The next generation of flexible displays and electronic paper could build themselves, thanks to the discovery of a way of getting the materials comprising them to self-assemble.
A compound known to switch from transparent to opaque when triggered by an electric current can be programmed to assemble itself, says Scott Jones from the University of New South Wales. The resulting arrays could form the pixels of an e-paper display.
Electroactive self-assembling hydrogels for flexible display technology
Scott Jones1, Kok Hou Wong1, Pall Thordarson2, Francois Ladouceur1
1School of Electrical Engineering & Telecommunications, University of New South Wales, Sydney, NSW, 2052, Australia
2School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia
The well-known redox-active molecule anthraquinone has been functionalised with glycine to produce an electroactive self-assembling molecular gel. Spectroelectrochemical experiments show a reversible variation in transmittance over the entire visible spectrum suggesting direct electrical control over the self-assembly process. This may have applications in flexible conformal displays.
The next generation displays aim to be flexible, light, require little energy to operate and support video playback. Such displays could exhibit the visual quality of printed paper but extend its capabilities into the high-performance colour video realm to which modern generations have become accustomed. This may herald a new technological age where many surfaces in the modern world, flat or curved, can display a moving picture.
One promising new avenue of research is the use of electroactive self-assembling hydrogels as materials that can reversibly change their transparency under electrical stimulus . Such a material, integrated into a flexible semi-conducting polymer base, may be a cheap, effective and reliable medium for mass producible conformal displays.
We consider herein self-assembling hydrogels (or molecular gels) that are materials composed of relatively small amphiphilic organic molecules .
Through intermolecular forces such as - stacking and hydrogen bonding they are capable of assembling into three-dimensional networks in an appropriate solvent system. This assembly is a process of physical alignment and involves no chemical reactions. Being merely a reorientation of the molecules, the transition is of low energy. This process is quite different from the more common polymer gel systems where polymer chains encapsulate a solvent through irreversible reactions .
Upon gelation the network of fibres, when of the correct spacing, scatter light and render the material opaque. This is the key mechanism for making passive ‘black and white’ pixels as a proof of principle for self-assembling molecular gel displays.
Appropriately functionalised hydrogels are electrically sensitive [4,5,6,7] and thus their self-assembling characteristic can be electrically controlled. A redox group functionality is one such source of electrical sensitivity. Anthraquinones, for instance, undergo well-known redox reactions and have previously been shown to function in electrochromic displays . This is through their ability to reversibly change colour as a result of their reduction/oxidation cycle.
Our research shows that such anthraquinone molecules can be functionalised to become capable of self-assembly and therefore act as a hydrogel. The initiation of the redox process can then alter the molecules and their environment and consequently induce or reverse gelation.
We present a novel robust high pH molecular gel system that demonstrates a potential for reversible variation in transmittance due to a redox process on the gelator molecules themselves.
Hydrogel concentration is set by weight of dry gelator crystals per volume of solvent (g/mL) and shown as a percentage (% w/v). MilliQ water is always used.
Initial gel formation is performed by the dissolution of gelator crystals in DMSO by gentle heating and the subsequent addition of an equal portion of water (0.1 M KCl as the electrolyte) to form a partial gel (6% w/v). A portion of 1 M NaOH is added equal to one fifth of the mixture’s previous volume. This causes a distinct orange/red colouration. The resulting mixture is then heated to produce a brilliant red/purple clear solution. Upon cooling a robust white/orange gel is formed.
Spectroelectrochemistry is performed with platinum working and counter electrodes with a reference electrode of Ag/AgCl.
A Cary 50 UV-Vis spectrometer was employed to record the visible spectrum. The wavelength range of 400 nm to 700 nm was scanned approximately every six seconds through a path-length of 1 mm as the electrical potential was applied.
III. RESULTS AND DISSCUSSION
Spectroelectrochemical methods allow the transparency of a material to visible light to be measured while the sample is influenced by an applied voltage. The UV-Vis beam is shone through the sample which is in contact with a platinum mesh working electrode to measure changes in transmittance caused directly by a chemical change at the surface.
For the spectroelectrochemical runs shown in figure 3 the warm gel sample is injected into the cell and quickly forms a gel due to cooling to room temperature. In this state a transmittance of approximately 0% is obtained over all visible wavelengths: this is due to scattering of light by the fibres throughout the gel material. The material is more opaque in the region of ~400–550 nm as absorption due to chromophore effects by the anthraquinone group dominate. It is due to this factor that 700 nm is the most convenient wavelength to probe self-assembly related effects.
After a suitably log rest period after gelation an initial potential of -1 V is applied for 900 seconds to reduce the gel molecules at the working electrode. A trend towards even more opacity is registered (insert to Figure 3 shows decrease from ~0.15% transmittance). At 900 seconds a 0 V potential is applied and a small visible effect is observed where the transmittance increases back to approximately 0.15%. At 1800 seconds the sample is reduced again by application of -1 V. Figure 3 shows a sequence of 900 second switches and a trend towards more effective switching is seen after repeated reversals. At 9000 s into the experiment the sample is seen to switch between approximately 5% and 60% transmittance. This trend to larger effects over time will be explored in future experiments and further results will be presented.
Figure 3. Electrochemical switching as indicated by absorbance changes at 700 nm. Periods of -1 V application (dark grey) cause decreases in transmittance and periods of 0 V application (light grey) cause transmittance to increase.
We have synthesised five novel molecules that are capable of undergoing two separate processes: a reversible redox reaction and a reversible self-assembly.
We have demonstrated a self-assembling molecular gel system that is capable of reversible transmittance changes controlled by redox reactions of the molecules. Variations of approximately 55% transmittance at 700 nm are visible. These results also indicate a potential for more effective switching under optimized conditions.
Future work will include the optimisation of electrochemical switching in electroactive self-assembling hydrogels. Of particular interest is the exact nature of their redox reactions and the related potential control of colour changes, the dynamics of the phase transition (switching speed), highest contrast achievable and the relative energies involved to achieve switching.
This project is supported by the International Science Linkages programme, an ARC Discovery Project (DP0985059) and DEST ISL GC120021. We also thank Dr. Joris T Meijer and Mr. Daniel C Goldstein for assistance with synthesis and spectroelectrochemistry, respectively.
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