Three-Dimensional Printable Viologen-Based Ionogel for Visible Sensing and Display

Open AccessCCS ChemistryRESEARCH ARTICLES16 Aug 2024Three-Dimensional Printable Viologen-Based Ionogel for Visible Sensing and Display Zhikang Han, Huan Yuan, Heng Zhang, Yueyan Zhang, Jian Lv, Xinyi Zhang, Zengrong Wang, Naiyao Li, Chenxu Liang, Ni Yan, Maxim Maximov, YongAn Huang and Gang He Zhikang Han Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Huan Yuan School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Heng Zhang Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Yueyan Zhang Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Jian Lv Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Xinyi Zhang School of Materials Science & Engineering, Engineering Research Center of Transportation Materials, Ministry of Education, Chang'an University, Xi'an, 710064 Shaanxi , Zengrong Wang Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Naiyao Li Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Chenxu Liang School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi , Ni Yan School of Materials Science & Engineering, Engineering Research Center of Transportation Materials, Ministry of Education, Chang'an University, Xi'an, 710064 Shaanxi , Maxim Maximov Peter the Great Saint-Petersburg Polytechnic University, Saint Petersburg, 195251 , YongAn Huang State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, 430074 Hubei and Gang He *Corresponding author: E-mail Address: [email protected] Frontier Institute of Science and Technology, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Future Industrial Innovation Institute of Emerging Information Storage and Smart Sensor, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi Cite this: CCS Chemistry. 2024;0:1–13https://doi.org/10.31635/ccschem.024.202404393 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Viologen has long been explored as an organic electrochromic material. However, conventional viologen (RV2+) often generates free radicals under photo-irradiation, interfering with the polymerization of monomers during digital light processing (DLP) three-dimensional (3D) printing when incorporated into ionogels. In this study, we synthesized a phenyl viologen ( (SPr)2PhMeV) capable of simultaneous two-electron transfer through molecular manipulation, effectively avoiding the formation of photogenerated radicals under illumination. This novel phenyl viologen demonstrated exceptional redox performance and cycle stability and could be seamlessly incorporated into ionogels via 3D printing technology. This innovative approach has facilitated the first-time acquisition of finely structured viologen-based ionogels, featuring high transparency (transmittance: 85%), robust stretchability (17 times), and self-healing capabilities (resistance recovers after contact) simultaneously. Notably, the material demonstrated exceptional visual responses to temperature and strain changes, rendering it ideal for visual temperature (30–90 °C, TCR = 36.09% °C−1) and strain (ΔT = 0 at strains of 300%) sensing applications. Additionally, we have designed a viologen ionogel display device that could independently showcase all 26 letters and 10 numbers within seconds. This breakthrough not only enhances the functionality of electrochromic materials but also paves the way for advanced sensory and display applications in the future. Download figure Download PowerPoint Introduction Various viologens have been developed as color-controllable smart materials, with applications ranging from smart windows, antiglare rearview mirrors, wearable sensors, and smart displays.1–12 However, solution-based viologen electrochromic devices face challenges such as shortened lifespan and circuit corrosion due to electrolyte leakage,13,14 significantly hindering the practical application of electrochromism. In response to this issue, viologen-based ionogels with high ionic conductivity and significant stretchability have emerged. Nevertheless, the challenge of achieving fine structures for exquisite displays persists.15 Previous approaches have primarily relied on polymer addition to increase viscosity and form viologen-based ionogels. Despite their promise, a key obstacle has been the inability of their constituent monomers to polymerize fully, which is crucial for establishing a stable structural framework. This