Microchemical Journal 182 (2022) 107870 Contents lists available at ScienceDirect Microchemical Journal journal homepage: www.elsevier.com/locate/microc Spectroelectrochemical studies of electrochromic diarylethene ionic Liquids: From solution to ionogel based devices Sara Santiago a, b, Hugo Cruz c, *, Noemi Jordão c, Pablo Giménez-Gómez b, Karolina Zalewska c, Xavier Muñoz-Berbel b, Andreia F.M. Santos c, Jordi Hernando a, Luís C. Branco c, *, Gonzalo Guirado a, * a Departament de Química, Universitat Autònoma de Barcelona, Bellaterra (Barcelona) 08193, Spain b Instituto de Microelectrónica de Barcelona (IMB-CNM, CSIC), Bellaterra, (Barcelona) 08193, Spain c LAQV-REQUIMTE, NOVA School of Science and Technology, NOVA University of Lisbon, 2829-516 Caparica, Portugal A R T I C L E I N F O A B S T R A C T Keywords: This study establishes the photoelectrochromic properties of four diarylethene ionic liquids (DAE-ILs) composed Diarylethene ionic liquids of a diarylethene dicarboxylate (DAE) moiety, as counter-anion, and different organic cations ([C10mim]+, Spectroelectrochemistry [C16Pyr] +, [N8,8,8,1]+, [P6,6,6,14]+). The synthetized ILs viscous fluids maintain the photochemical and elec­ Ionogels trochemical properties of this well-known family of photoswitches, as well as the typical features of ILs (e.g., low Photoelectrochemical devices melting point, and high thermal and electrochemical stability). Our strategy of making multi-stimuli-responsive ionic liquids by ionic exchange opens the door to easily tune the physicochemical properties of smart DAE-ILs in function of the selected cation. Finally, those DAE-ILs are used to formulate smart ionogels (IGs), a type of materials that, by showing good conductive properties and retaining the stimuli-responsive behavior of the embedded DAE molecules, allow the fabrication of photoelectrochemical devices. 1. Introduction groups, including our team. In this research area, diarylethene de­ rivatives (DAEs) are excellent candidates to develop IL-based photo­ In the last decades, ionic liquids (ILs), which are organic salts with electrochomic devices, since they are organic P-type, high fatigue- low melting points below 100 ◦ C, have been explored to be applied in resistant photochromic molecules whose isomerization between their different academic and industrial fields, including material science open and closed isomers can also be electro-induced [1,3]. Most of the [1,2]. Indeed, ILs have been reported as potential candidates to be used previous studies on DAE-containing ILs (DAE-ILs) relied on the intro­ as a clean, efficient and eco-friendly alternative to conventional organic duction of cationic groups to diarylethene scaffolds - e.g., by appending solvents, mainly due to their tunable physical, chemical, and thermal N-methyl pyridinium moieties to one or both of the lateral aryl rings properties [3]. Besides these characteristics, other peculiar properties [27] or replacing one of them by an imidazolium ion [29]. As expected, such as high ionic conductivities, large electrochemical windows, as these DAE-ILs did not only show photochromic properties upon irradi­ well as excellent dissolution of organic, inorganic and polymeric mate­ ation, but also demonstrated a rich electrochemical behavior, as they rials [4–6], have contributed to an extensive range of applications from could undertake dimerization, ring closure and/or ring opening pro­ energy storage and conversion processes [7] to pharmaceutical in­ cesses upon application of electric currents. Since these compounds gredients [8–10]. often present an electron-rich nature, most of their electrochemical Further uses can be envisaged for ILs if additional, stimuli-responsive transformations were fulfilled upon oxidation. [30,31]. functionalities are incorporated into their anion and/or cation struc­ In our research group we have recently developed a different tures, which can be exploited for the preparation of smart functional approach towards DAE-ILs that