Supramolecular Organic Radical Gels Formed with 2,2,6,6

Jun 21, 2017 - A supramolecular gelator, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-substituted cyclohexanediamine derivative, was synthesized, and i...
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Supramolecular Organic Radical Gels Formed with 2,2,6,6Tetramethylpiperidin-1-oxyl-Substituted Cyclohexanediamines: A Very Efficient Charge-Transporting and -Storable Soft Material Yoshito Sasada, Rieka Ichinoi, Kenichi Oyaizu, and Hiroyuki Nishide* Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan S Supporting Information *

ABSTRACT: A supramolecular gelator, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-substituted cyclohexanediamine derivative, was synthesized, and its excellent charge-transporting capability was explored. The gels with organic solvents and electrolytes, or with ionic liquids, were formed via reversible sol−gel phase transition at ca. 50 °C. The organogels displayed electrochemical redox responses at E1/2 = 0.72 V (vs Ag/AgCl) ascribed to the TEMPO moiety. Charge diffusion coefficient of the gel reached 3.3 × 10−7 cm2/s even in the quasi-solid state, which was comparable to those of the homogeneous solution (ca. 10−6). The high charge-transporting capability led to the tremendously large current density (a diffusion limited one) of ca. 1.0 mA/cm2 on a current collector and long distance for the charge-transporting beyond the organogel thickness of 50 μm. A half-cell of the organogel performed a plateau output voltage at the E1/2, very high rate, and almost quantitative charging−discharging, and it had cyclability without any additives such as conductive carbons and binder polymers.

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INTRODUCTION Redox-active organic molecules and their polymers or redox polymers have been widely studied, because of their less-toxic and safe properties, to be examined as a trigger, a mediator, and a functional body in electrochemical devices such as sensors,1,2 electrochromic devices,3,4 light-emitting electrochemical cells,5,6 dye-sensitized solar cells,7,8 and so on. To perform fast and reversible functions in the devices, nitroxide radicals such as TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) have been often utilized because of their high reactivity, reversibility, and robustness under ambient atmosphere in comparison with those of other organic redox molecules. 9−11 We have successfully extended the TEMPO radicals to their polymeric analogues, the polymers pendantly bearing the nitroxide radicals, (we called them “radical polymers”), such as poly(TEMPO-methacrylate) (PTMA)12,13 and poly(TEMPOsubstituted norbornene).14 The radical polymers displayed charge-transporting capability derived from their rapid electron self-exchange reaction between the redox sites.10,13 Boudouris et al. also reported electric conductivity of the PTMA thin film including the enhanced conductivity by adding a redox-active salt.15 We focused on and developed the radical polymers as the charge-transporting and reversibly storable site in the cathode of organic-based rechargeable devices or batteries.9,12 For example, rechargeable batteries fabricated with the radical polymer PTMA and vapor-phase-grown carbon fiber as a conductive additive as the cathode active material exhibited © 2017 American Chemical Society

high rate performance of a few minutes in fully charging− discharging and its 5000 cyclability.16 Charge-propagation or -transporting by the exchanging reaction in the redox polymers occurs when the distance between redox sites is close enough because the redox sites are immobilized on the polymer backbone and the physical diffusion of redox sites is prohibited in the redox polymers so that the charge propagation depends on the redox site concentration.17 Diffusion of counterions in the redox polymers also often influenced the rate of charge transporting.18 We discussed, for example, effect of the mobility of counterions in electrolytes on the charge transporting within the radical polymers, by using a charge diffusion coefficient in the PTMA gel swelled with the propylene carbonate electrolyte, and concluded that the charge-transporting capabilities in the radical polymers were also dominated with the mobility of counteranion besides the concentration of redox site.19 Polymer gel electrolytes have been developed for solidified electrolytes of lithium-ion batteries, where Li-ion electrolyte solutions held in the micrometer-scaled polymer network composed of highly polar polymers such as poly(vinylidene fluoride), poly(ethylene oxide), poly(acrylonitrile), and poly(methyl methacrylate).20 The polymer gels satisfied high ion conductivities (10−3−10−4 S/cm) even as quasi-solid electroReceived: April 13, 2017 Revised: June 21, 2017 Published: June 21, 2017 5942

