www.acsami.org Research Article Solid Multiresponsive Materials Based on Nitrospiropyran-Doped Ionogels Sara Santiago, Pablo Giménez-Gómez, Xavier Muñoz-Berbel, Jordi Hernando,* and Gonzalo Guirado* Cite This: ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The application of molecular switches for the fabrication of multistimuli-responsive chromic materials and devices still remains a challenge because of the restrictions imposed by the supporting solid matrices where these compounds must be incorporated: they often critically affect the chromic response as well as limit the type and nature of external stimuli that can be applied. In this work, we propose the use of ionogels to overcome these constraints, as they provide a soft, fluidic, transparent, thermally stable, and ionic-conductive environment where molecular switches preserve their solution-like properties and can be exposed to a number of different stimuli. By exploiting this strategy, we herein pioneer the preparation of nitrospiropyran- based materials using a single solid platform that exhibit optimal photo-, halo-, thermo-, and electrochromic switching behaviors. KEYWORDS: ionogel, molecular switches, spiropyrans, smart materials, ionic liquids, smart devices ■ INTRODUCTION Smart functional solid materials that exhibit multistimuli- needed to interconvert between the spirocyclic and open states of spiropyrans, which are hindered in rigid environments; and responsive behavior are of crucial importance for the (b) the strong interaction that takes place with the surrounding construction of novel dynamic systems and devices.1−5 A solid matrix, which may vary the relative energy of their major toolbox toward this goal are molecular switches.6−11 different states. Although these matrix effects could be Among them, spiropyrans are frequently preferred due to their exploited to develop new stimuli-sensitive responses for capacity to reversibly interconvert between states with spiropyrans,28,34,38 they eventually prevent direct transfer of strikingly different properties (e.g., color and polarity) upon the optimal switching properties found in solution to the final materials. One of the main strategies proposed to overcome application of a broad range of stimuli.12−15 On the one hand, this drawback comprises properly selecting the nature of the spiropyran switches are well known to photoisomerize between matrix as to warrant minimal interaction with the switch and/ their colorless spirocyclic (SP) and colored merocyanine or provide it with sufficient free volume as to fairly preserve its isomers (MC).15−17 On the other hand, they have also been solution-like stimulus-sensitive response.37 This is the case of found to respond to other external stimuli18−20 such as nanoporous solids (e.g., metal−organic39,40 and covalent pH,21−23 metal ions,24−27 solvent polarity,28−30 and redox organic41 frameworks) and soft polymeric matrices (e.g., low- potentials.31−33 In some cases, this allows the formation of Tg polymeric domains,36,42,43 polymer gels44−46). other states apart from SP and MC (e.g., the protonated However, even if a suitable spiropyran−matrix combination merocyanine state MCH+, the spiropyran dimer SP−SP) that is chosen to reach optimal switching, the number and type of additionally modify the photochromic response of the stimuli that can be applied to the resulting material are system,21,23,25−27,30 which further enriches the stimulus- ultimately limited by the properties of the matrix, i.e., opaque sensitive activity of spiropyran switches. materials will restrict spiropyran photochromism to the surface Despite their broad functionality and versatility, the application of spiropyrans (and other switches) to the fabrication of truly smart materials and devices suffers from a Received: March 4, 2021 major bottleneck: the influence of the surrounding matrix Accepted: May 19, 2021 when these compounds are transferred from solution to a solid Published: May 31, 2021 state (the so-called matrix effect), which often dramatically alters their switching performance.34−37 Two main factors account for this behavior: (a) the large geometrical changes © 2021 American Chemical Society https://doi.org/10.1021/acsami.1c04159 26461 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article Scheme 1. Multistimuli-Responsive Spiropyran-Based Ionogel Membranes (NO2BIPS@IG) to Be Prepared in This Study, Which Respond to Different Types of Input Signals: Light, Temperature, pH, and Electrical Current Figure 1. Key components for the fabrication of multistimuli-responsive NO2BIPS@IG films, which were found to be transparent, flexible, and elastic. layer, whereas electroinduced responses could only be variety of applications such as optical memories, electro- obtained for conductive substrates. In fact, the latter most chemical sensors, biosensors, and molecular actuators. For this, probably justifies why the electrochromic response of we focused our attention on 1′,3′-dihydro-1′,3′,3′-trimethyl-6- spiropyrans has only been explored to date in solution with nitrospiro[2H]-1-benzopyran-2,2′-(2H)-indole (NO2BIPS) as a proper supporting electrolyte and organic solvents.31−33 a benchmark system, a well-known commercial nitrospiropyran Therefore, to fully unleash the potential of the multistimuli- derivative capable of responding to a plethora of external responsive behavior of spiropyrans, the development of stimuli (Scheme 1).21,31,33,52−54 versatile platforms that allow both solution-like switching and multiple operations under different input signals (e.g., light, electricity, pH, temperature, ions) is required. To reach this ■ RESULTS AND DISCUSSION Fabrication of Spiropyran-Based Ionogel Mem- goal, we propose, herein, the use of ionogels (IGs), solid-gel branes. Based on our previous experience on the preparation polymer electrolytes that are attracting increasing attention for of ionogels loaded with molecular switches,55 IG membranes the fabrication of functional devices because of their unique were prepared by blending a fluorinated polymer (poly- combination of properties (i.e., elasticity, flexibility, easy (vinylidene fluoride-co-hexafluoropropylene), P(VDF-co- preparation methodology, transparency, high ionic conductiv- HFP)), the trimethylbutylammonium bis- ities, and large electrochemical and chemical stability).47−50 (trifluoromethylsulfonyl)imide ionic liquid ([N1114][TFSI]), Although a very recent example of spiropyran-based IG has and NO2BIPS in acetone. After solvent evaporation, rubbery been reported, only its light-sensitive operation has been IG films containing free NO2BIPS molecules were obtained described so far.51 Accordingly, in this work, we aim to (NO2BIPS@IG, Figure 1), which were found to be trans- demonstrate for the first time the multi-addressability of this parent, flexible, and stretchable. These features arise from the class of materials, which would open the door for the convenient choice of the polymer network and the ionic liquid, fabrication of spiropyran-based smart devices with a broad where [N1114][TFSI] acts as a plasticizer favoring segmental 26462 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 2. (a) Photochromic behavior of NO2BIPS@IG membranes when irradiated with UV and visible light to interconvert between the SP and MC states (cNO2BIPS = 0.05 mg NO2BIPS/g IG). (b) UV−vis absorbance spectra of the SP (initial) and MC states (λexc = 365 nm until the photostationary state (PSS) is achieved) of