ complex. As a reference point, theoretical calculations place the Au-CF₃ ³MLCT₁ state at 2.46 eV (corresponding to 504 nm, or ~237.8 kJ/mol). Based on the work by Hou et al.<sup>29</sup>, we identified 1-naphthoic acid (NCA) as a suitable mediator. NCA features the required carboxylic moiety and exhibits a ³ππ* triplet state at 2.5 eV (496 nm, ~241 kJ/mol), which is energetically favorable for transferring excitation to the Au-CF₃ ³MLCT₁ state.

This interpretation is reinforced by detailed photoluminescence measurements of the Au-CF<sub>3</sub>/NCA system (Figures S18–S21). Mixing NCA with Au-CF<sub>3</sub> produces a modest red-shift in the NCA triplet emission, consistent with quenching by Au-CF<sub>3</sub> and the appearance of its ³MLCT<sub>1</sub>phosphorescence. Subtracting the NCA-only spectrum reveals a distinct negative feature at higher energies (Figure S20), confirming attenuation of NCA fluorescence. A Stern–Volmer analysis (Figure S21) further demonstrates measurable quenching of NCA by Au-CF<sub>3</sub> in acetonitrile (λ<sub>exc.</sub> = 295 nm), giving a linear dependence with K<sub>sv</sub> = 4.8 × 10<sup>3</sup> M<sup>-1</sup> (Figure S22), characteristic of dynamic, collisional energy transfer. The overall extent of quenching is limited by the inherently low absorption coefficient and <6% phosphorescence quantum yield of naphthoic acids in solution, as well as the modest solubility of Au-CF<sub>3</sub> in acetonitrile, which prevents the use of large quencher excesses. Even so, the combined photophysical data substantiate the suitability of NCA as an energetically compatible mediator capable of enabling triplet-energy transfer between AuNPs and Au-CF<sub>3</sub>.

To confirm the electrostatic interaction between the AuNP and NCA via the carboxylic acid group, we performed UV-Vis and Raman spectroscopy. The UV-Vis spectra (Fig. S23) show a narrowing of the LSPR peak without a significant shift in the maximum wavelength. This is indicative of a more uniform nanoparticle surface environment (i.e., reduced surface heterogeneity) after NCA addition, consistent with a coating event which renders the ensemble more homogeneous. Plasmonic linewidth narrowing by encapsulation in a lossy medium demonstrated that changes in the dielectric environment (encapsulation in a lossy medium) can lead to narrower plasmonic resonance lines (reduced full-width at half maximum) even without a large shift in the resonance wavelength<sup>30</sup>, supporting this interpretation.

Raman spectra in the carboxylic region (Fig. S24) reveal a decrease in peak intensity upon NCA addition, supporting interaction through the carboxylic group. While NCA is expected to bind similarly to citrate on the AuNP surface, partial ligand exchange likely reduces the number of surface-bound carboxylic groups due to the greater steric bulk of NCA, as previously reported<sup>31</sup>. Together, these results, though indirect, support the formation of an AuNP–NCA interaction, a critical prerequisite for enabling the subsequent energy transfer process.

Excitation of the AuNP at 490 nm produced a small negative feature centered at 575 nm (Fig. 2a). The temporal profile of this band closely follows the laser excitation pulse, indicating that the observed dynamics are not governed by the intrinsic lifetime of the species. Given the negative nature of the signal, its ultrafast decay coinciding with the excitation pulse envelope, and its spectral position, we attribute this feature to photoluminescence from the AuNPs, arising from the relaxation of hot holes to the *d*-band edge.<sup>32,33</sup> This assignment is consistent with the report by Lee et al.<sup>34</sup>, who observed a photoluminescence lifetime of less than 100 fs for similar systems. Addition of NCA to AuNP did not yield significant changes in the TAS spectrum (Fig. 2a).

