# Design and Investigation of Carbazole–Dithienylethene Compounds for Switchable Organic Radical Systems

Anastasiia Rysich, Ludovic Favereau, Julien Boixel

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Anastasiia Rysich, Ludovic Favereau, Julien Boixel. Design and Investigation of Carbazole–Dithienylethene Compounds for Switchable Organic Radical Systems. Journal of Physical Chemistry Letters, 2025, 16 (19), pp.4796-4803. 10.1021/acs.jpclett.5c00773. hal-05119456

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Distributed under a Creative Commons Attribution 4.0 International License
Design and Investigation of Carbazole-
Dithienylethene Compounds for Switchable Organic
Radical Systems

Anastasiia Rysich,ª Ludovic Favereau*ª and Julien Boixel*ª

ª Univ Rennes, CNRS, ISCR-UMR 6226, F-35000 Rennes, France.

* julien.boixel@univ-rennes.fr, ludovic.favereau@univ-rennes.fr

Photo-switchable radicals incorporating dithienylethene (DTE) moieties offer unique opportunities for dynamic control in various chemical and materials applications. By exploiting light-triggered isomerization processes, researchers can achieve precise modulation of radical reactivity, enabling spatiotemporal control of chemical transformations. Despite growing interest in this field, challenges remain in controlling spin-orbit coupling and spin dynamics, which influence the electronic structure and transitions induced by light absorption, as well as achieving photostability for repeated photoisomerization cycles. To address these challenges, we have designed a new family of photo-switchable organic radicals that incorporate DTE moieties and carbazole radical centers. We studied the influence of spin delocalization through the molecular structure and spin evolution upon light irradiation using UV/Vis absorption, EPR, and NMR spectroscopy. Our findings demonstrate the photochromic behavior of the neutral form and reversible radical formation, contributing to advancements in this promising field.
TOC GRAPHICS

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**KEYWORDS.** Dithienylethene, Carbazole, Photochromism, Organic Radical.

Organic radicals continue to attract significant interest in chemistry, biochemistry, and optoelectronics due to their open-shell electronic structure, which imparts unique properties such as narrow HOMO-LUMO energy gaps, low-lying doubly excited states, and redox amphotericism. Controlling and modulating spin delocalization in response to external stimuli is therefore crucial for the development of next-generation spintronic components, as it enables access to switchable bistable states at the molecular level.<sup>1–5</sup> A particularly promising approach for modulating spin-state delocalization is the use of light as a stimulus, which offers the advantages of being contactless, spatially resolved, and tunable in energy. The integration of organic radicals with molecular photoswitches has been previously explored by Irie et al.,<sup>6–11</sup> who functionalized a dithienylethene (DTE) unit with persistent organic radicals such as nitronyl nitroxide. This design enabled the photoswitching of intramolecular spin interactions from –2 K to approximately –12 K. Beyond DTE-based structures, other fully organic molecules capable of spin-state switching have been reported. Depending on their molecular design, both transient<sup>12,13</sup> and bistable<sup>14–16</sup> diradicals have been observed, exhibiting a combination of photo- and thermally activated
switching. While these results serve as compelling proof of concept, the localized spin density on the N and O atoms of the radical units used in these examples combined with the lack of a π-conjugated linkage to the DTE unit, limits the magnitude of photoswitchable intramolecular spin interactions.

Building on these pioneering works, we propose that functionalizing the DTE system with radical organic units capable of delocalizing spin density more effectively along the molecular backbone could result in stronger spin interactions in the closed form of DTE compared to the open form. This design requires a delicate structural and electronic balance, as enhancing radical delocalization on DTE can potentially impair its photoswitching properties. Leveraging our expertise in the photomodulation of linear and nonlinear optical outputs$^{17-21}$ and the preparation of persistent carbazole-based organic molecular radicals,$^{22,23}$ we aim to explore the potential of novel open-shell photoswitchable compounds. These compounds could serve as valuable building blocks for designing advanced open-shell photoswitchable materials.