NO2BIPS@IG membranes. (c) Optical images of a NO2BIPS@IG membrane exposed to UV light (λexc = 365 nm) through a photoprotective patterned mask, after the PSS is reached (left) and after 1 h under dim UV irradiation (right) to minimize the thermal back-isomerization process. The black rectangle indicates the region of the membrane that was analyzed. (d) Color intensity cross section of the images in (c) for the membrane region within the rectangle. mobility of the polymer chains and, hence, accounting for the methodology for the preparation of spiropyran-doped IGs as final mechanical properties of the mixture. In addition, the versatile switching platforms drops the fabrication costs and ionic liquid provides free carriers to the material and explains complexity while granting access to the manufacture of flexible the large ionic conductivities measured for the IG films (0.3 and stretchable smart devices. mS cm−1). As for the spiropyran content in NO2BIPS@IG, it Photochromism of NO2BIPS@IG Membranes. The was selected to meet two important criteria: (a) high color photochromic interconversion between SP and MC states contrast upon application of different stimuli that could be still remains the most exploited switching mechanism for properly quantified by means of ultraviolet−visible (UV−vis) spiropyrans,11 and it has been widely studied in solution for absorption spectroscopy, and (b) good solubility in the liquid NO2BIPS.54,56 Therefore, it must be accurately preserved in phase of the ionogel to enable optimal switching performance the ionogels prepared for these materials to be of relevance. As (c = 0.05−0.5 mg NO2BIPS/g IG). Actually, the formation of observed in solution, the most stable isomer of NO2BIPS microscopic aggregates of the spiropyran molecules in this found in NO2BIPS@IG was the SP form, which mainly work was not observed for any of the concentrations tested absorbs in the UV region (λabs = 346 nm) and makes the when inspecting the ionogels under the optical microscope, ionogel films essentially colorless and transparent at naked eye which contributes to their high optical transparency. (Figure 2a,b). Actually, the UV−vis absorption spectrum Other advantages arise from our fabrication method of measured for NO2BIPS@IG fairly reproduced the behavior in spiropyran-based IGs. On the one hand, it is very simple and, the [N1114][TFSI] solution and other aprotic polar solvents in contrast to previous reports,44,45,51 it can be directly applied (Figure S2), thus indicating that the spiropyran molecules are to commercially available switches such as NO2BIPS without mainly solvated by the ionic liquid in the ionogel. the need of further derivatization to warrant functionalization After UV irradiation (λexc = 365 nm), intense purple of the liquid or solid phases of the gel. Instead, the spiropyran coloration of NO2BIPS@IG was observed, which is indicative molecules just lie dissolved in the ionic liquid phase of our of extensive photoisomerization to the ring-opening isomer ionogels, which provides them with a soft, fluidic and MC of the switch (Figure 2a).54,56 In particular, a new conductive environment to facilitate NO2BIPS switching absorption band in the visible region characteristic of MC upon illumination, addition of a chemical agent (e.g., acid), formation was found (λabs = 548 nm, Figure 2b), which or application of an electrical current. In spite of this, it must preserves the same spectral features registered in acetonitrile be noted that no leakage of NO2BIPS molecules from and [N1114][TFSI] solutions (Figure S2). This is a clear proof NO2BIPS@IG films was observed in our experiments even that polar MC molecules also lie well dissolved in the liquid when they were put in contact with external solutions (Figure phase of the ionogels at the concentrations studied in this S1). On the other hand, owing to the mechanical strength and work, as significant spectral changes should have occurred in the self-standing character of the IGs prepared, they can be the case of aggregation.57 To assess the efficiency of the easily shaped by cutting, thus offering significant advantages in photocoloration process in the IG films, two different the design and fabrication of smart devices using different parameters were evaluated and compared to the behavior of printing methods (e.g., screen-printing or inkjet). Overall, our NO2BIPS in solution. First, the total conversion from SP to 26463 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article MC was found to be around 22% for the photostationary state interconverted upon carbon−carbon double bond photo- (PSS) generated in NO2BIPS@IG upon irradiation at 365 nm, isomerization with UV and violet-blue light, respectively a value rather similar to that measured for [N1114][TFSI] (Scheme 1). solutions (28%). Second, the quantum yield of the ring- Interestingly, when embedded in IG membranes, NO2BIPS opening photoisomerization process was calculated to be molecules show a very similar photohalochromic behavior, ΦSP‑MC = 0.15 ± 0.03 for NO2BIPS@IG membranes, which is probably due to the fact that they are principally solvated by in good agreement with the behavior reported for NO2BIPS in the ionic liquid (Figure 3a,b). In particular, no change in color polar aprotic solvents (ΦSP‑MC = 0.24 and 0.12 in acetone and acetonitrile, respectively).54 Therefore, no detrimental effects on SP-to-MC photoisomerization were observed upon introduction of NO2BIPS molecules in ionogel films. As for the reverse back-isomerization process, it was investigated both thermally and photochemically for NO2BIPS@IG. On the one hand, we observed that MC-to- SP back-isomerization in the dark followed a first-order kinetics with a rate constant of kSP‑MC = 9.2 × 10−4 s−1 at room temperature (Figure S3). This value is rather similar to those measured in the [N1114][TFSI] solution (1.90 × 10−3 s−1, Figure S3) and in solvents of high polarity (kSP‑MC = 1.0 × 10−3 s−1 in ethanol at 25 °C58), which further confirms that NO2BIPS molecules lie nonaggregated in solution-like domains within the ionogel films where their intrinsic photochromic properties are preserved. In addition, because of the high polarity of the ionic liquid phase of the ionogel that favors stabilization of the MC form, NO2BIPS@IG shows a rather slow thermal decoloration process, which might be exploited for the preparation of long-lived printed patterns on the ionogels (Figure 2c,d). This is favored by the restricted diffusion mobility of spiropyran molecules within the membranes, which is much slower than that in liquid solution. As a result, embedding NO2BIPS inside the ionogel matrix allows spatial confinement of the photoisomerized molecules within the irradiated areas for rather long periods (Figure 2c,d). On the other hand, if color fading is to be accelerated, irradiation with visible light can be exploited to induce fast Figure 3. (a) Optical response of NO2BIPS@IG (cNO2BIPS = 0.05 mg MC-to-SP photoisomerization, which we found to occur in NO2BIPS/g IG) upon light and pH changes. (A) initial color (SP NO2BIPS@IG at similar rates as in the [N1114][TFSI] solution. form), (B) drop of 10 mM H2SO4 solution, (C) irradiation at λexc = Actually, this allowed conducting repetitive SP−MC photo- 365 nm. (B′, C′) UV irradiation of A (λexc = 365 nm) followed by addition of H2SO4, (D′) irradiation of C′ at λexc = 445 nm, (D″, E″) conversion cycles by sequential illumination with UV and addition of