Ultrastable Viologen Ionic Liquids-Based Ionogels for Visible Strain Sensor Integrated with Electrochromism, Electrofluorochromism, and Strain Sensing

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES14 Nov 2022Ultrastable Viologen Ionic Liquids-Based Ionogels for Visible Strain Sensor Integrated with Electrochromism, Electrofluorochromism, and Strain Sensing Yueyan Zhang, Mengying Guo, Guoping Li, Xiaoliang Chen, Zishun Liu, Jinyou Shao, YongAn Huang and Gang He Yueyan Zhang Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710054 , Mengying Guo Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710054 , Guoping Li Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710054 , Xiaoliang Chen Micro-/Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710054 , Zishun Liu International Center for Applied Mechanics, State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi'an Jiaotong University, Xi'an 710049 , Jinyou Shao Micro-/Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710054 , YongAn Huang State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074 and Gang He *Corresponding author: E-mail Address: [email protected] Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710054 https://doi.org/10.31635/ccschem.022.202202310 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Flexible electronics play a key role in the development of human society and our daily activities. Currently they are expected to revolutionize personal health management. However, it remains challenging to fabricate smart sensors with high robustness, reliability, and visible readout. Herein, high-performance electrochromic (EC), electrofluorochromic (EFC), and double-network ionogels with excellent transmissivity, high mechanical robustness, and ultrastable reversibility are prepared by combination of thienoviologen-containing ionic liquids with poly(ethyl acrylate) elastomer. The ionogels exhibit good mechanical properties (1000% stretchability and 3.2 kJ m−2 fracture energy). The ionogel-based EC devices have a significantly simplified device fabrication process as well as superior cycling stability in which 88% of the contract ratio is maintained at 88% at 500 cycles, even after being stored for 2 years under ambient atmosphere (relative humidity: 30%∼40%, 25 °C). The conductivity of ionogels showed a fast and reproducible response to strain, and the conductivity decreased with increased strain. By virtue of the EC and EFC properties of the thienoviologen component, the EC and EFC efficiency decreased with the increased strain loaded on the ionogels, and almost no EC or EFC phenomena were observed when the strain was above 300%. This feasible strategy provides an opportunity for the development of visible strain sensors to monitor the body's movements through color and fluorescence emission. Download figure Download PowerPoint Introduction Flexible electronics with detecting, actuating, adapting, memorizing, communicating, and displaying capabilities have arisen in both academia and industry over the past two decades.1–5 As the core component of flexible electronics, stimuli-responsive soft materials have received considerable attention due to their ability to monitor vital biometric signs and thus can be applied in personal health management to improve the quality of daily life.6–11 Ionogels, typically composed of ionic liquids and three-dimensional polymeric networks, have recently been recognized as promising sensitive materials for use in flexible electronics due to their high ionic conductivity, transparency, stretchability, and reliability.12–16 In addition, the conductivity of ionogels varies with the tensile strain which enables the transduction of mechanical signals into electrical signals.17–19 For instance, crosslinked polyacrylates are often selected as the polymeric matrix because of their superior transparency and mechanical properties. Ionogels based on polyacrylates have been studied extensively for stress monitoring.20–22 Although the application of ionogels in the smart sensor has been rapidly developed, their functions and convertible signals are still