limitation has impeded the creation of ionogels with the required durability and integrity needed for practical applications.16 While recent efforts have involved the photopolymerization of monomers followed by immersion in a viologen solution to create a stretchable and stable ionogel,6 issues like swelling and deformation of crosslinked monomers during soaking hinder the attainment of fine structures.17 The advancement of viologen-based ionogels for high-precision display necessitates the exploration of novel strategies in the gel solidifying process.18 Digital light processing (DLP) three-dimensional (3D) printing technology, which translates digital models into tangible objects through a layer-by-layer transformation process, constitutes an innovative approach to polymer manufacturing. This technology enables the creation of intricate patterns with precision and efficiency.19–21 Recognized for its rapid prototyping capabilities, accurate processing, and extensive design flexibility, DLP technology is commonly employed in manufacturing sensors and creating visually striking displays.22–24 The fundamental principle of 3D printing technology hinges on the photoinitiated free radical polymerization upon exposure to light.25,26 However, when it comes to viologen-based ionogels, the photochromic properties of viologen pose a challenge to the implementation of this technology.27,28 Viologen possesses the property of absorbing photons when exposed to light, leading to a transition into a free radical state. This transition can disrupt the free radical polymerization process, causing complications in the manufacturing process (Figure 1).29–32 To mitigate this issue, the two-electron transfer process of viologen shows promise in avoiding the generation of free radicals under illumination. More recently, it has been reported that introducing a phenyl ring in pyridine units has been shown to facilitate a concerted two-electron transfer process in viologen molecules.33,34 Nonetheless, the study of the phenyl viologen (PhV2+) has been primarily focused on charge transfer during the redox process, lacking exploration of the photochemistry-related reactions. It is envisioned that leveraging the advantages of PhV2+ with its two-electron transfer process alongside a conductive ionogel could effectively overcome the challenges associated with polymerized conductive ionogels for DLP 3D technology. This innovation could pave the way for the development of novel visible sensors and display devices. Figure 1 | The technical route of one-step preparation of electrochromic ionic gel (compared with previous work) and the synthesis route of (SPr)2PhMeV. Download figure Download PowerPoint Based on these considerations, we have successfully achieved the 3D printing of viologen-based ionogels through molecular manipulation, resulting in ionogels with finely structured architectures. The phenyl viologen (3,3′-(1,4-phenylenebis(2,6-dimethylpyridine-1-ium-4,1-diyl))bis(propane-1-sulfonate) [ (SPr)2PhMeV]) was synthesized by introducing a phenyl ring into parent viologen skeletons. The incorporation of a phenyl ring effectively prevented the generation of free radicals, thus avoiding interference by photopolymerization of the monomers. Additionally, PhV2+ acted as an internal salt through the introduction of propanesulfonate groups, enhancing steric hindrance and inhibiting the formation of by-products during the redox process.35,36 To address the issue of liquid leakage, deep eutectic solvents (DES) (composed of choline chloride and urea) were utilized as conductive media.37,38 Through 3D printing, successfully obtained PhV2+-based ionogels with superior mechanical performance, intricate structures, and excellent electrochromic functionality. These ionogels are well-suited for visible temperature and strain-sensing applications. Moreover, we have developed an innovative display device capable of independently showcasing all 26 alphabets and 10 digits, broadening viologens' applicability in high-resolution sensory and display technologies. Experimental Methods Materials and instrumentation All reactions were conducted utilizing standard Schlenk and glovebox (Vigor) techniques under an argon atmosphere. All chemicals were purchased from Energy Chemical Inc. (Anhui, China), and stored in an Argon glovebox. Toluene was distilled from sodium/benzophenone prior to