An Electrochemical Cholesteric Liquid Crystalline Device for Quick and Low-Voltage Color Modulation

Summary

A protocol for the fabrication of a reflective cholesteric liquid crystalline display device containing a redox-responsive chiral dopant allowing quick and low-voltage operation is presented.

Abstract

We demonstrate a method for fabricating a prototype reflective display device that contains cholesteric liquid crystal (LC) as an active component. The cholesteric LC is composed of a nematic LC 4'-pentyloxy-4-cyanobiphenyl (5OCB), redox-responsive chiral dopant (^FcD), and a supporting electrolyte 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIm-OTf). The most important component is ^FcD. This molecule changes its helical twisting power (HTP) value in response to redox reactions. Therefore, in situ electrochemical redox reactions in the LC mixture allow for the device to change its reflection color in response to electrical stimuli. The LC mixture was introduced, by a capillary action, into a sandwich-type ITO 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 perchlorate (PEDOT⁺). Upon application of +1.5 V, the reflection color of the device changed from blue (467 nm) to green (485 nm) in 0.4 s. Subsequent application of 0 V made the device recover the original blue color in 2.7 s. This device is characterized by its fastest electrical response and lowest operating voltage among any previously reported cholesteric LC device. This device could pave the way for the development of next generation reflective displays with low energy consumption rates.

Introduction

Cholesteric liquid crystals (LCs) are known to exhibit bright reflection colors due to their internal helical molecular arrangements^1,2,3,4. The reflection wavelength λ is determined by the helical pitch P and the average refractive index n of the LC (λ = nP). Such LCs can be generated by doping chiral compounds (chiral dopants) to nematic LCs and its helical pitch is defined by the equation P = 1/β_MC, where β_M is the helical twisting pow....

Protocol

1. Preparation of the cholesteric LC mixture

1. Add 84.6 mg of 5OCB and 5.922 mg of ^FcD¹⁹ (3.1 mol% to 5OCB) into a clean 10 mL glass vial.
2. Add 12.9 mg of EMIm-OTf and 10 mL of dichloromethane (CH₂Cl₂) into a new clean 10 mL glass vial and mix well. Transfer 2.1 mL of the EMIm-OTf solution into the 5OCB- and ^FcD-containing glass vial. Gently shake the vial to let all th.......

Discussion

Upon application of +1.5 V to the top ITO electrode (Figure 1C), ^FcD undergoes an oxidation reaction to generate ^FcD⁺. As the helical twisting power of ^FcD⁺ (101 µm^-1, Figure 1B) is lower than that .......

Materials

Name	Company	Catalog Number	Comments
1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate, 98%	TCI	E0494
4-Cyano-4'-pentyloxybiphenyl, 98%	TCI	C1551
Diamond tipped glass cutter	AS ONE	6-539-05
Dichloromethane, 99.5%	KANTO CHEMICAL	10158-2B	HPLC grade
Differential Scanning Calorimeter	METTLER TOLEDO	DSC 1
Digital microscope	KEYENCE	VHX-5000
Extran MA01	Merck	107555
Fully ITO-coated glass plate			Costum order, Resistance: ~30Ω
Glass beads	Thermo Fisher Scientific	9005	5 ± 0.3 μm in diameter
Hot stage	INSTEC	mK1000
ITO-patterned glass plate			Costum order, Resistance: ~30Ω
Oil rotary vacuum pump	SATO VAC	TSW-150	Pressure: ~5 Pa
Optical adhesive	Noland	NOA81
Poly(3,4-ethylenedioxythiophene), bis-poly(ethyleneglycol), lauryl terminated	Sigma Aldrich	687316	0.7 wt% (dispersion in nitromethane)
Potentiostat	TOHO TECHNICAL RESEARCH	PS-08
Rubbing machine	EHC	MRJ-100S
Spectrophotometer	JASCO	V-670 UV/VIS/NIR
Spin coater	MIKASA	1H-D7
Ultrapure water	Merck	Milli-Q Integral 3
Ultrasonic bath	AS ONE	ASU-2	Power: 40 W
Ultrasonic soldering	KURODA TECHNO	SUNBONDER USM-IV
UV lamp	AS ONE	SLUV-4	Power: 4 W