The key motivation behind designing these novel open-shell switch derivatives lies in their ability to modulate radical delocalization across the molecule. In contrast to previous studies on organic radical-based DTEs, where radicals were deliberately kept localized throughout photochromic cycles,$^{7,8,11}$ this study employs carbazole moieties as radical units. These moieties are directly attached to the DTE core, enabling controlled radical delocalization during the photo-switching process. For the synthesis of the target molecules **M1** and **M2**, a convergent synthetic route is employed, involving the coupling of a pre-functionalized brominated DTE core with boronic carbazole units (Scheme 1).
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthetic route of **M2** and **M1**.</figcaption></figure>

The starting materials 3-OMeCbz pinacol borane **5** and bisBr-dithienylethene derivative **9** were synthesized according to the reported works.<sup>22,24</sup> Briefly, electrophilic iodination of a 3-OMeCbz following by palladium-catalyzed Miyaura borylation reaction was performed to obtain intermediate 5 in 60% yield. A final palladium-catalyzed Suzuki cross-coupling reaction of **5** with DTE-Br **9** was performed to afford in the same reaction mixture bis-substituted **M2** and mono-substituted **M1** in 25% and 32% yield, respectively. More detailed synthetic procedures and characterizations of the compounds are described in the Supporting Information (SI).

Electronic absorption of the neutral forms of **M1** and **M2** have been first investigated, their absorption spectra are presented on Figure 1 and the relative data are summarized in Table 2. The open-form isomers, **M1(o)** and **M2(o)**, exhibit very similar absorptions with intense bands at maxima 311 nm and 314 nm, respectively. Despite the additional Cbz unit in **M2(o)**, no red-shift is observed in comparison to **M1(o)**, indicating a limited electronic interaction between the two Cbz along the DTE unit, as usually observed for this type of system. These absorption bands encompass several excited states including the transitions at 315 nm (*f* = 0.27), 299 nm (*f* = 0.39) and 293 nm (*f* = 0.32) being the most intense and mainly correspond to HOMO→LUMO+2 (54%), HOMO-2→LUMO (54%) and HOMO-1→LUMO+1 (67%), respectively (Figure S20 and S22). They are essentially admixture of charge transfer (CT) character from the Cbz to ethylene-
thiophene parts and π-π* transitions. Indeed, the HOMO and HOMO-1 are almost exclusively
localized on the carbazole-thiophene part for both derivatives, while the LUMO and LUMO+1 are
spread out either on the DTE or the carbazole units, respectively (Table 1). The photochromic
behavior was investigated by absorption spectroscopy using 350 nm and 580 nm light sources (LS
Series Light Source from ABET Technologies, Inc., equipped with a 150 W xenon lamp) for ring-
closing and cycloreversion processes, respectively. Upon 365 nm light exposure, the solutions of
**M1** and **M2** both turn to deep-blue coloration, consequently to the appearance of large absorption
bands in the visible range (S₀→S₁ ≈ 500–750 nm and S₀→S₂ ≈ 410 nm), indicating the conversion
to the closed isomers **M1(c)** and **M2(c)** (Figure 1). The bis-Cbz DTE **M2(c)** exhibits a 50 nm
redshift of the lower-lying absorbance maxima, compare to **M1(c)**, indicative of enhanced
electronic delocalization over the closed DTE core. 580 nm light irradiation recovers back the
initial absorbance for both **M1** and **M2** compounds (Figure 1). This reversibility, together with the
identification of isosbestic points, are indicating of a stable photochromism.