tetrabutylammonium hydroxide (TBAOH 10 mM) visible light, which demonstrate the reversible and robust followed by visible light exposure (λexc = 445 nm). (b) UV−vis photoresponse of NO2BIPS in the ionogels prepared (Figure absorption spectra of the initial state of NO2BIPS@IG (SP) and upon S4). acidification ((Z)-MCH+), and UV irradiation of (Z)-MCH+ ((E)- Photohalochromism and Thermochromism of MCH+) and SP (MC). NO2BIPS@IG Membranes. When dissolved in the ionic liquid [N1114][TFSI], NO2BIPS preserves the photohalochro- was observed when a droplet of diluted H2SO4 was placed on mic behavior already described in other polar solvents such as top of NO2BIPS@IG, though an increase in the absorption at acetonitrile (Figure S5), which is attributed to the basicity of λabs < 400 nm was registered, which is compatible with (Z)- the 4-nitrophenolate group of its open form.21 Thus, upon MCH+ formation. Unfortunately, competitive absorption by addition of a strong acid (e.g., HClO4) in the dark, the the ionogel matrix prevented proper determination of the spirocyclic structure of the SP isomer opens to yield the (Z)- absorption spectral maximum of this species at λabs ∼310 nm. MCH+ species, where the exocyclic carbon−carbon double In spite of this, (Z)-MCH+ generation could be corroborated bond maintains the cis configuration of the initial compound by subsequent UV irradiation (λexc = 365 nm). While most of and its phenolate moiety is protonated (Scheme 1). This the membrane turned purple colored because of SP-to-MC process, which can be reverted by the addition of a base, leads photoisomerization, the area in contact with the acid droplet to a new absorption band at λabs = 303 nm characteristic of the became yellow colored as expected for the (Z)-MCH+-to-(E)- (Z)-MCH+ form. As a consequence, the solution remains MCH+ photoconversion process. The same effect was essentially colorless. Similarly, acid−base titration of a solution observed when the chemical and optical stimuli were applied of the MC isomer results in reversible formation of its in inverse order, which proves the capacity of the spiropyran protonated state (E)-MCH+ with trans configuration and λabs = molecules within the ionogel to undergo MC-to-(E)-MCH+ 392 nm, which makes the system turn from purple to yellow transformation. Furthermore, all of these processes could be color (Scheme 1). In addition, the (Z)-MCH+ and (E)-MCH+ reverted by illumination with visible light and/or addition of a forms preserve the photochromic properties of the non- base, thus eventually recovering the initial colorless and protonated SP−MC couple, and they can be reversibly transparent state of NO2BIPS@IG. Therefore, our results 26464 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article demonstrate the potential of spiropyran-loaded IGs for the preparation of photohalochromic solid materials, as they allow the properties of the embedded switches to be preserved, warrant the access of wet chemicals (i.e., acid and base solutions) to the matrix, and enable confinement of the halochromic behavior to the regions of the system that are in contact with those chemicals. However, it must be mentioned that prolonged acid−base treatment of the IG membranes affected its chemical stability, which we attribute to the base- induced Hofmann elimination reaction of the quaternary ammonium cation of the [N 1114 ][TFSI] IL. 58 As a consequence, a limited number of halochromic and photo- halochromic cycles could be conducted before observing the degradation of the material (Figure S6). Another stimulus to which spiropyrans can respond is temperature, as thermal heating can induce heterolytic cleavage of the C−Ospiro bond of SP to yield the corresponding MC isomer. Although this process is not typically favored in organic media where SP is the most stable isomer, it could eventually occur if two main conditions are fulfilled: (a) the presence of electron-withdrawing groups stabilizing the negative charge of the phenolate moiety of MC, as it is the case of the nitro substituent in NO2BIPS; and (b) dissolution in highly polar media that further contributes to the stabilization of the zwitterionic MC isomer (e.g., in water− methanol mixtures).59 In view of this, the thermochromic behavior might be exhibited by NO2BIPS@IG when the spiropyran molecules lie solvated by the highly polar ionic liquid [N1114][TFSI]. Indeed, clear coloration both in the [N1114][TFSI] solution (Figure S7) and in the ionogel films was observed by just heating above 30 °C, and maximum MC absorption was registered at 45 °C for NO2BIPS@IG that did not further increase at higher temperatures (λabs = 552 nm, Figure 4a,b). From the UV−vis absorption data, the maximum thermal isomerization yields in the [N1114][TFSI] solution and in the membranes were estimated to be 4 and 5%, respectively. Figure 4. (a) Color change of NO2BIPS@IG membranes (cNO2BIPS = This demonstrates that the thermochromic conversion from 0.5 mg NO2BIPS/g IG) when increasing the temperature gradually. SP to MC is less efficient than that achieved upon exposure to (b) Variation of the UV−vis absorbance spectrum of NO2BIPS@IG light, probably due to the insufficient stabilization of the with temperature. (c) Variation of the absorbance at the spectral merocyanine form by the surrounding ionic liquid; however, maximum of the MC isomer in NO2BIPS@IG upon five consecutive the color change induced was clear and vivid enough as to be thermochromic cycles. Average data is shown for three independent measurements in different membranes. easily seen with naked eye. It must be noted that a minor spectral shift was measured in our thermochromic experiments relative to the absorption of Electrochromism of NO2BIPS@IG Membranes. In photochemically generated MC molecules (λabs = 548 nm). contrast to their photochromic, photohalochromic, and Although this must be ascribed to a simple thermal effect on thermochromic behaviors, the electrochromic properties of absorption, it cannot be overlooked that the thermal ring- spiropyrans have been less exploited, probably because of the opening of the SP form of NO2BIPS has been reported to yield difficulty to achieve redox-induced switching in solid materials. different stereoisomers of MC bearing distinct optical proper- It is, therefore, in this area where the ionogels prepared in this ties instead of just the predominant (E)-MC structure obtained work are expected to have a greater impact, as their large upon photoisomerization. Independent of this, fast decolor- conductivities should enable the electrochemical operation of ation of NO2BIPS@IG was measured after subsequently spiropyran switches. In light of this, the electrochromic and cooling the ionogels down to room temperature, which electrochemical properties of NO2BIPS@IG were thoroughly demonstrates the reversibility of the thermochromic behavior. investigated, for which we built on the previous findings about This was possible owing to the exceptional stability of the IGs the redox-induced behavior of NO2BIPS in solution: it prepared even at high temperatures, which results from the dimerizes upon oxidation (Figure 5a).33,60,61 negligible vapor pressure and intrinsic thermal stability of ionic Based on that, we focused on analyzing the response of liquids (ILs). Indeed, multiple thermochromic cycles could be NO2BIPS@IG upon electrochemical oxidation. Figure 5b measured for NO2BIPS@IG membranes without apparent shows the anodic region of the cyclic