Chemistry
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An Electrochemical Cholesteric Liquid Crystalline Device for Quick and Low-Voltage Color Modulation
Summary February 27th, 2019
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.
Transcript
Cholesteric liquid crystals are known to have bright reaction colors, and the fast color modulation is important for the development of next generation of reflective displays. Our method enables the fastest color modulation with the lowest operation voltage ever reported for cholesteric liquid crystals. The key is FcD.
A chiral dopant with redox responsive helical twisting power. Demonstrating the procedure will be Shoichi Tokunaga, a postdoc, and Mengyan Zeng, a graduate student from our laboratory. To begin preparing the cholesteric liquid crystal mixture, place 84.6 milligrams of 5OCB and 5.922 milligrams of FcD in a 10 milliliter glass vial.
In a separate vial, dissolve 12.9 milligrams of EMIm-OTf in 10 milliliters of dichloromethane. Transfer 2.1 milliliters of the EMIm-OTf solution to the liquid crystal mixture, and mix the components well by gently shaking the vial. Then, cover the vial with aluminum foil, and punch several holes in the foil with a needle.
Heat the solution at 80 degrees celsius for one hour to evaporate most of the DCM. Put the vial under vacuum, and continue heating it for another hour to remove the remaining DCM to obtain a clear, orange liquid crystal mixture. Next, to begin preparing the ITO glass, cut a 10 by 10 millimeter piece of glass patterned with ITO in the desired shape, and a 10 by 12 millimeter piece of standard ITO glass.
Place both pieces of ITO glass in the diluted surfactant without letting them touch. Combine 60 milliliters of an alkaline surfactant solution, and 240 milliliters of ultra pure water in a 500 milliliter glass container. Sonicate the plates in dilute alkaline surfactant for 30 minutes.
Then, decant the surfactant, and rinse the plates with 200 milliliter portions of ultra pure water. Perform this rinsing procedure three times in total. Refill the container with 300 milliliters of ultra pure water.
Sonicate the plates for 20 minutes. And decant the water. Wash the plates in ultra pure water in this way three times in total.
Finally, dry the clean plates with a stream of nitrogen gas and store them in clean, disposable petri dishes. Keep the dishes in a clean desiccator. To begin preparing the cell, sonicate a 0.7 percent by weigh dispersion of PEDOT in nitromethane for 60 minutes to ensure that the polymer is well dispersed.
Then, fix the clean standard ITO glass on a spin coater, and remove dust from the ITO coated surface with a nitrogen blow gun. Carefully apply 50 microliters of freshly sonicated PEDOT dispersion to the ITO surface and spin coat the plate at 1000 RPM for 60 seconds. Let the coated plate sit in ambient air for one hour afterwards.
Next, fix the patterned ITO plate in a rubbing machine equipped with a rayon cloth. Thoroughly rub the ITO patterned surface under a stream of nitrogen gas. Then, in a dust-free area, mix a drop of optical adhesive with a similarly sized quantity of 5 micrometer borosilicate glass beads.
Lay the PEDOT coated plate face up on the work space. Apply about 0.2 cubic millimeters of the adhesive mixture to each corner of one narrow side of the plate. Apply two more portions of adhesive eight millimeters away from that side to form an eight by 10 millimeter rectangle on the plate.
Place the patterned ITO plate face down on the adhesive with one edge aligned with the drops partway along the PEDOT coated plate so that the plates are offset by about two millimeters. Gently press down the corners of the cell to achieve a uniform gap between the plates as indicated by the disappearance of the fringe pattern in the cell. Eradiate the cell with 365 nanometer UV light for 20 seconds to set the adhesive.
Heat the cell on a hot plate at 100 degrees celsius for three hours to finish curing the adhesive. Lastly, connect conducting wires fitted with aligator clips to the exposed ITO surface on each plate by ultrasonic sautering. Use insulating tape to fix the wires of the glass cell to a microscope slide for easier handling later.
Heat the ITO glass, a small spatula, and the cholesteric liquid crystal mixture at 80 degrees celsius for 10 to 15 minutes. Then, quickly transfer a small amount of the hot cholesteric liquid crystal mixture to the gap between the plates with the heated spatula. The cell will fill by capillary action in about 60 seconds.
Once the cell is full, reduce the stage temperature to 37 degrees celsius and wait for the cell to stabilize at that temperature. Apply sheer force by gently pressing the center of the liquid crystal device to see the bright reflected color. Then, place the device on top of the hot stage under a digital optical microscope with the patterned side of the device facing the lens.
Connect the patterned and PEDOT coated plates to the positive and negative terminals of a poteniostat respectively. Configure the poteniostat to alternate between applying 1.5 volts for four seconds, and 0 volts for eight seconds. Observe and record the color change of the liquid crystal device with the digital microscope while cycling the voltage.
Next, set up a UV-vis spectrophotometer to scan transmittance from 800 to 300 nanometers. Put a small jack in the sample compartment to hold the hot stage. Place the liquid crystal device on a hot plate to keep warm and acquire a background measurement of the empty hot stage.
Then, load the device back into the hot stage and place it in the spectrometer with the patterned side facing the beam. Use a dark cloth to cover gaps between the sample chamber door and the wires. Wait a few minutes for the device to stabilize at 37 degrees celsius.
If necessary, adjust the position of the hot stage to maximize transmittance. Then, acquire an initial spectrum of transmittance through the device. After that, apply a 1.5 volts to the device for four seconds and immediately acquire a transmittance spectrum.
When the measurement finishes, apply 0 volts to the device for eight seconds and acquire another spectrum. Finally, record percent transmittance at 510 nanometers over time while cycling the voltage between 1.5 volts for four seconds and 0 volts for eight seconds five times. The cholesteric liquid crystal doped with a chiral ferrocene binaphtol complex, appeared bright blue and had a reflection band centered at 467 nanometers.
Applying 1.5 volts to an ITO electrode in contact with the liquid crystal solution, shifted the reflectance band to center on 485 nanometers. The oxidized cholesteric liquid crystal appeared bright cyan to green against the unoxidized blue surroundings. The reflectance band shifted back to 467 nanometers upon applying 0 volts to the electrode with a corresponding recovery of the blue reflection color.
Transmittance decreased modestly with repeated cycles because of increasing orientational disorder, but this was repaired by applying a sheer force to the device. The color change from blue to cyan occurred in 0.4 seconds, and the return change took 2.7 seconds. Alternately applying 1.5 volts for 0.5 seconds, and 0 volts for five seconds resulted in a blinking electrode.
When attempting the procedure, please take care when assembling the cell. The quality of the cell determines the quality of the display image of the device. You can identify the appropriate voltage range for operating the device using cyclic voltammetry.
X-ray for the electron spectroscopy can confirm that emergence of the oxidized part in the liquid crystalline material. By precisely tuning the chemical structure of the redox responsive chirol dopant and other components, we can develop new types of reflective displays, such as full color e-paper. Further investigation of the mechanism of our chirol dopant, can deepen the fundamental science of cholesteric liquid crystals where the molecular mechanism of chirol induction is still unclear.
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