limited.23–26 Particularly, most applications of ionogels in sensors are based on converting external stimuli to electronic signals, which severely limit the performance of devices in terms of visible readout.27–29 Therefore, developing stimuli-responsive materials with tunable optical properties under external stimuli is essential for smart sensors with easy readout, fast response, low cost, and simple fabrication processes. Electrochromic (EC) and electrofluorochromic (EFC) materials are a type of important photoelectric functional material whose optical absorption or luminescence emission can be switched reversibly under applied potentials.30–33 As a typical EC material, viologen and its derivatives (1,1′-disubstituted-4,4′-bipyridinium dications) have sparked tremendous interest and success in intelligent windows, antiglare rearview mirrors, displays, and other applications due to their distinct color change upon the applied voltage with high contrast, tunable EC properties, and ease of device fabrication.34,35 The viologen derivatives have also been incorporated with ionogels to make electrochromic devices (ECDs) which demonstrate fast response and high durability.36,37 For instance, Myoung et al.36 reported a series of EC gels with viologen derivatives, poly(ethylene glycol) diacrylate matrix, and ionic liquids. These ionogels exhibited stable properties without degradation during repeated operation and with high durability, even after 1000 cycles of mechanical bending tests. Liu et al.37 demonstrated all-in-one gel ECDs powered by perovskite solar cells. The combined devices achieved automatic light adjustment with high stability (up to 70,000 cycles). Moreover, viologens incorporated in conjugated units exhibited intense fluorescence emissions, thereby opening a powerful way to design optoelectronic devices for intelligent display and information storage.38–40 However, to the best of our knowledge, the application of viologen-based ionogels as sensitive materials in stimuli sensing has not been reported. We envision that the combination of the EC/EFC properties of viologen derivatives with the advantages of ionogel should achieve a significant breakthrough in transforming other stimulus signals into visual optical signals and fabricating novel visible strain sensors. Based on the above considerations, a series of double-network ionogels based on polymer matrix, ionic liquid, and viologen derivatives with superior stability and tensile properties were prepared (Figure 1a–g). The EC and EFC ionogels were used to fabricate ECDs and EFCDs free of the sealing process due to their adhesion ability. The polymeric double network alleviated the aggregation of the viologen radical cations and gave rise to better cycling ability. At the same time, the conductivity of the ionogel changes with tensile properties transformed the mechanical signal into the EC and EFC signals, thus laying a foundation for the visualization of smart sensors. Figure 1 | (a) Schematic illustration of the fabrication procedure of the ionogel. (b) Illustration of electrochromism and (c) digital picture of ionogel-based ECD. Illustration of EC properties of stretchable ionogel-based ECD at relaxed (d) and stretched state. (e) Digital pictures of electrochromism and electrofluorochromism of stretchable ionogel-based ECD at (f) relaxed and (g) stretched state. Download figure Download PowerPoint Experimental Methods Materials All reactions were performed with standard Schlenk and glovebox (Vigor) techniques under an inert atmosphere. 2,5-Dibromothiophene (98%), methyltrioctylammonium chloride (Aliquant 336, 98%), tetrakis(triphenylphosphine)palladium(0) (Pd (PPh3)4), 99%), ethyl acrylate (99%), ethylene dimethacrylate (99%), phenylbis(2,4,6-trimethyl benzoyl)phosphine oxide (PBPO, 97%), 1-methylimidazole (99%), 1,3-dibromopropane (98%), and bis(trifluoromethane)sulfonamide lithium salt (98%) were purchased from Energy Chemical Inc. (Shanghai, China). 