voltammogram of degradation (Figure 4c). Therefore, these results pave the way NO2BIPS@IG, which presents a one-electron and irreversible for the fabrication of low-cost thermochromic materials based oxidation wave at +0.95 V (vs Ag/AgCl) associated with the on spiropyrans for smart labeling and packaging. oxidation of the amino group of the indoline moiety to the 26465 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article overall redox-induced SP-to-[SP−SP]2+ transformation (Fig- ure 6a). Second, the UV−vis absorption spectrum of the dicationic dimer [SP−SP]2+ is bathochromically shifted with respect to the SP species (λabs = 416 and 500 nm for [SP− SP]2+; Figure 6b) and, therefore, the ionogel becomes colored. As SP electrodimerization is irreversible,33,60,61 the colo- ration effect observed in NO2BIPS@IG at Eap = +1.2 V (vs Ag/ AgCl) could not be reverted and a permanent modification of the initial colorless membrane was provoked, i.e., the material presents “electrochromism with memory” under these conditions (Figure 6a). Spectroelectrochemical measurements were conducted to characterize this electrochromic behavior, for which we fabricated a flexible device with a built-in three- electrode electrochemical cell where a 4 mm in diameter circular NO2BIPS@IG membrane was deposited onto an ITO- SPE working electrode (Figures 6a and S10). On the one hand, a high color contrast was found in this process, as proven by the large change in transmittance measured at λabs = 500 nm when transforming SP into [SP−SP]2+ (ΔT1 = 90%, Figure Figure 5. (a) Electrochemical conversion of NO2BIPS into the S9). On the other hand, the switching time needed to produce dimeric species SP−SP upon oxidation.33,60,61 (b) Cyclic voltammo- 90% of such color change was observed to be rather long (ta = grams of NO2BIPS@IG (cNO2BIPS = 0.5 mg NO2BIPS/g IG) registered 125 s, Figure 6c), probably because the electrochromic for a freshly prepared ionogel (solid black line) and after 30 conversion implies a dimerization reaction that is limited by consecutive oxidation cycles (red solid line, scan rate: 20 mV s−1). the spatial encounter between two SP molecules within the Gray dashed lines correspond to intermediate voltammograms ionogel matrix where diffusion is restricted. Finally, the measured. electrochromic efficiency (η) of the SP-to-[SP−SP]2+ trans- formation was extracted from the slope of the linear region of corresponding radical cation. After 30 consecutive anodic the plot between the change in the optical density (ΔOD) and cycles at 50 mV s−1, this wave disappeared, while two new the current density needed to produce this change (Figure 6d). oxidation signals at +0.77 and +1.10 V (vs Ag/AgCl) emerged The obtained value, η = 245.9 cm2 mC−1, is somewhat lower with half the intensity. These observations are in good than other reported results for electrochromic devices,63 due to agreement with previous electrochemical results in solu- the large capacitive current of both the NO2BIPS@IG tion,32,33,60,61 and they are indicative of a dimerization process membrane and the screen-printed ITO electrode used. of NO2BIPS to yield an SP−SP dimer via oxidative carbon− The electrochromic behavior of NO2BIPS@IG can be carbon bond formation. In fact, the oxidation waves at +0.77 further expanded by taking advantage of the reversible and +1.10 V (vs Ag/AgCl) are attributed to the formation of reduction of [SP−SP]2+ to its radical cation [SP−SP]·+ and the radical cation and dication of the dimer, respectively, while its neutral form [SP−SP] (e.g., at Eap = +0.2 V (vs Ag/AgCl)). two new signals are also observed in the cathodic region of the These species show new different absorption bands in the voltammogram that correspond to the sequential reduction of visible (λabs = 360 and 473 nm for [SP−SP]·+ and λabs = 385 the two nitro groups of the dimer (Ered = −0.8 and −1.1 V (vs nm for [SP−SP]; Figure 6b) and near-IR regions (λabs = 874 Ag/AgCl), Figure S8). Therefore, these results prove that the and 984 nm for [SP−SP]; Figure 6b), some of which might be electrochemical behavior of NO2BIPS@IG in solution can be related to the intervalence charge transfer between the directly transferred to the solid state by means of ionogel monocation and dication species.64 As a consequence, a clear matrices. Actually, SP electrodimerization in these materials color change from reddish-orange to yellow takes place upon might be even favored by two additional factors. First, since the [SP−SP]2+-to-[SP−SP] transformation, which can be reverted diffusion of NO2BIPS molecules is largely restricted in the IL after subsequent oxidation (Figure 6a). Therefore, this opens phase of the gel, the reactivity between the nearby SP radical the door to use previously oxidized NO2BIPS@IG membranes cations must be further promoted. In fact, this effect has as “true electrochromic” systems, a behavior that we also already been observed upon immobilization of NO2BIPS onto characterized by means of spectroelectrochemical measure- surfaces, which assisted the electrochemical oxidative C−C aryl ments on the flexible device shown in Figure 6a. In this case, coupling of the switch. Second, the use of ionic liquids in IGs the color contrast for the reversible electroswitching between should also increase the stability of the reactive radical cation [SP−SP]2+ and [SP−SP] was associated with a transmittance species through solvation, thus aiding the dimerization change at λabs = 500 nm of ΔT2 = 47% (Figure S9), while the reaction.62 response time determined was significantly faster (tc = 60.4 s, It is important to highlight that when applying +1.2 V (vs Figure 6c). As for the electrochromic reversibility and fatigue Ag/AgCl) using either a carbon screen-printed electrode resistance of the [SP−SP]2+ -[SP−SP] system, it was (SPE) or an ITO-SPE electrode as a working electrode (WE), investigated upon consecutive applied voltages of Eap = +1.2 the electrochemical formation of the dimer in NO2BIPS@IG is and +0.2 V (vs Ag/AgCl) for 130 and 70 s, respectively accompanied by a pronounced change in the color of the (Figure 6e). After five oxidation−reduction cycles, the material, which turns intensely reddish-orange. Two main electrochromic response of the membranes remained rather factors account for this behavior. First, SP dimerization is stable as proven by the similar absorbance values measured at immediately followed by oxidation to the dicationic state of the λabs = 500 nm for the dication and neutral forms of the dimer [SP−SP]2+ at the applied potential, thus leading to the spiropyran dimer. These results, together with the flexibility of 26466 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 6. (a) Flexible display with a three-electrode compartment. The NO2BIPS@IG membrane (cNO2BIPS = 0.5 mg NO2BIPS/g IG) is in contact with an ITO-SPE working electrode (WE) and changes its color when applying electric potentials to induce irreversible SP-to-[SP−SP]2+ and reversible [SP−SP]2+-[SP−SP] transformations. (b) UV−vis absorption spectra of the initial state of NO2BIPS@IG (SP) and of the different redox products formed: [SP−SP], [SP−SP]·+, and [SP−SP]2+. (c) Change in absorbance monitored at λabs = 500 nm for the electrochromic device shown in (a) before applying any electric potential (OCP, 0−100 s), at Eap = +1.2 V (vs Ag/AgCl, 100−475 s), and at Eap = +0.2 V (vs Ag/AgCl, 475−655 s). (d) Plot of the optical density difference at λabs = 500 nm against the current density