Communication Cite This: J. Am. Chem. Soc. 2018, 140, 10946−10949
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Redox-Responsive Chiral Dopant for Quick Electrochemical Color Modulation of Cholesteric Liquid Crystal Shoichi Tokunaga,† Yoshimitsu Itoh,*,† Hiroyuki Tanaka,† Fumito Araoka,‡ and Takuzo Aida*,†,‡ †
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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S Supporting Information *
ABSTRACT: Here, we report the first redox-active chiral dopant FcD, which electrically alters its helical twisting power (HTP) for a cholesteric liquid crystalline (LC) medium and quickly changes the reflection color. FcD is composed of an axially chiral binaphthyl unit in conjunction with a redox-active ferrocene unit. A cholesteric LC phase of 4′-pentyloxy-4-cyanobiphenyl, doped with FcD (3.0 mol %), developed a blue reflection color. When nitrosyl tetrafluoroborate, a one-electron oxidant, was added to this cholesteric LC phase, FcD was oxidized to decrease its original HTP value by 13%, so that a green reflection color was developed. In the presence of a supporting electrolyte, the reflection color was electrochemically modulated using a sandwich-type glass cell with indium tin oxide electrodes. In quick response to the applied voltage of +1.5 V, the reflection color changed from blue to green within 0.4 s. When 0 V was applied, the reflection color returned to its original blue color. The FcD-doped cholesteric LC is characterized by its fastest electrochemical response and lowest operating voltage among those reported for electrically driven cholesteric LC devices.
Figure 1. Chemical structures of (a) ferrocene-appended chiral dopant Fc D and (b) its oxidized form FcD+. Schematic representations of (c) Fc D- and (d) FcD+-doped cholesteric LCs comprising 5OCB sandwiched between two glass substrates.
C
holesteric liquid crystals (LCs) are known to exhibit bright reflection colors,1 and have the great potential in various applications such as sensors2 and lasers,3 among others.4 Such LC-based materials are derived from nematic LCs by doping them with chiral molecules5 and adopt a helical geometry that selectively reflects the incident light of a wavelength corresponding to the helical pitch (P). Because the helical pitch (P, μm) is determined by the helical twisting power (HTP; βM, μm−1) of a chiral dopant added and its mole fraction (C) through the relationship P = 1/βMC, the wavelength of the reflection light (λ, μm) can be estimated by λ = nP = n/βMC (where n is the average refractive index of the liquid crystal). On the basis of this principle, chiral dopants that can respond to a variety of stimuli such as light,6 heat,7 magnetic field,8 electric field,9 and gas10 have been developed. Here, we report the first chiral dopant FcD (Figure 1a) with a redox-active ferrocene unit that can change its HTP (βM) value electrochemically. Recently, we reported an unprecedented type of chiral dopant that is ionic and allows for its host LC to change and even memorize the reflection color by electrophoretic deposition onto the electrode surface.9a This finding prompted © 2018 American Chemical Society
us to estimate the HTP (βM) values of a variety of ionic and nonionic chiral dopants and provided a general tendency that ionic dopants show lower HTP values than nonionic ones (Figure S3). Chiral dopant FcD (Figure 1a), designed on the basis of this notion, can quickly and reversibly change its HTP value in response to the in situ electrochemical oxidation and reduction of the ferrocene unit. In the presence of a supporting electrolyte, a cholesteric LC doped with FcD in a sandwich-type glass cell with indium tin oxide (ITO) electrodes altered its reflection color only within 0.4 s and recovered its original color in 2.7 s in response to the applied voltages of +1.5 and 0 V, respectively (Figure 3). Fc D (Figure 1a)11 consists of a binaphthyl motif, known to provide a high HTP value,12 and a ferrocene unit, known to be active electrochemically.13 Because ferrocene undergoes a reversible one-electron oxidation reaction to generate its Received: June 25, 2018 Published: August 2, 2018 10946
DOI: 10.1021/jacs.8b06323 J. Am. Chem. Soc. 2018, 140, 10946−10949
Communication
Journal of the American Chemical Society ferrocenium derivative, FcD can reversibly switch its ionicity between the nonionic and ionic states. A cyclic voltammogram (CV) of FcD in MeCN (Figure S6) showed a reversible redox activity with a half-wave potential (E1/2) of +0.27 V (vs Ag+/Ag). Furthermore, the titration of FcD in MeCN using a one-electron oxidant nitrosyl tetrafluoroborate (NOBF4)14 caused an increase in the peak intensity at 632 nm, indicating the formation of a ferrocenium ion15 (Figure S4a). A FcD-doped (3.1 mol %) cholesteric LC containing 3.0 mol % of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIm-OTf) as a supporting electrolyte (Figure S15) in 4′pentyloxy-4-cyanobiphenyl (5OCB) was employed for the electrochemical experiments. This mixture was introduced into a sandwich-type glass cell comprising two glass slides with patterned indium tin oxide (ITO) electrodes, one of which was coated with poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) doped with perchlorate16 (PEDOT+) as a redox couple (Figure 2).11 The reflection colors were observed
Figure 3. Photographs (a) and transmittance spectra (b) of the FcDdoped LC device containing 3.0 mol % of EMIm-OTf in 5OCB in its initial state (I), after the application of 1.5 V for 4 s (II), and subsequent application of 0 V for 8 s (III) at 37 °C. (c) Changes in the transmittance of the LC device at 510 nm upon switching the applied voltage between +1.5 and 0 V. (d) Details of the transmittance change of the LC device at 510 nm.