The addition of the Au-CF₃ complex to the AuNP–NCA system led to pronounced changes upon excitation at 490 nm. The AuNP photoluminescence signal exhibited noticeable spectral broadening and increase in intensity (Fig. 2a) while remaining temporally confined within the excitation pulse envelope, as evidenced by the strong modulation of the kinetic trace (Fig. 2b) and an estimated lifetime of 7.8 ± 0.1 ns, considerably shorter than the excitation pulse duration (~ 10 ns). The observed spectral broadening suggests modifications in the hole-cooling dynamics and, consequently, in the rate of Auger scattering events, which constitute the dominant nonradiative relaxation pathway<sup>35</sup>.

The nsTAS spectrum exhibits a distinct positive feature below 450 nm, which diminishes upon exposure to O₂. This signal appears exclusively in the presence of Au-CF₃ and, in conjunction with the observed changes in the AuNP photoluminescence, is reasonably attributed to the formation of the Au-CF₃ ³MLCT₁ state through a resonant sensitization process mediated by NCA. Although the signal is strongly influenced by the excitation pulse, the lifetime of the photogenerated Au-CF₃ ³MLCT₁ state following AuNP interband excitation was determined to be 49.6 ± 1.2 ns. This substantially shorter lifetime, compared to the 243.1 ± 2.7 ns measured upon direct photoexcitation by Ir-F, indicates that approximately 80% of the formed Au-CF₃ ³MLCT₁ population undergoes rapid quenching, most likely through a highly competitive energy back-transfer process.

In summary, in the presence of a mediator such as NCA, the interband-excited charge carriers in the AuNP can effectively sensitize Au-CF₃, leading to the photogeneration of the Au-CF₃ ³MLCT₁ state through two sequential Dexter-type energy-transfer processes<sup>9</sup>. This mechanism involves the initial transfer of triplet energy from the excited AuNP to NCA, followed by subsequent energy transfer from NCA to Au-CF₃. Raman analysis further reveals that NCA is electrostatically associated with the citrate capping layer, indicating a reduced donor-acceptor distance and a constrained molecular orientation<sup>28</sup>. These structural features are expected to enhance the electronic coupling between components, thereby improving the overall efficiency of the energy-transfer process and creating favorable conditions for sensitization to occur.

Interband excitation at 490 nm promotes both 5 *d* and 6*p* electrons, which possess sufficient energy to resonantly interact with the NCA triplet state (centered at ~496 nm)<sup>31</sup>. Such processes are analogous to those observed in quantum dots and molecular systems where triplet-state energetics enable efficient sensitization. However, this observation raises the intriguing hypothesis that similar energy transfer could be achieved non-resonantly through plasmonic excitation, a more efficient light-absorption process that may circumvent the strict energetic matching typically required for conventional sensitization.

To further evaluate this hypothesis, analogous experiments were conducted using plasmonic excitation at 600 nm. Based solely on single-photon energetics, this excitation wavelength is insufficient to directly
<figure><img src="image_6.png" /><figcaption>b</figcaption></figure>

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<figure><img src="image_8.png" /><figcaption>d</figcaption></figure>

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**Fig. 2 | Energy transfer dynamics following non-resonant excitation of gold nanoparticles.** a ns-TAS spectra at 100 ns for the AuNP/NCA/Au-CF₃ and AuNP/NCA mixtures, and AuNPs alone after interband excitation at 490 nm under argon; b Kinetic traces extracted at selected wavelengths for the AuNP/NCA/Au-CF₃ mixture under argon following 490 nm excitation; c ns-TAS spectra at 100 ns for the

AuNP/NCA/Au-CF₃ and AuNP/NCA mixtures, and AuNPs alone after plasmonic excitation at 600 nm under argon; and d Kinetic traces extracted at selected wavelengths for the AuNP/NCA/Au-CF₃ mixture under argon following 600 nm excitation.