use, and other chemicals were used as commercially available without further purification. Deionized water was purged overnight using Ar before use. Nuclear magnetic resonance (NMR) spectroscopy was performed using a Bruker 400 MHz NMR spectrometer (Bruker, Massachusetts, USA). UV–vis measurements were conducted using a DH-2000-BAL Scan spectrophotometer (OceanOptics, Florida, USA). The cyclic voltammetry (CV) and differential pulse voltammetry in solution were measured using the electrochemical workstation CHI660E B157216 (Chen Hua, Shang Hai, China). High-resolution mass spectra (HRMS) were performed on a Bruker maxis UHR-TOF mass spectrometer (Bruker, Massachusetts, USA) in positive electrospray ionization (ESI) mode. All photographs were taken using a Nikon D5100 digital camera (Nikon, Tokyo, Japan). Single crystal X-ray diffraction data collection of the compounds were recorded on Bruker D8 Venture photon II diffractometer (Bruker, Massachusetts, USA). Electron paramagnetic resonance (EPR) was measured using a Bruker EMXPLUS6/1 instrument (Bruker, Massachusetts, USA) at room temperature in their aqueous solution. Thermogravimetric analysis (TGA) measurements were conducted using a Mettler-Toledo TGA1 thermal analyzer (Mettler Toledo, Zurich, Switzerland) in air, at a heating rate of 10 °C min−1 in the temperature range of 30–800 °C. The morphology and chemical elements of the samples were characterized using a scanning electron microscope (SEM, MAIA3 LMH; TESCAN, Prague, Czech) equipped with an energy dispersive X-ray spectroscopy (EDX) Analyzer (Aztec X-max 50, Oxford, Oxfordshire, UK). Synthesis of 1,4-bis(2,6-dimethylpyridin-4-yl)benzene (PhDMeP) To a mixture of 1,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene (1.00 g, 3.03 mmol), 4-bromo-2,6-dimethylpyridine (1.41 g, 7.57 mmol), Pd(PPh3)4 (0.140 g, 0.121 mmol), K3PO4 (3.22 g, 15.51 mmol), and 25 mL N,N-dimethylformamide were added. The solution was sealed in a pressure vial with a Teflon bushing and heated at 100 °C for 2 days. Upon completion, the reaction mixture was allowed to cool to room temperature and filtered to remove the catalyst. The filtrate was subjected to three extractions with water and chloroform, dried over Na2SO4, and filtered. The addition of ethyl ether (50 vol %) led to the precipitation of a faint yellow solid, which was filtered off and washed with hexane, ethanol, and ether. The solid was dried in a vacuum oven overnight to obtain a needle crystal of 1,4-bis(2,6-dimexthylpyridin-4-yl)benzene (0.68 g, 82% yield). 1H NMR (400 MHz, CDCl3, δ): 7.71 (d, J = 2.2 Hz, 4H, ArH), 7.22 (s, 4H, ArH), 2.61 (d, J = 2.0 Hz, 12H, CH3). 13C NMR (101 MHz, CDCl3, δ): 158.34 (s), 148.17 (s), 139.07 (s), 127.59 (s), 118.23 (s), 24.62 (s). Synthesis of 3,3′-(1,4-phenylenebis(2,6-dimethylpyridine-1-ium-4,1-diyl))bis(propane-1-sulfonate) [(SPr)2PhMeV] The dry N,N-dimethylformamide solution of 1,4-bis(2,6-dimethylpyridin-4-yl)benzene (PhDMeP; 1.00 g, 3.47 mmol) and 1,3-Propanesultone (4.24 g, 34.67 mmol) was heated at 155 °C for 24 h under an inert atmosphere. After the reaction was complete, the mixture was cooled to room temperature and filtered. The orange precipitate was washed with acetone and dried in a vacuum oven overnight to obtain a needle crystal of (SPr)2PhMeV (1.67 g, 90% yield). 1H NMR (400 MHz, D2O, δ): 8.07 (s, 4H, ArH), 8.04 (s, 4H, ArH), 4.69–4.64 (m, 4H, CH2), 3.13 (t, J = 6.9 Hz, 4H, CH2), 2.90 (s, 12H, CH3), 2.32–2.26 (m, 4H, CH2). 13C NMR (101 MHz, D2O, δ): 155.53 (s), 153.65 (s), 136.82 (s), 128.73 (s), 125.11 (s), 50.75 (s), 47.36 (s), 23.08 (s), 20.51 (s). HRMS (ESI) m/z: [M+Na]+ calcd for C26H32N2O6S2 555.1594; found 555.1595. Synthesis of ionogels based on deep eutectic solvent First, choline chloride and urea were weighed according to the molar ratio of 1:2 to form a deep eutectic solvent. Subsequently, acrylamide was introduced as the monomer, comprising 30% of the solvent's mass. To initiate the polymerization process, 2,4,6-trimethylbenzoyldiphenyl phosphine oxide (TPO) was added as a photoinitiator at a concentration of 5/10,000 of the monomer's mass, while methylene-bis-acrylamide (MBAA) served as the crosslinker at 2/1000 of the monomer's mass. Further, the electrochromic elastomer was synthesized through free radical polymerization, augmented by the addition of (SPr)2PhMeV as a discoloration material at a ratio of 1/1000 of