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Chemistry of Materials Scheme 1. Synthetic Route of TEMPO-Substituted Cyclohexanediamine

lytes.21,22 However, the ion conductivities were still lower than those of electrolyte solutions due to high solution viscosity in the polymer gels. Supramolecules provide nanometer-scaled macromolecular structures through self-assembling of the component molecules with interactions such as a hydrogen bonding, molecular stacking, hydrophobic interaction, and van der Waals force.23−25 Supramolecular polymers have been extensively studied to form gels with solvents.26,27 The component molecules to form supramolecular gels are called “gelators”, which could solidify solvents with their small amount of content. The gels of electrolyte solutions have been characterized with their high ion conductivities.28,29 Among the supramolecular gelators, cyclohexanediamine derivatives30,31 are known to form a 3D network of supramolecular chains which contain or solidify a large amount of solvents including not only common organic solvents but also electrolytes, ionic liquids, and liquid crystals. TEMPO-substituted supramolecular gelators have been reported as a magnetically active gel which showed a paramagnetic behavior based on unpaired electrons of the TEMPO moiety.32 A TEMPO-substituted and redox-active gelator has been also reported;33 however, its electrochemical properties such as being charge-transporting and chargestorable have not been elucidated yet. In this paper, the charge-transporting supramolecular gelator was designed with the cyclohexanediamine derivative as a selfassembling moiety and a TEMPO as a charge-transporting site. 1,2-Cyclohexanediamine derivatives have been studied to exhibit excellent gelation ability for the polar organic solvents and the electrolyte solutions commonly used for electrochemical devices,29 and functional molecules could be substituted on the alkyl chain ends of the derivatives. The designed and synthesized gelator molecule satisfies the following four points: the cyclohexane skeleton for a stacking moiety, the diamine for a hydrogen-bonding moiety, the long alkyl chains for crystallization inhibition, and the TEMPO for charge-transporting function. Isomers of the gelator were also synthesized because a steric structure of gelators has often affected the molecular alignment. To elucidate superior chargetransporting performance of the supramolecular gels in comparison with those of previously reported redox-active TEMPO radical polymers, charge-transporting capability of the supramolecular gel was analyzed in terms of the diffusion coefficient of charges and bulk current density in and of the gel. Application of the supramolecular organic gels as a cathode-

active, soft organic material in rechargeable devices was also described. We anticipated that this TEMPO-substituted supramolecular gel could be a new class of charge-transporting and -storable soft organic materials applicable for a wide variety of electrochemical or wet-type devices such as rechargeable batteries and sensors (Scheme 1).

