# Unconventional photoswitching behavior of a diarylethene-perylenediimide dyad in nanoparticles emerging from multiple intermolecular energy transfer processes

Nicolas Fabre, Tuyoshi Fukaminato, Issei Ikariko, Léa Chocron, Arnaud Brosseau, Rémi Métivier

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Nicolas Fabre, Tuyoshi Fukaminato, Issei Ikariko, Léa Chocron, Arnaud Brosseau, et al. Unconventional photoswitching behavior of a diarylethene-perylenediimide dyad in nanoparticles emerging from multiple intermolecular energy transfer processes. Advanced Optical Materials, 2024, 12 (22), pp.2400452. 10.1002/adom.202400452. hal-04797038

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# Unconventional Photoswitching Behavior of a Diarylethene-Perylenediimide Dyad in Nanoparticles Emerging from Multiple Intermolecular Energy Transfer Processes

Nicolas Fabre, Tuyoshi Fukaminato,* Issei Ikariko, Léa Chocron, Arnaud Brosseau, and Rémi Métivier*

Photoswitchable fluorescent systems based on photochromic derivatives have various applications, including high-resolution imaging and information processing. In this context, a key objective is to control, manipulate, and improve the fluorescence photoswitching of molecular systems. Herein, the intriguing zero-order photokinetics properties of a photochromic-fluorescent dyad made of one perylenediimide fluorophore (PDI) and two diarylethene photochromes (DAE) are reported, in the nanoparticles state, resulting from efficient intra- and intermolecular energy transfer processes from the PDI to the DAE, well-rationalized by modified photokinetics equations for photochromism. It allowed the fine-tuning of the photokinetics, leading to either classical or unconventional behavior, depending on the irradiation wavelength. Fluorescence photoswitching is investigated under a microscope, demonstrating a sigmoidal profile for fluorescence recovery which can be satisfyingly reproduced by a suitable mathematical model that is developed. These findings represent a significant advance in the development of enhanced photoswitchable fluorescent systems, showing high contrast, driven by few photons with tunable switching rates.

N. Fabre, L. Chocron, A. Brosseau, R. Métivier

ENS Paris-Saclay

CNRS

PPSM

Université Paris-Saclay

Gif-sur-Yvette 91190, France

E-mail: remi.metivier@ens-paris-saclay.fr

T. Fukaminato, I. Ikariko

Department of Applied Chemistry and Biochemistry

Kumamoto University

Kumamoto 860-8555, Japan

E-mail: tuyoshi@kumamoto-u.ac.jp

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202400452

© 2024 The Author(s). Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adom.202400452

## 1. Introduction

Molecular switches are widely used in various fields such as optical data storage, molecular logic gates, and smart imaging technologies.[1] Fluorescence-switching materials are particularly appealing for super-resolution imaging methodologies.[2-8] A highly efficient way to obtain such systems is to combine a photochromic molecule with a fluorophore.[9-12] Diarylethenes (DAEs) are extensively used to obtain switches due to their excellent fatigue resistance and the high stability of both isomers.[13] When combined in a dyad with a suitable fluorophore, through a non-conjugated link, fluorescence photoswitching can be achieved by means of intramolecular Förster resonance energy transfer (FRET).[14] Such dyads are typically fluorescent in the open-form (OF) but quenched in the closed-form (CF), where the DAE-CF plays the role of energy acceptor while the fluorophore assumes the role of donor.[9,15]

This strategy has been successfully exploited, and many molecular devices based on this concept have been already described in the literature.[16-20] To take full advantage of energy transfer, gathering dyads into nanoparticles (NPs) or polymer matrices can be an excellent strategy,[6,21-28] We recently discovered that molecular NPs made of DAE linked to a benzothiadiazole fluorophore displayed a "giant amplification of fluorescence photoswitching" (GAFP): NPs are strongly emissive when the dyad is in the OF and fluorescence is totally quenched even at low conversion yield.[29] To enhance the performance of dyads, perylenediimide (PDI) was reported as an excellent fluorophore candidate due to its bright and stable emission.[30] DAE-PDI dyad also exhibits a GAFP effect, based on efficient and ultrafast energy transfer, with high photoresistance.[31-33] To achieve fluorescence photoswitching with a minimal number of photons, improving the photochromic quantum yield is a key issue. The cycloreversion reaction (CF→OF) is known to be affected by the molecular design of the photochromic unit and by external parameters such as the temperature or irradiation wavelength.[34-36]
<figure><img src="image_4.png" /><figcaption>**Figure 1.** a) Chemical structure and photochromism of dyad 1. b) Absorption (full lines) and emission (dashed lines) spectra of a colloidal suspension of NPs of 1 ($1 \times 10^{-5} \text{ mol L}^{-1}$ in THF/H$_2$O 20:80, optical path 0.2 cm) in the OF (red lines) and at the photostationary state (PSS, blue lines) under irradiation at 313 nm.</figcaption></figure>

Determination of the photochromic quantum yield was reported for compounds in solution or thin film,[37,38] but its measurement and optimization in the NPs state have not been described to date. Moreover, the strong effect of multiple energy transfer processes on the photoswitching ability of NPs remains unexplored.

