Research Article ChemElectroChem doi.org/10.1002/celc.202101559 www.chemelectrochem.org Electrocarboxylation of Spiropyran Switches through Carbon-Bromide Bond Cleavage Reaction Sara Santiago, Clara Richart, Silvia Mena, Iluminada Gallardo, Jordi Hernando,* and Gonzalo Guirado*[a] This manuscript deals with carbon capture and utilization to atmosphere. Once the role of the cathode in the reduction synthetize high-added chemicals using CO2 as a C1-organic carbon-bromide bond cleavage is understood, carboxylated building block for C C bond formation. The study focuses on spiropyran derivatives can be synthesized in moderate yields the electrocarboxylation of 1,3,3-trimethylindolino-6’-bromo- and conversion rates through an electrocarboxylation process benzopyrylospiran switch (Br-BIPS). Prior to the electrocarbox- using CO2 silver cathode and polar aprotic solvents. The “green” ylation process, the electrochemical reduction mechanism of efficient route described in the current work would open a new Br-BIPS and CO2 is disclosed in polar aprotic solvents using two sustainable strategy for designing and building “smart” surfaces different cathodes (glassy carbon and silver) under nitrogen with switchable physical properties. Introduction Photoswitches are a class of molecules or organometallic complexes that undergo a reversible interconversion between different states triggered by light.[1] Spiropyrans are amongst the most studied of these systems, which toggle between spirocyclic (SP) and merocyanine (MC) isomers upon light- induced ring-opening and ring-closing reactions, respectively Figure 1. Photoisomerization of spiropyrans between the closed SP form and (Figure 1).[2–5] These two states show markedly distinctive geo- the open MC structure. metrical and physicochemical properties, such as different effective pKa values,[6–9] dipole moments (from ~ 4–6 D (SP) to ~ 18 D (MC))[10] or emission properties.[11] However, the most surfaces,[26,27] which requires the introduction of suitable groups remarkable change observed upon spiropyran photoisomeriza- in spiropyrans to ensure chemical functionalization. Carboxylic tion is the difference in UV-vis absorbance: while SP is trans- acid groups are amongst the most preferred for this purpose, as parent in the visible range, MC shows a very intense absorption they confer molecules with a broad reactivity that enables the peak in that region. This means that light-induced SP-MC covalent bonding to substrates by simple coupling reactions interconversion is accompanied by an intense variation in (e. g., amide bond formation).[26,28,29] However, the incorporation coloration, a phenomenon known as photochromism.[12,13] of carboxylic acids in spiropyrans is not straightforward, Because of the large change in properties upon photo- typically involves several synthetic steps (e. g., condensation of isomerization, spiropyrans have been exploited for the prepara- an aromatic aldehyde with enamine indole) and moderate tion of a wide variety of smart functional materials whose yields.[30–32] properties can be very precisely controlled using light as an Actually, the chemical synthesis of carboxylic acids and their external stimulus.[14–18] derivatives (e. g., esters or amides) imply the use of many For most of these applications, the photoswitchable mole- reagents, catalysts[33] or organolithium compounds[34] at mild or cules must be covalently attached to the final materials, such as strong conditions.[35,36] In this context, the electrochemical polymers,[19–21] nanoparticles,[22,23] biomacromolecules[24,25] and carboxylation of organic molecules to produce carboxylic acids by fixing carbon dioxide has emerged during the last years as an efficient green route compared to conventional chemical [a] S. Santiago, C. Richart, Dr. S. Mena, Prof. I. Gallardo, Dr. J. Hernando, Dr. G. Guirado synthetic methods, especially since this process can be Departament de Química performed efficiently under mild conditions at atmospheric Universitat Autònoma de Barcelona pressure and avoiding the use of additional reagents.[37–42] One Bellaterra (Barcelona), 08193, Spain E-mail: Jordi.Hernando@uab.cat of the most popular electrocarboxylation strategies of organic Gonzalo.Guirado@uab.cat compounds is based on the in situ formation of a carbanion via Supporting information for this article is available on the WWW under reduction, which in turn undergoes a nucleophilic addition to https://doi.org/10.1002/celc.202101559 CO2 to yield the carboxylate functional group (Figure 2).