To gain insight into the structure-properties relationship, ground-state geometry optimizations
and vibrational frequency calculations were performed using DFT/TD-DFT at the B3LYP/6-
31G+(d,p) level of theory. The computed electronic spectra for **M1(c)** and **M2(c)** indicate that for
both, the lower-lying S₀→S₁ transitions at 573 and 626 nm, respectively, are between the
molecular frontier orbitals (Table 1). In contrast to the open forms, the HOMOs of both **M1(c)** and
**M2(c)** are localized mainly on the DTE core with minor contributions of the Cbz moieties, and the
LUMOs show similar electronic density repartition with a stronger contribution on the C₅F₆ bridge,
which remains typical of closed DTE.²⁵ The second characteristic S₀→S₂ transitions are found at
around 410–430 nm and computed as HOMO-1→LUMO and HOMO-2→LUMO for **M1(c)** and
**M2(c)**, respectively (Figure S21 and S23).
<figure><img src="image_5.png" /><figcaption>**Figure 1.** UV-Vis absorption spectra of **M1** (top) and **M2** (bottom) in dichloromethane solution upon irradiations at 350 nm (cyclization) and 580 nm (cycloreversion) ($C \approx 10^{-5}M$).</figcaption></figure>

The photochromic reaction was followed by ¹H and ¹⁹F NMR spectroscopies, using a Rayonet® with 350 nm light emitting lamps (Figures S9 to S12). For **M1** and **M2**, characteristic ¹H NMR
spectral changes are observed, with shielded singlets shifting from 7.31 and 7.22 ppm to 6.80 and 6.28 ppm, respectively, corresponding to the thienyl protons of the DTE core. Additionally, there are high-frequency shifted singlets moving from around 1.98 to 2.17 ppm, corresponding to the methyl protons. The relative integration of these methyl signals between the open and closed forms indicates 85% of photoconversion at the PSS for **M1**. Interestingly, ¹⁹F NMR measurements clearly show the appearance of geminal couplings within each pair of fluorine atoms, indicative of the non-equivalence of the fluorine atoms upon ring closure (Figure S10). For **M2**, the ¹H NMR spectra following UV irradiation lead to similar conclusions, with a photoconversion of 95%. Additionally, the evolution of the ¹⁹F NMR spectrum exhibits similar features (Figures S11 and S12).

<figure><img src="image_6.png" /><figcaption>**Table 1.** Frontier molecular orbitals (HOMO/LUMO) of **M1** and **M2** in their open (left) and closed (right) forms (Isosurface value 0.02). The percentage contributions of the fragments (Cbz and DTE core) are specified.</figcaption></figure>
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HOMO

16.7 % DTE / 83.3 % Cbz

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HOMO

82.3 % DTE / 17.7 % Cbz

M2(o)

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LUMO

1.9 % Cbz / 96.4 % DTE / 1.7 % Cbz

M2(c)

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LUMO

12.0 % Cbz / 75.8 % DTE / 12.2 % Cbz

<figure><img src="image_11.png" /></figure>

HOMO

51.3 % Cbz / 20.9 % DTE / 27.8 % Cbz

<figure><img src="image_12.png" /></figure>

HOMO

12.3 % Cbz / 74.8 % DTE / 12.8 % Cbz
This outstanding photochromic behavior is further supported by geometry optimization calculations, which reveal that, in their open forms, both compounds meet the key criteria: (i) the antiparallel conformation is the most stable, and (ii) the distance between the two central carbon atoms is 3.55 Å.<sup>26,27</sup> Similarities of **M1** and **M2** extend to the torsion angles between the thiophen and the Cbz moieties. For both open forms of **M1(o)** and **M2(o)** this torsion angle is 28°deg and decreases at 12° and 16°deg for **M1(c)** and **M2(c)**, respectively. One can conclude that mono- or bis-Cbz functionalization give similar conformational and photochromic behavior.