We aim to fill the aforementioned gaps by conducting a comprehensive investigation and rationalization of the photoswitching behavior of NPs. Specifically, we focused on a novel dyad composed of a PDI moiety linked to two DAE units, as shown in **Figure 1a**. We investigate the impact of energy transfer on the photochromic transformation and efficiency. We report novel photokinetic properties, finely tunable by changing the irradiation wavelength. Furthermore, we successfully demonstrate GAFP in colloidal suspensions of NPs, as well as in single NPs. Notably, fluorescence recovery revealed a threshold effect rationalized with a suitable mathematical model.

## 2. Results and Discussion

The molecular dyad 1, whose structure and photochromic reaction are depicted in Figure 1a, was synthesized through a zinc

coupling reaction with DAE and PDI precursors (Section S1.2, Supporting Information). Moreover, 1 exhibits thermal stability until 350 °C (Section S1.3, Supporting Information) Absorption and fluorescence spectra of 1 in the OF and CF in THF solution are given in Figure S1 (Supporting Information) (Section S2.1, Supporting Information). Colloidal suspensions of NPs were prepared by the conventional reprecipitation method (Section S1.4, Supporting Information).[39] Atomic force microscopy (AFM) imaging of NPs deposited on a glass substrate revealed flattened sphere geometry with lateral diameters ranging from 30 to 100 nm and heights from 10 to 30 nm. The average number of molecules per single NP was estimated to be approximately $\eta \approx 1.4 \times 10^4$ (Sections S2.2–S2.3, Supporting Information).[40]

The absorption and emission spectra of 1-OF in NPs are shown in Figure 1b (red lines), with a structured absorption band from 500 to 630 nm and another from 400 to 500 nm, both attributed to the PDI moiety. The absorption band in the UV range with a maximum of 290 nm corresponds to both DAE and PDI units. The 1-OF isomer emits in the red range (600–800 nm, from the PDI fluorophore). Upon irradiation at 313 nm, a photostationary state (PSS) was reached, and an absorption band related to DAE-CF rose from 480 to 710 nm, while the PDI fluorescence completely vanished (Figure 1b, blue lines). The fatigue resistance of compound 1 in THF solution and NPs (irradiation cycles at 313 nm then visible light, Section S2.1, Supporting Information) is satisfactory. Compound 1 also demonstrates excellent photostability

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Photokinetics of cycloversion reaction of 1 in NPs, under irradiation (1 mW) at a) 640 nm, b) 575 nm, and c) 435 nm. Curve fits are added in full red lines and interval errors are visualized in the grey area.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Schematic representation of the photocycloreversion mechanisms, involving either a) direct absorption of light by the DAE-CF units (irradiation at 640 nm), or b) indirect excitation (i.e., antenna-driven, irradiation at 575 or 435 nm) via absorption of light by the PDI units followed by intramolecular or intermolecular FRET processes to the DAE-CF. The first mechanism leads to conventional cycloreversion photokinetics, whereas the second mechanism provides the basis for unconventional photochromic zero-order photokinetics.</figcaption></figure>

(see Section S2.1, Supporting Information). The absolute fluorescence quantum yield of 1-OF was measured at $\Phi_F = 0.93 \pm 0.07$ (THF solution) and $\Phi_F = 0.10 \pm 0.01$ (NPs). This decrease of $\Phi_F$ in NPs is likely due to some degree of aggregation-caused quenching but is limited by the bulky substituents at the bay position of PDI.[31,32] The Förster radius $R_0$ of the PDI→DAE-CF excitation energy transfer was estimated to be 72 Å in THF,[41] leading to a FRET efficiency approaching unity (>0.99) (see details in ESI, Section S2.4, Supporting Information). Although $R_0$ is lower in NPs (32 Å), the FRET is expected to be more efficient since each PDI fluorophore can be quenched by multiple DAE units located in its close vicinity, through multiple intermolecular energy transfer processes.