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EXPERIMENTAL SECTION

Synthesis of TEMPO-Gelators. 11-(4-Oxy-TEMPO)-undecanoic Acid (1). 11-Bromoundecanoic acid (15.4 g, 58.0 mmol) and tetrabutylammonium hydrogensulfate (980 mg, 2.9 mmol) were dissolved in 1.5 N NaOH (40 mL). 4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (10.0 g, 58.0 mmol) was added to the solution and stirred for 2 days at 80 °C. The resulting mixture was neutralized with 1 N HCl to pH 5−6 and extracted with CHCl3. The organic layer was washed with water and brine, successively, and dried over Na2SO4. After the solvent was removed under reduced pressure, the crude product was purified by silica gel column chromatography with diethyl ether/hexane (4/1 in v/v) as an eluent to yield 1 (5.97 g, 29%) as a pink solid. The melting point (powder) is 76 °C. 1H NMR for the quenched 1 (500 MHz, CDCl3 with pentafluorophenylhydrazine) δ = 3.65 (m, 1H, TEMPO-4−), 3.42 (m, 2H, −CH2O−), 2.30 (dt, J = 7.2 Hz, 2.9 Hz, 2H, −COCH2−), 1.1−2.1 (m, 32H). IR (cm−1): 2920, 2852, 1712, 1470, 1096, 939. FAB-MS (m/z): M+ 356.5. Found: 356.3. Anal. Calcd for C20H38NO4: C, 67.4; H, 10.7; N, 3.9. Found: C, 67.5; H, 11.1; N, 3.9. trans-N-Phthaloyl-N′-tetradecanoyl-1,2-cyclohexanediamine (3). To the CH2Cl2 (58 mL) solution of N-phthaloyl-1,2-cyclohexanediamine 2 (7.00 g, 28.7 mmol) and pyridine (15 mL) were added myristoyl chloride (15.5 mL, 57.4 mmol) dropwise. The reaction mixture was stirred for 3 h at 30 °C. To the resulting mixture was added 1 N HCl to pH 1, and the product was extracted with CHCl3. The organic layer was washed with water and brine, successively, and then dried over Na2SO4. After evaporating the solvent, hexane was added to precipitate 3 (8.97 g) as a white solid. 3 was used in the next reaction without further purification. 1H NMR (500 MHz, CDCl3) δ = 7.81 (m, 2H, ArH), 7.68 (m, 2H, ArH), 5.20 (d, J = 9.2 Hz, 1H, −NHCO−), 4.51 (m, 1H), 3.91 (m, 1H), 2.56 (m, 1H), 2.11 (m, 1H), 1.78−1.98 (m, 5H), 0.98−1.37 (m, 25H), 0.88 (t, J = 7.2 Hz, 3H, −CH3). IR (cm−1): 3380, 2921, 2852, 1704, 1663, 1470, 1399, 722. FAB-MS (m/z): M+ 454.6. Found: 455.2. trans-N-Tetradecanoyl-1,2-cyclohexanediamine (4). Hydrazine monohydrate (19.2 mL, 396 mmol) was added to the ethanol (180 mL) solution of compound 3 (9.00 g, 19.8 mmol). The reaction solution was stirred for 2 h at 70 °C. After precipitation of a white byproduct, water was added to the reaction mixture until the byproduct was completely dissolved. After continuing to add water, another white solid precipitated. The precipitate was collected by filtration, washed with water, and dried in vacuo to yield 4 (6.18 g) as a white solid. 4 was used in the next reaction without further 5943