1-Ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide (98%) was purchased from Bide Pharmatech Ltd. (Shanghai, China). Ethyl acrylate (EA) was filtrated through a column of basic alumina to remove the inhibitor. Toluene was distilled from the sodium-benzophenone system before use. Other reagents and solvents were used without any further purification. Synthesis of [MV][TFSI] The dry acetonitrile solution of 4,4′-bipyridine (1.78 g, 11.4 mmol) and iodomethane (1.50 mL, 24.1 mmol) was heated at 40 °C for 24 h under an inert atmosphere. The orange precipitate was filtered, washed with acetone, and dried in a vacuum oven overnight to obtain a needle crystal of 1,1′-dimethyl-4,4′-bipyridinium diiodide. To exchange the iodide for bis((trifluoromethyl)sulfonyl)imide (TFSI) anion, lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) (1.50 g, 5.2 mmol) was added into the [MV][I] (0.50 g, 2.1 mmol) aqueous solution, and the mixture was stirred for 24 h at room temperature. The colorless precipitate of [MV][TFSI] was obtained (1.40 g, 85% yield). Synthesis of [MTV][TFSI] 2,5-Di(pyridin-4-yl)thiophene was synthesized through previously reported procedures.1 The dry acetonitrile solution of 2,5-di(pyridin-4-yl)thiophene (1.90 g, 8.0 mmol) and iodomethane (1.20 mL, 19.3 mmol) was heated at 40 °C for 24 h under inert atmosphere. The orange precipitate was filtered, washed with acetone, and dried in a vacuum oven overnight to obtain a needle crystal of [MTV][I]. To exchange the iodide for TFSI anion, LiTFSI (1.50 g, 5.2 mmol) was added into the [MTV][I] (0.5 g, 2.1 mmol) aqueous solution, and the mixture was stirred for 24 h at room temperature. The yellow precipitate of [MTV][TFSI] was obtained (1.49 g, 87% yield). Synthesis of [ImTV][TFSI] 1,3-Dibromopropane (76 mL, 752.7 mmol, 20 equiv) was dissolved in 40 mL of acetonitrile, and a solution of 1-methylimidazole (3 mL, 37.6 mmol) in acetonitrile (10 mL) was added dropwise. The reaction mixture was refluxed for 24 h. The product was precipitated and isolated by filtration. 2,5-Di(pyridin-4-yl)thiophene (1.0 g, 4.2 mmol) and ImBr (3.58 g, 12.6 mmol) were dissolved in methyl nitrile (20 mL). The reaction mixture was stirred at 60 °C for 4 h under inert atmosphere. The crude product was isolated by centrifugation and washed with dichloromethane to give an orange solid. [ImTV][Br] (1.0 g, 1.25 mmol) was dissolved in H2O (20 mL), and LiTFSI (1.5 g, 5.2 mmol) was added. The product was isolated by centrifugation to give range viscous liquid (2.3 g, 80% yield). Synthesis of the poly(ethyl acrylate) elastomers Single-network films were prepared by photopolymerization of a solution containing EA monomer, ethylene glycol dimethacrylate (EGDMA) as cross-linker (0.68 mol % relative to monomer), and PBPO as photoinitiator (1.16 mol % relative to monomer). After the reactants were dissolved in toluene (1/1 v/v relative to monomer), the mixture was poured into a 1 mm-thick poly(vinylidene difluoride) mold. The polymerizations were initiated by the UV lamp (0.9 mW/cm2 where the sample was placed) and left to proceed for 1 h. The samples were then extracted from the mold and immersed in toluene/cyclohexane mixtures to extract any unreacted species. The single-network films were finally dried under vacuum at 80 °C overnight and then stored at room temperature until later use. Starting from the single-network film, poly(ethyl acrylate) (PEA) elastomer was prepared following swelling and the polymerization sequence. Briefly, a piece of single-network film was swollen in a bath composed of EA as the monomer, EGDMA as cross-linker (0.01 mol % of monomer), and PBPO as photoinitiator (0.01 mol % of monomer). Once swollen to equilibrium, the sample was carefully extracted from the monomer bath and placed between the poly(ethylene terephthalate) sheet and glass plate. Then, the whole sample's holder was exposed to the UV light for one hour to complete the polymerization. After being dried in a vacuum at 80 °C overnight, the PEA elastomer was stored at room temperature for later use. Preparation of the double network ionogels The PEA elastomers (m0) were immersed in the ethanol solution containing viologen ionic liquid (10 mg/mL) and 1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide (20% v/v). The