The impact of FRET on the photokinetic reaction was examined in detail. **Figure 2** displays the photokinetics observed for the cycloreversion reaction (DAE-CF→DAE-OF) induced at various irradiation wavelengths (640, 575, and 435 nm) with identical power values (1 mW) in the NPs state (see ESI, Section S2.5 (Supporting Information), for additional photokinetics curves). The profile obtained under irradiation at 640 nm (Figure 2a) shows a conventional shape and can be fitted numerically with a standard photokinetic model.[37] Most interestingly, absorption timeevolutions obtained when irradiating at 575 and 435 nm (Figure 2b,c) proved to be unconventional, showing unprecedented zero-order photokinetic profiles. To explain this observation, we developed a novel photokinetic model, considering the potential impact of FRET-induced DAE-CF → DAE-OF photoreaction:

$$ \frac{dC_{\text{OF}}(t)}{dt} = -\Phi_{\text{OF}\to\text{CF}} F(\lambda_{\text{irr}}) I_0(\lambda_{\text{irr}}) n_p \epsilon_{\text{OF}} l C_{\text{OF}}(t) \\ +\Phi_{\text{CF}\to\text{OF}} F(\lambda_{\text{irr}}) I_0(\lambda_{\text{irr}}) n_p \epsilon_{\text{CF}} l C_{\text{CF}}(t) \\ +\Phi_{\text{FRET}} \left[1 - (1 - \alpha_{\text{CF}})^N\right] \\ \Phi_{\text{CF}\to\text{OF}} F(\lambda_{\text{irr}}) I_0(\lambda_{\text{irr}}) n_F \epsilon_F l C_{\text{tot}} \quad (1) $$

with $\Phi_{\text{OF}\to\text{CF}}$ and $\Phi_{\text{CF}\to\text{OF}}$ the photoinduced cyclization and cycloreversion quantum yields, $\Phi_{\text{FRET}}$ the FRET efficiency, and N the number of DAE contained in the Förster sphere of the PDI fluorophores (see ESI, Section S2.5, for full description of the equation). The proposed Equation (1) consists of three terms. The first and second ones are related to the photocyclization and photocycloreversion reactions induced by direct absorption of photons by the DAE unit (Figure 3a). The third one corresponds to the cycloreversion triggered by indirect excitation, i.e., antenna-driven photoswitching effect allowed by multiple intramolecular and intermolecular FRET processes from the PDI to the DAE-CF units contained in the Förster sphere (Figure 3b). The curve fitting analysis demonstrates that this model provides an accurate description of the experimental data, as shown in Figure 2b,c. At 640 nm, only the DAE units absorb the irradiation light, and the cycloreversion induced by energy transfer cannot occur (direct absorption only, Figure 3a). By using irradiation at 575 nm, both the DAE and PDI units absorb light, and the ring-opening reaction is predominantly induced by the energy transfer from PDI to DAE-CF in NPs (mostly indirect absorption, Figure 3b). This phenomenon leads to the observed zero-order photokinetic profile in NPs, explained by the constant absorption of PDI throughout the irradiation period. This effect fully governs the behavior obtained under irradiation at 435 nm, where only the PDI unit absorbs light (only indirect absorption, Figure 3b). The values of both $\Phi_{\text{CF}\to\text{OF}}$ and N obtained by fitting the experimental curves using Equation (1) are summarized in Table 1. $\Phi_{\text{CF}\to\text{OF}}$, obtained

<table><caption>**Table 1.** $\Phi_{\text{CF}\to\text{OF}}$ and number of DAE units in Förster sphere (N) for NPs of 1 at various irradiation wavelengths.</caption><thead><tr><th>λ<sub>irr</sub>/nm</th><th>Φ<sub>CF→OF</sub></th><th>N</th></tr></thead><tbody><tr><td>435</td><td>3.3 (± 0.03) × 10<sup>-3</sup></td><td>20–400</td></tr><tr><td>575</td><td>2.3 (± 0.03) × 10<sup>-3</sup></td><td>20–400</td></tr><tr><td>640</td><td>1.5 (± 0.04) × 10<sup>-3</sup></td><td>—</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Figure 4. Correlation plot of the normalized fluorescence versus OF→CF conversion yield in THF (blue circles) and NPs (red squares). Curve fitting is added in full black lines.</figcaption></figure>

from the linear portion of the photokinetic curves, increases at higher irradiation photon energy, from 1.5 × 10⁻³ at 640 nm to 3.3 × 10⁻³ at 435 nm, which is consistent with previously reported on DAE systems.[35] The number N of potential DAE quenchers in the Förster sphere, responsible for the curvature of the photokinetic profile reaching zero, was estimated to be in the range 20–400 DAE units with a relatively large interval error (grey areas in Figure 2): the larger the N value, the more linear the absorption time-profile is. It is noteworthy that the fastest cycloreversion in NPs occurs under illumination at 575 nm, which is consistent with the antenna-driven photoswitching effect, predominantly observed at the highest absorption efficiency of the PDI fluorophores.