the monomer's mass. The above substances were mixed at 70° and heated and stirred in an argon atmosphere to form a yellowish transparent solution, that was the precursor solution of ionogels. The subsequent 3D printing process was executed using an Elegoo printer equipped with a UV projector (405 nm) as the light source. Firstly, the designed 3D structure was sliced by the software to form the corresponding 2D images of each layer, and then the patterned ultraviolet light was irradiated on the elastomer precursor solution by the projector, and the electrochromic ionogel was solidified layer by layer. The electrochemical characterization Redox potential was referenced to the normal hydrogen electrode (NHE). The glassy carbon electrode (d = 3 mm) was used for the working electrode, which was polished using Al2O3 suspended in deionized H2O, then rinsed with deionized H2O, and dried with airflow. The platinum sheet (1 cm2) was used for the counter electrode. The reference electrode consisted of a silver wire coated with a layer of AgCl and suspended in a solution of 3 M KCl electrolyte (Ag/AgCl, vs NHE). Density functional theory (DFT) calculations The geometries for the ground state of these compounds were optimized at the B3LYP hybrid functional and 6-311+G(d) basis set for all atoms.39 The calculated molecular orbitals involved in the main transitions were reported in this work. It should be pointed out that the structures of all stationary points were fully optimized; frequency calculations were performed at the same level, which confirmed the nature of all revealed equilibrium geometries without imaginary frequencies. All of the above computational calculations reported in this work were performed using the Gaussian 09 code ( https://gaussian-09w.software.informer.com/). Results and Discussion Synthesis and structural characterization of (SPr)2PhMeV The synthesis method was detailed in the supporting information.34,40–42 2,5-di(pyridin-4-yl)thiophene (TDP), 2,5-di(pyridin-4-yl)furan (FDP), 1,4-di(pyridin-4-yl)benzene (PhDP), and PhDMeP were synthesized via the Suzuki coupling reaction. Subsequently, they were ionized using iodomethane to produce diiodide. Tetrabutylammonium chloride was used to exchange anions to obtain 4,4′-(thiophene-2,5-diyl)bis(1-methylpyridin-1-ium) dichloride (MTV)Cl2, 4,4′-(furan-2,5-diyl)bis(1-methylpyridin-1-ium) dichloride (MFV)Cl2, 4,4′-(1,4-phenylene)bis(1-methylpyridin-1-ium) dichloride (MPhV)Cl2. (SPr)2PhMeV was obtained by the reaction of 1,3-Propanesultone with PhDMeP. The structures were confirmed through NMR ( Supporting Information) and HRMS ( Supporting Information). The molecular structure was further characterized by a single-crystal X-ray diffraction analysis ( Supporting Information Figure S1 and Table S1). The single crystal was prepared by heating and dissolving (SPr)2PhMeV in water until the solution reached saturation, followed by cooling and precipitation at room temperature. The supplementary crystallographic data was deposited at the Cambridge Crystallographic Data Centre (CCDC 2333264) and can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif. The angle between the pyridine plane and the phenyl ring in (SPr)2PhMeV was 176.61°, indicating that these three groups were not coplanar. The linear structure nature of the molecule was attributed to the stabilizing influence of the four methyl groups. As an internal salt, the molecules carried the negative charges after reduction, with the linear configuration promoting charge repulsion. This effectively mitigated the likelihood of side reactions.35,36 The thermal stability of four viologens was characterized by TGA. The decomposition occurred above 270 °C, underscoring their capability for applications in high temperatures ( Supporting Information Figure S2). The UV–vis spectra of the four viologens were tested in H2O to evaluate their light absorption characteristics ( Supporting Information Figure S3). The concentration of viologen was 10−4 mol/L. The absorption maxima of (SPr)2PhMeV and (MPhV)Cl2 were at 322 and 321 nm, respectively, and (MTV)Cl2 and (MFV)Cl2 exhibited peaks at 370 nm. This indicated that the light absorption characteristics of viologen were mainly affected by the bridging group. The electrochromic ability of (SPr)2PhMeV was investigated by spectroelectrochemistry at a voltage of 2.8 V ( Supporting Information