[43] The An invited contribution to a Special Collection on Current Trends in Electrochemistry 2021 for the 1st French-Spanish Workshop on success of this approach is strongly related to the stability (i. e., Electrochemistry. lifetime) of the anion formed in the electrolytic medium. In ChemElectroChem 2022, 9, e202101559 (1 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 Instrumentation and procedure Electrochemical studies were performed by cyclic voltammetry (CV) using the potentiostat Workstation CHInstrument and controlled by the software CHI660e V14.08. For these measurements, a three- electrode system and a thermostatic electrochemical cell was used. Glassy carbon (WEC, 1 mm of diameter) or silver (WEAg, 3 mm of diameter) were used as working electrodes, while a platinum wire and a saturated calomel electrode (SCE) were used as counter (CE, Figure 2. Representation of the electrochemical carboxylation of aryl halides and reference (Ref) electrodes, respectively. Both CE and Ref using CO2. electrodes were separated from the solution by a salt-bridge. Solutions at different concentration of Br-BIPS (in the range of 3– 10 mM) were prepared in DMF/0.1 M of TBAPF6. Prior to each measurement, the solutions were purged with either Ar or CO2 depending on the type of study in each case. In the case of CO2 order to increase its lifetime, it is highly desirable to decrease gas, the flow was controlled using a Mass Flow Meter. the reduction potential at which the carbanion is produced. This goal can be achieved by the use of aryl halides, which can The Bulk Electrolysis were carried out at controlled constant potential using the potentiostat EG&G Princeton Applied Research be electrochemically reduced leading to the desired anionic model 273 A. For these experiments, we used the same electro- species after C X bond cleavage at relatively low potentials in chemical setup as for CV measurements but replacing the small organic aprotic solvents.[44] diameter sized WEC or WEAg for a large area graphite rod (area: Given the advantages of electrocarboxylation processes and ~ 8 cm2) or silver foil (area: ~ 6 cm2), respectively. For the electrolysis their reported success on simple, model aryl halides such as of Br-BIPS under inert atmosphere, the solution was purged with Ar bromobenzene,[43] herein we attempted the carboxylation for 10 min and then the corresponding reduction potential was applied until a total charge of 2 F was passed ( 2.35 V when using reaction of much more challenging spiropyran substrates by WEC and 2.00 V for WEAg vs SCE). For the purification of the electrochemical reduction of a commercially available bromo- products after the electrolysis, firstly the solvent of the reaction spiropyran derivative under CO2 atmosphere. In addition, the mixture was removed under vacuum and then an extraction in effect of the nature of the cathode (carbon and silver) was diethyl ether: H2O (2 : 1) was performed. Then, the organic phase evaluated in terms of performance and energy efficiency, since was washed for three times with water (2 : 1 v/v ratio), dried with it is well known the electrocatalytic effect of using silver in the anhydrous Na2SO4 and the solvent evaporated in vacuo. reduction of organic halides.[45–48] Our work therefore showcases Analogously, for the electrolysis under CO2 atmosphere, the a new one-pot and facile strategy for the functionalization of solution was saturated in this gas by bubbling for 20 minutes. After the spiropyran backbone using CO2 (as a C1-buidling block) and that, a constant reduction potential was applied until 2 F of total charge were reached ( 2.35 V when using WEc and 2.00 V for electrochemical techniques. Such an attractive and environ- WEAg vs SCE). Immediately after, 10 equivalents of MeI were added mentally friendly approach could grant access to a broad to the solution to derivatize the carboxylate goup introduced and variety of carboxylated spiropyran derivatives, which could be facilitate the isolation of the resulting methylated product BIPS- eventually applied for the design of smart surfaces and devices. COOMe. The solvent of the reaction mixture was removed under vacuum and purified performing a preparative thin layer chroma- tography (TLC). A mixture of n-hexane and ethyl acetate (80 : 20 v/v ratio) was used as a mobile phase. Experimental Section The products BIPS and BIPS-COOMe obtained after the electrosyn- Reagents thesis of the reagent Br-BIPS under Ar and CO2 atmosphere, respectively, were characterized by attenuated total reflectance 1,3,3-trimethylindolino-6’-bromobenzopyrylospiropyran (Br-BIPS, infrared spectroscopy (ATR-FTIR), H 1NMR on a Brucker spectrom- pur. 98.0 %) was purchased from TCI Chemicals and used without eter DPX360 and electrospray mass spectrometry (ESI-MS). further purification. High purity CO2 and Ar gases (IC 5.0) used for the electrochemical measurements and electrosynthesis were Br-BIPS: 1H NMR (360 MHz, Chloroform-d) δ 1.16 (s, 3H), 1.29 (s, 3H), supplied by Linde. 