# Bifunctional Molecular Switches in Fused Diarylethene–Rhodamine Systems

Bassima Hoteit, Shunan Zhao, Corentin Poidevin, Arnaud Fihey, Julien Boixel

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Bassima Hoteit, Shunan Zhao, Corentin Poidevin, Arnaud Fihey, Julien Boixel. Bifunctional Molecular Switches in Fused Diarylethene–Rhodamine Systems. ChemPhotoChem, 2025, pp.e202500304. 10.1002/cptc.202500304. hal-05408495

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RESEARCH ARTICLE

OPEN ACCESS

# Bifunctional Molecular Switches in Fused Diarylethene-Rhodamine Systems

Bassima Hoteit¹ | Shunan Zhao² | Corentin Poidevin¹ | Arnaud Fihey¹ ID | Keith Man-Chung Wong² ID | Julien Boixel¹ ID

¹Univ Rennes, CNRS, ISCR-UMR 6226F, Rennes, France | ²Department of Chemistry, Southern University of Science and Technology, Shenzhen, P. R. China

**Correspondence:** Arnaud Fihey (arnaud.fihey@univ-rennes.fr) | Keith Man-Chung Wong (keithwongmc@sustech.edu.cn) | Julien Boixel (julien.boixel@univ-rennes.fr)

**Received:** 9 September 2025 | **Revised:** 14 November 2025 | **Accepted:** 17 November 2025

**Keywords:** chemosensing | diarylethene | photochromism | rhodamine | TD-DFT

## ABSTRACT

A synthetic route for a novel rhodamine-based diarylethene (DAE) systems 1 and 2 is reported. The key synthetic steps involve the preparation of 6'- and 7'-bromo rhodamine precursors, their conversion into azide derivatives, and subsequent Huisgen 1,3-dipolar cycloaddition with a perfluorocyclopentene acetylene unit. The target compounds, 1 and 2, isolated in their “closed” rhodamine and “open” DAE forms, are characterized through nuclear magnetic resonance (NMR), infrared, and UV-vis absorption spectroscopies, revealing distinct electronic environments for the triazole moiety in both derivatives. Spectroscopic studies highlight their intense UV-vis absorption with minimal visible-light absorption, supported by theoretical time-dependent density-functional theory (TD-DFT) calculations. Investigation of chemo-switching behavior with trifluoroacetic acid and metal ions in dichloromethane reveals a two-step protonation process involving the triazole and rhodamine moieties, confirmed by UV-vis absorption and NMR spectroscopies. Such a process is reversible upon triethylamine addition. Selectivity tests show strong responses toward Hg²⁺ and Zn²⁺, for compound 1 displaying an additional response with Fe³⁺. Photoswitching experiments and TD-DFT calculations reveal distinct behaviors for compounds 1 and 2. UV irradiation of 1 results in the formation of an unexpected open-spiro rhodamine species, potentially due to photoinduced acid production. In contrast, 2 undergoes a different spectral feature arising from a degraded product with lower-lying transitions.

## 1 | Introduction

Fluorescent molecular switches, which undergo reversible transitions between states in response to external stimuli such as light, chemical inputs, or electric signals, have attracted considerable attention for their potential applications in sensors [1], bioimaging [2], high-density optical data storage, and molecular logic gates [3–5]. Broadly, molecular switches can be classified into two categories: (i) kinetic molecular switches (KMS), which operate under kinetic control, and (ii) thermodynamic molecular switches (TMS), which are governed by thermodynamic equilibrium. KMS are typically photochromic systems that interconvert between two kinetically stable states upon irradiation. The

photogenerated state of a KMS would persist without further stimulus. In contrast, TMS responds to the presence of specific ions by adopting a new thermodynamically stable configuration that remains in equilibrium with the environment, while the removal of the ions restores the system to its original state.

