Command Surface of Self-organizing Structures by Radical Polymers with. Cooperative Redox Reactivity

Transcription

1 Supporting Information Command Surface of Self-organizing Structures by Radical Polymers with Cooperative Redox Reactivity Kan Sato, Takahiro Mizuma, Hiroyuki Nishide*, and Kenichi Oyaizu* Department of Applied Chemistry, Waseda University, Tokyo , Japan. 1. Experimental section... 2 (1) Materials... 2 (2) Fabrication of charge storage devices Ionic conductivity of liquid crystal electrolytes Alignment of liquid crystals by redox-active polymers Characteristics of charge storage devices Determination of apparent rate constants... 9 (1) Impedance spectroscopy (2) Chronoamperometry References S1

2 1. Experimental section (1) Materials PTGE, 1 chemically oxidized PTGE, 2 polyviologen, 3 LC1, 4 and LC2 5 were synthesized according to previous reports. The polyimide layer was formed after the condensation reaction of pyromellitic dianhydride and 4,4 -oxydianiline. 4 LC3 was obtained from the Kanto Chemical Co. EMIM TCB was purchased from Merck. Other chemicals were purchased from Tokyo Kasei Co., Sigma-Aldrich Japan, or Kanto Chemical Co, and used as received. To prepare liquid crystal electrolytes, 4 mol % lithium trifluoromethanesulfonate (LiTf, Tokyo Kasei Co.), 4 mol % EMIM TCB, and 1.4 mol % EMIM TCB were added to LC1-3, respectively. LiTf was reported to exhibit high anisotropic ionic conductivity when mixed with LC1. 4 Similarly, EMIM TCB gave excellent miscibility and conductivity when added to aromatic liquid crystals, 3 and was selected as the electrolyte salt for the mixture of LC2-3. The concentration of the electrolyte salt in LC3 was reduced to 1.4 mol % because the excess addition of EMIM TCB induced an unfavorable decrease in the phase transition temperature 3 (i.e., less than room temperature). The liquid crystal electrolytes exhibited transition (smectic-isotropic or nematic) temperatures of 65 C, 71 C, and 34 C for LC1-3, respectively (Figure S1). Figure S1 DSC curves for liquid crystal electrolytes measured by a Q200 instrument (TA Instruments). (a) 4 mol % LiTf in LC1. (b) 4 mol % EMIM TCB in LC2. (c) 1.4 mol % EMIM TCB in LC3. Scan rate, 10 C/min. S2

3 (2) Fabrication of charge storage devices PTGE solution (0.5 wt %) in chloroform was spin-coated on ITO substrates at 1500 rpm to obtain the cathodes. The electrodes were vacuum dried. The electrode thicknesses were nm as measured by a KLA Tencor profilometer. Viologen electrodes were prepared according to a previous report. 3 All cells were sealed under an ambient atmosphere using conventional hot-melt adhesive with a thickness of 15 µm (Figure S2). The electrode area was set to 0.5 cm 2 for AC impedance measurement. Electrochemical responses were measured using a conventional potentiostat (PGSTAT 128N, AUTOLAB) at room temperature unless stated otherwise. For the measurements of ionic conductivity, the standard aqueous KCl solution was used for calibration. Polarized optical microscopy (POM) was performed using a BX53 OLYMPUS system using a conventional hot stage. UV-Vis spectra were measured by a USB4000-UV-VIS Ocean Optics spectrometer. Before measurements, the cells were thermally annealed at the temperature of the isotropic or nematic phases of the LC electrolytes, and cooled at a rate of about 50 C/min. Chemical switching of the liquid crystal was conducted by applying the voltages to the cell at the temperature of isotropic or nematic phases, and cooling it in the same way as the annealing process. Mechanical switching was done by pushing the cell by fingers several times at room temperature (i.e., shear alignment of liquid crystals to the homeotropic direction 6 ). (a) (b) LC1 (c) LC2 (d) LC3 Figure S2 (a) Structure of the cell. (b d) UV-vis spectra of charge storage devices measured in the charged and discharged states. S3

4 2. Ionic conductivity of liquid crystal electrolytes For ionic conductivity measurement, liquid crystal electrolytes LC1-3 were sandwiched between polyimide-coated or bare ITO substrates to align the liquid crystals in polydomainal planar or homeotropic directions, respectively. All the POM images showed that alignment was controlled by the surfaces of the substrates (Figure S3). Ionic conductivity was estimated via AC impedance measurement for each category of alignment (Figure S4). A series connection of RC components was used to simulate the equivalent circuit. [Planar] [Planar] [Planar] [Homeotropic] [Homeotropic] [Homeotropic] Figure S3 POM images for LC1-3 sandwiched between polyimide (polydomainal planar alignment) or ITO substrates (homeotropic alignment). Insets: Conoscope images. (a) Planar (b) Homeotropic Figure S4 Nyquist plots for LC1-3 with polydomainal planar and homeotropic alignment (10 6 to ca Hz). S4

5 3. Alignment of liquid crystals by redox-active polymers Liquid crystal electrolytes were sandwiched between PTGE layers in a series of oxidation levels (Figure S5). The mixing ratio of normal and chemically oxidized PTGE was adjusted to change the oxidation level. Oxidation level (%) LC1 LC2 LC3 Figure S5 POM images for LC1-3 sandwiched between PTGE layers in a series of oxidation states. Insets: Conoscope images. S5

6 Polyviologen layers were revealed to induce homeotropic alignment of LC1-3 due to the dication (or cation) states of the redox sites (Figure S6). Therefore, the liquid crystals may have always been vertically aligned near the viologen electrode in the electrochromic cells. LC1 LC2 LC3 Figure S6 POM images for LC1-3 sandwiched between polyviologen layers. Insets: Conoscope images. S6