does not require the tedious synthesis of materials [11–13]. Indeed, “smart” ILs exhibiting photochromic [9,14], complex diarylethene ionic derivatives. Instead, they are composed of electrochromic [15–21] luminescent [22–26], magnetic [16,27] and (i) an anionic diarylethene scaffold obtained from the deprotonation of a rheological properties [28] have been explored by different research dicarboxylated DAE that can be produced in a large scale through a * Corresponding authors. E-mail addresses: hg.cruz@fct.unl.pt (H. Cruz), l.branco@fct.unl.pt (L.C. Branco), gonzalo.guirado@uab.cat (G. Guirado). https://doi.org/10.1016/j.microc.2022.107870 Received 14 May 2022; Received in revised form 5 August 2022; Accepted 6 August 2022 Available online 17 August 2022 0026-265X/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). S. Santiago et al. Microchemical Journal 182 (2022) 107870 rather simple route [32–34], and (ii) conventional IL ammonium and N-Trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide phosphonium cations [35]. The two DAE-ILs that we reported based on ionic liquid ([N1114][TFSI]) was acquired from Solvionic and dried with this approach showed good photochromic properties, which were molecular sieves under vacuum prior to use. Extra dry and high purity somewhat influenced by the type of the selected cation (e.g. the colors of (≥99.5 % pur) acetone and acetonitrile (ACN) were purchased from their closed states) [35]. However, their electrochemical properties Acros. The basic anion-exchange resin Amberlyst A26-(OH) (ion-ex­ were scarcely investigated, and the use of spectroelectrochemical tech­ change capacity 1.4 eq. mL− 1, Alfa Aesar) and the silica gel for flash niques to characterize their redox-induced behavior was totally column chromatography (230–400 mesh, Sigma-Aldrich) were used as neglected. received. Trihexyltetradecylphosphonium chloride ([P6,6,6,14]Cl) Spectroelectrochemical measurements, which allow monitoring the (Cytec, > 98 %), hexadecylpyridinium chloride ([C16Pyr]Cl), 1-decyl-3- light absorbance or transmittance of a sample while applying electrical methylimidazolium bromide ([C10mim]Br) and tri­ currents, have become the most suitable tools for the characterization of octylmethylammonium bromide ([N8,8,8,1]Br) were purchased from electrochromic compounds [34,36–38]. These techniques allow the Solchemar and used without further purification. massive oxidation or reduction of the electroactive compounds present in a sample, while the spectroscopic changes caused by the application 2.2. Ionogel membranes preparation of the electrical potential can be measured in real time. This enables to establish the electro-induced reaction mechanisms, their kinetics, as P(VDF-co-HFP), [N1114][TFSI] ionic liquid and acetone were mixed well as electrochromic parameters such as response times, color contrast in a 1:5:18 wt ratio, respectively. The synthesized DAE-ILs were added to and efficiencies. In addition, the experimental system can be easily this solution to obtain a concentration of 5 mM for the case of cyclic adapted to perform measurements in liquid, solid or gel samples. This is voltammetry experiments, or 0.5 mM for optical measurements. The especially relevant given the evolution of new electrochromic systems resulting mixture was stirred overnight until a clear solution was ob­ towards the formulation of solid materials, while maintaining the un­ tained. 160 µL of this solution was casted onto a circle-shaped mold with altered electrochromic properties. This is the case of the smart (photo) of 1 cm in diameter and, after acetone evaporation for 1 h at ambient electrochromic ionogels based on a polymeric solid matrix and an IL conditions, a flexible and transparent membrane was obtained. phase doped with the (photo)electrochromic compounds of interest [39,40]. 