Numerous efforts have been devoted to integrating both kinetic and thermodynamic switching abilities into a single molecule, thereby enabling independent modulation of fluorescence by light irradiation and ion stimuli [6–9]. Among the various photo-responsive KMS, diarylethenes (DAEs) are particularly attractive due to their exceptional thermal stability, outstanding fatigue resistance, high photoisomerization efficiency, and fast response. DAEs undergo reversible transitions between open-ring and

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© 2025 The Author(s). *ChemPhotoChem* published by Wiley-VCH GmbH.
closed-ring isomers under UV/visible light, corresponding to colorless and colored states, respectively. This can mimic binary code "on/off" optical switches [10]. In contrast, rhodamine-based dyes have emerged as prototypical TMS systems owing to their high fluorescence quantum yields, large molar extinction coefficients, and excellent chemosensitivity [11, 12]. They exist in two interconvertible conformations: a colorless, nonemissive spirocyclic form and a highly fluorescent, deeply colored open form.

The design of bifunctional diarylethene-rhodamine (DAE-Rho) systems typically involves covalent linkage of photochromic DAE units with ionochromic rhodamine moieties, often achieved through conjugation reactions to promote synergistic interactions between the two components [2, 13–16]. In many reported systems, dimethylene spacers are employed to connect the DAE and rhodamine units, enabling efficient electronic communication [17]. The nature of the linker critically influences the overall photophysical performance. For instance, pyrroledione bridges have been shown to enhance both spectral and kinetic properties of the photochromic and ionochromic responses [17], while triazole linkers have been utilized to facilitate strong DAE-Rh interactions, thereby enabling multi-addressable switching behaviors [18].

To date, most reported DAE-Rho systems achieve photoswitchable fluorescence through fluorescence resonance energy transfer (FRET) from the rhodamine dye to the ring-closed isomer on the DAE unit [13–18]. However, involvement of the FRET process in the photoregulation of luminescence renders these systems highly sensitive to external factors such as polarity and electric fields, which may compromise switching efficiency [19, 20]. Moreover, the photoswitching function of DAE has not yet been fully exploited to modulate the chemosensitivity of rhodamine. To address these challenges, a redesign of a system from the combination of DAE and rhodamine is necessary to achieve efficient photoregulation independent of FRET, thereby enhancing stability and functionality [21, 22].

Herein, we report a novel molecular design strategy inspired by our previous work on rhodamine-based platinum(II) complexes, in which rhodamine functions as an alkyne ligand [23]. In that

study, we demonstrated that the position of the alkyne bridge on the rhodamine framework (6' or 7') plays a crucial role in modulating electronic delocalization across the entire molecule, while maintaining the intrinsic chemo-switching ability of the rhodamine core.

In our pursuit of novel photoactivatable molecular systems, we conceived the idea of combining a rhodamine moiety with a photochromic diarylethene unit, inspired by the "click" chemistry approach reported by Jäschke et al. [24]. This integration would provide a single molecule with two potential stimuli (chemo and photo), which can be activated independently, i.e., without interference. The independent activation of the photochromic and chemoswitching functions within a single molecule offers a conceptual framework for multi-input logic operations. Such bifunctional systems can thus serve as minimal molecular prototypes for AND/OR gate functions and memory devices, as discussed in related studies of photo-acid-base controlled switches [25, 26]. Given our objective to explore the photo- and chemo-switching behaviors without the FRET mechanism, it was imperative to electronically fuse the two chromophores.

## 2 | Results and Discussion

### 2.1 | Synthesis and Characterization

The synthesis route for rhodamine-based DAE compounds **1** and **2** is shown in Scheme 1. The 6'- and 7'-bromo rhodamine precursors are prepared following the reported condensation reaction [27]. Their azide-terminated rhodamine derivatives, 6'-RhoN₃ and 7'-RhoN₃, were then formed from the precursors in classical conditions with the presence of sodium azide and copper iodide as catalyst. In parallel, a Huisgen 1,3-dipolar cycloaddition reaction between 6'-RhoN₃ or 7'-RhoN₃ and the reported perfluorocyclopentene acetylene (Alc) [24], afforded the target rhodamine-based DAE **1** and **2**. It is noteworthy that compounds **1** and **2** are isolated in their rhodamine "closed" and DAE "open" forms, as evidenced by nuclear magnetic resonance (NMR), infrared (IR), and absorption spectroscopies.