samples were removed from the ethanol solution and dried in a vacuum at 70 °C overnight. The weight of the prepared ionogel (m) was measured. The IL content in each ionogel was calculated by the following equation: IL content ( wt % ) = ( m − m 0 ) / m × 100 % (1) Characterization and measurement NMR spectra were recorded on a Bruker Avance-400 spectrometer (Bruker, Switzerland) in the indicated solvents. Chemical shifts were reported in ppm by assigning tetramethylsilane (TMS) resonance in the 1H spectra as 0.00 ppm, dimethyl sulfoxide-d6 resonance in the 1H spectra as 2.50 ppm, and in the 13C spectra as 39.50 ppm. Coupling constants were reported in Hz, and multiplicities were denoted as s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). UV–Vis spectra were measured using a DH-2000-BAL scan spectrophotometer (OceanOptics, USA). The cyclic voltammetry (CV) in solution was measured using CHI660E B157216, with a polished glassy carbon electrode as the working electrode, a platinum electrode as the counter electrode, and a silver electrode as the reference electrode, with ferrocene/ferrocenium (Fc/Fc+) as the internal standard. Impedance analysis was performed using an Autolab electrochemical workstation (PGSTAT302N, Metrohm Instruments, Herisau, Switzerland). Thermogravimetric analysis (TGA) measurements were conducted using a Mettler-Toledo TGA1 thermal analyzer in air at a heating rate of 10 °C min−1 in the temperature range of 30–700 °C. Differential scanning calorimetry (DSC) was utilized to determine the thermal transition properties using a DISCOVER DSC250 (TA Instruments, USA) in a temperature range of −150–200 °C with a ramp rate of 10 °C min−1 under an inert atmosphere. The heating and cooling cycles were repeated three times. High-resolution mass spectra (HRMS) were recorded on a Bruker maXis UHR-TOF mass spectrometer (Bruker, USA) in an electrospray ionization-positive mode. Fluorescence measurements were performed on the FLS980 system (Edinburgh Instruments, UK). Fourier transform infrared spectroscopy was collected with a Nicolet 6700 FT-IR Instrument (USA). Electron paramagnetic resonance (EPR) was measured using a Bruker EMX PLUS6/1 Instrument (Switzerland) at room temperature in dry degassed dimethylformamide (DMF). The structure and morphology of the samples were characterized using a scanning electron microscope (SEM, MAIA3 LMH, USA) equipped with an energy dispersive X-ray analyzer (AztecX-MaxN50mm2, UK). In the ECD, indium tin oxide (ITO)-coated glass or ITO-coated Polyethylene Terephthalate (PET) film was utilized as the electrode, and the EC ionogel was used as an active component. The ionogel was sandwiched between two slides of ITO glass or films without other sealing methods. Photographs were taken with a Nikon D5100 digital camera. Results and Discussion Ionic liquids based on viologen derivatives Different EC substances, methyl viologen (MV2+), methyl thienoviologen (MTV2+), and imidazolium thienoviologen (ImTV2+), were synthesized to prepare the EC/EFC ionogels ( Supporting Information Schemes S1–S7). The obtained [MV][TFSI], [MTV][TFSI], and [ImTV][TFSI] were verified by 1H NMR, 13C NMR, and HRMS ( Supporting Information Figures S37–S51). TGA analysis showed that [MV][TFSI], [MTV][TFSI], and [ImTV][TFSI] started to decompose around 300 °C ( Supporting Information Figure S1). [MV][TFSI] is a white solid with a melting point of 51 °C, and [MTV][TFSI] is an orange solid with a melting point of 86 °C. DSC analysis showed that the phase transition temperature of [ImTV][TFSI] was about −15.4 °C (Figure 2b), which confirmed the ionic liquid nature of [ImTV][TFSI]. The light absorption characteristics of [MV][TFSI], [MTV][TFSI], and [ImTV][TFSI] were studied by UV/Vis spectroscopy ( Supporting Information Figures S2–S4). The wavelength with maximum absorption of [MV][TFSI] in DMF was obtained from the spectrum to be 365 nm. After reduction reaction with Zn powder, the solution of [MV][TFSI] turned from colorless to navy, and new absorption peaks at 557, 605, and 875 nm appeared in the UV/Vis spectrum. The wavelength with maximum absorption