Colored Ionic Liquid Based on Stable Polycyclic Anion Salt Showing

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Colored Ionic Liquid Based on Stable Polycyclic Anion Salt Showing Halochromism with HCl Vapor Hideo Enozawa, Shusaku Ukai, Hiroshi Ito, Tsuyoshi Murata, and Yasushi Morita* Department of Applied Chemistry, Faculty of Engineering, Aichi Institute of Technology, Toyota, Aichi 470-0392, Japan

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S Supporting Information *

ABSTRACT: A sodium salt of a polycyclic trioxotriangulene (TOT) anion with six triethylene glycol chains exhibiting the formation of a colored ionic liquid at room temperature was synthesized. The ionic liquid is air- and water-stable, reflecting thermodynamic stabilization of a charge-delocalized TOT anion. Upon protonation of the TOT anion, the salt shows halochromic behaviors in solution and even in the neat liquid state with HCl vapor. The ionic liquid shows no morphological change with the chromism, presumably as a result of poor intermolecular interactions between π skeletons.

battery.11c Since TOTs have a large π-conjugated plane to stack with each other by cooperative spin−spin and π−π interactions, they form one-dimensional columns to be electron-conductive pathways.11d,e TOT is able to undergo one-electron reduction to produce an enolate-type polycyclic anion with a charge-delocalized closed-shell structure.11a,12 Since charge delocalization causes thermodynamic stabilization of the anion, TOT anion is a candidate for the parent skeleton of stable ILs (Figure 1a). On

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sotropic liquid materials with a variety of functions have recently attracted increasing interest in a wide range of research areas because of their facility of fabrication and the sustainability of the morphology.1−3 Especially, stimuliresponsive liquids are of importance because of their potential application to switches and sensors.4 Ionic liquids (ILs) are salts that are in the liquid state at room temperature or below 100 °C usually, and they have various potential applications5 such as electrolytes in batteries,5b solvents for organic reactions,5c and lubricating agents in space/vacuum technology5d because of their ionic conductivity, nonvolatility, and fluidity.5e Although a huge number of ILs have been reported to show a variety of functionalities,6 the parent skeletons of stable ILs are limited because they inherently require a stable anion or cation with fluidity.7 Anions of aromatic alcohols such as phenolate are usually unstable because of the low acidity of the OH proton. However, anionic polycyclic systems produced by reductions of perylene bisimide derivatives were recently highlighted to show excellent stability even in water under air at room temperature.8 Such stability might be derived from several factors, including charge delocalization on electron-deficient enolate forms.9 This finding has opened applications of airstable polycyclic anions to functional materials.10 Trioxotriangulene (TOT) is a non-Kekulé-type polycyclic system with an open-shell structure in the neutral state11a and is an extremely stable neutral radical. It has a decomposition temperature above 300 °C under air and is easy to handle at room temperature without any steric protections around the skeleton.11b This unusual stability might be based on thermodynamic stabilization derived from spin delocalization over the large π-conjugated molecular skeleton of TOT. Furthermore, TOT shows multiredox behavior involving four electrons to be utilized as a cathode material in a secondary © XXXX American Chemical Society

Figure 1. (a) TOT anion and its protonated derivative TOT-H. (b) TOT anion 1 having six TEG chains via terephthalic acid esters.

the other hand, a carbonyl oxygen of TOT anion can be protonated to form TOT-H under acidic conditions. This might cause a drastic change in the electronic structure of the TOT skeleton, leading to stimuli-responsive functions such as halochromism.13,14 Triethylene glycol (TEG) is a flexible-chain molecule with ether-type oxygens that shows good affinity for a variety of organic solvents as well as water based on the amphiphilic Received: February 5, 2019