<figure><img src="image_3.png" /><figcaption>**SCHEME 1** | Synthetic route for compounds **1** and **2**. i) H₂SO₄, 100°C, 3 h. ii) N,N' DMEDA, EtOH/H₂O, CuI, NaN₃, 100°C, 16 h. iii) NaNO₂, HCl, H₂O, DMF, NaN₃, 0°C, 1 h. iv) ascorbic acid, CuSO₄·5H₂O, DMF, 50°C, 72 h.</figcaption></figure>
Particularly, the ¹H signal of the triazole moiety appears upfield shifted for **2** at 7.89 ppm, compared to that of **1** at 7.63 ppm (Figure 1, left). This difference in chemical shift reflects the higher electronic density at the para-position of the oxygen of the xanthene backbone. Electronic absorption spectra of **1** and **2** are presented in Figure 1 right, and their data are summarized in Table 1. Intense absorption in the UV region (band at 280 nm and a shoulder at around 340 nm, typically assigned to π-π*) is observed in **1** and **2**. There is almost negligible transition within the visible window for compound **1**, while an absorption tail to 450 nm is found for compound **2**. The weak absorption tail extending to 450 nm for compound **2** likely arises from a modest extension of π-conjugation

through the 7'-oxygen position of the xanthene framework, which enhances electronic delocalization toward the DAE core.

The computed spectra (see computational details in ESI) for **1** and **2** nicely agree with those experimental results, with a maximum of the lowest absorption band found at 304 nm and 306 nm, respectively (Figure 2, vide infra for a description of the transition).

## 2.2 | Chemo-switching

The investigation of the chemo-switching behavior of the rhodamine part in **1** and **2** is conducted in dichloromethane solution in

<figure><img src="image_4.png" /><figcaption>**FIGURE 1** | *top*: Partial ¹H NMR spectra of **1** and **2** (CDCl₃, 298 K). *bottom*: UV-vis absorption spectra of **1** and **2**, in CH₂Cl₂ (C ≈ 10⁻⁵ M, 298 K).</figcaption></figure>
<table><caption>**TABLE 1** | UV-vis absorption data and description of the computed low-lying electronic transitions (oscillator strength *f*) for **1** and **2** in the form of Rho<sub>c</sub>-DTE<sub>o</sub>.</caption><thead><tr><th>Cpd</th><th>λ<sub>abs</sub><sup>a</sup> (nm) (ε × 10<sup>3</sup> (M<sup>-1</sup> cm<sup>-1</sup>))</th><th>λ<sub>abs</sub> calc<sup>b</sup> (nm)</th><th>f<sup>c</sup></th><th>Orbital contribution</th></tr></thead><tbody><tr><td>1</td><td>279 (60.3), 331 (40.2), 348 (46.6), 403 (33.2), 422 (32.8)</td><td>306</td><td>0.23</td><td>HOMO-1 -> LUMO</td></tr><tr><td>2</td><td>280 (41.3), 333sh (23.0), 350 (34.4)</td><td>304</td><td>0.31</td><td>HOMO-1 -> LUMO</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**FIGURE 2** | Calculated absorption spectra for **1** (red) and **2** (blue). In the inserts are presented the frontier molecular orbitals relevant for the first electronic transition.</figcaption></figure>

the presence of trifluoroacetic acid (TFA) and various metal anions (alkali, alkaline-earth, and transition metal). Both compounds **1** and **2** exhibit essentially the same spectral changes with TFA, i.e., decrease of the absorbance at 327 nm and the emergence of structured absorption bands centered at around 380 and 500 nm, typical of the spiro ring-opening process in the rhodamine unit (Figure 3, top). Such spectral changes are also associated with the drastic color change from colorless to pink in solution. However, no isosbestic point was observed, suggesting that more than two species coexist during the protonation. The close scrutinization of the electronic absorption profiles at 327 and 502 nm with the concentration of protons (Figure 3, bottom) suggests a two-step protonation process. One can notice that the absorption at 327 nm rapidly drops until the addition of about 1 equivalent of TFA, before re-increasing slowly. On the contrary, the absorbance at 500 nm showed a gradual increase with the TFA acid, but with a distinct acceleration after 1.5–2 equivalents of TFA. This difference in titration profile with the absorption bands at 327 and 500 nm is ascribed to the presence of two protonation sites of the molecule. The triazole and the diethylamine on the rhodamine part are identified as these two sites of protonation. As the absorbance at 500 nm is characteristic of the rhodamine absorption in open form (Rho<sub>o</sub>), the one at 327 nm can be reasonably assigned to the triazole moiety. In view of the absorption profile showing that the 327 nm absorption band decreases prior to the emergence of the rhodamine absorption, it is anticipated that the first protonation occurs on the triazole moiety while the second protonation is responsible to the spiro-ring opening on rhodamine, i.e. pK<sub>a</sub> (triazole) > pK<sub>a</sub> (NEt<sub>2</sub>). The

