Chapter 2. Design of organic single-ion conductor based on relaxation dynamics
2.1. Ion relaxation dynamics of G-quadruplex for single-ion conduction
2.1.2. Experiment
Monomer synthesis. All chemicals and reagents were purchased from Sigma-Aldrich, Alfa Aesar Chemical Company, and Tokyo Chemical Industry Co., Ltd. and used without any further purification.
All solvents are ACS and anhydrous grade by distillation. The 1H and 13C NMR spectra were recorded on a Bruker AVANCE III HD 400 spectrometer by using deuterated chloroform (CDCl3) as solvent and tetramethylsilane as an internal standard, respectively. Elementary analyses were carried out with a Flash 2000 element analyzer (Thermo Scientific, Netherlands) and MALDI-TOF MS spectra were checked by Ultraflex III (Bruker, Germany). The synthesis route and yield for each step to synthesize the intermediates and target monomer are presented in Figure 2-2. The results of 1H and 13C NMR spectroscopy and elemental analysis of the monomer are provided here to verify the right structure. The monomer 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 12.18 (s, 1H), 7.34 (d, J = 3.9 Hz, 1H), 7.21 (d, J = 4.0 Hz, 1H), 7.18 (d, J = 3.8 Hz, 1H), 7.15 (d, J = 3.7 Hz, 1H), 6.79 (s, 2H), 6.17 (s, 2H), 4.23 (t, J
= 7.0 Hz, 2H), 4.04 (t, J = 6.4 Hz, 4H), 3.99 (t, J = 6.4 Hz, 2H), 1.85 – 1.74 (m, 8H), 1.51 – 1.46 (m, 6H), 1.27 (m, 58H), 0.90 – 0.86 (m, 12H). 13C NMR (100 MHz, CDCl3): δ (ppm) 158.90, 153.86, 153.64, 153.42, 153.06, 139.33, 139.16, 138.61, 135.33, 130.92, 129.22, 127.11, 125.55, 123.87, 123.76, 116.95, 104.80, 73.75, 69.43, 43.66, 32.11, 32.09, 31.91, 30.54, 29.94, 29.92, 29.89, 29.87, 29.84, 29.81, 29.64, 29.57, 29.54, 29.33, 29.21, 26.78, 26.32, 22.85, 22.78, 14.28, 14.25. MALDI-TOF MS (m/z) Calcd:
1055.73. Found: 1056.66 (MH+). Anal. Calcd. for C63H101N5O4S2: C, 71.61; H, 9.63; N, 6.63, O, 6.06;
S, 6.07; Found: C, 71.54; H, 9.60; N, 6.64, O, 6.10; S, 5.93.
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Figure 2-2. Synthesis routes and chemical structures of monomer and the intermediates.
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Self-assembly of LiGQ. The LiGQ was obtained as follows: the monomer (50 mg) was dissolved in THF (0.5 mL) at room temperature in a capped 5 mL vial, yielding a yellow solution. Into the monomer solution, the lithium trifluoromethanesulfonate (LiOtf, monomer:salt = 4:1, (mol/mol)) was added and stirred for overnight. All solution preparation was conducted in an Ar-filled glove box. The vial was placed in a 30 mL vial which contains acetone (3 mL). Vapor diffusion occurred in this vial for 48 h until the LiGQ precipitates. The dark orange solid powders were obtained after the sequential washing with water and acetone.
2D-grazing incidence X-ray diffraction (2D-GIXD) measurements were performed at PLS-II 3C SAXS I and PLS-II 9A U-SAXS beamline of Pohang Accelerator Laboratory in Korea. The X-rays coming from the in-vacuum undulator (IVU) were monochromated (Ek = 9.81 keV and λ = 1.26 Å for 3C beamlines and Ek = 11.02 keV and λ = 1.13 Å for 9A beamlines) using a Si (111) double crystal monochromator. The incidence angle of X-rays for 2D-GIXD was adjusted in the range of 0.12o, which was close to the critical angle of samples. The 2D-GIXD patterns were recorded by a 2D CCD detector (SX165, Rayonix, USA). The raw data were processed and analyzed using Igor-Pro software package.