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DOI: 10.1021/acs.orglett.9b00468 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters character,8c,15a−c and molecules having several TEG chains are known to show enhanced fluidity.15d Previously synthesized TOT derivatives have rigid hydrophobic substituents designed for exploring functionalities in the solid crystalline state.11 To produce a stable IL with functionalities, the novel TOT anion 1 having six TEG chains via terephthalic acid esters was designed (Figure 1b). The bulky TEG chains might prohibit strong intermolecular interactions between TOT planes and thus are expected to make 1 fluidic.1 Herein we report the synthesis of a colored ionic liquid based on a sodium salt of a polycyclic anion that is thermodynamically stabilized by charge delocalization. Upon protonation of the TOT anion (Figure 1a), the salt shows halochromic behavior in solution and even in the neat liquid state with HCl vapor. Notably, no morphological change with the chromic behavior was observed, presumably because of the poor intermolecular interactions between the π skeletons as well as the small change in molecular structure. To our knowledge, there are no examples of stable ILs showing such sustainable chromic behavior, though neutral liquids showing halochromism with a morphological change were recently reported.4 This feature will provide a new design strategy for the development of stimuli-responsive ionic liquid materials. 1-Na, the sodium salt of the designed TOT anion 1, was synthesized according to Scheme 1.16 Terephthalic acid ethyl

Figure 2. (a) TGA profile of 1-Na. A 5.300 mg sample of 1-Na in an aluminum pan was heated in flowing N2 at a heating rate of 5 K/min. (b) XRD profile of a film prepared by painting the neat liquid of 1-Na over a glass plate at room temperature. (c) Normalized electronic spectra of 1-Na in triglyme (0.1 mM, red line) and of a film prepared by painting the neat liquid of 1-Na over a glass plate (blue line) together with simulated electronic transitions of TOT anion (green lines). The spectra were measured at room temperature. The transitions for the parent TOT anion were calculated using the TDDFT method at the B3LYP/6-31++G(d,p)//B3LYP/6-31G(d,p) level.

Interestingly, 1-Na forms a sticky liquid with a dark-green color at room temperature. Differential scan calorimetry (DSC) analysis of 1-Na shows only a broad baseline shift at around −35 °C in both the cooling and heating processes, which is a typical signal for a glass transition (Figure S1). 1-Na was easily painted over a glass plate by rubbing to produce a green liquid film (Figure S2).16 In X-ray diffraction (XRD) measurements of the liquid film at room temperature, two broad peaks typical of amorphous materials were observed (Figure 2b). The peak at d = 1.9 nm might correspond to the average distance between molecules (Figure S3a), while the signal for the distance between TEG chains was observed at d = 0.39 nm (Figure S3b). Additionally, polarized optical microscopy (POM) of 1-Na at room temperature showed a completely dark field, indicating an isotropic phase in which molecules are randomly dispersed (Figure S4a). As expected from the molecular design, 1-Na shows a high solubility in a variety of organic solvents and water. Molecular modeling of 1 shows a structure in which six TEG chains surround a TOT skeleton and are able to wrap it depending on the conformations of the chains (Figure S5). On the other hand, the phenyl rings of the terephthalic acid esters of 1 have biphenyl-type steric repulsions with adjacent TOT planes at the ortho position to produce possible two isomers. However, 1 H NMR spectra of 1-Na in solution show symmetrical peaks for all of the sp2 protons at room temperature (Figure S6), indicating that averaged spectra are observed on the NMR time scale as a result of fast rotation of the phenyl rings. An electronic spectrum of 1-Na in triglyme at room temperature shows two kinds of strong absorptions in the visible region (Figure 2c, red line). On the basis of density functional theory (DFT) calculations,16 TOT anion shows a D3h-symmetric planar structure (Figure S8a and Table S1) having two degenerate LUMOs and one HOMO (Figure S9a), similar to those of the neutral radical. The HOMO of TOT anion spreads over the entire skeleton of TOT, resulting in robust charge delocalization, which is supported by an electrostatic potential (ESP) distribution mapped on the isodensity surface (Figure S10a). Time-dependent DFT (TDDFT) calculations on the parent TOT anion showed two intense transitions in the visible region (Figure 2c, green lines) and imply that the absorption whose maximum is located at 666 nm is mainly contributed by transitions from the HOMO to the two degenerate LUMOs (Table S3). The solution

