English

• Viologen is a class of disubstituted pyridinium salts, with the 4,4′-disubstituted pyridinium salt being the most common form [1]. These compounds exhibit three oxidation states. The most stable state of the viologen is the cation (RV²⁺), which undergoes two distinct, stable, and reversible single-electron reduction processes to form the radical (RV^+•) and neutral species (RV), each accompanied by significant color changes [2]. When viologen is reduced from the cation to the radical state, the molecule becomes planar, increasing the overlap of π-π orbitals and enhancing the conjugation between the pyridine rings [3,4]. At this stage, the radical electron is delocalized not only on a single pyridine ring but also on the π-π conjugated orbitals of the entire molecule, making the viologen radical more stable [5-7]. Their excellent redox properties make them ideal for use as color-changing centers in the preparation of smart color-changing devices [8-10], as sensing elements in the manufacture of biosensing [11,12], as electrode materials to manufacture flow batteries [13-15], and as photosensitizers to construct photocatalytic reduction systems [16,17], etc. However, challenges such as the formation of radical cation dimers and weak emission still hinder the development of viologen-based applications [18-20].

  Traditional viologen derivatives have a large dihedral angle between the two pyridine rings, leading to low overlap of π electron orbitals, minimal conjugation between the pyridine rings, and weak visible light absorption [21]. Numerous researchers have previously conducted various modifications on viologens, such as modifying the framework with "VIA" elements [22-24], connecting different groups to the N atom of the pyridine through substitution reactions [25,26], combining viologen with polymer matrices [27-29], and bridging conjugated groups between the pyridine rings [30,31], to improve the photophysical and photochemical properties of viologens.

  Pyrene is a polycyclic aromatic hydrocarbon consisting of four fused benzene rings and is recognized as a classic electron-donating group. Its unique molecular structure and optical properties have garnered considerable interest in both fundamental research and industrial applications [17,32]. Pyrene exhibits deep blue fluorescence, a long lifetime, and a high quantum yield when dissolved in nonpolar solvents. As an outstanding fluorophore, it is extensively utilized in the fabrication of fluorescence materials [33,34]. Leveraging the interaction between the electron donor and acceptor, pyrene has been extensively utilized as an electron donor in the preparation of high-emissive materials [35-39]. Introducing pyrene into viologen molecules in a donor-acceptor (D-A) configuration can effectively enhance charge transfer and narrow the energy gap of the viologen, subsequently enhancing its optical and electrical properties [40-44].

  Based on these considerations, this study introduces a pyrene featuring large conjugation and robust fluorescence into the 4,4′-bipyridine structure. By narrowing the energy gap of the viologen, it becomes capable of absorbing visible light and exhibiting more abundant fluorescence properties. Furthermore, S_N2 substitution reactions were conducted on the N atoms of pyridine to synthesize pyrenoviologen derivatives (PyV²⁺) 3a, 3b, and 3c, substituted with methyl, benzyl, and naphthylmethyl groups, respectively (Scheme 1). Utilizing these three pyrenoviologen derivatives as active materials, smart electrochromic devices (ECDs) were developed, and their fundamental performance was systematically evaluated. Due to the unique D-A-D-A-D structure of 3c, which enhances radical stability, the ECD prepared with 3c exhibits superior color retention capability. Additionally, flexible ECDs capable of bending at certain angles were fabricated, along with low-voltage-driven fluorescent signs for dynamic information display. The pyridine ring exhibits a special electronic interaction with picric acid (PA), and by leveraging the strong fluorescence properties of pyrenoviologen, they function as fluorescent probes for PA detection. Experimental results validate that 3c demonstrates the best responsiveness to PA. Consequently, a fluorescent color-changing detection film specifically responsive to PA was prepared. The pyrenoviologen 3c, featuring a D-A-D-A-D conjugated structure, exhibits exceptional electrochromic performance and heightened sensitivity, particularly towards PA aqueous solutions.

  Scheme 1

  

  Scheme 1.  Design strategy and synthetic route to the pyrenoviologens.