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Chemistry of Materials purification. 1H NMR (500 MHz, CDCl3) δ = 5.39 (d, J = 8.6 Hz, 1H, −NHCO−), 3.52 (m, 1H), 2.36 (m, 1H), 2.19 (m, 2H), 1.97 (m, 2H), 1.06−1.37 (m, 26H), 0.88 (t, J = 7.2 Hz, 3H, −CH3). IR (cm−1): 3295, 3075, 2920, 2850, 1638, 1463. FAB-MS (m/z): M+ 324.5. Found: 325.1. trans-N-(4-TEMPO-oxyundecanoyl)-N′-tetradecanoyl-1,2-cyclohexanediamine (TCD). 1-(3-(Dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (886 mg, 4.62 mmol) and 4-dimethylaminopyridine (152 mg, 1.24 mmol) were added to the CHCl3 (30 mL) solution of 1 (1.1 g, 3.08 mmol) and 4 (1 g, 3.08 mmol). The reaction mixture was stirred overnight, and the resulting precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and the crude product was purified by silica gel column chromatography with CHCl3/diethyl ether (4/1 in v/v) as an eluent to yield TCD (1.06 g, 52%) as a pink solid. The melting point (powder) is 110 °C. 1 H NMR (500 MHz, CDCl 3 with pentafluorophenylhydrazine) δ = 5.98 (s, 2H, −NHCO−), 3.65 (br, 2H, cHx-1,2), 3.55 (m, 1H, TEMPO-4), 3.41 (t, J = 6.6 Hz, 2H, −CH2O−), 2.11 (m, 4H, −COCH2−), 1.91−2.00 (m, 4H, TEMPO3,5), 1.73 (br, 2H, −CH2CH2O−), 1.55 (m, 8H, cHx-3,4,5,6), 1.1−1.4 (m, 48H), 0.88 (t, J = 7.2 Hz, 3H, −CH3). IR (cm−1): 3285, 3079, 2922, 2852, 1637, 1547, 1468, 1103. FAB-MS (m/z): (M + H+) 664.1. Found: 664.8. Anal. Calcd for C40H76N3O4: C, 72.5; H, 11.5; N, 6.3. Found: C, 72.4; H, 11.3; N, 6.3. (1R,2R)-N-(4-TEMPO-oxyundecanoyl)-N′-tetradecanoyl-1,2-cyclohexanediamine (R-TCD) and (1S,2S)-N-(4-TEMPO-oxyundecanoyl)-N′-tetradecanoyl-1,2-cyclohexanediamine (S-TCD). R- and STCD were synthesized by the same method as that for the racemicTCD, starting from (1R,2R)- and (1S,2S)-trans-1,2-cyclohexanediamine. R-TCD. The melting point (powder) is 160 °C. 1H NMR (500 MHz, CDCl3 with pentafluorophenylhydrazine) δ = 5.92 (s, 2H, −NHCO−), 3.64 (br, 2H, cHx-1,2), 3.56 (m, 1H, TEMPO-4), 3.40 (t, J = 6.6 Hz, 2H, −CH2O−), 2.10 (m, 4H, −COCH2−), 1.95−2.02 (m, 4H, TEMPO-3,5), 1.73 (br, 2H, −CH2CH2O−), 1.56 (m, 8H, cHx3,4,5,6), 1.1−1.4 (m, 48H), 0.87 (t, J = 7.2 Hz, 3H, −CH3). IR (cm−1): 3283, 3081, 2923, 2852, 1636, 1545, 1468, 1102. FAB-MS (m/z): (M + H+) 664.1. Found: 664.6. Anal. Calcd for C40H76N3O4: C, 72.5; H, 11.5; N, 6.3. Found: C, 72.7; H, 11.2; N, 6.2. S-TCD. The melting point (powder) is 160 °C. 1H NMR (500 MHz, CDCl3 with pentafluorophenylhydrazine) δ = 5.93 (s, 2H, −NHCO−), 3.65 (br, 2H, cHx-1,2), 3.57 (m, 1H, TEMPO-4), 3.41 (t, J = 6.6 Hz, 2H, −CH2O−), 2.10 (m, 4H, −COCH2−), 1.95−2.03 (m, 4H, TEMPO-3,5), 1.74 (br, 2H, −CH2CH2O−), 1.56 (m, 8H, cHx3,4,5,6), 1.1−1.4 (m, 48H), 0.88 (t, J = 7.2 Hz, 3H, −CH3). IR (cm−1): 3285, 3081, 2924, 2852, 1636, 1546, 1467, 1102. FAB-MS (m/z): (M + H+) 664.1. Found: 664.5. Anal. Calcd for C40H76N3O4: C, 72.5; H, 11.5; N, 6.3. Found: C, 73.1; H, 11.8; N, 6.2. Other Materials. All starting materials, organic reagents, catalysts, electrolytes, and organic solvents were purchased from Tokyo Chemical Industry Co., Kanto Chemical Co., and Sigma-Aldrich, Japan, and were used without further purification. N-Phthaloyl-1,2cyclohexanediamine 2 was prepared according to the literature procedure.34 Preparation of the Supramolecular Gels. Supramolecular gels with organic solvents, or organogels, were prepared as follows: 10−20 mM TCD was dissolved completely in hot organic solvents. The clear solution was cooled on standing to room temperature to form gel states. The gels with ionic liquids were prepared as follows: 5−20 mM TCD (vs an ionic liquid) and an ionic liquid were dissolved in CHCl3. The CHCl3 was removed under reduced pressure to yield the ionic liquid gels. Cell Fabrication. Two glassy carbon substrates were bonded with a separator (15, 50, and 100 μm thickness). The cell sandwiched with substrates was sintered in a hot acetonitrile solution of 20 mM R-TCD with 0.5 M tetrabutylammonium perchlorate. The electrolyte penetrated into the sandwich cell under reduced pressure, and then the cell was taken out and cooled to room temperature. The sandwich cell was also used for measurement of a half-cell property.