Scheme 1. Synthesis of 1-Na

esters were introduced into starting material 211c by the Suzuki−Miyaura cross-coupling reaction with 3,17 and subsequent hydrolysis of the esters and then acidic workup produced hexacarboxylic acid 4 in good yield. Six TEG chains were then attached to 4 by using excess amounts of TEG-Br under anhydrous basic conditions. Finally, cation exchange of the precursor afforded the target 1-Na in 67% total yield based on 2. The obtained 1-Na is water-stable at room temperature and contains one water molecule in the composition of 1-Na, as confirmed by elemental analysis.16 Thermogravimetric analysis (TGA) of 1-Na shows only 1.5% weight loss by 200 °C (Figure 2a), which might correspond to evaporation of the water molecule. This composition as well as the spectra of 1Na described later were invariant after 1-Na was allowed to stand for more than a year at room temperature in air under room light, reflecting the thermodynamic stability of the TOT anion. B

DOI: 10.1021/acs.orglett.9b00468 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

detail. The obtained sigmoidal curve of the plot was then fitted by the Henderson−Hasselbalch equation21 to give 2.58 as the value of pKa (Figure 3c and Table S5).16 Although the OH group of 1-H has a phenol-type partial structure, 1-H is a 2.5 × 107 times stronger acid than phenol (pKa = 9.95).22 Compared with tropolone (pKa = 6.92)23 which has similar resonance forms as TOT-H, the acidity of 1-H is still 2.0 × 104 times stronger, indicating enhanced stability of the conjugate base 1. On the other hand, the pKa value of TOT-H was evaluated to be 1.47 on the basis of DFT calculations (Table S6 and Figure S12).24,16 The calculated value seems to be comparable to the experimental value of 1-H considering the accuracy of the calculations (Table S7). Finally, to demonstrate the halochromism of the neat liquid, 1-Na was exposed to HCl vapor for 30 min at room temperature (Figure S13).16 The color of the liquid turned from green to blue. A film prepared by painting the exposed liquid over a glass plate gave an electronic spectrum (Figure 3d, blue line) similar to that of 1-Na in water at low pH (Figure 3b, red line), indicating the formation of 1-H. In addition, the electronic spectrum of 1-Na in triglyme with an excess amount of HCl is also similar to that of the liquid film except for a shift25 and a broadening of the peaks (Figure 3d, red line). These results indicate that intermolecular interactions between the π skeletons in the neat liquid are still poor for 1-H and that a main driving force for the chromism is a change in the electronic state of TOT anion itself upon protonation.26 The obtained bluish liquid gradually changed color to be a greenish liquid after 20 h in air, presumably because of evaporation of HCl (Figure S14b).16 However, a change in the electronic spectra of the liquid film was unfinished even 20 h later in air.27 On the other hand, the resulting liquid was dissolved in triglyme, followed by additon of an excess amount of NaHCO3 for a neutralization, to give a green solution (Figure S15).16 The electronic spectrum of the resulting solution was almost identical to that of 1-Na (Figure S16). Furthermore, both XRD (Figure S3c) and POM (Figure S4b) data for the exposed liquid suggest a similar isotropic phase as 1-Na. Thus, we achieved control of the electronic state of TOT anion using HCl vapor as an external stimulus without changing the morphology of the liquid.28 Halochromism in previously reported systems15 is based on strong intermolecular interactions between N-heteroacenes as chromophores after protonation of the imino N atoms and thus is accompanied by a morphological change, in contrast to our system. In conclusion, we have designed and synthesized a sodium salt of a polycyclic anion with fluidity that forms a colored ionic liquid at room temperature. Reflecting the thermodynamic stability of the TOT anion based on its robust charge delocalization, the ionic liquid is water-stable and does not degrade after more than a year at room temperature in air. Upon protonation of the TOT anion by exposure to HCl vapor, the color of the ionic liquid turned from green to blue. Surprisingly, the ionic liquid showed no morphological change with the chromism, presumably because of the poor intermolecular interactions between the π skeletons. Although no examples of stable ILs showing such a sustainable chromic behavior have been reported to our knowledge, they might be applicable to sensors.29 In addition, 1 might also be applicable to symmetric organic flow batteries30 because of the high solubility and ambipolar redox ability of TOT.11 On the other hand, since TOT anion is stable and easy to handle at room

spectrum of 1-Na shows no concentration dependence in the range of 0.025−0.1 mM, and a film prepared by painting the neat liquid of 1-Na over a glass plate gave an electronic spectrum similar to that of the solution except for a vibrational structure18 as well as a small shift and a broadening of the peaks (Figure 2c, blue line). These results indicate that 1-Na molecules have poor intermolecular interactions between the π skeletons in the neat liquid, presumably hampered by the bulky TEG chains. To investigate the halochromic behavior of 1-Na in water, the pH dependence of the electronic spectrum was investigated using buffer solutions of citrate, HCl, and NaHCO3. The color of the solution turned from green to blue in the region of pH < 2 (Figure 3a). In the electronic spectra, the two strong