2.71 (d, J = 1.6 Hz, 3H), 5.72 (d, J = 10.3 Hz, 1H), 6.53 (d, J = 7.7 Hz, 1H), 6.60 (d, J = 9.3 Hz, 1H), 6.78 (d, J = 10.2 Hz, 1H), 6.85 (td, J = 7.4, Anhydrous and extra pure tetrabutylammonium hexafluorophos- 1.0 Hz, 1H), 7.07 (dd, J = 7.3, 1.3 Hz, 1H), 7.16 (ddt, J = 5.4, 3.9, phate, (TBAPF6; pur. > 99.0 %), iodomethane (MeI; pur. > 99.0 %) 1.8 Hz, 3H). ESI-MS: m/z found for C19H18BrNO (M + H): 358.0781 and anhydrous sodium sulfate (Na2SO4; pur. > 99.0 %) were (100 %); 359.0809 (24.4 %); 360.0773 (97.2 %); 361.0796 (19.9 %). acquired from Merck. Anhydrous N,N-dimethylformamide (DMF; pur. > 99.5 %) and anhydrous dimethylsulfoxide (DMSO; pur. BIPS 1H NMR (360 MHz, CDCl3) δ 1.18 (s, 3H), 1.32 (s, 3H), 2.74 (s, > 99.8 %) were purchased from Acros Organics. Both solvents were 3H), 5.68 (d, J = 10.2 Hz, 1H), 6.54 (d, J = 7.7 Hz, 1H), 6.72 (d, J = dried under activated molecular sieves of 4 Å prior to use to ensure 8.1 Hz, 1H), 6.78–6.89 (m, 3H), 7.01–7.14 (m, 3H), 7.19 (td, J = 7.6, water content < 100 ppm for the electrochemical measurements 1.3 Hz, 1H). ESI-MS: m/z found for C19H19NO (M + H): 278.1539 and electrosynthesis. (100 %); 279.1572 (21.1 %); 280.1604 (2.3 %). Dichloromehtane (DCM, pur. > 98 %), n-hexane (pur. > 98 %) and BIPS-COOMe: 1H NMR (360 MHz, CDCl3) δ 1.19 (s, 3H), 1.33 (s, 3H), ethyl acetate (pur. > 98 %) used for the purification of the products 2.74 (s, 3H), 3.90 (s, 3H), 5.77 (d, J = 10.1 Hz, 1H), 6.55 (d, J = 7.6 Hz, prepared by electrosynthesis were obtained from SDS and used as 1H), 6.73 (d, J = 7.6 Hz, 1H), 6.82–6.91 (m, 2H), 7.10 (d, J = 7.2 Hz, received. 1H), 7.17–7.23 (m, 2H), 7.8 (s, 1H). ESI-MS: m/z found for C21H21NO3 (M + H): 336.1581 (100 %); 337.1616 (23.2 %); 338.1701 (7.7 %). ChemElectroChem 2022, 9, e202101559 (2 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 Results and Discussion Irrespective of the working electrode used, the irreversibility of the two-electron reduction wave measured for Br-BIPS in Electrochemical reduction of Br-BIPS in DMF DMF under Ar atmosphere indicates that there is an irreversible chemical reaction coupled to the electron transfer to this Electrochemical study under inert atmosphere (Ar) compound. To determine the nature of this chemical reaction, the dependence of Epc with the scan rate (v) and concentration The electrochemical behavior of the compound Br-BIPS was were examined by CV. From the plot of Epc vs log v, a linear studied by CV at different concentrations (1–5 mM) and scan relationship was found with a slope value of 35 mV, whereas no rates (0.1–1.0 V/s) under inert conditions (Ar) using N,N-dimeth- variation of the cathodic peak potential was observed with the ylformamide (DMF) as a solvent and 0.1 M of TBAPF6 as a Br-BIPS concentration in solution (Figure S1). Hence, it is supporting electrolyte. As an example, Figure 3a shows the possible to conclude that the chemical reaction coupled to the electrochemical response of a 5 mM solution of Br-BIPS first reduction process is a first order reaction. recorded at 0.1 V/s scan rate and using two different working In order to disclose the nature of the product irreversibly cathodes: glassy carbon (WEC) and silver (WEAg). Figure 3b shows formed after the electrochemical reduction of Br-BIPS, electro- the cyclic voltammogram in a first cathodic scan, from 1.60 V lytic processes at a controlled constant potential (vs SCE) of to 2.70 V vs SCE, for a 5 mM solution of Br-BIPS using WEC. 2.35 V (for a carbon graphite rod electrode) or 2.00 V (for a Two irreversible peaks were found at Epc = 2.23 V and 2.47 V silver foil electrode) were conducted in DMF and under inert (vs SCE), which could be attributed to two- and one-electron atmosphere. These electrolysis experiments were simultane- reduction processes, respectively (Figure 3b). It is worth noting ously monitored by CV with WEC, where we observed that the that, when WAg was used as a working