slow increase of the 327 nm absorption after 1 eq. of TFA is due to the spiro-ring opened compounds, which also absorb in the UV region.

To put the observed two-step protonation into context, we note that the intrinsic basicities of the two sites differ markedly in conventional solvents. In aqueous media, the conjugate acid of a simple substituted triazole is rather weak (pKa ≈4–5), whereas tertiary aliphatic amines (like diethylamino) typically have pK<sub>a</sub> of their conjugate acid in the range ≈ 9–11 [28, 29]. These values, however, cannot be directly transferred to low-polarity solvents such as dichloromethane – pKa values are strongly solvent dependent, and protonation equilibria in nonpolar media are shifted and can be governed by specific solvation and local electrostatic effects. Consequently, while the absolute pKa of an isolated triazole is lower than that of a diethylamino group in water, the protonation sequence observed experimentally for **1** and **2** in dichloromethane (first event assigned to the triazole, second to the rhodamine-NEt<sub>2</sub>) can be rationalized by: i) solvent-dependent stabilization of the corresponding conjugate acids and ii) local electronic effects introduced by the xanthene–DAE linkage that modulate the apparent basicity of the triazole. In short, the apparent pKa values in dichloromethane differ from aqueous pKa benchmarks; this explains the experimental observation that the triazole site is protonated prior to the rhodamine NEt<sub>2</sub> under our measurement conditions.

The reversibility of the protonation reactions is verified upon addition of triethylamine (Et<sub>3</sub>N) in the same solution. Figure 4 shows the absorption spectral evolution of compounds **1** and **2** from their protonated forms (100 eq. of TFA), upon progressive addition of Et<sub>3</sub>N. The features of spectral changes for **1** and **2** are opposite to the corresponding spectral changes upon protonation. At 327 nm, the observation of slow decrease in absorbance is attributed to the progressive neutralization of the rhodamine part. After the addition of around 90–95 equivalents of Et<sub>3</sub>N (100 eq. of TFA was previously added), the triazole moiety starts to be neutralized leading to a rapid increase in absorbance at 327 nm, concomitant with the absorbance diminishment in 500 nm.

This stepwise protonation was also monitored by ¹H-NMR spectroscopy. Upon the addition of TFA (≈100 eq.), the aromatic protons on rhodamine part are perturbated or even vanished, reasonably due to the presence of two protonated sites (Figure S2). This behavior is distinct from what observed in DAE-free control rhodamine, in where only a slight de-shielding effect was observed (Figure S3). Importantly, the initial spectrum is recovered upon neutralization with triethylamine, demonstrating the reversibility of both protonated sites.

The response to metal ions has also been tested with several alkali, alkaline-earth, and transition-metal ions (Figure S4).

<footnote><sup>a</sup>In dichloromethane solution (C ≈ 10<sup>-6</sup> M) at 298 K;</footnote>

<footnote><sup>b</sup>See the Computational details section in the ESI for additional information;</footnote>

<footnote><sup>c</sup>Oscillator strengths (*f*) indicate the transition probabilities; higher values correspond to more intense electronic transitions.</footnote>
<figure><img src="image_6.png" /><figcaption>**FIGURE 3** | Top: Electronic absorption spectral changes of **1** (A) and **2** (B) upon progressive addition of TFA, in dichloromethane at 298 K with C ≈ 10⁻⁵ M. Bottom: Plots of absorbance as a function of the concentration of TFA, at 327 and 500 nm for **1** (A) and **2** (B).</figcaption></figure>