sensitize either NCA or Au-CF₃. Plasmonic excitation of AuNPs alone, or in the presence of NCA, produced no measurable spectral changes (Fig. 2c). Notably, under these conditions, no photoluminescence signal associated with AuNPs was observed. However, the addition of Au-CF₃ to the AuNP/NCA solution resulted in significant spectral changes. Once again, the nsTAS spectrum revealed a distinct positive feature below 450 nm, which diminished upon exposure to O₂. This feature is attributed to the formation of the Au-CF₃ ³MLCT₁ state via two sequential Dexter-type energy-transfer processes³, though, in this case, initiated by a non-resonant energy transfer step, which will be discussed in greater detail in the mechanistic proposal. The lifetime of the Au-CF₃ ³MLCT₁ state following plasmonic excitation was determined to be 37.0 ± 0.4 ns (Fig. 2d), indicating that approximately 84.8% of the photogenerated population undergoes rapid back energy transfer.

A qualitative comparison of the Au-CF₃ ³MLCT₁ state absorption intensity photogenerated by Ir-F reveals that the signal is approximately 30–40% weaker when the state is produced via non-resonant AuNP excitation. This reduction can be partially attributed to the short lifetime of plasmonic hot carriers, which typically result in non-negligible but relatively low-yield photoreactions³⁶. However, considering that this excited state is generated using photons that are not sufficiently energetic to directly promote the transition, the observation clearly indicates a distinct plasmonic

effect. An additional and unexpected observation was the emergence of an AuNP photoluminescence signal upon addition of Au-CF₃ to the AuNP/NCA solution, a phenomenon that will be discussed in greater detail in the mechanistic proposal.

To further substantiate the hypothesis that energy transfer can proceed via non-resonant plasmonic excitation, we investigated the sensitized [2 + 2] cycloaddition of styrene in acetonitrile to form 1,2-diphenylcyclobutane, a classical benchmark reaction for probing triplet energy-transfer mechanisms³. Styrene possesses a vertical triplet energy of approximately 471 nm (2.64 eV, or ~254.4 kJ·mol⁻¹)⁵,³⁷, which lies near the upper limit of what NCA can effectively mediate³¹. The experimental conditions are summarized in Table S1, and the corresponding ¹H-NMR spectra are presented in Figs. S25–S26. In the absence of AuNP, no reaction was observed, as the ¹H-NMR spectra were dominated by signals from the substrate and NCA (when present) after solvent suppression. The presence of styrene is clearly identified by two doublets at 5.76 and 6.72 ppm, corresponding to the olefinic protons (Fig. S27), while the NCA displays its characteristic carboxylic proton signal at 13.2 ppm (Fig. S28).

In the presence AuNP and under excitation at 635 nm, a distinct quartet signal appeared between 3.6 ppm, corresponding to the cyclobutane proton on the carbon bearing the aryl substituent (Fig. S29)³⁸. The remaining cyclobutane resonances related to protons in carbon positions 3
<figure><img src="image_10.png" /><figcaption>**Fig. 3** | Schematic representation of the electronic processes involved in non-resonant plasmonic energy transfer. a Illustration of the molecular structures and the two consecutive energy transfer (EnT) steps leading to the formation of the Au-CF₃ ³MLCT₁ state upon AuNP excitation; b Proposed electronic transitions and spin-allowed EnT pathway from AuNPs to NCA following interband excitation;</figcaption></figure>

c Proposed electronic transitions and spin-allowed EnT pathway from AuNPs to NCA following localized surface plasmon resonance (LSPR) excitation; and
d Consecutive EnT processes within the AuNP/NCA/Au-CF₃ assembly culminating in the formation of the Au-CF₃ ³MLCT₁ state.

and 4 are not visible due to solvent suppression. Since olefins are known to undergo [2 + 2] cycloaddition predominantly through its sensitized triplet state, the major product under these conditions is expected to be the *trans*-1,2-diphenylcyclobutane isomer due to π-stacking<sup>39-41</sup>. In the case of styrene an alternative product can be formed, namely 1-phenyl-1,2,3,4-tetrahydronaphthalene<sup>42</sup>, however the absence of a ¹H-NMR signal at 4.13 ppm excludes this possibility (Fig. S30).