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  Molecular synthesis was conducted according to previous work [43,44]. The starting material, 2,7-di(pyridin-4-yl)pyrene 1, was synthesized through Suzuki-Miyaura coupling reactions between 2,7-dibromopyrene and 4-pyridineboronic acid pinacol ester, yielding 66%. Compounds 3a, 3b, and 3c were synthesized through S_N2 substitution reactions between 1 and methyl triflate (4a), benzyl bromide (4b), and 1-methylnaphthalene (4c), respectively, with good yields (2a: 97%, 2b: 90%, and 2c: 89%). The desired pyrenoviologens 3a, 3b, and 3c were synthesized by stirring an excess of NH₄PF₆ with 3a, 3b, and 3c in an aqueous solution overnight, followed by conversion of all anions to PF₆⁻. After a simple purification procedure, the pyrenoviologens were obtained. The targeted compounds were verified by ¹H NMR, ¹³C NMR, high-resolution mass spectrometry (HRMS), and the details of the synthesis are provided in Supporting information. The melting points of 1,3a, 3b, and 3c were determined after drying using a micro melting point apparatus, and TGA analysis showed that the pyrenoviologens are stable at 250 ℃ (Fig. S1 in Supporting information).

  The electrical properties of the pyrenoviologens were determined using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). At different scanning speeds, the redox curves of the pyrenoviologens were measured, as shown in Fig. 1a and Fig. S2 (Supporting information). The pyrenoviologens exhibited three pairs of redox peaks, labeled A, B, and C, which correspond to single-electron redox reactions between radical cations and neutral molecules (A), π-π stacked electron transfer of pyrene groups (B) [45,46], and electron transfer between bivalent cations and cationic radical states (C). The three processes are illustrated in Scheme S1 (Supporting information). This suggests that like traditional viologen derivatives, the pyrenoviologens also undergo two single-electron redox processes. It is noteworthy that the introduction of pyrene groups results in a new redox potential.

  Figure 1

  

  Figure 1.  (a) The CV and DPV curves of 3c at different scan rates in DMF with tetrabutylammonium hexafluorophosphate (0.1 mol/L) as supporting electrolyte, potential E referenced to Fc/Fc⁺ (c = 5 × 10⁻⁴ mol/L). (b) UV–vis spectra and (c) fluorescence spectra of 3a, 3b and 3c in DMF (c = 2 × 10⁻⁵ mol/L, λ_{ex, 3a} = λ_{ex, 3b} = 360 nm, λ_{ex, 3c} = 380 nm). The inset photographs of 3a, 3b and 3c in DMF upon daylight and UV light (365 nm). (d) UV–vis spectra and (e) fluorescence spectra of three redox states of 3c (c = 2 × 10⁻⁵ mol/L). The inset photographs of 3c (3c' and 3c'') in DMF upon daylight and UV light (365 nm). The "×10" on the blue line which reduced with Na means that its strength has been artificially expanded tenfold when drawing to facilitate the analysis of data. (f) EPR spectra of the biradical species of 3a', 3b' and 3c' at room temperature.

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  The B-peaks of the pyrenoviologens were not obvious in CV tests, and their electrical properties were further shown in DPVs in Fig. S3 (Supporting information). Taking 3c as an example, the reduction potentials of 3c were E_A = −0.43 V, E_B = −1.12 V, E_C = −1.44 V, respectively. The reduction potential in the CV curves basically corresponds to the peak position in the DPV curves, which can mutually confirm their accuracy (Table S1 in Supporting information). In addition, the electron transfer constant (K_ET) of pyrenoviologens could be calculated by using CVs data and the Nicholson method. K_ET1 = 2.78 × 10⁻⁵ cm/s, K_ET2 = 8.9 × 10⁻³ cm/s for 3a, K_ET1 = 1.17 × 10⁻⁵ cm/s, K_ET2 = 2.7 × 10⁻² cm/s for compound 3b, and K_ET1 = 2.56 × 10⁻⁶ cm/s, K_ET2 = 2.9 × 10⁻² cm/s for 3c. The differences in reduction potentials and K_ET of these three kinds of pyrenoviologens are due to the substituents on the side, which agreed with previous work (Fig. S4 and Table S2 in Supporting information) [30,47].