Both compounds **1** and **2** exhibit similar and distinctive responses toward Hg²⁺ and Zn²⁺ ions, with a more pronounced color change observed for Hg²⁺. In contrast, Pb²⁺ ions produced only faintly colored solutions, which can be attributed to their weaker

interaction with the rhodamine moiety of compounds **1** and **2**. A higher affinity of rhodamine derivatives toward Hg²⁺ has already been reported in the literature [30, 31]. Interestingly, only compound **1** exhibited a measurable response to Fe³⁺, suggesting that
<figure><img src="image_7.png" /><figcaption>**FIGURE 4** | Top: Electronic absorption spectral changes of 1 (A) and 2 (B) upon progressive addition of Et<sub>3</sub>N, in dichloromethane at 298 K with C ≈ 10<sup>-5</sup> M. Bottom: Plots of absorbance as a function of the concentration of Et<sub>3</sub>N, at 327 and 500 nm for 1 (A) and 2 (B).</figcaption></figure>

the positional variation of the DAE unit on the rhodamine scaffold modulates the electronic communication through the 6'-position, thereby enhancing metal-ion binding affinity and conferring cation selectivity.

**2.3 | Photoswitching**

To assess the photoswitching behavior, the nature of the computed S<sub>0</sub> → S<sub>1</sub> electronic transition and the potential energy
surfaces of the states along ring closure were further investigated for this series. Although the photochromism mechanism of DAE has been extensively studied for the related compounds featuring two methyl-thiophene rings as aryl units [10], there is a scarcity of theoretical investigations on DAEs with a combination of different aryl groups (e.g., thiophene + pyrrole or furan) [32–34]. Additionally, only one prior study was reported to explore a DAE molecule incorporating a thiophene and a triazole as its aryl constituents [24]. Here, in addition to **1** and **2**, a reference compound (**Ph-DAE**) where the triazole ring is substituted with a phenyl instead of a rhodamine, is also taken to pinpoint any perturbation of the typical DAE optical properties. The first excited state for **Ph-DAE** corresponding essentially to a $\pi \to \pi^*$ highest occupied molecular orbital (HOMO) → lowest occupied molecular orbital (LUMO) transition (at 303 nm) was computed, with the latter orbital centered on the DAE core and exhibiting suitable bonding interaction between the reactive carbons for ring-closure (Figure 5, middle). **1** and **2** show a similar $\pi \to \pi^*$ first vertical excitation, corresponding to a HOMO-1 → LUMO (see Figure 2) at 306 and 304 nm, respectively. While the HOMO-1 is spread along the whole backbone, the LUMO is DAE-centered and very similar to the LUMO of **Ph-DAE**. In view of these, it is clear that the introduction of rhodamine results in only minor perturbation on the expected topology of the first excited state, which is associated with ring-closure. In addition, the variation of anchoring position in **1** and **2** with trifling modification leads to similar first excited states. To further investigate the impact of the rhodamine moiety, the intrinsic ability for photoisomerization, potential energy scans for the ground ($S_0$) and the first excited state ($S_1$) of **Ph-DAE** and **1** have been conducted along the reaction coordinate, as shown in Figure 5. In both cases, a crossing point is located at the same C–C distance of slightly less than 2.1 Å, which allows for a barrierless ring-closure reaction upon vertical excitation of their open isomers through a conical intersection. Both the vertical calculation and the potential energy scans confirm that the incorporation of rhodamine chromophore results in negligible disruption of the potential photochromic reactivity. It is noteworthy that similar theoretical results have been reported in triazole-based DAEs, which also exhibit experimental photochromic behavior [24].

Irradiation measurements were performed in dichloromethane solutions of **1** and **2** ($C \approx 10^{-5}$ M, 293 K) at 270 nm to initiate

the photocyclization reaction. Surprisingly, instead of the DAE ring-closed conversion upon, formation of spiro-ring open form in rhodamine unit was observed upon excitation of **1**, as revealed in Figure 6a. Monitoring the absorbance during UV irradiation clearly demonstrates the appearance of characteristic absorption bands unit in the visible region (around 500 nm), accompanied by the emergence of a luminescence signal centered at 570 nm, typical of rhodamine absorption and emission, respectively. The addition of triethylamine results in the disappearance of both visible absorption and emission bands. Based on these observations, two hypotheses have been made, e.g., either the formation of open-ring rhodamine derivative facilitated by a zwitterionic equilibrium upon UV-light irradiation (Figure S8), or the occurrence of a side reaction involving the photoproduction of acid.