Figure S4). The molecule showed strong absorption at 474, 554, and 900 nm, with the solution color changing from colorless to purplish red. This result proved the capability of (SPr)2PhMeV in electrochromic applications. Electrochemical characterization and electron paramagnetic resonance of viologens To underscore the distinctive redox capabilities of (SPr)2PhMeV, three commonly encountered methylated conjugated viologen ( (MPhV)Cl2, (MTV)Cl2, and (MFV)Cl2) were selected and subjected to CV in a 0.5 M sodium chloride solution. (MTV)Cl2 and (MFV)Cl2exhibited two sets of redox peaks, while (SPr)2PhMeV and (MPhV)Cl2 displayed a single peak (Figure 2a and Supporting Information Figure S5). This indicated that (SPr)2PhMeV and (MPhV)Cl2 could undergo two-electron transfer at one potential, whereas (MTV)Cl2 and (MFV)Cl2 required an increase in voltage to acquire the second electron. The capability of (SPr)2PhMeV to undergo two-electron transfer was attributed to the blocking effect of the phenyl ring in bipyridine. The phenyl ring separated electron interactions between two pyridine rings, ensuring non-interfering electron transfers.34 Figure 2 | (a) Cyclic voltammograms of 2.0 mM (SPr)2PhMeV and (MTV)Cl2 with 1 V/s in 0.5 M NaCl solution. (b) 1000 CV tests of (SPr)2PhMeV at 0.1 V/s and comparison of 1st and 1000th cycles. (c) The EPR spectra of (MTV)Cl2 and (SPr)2PhMeV after thirty minutes of illumination. (d) the β-HOMO and LUMO energy difference of (MTV)Cl2, and (MFV)Cl2, (MPhV)Cl2 and (SPr)2PhMeV. Download figure Download PowerPoint To further characterize the redox capabilities of phenyl viologen ( (SPr)2PhMeV, (MPhV)Cl2), their CVs were measured at varied scanning speeds ( Supporting Information Figures S6 and S7). The redox peaks for (SPr)2PhMeV and (MPhV)Cl2 were observed at −0.87 and −0.69 V, respectively. The redox potentials of (SPr)2PhMeV were significantly lower than that of (MPhV)Cl2. This discrepancy was attributed to the presence of four methyl groups on the pyridine as electron donor groups, with the propanesulfonate groups on the branched chain exhibiting stronger electron donor ability than the methyl group. This led to a lower redox potential for (SPr)2PhMeV.43 The peak current of the redox process of (SPr)2PhMeV and (MPhV)Cl2 were linearly proportional to the square root of the scan rate, indicating the reversibility and diffusion-controlled process of the redox reaction. The electron-transfer constant (kET) of (SPr)2PhMeV (kET = 0.1172 cm/s), and (MPhV)Cl2 (kET = 0.1037 cm/s) were calculated according to the Nicholson method ( Supporting Information Table S2). The electron transfer constant of (SPr)2PhMeV was higher than that of (MPhV)Cl2, which indicated that the electrochromic behavior of (SPr)2PhMeV was faster under the same conditions. The cyclic stability of the two phenyl viologens was assessed in a 0.5 M sodium chloride solution at a scanning rate of 0.1 V/s. (SPr)2PhMeV showed good cyclic performance (Figure 2b), while (MPhV)Cl2 exhibited a significant decline, with nearly disappeared oxidation and reduction peaks at the 1000th cycle ( Supporting Information Figure S8). This divergence arose from the increased steric hindrance of the four methyl and propanesulfonate groups in (SPr)2PhMeV, effectively impeding side reactions during the redox process.42 EPR was measured in their aqueous solution. Detailed experimental methods can be found in the Supporting Information. The EPR signals of (MTV)Cl2 and (MFV)Cl2 were observed after one minute of illumination, while (MPhV)Cl2 and (SPr)2PhMeV showed no EPR signal ( Supporting Information Figure S9). After increasing the light exposure time to 30 min, (MTV)Cl2 and (MFV)Cl2 exhibited a more pronounced EPR signal, whereas (SPr)2PhMeV and (MPhV)Cl2 continued to show no detectable EPR signal (Figure 2c and Supporting Information Figure S9). This showed that (MPhV)Cl2 and (SPr)2PhMeV prevented free radicals production under illumination, and they can be used for DLP 3D printing to prepare ionogels with fine structures. DFT calculations of four viologens DFT calculations for the four viologens were implemented to confirm the results of electrochemical characterization ( Supporting Information Figure S10). Even with a phenyl ring, the bandgap of (SPr)2PhMeV was reduced to 3.48 eV. Typically, the phenyl ring contributed to a larger bandgap, as evidenced by (MPhV)Cl2, which