  The pyrenoviologens in DMF (0.1 mmol/L) were light yellow liquids, and their UV–vis spectra are shown in Fig. 1b, with characteristic absorption bands at 400–500 nm. Notably, the absorption peak of 3a (428 nm), 3b (432 nm) and 3c (435 nm) shifted gradually to the red, accompanied by decreasing of HOMO-LUMO energy gaps (3a for 2.573 eV, 3b for 2.541 eV and 3c for 2.536 eV). The compounds from 3a to 3c have a smaller energy gap and the compounds exhibited smaller energy gaps and lower energies, possibly due to the increasingly complete D-A-D-A-D structures, and the electron delocalization range is increasing. The related photophysical computational results are shown in Fig. S5 and Table S3 (Supporting information). Additionally, the experimental data were in accordance with the first singlet excitation energy, which was calculated by time-dependent density functional theory (TD-DFT) calculations (Tables S4-S6 and Figs. S6-S8 in Supporting information).

  Ordinary 4,4′-bipyridine exhibits very weak emission. On the other hand, pyrene is a strong emitting group with a high quantum yield. When 4,4′-bipyridine is modified with pyrene, it also displays strong fluorescence emission. The emission spectra of pyrenoviologens are shown in Fig. 1c. Compared with compound 3a, the emissions of compounds 3b and 3c are redshifted. This is attributed to the pyridine ring and aromatic ring connected by methylene in 3b and 3c, which elongates the hyperconjugated system and increases conjugation [48-51]. The pyrene molecules exhibit vibrational excitation bands between 370 nm and 410 nm. Among these, the first and third excitation bands are the most significant, with the first excitation spectral band occurring at about 373 nm, where fluorescence enhancement occurs under polar conditions [52]. The excitation spectra of pyrenoviologens in DMF (0.02 mmol/L) also display multiple vibrational excitation bands within the range of 320–380 nm. The excitation wavelength of 3c is redshifted due to its D-A-D-A-D structure. Furthermore, these compounds possess a long fluorescence lifetime and a high quantum yield, especially compound 3c. These results suggest that pyrenoviologens exhibit superior photophysical and photochemical activities akin to those of pyrene molecules (Figs. S9 and S10, and Table S7 in Supporting information).

  By gaining and losing electrons, the viologen derivatives can reversibly convert between cations, radical cations, and neutral states. To achieve this reversible transformation, we utilized the reductive properties of Zn and Na. The pyrenoviologens were reduced using Zn and Na in the glove box, with the reactants dissolved in DMF (1 mmol/L). Their UV–vis spectra were measured individually, and the solutions exhibited different colors (Fig. 1d, Figs. S11a and c in Supporting information), indicating that the pyrenoviologens had transitioned into different states. Taking compound 3c as an example, after reduction with Zn, its color changed from yellow to green, and the UV–vis spectrum showed strong absorption at 670 nm. This indicates that the cation state transformed into the radical cation state, and the conjugation of the molecule was enhanced. Upon further reduction with Na, the color turned brown, signifying that the compound was reduced to a neutral molecule and absorbed light at all wavelengths. The phenomena observed for 3a and 3b were essentially the same as those for 3c. These results confirm that pyrenoviologens possess similar redox properties to classical viologens. The reduction process was accompanied by distinct color changes, providing fantastic support for their application in electrochromic devices (ECDs).

  To further investigate the fluorescence properties of pyrenoviologens at three distinct redox states, we tested the emission of 3a, 3b, and 3c in DMF (0.02 mmol/L) after chemical reduction, and the results are presented in Fig. 1e and Figs. S11b and d (Supporting information). When Zn was used as the reductant, the fluorescence intensity of the pyrenoviologens decreased. Subsequently, the emission was further quenched after reduction with Na. This indicates that the fluorescence properties change with electron transfer, which holds potential application value in electrofluorescent devices (EFCDs).

  Electron paramagnetic resonance (EPR) spectroscopy was employed to analyze the properties of free radicals. The pyrenoviologens solution, reduced by Zn and Na, was encapsulated in capillaries and sealed with vacuum grease. EPR spectra revealed that the pyrenoviologens exhibited significant and similar radical signals, with g-factors of 2.0033 for 3a, 2.0036 for 3b, and 2.0031 for 3c after reduction with Zn (Fig. 1f). This radical signal originated from the bipyridine moiety. Upon further reduction with Na, the free radical signal persisted. This is attributed to the reduction of the aromatic pyrene by alkali metals (Na), resulting in the formation of the fully doubly reduced diamagnetic di-anion Py²⁻ (Fig. S12 in Supporting information). Under light irradiation, this further generates a paramagnetic electron and a radical anion Py^−• [53,54].