electrode instead, the reduction peak at Epc = 2.23 V (vs SCE) associated to the two-electron irreversible reduction peak of Br-BIPS was regis- reduction of Br-BIPS gradually decreased, and a new oxidation tered at Epc = 1.92 V (vs SCE) (Figure 3a). This potential shift of peak at Epa ~ + 0.70 V (vs SCE) concomitantly increased with the ca. 280 mV to less negative values indicates the electrocatalytic injection of charge (Figure S2). After the passage of 2 F, the effect of the use of silver as a cathode (Figure 3a). electrolytic mixture was treated and the irreversible product formed was isolated and analyzed by 1H NMR, ESI-MS, ATR-FITR and UV-vis spectroscopy. This allowed us to assign this product to the debrominated compound BIPS, which was obtained in a 56 % yield when using the carbon graphite rod as a WE and a 91 % yield for WEAg. The remaining 30 % and 9 % respectively correspond to unreacted starting material. Interestingly, this assignment was consistent with two significant features observed in our CV measurements. On the one hand, the anodic peak emerging during the electrolysis experiments at Epa ~ + 0.70 V (vs SCE) for WEC could be attributed to the bromide anion produced during the debromination of Br-BIPS (Fig- ure S2).[49] On the other hand, the second one-electron reduc- tion wave registered for Br-BIPS at Epc = 2.47 V (vs SCE) with WEC matches the cathodic peak measured for a solution of pure BIPS in DMF under Ar atmosphere (Epc = 2.50 V (vs SCE) using WEC, Figure S3) Therefore, the latter further supports the formation of BIPS after the two-electron reduction process of Br-BIPS. From all this data, we conclude that the electrochemical reduction of Br-BIPS proceeds through an ECE mechanism (Scheme 1). First, an electron transfer (E) takes place where the corresponding radical anion is formed ([Br-BIPS] ). Then, a first * order chemical reaction (C) leads to the cleavage of the C Br bond of the compound, thus producing the radical intermedi- ate species [BIPS] . Subsequently, this radical species is reduced * again by a second electron transfer (E) yielding an anionic form ([BIPS] ). This second reduction step must present a less Figure 3. (a) CVs registered for a 5 mM solution of Br-BIPS in DMF/0.1 M TBAPF6 under Ar atmosphere and using either glassy carbon or silver as negative potential than the first electron transfer (Eo2 > Eo1), working electrodes (scan rate = 0.1 V/s). (b) CVs registered for a 5 mM which would explain why a single cathodic wave associated to solution of Br-BIPS in DMF/0.1 M TBAPF6 under Ar atmosphere and using a two-electron reduction process was observed by CV. Finally, glassy carbon as a WE (scan rate = 0.1 V/s). Two different independent measurements are shown, which differ in the minimum potential of the the anionic form [BIPS] is protonated by subtracting a proton scan. In both cases, a two-electron irreversible wave at Epc = 2.23 V was from the solvent or the tetrabutylammonium ion of the observed. supporting electrolyte, yielding the final product BIPS. ChemElectroChem 2022, 9, e202101559 (3 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 Scheme 1. Electrochemical reduction mechanism for Br-BIPS under inert atmosphere in DMF/0.1 M TBAPF6. (SH is the solvent or the supporting electrolyte). Electrochemical study under saturated atmosphere of CO2 As the reduction of Br-BIPS leads to the formation of the carbanionic intermediate BIPS , we envisaged that this species could react with CO2 to produce the target electrocarboxylated spiropyran derivative. To validate this hypothesis, we first investigated by CV the electrochemical properties of Br-BIPS in a 75 mM CO2 DMF solution (Figure 4). Mainly, the same behavior previously observed under inert atmosphere was registered, and a two-electron irreversible reduction peak at Epc = 2.23 V (vs SCE) or 1.92 V (vs SCE) was detected for WEC or WEAg, respectively. Therefore, the electrocatalytic effect of the silver electrode was again demonstrated. When using WEAg, an additional irreversible cathodic wave at Epc = 2.60 V (vs Figure 4. CVs (scan rate = 0.1 V/s) of a 5 mM solution of Br-BIPS in DMF/0.1 M SCE) was measured, which can be attributed to the electro- TBAPF6 under the presence of CO2 using either glassy carbon (black solid chemical reduction of the CO2 present in solution.