possessed the highest bandgap at 4.2 eV, while (MTV)Cl2 and (MFV)Cl2 displayed bandgaps of 3.47 eV. Since the methyl and propanesulfonate groups of (SPr)2PhMeV reduced the bandgap and weakened the effect of the phenyl ring.44 The energy difference between the β-highest occupied molecular orbital (β-HOMO) of the singly reduced state and the lowest unoccupied molecular orbital (LUMO) of the ground state served as an indicator of the barrier for a molecule to gain a second electron (Figure 2d). Thus, the introduction of the phenyl ring resulted in a reduction of this difference, accounting for the reason why phenyl viologen exhibited a single redox peak. It is noteworthy that the two orbital energy difference of (SPr)2PhMeV was the smallest. This phenomenon was attributed to the high delocalization of electrons in the free radical state.45–47 The quinone structure of the (SPr)2PhMeV after reduction was the most probable configuration, owing to the lowest energy state among all potential structures ( Supporting Information Table S3). On the isosurfaces of their electron density, the local electrostatic potential values of (SPr)2PhMeV were shown by color ( Supporting Information Figure S11). The electrostatic potential near pyridine was decreased, indicating that pyridine was the primary site of the reduction reaction. Additionally, the study indicated that the entire molecule adopted a linear structure, consistent with the results from the single-crystal analysis.42 It is noteworthy that the molecular structure underwent rotation after electron gain, with the benzene ring and the two pyridines being in a coplanar state.34 Fabrication and characterization of ionogels Utilizing MBAA as the cross-linker, TPO as the photoinitiator, acrylamide as the monomer, and (SPr)2PhMeV as the electrochromic discoloration agent, the homogeneous mixture was photopolymerized by the UV light. This indicated that (SPr)2PhMeV avoided the generation of free radicals under ultraviolet light and the disturbance to the polymerization of the monomer. Consequently, the DLP 3D printing of ionogel based on this viologen could proceed. The viologen-based ionogels with fine structures and strong fluorescence were prepared (Figure 3a), which exhibited high transparency and exceptional tensile properties (Figure 3b). EDX analysis revealed a uniform distribution of C, O, Cl, N, and S elements on the cross-sectional surface, affirming the homogeneous distribution of DES and viologen derivatives in the ionogel ( Supporting Information Figure S12). Figure 3 | (a) Ionogel prepared by three-dimensional (3D) printing technology. (b) Stretch photographs of ionogel. (c) Photographs of ionogels prepared by the 3D printing method and the previous soaking approach. (d) Stress–strain curves of the original and self-healed ionogels (60 °C, 1 h). (e) Stress–strain curves of the original and saturated ionogels. (f) Electrical properties of the original and self-healed ionogels. Download figure Download PowerPoint To characterize the mechanical properties of the ionogel, its tensile properties were tested at a strain rate of 10 mm/min. It could be stretched nine times longer than its original length without rupture. Upon cutting and subsequent heating to 60 °C for 1 h, the ionogel demonstrated the ability to restore up to 79.8% of its initial tensile stress and 62.8% of its initial tensile strain, showcasing excellent self-healing capabilities (Figure 3d). This was attributed to the presence of −C=O, −NH2, and −OH groups in the ionogel that formed a robust hydrogen-bond network. As a special interaction, the hydrogen bonds were restored after being destroyed.48,49 The ionogel delivered pronounced hygroscopicity, attributed to the presence of choline chloride. At 25 °C and 40% humidity, the water content of the ionogel gradually tended to saturate following long-term storage ( Supporting Information Figure S13). Meanwhile, the resistance of the ionogel (10 mm × 10 mm × 1 mm) exhibited a gradual decrease and remained stable ( Supporting Information Figure S14). Stable resistance was extremely valuable for sensors to realize repeatability. TGA of the ionogel at 150 min revealed a weight loss of ∼25% before reaching 100 °C, corresponding to the percentage of absorbed water. The decomposition of the ionogel started at 200 °C, demonstrating its suitability for high-temperature con