  To better observe the electrochromic properties of pyrenoviologens, we manufactured some proof-of-concept electrochromic devices (Fig. 2a). Solutions of 3a, 3b, and 3c in DMF (2 mmol/L) were used as the active substance for the electrochromic devices, with fluoride tin oxide (FTO) conductive glass serving as the electrode, and nano double-sided tape was used for sealing. The construction of the solution-based ECD was completed, and the detailed operation procedure is provided in Supporting information. When a positive voltage is applied, the cationic pyrenoviologens extract electrons at the cathode (FTO conductive glass) and reduce to the free radical state.

  Figure 2

  

  Figure 2.  (a) Electrochromic device assembly diagram. UV–vis spectra of the 3c in DMF from 3c to 3c' (b) and from 3c' to 3c'' (c) in ECD. The inset photographs of ECD in different color states. (d) The photographs of change in color of the flexible ECD assembled by 3c under bending stress.

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  Consequently, the cathode part of the ECD changes from yellow to green. The anodic conductive glass acts as part of the electronic path, completing the electronic transmission circuit. Combined with visual observations (Figs. 2b and c), the process of color conversion was analyzed. The device made of 3c was yellow in its initial state. After applying a voltage of 3.6 V, the absorption peaks at 420 and 688 nm increased gradually, indicating that a significant amount of light of corresponding wavelengths (red light) was absorbed, and thus the device appeared green to the naked eye, which is the complementary color of red (Fig. 2b). The color change of the device was consistent with that observed when using Zn for chemical reduction. Subsequently, the color returned to its initial state when the voltage was removed. By continuing to apply a voltage of 5.4 V to the ECD, it was found that the color of 3c further changed from green to yellowish-brown. In the electrochemical spectrum, the peak at 682 nm decreased, and the peak shape and position were consistent with the spectrum obtained after chemical reduction with Na. The color difference may be due to the reddish-brown flocculent precipitate produced during the chemical reduction of Na with DMF. The electrochromic behavior of 3a and 3b was similar to that of 3c (Figs. S13 and S14 in Supporting information). The reduction voltage of 3c was slightly higher than that of the other two compounds, possibly due to its lower solubility.

  Coloring efficiency (η) indicates the amount of power required to be injected when the color of the device changes to a certain degree. The higher the η of the ECD (electrochromic device), the less energy is consumed to achieve a certain color change. A relationship between the change in transmittance (ΔOD) and the electricity required per unit area (ΔQ) has been established. The tangent line of the curve represents the η. The ΔOD-ΔQ curves of pyrenoviologen electrochromic devices show that the η of 3a, 3b, and 3c are 50.46, 134.78, and 87.89 cm²/C, respectively (Fig. S15 in Supporting information). The color change velocity, contrast, and recovery time after voltage removal are important parameters for evaluating the performance of ECDs. By monitoring the transmittance (T) at the wavelength that undergoes the most change, a relationship between T and time was established (Fig. S16 in Supporting information). Before voltage application, T was at its highest. After applying voltage for a while, the color of the device gradually turned green, and T gradually decreased. Referring to previous work, a change of 90% in T is typically used as the standard to calculate the time for color change [55,56]. All three compounds completed the color change within approximately 14 s. The contrast of the color change was over 50%. However, in terms of color recovery time, 3c was significantly longer than 3a and 3b (3a was 18.4 s, 3b was 24.9 s, and 3c was 44.1 s). This is because the D-A-D-A-D structure gives 3c a more stable radical, meaning that 3c has a better ability to maintain its color and is more aligned with the concept of energy saving.