[26,49] line) or silver as a WE (blue solid line). For the case of WEAg, the electrochemical response of CO2 in a neat DMF/0.1 M TBAPF6 solution is also This was further proven by analyzing a pure DMF sample given (blue dashed line). saturated with CO2 by CV, where the same reduction peak at Epc = 2.60 V (vs SCE) also appeared. In view of these results, it is possible to conclude that, in the presence of CO2, the reduction of Br-BIPS follows the same ECE mechanism after irreversible producing BIPS ), two different oxidation previously established under inert atmosphere, where the waves were observed in the anodic region (Figure 5). One of anionic intermediate BIPS is electrogenerated. them appeared at Epa ~ + 0.70 V (vs SCE) for WEC, which as Noticeably, when recording the full voltammogram of Br- previously mentioned, can be assigned to bromide anions and, BIPS under CO2 atmosphere after the first cathodic scan (i. e., therefore, is compatible with the reductive debromination of ChemElectroChem 2022, 9, e202101559 (4 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 obtained through the nucleophilic attack of the electrogener- ated anionic form BIPS to the CO2 molecules present in solution (Scheme 2). The low oxidation potential value obtained for the carboxylate BIPS-COO derivative is due to the aromatic electron rich system of the indole moiety. Similar anodic peak potential values were observed for other aromatic electron rich heterocycles, such as thiophenecarboxylate derivatives.[50] To confirm the nature of the product obtained, bulk electrolysis of Br-BIPS in DMF and under CO2 atmosphere were performed applying 2.35 V (vs SCE) or 2.00 V (vs SCE) for graphite rod or silver foil cathodes, respectively. The electrosyn- thesis was stopped after the passage of 2F (ca. 20 minutes) to Figure 5. Cyclic voltammograms recorded to monitor the electrochemical avoid the obtention of non-desired overreduced compounds reaction of Br-BIPS performed at a controlled constant potential in a partially despite the fact that the reactant was not fully consumed, as saturated solution in CO2 using glassy carbon as a WE. The CVs were recorded at 0.30 V/s in DMF/0.1 M of TBAPF6 from 0.20 V to 0.80 V. further discussed below. To facilitate the isolation and identi- fication of the electrocarboxylation product, 10 equivalents of CH3-I were added to the solution after the electrosynthesis to further derivatize BIPS-COO and obtain the methyl ester BIPS- Br-BIPS. As for the second of the anodic peaks, it was registered COOMe. The resulting samples were treated and the products at Epa ~ + 0.40 V (vs SCE) for WEC and, more importantly, was generated were purified and analyzed by 1H NMR, ESI-MS, ATR- not observed neither in the case of the electrochemical FTIR and UV-vis absorption spectroscopy. As shown in Figure 6 measurements of Br-BIPS nor the electrosynthesis of BIPS three main compounds were identified in this way: (i) the under inert atmosphere. unreacted starting material Br-BIPS, which was recovered; (ii) Consequently, this suggests that a different product is the byproduct BIPS in ca. 30 % yield for both electrodes, which obtained when conducting the electrochemical reduction of Br- should arise from the competing protonation reaction of BIPS BIPS in the presence of CO2, which must give rise to such an with the solvent or the supporting electrolyte; (iii) the desired additional oxidation wave. We tentatively attributed this new carboxylated product BIPS-COOMe, which was obtained in peak to the oxidation of the carboxylate anion (BIPS-COO ) 16 % or 35 % yield when either carbon or silver cathodes were Scheme 2. Electrochemical reduction mechanism for Br-BIPS in DMF/0.1 M TBAPF6 saturated with CO2, which eventually leads to the formation of the electrocarboxylated product BIPS-COO . ChemElectroChem 2022, 9, e202101559 (5 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 Figure 6. Representation of the different products obtained depending on the conditions of the electrolysis and the yield resulted in each case. employed (31 % and 54 % yield if the recovered initial reagent is taken into account). From this data, we could not only verify the electrocarboxylation of Br-BIPS as shown in Scheme 2, but also conclude that the use of silver cathodes to conduct this process presents several advantages resulting from their electrocatalytic properties. First, the reductive cleavage of the C Br bond of the starting material takes place at lower potentials, which makes the reaction less energy consuming and, more importantly, increases the lifetime of the resulting reactive carbanion. Second, this leads to both