Electrochemical Measurements. Electrochemical measurements of the organogels were performed with 0.5 M tetrabutylammonium perchlorate in acetonitrile as the electrolyte. A potentiostat system (BAS Inc. ALS760E) was used for the cyclic voltammetry, chronopotentiometry, and chronoamperometry. A platinum disk and coiled platinum wire were used as the working and counter electrodes, respectively, and the working potential was measured vs an Ag/AgCl reference electrode. Other Measurements. 1H and 13C NMR spectra were recorded on a JEOL ECX-500 spectrometer with chemical shifts downfield from tetramethylsilane as the internal standard. Mass spectra were obtained using a JMS-GCMATE II or a Bruker Daltonics Autoflex. Elemental analyses were performed using a PerkinElmer PE-2400 II and a Metrohm 645 multi-DOSIMAT. Two parallel analyses were performed for each sample. Electron spin resonance (ESR) spectra of the compounds were taken using a JEOL JES-TE200 ESR spectrometer with a 100 kHz field modulation. The radical concentration was estimated on the basis of the assumption of being paramagnetic at room temperature by integration of the ESR signal standardized with that of TEMPO solution. Infrared measurement was operated with a JASCO FT/IR-6100 spectrometer. The IR spectra of solid samples were obtained by a KBr method, and the spectra of the solution and gel states of TCD were obtained using a cell (MagCELL, JASCO Engineering Co., Ltd.) with CaF windows. Optical rotations were measured with a JASCO JW-2400 spectrometer. Melting points of the compounds and thermal properties of the TCD gels were recorded using different scanning calorimetry (Seiko Instruments Inc.) with alpha-alumina as a reference at the scan rate = 10 K/min. A xerogel of TCD for scanning electron microscopy (SEM) (HITACHI High-Tech S-3000N) was prepared as follows: 10 mM acetonitrile gel was immersed in water to replace the solvent from acetonitrile to water, and then water in the gel was removed by freeze-drying under reduced pressure.

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RESULTS AND DISCUSSION Only 10 mM (0.84 wt %) of TEMPO-substituted cyclohexanediamine derivatives, R-, S-, and racemic-TCD, efficiently gelled common organic solvents such as acetonitrile (AN), γbutyrolactone, and ethylene carbonate/diethyl carbonate mixture often used for electrochemical electrolytes (see Figure S1 of the Supporting Information). Sol−gel transition of the gels was reversible with melting and gelation temperatures given in Table S1 of the Supporting Information. This gelation ability of TCD was almost similar to those of cyclohexanediamine derivatives reported previously,30 and the cyclohexanediamine moiety could contribute to a supramolecular polymer chain formation to gelate the solvents. The SEM image for the corresponding xerogel prepared from the TCD gel indicated a 3D network probably composed of bundles of the supramolecular polymer chains (Figure 1).

Figure 1. SEM image of the xerogel prepared from the TCD/AN gel. 5944

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Chemistry of Materials FT-IR spectra of the solution and gel of TCD are given in Figure 2 to study a driving force to form the supramolecular

Figure 3. Cyclic voltammograms of the 20 mM TCD/AN gel with 0.5 M tetrabutylammonium perchlorate (solid) and of the 10 mM TCD/ EMImTFSI gel (dotted).