  When pyrenoviologens are used as electrochromic device materials, they respond to voltage rather than shape, external stress, temperature, and other factors. A flexible electrochromic device was assembled using a conductive flexible film instead of rigid FTO (fluorine-doped tin oxide) glass, specifically a polyethylene terephthalate (PET) film coated with indium tin oxide (ITO). Additionally, hydrogen generated by the reaction of zinc and dilute hydrochloric acid was used to partially reduce the ITO coating, creating a specific pattern (such as a star or dog paw). As shown in Fig. 2d and Fig. S17 (Supporting information), similar to traditional ECDs, when a positive voltage is applied, the pyrenoviologens gain electrons on the negative PET film and change its color. Meanwhile, the etched pattern on the negative electrode remains yellow, making the "dog paw" pattern visible. When a negative voltage is applied, the positive and negative electrodes of the flexible ECD switch, and the "star" pattern on the other side is displayed [54,55]. Pyrenoviologens used in flexible ECDs can quickly respond to voltage and easily bend to different angles while maintaining their display capabilities, demonstrating good application potential in the display industry, flexible wearable devices, and information encryption transmission.

  Through chemical reduction, it can be observed that as the pyrenoviologens undergo electron transfer, their fluorescent emission is quenched. This phenomenon also occurs when the pyrenoviologens are treated by electroreduction. This property has important implications for the design of electrofluorescent devices (EFCDs) that are more visible at night. A device similar to an ECD was prepared to investigate whether electrical reduction could quench the emission of pyrenoviologens. Fig. 3a and Fig. S18 (Supporting information) show the electrofluorescence emission spectra of compounds 3a, 3b, and 3c as they transition from their cationic form to their free radical form. As the voltage increases, the emission of the pyrenoviologens gradually weakens until it is quenched. Notably, within the voltage range of 2.8–3.4 V, the fluorescence attenuation is the fastest. To confirm that this electrofluorochromism is an inherent property of the compounds rather than a general function of EFCDs, we conducted a blank control experiment using an EFCD filled with pure solvent (Fig. S19 in Supporting information). Upon applying voltage, the emission intensity at 550 nm in the blank group remained largely unchanged, indicating that the previously observed phenomenon is indeed an intrinsic property of the compounds themselves. Pyrenoviologens exhibit electrofluorescence and are ideal candidate materials for the preparation of night-vision EFCDs.

  Figure 3

  

  Figure 3.  (a) Fluorescence spectra of spectra of the 3c in DMF with the voltage gradually from 0 to 3.6 V. The inset photographs of 3c in DMF upon UV light (365 nm) in ECD (λ_ex = 380 nm). (b) The assembly diagram of the fluorescent warning sign and the actual recording picture of the assembly process. (c) Schematic diagram of the working process of electronic warning signs.

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  For the electrofluorescent discoloration warning board, two pieces of FTO glass (10 × 2.5 cm²) were etched with the word "Beware!" and mirrored the words "Deep water" on their conductive surfaces (Fig. 3b). Fig. 3c and Fig. S20 (Supporting information) demonstrate how the EFCDs work. Under light conditions, the words "Beware!" applied, and "Deep water" are displayed in turn. Just same as the principle of flexible electrochromic device described earlier. However under dark conditions, while the reduction occurs, the etched area retains its original emission, which means the device displays a dark background with bright text that is noticeable. In this way, by controlling the direction of the voltage (positive or negative) and the duration of each voltage application, the messages "Beware!" and "Deep water" can be dynamically displayed at night (Fig. 3c and Fig. S20). The electronic warning signs have high color contrast and can clearly display the warning words. Dynamic text is more likely to attract people's attention. Pyrenoviologen can be used both during the day and at night, making it a potential candidate for use in electrofluorescent discoloration applications.