The correlation plots of the normalized fluorescence intensity ($I_{Fluo}$) as a function of the conversion yield OF→CF ($\alpha_{CF}$) in THF solution and NPs are presented in Figure 4, with their fitting curves using Equation (2) (Section S2.6, Supporting Information):

$$I_{Fluo} (\alpha_{CF}) = (1 - \alpha_{CF})^N \quad (2)$$

Different behaviors are observed comparing the two situations. In the THF solution, a linear relationship could be expected, based on our previous reports.[29,31] However, the initial slope intercepts the x-axis at 0.5, consistent with the presence of two DAE units available to quench the PDI (consistent with Equation (2) and N = 2). In contrast, a GAFP was observed in the NPs state, where only a few DAE-CF units (<2% conversion yield) were required to completely turn OFF the fluorescence of the entire NP (in agreement with N reaching 400, cf. Table 1). The plots of the fluorescence recovery as a function of the irradiation energy supplied to the system are displayed in Section S2.8 (Supporting Information). A classical situation appears in the THF solution while in the NPs state, a threshold effect occurs. A certain amount of energy has to be provided to the system until a sufficient number of DAE units is converted to the OF before the PDI units can emit again. The combination of Equations (1) and (2) allows to obtain the following expression of the normalized fluorescence intensity $I_{Fluo}$ versus the energy E delivered to the system:

$$I_{Fluo} (E) = (1 - \alpha_{CF}^0 \times e^{-K \times N \times E})^N \quad (3)$$

with $\alpha_{CF}^0$ the conversion yield before irradiation and K a constant related to the molecular system (Section S2.7, Supporting Information). Curve fitting was performed using Equation (3) (Section S2.8, Supporting Information) and confirmed the previously determined values of numbers of DAE contained in the Förster sphere: N = 2 for the dyad 1 in THF solution and N = 450 for 1 in NPs.

To demonstrate the persistence of such an unconventional behavior of dyad 1 at the single NP level, the fluorescence photoswitching of NPs deposited on glass was examined by microscopy. To ensure that individual NPs were observed, AFM images were recorded and correlated with fluorescence images as shown in Figure 5a,b. Individual NPs demonstrated efficient fluorescence photoswitching: fluorescence was quenched in ≈3 ms on average ($\lambda_{irr}$ = 375 nm), and could be recovered using light at 514 nm to induce the cycloreversion photochromic reaction (Figure 5c). About 70 ± 15 ms were required to recover 50% of the final fluorescence level. The number of DAE in the Förster sphere was estimated using Equation (3) to be N = 400, leading to sigmoidal fluorescence recovery profiles. No relationship was found between the number of dyads per NPs and the time to recover 50% of the initial fluorescence signal (Section S2.9, Supporting Information). Further studies on a larger number of NPs will be carried out in the future to provide more statistical information.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** a) AFM and b) confocal fluorescence images of five NPs, together with c) their fluorescence photoswitching profiles (probe beam: 514 nm, 500 µs at 2 mW) under irradiation at 375 nm (violet box, irradiation beam: 500 µs at 25 mW).</figcaption></figure>

3. Conclusion

In conclusion, our study provides a comprehensive investigation of the photoswitching behavior of a new DAE-PDI dyad in the NPs state, showing how the multiple intramolecular and intermolecular FRET processes affect the photokinetics and lead to novel antenna-driven photoswitching effects. Our findings reveal an unprecedented zero-order behavior, which accelerates the rate of the photoinduced cycloreversion reaction, tunable with the irradiation wavelength, and we propose a suitable photokinetic model to account for this effect. Furthermore, dyad 1 exhibits a GAFP in the NPs state, leading to efficient fluorescence quenching, one DAE-CF being able to quench ≈400 PDI fluorophores: less than 2% conversion yield is required to turn OFF the entire

fluorescence signal. A promising threshold effect in the fluores-
cence recovery profile was observed, suggesting that the system
requires a certain amount of irradiation energy before the revival
of fluorescence. We finally demonstrate that such amplified fea-
tures can be observed down to the single NP level under a micro-
scope, with highly efficient and fast quenching (≈3 ms to switch
OFF) and threshold-based fluorescence recovery (≈70 ms for the
reappearance of 50 % of the final fluorescence level). All these
amplified properties are governed by the multiple intermolecu-
lar FRET. Our study sheds light on the unique properties of NPs
made of photochromic-fluorescent dyads, offering a new perspec-
tive on the exploration and development of switching materials
or devices which require high efficiency, brightness, contrast and
non-linear behavior.
**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

The authors thank Lysa Dhamani for her help in the preliminary study of the molecular system. Funding from the Agence Nationale de la Recherche (ANR-17-CE07-0056-01) is acknowledged. The authors are grateful to the CNRS for supporting this work in the framework of the NanoSynergetics International Research Project (IRP).

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords**

fluorescence, microscopy, nanoparticles, photochromism, photokinetic

Received: February 27, 2024

Revised: April 16, 2024

Published online: June 25, 2024

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