2.3. Techniques In light of this, the aim of this work is to exploit spectroelec­ trochemical measurements to fully determine the electrochromic prop­ A spectrophotometer Hammatsu L10290 for absorbance UV–vis erties of several DAE-ILs based on a dicarboxylated anionic derivative of measurements and spectroelectrochemical experiments was used. The diarylethenes (Fig. 1): two new DAE-ILs systems bearing 1-decyl-3- equipment was controlled by the Bio-Kine32 V4.16 software. For elec­ methylimidazolium (1-DAE; imidazolium) and 1-hexadecylpyridinium trochemical and spectroelectrochemical studies, a VSP100 potentiostat (2-DAE; pyridinium) counter-cations and the two previously described was used. In the case of spectroelectrochemical measurements, the by us with trioctylammonium (3-DAE; ammonium) and trihexylte­ equipment was synchronized with the UV–vis spectrometer and tradecylphosphonium (4-DAE; phosphonium) counter-cations [35]. controlled with the EC-Lab V9.51 software. Cyclic voltammetry (CV) Once their electrochromic behavior is fully disclosed in solution, it is measurements in solution were done with a conical glass cell using a investigated in the solid state by synthesizing new smart photo­ 1 mm-in-diameter glassy carbon working electrode (WE), a Pt wire as electrochromic DAE-IL-based ionogels. Finally, a proof of concept pho­ counterelectrode (CE) and a reference Ag/AgCl electrode. The solutions toelectrochromic flexible device based on this material has been were prepared in acetonitrile (ACN) and adding 0.1 M concentration of fabricated and investigated. the supporting electrolyte TBAPF6. For 4-DAE, 5–10 % of methanol was added to improve solubility. The number of electrons related to electron 2. Material and methods transfer is determined by comparison with known one electron redox probes under the same electrochemical conditions. For spectroelec­ 2.1. Reagents and chemicals trochemical measurements in solution, a quartz cell of 0.1 cm in optical path length was used along with a Pt mesh as WE, a Pt wire as CE and a Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP), reference Ag/AgCl electrode (Ref). (Spectro)electrochemical measure­ Mw ~ 400000 and Mn ~ 130000) and tetrabutylammonium hexa­ ments of IG membranes were performed using screen printed electrodes fluorophosphate (TBAPF6) (>99.5 % pur.) were purchased from Merck. purchased from DropSens, where: (i) ITO working electrodes (ref. Fig. 1. Chemical structures and photoelectrochemical conversion between the open and closed isomers of all the DAE-ILs investigated in this work. 2 S. Santiago et al. Microchemical Journal 182 (2022) 107870 1 ITO10) were used for spectroelectrochemical measurements, and (i) H NMR (400.13 MHz, MeOD, 25 ◦ C) 9.03 (d, 3J = 4.0 Hz, 4H), 8.61 carbon WEs were employed for electrochemical measurements (ref. (t, J = 8.0 Hz, 2H), 8.14 (t, J = 4.0 Hz, 4H), 7.43 (s, 2H), 4.64 (t, DRP-110). J = 8.0 Hz, 4H), 2.83 (t, J = 8.0 Hz, 4H), 2.10 (m, 2H), 2.03 (m, 4H), NMR spectra were recorded at 25 ◦ C on a Bruker AMX 400 spec­ 1.90 (s, 6H), 1.39–1.30 (m, 52H), 0.92 (t, J = 8.0 Hz, 6H). trometer operating at 400.13 MHz (1H) and 100.61 MHz (13C) and using 13 C NMR (100.61 MHz, MeOD, 25 ◦ C) = 166.86, 145.44, 144.53, TMS as internal standard. 140.06, 136.30, 135.48, 134.59, 132.01, 128.11, 61.73, 38.19, 31.70, 31.16, 29.41, 29.39, 29.35, 29.26, 29.14, 29.11, 28.75, 25.81, 22.43, 2.4. Synthesis of DAE-ILs 22.37, 13.41, 13.10. 3-DAE and 4-DAE: they were prepared according to a synthetic 1,2-Bis(5′-carboxy-2′-methylthien-3′-yl)-cyclopentene (DAE): procedure previously reported by the team [35]. This compound was prepared according to a previous synthetic pro­ cedure developed in our group [36]. 