  Picric acid (2,4,6-trinitrophenol, PA) is a kind of high explosive, which primarily exists in aqueous solutions [57,58], widely utilized in industries, chemical medicine, and chemical laboratories, among others [58-60]. Therefore, it is very important to develop a kind of new sensor capable of detecting low concentrations of PA [38,63-67]. Previous studies have shown that cationic pyridine could be used to detect PA in solution [65,69]. There is a special static electronic interaction between cationic bipyridine salt and PA, which facilitates the electron transfer from pyridine to PA [48,69]. Pyrenoviologens exhibit strong fluorescence emission in solutions and have a specified pyridine structure. We investigated the ability of pyrenoviologens as a fluorescent probe to detect PA in aqueous solutions (Fig. 4a). Fig. 4b and Fig. S21 (Supporting information) show the changes in fluorescence emission of pyrenoviologens in DMF after the addition of PA. In this process, the emission intensity of compounds gradually decreased. The emission of three compounds was quenched completely when the concentration of PA was 270 µmol/L. To confirm that the fluorescence quenching was not caused by the change in pyrenoviologens concentration resulting from variations in solution volume during titration, we conducted emission titration of pyrenoviologens using original solvent DMF (Fig. S22 in Supporting information). It was observed that with the addition of DMF, there was no significant change in its emission intensity, indicating that the previous fluorescence quenching phenomenon was induced by the addition of PA. Furthermore, we delved into the mechanism underlying the use of pyrenoviologens as a fluorescent probe for PA detection. In ¹H NMR, it was observed that upon the addition of PA, there were no shifts in the peak positions of the compounds, with relevant data provided in Fig. S23 (Supporting information). This observation suggests that the structure of the compounds remains unchanged, thereby eliminating the possibility of fluorescence quenching arising from compounds decomposition. Meanwhile, the change in quantum yield of pyrenoviologens when the react with PA is supported in Fig. S24 (Supporting information). In the process of PA titration, fluorescence quenching occurs, which is essentially the reduction of the number of photons emitted by fluorescent substances, and the quantum yield of pyrenoviologens in solution decreases with the addition of PA. Additionally, we investigated the fluorescence lifetime (τ₀/τ = 1) of mixed solutions containing varying amounts of PA, which serves as a crucial indicator for fluorescence static quenching (Fig. 4c and Fig. S25 in Supporting information). Based on these findings, we propose the quenching mechanism of pyrenoviologens for PA detection: Pyrenoviologens, a kind of strongly fluorescent substance, interact with PA through electrostatic forces, leading to electron transfer. As the concentration of PA increases, their emission intensity gradually decreases. The calculation of energy levels also theoretically ensures the feasibility of electron transfer (Fig. S26 in Supporting information). The Stern–Volmer equation was used to fit the fluorescence quenching, I₀/I = K_SV[PA]+1, where K_SV is the Stern–Volmer constant [54], and I₀ or I are the emission intensity before or after adding PA (Fig. 4c and Fig. S25), K_{SV, 3a} = 2.77 × 10⁴ L/mol, K_{SV, 3b} = 4.95 × 10⁴ L/mol, K_{SV, 3c} = 1.24 × 10⁵ L/mol. The energy gap of compounds 3a, 3b and 3c decreased gradually, so K_SV showed an increasing trend. Meaning that a higher quenching efficiency, and the sensitivity of compounds to PA (Fig. S24). At the same time, compound 3c has D-A-D-A-D structure, large conjugated and electron-rich naphthalene is more favorable for PA to bind with pyridine, so the K_SV of 3c is increased greatly. Compared with some other viologen systems to detected of PA, D-A-D-A-D structure's pyrenoviologens have a better quenching rate (Table S8) in Supporting information [47,61-73].

  Figure 4

  

  Figure 4.  (a) The electron-transfer mechanism for the detection of PA by pyrenoviologens. (b) Emission spectra of 3c were titrated by different concentrations of picric acid (λ_ex = 380 nm). (c) The Stern–Volmer constant and lifetime of 3c to PA. (d) Quenching efficiencies of 3c to PA and other analogs at different concentrations.

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  In addition to sensitivity, selectivity is also a key factor in evaluating sensor performance. To test the selectivity of pyrenoviologens for PA, trinitrotoluene (TNT), dinitrotoluene (DNT), p-dinitrobenzene (DNB), and eight other common solvents were used to perform fluorescence titration tests. As shown in Fig. 4d and Fig. S27 (Supporting information), pyrenoviologens exhibited high selectivity for PA. In contrast, the other three common explosives only caused a small reduction in fluorescence, and the eight other kinds of solvents had little effect on the primary emission intensity. These results indicated that pyrenoviologens exhibit good selectivity and high sensitivity to PA, and can be used as sensing materials for PA detection.

  In order to further expand the application of pyrenoviologens in detection, and simplify its operation process meanwhile, the pyrenoviologens film was prepared for PA detection. The cut glass sheet (2.5 × 0.9 cm²) was soaked in ethanol, ultrasonicated for 60 min. After the glass substrate was dry, pyrenoviologens in acetonitrile solution (100 µL, 1 mmol/L) was dripped on it. The glass substrate was placed in a ventilated place to air dry naturally, so that the pyrenoviologens dried to form a film on the glass substrate. The glass sheet attached with pyrenoviologens film was placed in a quartz cuvette with 1 mL deionized water, and the glass substrate will close to the inner wall of the cuvette via surface tension. The cuvette was fixed on the solid sample holder of the fluorometer so that the glass substrate was facing the light source to ensure that the position of the film was stable during each step of operation (Fig. 5a).