Additionally, the ring-closure and ring-opening photokinetics were monitored under continuous irradiation at 350 nm and 580 nm, respectively, using a custom-built spectroscopic setup. This setup enables high-rate acquisition of absorption spectra under continuous irradiation within a thermostated cell holder maintained at 20 °C (Figure S25). The resulting time-profiles were analyzed numerically, yielding quantum yield values of 0.069 and 0.034 for $\Phi_{o \to c}$, and 0.034 and 0.003 for $\Phi_{c \to o}$, for **M1** and **M2**, respectively (Table 2). While these values fall within the common range for chromophore-based DTEs, a closer examination reveals that **M2** has a twofold lower $\Phi_{o \to c}$ and a tenfold lower $\Phi_{c \to o}$ compared to **M1**. This significantly lower $\Phi_{c \to o}$ for **M2** reflects an enhanced stabilization of the closed isomer **M2(c)** compared to **M1(c)**, consistent with greater electronic delocalization facilitated by bis-carbazole functionalization. This agrees with the observed and computed red-shifted, lower-lying absorption for **M2(c)** relative to **M1(c)**, which correlates with a lower stabilization energy for **M2(c)**.

Fatigue resistance over multiple open-closed cycles and thermal stability at the photostationary state (PSS) were evaluated (Figures S26 and S27, respectively). Compounds **M1** and **M2** exhibited no loss of photochromic activity over 10 switching cycles, indicating excellent long-term
photostability. Furthermore, the closed forms **M1(c)** and **M2(c)** demonstrated outstanding thermal stability at their PSS, remaining unchanged for several hours in acetonitrile solution at 298 K.

<table><caption>**Table 2.** Photochromic properties of **M1** and **M2**.</caption><thead><tr><th colspan="2">Open form</th><th colspan="2">PSS</th><th></th></tr><tr><th>Cpd</th><th>λ<sub>abs</sub><sup>a</sup> [nm]<br>(ε×10<sup>3</sup> [M<sup>-1</sup>cm<sup>-1</sup>])</th><th>λ<sub>abs</sub><sup>a</sup> [nm]</th><th>Conversion<sup>b</sup> [%]</th><th>Φ<sub>o</sub>→c<br>Φ<sub>c</sub>→o</th></tr></thead><tbody><tr><td><b>M1</b></td><td>279 (60.3), 331 (40.2), 348<br>(46.6), 403 (33.2), 422 (32.8)</td><td>279 (37.3), 345 (30.0),<br>368sh (22.7), 411 (5.0),<br>572 (5.5)</td><td>85</td><td>0.069±0.010<br>0.034±0.010</td></tr><tr><td><b>M2</b></td><td>280 (41.3), 333sh (23.0), 350<br>(34.4)</td><td>276, 327, 412sh, 622</td><td>95</td><td>0.037±0.010<br>0.003±0.010</td></tr></tbody></table>

The electrochemical behavior of both the open and closed forms of compounds **M1** and **M2** was investigated using cyclic voltammetry (Figure S13 and Table 3). For **M1(o)**, the first oxidation event occurs at approximately 1 V vs. SCE, is reversible, and involves mainly the Cbz-thienyl fragment, in line with our previously mentioned DFT calculations (①, Figure S13 and Table 1). This reversibility highlights the stability of the radical monocation **M1(o)**·⁺ in dichloromethane at room temperature. At higher potentials, irreversible oxidation processes are observed at approximately 1.5 V and 1.9 V vs. SCE (② and ③), corresponding to the second oxidation of the Cbz-thienyl fragment and the oxidation of the unsubstituted thienyl unit, respectively. This latter thienyl oxydation event is likely responsible for the observed irrversibility process due to unintended chemical reactions.

<table><caption>**Table 3.** Electrochemical data of **M1** and **M2**, in the anodic region.</caption><thead><tr><th>Cpd</th><th>Open form [V vs SCE]<sup>[a]</sup></th><th>PSS [V vs SCE]<sup>[a]</sup></th></tr></thead></table>