larger conversion of the initial starting product (up to 65 %) and formation of the target electrocarboxylated compound, which is eventually obtained at moderate yields (about 50 % taking into account Figure 7. Normalized CV representing the electrochemical reduction re- the reactant consumption). sponse of Br-BIPS, BIPS and BIPS-COOMe in DMF/0.1 M TBAPF6 with WEc. The electrochemical response of the formed product BIPS- Please note that the second reduction peak observed for Br-BIPS corresponds to the reduction of the BIPS molecules formed after the first COOMe was compared to those of the precursor Br-BIPS and electron transfer reaction. the byproduct of the reductive electrolysis BIPS. As shown in Figure 7, the potential of the cathodic peak associated with the first electron transfer toward these compounds varies depend- ing on the nature of the substituent at the benzopyran ring. values could lead to the formation of undesired hydrogenated This behavior is related to the electronic character of each of byproducts of Br-BIPS. Finally, in the case of BIPS-COOMe, a these groups. In the case of Br-BIPS, Epc is about 249 mV less further decrement in the absolute value of Epc was registered negative than BIPS; i. e., it shows a larger tendency to be (447 mV relative to BIPS), which can be attributed to the reduced than BIPS. This means that the inductive electron presence of the strong electron withdrawing ester substituent. withdrawing effect of the bromide substituent in Br-BIPS is more important than its mesomeric electron donating effect. The nature of the irreversible two-electron transfer reduction Photochromism of electrochemically synthesized molecular process of BIPS was investigated by conducting a controlled switches potential electrolysis of this product at 2.6 V vs. SCE under nitrogen atmosphere. This revealed the formation of hydro- The photochromic response of spiropyrans BIPS, BIPS-COOMe genated byproducts by reduction of BIPS olefinic bonds, a and Br-BIPS (0.5 mM) was explored in dimethyl sulfoxide finding that corroborates the need of carefully controlling the (DMSO) at room temperature (Figure 8). As expected, the most conditions applied during the reductive activation of Br-BIPS to thermodynamically stable isomer, for all the three compounds product the reactive species BIPS : because of the small in the dark, was found to be the spirocyclic form SP, with a difference in potential of the first cathodic waves of Br-BIPS maximum absorption peak located at ~ 295 nm for all three and BIPS, the use of too long electrolysis times or large Eap samples and a molar absorption coefficient of ɛ = 3310– ChemElectroChem 2022, 9, e202101559 (6 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 Figure 8. Absorbance UV-vis spectra of a 0.5 mM in DMSO of the reactant Br-BIPS showing the photochromic response of the molecular switch and the photochromic response of the switches, BIPS and Br-BIPS 0.5 mM in DMSO, electrochemically obtained. The switching between the closed (SP) to the colored (MC) forms for the different molecular switches were achieved when irradiating at @312 nm. The back conversion to the SP form was totally reached spontaneously over time in the dark. 4586 M 1cm 1 (Table 1). After irradiation of the samples with UV isomerization, obtaining a 100 % yield back conversion. Some light (312 nm), the closed SP isomer opens via Cspiro O bond differences were found in the back conversion rates from MC to breakage leading to the formation of the open MC form. This SP. This fact is directly related to the stability of the MC form, results in coloration of the solution as shown in Figure 8, which in the case of BIPS was lower compared than for Br-BIPS because the merocyanine isomer formed presents a new and BIPS-COOMe. The presence of the electron-withdrawing absorption band at 573–601 nm.