Figure 3. (a) Solution colors and (b) electronic spectra of 1-Na (0.1 mM) in water depending on pH using buffer solutions of citrate, HCl, and NaHCO3 at room temperature. (c) Plot prepared from the intensity at 640 nm in the electronic spectra against pH, fitted by the Henderson−Hasselbalch equation (R = 0.99934). (d) Normalized electronic spectra of 1-Na in triglyme (0.1 mM, ∼4 mL, red line) with a drop (∼0.01 mL) of 6 M aqueous HCl and of a film prepared from 1-Na liquid exposed to HCl vapor for 30 min (blue line) together with simulated electronic transitions of TOT-H (green lines). The spectra were measured at room temperature. The transitions for TOT-H were calculated using the TD-DFT method at the B3LYP/631++G(d,p)//B3LYP/6-31G(d,p) level.

absorptions of 1-Na were gradually attenuated with decreasing pH to produce a new spectrum having the absorption maximum at 597 nm (Figure 3b).19 This indicates that the electronic structure of 1 drastically changed upon the addition of a proton at an oxygen atom of the TOT skeleton to form 1H.20 The optimized structure of TOT-H is also planar, but the symmetry is changed to Cs (Figure S8b and Table S2). ESP mapping of TOT-H indicates a polarized structure with a dipole moment of 5.69 D in the molecular plane due to the localized charge of lone-pair electrons on two carbonyl oxygens of the TOT skeleton (Figure S9b). It should be noted that the two degenerate LUMOs of TOT anion are split into different energy levels upon protonation (Figure S10b). TD-DFT calculations on TOT-H show one intense transition in the visible region (Figure 3d, green lines) and indicate that the HOMO−LUMO transition mainly contributes the observed absorption having the maximum at 597 nm (Table S4). A decrease in oscillator strength and a shift to higher energy for the transitions of TOT-H compared with those of TOT anion also support the experimental results. The intensities at 640 nm in the electronic spectra of 1-Na were plotted against pH to reveal the protonation behavior in C

DOI: 10.1021/acs.orglett.9b00468 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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temperature under air, various applications are expected for other soft matters such as a liquid crystal and a gel. Moreover, one-electron oxidation of the TOT anion to generate the neutral radical may produce novel soft matters having a variety of functionalities such as electron conductivity and magnetic properties. The preparation of soft matters based on TOT neutral radical as well as the anion are ongoing in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00468. Experimental procedures and computational details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Tsuyoshi Murata: 0000-0001-6861-5456 Yasushi Morita: 0000-0002-2124-0201 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. S. Sakakibara and Mr. T. Minbu (Aichi Institute of Technology) for their support in the synthesis. This work was supported by Grants-in-Aid for Scientific Research B (25288022 and 16H04114) and the Core Research for Evolutionary Science and Technology (CREST) Basic Research Program “Creation of Innovative Functions of Intelligent Materials on the Basis of Element Strategy” of the Japan Science and Technology Agency (JST).