passed at Eap = +1.2 V (vs Ag/AgCl) for the electrochromic device shown in (a). (e) Switching color reversibility measured at λabs = 500 nm for the same electrochromic device after applying consecutive cycles of Eap = +1.2 V (vs Ag/AgCl) for 130 s and Eap = +0.2 V (vs Ag/AgCl) for 70 s. Average data is shown for three independent measurements in different membranes. the electrochemical device prepared, make NO2BIPS@IG membranes very appealing electrochromic materials for the fabrication of systems of great technological interest, such as wearable sensors65 and flexible panels.66 Multistimuli-Responsive Displays Based on NO2BIPS@IG. To demonstrate the feasibility of spiropyran- loaded ionogels for practical applications, we built a simple microfluidic architecture with multistimuli-responsive perform- ance (Figure S11). In this device, we introduced four different NO2BIPS@IG membranes cut with distinct complex shapes using a CO2 ablation laser, for which we took advantage of Figure 7. Multistimuli-responsive microfluidic prototype device based their high mechanical strength and high thermal and chemical on NO2BIPS@IG shaped into letters and a square (cNO2BIPS = 0.5 mg stability. Each one of those membranes could then be exposed NO2BIPS/g IG). Each form was exposed to different external stimuli: independently to stimuli of variable nature, as shown in Figure T, pH, light, and electric potential. 7. For the U-shaped membrane, a hot liquid flow (T = 40 °C) was passed through the microfluidic cell to induce the thermochromic conversion into a pink-colored MC isomer. hot acidic solution (T = 40 °C), which led to the The A-shaped membrane was instead put into contact with a thermohalochromic formation of the yellow-colored (E)- 26467 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article MCH+ form. In the case of the B-shaped membrane, the pink- aim, we formulated ionogels from a polyfluorinated polymer colored MC state was reached upon UV irradiation (λexc = 365 and an ionic liquid that were loaded with the photo-, halo-, nm) through a near-UV and visible transparent window. thermo-, and electrosensitive NO2BIPS spiropyran derivative. Finally, to trigger the electrochromic response of the system, The advantages of the resulting NO2BIPS@IG membranes the square-shaped NO2BISP@IG membrane was placed on were found to be multifold. First, the solution-like photo- top of a platinum (Pt) electrode and an irreversible color chromic properties of the switch were preserved in the ionogels change from transparent to orange was obtained because of SP thanks to the light transparency of these materials and the fluid electrodimerization to yield [SP−SP]2+ at Eap = +1.0 V (vs Pt). nature of their ionic liquid phase where NO2BIPS molecules Hence, selective and reversible thermo-, halo-, photo-, and lie, which favors the large conformational changes needed for electrochromic responses could be measured for each one of the switching process to occur. The latter also grants fast access them by appropriately selecting the stimulus of interest. of chemicals to NO2BIPS@IG (e.g., acids and bases), which To further demonstrate the potential of spiropyran-loaded we exploited to trigger the characteristic halo- and photo- ionogels for the fabrication of smart devices, we decided to halochromic operations of the embedded switch molecules. In exploit their good mechanical properties for the preparation of addition, because of the nonvolatility of ionic liquids, the multiresponsive flexible displays. With this aim, a microfluidic ionogel membranes obtained could be heated without channel was incorporated to the flexible electrochromic device degradation to stimulate the thermochromic behavior of shown in Figure 6a, thus enabling the application of other NO2BIPS. More importantly, NO2BIPS@IG strongly benefits stimuli different from electrical potentials to NO2BISP@IG from the adequate ionic conductivity provided by the ionic (Figures 8 and S10). Thus, the introduction of hot water and liquid phase of the material, which enabled the electroinduced operation of the molecular switch. All of these features, in combination with the facile preparation, flexibility, mechanical strength, self-standing nature, and shapeability of ionogels, make these materials promising candidates for the fabrication of a range of stimuli-sensitive systems and devices for optoelectronic applications (e.g., smart displays, chemical sensors, security inks, data storage). As a proof of concept, we constructed rigid and flexible microfluidic prototype devices containing different NO2BIPS@IG membranes that could be independently exposed to external stimuli to selectively promote their photo, halo-, thermo-, and electro- chromic response. ■ EXPERIMENTAL SECTION Materials. 1′,3′-Dihydro-1′,3′,3′-trimethyl-6-nitrospiro[2H-1-ben- zopyran-2,2′-(2H)-indole] (NO2BIPS) was purchased from TCI Chemicals (≥98% pur.) and used without further purification. Poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) Figure 8. Flexible multistimuli-responsive microfluidic prototype and HCl 36.5−38% were acquired from Merck and used as received. based on NO2BIPS@IG (cNO2BIPS = 0.5 mg NO2BIPS/g IG). The Trimethylbutylammonium bis(trifluoromethylsulfonyl)amide material shows different color responses upon application of distinct ([N1114][TFSI]) was purchased from Solvionic and dried under external stimuli in a single device: T, pH, light, and electrical potential. vacuum using molecular sieves for 24 h to ensure a total water content lower than 0.001%. Acetone was purchased from Acros (≥99.5% pur.) and used as provided. aqueous acidic solutions resulted in color changes compatible Preparation of NO2BIPS@IG Membranes. For NO2BIPS@IG with the transformation of the colorless initial SP molecules in preparation, P(VDF-co-HFP) and the ionic liquid [N1114][TFSI] were the ionogel to the pink MC isomer and the yellow (E)-MCH+ mixed in acetone in a 1:5 weight ratio. This mixture was stirred protonated species. In addition, SP-to-MC conversion could overnight at room temperature and under a N2 atmosphere until the also be induced under UV irradiation, while the system polymer was fully dissolved, and it was finally sonicated for 3 min. Later, the desired amount of NO2BIPS was added to the solution and preserved its capacity to turn reddish-orange when applying a dissolved by stirring. The concentration of NO2BIPS in the ionogel potential of +1.2 V (vs Ag/AgCl) to induce [SP−SP]2+ membranes (NO2BIPS@IG) was selected taking into account the film formation. These results, together with those shown for the thickness (typically, ∼60 μm) to obtain absorbance values below 1 in rigid microfluidic prototype, are unambiguous proofs that the UV−vis spectra. In most of the cases, this was observed for ionogels offer a wide range of advantageous properties (i.e., NO2BIPS contents in the range of cNO2BIPS = 0.05−0.5 mg NO2BIPS/ facile formulation, high ionic conductivities and optical g IG. The resulting viscous solution was cast into a ceramic transparency, chemical and thermal stability, access to evaporating dish and was left at room temperature for 24 h until the chemicals from external solutions, and mechanical strength solvent was completely evaporated. Eventually, a transparent, flexible, and flexibility) that make them a very promising platform for and elastic thin film was obtained. Films could be stored for weeks in the fabrication of smart devices based on multistimuli- a glovebox