7 4. Characteristics of charge storage devices LC1-3 were aligned homeotropically by chemical and mechanical switching (Figure S7). In the case of chemical switching, defects in liquid crystal alignment due to non-uniformity of electrode geometry, chemical reactions, and other factors are more likely to occur. In contrast, mechanical switching gave a more uniform vertical orientation over a wider area and gave a larger change in electrolyte resistance for LC1. Therefore, unless stated otherwise in the following discussion, LC1-2 were vertically aligned by mechanical switch and LC3 was vertically aligned by chemical switch. [Planar] [Planar] [Planar] [Homeotropic] (Chemical) [Homeotropic] (Chemical) [Homeotropic] (Chemical) (No capability for homeotropic alignment) [Homeotropic] (Mechanical) [Homeotropic] (Mechanical) [Homeotropic] (Mechanical) (No capability for homeotropic alignment) Figure S7 Nyquist plots for charge storage devices at charged and discharged states (10 6 to ca Hz). S7

8 (a) LC 1 (b) LC2 (c) LC3 Figure S8 Electrochromic characteristics of the charge storage devices (absorbance measured at 530 nm). Figure S9 Open circuit voltage and absorbance (530 nm) of the devices after constant potential charging. Just after coloring 3 hours later Shown in Figure 2(d) (a) LC1 (b) LC2 Figure S10 Photographs of electrochromic devices to compare self-discharging behaviors. S8

9 5. Determination of apparent rate constants The apparent rate constants for charge diffusion, D app, and heterogeneous charge transfer, k 0,app, of PTGE were estimated by subjecting the fabricated cells to both electrochemical impedance spectroscopy and chronoamperometry (Table S1). The electron self-exchange rate constant, k ex.app, was estimated using the Dahms-Ruff equation (D app = k ex.app δ 2 C*/6, where δ is the site distance and C* is the concentration of redox species in PTGE). 7,8 Although the measurements were not conducted in a three-electrode system (e.g., Ag/Ag + as a reference and platinum as a counter electrode), where a more accurate estimation could be expected, the data estimated by the two methods were similar and suggested the evaluation was valid, at least semi-quantitatively. A three-electrode configuration is inappropriate in this particular study because the use of additional reference and counter electrodes would disturb the order of the liquid crystals. The constants obtained for LC1-3 were about 10 3 times lower than those for conventional electrolytes such as acetonitrile solution. 7 The high viscosity of the liquid crystal electrolytes lowered the rate constants. 9 The greater rate constant of LC3 compared with those of LC1-2 can also be explained by the viscosity of the electrolytes (i.e., transition temperature of LC3 was 34 C, whereas that of LC1-2 was ~70 C). Table S1 Estimated kinetic parameters for charge storage devices Method LC1 LC2 LC3 Planar Ionic conductivity EIS Homeotropic (10-4 S/cm) Ratio Planar Heterogeneous EIS Homeotropic electron-transfer rate Ratio constant Planar k 0,app (10-8 cm/s) Chronoamperometry Homeotropic Ratio Planar EIS Homeotropic Diffusion coefficient Ratio D app (10-14 cm 2 /s) Planar Chronoamperometry Homeotropic Ratio Planar EIS Homeotropic Electron self-exchange Ratio rate constant Planar k ex,app (1/M s) Chronoamperometry Homeotropic Ratio S9

10 (1) Impedance spectroscopy Electrochemical impedance spectra were measured at a series of DC bias voltages for each device and alignment (Figure S11). The spectra were analyzed using Randles circuit with a constant phase element. 10 An additional RC circuit was connected in series in the case of homeotropic alignment owing to the emergence of semicircles that originated from electrolyte resistance. The diameters of the capacitive semicircles corresponding to heterogeneous charge transfer resistance, R ct, changed along with the applied voltage of E E 1/2. The exchange current density, i 0, was calculated from R ct and equation (1) 10 (Figure S12). Fitting of the experimental results with equations (2) and (3) led to the determination of k 0,app. To evaluate D app, the diffusive response for low frequency at a voltage of E = E 1/2 was analyzed by Warburg impedance. 11 [Planar] [Planar] [Planar] [Homeotropic] [Homeotropic] [Homeotropic] Figure S11 Dependence of charge transfer resistance, R ct, on applied DC voltage, E (ca Hz). S10

11 (1), (2) (3) (R: gas constant; T: temperature of the system; A: electrode surface area; n: number of moles of electrons; F: Faraday constant; k 0,app : rate constant for heterogeneous electron transfer; C O : concentration of oxidized species; C R : concentration of reduced species; α: transfer coefficient) Figure S12 Dependence of exchange current density, i 0, on applied DC potential, E with polydomainal planar (red line) and homeotropic (blue line) alignment. Dashed lines show the fitted results. S11

12 (2) Chronoamperometry To estimate the diffusion coefficient, the relationship between current density i and time t during chronoamperometry was analyzed by the Cottrell equation (6) assuming that semi-infinite diffusion occurred at the early stage of electrolysis (Figure S13). 10,12 (4) (i: current density; C*: initial concentration of reduced species; t: time.) Figure S13 Cottrell plots for the charge storage devices after applying a potential pulse from 0 to 1.5 V with polydomainal planar (red line) and homeotropic (blue line) alignment. Dashed lines show the experimental results. The heterogeneous rate constant was evaluated by analyzing the initial current density, i t=0, during measurements at a series of applied voltages and using equations (5) and (6). 10 (5) ln, ln, / (6) Figure S14 Dependence of k f on applied DC voltage. S12

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