After 44 hours of reaction under chopped illumination (0.125 Hz cycle), the yield of 1,2-diphenylcyclobutane was below 3% (estimated using a ration cyclobutane proton in carbon position 1 and 2 and between NCA carboxylic proton peak). The product formation was cross validated with substrate combustion (estimated using a ration styrene olefin proton and between NCA carboxylic proton peak). This low yield is consistent with the short lifetime of the styrene triplet state (~ 23 ns)<sup>39</sup> and the presence of a highly competitive energy back-transfer process. While optimization of the reaction conditions was beyond the scope of this study, we speculated that heat accumulation at the electrode surface during illumination could accelerate back-energy transfer and thereby suppress product formation. To mitigate thermal buildup, the illumination frequency was increased to 0.25 Hz and the laser power reduced to 10–12% (using analogous data analysis as aforementioned). Under these modified conditions, the product

yield increased by almost six-fold (ca. 10-11% estimated using a ration cyclobutane proton in carbon position 1 and 2 and between NCA carboxylic proton peak), thereby confirming our hypothesis that localized heating significantly improves the efficiency of the energy-transfer process. More importantly, these findings provide direct experimental evidence supporting the spectroscopic observations, confirming that energy transfer from AuNP upon non-resonant plasmonic excitation indeed occurs and can be harnessed to drive chemical reactions.

We next propose a mechanistic framework to rationalize the electronic processes underlying the spectroscopic observations that lead to the photogeneration of the Au-CF₃ ³MLCT₁ state upon AuNP excitation. Figure 3a illustrates the molecular components involved and the two sequential Dexter-type EnT steps that culminate in the formation of the Au-CF₃ ³MLCT₁ state. These forward energy-transfer pathways are in dynamic competition with an energy back-transfer process from NCA to the AuNP, which collectively defines the overall efficiency of the system.

In this study, the AuNPs were excited through two distinct pathways: interband excitation (resonant with the mediator) and plasmonic excitation (non-resonant with the mediator). Under interband excitation (Fig. 3b), absorption of photons with sufficient energy promotes electrons to the 6sp conduction band while generating holes in the 5d and 6sp valence bands.
This process resembles the excitation observed in quantum dot systems, wherein absorption of a single photon produces an electron-hole pair separated by an energy gap. The excited charge carriers rapidly undergo spin relaxation<sup>43</sup>, establishing a population with the appropriate spin configuration required for a Dexter-type, a concurrent two-electron exchange mechanism<sup>44</sup>. For clarity and simplicity, Fig. 3b depicts only the electronic states anticipated to participate in the EnT interaction with NCA.

Under plasmonic excitation (Fig. 3c), the collective oscillation of conduction electrons in the metal nanoparticle absorbs incident light and decays non-radiatively, generating a highly non-equilibrium population of energetic (“hot”) carriers. Immediately following excitation, this population does not follow a Fermi-Dirac distribution, reflecting a non-thermalized electronic state<sup>45</sup>. Through rapid electron-electron scattering (on the order of tens to hundreds of femtoseconds), the carriers thermalize to form a hot Fermi-Dirac distribution characterized by an elevated electronic temperature that can reach up to ~3000 K.<sup>46,47</sup> Subsequent electron-phonon coupling transfers this energy to the lattice, completing the relaxation process and dissipating the plasmonic energy as heat or enabling energy transfer to nearby acceptor species.

Once the hot Fermi-Dirac distribution is established, a broad range of energetic carriers is generated predominantly within the 6sp band, without the direct formation of 5d-band holes, thus lacking the electronic configuration necessary for AuNP photoluminescence<sup>48</sup>. Among the carrier multiplication processes that can occur under plasmonic excitation, there is Auger heating<sup>49</sup>. In this mechanism, the multiplication of high-energy carriers promotes the generation of 5d-band holes via Auger processes, accompanied by the thermalization of 6sp holes to conserve energy principle. Consequently, Auger heating enhances the number of d-band holes at the cost of hot 6sp carriers, as shown in the middle panel of Fig. 3c. Although this process occurs with relatively low probability, its presence has been experimentally verified and is recognized as a contributing pathway to plasmon-induced electronic relaxation dynamics<sup>50</sup>.