TCD of high concentration probably formed thick bundles of the supramolecular polymer which relatively reduced the active surface of the polymer bundle to result in low redox response or low counteranion mobility. Diffusion coefficient (D) of charge in the TEMPO redox reaction was estimated with the Cottrell plots obtained from chronoamperograms in the potential range of 0−1.0 V vs Ag/ AgCl (Figure 4). The slope of the Cottrell plots gave the D

Figure 2. IR spectra of TCD: the CHCl3 solution (solid), the solid compound (broken), and the AN gel (dashed).

polymer, where CHCl3 was used as the solution solvent because of its high solubility of TCD. The spectrum of the 20 mM CHCl3 solution showed CO stretching vibration at 1654 cm−1, which was shifted to the lower frequency of νC=O 1635 cm−1 for the 20 mM TCD/AN gel (as has been assigned on the similar gelator30). The latter for the gel was almost the same for the solid TCD sample. These results indicated that an intermolecular hydrogen bonding of TCD was one of the forces to form the supramolecular polymers in the gel. A broadened ESR signal of the gel in comparison with a sharply split signal of the homogeneous solution (see Figure S2 of the Supporting Information) supported the assertion that the TEMPO moiety existed at a high concentration in the gel state, as has been often described for the radical polymers.35 The TCD organogels were stable around room temperature. Isolated enantiomers of TCD, synthesized along with the optical activity of the starting (1R,2R)- and (1S,2S)-cyclohexanediamine (see Figure S3 of the Supporting Information), gave the AN gels whose thermal stabilities were ca. 10 °C higher than that of the racemic one (Table S1). This result suggested that the enantiomers favorably formed the molecular alignments with more efficient hydrogen bonding of the two equatorial NH and CO and stack of the cyclohexane sites, as has been discussed in the previous paper.30 The TCD also formed gels with organic solvents containing electrolytes such as tetrabutylammonium salts and also for ionic liquids (ILs) in similar to organogels. The TCD/electrolyte and TCD/IL gels were examined as redox-active supramolecules, and electrochemical measurements were carried out using R-TCD gels because of its high gelation ability. Both of the TCD organogels and the TCD/IL gels were redox active, which was ascribed to the TEMPO moiety of TCD. For example, cyclic voltammograms of the TCD/AN gel with 0.5 M tetrabutylammonium perchlorate displayed reversible redox responses at E1/2 = 0.72 V vs Ag/AgCl (Figure 3), with a broaden redox peak separation (ca. 100 mV) in comparison with the voltammogram of TEMPO solution. The TCD/IL gel, composed of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) as both the solvent and the supporting electrolyte, gave the reversible redox responses almost similar to that of the AN gel (Figure 3). The peak current density increased with the TCD concentration but was saturated at around a TCD concentration of 20 mM for the AN gel and 10 mM for the EMImTFSI gel. The

Figure 4. Chronoamperogram of the 20 mM TCD/AN gel with 0.5 M tetrabutylammonium perchlorate (solid) and the 10 mM EMImTFSI gel (dotted). Inset: Cottrell plots.

value using Cottrell equation (J = nFD1/2CT/(πt)1/2 where J, n, F, and CT are the current density, the number of electrons, the Faraday constant, and concentration of the redox site, respectively).36,37 The D values of the TCD/AN gel and TCD/EMImTFSI gel were estimated to be 3.3 and 2.2 × 10−7 cm2/s, respectively, which were larger than those of the previously reported ones for the layers of TEMPO-bearing polymers (ca. 10−9−10−10 cm2/s). Cells composed of the 20 mM TCD/AN gel and sandwiched with two grassy carbon substrates with 15 or 50 μm thickness were fabricated, to measure bulk current density using the cyclic voltammetry with four electrodes. In the voltammograms (Figure 5), the current was observed from the potential of E1/2 by applying a voltage to the gel and then reached to a diffusionlimited current density JLIM at the sufficient overpotential. The calculated JLIM value was estimated according to the previous literature38 using the equation39 JLIM = ω0nFDCT/d where d is the thickness. ω0 is a correction factor for an electrostatic coupling, which was estimated to be 1.50 for the TEMPO site. JLIM = 0.83 and 0.29 mA/cm2 for the gel with the thicknesses of 15 and 50 μm, respectively. These values are not inconsistent with the JLIM calculated with the diffusion coefficient of charge (D = 3.3 × 10−7 cm2/s) described above: 0.63 and 0.19 mA/ cm2 for the gel with the thicknesses of 15 and 50 μm, respectively. These results supported that the charge transporting in the TCD gel was based on the redox conduction in 5945