Detailed analysis of the spectral changes in the solution of **1** upon UV irradiation has been conducted (Figure S5). Photokinetic measurements were carried out under continuous UV irradiation, in which the electronic absorption spectra were recorded in 30 s intervals over a period of 4 h. Kinetic traces were plotted at the wavelengths of 270 nm, 400 nm, 500 nm, and 600 nm. One would expect an increase in absorbance at 400 nm ($S_0 \to S_2$ transition), as well as at 500 nm and 600 nm ($S_0 \to S_1$ transition) [24], for the formation of the closed form of the DAE moiety. Indeed, the expected decrease in absorbance at 270 nm and simultaneous increases at 400 nm, 500 nm, and 600 nm are observed within the initial tens of seconds. These changes are followed by an inflection point at $\approx 50$ s, marking the onset of exponential increases in absorbance. This phenomenon strongly suggests the formation of another species, which can reasonably be attributed to the open-spiro compound, **Deg-Rho1** (Figure S9). Previous work of Jäschke et al. also mentioned similar spectral evolution on non-functionalized triazole-based DAEs, which were tentatively explained through the photoelimination of H-F [24]. The formation of **Deg-Rho1** is further supported by the time-dependent density-functional theory (TD-DFT) computed vertical transitions, which closely resemble the recorded absorption features (Figure S6, Table S1). Likely, the presence of a rhodamine moiety on **1** plays the role of a very efficient acid-sensor and unambiguously facilitates the photoproduction of acid upon UV exposure.

Notably, acid production was not observed in similar irradiation experiments for **2**, as evidenced by the absence of a characteristic absorption band from the spiro-ring open on the rhodamine unit.

<figure><img src="image_8.png" /><figcaption>**FIGURE 5** | Left: frontier molecular orbitals of Ph-DAE. Potential energy scan for the first excited state ($S_1$) and the ground state ($S_0$) following the reactive C–C bond for **Ph-DAE** (middle) and **1** (right). The orange circle denotes the crossing point of the surfaces where a conical intersection is expected.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**FIGURE 6** | (a) Electronic absorption spectra of **1** in DCM (C = 1 × 10⁻⁵ M) before irradiation (black curve), with UV irradiation at λ = 250 nm until reaching the photostationary state (PSS, 10 min) (red curve), and after the addition of triethylamine (dotted blue curve). Electronic emission properties (λexc = 340 nm) of **1** (green curve) upon UV irradiation (λ = 250 nm). (b) Electronic absorption spectra of **2** in DCM (C = 3.81 × 10⁻⁵ M) before irradiation (light blue curve), with UV irradiation at λ = 250 nm until reaching the PSS (red curve), and with visible light irradiation for reopening (dashed black).</figcaption></figure>

Instead, the absorption spectrum undergoes a distinct evolution with an isosbestic point at 306 nm, accompanied by the emergence of a weak band at 500 nm (Figure 6b). On the other hand, restoration of the initial absorption spectrum could not be achieved with exposure to visible light at 480 nm. Good agreement of mono-exponential fitting was obtained from the kinetic traces of absorbances at 350 and 500 nm, suggesting the formation of a unique photoproduct (Figure S6). The frontier molecular orbitals of **Deg-Rho2** clearly show the charge-transfer character of the first allowed transition (HOMO-1 → LUMO Figure S7), which correlates well with the experimental absorption observed in the visible region. The strong agreement between experimental and theoretical absorption spectra (Figure S6, Table S1)

further supports the hypothesis of a degradation process resulting in a species with a lower-lying transition, identified as compound **Deg-Rho2**.

The divergent fluorescence responses observed for compounds **1** and **2** can be rationalized by their distinct photochemical pathways. Upon UV irradiation, compound **1** generates an open-spiro rhodamine species through a photoinduced acid formation process, resulting in the emergence of a visible emission band around 570 nm. In contrast, compound **2** undergoes an irreversible photoreaction that produces a nonfluorescent degraded species. The difference reflects the subtle influence of the anchoring position of the DAE unit on charge delocalization and on the stability of the photoexcited state (Scheme 2).