As in the case of interband excitation, the hot electrons in the 6sp band undergo spin relaxation, producing a carrier population with the appropriate spin configuration to engage in the EnT process. In a classical Dexter-type mechanism, the donor (AuNP) transfers an excited electron to the lowest unoccupied molecular orbital (LUMO) of the acceptor (NCA), while the acceptor simultaneously donates an electron from its highest occupied molecular orbital (HOMO) back to the donor, thereby generating an excited triplet state in the acceptor. In both modes of AuNP excitation, the transferred excited electron originates from the 6sp band (Fig. 3d, Step 1). However, for the corresponding hole transfer (i.e., electron donation from the acceptor to the donor), two possible acceptor states exist within the AuNP - either the 6sp band or the 5d band.

To elucidate which holes participate in the EnT process from AuNP to NCA, we examined the photoluminescence response. If the holes involved originated from the 5d band, a reduction in photoluminescence intensity would be expected due to the introduction of an additional non-radiative decay pathway competing for these holes. However, the experimental observations reveal the opposite behavior, an enhancement and spectral broadening of the photoluminescence signal. This finding indicates an increase in the lifetime of the d-band holes, rendering them more susceptible to radiative recombination. Furthermore, the observed spectral broadening is consistent with scattering processes that facilitate energy relaxation of the d-holes while simultaneously increasing their population<sup>55</sup>. Therefore, one can conclude that is the 6sp holes that are involved in the EnT mechanism leading to the sensitization of NCA. The sensitized NCA undergoes a second Dexter-type EnT either to the catalyst or to a substrate.

## Conclusions

Our results establish a clear mechanistic framework in which excitation of AuNP - either resonantly or non-resonantly - generates 6sp electron-hole pairs that transfer energy via a Dexter-type mechanism to an NCA acceptor, producing its triplet excited state. The latter can subsequently sensitize catalytic or substrate species, enabling downstream photochemical

transformations. Notably, non-resonant excitation decouples light absorption from the strict energy-level matching required in conventional EnT systems, thereby broadening the operational scope of photocatalysis. The AuNP photoluminescence serves as an indirect yet sensitive probe of this process, confirming the operation of the EnT pathway even in systems lacking direct optical signatures.

Collectively, these findings reveal that plasmon-mediated energy transfer enables direct activation of catalytically relevant gold complexes under both interband and plasmonic excitation. This dual-mode activation highlights a key advantage of plasmonic systems over molecular photosensitizers - their capacity for selective excited-state activation independent of donor-acceptor energy constraints. More broadly, this approach establishes plasmonic EnT as a generalizable and robust platform for light-driven catalysis, offering new opportunities for bond activation, energy conversion, and synthetic transformations beyond the limits of traditional photochemical systems.

## Methods

The methods are described in detail in supplementary information. Briefly: Tannic acid (Na₃C₆H₅O₇), sodium citrate tribasic dihydrate (Na₃C₆H₅O₇·2H₂O), hydrogen tetrachloroaurate (III) (HAuCl₄), 1-naphthoic acid (C₁₁H₈O₂; NCA) and (*p*-CF₃Ph)₃PAuCl were acquired in high purity from Sigma-Aldrich. Anhydrous K₂CO₃ was purchased from Alfa Aesar. Ir[d(CF₃ppy)₂(dtbbpy)PF₆ [(Ir-F)] was purchased from BLDpharm. 2-(*p*-Tolylethynyl)phenol was synthesized following the previous reported procedure.<sup>15</sup>, Acetonitrile (Acetonitrile) was obtained free of water and kept under argon atmosphere using a Solvent Purification System (SPS) (PureSolv MD 5). Milli-Q water was obtained from a Milli-D<sup>®</sup> System. Argon and Air gases used for purging the solution were obtained in high purity from Air Liquid.