Synchrotron wide-angle X-ray diffraction (WAXD) measurements were conducted at PLS-II 6D UNIST-PAL beamline of Pohang Accelerator Laboratory in Korea. The X-rays coming from the IVU were monochromated (Ek = 11.6 keV and λ = 1.069 Å ). The WAXD patterns were recorded by a 2D CCD detector (SX165, Rayonix, USA). The raw data were processed and analyzed using Igor-Pro software package.
Magic angle spinning 1H and 7Li nuclear magnetic resonance (MAS 1H and 7Li NMR) experiments were performed with an Agilent VNMRS 600 MHz narrow NMR spectrometer and 1.6 mm HXY Fast MAS T3 probe. The 1H NMR spectra were recorded using one-pulse (or DP, direct polarization) with recycle delay of 5 s, 90 degree pulse width of 6 us, acquisition time of 0.0344 s, 4096 complex points under 35 kHz spinning rate to the resonance frequency of 599.83 MHz. The total number of transients was 16. 7Li NMR spectra were recorded using one-pulse (or DP, direct polarization) with recycle delay of 45 s, 90 degrees pulse width of 2 us, acquisition time of 0.3539 s, 16384 complex points under 35 kHz spinning rate to the resonance frequency of 233.12 MHz. The total number of transients was 512.
The chemical shift is referenced to a hexamethylbenzene at 1H (2.2 ppm) and 1.0 M aqueous LiCl solution at 7Li (0 ppm).
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Transmission electron microscopy and scanning electron microscopy (SEM and TEM) measurements were conducted using a Hitachi S-4800 field-emission SEM and a JEOL JEM-2100F TEM for morphological analyses.
Cyclic voltammetry (CV) was conducted with a Stainless steel (Sus)|LiGQ|Li asymmetric cell operated under a sweep rate of 1 mV s-1 in a voltage range of -0.5 ~ 4.0 V (vs. Li/Li+) at room temperature using a Biologic VSP classic potentiostat. The LiGQ self-standing pellet was prepared using a cold-pressing method in a glove box. The obtained pellet was then soaked in fluoroethylene carbonate (FEC) for 24 h prior to the electrochemical characterizations, in which FEC was used to mitigate grain boundary resistance of LiGQ domains that could be generated during the pellet fabrication. The FEC content in the pellet was estimated to be 3.1 vol%.
Chronoamperometry (CA) was carried out at 100 mV bias using blocking (Sus|LiGQ|Sus) and non- blocking (Li|LiGQ|Li) cells.
Ion conductivity (σ) was recorded with a Sus|LiGQ|Sus symmetric cell based on an electrochemical impedance spectroscopy analysis at a frequency range 10-2 to 106 Hz with an applied amplitude of 10 mV. The σ was determined by following equation:
𝜎 = 𝑙 𝑅𝐴
where l is the pellet thickness, R is the resistance, and A is the area in contact with the electrodes. The activation energy (Ea) was determined from the slope of the T-σ plot (Arrhenius plot).
Li+ transference number (tLi+) was evaluated using a potentiostatic polarization method.27 The DC current flowing through the Li|LiGQ|Li symmetric cell and the AC impedance (EIS) of the cell before and after polarization were measured to determine the tLi+ value of LiGQ according to following equation:
𝑡𝐿𝑖+= 𝐼𝑠𝑠(∆𝑉 − 𝐼𝑜𝑅𝑜) 𝐼𝑜(∆𝑉 − 𝐼𝑠𝑠𝑅𝑠𝑠)
where ISS is the steady-state current, I0 is the initial current, ∆V is the applied potential, R0 and RSS are the interfacial resistances before and after the polarization, respectively.
6Li symmetric cell test was conducted with the 6Li|LiGQ|6Li symmetric cell under a current density of 5 μA cm-2 for 5 min per cycle at room temperature. 6Li chunk was purchased from Sigma-Aldrich and used after cutting out the oxidized shell.
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Local electrochemical impedance spectroscopy (LEIS) was performed using a Biologic M470 scanning electrochemical workstation connected with a LEIS scanning probe, μTriCell beaker-type analytic cell filled with a FEC, and SP-300 potentiostat at a frequency 100 to 106 Hz with an applied amplitude of 100 mV.
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