[51] An hypsochromic shift in bromo and ester substituents at the para position of the absorption was noticeable when comparing the λmax of the MC phenolate group of the MC form account for this result, as they form depending on the nature of the substituting group at the aid stabilizing the negative charge of this group. Actually, this 6 position in the benzopyran moiety; i. e., for BIPS-COOMe explains why, in the case of Br-BIPS and BIPS-COOMe, greater relative to BIPS and Br-BIPS. conversions to MC could be obtained compared to BIPS It was found that, after irradiation at 312 nm for 2 minutes, (Table 1 and Figure S5). Thus, it can be determined that it is all three molecular switches reached their photostationary state possible to electrochemically access a variety of molecular (PSS). The photochromic behavior of the switches was totally switches with photochromic properties. reversible since the initial SP form was recovered when the solution was let in the dark over time due to thermal back Table 1. Photochemical and electrochemical data extracted from the absorbance UV-Vis spectra and CV of Br-BIPS, BIPS, BIPS-COOMe upon irradiation and acidic conditions. Name State λmax, nm (ɛ, M 1 cm 1) k (s 1) (MC!SP) Ep,c (V) Br-BIPS SP 297, shoulder (6293) 0.01986 2.23 SPH + 297 MC 601 MCH + 428 BIPS SP 296 (8274) 0.0362 2.53 SPH + 295 MC 585 MCH + 418 BIPS-COOMe SP 292 (8719) 0.0186 2.08 SPH + 286 MC 573 MCH + 409 ChemElectroChem 2022, 9, e202101559 (7 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 Figure 9. Switching properties of Br-BIPS, BIPS and BIPS-COOMe in slightly acidic DMSO when irradiating at 312 nm (c = 0.5 mM). Formation of the yellow- colored, protonated MCH + form is observed by means of absorption spectroscopy for all three compounds, which can revert back to the unprotonated SP state upon irradiation at 442 nm. Photo-halochromism of electrochemically synthesized electrocarboylated products under mild conditions thanks to its molecular switches electrocatalytic properties. Finally, the green efficient electro- chemical route described in the current work would open a In addition, it was also determined the responsiveness of the new sustainable strategy for designing and building “smart” spiropyran photoswitches developed in this work upon changes surfaces with switchable physical properties by s using CO2 as a in pH, i. e., their photo-halochromism. For this, we consider the C1-organic building block. use of slightly acid media, where the SP form of spiropyrans is known to remain unprotonated because of its low pKa while the more basic MC isomer must become protonated into MCH + Acknowledgements .[52,53] As seen in Figure 9, this was proven by irradiating with UV light acidic DMSO solutions of Br-BIPS, BIPS and BIPS-COOMe: The authors thank the Ministerio de Ciencia e Innovación of Spain instead of observing the blue coloration arising from MC for financial support though the project PID2019-106171RB I00. formation with λabs, max > 550 nm, we obtained yellow solutions S.M. acknowledges the Autonomous University of Barcelona for with new absorption bands with a maximum wavelength her predoctoral PIF grant. located around 409–428 nm, which are generally ascribed to the protonated merocyanine form MCH (Table 1). This process was found to be totally reversible, as the MCH + form of Br- Conflict of Interest BIPS, BIPS and BIPS-COOMe could be fully converted to the initial, non-protonated spiropyran state when irradiating at The authors declare no conflict of interest. 442 nm. Data Availability Statement Conclusion The data that support the findings of this study are available The electrochemical reduction mechanism of Br-BIPS has been from the corresponding author upon reasonable request. disclosed under nitrogen and CO2 atmospheres using cyclic voltammetry and control potential electrolysis under nitrogen Keywords: Carbon-Bromide cleavage · Carbon dioxide · and carbon dioxide atmospheres using carbon and silver Electrocarboxylation · CO2 capture · spiropyran switches cathodes. This study proves that the electrochemical reduction follows an ECE mechanism involving a C Br reaction cleavage that takes place after the first electron transfer. Based on these [1] B. L. Feringa, Molecular Switches, WILEY-VCH, Weinheim, Germany, 2001. data, compound Br-BIPS has been used for activating, valoriz- [2] G. Berkovic, V. Krongauz, V. Weiss, Chem. Rev. 2000, 100, 1741–1754. ing and capturing CO2, since the electrogenerated BIPS anion [3] R. Tong, H. D. Hemmati, R. Langer, D. S. Kohane, J. Am. Chem. Soc. 2012, 134, 8848–8855. reacts with CO2 though a nucleophilic attack. The use of silver [4] N. Tamai, H. Miyasaka, Chem. Rev. 2000, 100, 1875–1890. cathodes allowed achieving moderate yields and efficiencies of [5] L. Kortekaas, W. R. Browne, Chem. Soc. Rev. 2019, 48, 3406–3424. ChemElectroChem 2022, 9, e202101559 (8 of 9) © 2022 Wiley-VCH GmbH Research Article ChemElectroChem doi.org/10.1002/celc.202101559 [6] N. Voloshin, E. Voloshina, M. Chernov’yants, A. Chernyshev, Russ. J. Gen. [31] M. Natali, C. Aakeröy, J. Desper, S. Giordani, Dalton Trans. 2010, 39, Chem. 2002, 72, 1468–1472. 