### Synthesis of Au-CF₃

(2-(*p*-Tolyl)benzofuranyl)((tri(*p*-trifluomethyl)phenylphosphine)gold was synthesized as previously<sup>15,51</sup> reported starting from 2-(*phenylethynyl*) phenol instead of methyl-*N*-(2-(*p*-tolylethynyl)phenyl)benzenesulfonamide. In a 25-mL Schlenk tube, 120 mg (171.9 µmol, 1 eq.) of (*p*-CF₃Ph)₃PAuCl, 35.8 mg (171.9 µmol, 1 eq.) of 2-(*phenylethynyl*)phenol and 59.4 mg (429.8 µmol, 2.5 eq.) of K₂CO₃ were dissolved in 7 mL of dry acetonitrile and stirred for 16 h at r.t., covered with aluminium foil from the light. The mixture was filtered over a small column of basic aluminum oxide and washed with 50 mL of dry acetonitrile. The solvent was removed in the rotary evaporator without heating, giving [(Au-CF₃)] as white solid, 136.4 mg, 91.2%.

### AuNP synthesis

to synthesize the gold nanoparticles used in this study (AuNP) in water, a modified version of the Turkevich method was employed<sup>52</sup>. The method employs hydrogen tetrachloroaurate (III) as the gold precursor, tannic acid to adjust the solution's pH, and sodium citrate as both the reducing agent and stabilizer. In a 100 mL round-bottom flask, 50 mL of an aqueous solution containing 97.0 mg (0.33 mmol; 6.60 mM; 13 200 equiv.) of sodium citrate dihydrate and 212.65 mg (0.13 mmol; 2.5 mM; 5000 equiv) of tannic acid were stirred in an oil bath at 70 °C for 10 min. Then, 4.25 mg (25 nmol; 0.5 µM; 1 equiv.) of a solution of 25 mM HAuCl₄ (1 mL) was rapidly added to the mixture and the suspension was stirred for additional 5 min, resulting in an abrupt change of color from light yellow to deep red. The mixture was then protected from light, cooled to room temperature and then stored in the fridge at 4 °C. If there was decantation of the AuNP after 4 days, the yellow supernatant was removed, and 60 mL water was added to resuspend the nanoparticles in water and reduce the salt concentration.

### AuNP phase transfer to acetonitrile

High salt concentration can significantly hinder the phase transfer of AuNPs from water to organic solvents. To address this, the initial step of the transfer process focused on reducing the salt content in the aqueous AuNP
suspension, following a procedure adapted from the literature⁵³. Briefly, 2 mL Eppendorfs (10 per batch) containing 1 mL AuNP each (10 mL) were centrifuged (12000 rpm, 5 min), the supernatant was removed (around 15 µL of the aqueous suspension remained), and water was carefully added to avoid resuspension of the nanoparticles. After 10 min, the process was repeated without the centrifugation step. This washing cycle was performed up to 2 times, after which the supernatant was removed and acetonitrile was added to resuspend the suspension (typically 1 mL was used to mix 5 Eppendorfs). The 2 remaining Eppendorfs (2 mL) were then centrifuged (12,000 rpm, 5 min), the supernatant was removed and Acetonitrile was added so that the final concentration of water in the mixture was less than 0.2% (v/v).

## AuNP functionalization

To functionalize the AuNPs with NCA in acetonitrile, a 40 mM NCA solution was pre-prepared in acetonitrile. Afterwards, in a 4 mL vial, NCA, AuNP and acetonitrile were added so that the final concentration of NCA was 20 mM and AuNP had an A = 0.6 at λ<sub>max</sub> = 540 nm. NCA and acetonitrile were added first to avoid high concentrations of NCA, and then the AuNP as this seems to prevent nanoparticles aggregation. The purple suspension was shaken gently for 20 s and allowed to rest at room temperature, protected from light for 24 h. After that period, the suspension was stored at 4 °C until further use.