of [MTV][TFSI] in DMF was 460 nm. The solution turned from light yellow to blue after the reduction reaction with Zn powder, and new absorption peaks at 570, 905, and 1050 nm appeared in the spectrum. The UV absorption of [ImTV][TFSI] was comparable with [MTV][TFSI] so that the wavelength with maximum absorption was 480 nm, and new absorption peaks at 573, 910, and 1050 nm appeared in the spectrum after Zn reduction. The new absorption peaks in the UV–Vis spectra indicated the generation of viologen radical cations, and the radical species were clarified with EPR spectroscopy ( Supporting Information Figure S5). The whole process to obtain the radical species was undertaken in a glove box. [MV][TFSI], [MTV][TFSI], and [ImTV][TFSI] were dissolved in dry DMF to form a solution (c = 5 × 10−3 M) and were reduced to radical species by adding zinc powder.41 The EPR results confirmed the existence of radical species. The presence of thiophene units provided [MTV][TFSI] and [ImTV][TFSI] with fluorescent emission properties. As shown in Supporting Information Figure S6, [MTV][TFSI] showed strong emission with the maximum emission around λem = 550 nm. The quantum yield of [MTV][TFSI] was 88.07%, and the lifetime was 2.52 ns. [ImTV][TFSI] showed strong emission with the maximum emission around λem = 580 nm. The quantum yield of [ImTV][TFSI] was 86.12%, and the lifetime was 2.65 ns. The absorption and emission properties of [MTV][TFSI] and [ImTV][TFSI] suggested that the imidazolium group did not affect the optical properties as well as the physical properties. The chemical properties of obtained [MV][TFSI], [MTV][TFSI], and [ImTV][TFSI] indicate that they are suitable for the fabrication of EC and EFC devices (ECDs and EFCDs). Figure 2 | (a) TGA analysis and (b) DSC analysis of PEA elastomer, [EMIM][TFSI], [ImTV][TFSI], and ionogels containing 21, 57, and 95 wt % ionic liquids. (c) Tensile stress–strain curve of the ionogel (10 mm/min stretching rate). (d) Swelling kinetics of the PEA polymer immersed in the solution of ionic liquid at room temperature. The inset is the plot of weight maintenance of the ionogels under vacuum versus time. Digital pictures of (e) PEA, ionogel containing [ImTV][TFSI], and ionogel under UV light of 365 nm and (f) the ionogel under knotting and then stretching extensively. (g) Interfacial adhesion of ionogel containing 57 wt % ionic liquid on different substrates (glass, PET film and silver foil) by the 90-deg peel test. Download figure Download PowerPoint Fabrication and characterization of double-network ionogels The ionogels were prepared by an ex situ approach according to previously reported procedures (Figure 1a).28 EA, EGDMA, and PBPO were used as the monomer, cross-linker, and photoinitiator, respectively. The first network of PEA was prepared by UV-initiated photopolymerization of EA and EGDMA. The single network films were then swelled in the EA monomer solution containing a small amount of EGDMA and PBPO, and the second photopolymerization was performed to obtain the double network PEA elastomer. Finally, the PEA elastomer was immersed in a solution of ionic liquid to generate the ionogels. [MTV][TFSI] and [ImTV][TFSI] exhibited a small amount of were incorporated into the ionogels to EC properties. used ionic liquid 1-ethyl-3-methylimidazolium was to good ionic conductivity of the ionogels. The obtained ionogels exhibited high transparency, ionic conductivity, and stretchability (Figure analysis of PEA elastomer and EC ionogels was conducted to the of the samples and the and demonstrated that and were at the that the viologen derivatives and the ionic liquid was in ionogels, and no phase was observed ( Supporting Information Figure The swelling kinetics of the PEA elastomer immersed in the solution of ionic liquid at room temperature is shown in Figure The ionic liquid content increased rapidly as the swelling increased and the maximum amount after 24 h. TGA analysis demonstrated the thermal stability of ionogels so that the weight at °C was under an inert atmosphere (Figure The glass transition temperature of the ionogels decreased