3. Results and discussion 1-DAE: Amberlyst A26 (OH) (~1.5 mL; 0.80 equivalents) was added to [C10mim]Br (0.1214 g, 0.40 mmol) in methanol solution (15 mL) and 3.1. Electrochemical behavior and spectroelectrochemical study of DAE- the suspension was stirred at room temperature during 1 h. The resin ILs in solution was removed by filtration and [C10mim]OH was slowly added to DAE (0.07 g, 0.20 mmol) in methanol solution (10 mL) and stirred at room Initially, the electrochemical behavior of the open state of the ILs 1- temperature during ca. 21 h in the dark. The solvent was removed and DAE and 2-DAE was studied in acetonitrile solution by CV (Fig. 2). As the desired product was dried at 40 ◦ C in vacuum to give a yellow liquid shown in the voltammograms, an irreversible two-electron oxidation (0.1958 g, 99 %). wave was registered at relatively high potentials for both compounds, 1 H NMR (400.13 MHz, MeOD, 25 ◦ C) δ = 8.98 (s, 2H), 7.64 (d, whose peak lies at Epa ~ 1.35 V vs Ag/AgCl (Table 1 and Fig. 2a and b). A J = 4.0 Hz, 2H), 7.58 (d, J = 4.0 Hz,2H), 7.35 (s, 2H), 4.21 (t, J = 8.0 Hz, comparison of the oxidation peak potential values of DAE-ILs open states 4H), 3.94 (s, 6H), 2.81 (t, J = 8.0 Hz, 4H), 2.06 (m, 2H), 1.89 (m, 4H), in solution indicates no significant differences, which means that the 1.84 (s, 6H), 1.37–1.31 (m, 28H), 0.91 (t, J = 8.0 Hz, 6H). chemical environment is very similar (Table 1). Once all these samples 13 C NMR (100.61 MHz, MeOD, 25 ◦ C) = 168.57, 138.98, 138.29, 136.06, 134.39, 130.52, 123.53, 122.22, 49.44, 38.39, 35.03, 31.63, 29.75, 29.21, 29.14, 29.03, 28.71, 25.88, 22.42, 22.32, 13.36, 13.03. Table 1 2-DAE: Amberlyst A26 (OH) (~1mL; 0.79 equivalents) was added to Photo and electrochemical parameters for DAE ILs in solution. [C16Pyr]Cl (0.1059 g, 0.29 mmol) in methanol solution (12 mL) and the Solution suspension was stirred at room temperature during 1 h. The resin was Compound Epa (V) vs Ag/AgCl λmax (nm) removed by filtration and [C16Pyr]OH was slowly added to DAE (0.05 g, 1-DAEo 1.32 350 0.143 mmol) in methanol solution (10 mL) and stirred at room tem­ 1-DAEc 0.42 510 perature during ca. 21 h in the dark. The solvent was removed and the 2-DAEo 1.34 352 desired product was dried at 40 ◦ C in vacuum to give a brown liquid 2-DAEc 0.49 and 0.68 508 (0.1742 g, 84 %). Fig. 2. a-b) Cyclic voltammograms recorded at 0.5 V/s of 1-DAE and 2-DAE in their open and closed form after UV irradiation. Experiments were performed in ACN 0.1 M TBAPF6. The concentration values of 1-DAE and 2-DAE were 13 mM and 5.3 mM, respectively. c-d) 3D spectroelectrochemical measurements performed for 0.5 mM solutions of 1-DAEc and 2-DAEc using TBAPF6 as supporting electrolyte. A constant potential of Eapp,ox = +1.00 V vs Ag/AgCl was applied and, after 140 s, this led to a major decrement of the absorption band at ~510 nm, which indicates electroinduced isomerization towards their DAEo isomer. 3 S. Santiago et al. Microchemical Journal 182 (2022) 107870 were irradiated at 312 nm, ring cyclization of the DAE counter-anion electron transfer). It is important to note that, because this latter specie was evidenced by the appearance of strong reddish coloration associ­ is more oxidizing that DAEc, the homogeneous electron transfer reac­ ated to the formation of the corresponding DAEc-containing ILs with tion between [DAEo]+ and DAEc may also contribute to the ring-opening λabs,max ~ 510 nm. As expected [36], the oxidation potential value of this reaction and the discoloration of the samples, which would make this new state was measured to be lower in comparison with the open isomer isomerization process electrocatalytic. of 1-DAE and 2-DAE. In particular, when 1-DAEo and 2-DAEo solutions These results were also confirmed by conducting controlled potential were irradiated, a new one-electron oxidation peak at Epa ~ 0.40 V vs electrolysis experiments on DAEc solutions. When quantitatively elec­ Ag/AgCl was registered. Table 1 summarizes the oxidation potentials for trolyzing these samples at Eapp,ox = 1.2 V, the bleaching of the original each compound, as well the photo-electrochemical parameters charac­ characteristic red color of the closed isomers was observed for all teristic for each sample. However, a closer look to the Cvs of 1-DAE (1- compounds. Chemical analysis of the electrolyzed samples by UV–vis DAEc) and 2-DAE (2-DAEc) ILs closed forms (Fig. 2a and b, blue line) Absorbance spectroscopy, cyclic voltammetry