## AuNP characterization

The nanoparticles' size was assessed using Dynamic Light Scattering (DLS) in a Quartz cuvette (3 mL; samples in acetonitrile) or a disposable cuvette (1.5 mL; samples in H₂O). The measurements were complemented with Transmission Electron Microscopy (TEM). TEM sample was prepared by dropping a diluted AuNPs suspension onto a copper grid coated with a carbon film and allowing the water to evaporate in air. The TEM images were obtained using a Tecnai Spirit G2 microscope operating at 120 kV. The size distribution histogram and average size were obtained by manually measuring at least 400 particles using the ImageJ software. Raman spectroscopy measurements were carried out using an inVia confocal Raman microscope with 532 nm laser excitation and a 20× objective lens. All samples were analyzed in solution.

## Nanosecond transient absorption spectroscopy (nsTAS)

Spectra were recorded in the 1–10,000 ns time range and 330–730 nm range using pump-probe spectroscopy in a Quartz cuvette (3 mL). The source of the pump was an EKSPLA NT242 and the probe source was a LP920 (Edinburgh Instruments). For experiments under Ar, samples in the cuvette were initially bubbled with Ar for at least 10 min, then closed with a lid and measured instantly. For experiments identified as “O₂”, the sample was measured without addition of Ar or air was bubbled in the sample if Ar had been previously added. In all experiments, the absorbance of the light absorber was maintained between 0.2 and 0.3 a.u. The excitation power was adjusted to excite approximately 5% of the light absorbers within the probing volume, ensuring an adequate signal-to-noise ratio while minimizing excited-state quenching from inter-molecular collisions.

## Sensitized styrene [2 + 2]-cycloaddition

The energy transfer from AuNP mediated by NCA to a target molecule was tested with a model photocycloaddition of styrene, yielding trans-1,2-diphenylcyclobutane (DCPB), according to Scheme S1. A 2 mL solution containing styrene (100 mM, 5 equiv.) and NCA (20 mM; 1 equiv.) in acetonitrile was prepared in a 4 mL vial. The solution was purged with argon for 2 min before being transferred to a photoelectrochemical cell. As windows we used two FTO glasses with Au NP deposited on it prepared via physical vapor deposition and annealing, average particle size 12–15 nm⁵¹. The sample was irradiated with a 635 nm laser with a pulse cycle of 0.125 Hz (4 s ON/4 s OFF) for 44 h. The sample was then collected and analyzed by H¹-NMR spectroscopy without addition of deuterated solvent using the

solvent suppression technique. To study the system in the absence of photothermal effects, a filter was introduced so that the final intensity of light reaching the sample was less than 15 mW (compared to 130 mW without the filter) and pulses were shortened to 0.25 Hz (2 s ON/2 s OFF) during the same reactional time.

## Data availability

All data are available in the main text or the supplementary materials.

Received: 11 September 2025; Accepted: 8 January 2026;

Published online: 24 January 2026

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## Acknowledgements

The authors gratefully acknowledge Dr. Rima Charaf and Ms. Sree Chithra for their valuable assistance and training with the optical setups, which were essential for the experimental work presented in this manuscript. This manuscript was prepared with the support of language editing services and AI-assisted technologies, including the use of OpenAI's ChatGPT, which contributed to grammar and clarity improvements during the drafting process. Knut & Alice Wallenberg Foundation grant 2019-0071 (AA, JS).

## Author contributions

Conceptualization: L.F., J.S. Methodology: A.A., J.S. Investigation: A.A., J.H., S.B.S. Visualization: A.A., J.S. Funding acquisition: J.S. Project administration: J.S. Writing—original draft: A.A., J.S. Writing—review & editing: A.A., J.H., V.M.M., C.O., L.F., J.S.

## Funding

Open access funding provided by Uppsala University.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s43246-026-01077-1.

**Correspondence** and requests for materials should be addressed to Arthur Andreis, Louis Fensterbank or Jacinto Sá.

**Peer review information** *Communications Materials* thanks the anonymous reviewers for their contribution to the peer review of this work.

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