<figure><img src="image_10.png" /><figcaption>**SCHEME 2** | Overview of the photo- and chemo-stimuli applied and the resulting compounds formed.</figcaption></figure>
In summary, two novel rhodamine-based diarylethene (DAE) systems (1 and 2) have been successfully synthesized and characterized through a Huisgen 1,3-dipolar cycloaddition reaction. The spectroscopic and computational analyses confirm their identities as in the “closed” rhodamine and “open” DAE forms. The structural differences between 1 and 2, evidenced by the distinct ¹H NMR chemical shifts of the triazole proton, were found to give the electronic influence of the xanthene backbone. Furthermore, the chemo-switching properties of 1 and 2 have been explored, revealing a stepwise protonation process that involves the triazole and diethylamine moieties. The reversibility of such protonation was demonstrated through neutralization with triethylamine, with the recovery of initial spectral features. Both compounds also exhibit selective responses to certain metal ions, particularly Hg²⁺ and Zn²⁺, with 1 showing additional sensitivity to Fe³⁺. The photochromic potential of 1 and 2 was illustrated by TD-DFT calculations, which reveal that both systems possess the electronic and structural prerequisites for an electrocyclic ring closure through a conical intersection. However, photophysical investigations demonstrate divergent behaviors under UV irradiation: 1 undergoes a transformation associated with acid production, likely due to a photoinduced side reaction, while 2 exhibits an irreversible photoreaction leading to a distinct degradation product. These variations demonstrate that subtle structural modifications can significantly impact the photochemical pathways of rhodamine-DAE conjugates. In particular, while the use of a hexafluorinated bridge is well known to enhance the thermal stability and photofatigue resistance of the closed DTE isomer, it can also lead to unexpected photoreactivity, such as the photorelease of fluoride (F⁻) ions. These findings highlight the intricate interplay between electronic structure, protonation dynamics, and photoreactivity in rhodamine-DAE hybrids. The insights gained from this study provide valuable guidelines for the rational design of multifunctional photochromic and chemosensitive molecular systems, with potential applications in sensing, imaging, and optoelectronic devices.

### Acknowledgements

The authors acknowledge the Ministère de l'Éducation Nationale, de la Recherche et de la Technologie, the Centre National de la Recherche Scientifique (CNRS). This work is funded by the Agence Nationale de la Recherche (ANR) on the project PhotoControl (ANR-19-CE07-0038-01). A.F. and C.P. are grateful to the ANR FALCON project, grant no. ANR-20-CE09-0002-01.

### Funding

This study was supported by Agence Nationale de la Recherche (ANR-19-CE07-0038-01 and ANR-20-CE09-0002-01).

### Conflicts of Interest

The authors declare no conflicts of interest.

### Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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### Supporting Information

Additional supporting information can be found online in the Supporting Information section. **Supporting Fig. S1:** ¹H-NMR spectra of **1** before (red), after addition of TFA (green) and after addition of Et₃N (blue), in CDCl₃. **Supporting Fig. S2:** ¹H-NMR of 7'-bromorhodamie before (bottom) and after (top) addition of TFA in CDCl₃. **Supporting Fig. S3:** Photographs of vials with **1** (top) and **2** (bottom) (in methanol 2 × 10⁻⁵ M) in the presence of 1 equivalent various cation. **Supporting Fig. S4:** Absorption spectral features evolution upon UV irradiation with kinetic traces at chosen wavelengths for A) **1** and B) **2**. (C ≈ 10⁻⁵ M, in CH₂Cl₂ at 293 K; λex = 270 nm). **Supporting Fig. S5:** Computed spectrum (see computational details) for **Deg-Rho1** and **Deg-Rho2**. **Supporting Fig. S6:** Frontier molecular orbitals of **Deg-Rho1** and **Deg-Rho2**. **Supporting Fig. S7:** Putative zwitterion form of **1**, under UV light irradiation. **Supporting Fig. S8:** Formation of the proposed degraded compounds **Deg-Rho1** and **Deg-Rho2**. **Supporting Table S1:** Description of the computed low-lying electronic transitions **Deg-Rho1** and **Deg-Rho2** showing non-negligible intensities.