Bio-inspired ultra-high energy efficiency bistable electronic billboard and reader

Bistable display has been a long-awaited goal due to its zero energy cost when maintaining colored or colorless state and electrochromic material has been highly considered as a potential way to achieve bistable display due to its simple structure and possible manipulation. However, it is extremely challenging with insurmountable technical barriers related to traditional electrochromic mechanisms. Herein a prototype for bistable electronic billboard and reader with high energy efficiency is demonstrated with excellent bistability (decay 7% in an hour), reversibility (104 cycles), coloration efficiency (430 cm2 C−1) and very short voltage stimulation time (2 ms) for color switching, which greatly outperforms current products. This is achieved by stabilization of redox molecule via intermolecular ion transfer to the supramolecular bonded colorant and further stabilization of the electrochromic molecules in semi-solid media. This promising approach for ultra-energy-efficient display will promote the development of switching molecules, devices and applications in various fields of driving/navigation/industry as display to save energy.


Supplementary Note 1. Light and thermal stability of Urea-N and Rh-M
In order to investigate the stability of the electrochromic materials under light irradiation, the UV-vis spectra of the mixture of Urea-N/Rh-M under room light and in dark were measured respectively. According to Supplementary Figure 34, the Urea-N/Rh-M mixture shows good stability on both ring-open and ring-close state of Rh-M when exposed to the room light, for the absorption spectra have no obvious change during 7 days, which indicates the good stability of the EC molecules under room light.
In order to test the thermal stability of the electrochromic materials, the UV-vis absorption spectra of both ring-open state and ring-close state of Rh-M + Urea-N, and Rh-M with 1 eq CF3COOH were measured under different temperature. The spectra in Supplementary Figure 36 showed that the absorption shows little change under different temperature from 25 o C to 75 o C, which indicates a good thermal stability of our materials at both ring-open and ring-close state, and the thermal stability provides the potential for future application to some extent.

Supplementary Note 2. Electrofluorochromic performance in liquid devices
For small organic electrochromic materials, the coloration efficiency, η, is defined by the change of injected charges per unit area (ΔQ) and the optical absorbance change (ΔA). And the η for the liquid device is about 150 cm 2 C -1 , as shown in Supplementary Figure 5d. More importantly, electrofluorochromic switching can be repeated 10000 times with no sign of degradation by consecutive on/off switching cycles (Supplementary Figure 6), which indicates that this device is extremely durable.

Supplementary Note 3. Optimum parameters of the solid device
Here we propose to use ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM]PF6) in PMMA for substituting both functions of electrolyte and plasticizer in our device. As a result, excellent conductivity (3.5×10 -6 S cm -1 , as shown in Supplementary Figure Table 3 Figure 9c, blue) can be read, the ratio of 60% and 80% exhibit good stability on colored state and short coloring time, which is suitable for bistable electrochromic device. However, the intensity is too weak when the ratio of PMMA is 80% (pink line in Supplementary Figure 9a and b). Thus, the appropriate condition of PMMA ratio is 60%. As for Supplementary Figure 9d-f, the bistable and coloring properties of solid devices with different thickness of conductive layer are investigated. Considering both bistability and coloring time, the thickness of 163 μm shows low percentage of decay and short coloring time among the five sets of data. Therefore, the proper condition of different thickness of conductive layer is around 163 μm. According to Supplementary Figure  9g-i, with the ratio range from 1:0.1 to 1:1.5, all the devices show excellent bistability, with the increase of ratio of Urea-N/Rh-M, the intensity of absorption increases. As for the coloring time, the devices exhibit short coloring time between the ratio of 1:0.1 to 1:1.5, compared with the ratio of 1:2 (1200 ms). Considering both the two aspects, the ratio of 1:1 is chosen as the best ratio of Urea-N/Rh-M. The influences of each factor on coloring time, fading time and the percentage of decay were investigated and PMMA/[BMIM]PF6=6:4, Urea-N/Rh-M=1:1 and thickness of conductive layer between 150 and 200 µm were considered as the optimum conditions according to the experiments above.

The mechanism of BQ as counter material in EC layer
In order to shorten the fading time of the device when under negative voltages, BQ was added to electrochromic layer due to its electrobase properties. In order to research the interaction among the three molecules, we tested the cyclic voltammetry and 1 H NMR spectra of the sole molecule and mixture under different conditions.  Figure 28, the 1 H NMR spectra also demonstrate the interaction between Urea-N and Rh-M is stronger than that between Urea-N and "BQ in neutral state". There are Urea-N, Rh-M and BQ in the EC layer of solid bistable device. Thus, when device changes from colorless to colored state, the main reaction in the EC layer occurs between Urea-N and Rh-M rather than BQ. And although there is BQ as counter material in device, the EC layer and counter layer are separated by a conductive and isolating membrane of conductive Li-Nafion or PMMA/[BMIM]PF6, which effectively prevent the contact and diffusion of the electroactive molecules from the other two layers to some extent, and consequently avoids the redox self-discharging reaction. Figure 29 and Supplementary Figure 30 shown, when oxidized Urea-N, Rh-M-H + and reduced BQ coexist in the device, the reduced benzoquinone will compete with Rh-M to accept the proton, which supports the perspective of the reviewer. When colored device begins to fade under negative voltage, the main active species in the electrochromic device are BQ, Rh-M-H + , and oxidized Urea-N. According to Supplementary Figure 30a, the redox peak of sole BQ and mixture of BQ and Rh-M show little change, however, when mixed with Rh-M-H + (Rh-M treated with 1 eq CF3COOH and then the residue of acid was removed), the reduction peak of BQ shifts 0.4 V (from -0.53 V to -0.13 V). Notably, the shape of the redox peak of BQ also changes when mixed with Rh-M-H + (Supplementary Figure 29b). The shift of the reduction peak of BQ and the departure of the redox peak reveals the proton transfer between the two components according literature [1], [2] , which is to say BQ can capture the proton from the Rh-M+H + . The redox peak of BQ/Rh-M+H + is similar to that of BQ/CF3COOH, which provides another evidence of the proton transfer between BQ and Rh-M+H + . The proton transfer can help accelerate the fading speed of the device to some extent. Compared with the redox peak of Urea-N/Rh-M (Supplementary Figure 30), the potential of reduction peak of BQ/Rh-M+H + is much lower and the oxidation peak is much higher. It can be inferred from the phenomena above that reduced BQ is able to give proton back to oxidized Urea-N to form neutral BQ and Urea-N after capturing proton from Rh-M+H + , which makes the system totally reversible.