<footnote>a in CH₂Cl₂ at 298 K. b irradiation at 350 nm in CD₂Cl₂ (NMR tube) at 298 K.</footnote>
<table><thead><tr><th></th><th>E<sub>p,a</sub></th><th>E<sub>p,c</sub></th><th>ΔE [mV]</th><th>E<sub>p,a</sub></th><th>E<sub>p,c</sub></th><th>ΔE [mV]</th></tr></thead><tbody><tr><td></td><td>1.00</td><td>0.93</td><td>70</td><td>0.87</td><td>0.60</td><td>270</td></tr><tr><td>M1</td><td>1.56</td><td>Irr<sup>[b]</sup></td><td>/</td><td>1.09</td><td>irr</td><td>/</td></tr><tr><td></td><td>1.92</td><td>irr</td><td>/</td><td>1.58</td><td>irr</td><td>/</td></tr><tr><td></td><td>0.97</td><td></td><td>/</td><td>0.62</td><td>0.55</td><td>70</td></tr><tr><td>M2</td><td>1.11</td><td>0.99</td><td>/</td><td>1.63</td><td>irr</td><td>/</td></tr><tr><td></td><td>1.48</td><td>irr</td><td>/</td><td></td><td></td><td></td></tr></tbody></table>

<sup>a</sup> in deaerated dichloromethane solution (C<sub>M1/M2</sub> ≈ 10<sup>-3</sup> M; C<sub>TBAF</sub> = 0.1 M). <sup>b</sup> Irr = irreversible.

For **M2(o)**, the first two oxidation events are attributed to the two Cbz-thienyl fragments (1.0 and 1.1 V vs. SCE; ① and ②, Figure S14). The anodic peak separation (ΔE = 140 mV) indicates stepwise oxidations, suggesting significant electronic interaction between the two Cbz-thienyl fragments, once one is oxidized.

In the photostationary state (PSS), both **M1(c)** and **M2(c)** exhibit their first oxidation processes at less anodic potentials compared to their open-form counterparts, indicating destabilization of the HOMO. Given the high open-to-closed conversion rates (≈ 90%), the contributions of residual open forms are negligible in the PSS. The calculated electronic densities for the closed forms, **M1(c)** and **M2(c)**, reveal a significant redistribution of electron density compared to the open isomers, with the HOMO and the first oxidation events (HOMO → LUMO) primarily localized on the closed DTE cores (Figures S21 and S23). Consequently, the first redox event for **M1(c)** at 0.8 V vs. SCE (④, Figure S13) is attributed to the oxidation of the closed DTE core, likely with contributions from the Cbz fragment. Due to the unsymmetrical functionalization of the DTE core, a second oxidation event occurs at a slightly higher potential (1.1 V vs. SCE, ⑤, Figure S13) and is localized on the unsubstituted thienyl unit. This second oxidation process lacks reversibility,
presumably due to side reactions involving the oxidized unsubstituted thienyl fragment, as observed for **M1(o)**·⁺ at potentials > 1.5 V vs. SCE.

In contrast, **M2(c)** exhibits a single cathodic peak accounting for its first two redox events, with unusual reversibility ($E_{cat} \approx 1.0$ V vs. SCE, **④**, Figure S13). Based on previous studies²⁸,²⁹ and the calculated HOMO densities, these oxidations are primarily localized on the closed DTE fragment, with participation from the lateral Cbz units. This unusual reversible redox process is rationalized by the stabilization of a metastable dicationic species through isomerization between the open and the closed forms. At potentials >1.5 V vs. SCE, the Cbz units undergo their second irreversible oxidation (**⑤**, Figure S13). Additionally, a quasireversible reduction process at -1.3 V (**⑥**, Figure S13) suggests more complex electrochemical behavior, likely arising from the redox-active Cbz moieties and potential intramolecular electron transfer.

The formation and stability of radical species were monitored using UV, NMR, and EPR spectroscopy, starting from both open and closed isomers. Chemical oxidation and reduction processes were performed *in situ* in dichloromethane solution. Following previous reports on similar radical unit, oxidation was achieved by adding 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and trifluoroacetic acid (TFA), while sodium bicarbonate ($NaHCO_3$) served as the reducing agent.