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Chemistry of Materials

Figure 5. Four-electrode cyclic voltammograms of the TCD/AN gel. The gel thickness: 15 (black) and 50 μm (red); scan rate: 1 mV/s.

Figure 7. Discharging curves of the 20 mM TCD/AN gels for the current densities of 10−50 C.

the supramolecular TCD similar to that of the TEMPO radical polymers. The diffusion limit of the TCD/AN gel was estimated by measuring the amount of active redox sites in the gel using chronopotentiometry (see Figure S4 of the Supporting Information). For example, the charging capacity was 21.4 mAh/g with the 100 μm thickness gel, which corresponded to 55% of the TEMPO moiety of the TCD gel. This suggested 55 μm for the distance limitation of effective charge-transporting. This charge-transporting distance was more than 100 times longer than those of the previously reported radical polymers; for example, all redox sites were available for the charge transporting for the lay thickness less than 260 nm in the poly(TEMPO-substituted norbornene) without any conductive additives.14 The results could promise that the supramolecular TCD gels work as an electric conductive purely organic material with the effective length up to ca. 50 μm and an electrode-active organic material even in the absence of conductive additives and binders. A charging−discharging performance of the TCD/AN gel was tested by chronopotentiometry using a half-cell (the gel thickness = 50 μm). The charging−discharging curves of the half-cell exhibited a plateau voltage at the E1/2 (0.72 V) (Figure 6). For the current densities from 10 to 50 C (1 C rate is

material even under a rapid charging−discharging current density.

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CONCLUSIONS The TEMPO-substituted cyclohexanediamine derivatives (TCD) formed their supramolecular bundles to yield organogels even after substitution of the bulky TEMPO moiety with organic solvents and ionic liquids. The gels exhibited very high charge-transporting capability with the current density of 1 mA/cm2 derived from rapid electron self-exchange reaction between the TEMPO moieties immobilized on the supramolecules. The gel was formed reversibly through sol−gel transition around 60 °C to be fulfilled even in microporous substrates and was conductive-additive free and transparent. The genuine organic and charge-transporting supramolecular gels could have a potential application in organic-based rechargeable devices, photovoltaic cells, and sensors.

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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.chemmater.7b01476. Gel image, ESR spectra, CD spectra, and an additional electrochemical result (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.N.) E-mail: [email protected]. ORCID

Kenichi Oyaizu: 0000-0002-8425-1063 Hiroyuki Nishide: 0000-0002-4036-4840 Notes

The authors declare no competing financial interest.

Figure 6. Charging−discharging curve of the 20 mM TCD/AN gel at 10 C. Inset: cycle performance.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (No. 24225003) from MEXT, Japan. Y.S. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University, from MEXT, Japan.

defined as the charging−discharging current which takes 1 h to charge and discharge the entire theoretical capacity) or from 28 to 134 μA/cm2, the specific capacity of the half-cell was 42 and 36 mAh/g for 10 and 50 C, which almost agreed with the molecular weight-based theoretical capacity (40.4 mAh/g) (Figure 7). The initial capacity was maintained over 80% after the 50 charging−discharging cycles at 10 C (inset of Figure 6). The half-cell performance indicated that the TCD gel was appropriate to store charge as an electrode-active organic

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b01476 Chem. Mater. 2017, 29, 5942−5947

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DOI: 10.1021/acs.chemmater.7b01476 Chem. Mater. 2017, 29, 5942−5947