8269. [7] A. A. García, S. Cherian, J. Park, D. Gust, F. Jahnke, R. Rosario, J. Phys. [32] J. G. Pargaonkar, S. K. Patil, S. N. Vajekar, Synth. Commun. 2018, 48, 208– Chem. A 2000, 104, 6103–6107. 215. [8] F. M. Raymo, S. Giordani, J. Am. Chem. Soc. 2001, 123, 4651–4652. [33] Y. Liu, J. Cornella, R. Martin, J. Am. Chem. Soc. 2014, 136, 11212–11215. [9] C. Berton, D. M. Busiello, S. Zamuner, E. Solari, R. Scopelliti, F. Fadaei- [34] T. E. Hurst, J. A. Deichert, L. Kapeniak, R. Lee, J. Harris, P. G. Jessop, V. Tirani, K. Severin, C. Pezzato, Chem. Sci. 2020, 11, 8457–8468. Snieckus, Org. Lett. 2019, 21, 3882–3885. [10] N. A. Murugan, S. Chakrabarti, H. Ågren, J. Phys. Chem. B 2011, 115, [35] K. Kobayashi, Y. Kondo, Org. Lett. 2009, 11, 2035–2037. 4025–4032. [36] B. T. Sargent, E. J. Alexanian, J. Am. Chem. Soc. 2016, 138, 7520–7523. [11] L. Zhu, W. Wu, M.-Q. Zhu, J. J. Han, J. K. Hurst, A. D. Q. Li, J. Am. Chem. [37] S. Mena, C. Louault, V. Mesa, I. Gallardo, G. Guirado, ChemElectroChem Soc. 2007, 129, 3524–3526. 2021, 8, 2649–2661. [12] H. Görner, Phys. Chem. Chem. Phys. 2001, 3, 416–423. [38] S. Mena, S. Santiago, I. Gallardo, G. Guirado, Chemosphere 2020, 245, [13] N. W. Tyer, R. S. Becker, J. Am. Chem. Soc. 1970, 92, 1289–1294. 125557. [14] A. Goulet-Hanssens, F. Eisenreich, S. Hecht, Adv. Mater. 2020, 32, [39] S. Mena, G. Guirado, C 2020, 6, 34. 1905966. [40] S. Mena, J. Bernad, G. Guirado, Catalysts 2021, 11, 880. [15] M.-M. Russew, S. Hecht, Adv. Mater. 2010, 22, 3348–3360. [41] S. Mena, I. Gallardo, G. Guirado, Catalysts 2019, 9, 413. [16] S. Santiago, P. Giménez-Gómez, X. Muñoz-Berbel, J. Hernando, G. [42] S. Mena, J. Sanchez, G. Guirado, RSC Adv. 2019, 9, 15115–15123. Guirado, ACS Appl. Mater. Interfaces 2021, 13, 26461–26471. [43] A. A. Isse, C. Durante, A. Gennaro, Electrochem. Commun. 2011, 13, 810– [17] S. Wu, J. Fan, W. Wang, D. Yu, Colloids Surf. A 2022, 632, 127760. 813. [18] R. Feringa, H. S. Siebe, W. J. N. Klement, J. D. Steen, W. R. Browne, Mater. [44] Z. Chami, M. Gareil, J. Pinson, J. M. Saveant, A. Thiebault, J. Org. Chem. Adv. 2022, 3, 282. 1991, 56, 586–595. [19] P. Hong, L. Wang, L. Bai, Z. Liu, Y. Liu, J. Yang, A. Ying, Dyes Pigm. 2022, [45] C. Durante, A. A. Isse, F. Todesco, A. Gennaro, J. Electrochem. Soc. 2013, 197, 109902. 160, G3073–G3079. [20] S. Tian, J. Zhang, Q. Zhou, L. Shi, W. Wang, D. Wang, Polymers (Basel). [46] C. Durante, A. A. Isse, G. Sandonà, A. Gennaro, Appl. Catal. B 2009, 88, 2021, 13, 2496. 479–489. [21] M.-Q. Zhu, L. Zhu, J. J. Han, W. Wu, J. K. Hurst, A. D. Q. Li, J. Am. Chem. [47] S. Arnaboldi, A. Bonetti, E. Giussani, P. R. Mussini, T. Benincori, S. Rizzo, Soc. 2006, 128, 4303–4309. A. A. Isse, A. Gennaro, Electrochem. Commun. 2014, 38, 100–103. [22] H. Zhang, Z. Chen, Y. He, S. Yang, J. Wei, ACS Appl. Mater. Interfaces [48] S. Mena, J. Sanchez, G. Guirado, RSC Adv. 2019, 9, 15115–15123. 2021, 4, 4340–4345. [49] I. Gallardo, G. Guirado, J. Marquet, J. Electroanal. Chem. 2000, 488, 64– [23] R. Tong, H. D. Hemmati, R. Langer, D. S. Kohane, J. Am. Chem. Soc. 2012, 72. 134, 8848–8855. [50] J. Massaad, J. C. Micheau, C. Coudret, C. L. Serpentini, G. Guirado, Chem. [24] K. Tomizaki, H. Mihara, Mol. BioSyst. 2006, 2, 580. Eur. J. 2013, 19, 12435–12445. [25] T. Sakata, Y. Yan, G. Marriott, Proc. Nat. Acad. Sci. 2005, 102, 4759–4764. [51] S.-R. Keum, R. Se-Jung, K. Sang-Eun, L. Sung-Hoon, C. Chul-Hee, K. Sung- [26] N. Wagner, P. Theato, Polymer 2014, 55, 3436–3453. Hoon, K. Kwang-Nak, Bull. Korean Chem. Soc. 2006, 27, 187–188. [27] T. Garling, Y. Tong, T. A. Darwish, M. Wolf, R. K. Campen, J. Phys. [52] L. Kortekaas, J. Chen, D. Jacquemin, W. R. Browne, J. Phys. Chem. B 2018, Condens. Matter 2017, 29, 414002. 122, 6423–6430. [28] J. Opel, L.-C. Rosenbaum, J. Brunner, A. Staiger, R. Zimmermanns, M. [53] L. Wimberger, S. K. K. Prasad, M. D. Peeks, J. Andréasson, T. W. Schmidt, Kellermeier, T. Gaich, H. Cölfen, J.-M. García-Ruiz, J. Mater. Chem. B 2020, J. E. Beves, J. Am, Chem. Soc. Rev. 2021, 143, 49, 20758–20768. 8, 4831–4835. [29] A. K. Bohaty, M. R. Newton, I. Zharov, J. Porous Mater. 2010, 17, 465–473. [30] R. da Costa Duarte, F. da Silveira Santos, B. B. de Araújo, R. Cercena, D. Manuscript received: November 19, 2021 Brondani, E. Zapp, P. F. B. Gonçalves, F. S. Rodembusch, A. G. Dal-Bó, Revised manuscript received: December 24, 2021 Chemosensors 2020, 8, 31. Accepted manuscript online: January 10, 2022 ChemElectroChem 2022, 9, e202101559 (9 of 9) © 2022 Wiley-VCH GmbH