The electronic absorption spectra of **M1(o)** and **M2(o)**, recorded during the chemical oxidation process, exhibit distinct electrochromic behaviors (Figures 3, top). Upon oxidation of **M1(o)**, characteristic absorption bands of a Cbz-based radical emerge, displaying a vibronic pattern with peaks at 600, 790, and 900 nm, along with a very broad near-infrared band extending from 1100 to 2000 nm (Figure 2, left/top). Comparison of this spectral signature with that of a reference cationic carbazole radical (tBuCbz⁺, Figure S16) confirms spin delocalization onto the thienyl
unit. Upon irradiation at 350 nm, the absorption spectrum of the **M1(o)**·⁺ solution undergoes a significant transformation, with the disappearance of the thienyl-carbazole signature and the emergence of a broad band spanning 500-1000 nm (Figure 2, left/top).

<figure><img src="image_13.png" /><figcaption>**Scheme 2.** Different pathways of sequential oxidation and photochromic reactions for **M1** and **M2**.</figcaption></figure>

Surprisingly, **M2(o)** exhibits distinct electrochromic behavior under similar oxidation conditions, without showing the characteristic spectral features associated with a localized Cbz radical. Instead, its absorption spectrum presents a broad band extending from 500 to 1000 nm (Figure 2, right/top). This spectral signature, combined with the absence of a localized thienyl-Cbz radical signal, suggests different radical delocalization, likely originating from a structure similar to the closed isomer: *M2(c)*²⁺. Indeed, consistent with previous studies on related compounds,²⁸⁻³⁰ the oxidation of both Cbz moieties in **M2(o)**·⁺ promotes radical delocalization, leading to the formation of two thienyl radicals that can subsequently facilitate the ring-closure reaction.

Oxidation experiments were performed on the closed isomers **M1(c)** and **M2(c)** (at the PSS) to further investigate the unusual behavior of **M2** upon oxidation. When starting from the closed isomer of **M1**, the addition of DDQ/TFA produces absorption features similar to those observed after UV irradiation (365 nm) of the oxidized open form (Scheme 2 and Figure 2, bottom left).
This indicates that the radical species $M1(c)•^+$ is generated upon oxidation followed by UV irradiation, regardless of the sequence in which these steps are applied (Scheme 2).

In the case of **M2**, oxidation of its closed isomer yields an absorption spectrum identical to that of the oxidized open form (Figure 2, bottom right). This peculiar electrochromic behavior suggests that, irrespective of the initial isomer – open or closed – oxidation of **M2** leads to the formation of either a single radical species or an equilibrium between two species, tentatively labeled $M2^{2+}$ (Scheme 2).

The reversibility of radical formation was verified by proton and fluorine NMR, as well as ESR experiments. As expected, oxidation of a solution of **M1(o)** resulted in no identifiable NMR signals. However, the ESR spectrum of $M1(o)•^+$ clearly exhibits a signal with faint hyperfine coupling, indicating significant delocalization of the radical over the thienyl-carbazole fragment (Figure S18). Notably, upon the addition of a reductant, the original NMR and ESR spectra of the neutral **M1(o)** compound are fully restored, confirming the excellent stability of the oxidized-open $M1(o)•^+$ species (Figures S14, top, and S18). Interestingly, when starting from the neutral closed form **M1(c)(PSS)**, the addition of DDQ/TFA produces an unresolved spectrum that remains unchanged even after the addition of a reductant (Figure S15). This observation suggests that, consistent with cyclic voltammetry experiments, the oxidized closed form $M1(c)•^+$ undergoes chemical degradation.

$M2(o)$ also exhibits reversible radical formation upon the sequential addition of DDQ/TFA and $NaHCO_3$ (Figure S14, bottom). However, in contrast to **M1**, radical reversibility is observed upon oxidation of the closed form $M2(c)$ (Figure 3). The unresolved $^1H$ and $^{19}F$ NMR spectra resulting from the oxidation of $M2(c)$ are restored to those of $M2(o)$ upon the addition of $NaHCO_3$ (Figures 3 and S17). Subsequent irradiation at 580 nm completes the ring-opening process, yielding the
fully open isomer. Subsequent electro-fatigue experiments were carried out using spectro-electrochemical measurements, with absorbance monitored at 585 nm over repeated electrochemical cycling between +1.0 V and -0.4 V (Figure S29). These electrochromic results further confirm the excellent electrochemical stability of **M2**, as no noticeable degradation was observed across multiple cycles.