without observing any detrimental effect in their stimuli- responsive molecular switches. responsive properties. ■ Characterization of NO2BIPS@IG Membranes. Ultraviolet− CONCLUSIONS visible (UV−vis) absorption spectra were recorded in a Hamamatsu L10290 spectrophotometer and a HP 8453 spectrophotometer. In this work, we have demonstrated the potential of ionogels to Spectroelectrochemical studies were performed coupling a VSP100 be used as platforms for the preparation of smart materials potentiostat controlled by EC-Lab V9.51 software to the Hamamatsu based on multistimuli-responsive molecular switches. With this L10290 spectrophotometer. Electrochemical and spectroelectrochem- 26468 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article ical measurements on NO2BIPS@IG membranes were performed for all cells, and the insertion of the two spring-loaded connectors (RS using screen-printed electrodes (SPE, DropSens), a three-electrode Components, Switzerland) used to contact the chips with the system composed of a carbon or optically transparent ITO as a measurement instrument. Finally, both structures were fixed using working electrode (WE), a carbon counter electrode (CE), and a Ag/ screws (2 mm diameter) to allow easy assembly and disassembly of AgCl reference electrode. Since NO2BIPS@IG membranes are the system (Figure S11d). Samples were flowed inside the device photoresponsive, the spectroelectrochemical measurements were using acidic diluted aqueous solution of HClO4 or basic diluted performed in discontinuous mode, recording the spectra in time for aqueous solution of TBAOH at different temperatures. However, in 0.5 s to avoid the long-time exposure of the sample to the light beam. the case of the electrochemical compartment, no aqueous solution An infrared probe (Laserliner ThermoSpot) was used for monitoring was flowed to avoid side reactions during the electrochromic the temperature of the IG membrane when heated. To estimate the performance. In this case, bare IG was used as a solid electrolyte to composition of the SP−MC mixtures prepared upon irradiation, the ensure adequate ionic conductivity. MC content in the resulting photostationary states was calculated by The flexible multistimuli-responsive device (37.5 mm in width and Lambert Beer’s equation using its absorption coefficient reported in 41 mm in length) was formed by five layers of fast-prototyped ref 54. The SP-to-MC photoisomerization quantum yield in the polymers mechanized with a laser writer (Figure S10). A 175 μm ionogel membrane was determined using the methodology reported polycarbonate layer was used to close the microfluidic device and to in refs 67 and 68, which is described in detail in the Supporting define the position of the 1 mm in diameter microfluidic inlet and Information.69 SP-to-MC photoisomerization was induced with a outlet (Layer 1). Layer 2 was made of a 175 μm thick PSA film, which Vilber Lourmat UV lamp equipped with two 4 W tubes emitting at defined the 1 mm in width microfluidic channels connecting the inlet 365 nm or the third harmonic of a Nd:YAG ns-pulsed laser (Brilliant, and outlet in Layer 1 with a 20 μL electrochemical cell. Layer 3 was Quantel, λexc = 365 nm), while MC-to-SP back-photoisomerization formed by a 175 μm thick PSA layer bonded to a 50 μm PMMA layer was triggered with a cw laser diode at λexc = 532 nm (Z-Laser). The and was used to contact the electrochemical cell to the electro- thermal back-photoisomerization MC-to-SP process in solution and chemical sensor. The electrochemical sensor was fabricated with a the membranes was investigated in the dark and at room temperature Dropsens screen-printed ITO electrode and positioned using the hole using the methodology described in the Supporting Information. For defined in Layer 4 (175 μm thick double-sided PSA layer +50 μm the photohalochromic study, an acidic 10 mM H2SO4 and a basic 10 PMMA layer). Finally, the flexible device was enclosed by a 50 μm mM TBAOH aqueous solutions were prepared, and a total volume of thick PSA layer used as a white back-cover. A 4 mm in diameter 20 μL was cast on the top of NO2BIPS@IG membranes. Depending circular NO2BIPS@IG membrane was deposited onto the ITO-SPE on the state aimed to reach, a combination of an acidic or basic working electrode of the electrochemical sensor. Samples were flowed solution was used along with the irradiation at λexc = 445 nm (sciTec), inside the device using an acidic diluted aqueous solution of HClO4 λexc = 365 nm, (Brilliant, Quantel, λexc = 365 nm), or λexc = 532 nm or a basic diluted aqueous solution of TBAOH at different (Z-Laser). temperatures. Fabrication of Multistimuli-Responsive Devices. The port- able rigid microfluidic system was designed and fabricated using low- cost polymers poly(methyl methacrylate) (PMMA), double-sided pressure-sensitive adhesive (PSA), and poly(dimethyl siloxane) ■ ASSOCIATED CONTENT * Supporting Information sı (PDMS) to demonstrate the feasibility of NO2BIPS@IG in real The Supporting Information is available free of charge at scenarios (Figure S11). The polymers were fast-prototyped with a https://pubs.acs.org/doi/10.1021/acsami.1c04159. CO2-laser writer (Epilog Mini 24, Epilog Laser). The total size of the NO2BIPS@IG membranes water stability; photoisome- fluidic system was 9 mm in height, 43 mm in width, and 70 mm in length. The system was formed by two structures (bottom and top) of rization in acetonitrile and [N1114][TFSI] solutions; different layers of polymers. The bottom structure (Figure S11b) was determination of photoisomerization quantum yields for formed by 2 PMMA layers bonded with a 175 μm thick PSA layer. A NO2BIPS@IG membranes; thermal-back isomerization bottom black-colored 3 mm thick PMMA layer was used to avoid the reaction in [N1114][TFSI] solution and the light beam losses during light exposure of the NO2BIPS@IG material. NO2BIPS@IG membrane; photoisomerization cycles The second 380 μm thick PMMA layer enabled the position of the in NO2BIPS@IG membrane; photohalochromism of chips used for the electrochromic tests. Two 11 × 9 mm2 silicon chips NO2BIPS in acetonitrile and [N1114][TFSI]; halochro- fabricated using standard photolithographic techniques were used in mic cycles in NO2BIPS@IG membranes; thermochrom- this system:6 a chip containing two in-parallel platinum (Pt) ism of NO2BIPS in [N1114][TFSI] solution; cyclic microelectrodes working as a working electrode (WE) and a chip voltammetry of NO2BIPS@IG membranes; electro- containing three in-parallel Pt microelectrodes working as a counter (CE) and pseudo-reference electrodes (p-RE). The microelectrodes chromic characterization of NO2BIPS@IG membranes; had an area of 2.5 or 5 mm2. The bottom structure is completed by a fabrication of rigid microfluidic devices; fabrication of 680 μm PDMS layer defining the outline of the four NO2BIPS@IG flexible devices (PDF) ■ material pieces used for the light, pH, temperature, and electric potential tests. This layer enabled the hosting of the NO2BIPS@IG pieces and their perfect alignment with the microfluidic cells defined AUTHOR INFORMATION in the top structure. Four shapes are defined for the tests: a letter U, a Corresponding Authors letter A, a letter B, and a square, which are used for the temperature, Jordi Hernando − Departament de Química, Universitat pH, light, and electric potential stimulus, respectively. Regarding the Autònoma de Barcelona, Barcelona 08193, Spain; electrochemical cell, other shapes