Comparison of different EC materials
In order to further prove the mechanism we proposed, the methyl ketone bridged molecule, viologen, H2Q/Rh-M was applied in EC layer of the device instead of Urea-N/Rh-M. As Supplementary Figure 24 shown, the methyl ketone bridged molecule and viologen shows poor bistability in the solid device and the H2Q/Rh-M decays about 20% during 100 s, while the Urea-N/Rh-M device exhibits superb bistability. When the mechanisms of the four systems are futher investigated, the differences among the four systems are figured out. For the viologen molecule, the redox of the viologen directly leads to the absorption and color change of the device, and there is no proton transfer process in the system. As for the methyl ketone molecule, there are both electron transfer and proton transfer, however, there is no hydrogen bonding in the system, which is proved to help stabilizing the electrochromic molecules. For the two electroacid systems, although they both have the electron transfer and proton transfer during the process, and there are hydrogen bonding in both systems, the H2Q system produces free radical which makes it not stable during the electrochromic process. The results provide evidence of the bistability source of Urea-N/Rh-M electroacid system.

Instrument characterization
UV-vis absorption spectra were measured using a Shimadzu UV-2550 double-beam spectrophotometer. Fluorescence spectra were obtained with a Shimadzu spectrofluorimeter RF-5301PC. Cyclic voltammograms were obtained from Bio-logic electrochemical work station. Solution experiments were performed under argon atmosphere. ESI-HRMS analysis was performed on an Agilent 1290-micrOTOF-Q II mass spectrometer. Accurate masses were reported for the molecular ion [M+H] + or [M] + . Nuclear magnetic resonance spectra ( 1 H NMR and 13 C NMR) were recorded with Varian Mercury (300 MHz) and Bruker 500 MHz NMR spectrometer. IR spectra studies were performed on Vertex 80/80V FT-IR spectrometer with LN-MCT Mid DC detector over the range of 4000-800 cm -1 using a KBr plate. Photo-etching was realized by layer mask machine FB30-Z HPWU0300-SKS. Fiber optical spectra were measured on a Maya 2000PRO fiber optical spectrometer with Ocean DH-2000-BAL UV-Vis-NIR lightsource.

The theoretical calculations
All the calculations were performed using the Gaussian 09 program. All molecules were fully optimized using the hybrid M06 functional with 6-311g(d,p) basis set [3] . Then the vibrational spectrum of each molecule has been calculated at the same level of theory to ensure that all structures correspond to true minima of the potential energy surface.
The electrochromic device was prepared by two ITO electrodes and a spacer (thickness: 0.1 mm). Then, the electrochromic solution was injected into electrochromic device as Supplementary Figure 5a.

Preparation of the photo-etched electrodes
ITO glass electrodes or PET-ITO electrodes are photo-etched by layer mask machine FB30-Z HPWU0300-SKS.
The the solid electrochromic devices with photo-etched ITO electrodes were fabricated according to the procedure of the preparation of solid electrochromic devices.

Preparation of the flexible solid electrochromic device
Firstly, the transparent counter film layer was deposited by spin coating on the first PET-ITO. Next, the transparent conductive film was deposited on the top of the counter film. The transparent colorless electrochromic film was deposited by spin coating on the second PET-ITO. Finally, the two PET-ITO are assembled together.

Preparation of the smart glasses device
Firstly, the transparent counter film layer was deposited by spin coating on the first glassesshape ITO. Next, the transparent conductive film was deposited on the top of the counter film. The transparent colorless electrochromic film was deposited by spin coating on the second glasses-shape ITO. Finally, the two glasses-shape ITO are assembled together.

Preparation of the multicolor bistable electrochromic device with photo-etched electrode
Firstly, the transparent counter film layer was deposited by spin coating on the first glassesshape ITO. Next, the transparent conductive film was deposited on the top of the counter film. The transparent electrochromic films with different acid sensitive molecules were deposited on the corresponding parts of the etched ITO electrode. Finally, the two ITO electrodes are assembled together. Solution of electrochromic layer was coated on an ITO glass electrode, and solution of counter layer was coated on another ITO glass electrode. After the vaporization of the solvent, uniform films were attached to the two electrodes. Li-Nafion membrane was then coated onto the film of counter layer. The electrodes with films were put together and the film was sandwiched between the electrodes to fabricate the solid devices.