with the content of ionic liquid due to the of (Figure The that one was that the ionogels are without any phase The PEA elastomer be stretched to of its without at a rate of 10 the of ionic liquid and viologen derivatives, the mechanical properties of ionogel the elastomer (Figure The ionogels containing [ImTV][TFSI] demonstrated better strain properties with [MV][TFSI] and [MTV][TFSI] ( Supporting Information Figures The ionogels also demonstrated good mechanical The ionogels with 57 wt % ionic liquid were stretched over 500 cycles under strain and over 300 cycles under strain. was observed in the first stress–strain cycles, and the stretchable stable in the cycles ( Supporting Information Figure As the ionic liquid content the stretchability, and of the ionogels decreased ( Supporting Information Figure The superior mechanical properties of the ionogel it to be by stretching (Figure The ionogels also exhibited good adhesion ability that the application of sensors or electronic The adhesion of ionogels on different substrates (glass, PET film, and silver foil) was measured by the peel test. As shown in Figure and Supporting Information Figure ionogels with 57 wt % ionic liquid content demonstrated the adhesion the containing exhibited adhesion with [MTV][TFSI] and to the and stability of ionic the ionogels exhibited stability so that no weight was observed when the ionogel was stored in the vacuum oven for of Figure EC and EFC devices The EC gels significantly simplified the fabrication process of ECDs so that the ECDs be by the ionogels between two ITO-coated or PET films (Figure sealing process was due to the adhesion ability of ionogels. In addition, the prepared polymer network and EC materials as a all-in-one polymer EC which the of The electrochemical properties of the were studied by at different scanning ( Supporting Information Figures of electrochemical peaks were observed for [MV][TFSI] and [ImTV][TFSI], the of the viologen and thienoviologen units one of reduction peaks was for The reduction for [MV][TFSI] were = and = and the reduction for [ImTV][TFSI] were = and = [MTV][TFSI] exhibited one of peaks at = which be to the reduction in which the first reduction process can two The EC of the ionogel-based devices containing [MV][TFSI], [MTV][TFSI], and [ImTV][TFSI] were by To the EC of ITO under the applied a was composed of double-network PEA matrix and ionic liquid was prepared and used as the sandwiched between the ITO was applied for 10 and the was to the and versus time, the optical of ITO did not change when the was applied ( Supporting Information Figure To the suitable for of ECD under different was As shown in Supporting Information Figure the of ECD did not change under and peaks started to when the increased to and was observed when was The color of the ECD from colorless to blue upon application of and the color turned to the under 0 This process the first of viologen As shown in the spectra ( Supporting Information Figure the light absorption at around and nm increased when a voltage of was to the of radical species. The electrochromism of the second was to voltage reactions on the ITO to the of the The ECDs containing [MTV][TFSI] and [ImTV][TFSI] exhibited EC in which the ionogels turned from light yellow to orange under and to the under 0 Figure | (a) reaction of viologens during EC (b) Digital pictures of the [MV][TFSI], [MTV][TFSI], and ECDs under different (c) of ECD under (d) of (e) and of [MV][TFSI], [MTV][TFSI], and (f) of the optical versus and the as efficiency for ECDs with EC gel under Download figure Download PowerPoint The showed that new absorption peaks at around 570, 910, and nm appeared upon application of which was in with the chemical reduction (Figure and Supporting Information Figure with different of were and the of ECDs was As shown in Supporting Information Figure ionogels with 2 wt % demonstrated in UV–Vis ionogels with 4 and wt % of exhibited EC However, the ionogels with wt % of showed color in the the ratio of ionogels with wt % of was the ionogels with 4 wt % of

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