<figure><img src="image_14.png" /><figcaption>**Figure 2.** *Left column:* Absorption spectra of **M1** upon sequential chemical oxidation then photo-irradiation (top), and photo-irradiation then chemical oxidation (bottom). *Right column:* Absorption spectra of **M2** upon sequential chemical oxidation then photo-irradiation (top), and photo-irradiation then chemical oxidation (bottom).</figcaption></figure>
This result, in agreement with cyclic voltammetry measurements and the oxidation of the closed isomer, confirms the formation of the $M2^{2+}$ radical depicted in Scheme 2. $M2^{2+}$ is likely in equilibrium between its closed and open forms, with significant charge delocalization along the molecular backbone. Within this equilibrium, as $M2^{2+}$ planarizes, electronic delocalization extends across the molecule, leading to the observed broad, low-energy absorption. This lower-energy electronic state accounts for the lack of photoreactivity under 580 nm light excitation. Further evidence for this equilibrium-induced electronic delocalization comes from the ESR spectrum, which exhibits a doublet signal without detectable hyperfine coupling (Figure S19).

<figure><img src="image_15.png" /><figcaption>**Figure 3.** Proton NMR spectra of **M2** upon sequential photo-irradiation then chemical oxidation and reduction (400 MHz, $CD_2Cl_2$).</figcaption></figure>

Undoubtedly, functionalizing DTE with radicals capable of delocalizing their spin density across the molecule leads to complex systems. In contrast to pioneering studies on nitronyl nitroxide-based DTE, where localized open-shell radicals do not interfere with photochromic behavior, the radical state of **M2** exhibits significant spin density delocalization, which hinders its photochromic reactivity. However, the radical compound $M2^{2+}$ demonstrates exceptional stability in solution at
room temperature, attributed to an equilibrium between pseudo-open and closed radical structures.
This unique behavior enables the conversion of **M2(o)** into its open-shell radical form **M2**<sup>2+</sup> either
directly *via* oxidation or through a sequential process involving light activation followed by radical
generation.

To conclude, original carbazole-based dithienylethenes capable of producing stable open-shell
radical systems have been investigated. Notably, the doubly functionalized molecule **M2**, in its
doubly open-shell state, undergoes a ring-closure reaction accompanied by delocalization of the
radicals across the molecule. Data from absorption, NMR, and EPR spectroscopy collectively
establish the unprecedented reversible formation of a bis-radical species. In contrast, the mono-
functionalized analogue **M1** does not produce the closed isomer in its mono-oxidized state,
retaining the radical localized on the carbazole unit. The ability to couple a parallel photochromic
reaction with the reversible formation of a bis-radical species makes this design both unique and
inspiring for the development of organic radical switches.

ASSOCIATED CONTENT

Supporting Information (SI): General information, synthesis way and characterization, NMR,
voltamperometry, EPR experiments.

AUTHOR INFORMATION

**Anastasiia Rysich** - Université de Rennes, ISCR - UMR 6226, F-35000 Rennes, France

Ludovic Favereau - Université de Rennes, ISCR - UMR 6226, F-35000 Rennes, France

Julien Boixel - Université de Rennes, ISCR - UMR 6226, F-35000 Rennes, France

Notes: The authors declare no competing financial interests.
ACKNOWLEDGMENT

We acknowledge the Ministère de l'Éducation Nationale, de la Recherche et de la Technologie, the Centre National de la Recherche Scientifique (CNRS). This project is cofunded by the European Union (ERC), SHIFUMI, 101041516 (L.F.). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them. We sincerely thank Dr. Olivier Galangau for granting us access to his custom-built spectroscopy setup for photokinetic time-profile measurements.

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