and thin layer chroma­ reveals significant differences. In the case of 2-DAEo, the first electron tography (TLC), confirmed that only DAEo was recovered. This process, transfer is slightly reversible, which indicates that the cation radical of results in a permanent modification of the color, which is known as 2-DAEo has a greater solvation effect when pyridinium is used as “electrochromism with memory”. counterion. Whereas, in the case of the rest of DAEc ILs possess one Note that for 2-DAEc IL, the absorption peak at 512 nm disappears electron irreversible wave, which indicates that a chemical reaction is following two different reaction pathways. At an applied potential of coupled to the electron transfer. Moreover, in this case it is possible to 1.00 V, the oxidation of the 2-DAEc sample generates the corresponding distinguish a second irreversible oxidation peak, which is related with dication (Scheme 1, 2nd electron transfer). In both cases, it seems that the formation of the corresponding dication of 2-DAEc. The same gen­ the chemical reaction linked to the electron transfer is a ring opening eral trend is observed for ILs 3-DAEo and 4-DAEo, as previously reported reaction. Hence, the corresponding cation radical and dication of the by our research group [35]. open isomer are generated. In the case of the electro generated 2-DAEo At this point, the use of spectroelectrochemical techniques is crucial cation radical, it reacts with the starting material following the mech­ to disclose the oxidation mechanism of the closed isomers of our DAE- anism previously described for 1-DAEc. However, in the case of the 2- ILs, to establish the nature of the chemical reaction linked to the elec­ DAEo dication, it will involve a homogenous electron transfer with the tron transfer processes, as well as their electrochromic behavior. For this starting material, leading to the open and closed 2-DAE cation radical. reason, spectroelectrochemical studies were performed for acetonitrile As a consequence, a plateau between 20 and 80 s in the spectroelec­ solutions of 1-DAEc and 2-DAEc, which were previously prepared by trochemical plot is seen, indicating that the concentration of 2-DTAC exposure of the 1-DAEo and 2-DAEo samples to 312 nm until a photo­ cation radical remains constant due to the competition between the stationary state was reached. As previously mentioned, this was homogeneous electron transfer and the ring opening reactions. confirmed by the emergence of a strong absorbance band at ca. 510 nm arising from the photogenerated closed isomer molecules. Fig. 2c and 3.2. Electrochemical behavior and spectroelectrochemical study of DAE- d show a 3-D plot where the optical response of the 1-DAEc and 2-DAEc IL-based ionogel membranes solutions was monitored in time upon applying a constant oxidation potential value of Eapp,ox = +1.00 V during 140 s - i.e., at a potential Further studies were performed on DAE-ILs to investigate whether it value that selectively oxidizes the closed state of these compounds. For is possible to transfer the photoelectrochemical results obtained in so­ both samples, a concomitant decrement of the absorbance band at λabs, lution to solid state. To do so, IG membranes doped with 1-DAE, 2-DAE, max ~ 512 nm was observed. 3-DAE and 4-DAE were prepared, and their photoelectrochemical As previously reported for similar DAEs [36], this result is consistent behavior was disclosed. As expected, an oxidation wave at high poten­ with a ring-opening electrochemically-induced process, where the tials (>+1.10 V vs Ag/AgCl) was initially observed for all the resulting electrogenerated cation radical of DAEc follows a ring-opening reaction IGs in the dark – i.e., for the open isomer of the embedded IL molecules leading to the corresponding cation radical of DAEo (Scheme 1, first (Fig. 3a–d, Table 2). Relative to the solution measurements, the oxida­ tion peaks registered were less well-defined, which we ascribe to the lower molecular diffusion within