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REFERENCES

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DOI: 10.1021/acs.orglett.9b00468 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters (13) For reviews, see: (a) Machado, V. G.; Stock, R. I.; Reichardt, C. Chem. Rev. 2014, 114, 10429−10475. (b) Shindy, H. A. Dyes Pigm. 2017, 145, 505−513. (c) Zhang, Y.; Tang, S.; Thapaliya, E. R.; Sansalone, L.; Raymo, F. M. Chem. Commun. 2018, 54, 8799−8809. (14) For selected examples, see: (a) Reichardt, C.; Che, D.; Heckenkemper, G.; Schäfer, G. Eur. J. Org. Chem. 2001, 2001, 2343− 2361. (b) Matsui, K.; Segawa, Y.; Itami, K. Org. Lett. 2012, 14, 1888− 1891. (c) Garcia-Amorós, J.; Swaminathan, S.; Raymo, F. M. Dyes Pigm. 2014, 106, 71−73. (d) Maeda, T.; Würthner, F. Chem. Commun. 2015, 51, 7661−7664. (e) Das, R. J.; Mahata, K. Org. Lett. 2018, 20, 5027−5031. (15) (a) Hirose, T.; Higashiguchi, K.; Matsuda, K. Chem. - Asian J. 2011, 6, 1057−1063. (b) Cheng, Z.; Xing, F.; Bai, Y.-L.; Zhao, Y.; Zhu, S.; Li, M. Asian J. Org. Chem. 2017, 6, 1612−1619. (c) Lin, H.A.; Sato, Y.; Segawa, Y.; Nishihara, T.; Sugimoto, N.; Scott, L. T.; Higashiyama, T.; Itami, K. Angew. Chem., Int. Ed. 2018, 57, 2874− 2878. (d) Motoyanagi, J.; Yamamoto, Y.; Saeki, A.; Alam, M. A.; Kimoto, A.; Kosaka, A.; Fukushima, T.; Seki, S.; Tagawa, S.; Aida, T. Chem. - Asian J. 2009, 4, 876−880. (16) See the Supporting Information for details. (17) Zhang, X.-T.; Fan, L.-N.; Zhao, X.; Sun, D.; Li, D.-C.; Dou, J.M. CrystEngComm 2012, 14, 2053−2061. (18) The vibrational structure of the electronic spectra of 1-Na is strongly affected by the solvent (Figure S11). (19) All of the electronic spectra were unchanged after the solutions were allowed to stand at room temperature for 24 h. However, a precipitate was formed for the solution at pH 1.1 after a week of standing, presumably as a result of slow hydrolysis of ester groups under acidic aqueous conditions. (20) 1H NMR spectra of 1-Na with excess amounts of HCl showed broad peaks, indicating a slow equilibrium of protonation/ deprotonation on the NMR time scale. (21) Po, H. N.; Senozan, N. M. J. Chem. Educ. 2001, 78, 1499−1503. (22) Fickling, M. M.; Fischer, A.; Mann, B. R.; Packer, J.; Vaughan, J. J. Am. Chem. Soc. 1959, 81, 4226−4230. (23) (a) Ito, A.; Muratake, H.; Shudo, K. J. Org. Chem. 2013, 78, 5470−5475. (b) Yui, N. Sci. Rep. Tohoku Univ., First Ser. Phys., Chem., Astron. 1956, 40, 102−113. (24) Matsui, T.; Baba, T.; Kamiya, K.; Shigeta, Y. Phys. Chem. Chem. Phys. 2012, 14, 4181−4187. (25) This peak shift might be due to partial evaporation of HCl from the exposed liquid before measurements in air (Figure S14a). (26) In the case where H2O was used instead of 6 M aqueous HCl as a vapor source, no spectral changes were observed for a liquid film of 1-Na, indicating a poor influence of humidity under the current experimental conditions at least, even though TEG chains are known to be hydrophilic (Figures S17 and S18).16 (27) In the case of using AcOH instead of HCl as a vapor, the color of the liquid also turned from green to blue but immediately returned to green under air before measurements, reflecting the large pKa value of AcOH compared to HCl. (28) For preliminary results, we confirmed that the lithium salt 1-Li showed similar chromic behaviors as 1-Na. (29) (a) Van der Schueren, L.; De Clerck, K. Color. Technol. 2012, 128, 82−90. (b) Trovato, V.; Colleoni, C.; Castellano, A.; Plutino, M. R. J. Sol-Gel Sci. Technol. 2018, 87, 27−40. (30) (a) Potash, R. A.; McKone, J. R.; Conte, S.; Abruña, H. D. J. Electrochem. Soc. 2016, 163, A338−A344. (b) Hagemann, T.; Winsberg, J.; Häupler, B.; Janoschka, T.; Gruber, J. J.; Wild, A.; Schubert, U. S. NPG Asia Mater. 2017, 9, No. e340.

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DOI: 10.1021/acs.orglett.9b00468 Org. Lett. XXXX, XXX, XXX−XXX