oxidation of FcD by accepting an electron to form PEDOT. When the applied voltage was varied between −1.0 V and +2.0 V, the LC device ([FcD] = 3.1 mol %, [EMIm-OTf] = 3.0 mol %), in its two-electrode CV, showed only one reversible oxidation wave at a half-wave potential (E1/2) of +0.61 V (Figure S10a). As calculated from the E1/2 values of FcD (+0.27 V vs Ag+/ Ag, Figure S6) and PEDOT+ (−0.28 V vs Ag+/Ag, Figure S8) in MeCN, the observed E1/2 value was consistent with the minimum voltage required for oxidizing FcD with PEDOT+ as a redox couple. We confirmed the formation of PEDOT by a decrease in transmittance at around 600 nm19 (green curve II in Figure 3b) at an applied voltage of +1.5 V. As expected, X-ray photoelectron (XP) spectroscopy of the electrically oxidized LC device showed the presence of Fe(III) and Fe(II) species (Figures S19 and S20).20,21 Thus, the electrical color modulation event includes the oxidation of FcD into FcD+. Quantitative analysis of the XP spectra in Figure S19 indicated the emergence of a stationary state with the FcD/FcD+ mole ratio of 72/28 under the applied voltage of +1.5 V. For the redox cycle to turn over, FcD+, generated on the anodic side as a consequence of the reduction of PEDOT+ into PEDOT, has to diffuse through the LC medium toward the cathodic side and be reduced into FcD (Figure 2). The emergence of the stationary state (Figure S19b) in regard to the conversion of FcD to FcD+, indicates that the diffusion of FcD+ (FcD) is the rate-determining step of the electrochemical cycle. A color modulation event similar to the above using the electrical cell also occurred when FcD was chemically oxidized with NOBF4 in the cholesteric LC medium. Thus, a CH2Cl2
Figure 2. Schematic representation and its photograph (inset) of the Fc D-doped cholesteric LC device containing EMIm-OTf in 5OCB. The illustration represents the mechanism of electrochemical modulation of the reflection color. The black dots at the corner of the device in the photograph (inset) are an optical adhesive containing a small amount of 5-μm glass beads used to fabricate the cell.
through the glass slide that was not coated with PEDOT+ (Figure 2). When a voltage of +1.5 V was applied to the LC device, its reflection color changed immediately from blue (467 nm, picture I in Figure 3a; blue curve I in Figure 3b) to green (485 nm, picture II in Figure 3a; green curve II in Figure 3b). On the other hand, the application of 0 V to the resulting LC device allowed for a quick recovery of the original blue color (picture III in Figure 3a; blue curve III in Figure 3b).17 A quantitative analysis based on the 90% transmittance change revealed that the forward and backward color changes were completed in only 0.4 and 2.7 s, respectively (Figure 3d). Considering the practical importance of electrically driven color display devices, it should be noted that this light-reflecting LC device is by far the fastest in response and lowest in operation voltage among those designed to be electrically driven.9,18 Also noteworthy, the color switching and recovery events, thus observed, could be repeated many times so long as the orientational disorder of the LC device was taken care of by applying a shear (Figures 3c and S12). In contrast, without PEDOT+ in the LC device, no color change took place (Figure S16). The role of PEDOT+ is to enhance the 10947
DOI: 10.1021/jacs.8b06323 J. Am. Chem. Soc. 2018, 140, 10946−10949
Communication
Journal of the American Chemical Society solution of NOBF4 was added to FcD-doped LC ([NOBF4] = [FcD] = 3.0 mol %), and the mixture was evaporated under reduced pressure. Then, the residue was introduced into a sandwich-type glass cell,11 whereupon its reflection color changed explicitly from blue (λ = 479 nm) to green (λ = 530 nm) (Figure 4). When [FcD] in 5OCB was changed from 3.0
As shown in Figure S21, we fabricated a prototype of 3 × 3 passive matrix display using the FcD-doped (3.1 mol %) cholesteric LC containing EMIm-OTf (3.0 mol %). When a pulsed voltage of +1.5 V was pixelwise applied to the 3 × 3 matrix in a designated order, the pixels blinked sequentially with a switching frequency of 10 Hz due to a temporal change in their reflection colors (Figure 5a, Movie S1). We also fabricated an
Figure 4. (a) Photographs and (b) transmission spectra of the FcD- and Fc + D -doped cholesteric LCs in a sandwich-type glass cell at 37 °C at varying concentrations of FcD and FcD+.
Figure 5. (a) Prototype of 3 × 3 passive matrix display and (b) an ITOpatterned cell, both containing FcD-doped (3.1 mol %) cholesteric LC in the presence of EMIM-OTf (3.0 mol %). When a pulsed voltage of +1.5 V was pixelwise applied to the 3 × 3 matrix in a designated order, the pixels blinked sequentially with a switching frequency of 10 Hz. The ITO-patterned cell can blink the figure of a star by switching the applied voltage between +1.5 and 0 V alternately.
mol % to 2.2, 2.5, 3.5, and 4.0 mol %, the center reflection wavelength of the transmittance spectrum, as expected, changed from 530 nm to 620, 550, 424, and