the semisolid ionogel matrix and the higher capacitive current shown by the membranes. In spite of this, the fact that the waves measured fairly reproduced the Epa values previously in solution corroborates that the DAE-ILs did not suffer from any chemical variation by incorporation into the IG matrix. After 1 min of direct irradiation at 312 nm, the ionogels red-colored and a clear change in their cyclic voltammograms was detected. For all the DAE-IL@IGs, the oxidation peak registered in the dark at ca. 1.10–1.40 V vs Ag/AgCl shifted to lower potentials (~0.60–0.90 V vs Ag/AgCl), which is consistent with the ring-closing photoisomerization reaction of the embedded molecules (Table 2). Again, the oxidation potential values recorded upon irradiation were in good agreement with the values previously determined in solution. A good match between the optical properties of DAE-ILs in solution and in the IGs was also observed. Initially, all the IGs were essentially colorless and only showed a defined absorption band in the UV region at 340–350 nm, as expected for the open isomer of their DAE-IL molecules (DAEo-IL@IG, Fig. 3e-h, Table 2). Once irradiated at 312 nm, a very pronounced increase of absorption around λabs,max ~ 515 nm was observed due to DAEc-IL formation (DAEo-IL@IG, Fig. 3e-h, Table 2). A larger variability in λabs,max for the photogenerated close isomer was observed in the IGs with respect to solution, which was attributed to the absence of solvent and to the distinctive stabilization effect of the DAEc Scheme 1. Electrochemical oxidation mechanism of 1-DAE ILs. excited state by each of the countercations. In spite of this, subsequent 4 S. Santiago et al. Microchemical Journal 182 (2022) 107870 Table 2 Photo and electrochemical parameters for DAE-IL in ionogel membranes. Longel membranes Compound Epa (V) vs Ag/AgCl λmax (nm) 1-DAEo 1.12 349 1-DAEc 0.77 527 2-DAEo 1.44 340 2-DAEc 0.95 506 3-DAEo 1.40 352 3-DAEc 0.93 527 4-DAEo 1.44 352 4-DAEc 0.57 521 irradiation with green light (532 nm) resulted in color bleaching for all the samples, thus revealing the reversible photochromic behavior of the membranes prepared with our DAE-ILs. Finally, the photoelectrochromic properties of the DAE-IL@IGs were fully determined. To do so, spectroelectrochemical experiments were conducted to demonstrate that the oxidation of DAEc-IL@IGs leads to redox-induced conversion into DAEo-IL@IG. Fig. 3 i-l shows the varia­ tion of the UV–vis absorbance spectra of all the IG membranes pre- irradiated at 312 nm when a potential of Eap = +1.20 V was applied. The observed decrease in the visible absorption band of these samples was associated with the electrochemical ring-opening of the DAEc-IL structure to generate the colorless form DAEo-IL, as previously shown in solution. It is important to highlight that, by subsequently irradiating the membranes at 312 nm, we regenerated the DAEc-IL state again. Hence, this demonstrated the reversible photoelectrochromic properties of DAE-based ionic liquids when those systems were transferred to solid materials. 3.3. DAE-IL-based membranes as smart materials for the fabrication of molecular logic gates Given the good rheological properties shown by the fabricated membranes, they could be used for the fabrication of flexible multi- stimuli-responsive devices able to respond to both the application of electric potentials and irradiation with UV or visible light. As a probe of concept, 2-DAE was selected for preparing the IG membrane due to its excellent photoelectrochemical properties. For this purpose, a device was made by cutting flexible pressure sensitive adhesive (PSA, 175 μm thick, Sertek Global Technology, Bar­ celona, Spain) and polycarbonate (PC, 175 μm thick, MicroPlanet Lab­ oratorios S.L., Barcelona, Spain) layers with a CO2 laser writer (Epilog Mini 24, Epilog Laser, United States). Two transparent PC layers (25 mm × 25 mm) were sandwiched by using two white PSA layers. The bottom PSA layer defined a 10 mm × 20 mm hold for