  Figure 5

  

  Figure 5.  (a) Schematic diagram of preparation of sensing film and schematic diagram of aqueous medium detection. (b) The film of 3a, 3b and 3c of Stern–Volmer constant to PA. The inset photographs of the films of 3a, 3b and 3c to PA. (c) The fluorescence spectra of films at repeated tests.

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  Added PA to the cuvette tardily, mixed well after each addition. With the increased PA concentration, the fluorescence intensity of the film gradually decreased (Fig. S28 in Supporting information). Stern–Volmer constant K_SV of films was still used to evaluate the detection effect (Fig. 5b). Among them, the fluorescence intensity of compound 3a did not change significantly when the PA concentration was low, then decreased when the PA concentration reached 30 µmol/L gradually. When reached at 70 µmol/L, the fluorescence quenching efficiency of compound 3a film was 46.9%, and the K_{SV, 3a} = 1.2 × 10⁴ L/mol. The fluorescence intensity of 3b and 3c films was more sensitive to the change of concentration (10 µmol/L). When the concentration of PA reaches 70 µmol/L, the amplitude of fluorescence quenching is large (58.1% for 3b, and 65.2% for 3c). Stern–Volmer constant K_SV of 3b and 3c higher than 3a correspondingly (K_{SV, 3b} = 2.2 × 10⁴ L/mol and K_{SV, 3c} = 3.0 × 10⁴ L/mol). The results showed that pyrenoviologen films were also fluorescently responsive to PA except in the solution state. With the improvement of electron-donating properties of R groups, its response to PA becomes higher and higher. This is a breakthrough application of pyrenoviologen. It is also noteworthy that the fluorescence lifetime of pyrenoviologens in films remained unaffected by the concentration of PA (Fig. S29 in Supporting information). The relevant laws observed in the solution state can be equally applied to the film state.

  In the process of detecting PA using pyrenoviologen films, the stability and repeatability of the films are crucial guarantees for smooth detection. To monitor the stability of the films in aqueous solution, the films were immersed in deionized water for 96 h, during which the emission intensity of the films was measured periodically (Fig. S30 in Supporting information). The emission intensity of the films did not change significantly during immersion, indicating that the pyrenoviologen films could maintain stability in a water environment. The reversible change of the sensing film was studied by alternately placing the film in a PA aqueous solution (0.1 mmol/L) and a propylene diamine solution. After adding the PA solution to the initial aqueous solution, perturbations allowed it to reach equilibrium. The fluorescence intensity at the maximum emission wavelength was recorded by continuous scanning for four times, and the ratio of this intensity to the intensity without PA was plotted against time as the ordinate (Fig. 5c). The results showed that compound 3a had poor reversibility for PA detection, while 3b and 3c had almost completely reversible responses to PA, which may be due to 3b and 3c having a smaller energy gap.