are defined in the PDMS to expose orcid.org/0000-0002-1126-4138; the microelectrodes used as CE and p-RE. Finally, two more Email: Jordi.Hernando@uab.cat rectangular shapes are defined to allow the electrical connection of the chip with the potentiostat equipment. The PDMS layer also Gonzalo Guirado − Departament de Química, Universitat disenabled the fluidic leakage between both structures during the Autònoma de Barcelona, Barcelona 08193, Spain; fluidic tests. The top structure (Figure S11c) is formed by four 500 orcid.org/0000-0003-2128-7007; μm thick layers bonded by 175 μm thick PSA layers. These layers Email: Gonzalo.Guirado@uab.cat defined three 75 μL and one 37.5 μL microfluidic cells used for temperature, pH, light, and electric potential stimulus, respectively. Authors The PMMA layers also enabled the fluidic connection between them, Sara Santiago − Departament de Química, Universitat the position of the microfluidic threads used in each inlet and outlet Autònoma de Barcelona, Barcelona 08193, Spain; Instituto 26469 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article de Microelectrónica de Barcelona (IMB-CNM, CSIC), (16) Kortekaas, L.; Browne, W. R. The Evolution of Spiropyran: Barcelona 08193, Spain Fundamentals and Progress of an Extraordinarily Versatile Photo- Pablo Giménez-Gómez − Instituto de Microelectrónica de chrome. Chem. Soc. Rev. 2019, 48, 3406−3424. Barcelona (IMB-CNM, CSIC), Barcelona 08193, Spain; (17) Berkovic, G.; Krongauz, V.; Weiss, V. Spiropyrans and orcid.org/0000-0003-3443-802X Spirooxazines for Memories and Switches. Chem. Rev. 2000, 100, Xavier Muñoz-Berbel − Instituto de Microelectrónica de 1741−1753. (18) Mondal, B.; Ghosh, A. K.; Mukherjee, P. S. Reversible Barcelona (IMB-CNM, CSIC), Barcelona 08193, Spain; Multistimuli Switching of a Spiropyran-Functionalized Organic Cage orcid.org/0000-0002-6447-5756 in Solid and Solution. J. Org. Chem. 2017, 82, 7783−7790. Complete contact information is available at: (19) Mialane, P.; Zhang, G.; Mbomekalle, I. M.; Yu, P.; Compain, J. https://pubs.acs.org/10.1021/acsami.1c04159 D.; Dolbecq, A.; Marrot, J.; Sécheresse, F.; Keita, B.; Nadjo, L. Dual Photochromic/Electrochromic Compounds Based on Cationic Author Contributions Spiropyrans and Polyoxometalates. Chem. - Eur. J. 2010, 16, 5572− The manuscript was written through contributions of all 5576. authors. All authors have given approval to the final version of (20) Meng, X.; Qi, G.; Li, X.; Wang, Z.; Wang, K.; Zou, B.; Ma, Y. the sending manuscript. Spiropyran-Based Multi-Colored Switching Tuned by Pressure and Mechanical Grinding. J. Mater. Chem. C 2016, 4, 7584−7588. Notes (21) Kortekaas, L.; Chen, J.; Jacquemin, D.; Browne, W. R. Proton- The authors declare no competing financial interest. ■ Stabilized Photochemically Reversible E/ Z Isomerization of Spiropyrans. J. Phys. Chem. B 2018, 122, 6423−6430. ACKNOWLEDGMENTS (22) Kong, L.; Wong, H.-L.; Tam, A. Y.-Y.; Lam, W. H.; Wu, L.; The authors thank the Ministerio de Ciencia e Innovación and Yam, V. W.-W. Synthesis, Characterization, and Photophysical the Agencia Estatal de Investigación of Spain for financial Properties of Bodipy-Spirooxazine and -Spiropyran Conjugates: support through the projects CTQ 2015-65439-R and Modulation of Fluorescence Resonance Energy Transfer Behavior PID2019-106171RB-100/AEI/10.13039/501100011033. via Acidochromic and Photochromic Switching. ACS Appl. Mater. ■ Interfaces 2014, 6, 1550−1562. REFERENCES (23) Chen, S.; Jiang, F.; Cao, Z.; Wang, G.; Dang, Z.-M. Photo, PH, and Thermo Triple-Responsive Spiropyran-Based Copolymer Nano- (1) Qi, Z.; Schalley, C. A. Multi-Stimuli Responsive Materials. In particles for Controlled Release. Chem. Commun. 2015, 51, 12633− Chemoresponsive Materials: Stimulation by Chemical and Biological Signals; Schneider, H.-J., Ed.; Smart Materials Series; Royal Society of 12636. Chemistry: Cambridge, 2015; Chapter 5, pp 98−135. (24) Liu, Z.; Jiang, L.; Liang, Z.; Gao, Y. Photo-Switchable (2) Zhuang, J.; Gordon, M. R.; Ventura, J.; Li, L.; Thayumanavan, S. Molecular Devices Based on Metal-Ionic Recognition. Tetrahedron Multi-Stimuli Responsive Macromolecules and Their Assemblies. Lett. 2005, 46, 885−887. Chem. Soc. Rev. 2013, 42, 7421−7435. (25) Fries, K. H.; Sheppard, G. R.; Bilbrey, J. A.; Locklin, J. Tuning (3) Ke, Y.; Wang, S.; Liu, G.; Li, M.; White, T. J.; Long, Y. Chelating Groups and Comonomers in Spiropyran-Containing Vanadium Dioxide: The Multistimuli Responsive Material and Its Copolymer Thin Films for Color-Specific Metal Ion Binding. Polym. Applications. Small 2018, 14, No. 1802025. Chem. 2014, 5, 2094−2102. (4) Fu, X.; Hosta-Rigau, L.; Chandrawati, R.; Cui, J. Multi-Stimuli- (26) Natali, M.; Aakeröy, C.; Desper, J.; Giordani, S. The Role of Responsive Polymer Particles, Films, and Hydrogels for Drug Metal Ions and Counterions in the Switching Behavior of a Carboxylic Delivery. Chem 2018, 4, 2084−2107. Acid Functionalized Spiropyran. Dalton Trans. 2010, 39, 8269−8277. (5) Jochum, F. D.; Theato, P. Temperature- and Light-Responsive (27) Paramonov, S. V.; Lokshin, V.; Fedorova, O. A. Spiropyran, Smart Polymer Materials. Chem. Soc. Rev. 2013, 42, 7468−7483. Chromene or Spirooxazine Ligands: Insights into Mutual Relations (6) Molecular Switches; Feringa, B. L.; Browne, W. R., Eds.; Wiley- between Complexing and Photochromic Properties. J. Photochem. VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2011. Photobiol., C 2011, 12, 209−236. (7) Zhang, J.; Wang, J.; Tian, H. Taking Orders from Light: Progress (28) Julià-López, A.; Ruiz-Molina, D.; Hernando, J.; Roscini, C. in Photochromic Bio-Materials. Mater. Horiz. 2014, 1, 169−184. Solid Materials with Tunable Reverse Photochromism. ACS Appl. (8) Natali, M.; Giordani, S. Molecular Switches as Photocontrollable Mater. Interfaces 2019, 11, 11884−11892. “Smart” Receptors. Chem. Soc. Rev. 2012, 41, 4010−4029. (29) Abdollahi, A.; Alinejad, Z.; Mahdavian, A. R. Facile and Fast (9) Moulin, E.; Faour, L.; Carmona-Vargas, C. C.; Giuseppone, N. Photosensing of Polarity by Stimuli-Responsive Materials Based on From Molecular Machines to Stimuli-Responsive Materials. Adv. Spiropyran for Reusable Sensors: A Physico-Chemical Study on the Mater. 2020, 32, No. 1906036. Interactions. J. Mater. Chem. C 2017, 5, 6588−6600. (10) Pu, S.-Z.; Sun, Q.; Fan, C.-B.; Wang, R.-J.; Liu, G. Recent (30) Rosario, R.; Gust, D.; Hayes, M.; Springer, J.; Garcia, A. A. Advances in Diarylethene-Based Multi-Responsive Molecular Solvatochromic Study of the Microenvironment of Surface-Bound Switches. J. Mater. Chem. C 2016, 4, 3075−3093. Spiropyrans. Langmuir 2003, 19, 8801−8806. (11) Bisoyi, H. K.; Li, Q. Light-Driven Liquid Crystalline Materials: (31) Jin Fang Zhi; Baba, R.; Hashimoto, K.; Fujishima, A. From Photo-Induced Phase Transitions and Property Modulations to Applications. Chem. Rev. 2016, 116, 15089−15166. Photoelectrochromic Properties of a Spirobenzopyran Derivative. J. (12) Kortekaas, L.; Browne, W. R. The Evolution of Spiropyran: Photochem. Photobiol., A 1995, 92, 91−97. Fundamentals and Progress of an Extraordinarily Versatile Photo- (32) Ivashenko, O.; Van Herpt, J. T.; Feringa, B. L.; Rudolf, P.; chrome. Chem. Soc. Rev. 2019, 48, 3406−3424. Browne, W. R. Electrochemical Write and Read Functionality through (13) Berkovic, G.; Krongauz, V.; Weiss, V. Spiropyrans and Oxidative Dimerization of Spiropyran Self-Assembled Monolayers on Spirooxazines for Memories and Switches. Chem. Rev. 2000, 100, Gold. J. Phys. Chem. C 2013, 117, 18567−18577. 