positioning and aligning the electrode in the device. The top PSA layer was used to define the 8 − mm diameter electrochemical cell. Before closing the device with the top PC layer, the flexible ionogel membrane of 2-DAE was adhered to the top of the electrode by placing it on the circle hole defined in the top PSA layer. Finally, the electrode was connected to a potentiostat to finish assembling the set up. The transparent circular Fig. 3. Top: Cyclic voltammograms obtained for ionogel membranes containing aperture constructed in the device allowed one to observe the color (a) 1-DAE, (b) 2-DAE, (c) 3-DAE and (d) 4-DAE. Carbon screen printed elec­ change of the membrane with the naked eye, as well as exposing the trodes were used in this case to record the electrochemical response. Middle: sample to UV or visible light (Fig. 4a). The main electrochromic features Absorbance spectra of (e) 1-DAE, (f) 2-DAE, (g) 3-DAE and (h) 4-DAE in ion­ of the devices were also assessed. It was determined a switching color ogel membranes before (dark line) and after irradiation (blue line) at 312 nm contrast, which is defined as the ratio in transmittance of the material in for 1 min. The images in the insets show the color change achieved in this way it bleached and colored form, of 70 % measured at λmax = 506 nm for 2- for the IGs..Bottom: Spectroelectrochemical measurements of (i) 1-DAE, (j) 2- DAE. The switching time needed to obtain the 90 % of the total DAE, (k) 3-DAE and (l) 4-DAE in ionogel membranes. Initially, these materials are colored because of pre-irradiation at 312 nm that generates the DAEc-IL bleaching color was 108 s, while 41 s was required to observe the 50 % form. After the application an oxidation potential of Eap = +1.20 V vs Ag/AgCl, of color change. Despite of the times obtained to produce a color change the area in contact with the conductive electrode is bleached due to the for­ are relatively higher compared to other electrochromes [41–43], it mation of DAEo-IL. This process can be reverted by further irradiation at would be possible to improve them by optimizing the thickness of the IG 312 nm to obtain the DAEc-IL form again. films. The coloration efficiencies of the device were also calculated, obtaining 171.3 cm2⋅C− 1 at λmax = 506 nm which. Those values are in 5 S. Santiago et al. Microchemical Journal 182 (2022) 107870 closed isomer was generated with UV radiation, while back- isomerization to the initial open state was produced both with visible light and oxidative potentials. This suggest that it is possible to access a wide range of smart ILs by incorporation of appropriate ions, which allows combining the very advantageous properties that ionic liquids possess (eco-friendliness, low melting point, low volatility, high ionic conductivity…) with the advanced stimuli-responsive features of the functional units introduced (e.g., photo- and electrochromic respon­ siveness). In addition, we showed that DAE-ILs are also compatible with organic polymeric matrices, which allows the formation of solid-gel type membranes that can be applied for the fabrication of functional devices such as sensors, smart labels, panels or displays. 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. Acknowledgements The authors are grateful for financial support from project PID2019- 106171RB-I00 from the Ministerio de Ciencia e Innovación (MICINN) of Spain. This work was also supported by the Associate Laboratory for Green Chemistry-LAQV (FCT/MCTES (UID/QUI/50006/2019)), Portu­ guese Foundation for Science and Technology/MCTES (POCI-01-0145- FEDER-016387 and MIT-EXPL/CS/0055/2021)). Finally, H.C. thanks financial suport in the scope of the framework contract foreseen in the numbers 4, 5, and 6 of article 23 of the Decree-Law 57/2016 of 29 August, changed by Law 57/2017 of 19 July. Fig. 4. a) Multistimuli device connected to a potentiostat. The center of the device contains the membranes formulated with 2-DAE. 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