  In this work, pyrenoviologens 3a, 3b, and 3c with excellent redox and fluorescence properties were synthesized. These pyrenoviologens serve as active materials for the preparation of electrochromic devices (ECDs) that exhibit different color changes at varying voltages. Specifically, pyrenoviologen 3c, with its donor-acceptor-donor-acceptor-donor (D-A-D-A-D) structure, demonstrated more stable free radicals, resulting in better color retention. An easily bendable flexible display was assembled, capable of alternate display of specific symbols or identifiers. Additionally, leveraging their strong fluorescence properties, night vision fluorescent warning signs that dynamically display specific text or patterns display perfectly. These results broaden the scope of electrochromic materials and their related ECD applications in future life. Furthermore, the unique electrostatic association effect between pyrenoviologen cations and PA anions imparts the probe with excellent selectivity. Pyrenoviologens were used as fluorescent probes for PA detection, exhibiting remarkable sensitivity (K_{SV, 3a} = 2.77 × 10⁴ L/mol, K_{SV, 3b} = 4.95 × 10⁴ L/mol, K_{SV, 3c} = 1.24 × 10⁵ L/mol). Notably, pyrenoviologen 3c displayed extremely high sensitivity to PA in solution due to its D-A-D-A-D structure. Additionally, films prepared with 3b and 3c exhibited good stability and recoverability. The incorporation of the D-A-D-A-D structure not only enhances the photoelectric performance of viologens but also extends their application in electrofluorescent color-changing devices and fluorescent probes.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Tianle Cao: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Ni Yan: Writing – review & editing, Funding acquisition, Conceptualization. Yawen Li: Writing – review & editing, Funding acquisition, Conceptualization. Xinyi Zhang: Visualization, Data curation. Yue Zhu: Formal analysis, Data curation. Naiyao Li: Methodology, Formal analysis. Zengrong Wang: Validation, Software, Resources. Gang He: Writing – review & editing, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was supported by the Shaanxi Province Technological Innovation Guidance Special (No. 2022QFY08-01), the National Key Research and Development Program of China (No. 2021YFB3200702), Natural Science Foundation of China (Nos. 22201228, 22205172, 52203240 and 22175138), China Postdoctoral Science Foundation (Nos. 2022M712530, 2023T160506, and 2022M712497), Fundamental Research Funds for the Central Universities (No. xzy012022017), Young Talent Fund of Association for Science and Technology in Shaanxi (No. 20230624), Shaanxi Province Postdoctoral Science Foundation (No. 2023bSHTBZZ04), and the Youth Innovation Team of Shaanxi Universities. We thank Dr. Gang Chang, and Dan He at the Instrument Analysis Center of Xi'an Jiaotong University for their assistance with 600 M NMR, fluorescence, and EPR measurements. The high-performance computing center at Xi'an Jiaotong University is especially acknowledged for providing the computational resources.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111021.

  
  
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• 

  Scheme 1  Design strategy and synthetic route to the pyrenoviologens.

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  Figure 1  (a) The CV and DPV curves of 3c at different scan rates in DMF with tetrabutylammonium hexafluorophosphate (0.1 mol/L) as supporting electrolyte, potential E referenced to Fc/Fc⁺ (c = 5 × 10⁻⁴ mol/L). (b) UV–vis spectra and (c) fluorescence spectra of 3a, 3b and 3c in DMF (c = 2 × 10⁻⁵ mol/L, λ_{ex, 3a} = λ_{ex, 3b} = 360 nm, λ_{ex, 3c} = 380 nm). The inset photographs of 3a, 3b and 3c in DMF upon daylight and UV light (365 nm). (d) UV–vis spectra and (e) fluorescence spectra of three redox states of 3c (c = 2 × 10⁻⁵ mol/L). The inset photographs of 3c (3c' and 3c'') in DMF upon daylight and UV light (365 nm). The "×10" on the blue line which reduced with Na means that its strength has been artificially expanded tenfold when drawing to facilitate the analysis of data. (f) EPR spectra of the biradical species of 3a', 3b' and 3c' at room temperature.

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  Figure 2  (a) Electrochromic device assembly diagram. UV–vis spectra of the 3c in DMF from 3c to 3c' (b) and from 3c' to 3c'' (c) in ECD. The inset photographs of ECD in different color states. (d) The photographs of change in color of the flexible ECD assembled by 3c under bending stress.

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  Figure 3  (a) Fluorescence spectra of spectra of the 3c in DMF with the voltage gradually from 0 to 3.6 V. The inset photographs of 3c in DMF upon UV light (365 nm) in ECD (λ_ex = 380 nm). (b) The assembly diagram of the fluorescent warning sign and the actual recording picture of the assembly process. (c) Schematic diagram of the working process of electronic warning signs.

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  Figure 4  (a) The electron-transfer mechanism for the detection of PA by pyrenoviologens. (b) Emission spectra of 3c were titrated by different concentrations of picric acid (λ_ex = 380 nm). (c) The Stern–Volmer constant and lifetime of 3c to PA. (d) Quenching efficiencies of 3c to PA and other analogs at different concentrations.

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  Figure 5  (a) Schematic diagram of preparation of sensing film and schematic diagram of aqueous medium detection. (b) The film of 3a, 3b and 3c of Stern–Volmer constant to PA. The inset photographs of the films of 3a, 3b and 3c to PA. (c) The fluorescence spectra of films at repeated tests.

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