1741−1754. (33) Preigh, M. J.; Stauffer, M. T.; Lin, F. T.; Weber, S. G. Anodic (14) Florea, L.; Diamond, D.; Benito-Lopez, F. Photo-Responsive Oxidation Mechanism of a Spiropyran. J. Chem. Soc., Faraday Trans. Polymeric Structures Based on Spiropyran. Macromol. Mater. Eng. 1996, 92, 3991−3996. 2012, 297, 1148−1159. (34) Kinashi, K.; Nakamura, S.; Imamura, M.; Ishida, K.; Ueda, Y. (15) Klajn, R. Spiropyran-Based Dynamic Materials. Chem. Soc. Rev. The Mechanism for Negative Photochromism of Spiropyran in Silica. 2014, 43, 148−184. J. Phys. Org. Chem. 2012, 25, 462−466. 26470 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471 ACS Applied Materials & Interfaces www.acsami.org Research Article (35) Tork, A.; Boudreault, F.; Roberge, M.; Ritcey, A. M.; Lessard, (56) Lenoble, C.; Becker, R. S. Photophysics, Photochemistry, R. A.; Galstian, T. V. Photochromic Behavior of Spiropyran in Kinetics, and Mechanism of the Photochromism of 6′-Nitro- Polymer Matrices. Appl. Opt. 2001, 40, 1180−1186. indolinospiropyran. J. Phys. Chem. A 1986, 90, 62−65. (36) Sharifian, M. H.; Mahdavian, A. R.; Salehi-Mobarakeh, H. (57) Uznanski, P. UV−Assisted Formation of Nanoaggregates from Effects of Chain Parameters on Kinetics of Photochromism in Photochromic Spiropyrans in Nonpolar Solvents. Langmuir 2003, 19, Acrylic−Spiropyran Copolymer Nanoparticles and Their Reversible 1919−1922. Optical Data Storage. Langmuir 2017, 33, 8023−8031. (58) Landini, D.; Maia, A.; Rampoldi, A. Stability of Quaternary (37) Gonzalez, A.; Kengmana, E. S.; Fonseca, M. V.; Han, G. G. D. Onium Salts under Phase-Transfer Conditions in the Presence of Solid-State Photoswitching Molecules: Structural Design for Isomer- Aqueous Alkaline Solutions. J. Org. Chem. 1986, 51, 3187−3191. ization in Condensed Phase. Mater. Today Adv. 2020, 6, No. 100058. (59) Shiraishi, Y.; Itoh, M.; Hirai, T. Thermal Isomerization of (38) Julià-López, A.; Hernando, J.; Ruiz-Molina, D.; González- Spiropyran to Merocyanine in Aqueous Media and Its Application to Monje, P.; Sedó, J.; Roscini, C. Temperature-Controlled Switchable Colorimetric Temperature Indication. Phys. Chem. Chem. Phys. 2010, Photochromism in Solid Materials. Angew. Chem. 2016, 128, 15268− 12, 13737−13745. 15272. (60) Kortekaas, L.; Ivashenko, O.; Van Herpt, J. T.; Browne, W. R. A (39) Schwartz, H. A.; Olthof, S.; Schaniel, D.; Meerholz, K.; Remarkable Multitasking Double Spiropyran: Bidirectional Visible- Ruschewitz, U. Solution-Like Behavior of Photoswitchable Spiropyr- Light Switching of Polymer-Coated Surfaces with Dual Redox and ans Embedded in Metal−Organic Frameworks. Inorg. Chem. 2017, 56, Proton Gating. J. Am. Chem. Soc. 2016, 138, 1301−1312. 13100−13110. (61) Browne, W. R.; Ivashenko, O.; Feringa, B. L.; Rudolf, P.; van (40) Healey, K.; Liang, W.; Southon, P. D.; Church, T. L.; Herpt, J. T. Oxidative Electrochemical Aryl C−C Coupling of D’Alessandro, D. M. Photoresponsive Spiropyran-Functionalised Spiropyrans. Chem. Commun. 2013, 49, 6737−6739. MOF-808: Postsynthetic Incorporation and Light Dependent Gas (62) André, F.; Hapiot, P.; Lagrost, C. Dimerization of Ion Radicals Adsorption Properties. J. Mater. Chem. A 2016, 4, 10816−10819. in Ionic Liquids. An Example of Favourable “Coulombic” Solvation. (41) Kundu, P. K.; Olsen, G. L.; Kiss, V.; Klajn, R. Nanoporous Phys. Chem. Chem. Phys. 2010, 12, 7506−7512. Frameworks Exhibiting Multiple Stimuli Responsiveness. Nat. (63) Mortimer, R. J.; Rosseinsky, D. R.; Monk, P. M. S. Commun. 2014, 5, No. 3588. Electrochromic Materials and Devices; Wiley-VCH Verlag GmbH & (42) Peterson, C.; Hillmyer, M. A. Fast Photochromic Dye Response Co. KGaA, 2013; Vol. 77, pp 1−638. in Rigid Block Polymer Thermosets. ACS Appl. Polym. Mater. 2019, 1, (64) Guichard, V.; Bourkba, A.; Poizat, O.; Buntinx, G. Vibrational 2778−2786. Studies of Reactive Intermediates of Aromatic Amines. 2. Free-Radical (43) Nam, Y. S.; Yoo, I.; Yarimaga, O.; Park, I. S.; Park, D. H.; Song, Cation and Dication Resonance Raman Spectroscopy of N,N,N′,N′- S.; Kim, J. M.; Lee, C. W. Photochromic Spiropyran-Embedded Tetramethylbenzidine and N,N,N′,N′-Tetraethylbenzidine. J. Phys. PDMS for Highly Sensitive and Tunable Optochemical Gas Sensing. Chem. B 1989, 93, 4429−4435. Chem. Commun. 2014, 50, 4251−4254. (65) Yan, C.; Kang, W.; Wang, J.; Cui, M.; Wang, X.; Foo, C. Y.; (44) Wang, W.; Hu, J.; Zheng, M.; Zheng, L.; Wang, H.; Zhang, Y. Chee, K. J.; Lee, P. S. Stretchable and Wearable Electrochromic Multi-Responsive Supramolecular Hydrogels Based on Merocyanine− Devices. ACS Nano 2014, 8, 316−322. Peptide Conjugates. Org. Biomol. Chem. 2015, 13, 11492−11498. (66) Kim, J.; Myoung, J. Flexible and Transparent Electrochromic (45) Li, C.; Iscen, A.; Palmer, L. C.; Schatz, G. C.; Stupp, S. I. Light- Displays with Simultaneously Implementable Subpixelated Ion Gel- Driven Expansion of Spiropyran Hydrogels. J. Am. Chem. Soc. 2020, Based Viologens by Multiple Patterning. Adv. Funct. Mater. 2019, 29, 142, 8447−8453. No. 1808911. (46) Francis, W.; Dunne, A.; Delaney, C.; Florea, L.; Diamond, D. (67) Lees, A. J. A Photochemical Procedure for Determining Spiropyran Based Hydrogels ActuatorsWalking in the Light. Sens. Reaction Quantum Efficiencies in Systems with Multicomponent Actuators, B 2017, 250, 608−616. Inner Filter Absorbances. Anal. Chem. 1996, 68, 226−229. (47) Sahrash, R.; Siddiqa, A.; Razzaq, H.; Iqbal, T.; Qaisar, S. PVDF (68) Pimienta, V.; Lavabre, D.; Levy, G.; Samat, A.; Guglielmetti, R.; Based Ionogels: Applications towards Electrochemical Devices and Micheau, J. C. Kinetic Analysis of Photochromic Systems under Membrane Separation Processes. Heliyon 2018, 4, No. e00847. Continuous Irradiation. Application to Spiropyrans. J. Phys. Chem. D (48) Le Bideau, J.; Viau, L.; Vioux, A. Ionogels, Ionic Liquid Based 1996, 100, 4485−4490. Hybrid Materials. Chem. Soc. Rev. 2011, 40, 907−925. (69) Giménez-Gómez, P.; Gutiérrez-Capitán, M.; Capdevila, F.; (49) Chen, N.; Zhang, H.; Li, L.; Chen, R.; Guo, S. Ionogel Puig-Pujol, A.; Jiménez-Jorquera, C.; Fernández-Sánchez, C. Compact Electrolytes for High-Performance Lithium Batteries: A Review. Adv. Analytical Flow System for the Simultaneous Determination of L- Energy Mater. 2018, 8, No. 1702675. Lactic and l-Malic in Red Wines. Sci. Rep. 2020, 10, No. 19404. (50) Kavanagh, A.; Byrne, R.; Diamond, D.; Fraser, K. J. Stimuli Responsive Ionogels for Sensing ApplicationsAn Overview. Membranes 2012, 2, 16−39. (51) Cao, Z.; Liu, H.; Jiang, L. Hydrogen-Bonding-Driven Tough Ionogels Containing Spiropyran-Functionalized Ionic Liquids. ACS Appl. Polym. Mater. 2020, 2359−2365. (52) Ivashenko, O.; van Herpt, J. T.; Rudolf, P.; Feringa, B. L.; Browne, W. R. Oxidative Electrochemical Aryl C−C Coupling of Spiropyrans. Chem. Commun. 2013, 49, 6737−6739. (53) Sueishi, Y.; Ohcho, M.; Nishimura, N. Kinetic Studies of Solvent and Pressure Effects on Thermochromic Behavior of 6- Nitrospiropyran. Bull. Chem. Soc. Jpn. 1985, 58, 2608−2613. (54) Görner, H. Photochromism of Nitrospiropyrans: Effects of Structure, Solvent and Temperature. Phys. Chem. Chem. Phys. 2001, 3, 416−423. (55) Villabona, M.; Benet, M.; Mena, S.; Al-Kaysi, R. O.; Hernando, J.; Guirado, G. Multistimuli-Responsive Fluorescent Switches Based on Spirocyclic Meisenheimer Compounds: Smart Molecules for the Design of Optical Probes and Electrochromic Materials. J. Org. Chem. 2018, 83, 9166−9177. 26471 https://doi.org/10.1021/acsami.1c04159 ACS Appl. Mater. Interfaces 2021, 13, 26461−26471