Reversible Bond/Cation-Coupled Electron Transfer on Phenylenediamine-Based Rhodamine B and Its Application on Electrochromism

Emerging Electrochromic Materials and Devices for Future Displays

With the rapid development of optoelectronic fields, electrochromic (EC) materials and devices have received remarkable attention and have shown attractive potential for use in emerging wearable and portable electronics, electronic papers/billboards, see-through displays, and other new-generation displays, due to the advantages of low power consumption, easy viewing, flexibility, stretchability, etc. Despite continuous progress in related fields, determining how to make electrochromics truly meet the requirements of mature displays (e.g., ideal overall performance) has been a long-term problem. Therefore, the commercialization of relevant high-quality products is still in its infancy. In this review, we will focus on the progress in emerging EC materials and devices for potential displays, including two mainstream EC display prototypes (segmented displays and pixel displays) and their commercial applications. Among these topics, the related materials/devices, EC performance, construction approaches, and processing techniques are comprehensively disscussed and reviewed. We also outline the current barriers with possible solutions and discuss the future of this field.

Synthesis and Optical Characterization of a Rhodamine B Spirolactam Dimer

Amide derivatives of xanthene dyes such as rhodamine B are useful in a variety of sensing applications due to their colorimetric responses to stimuli such as acidity changes and UV light. The optical properties of these molecules can be influenced by intermolecular associations into dimeric structures, but the exact impact can be hard to predict. We have designed a covalently linked intramolecular dimer of the dye rhodamine B utilizing p-phenylenediamine to link the two dyes via amide bonds. The doubly closed spirolactam version of this dimer, RSL₂, is isolated as a colorless solid. Under acidic conditions or UV exposure, RSL₂ solutions develop a pink color that is expected for the ring-opened form of the molecule. However, nuclear magnetic resonance (NMR) and single-crystal diffraction data show that the equilibrium still prefers the closed dimer state. Interestingly, the emission profile of RSL₂ shows solvatochromic blue fluorescence. Control studies of model compounds with similar structural motifs do not display similar blue fluorescence, indicating that this optical behavior is unique to the dimeric form. This behavior may lend itself to applications of such xanthene dimers to more sophisticated sensors beyond those with traditional binary on/off fluorescence profiles.

Dynamic Metal-Ligand Interaction of Synergistic Polymers for Bistable See-Through Electrochromic Devices

Bistable electrochromic materials are a promising alternative solution to reduce energy consumption in displays. Limited by the mechanism and lack of a design strategy, only a few electrochromic materials have truly been able achieve bistability. Herein, a novel strategy is proposed to design bistable electrochromic materials based on polymer-assisted dynamic metal-ligand coordination. The mechanism and materials of such unconventional electrochromic systems are proved by sufficient characterization. Synergistic stabilization of polymerized switchable dyes and the ionic ligand polymer are attracted to each other by supramolecular forces. The color states of the dye molecules are controlled and stabilized by valence changes of the metal ions. Meanwhile, through the polymerization of the electrochromic material and the nearby metal-ligand material, the metal ions of the electroinduced valence change are tightly fixed, and the related diffusion problem of the active EC component is also almost completely suppressed. This strategy successfully enables preparation of the corresponding transparent electrochromic displays with good performances, such as, the display information is clearly visible for more than 1.5 h without consuming energy. Furthermore, the new way of dynamic coordination or dissociation bistable displays could likely prosper the development of the electrochromic area and inspire other fields.

Bistable Elastic Electrochromic Ionic Gels for Energy-Saving Displays

The diversification of electrochromic materials greatly expands the application fields of electrochromic devices. However, highly flexible electrochromic materials remain challenging due to the inherent limitations associated with the existing electrochromic processes. Inspired by the hydrogen bonding effect in the hydrogel structure, a highly elastic and bistable electrochromic ionic gel based on a hydrogen bonding cross-linking network is prepared by solution polymerization having excellent tensile resilience, uniform coloring, reversible switching (≤24.3 s), maximum transmittance change (≥80%), bistability (54 h), reversibility (>500 cycles), and coloration efficiency (≥85.3 cm²·C^-1). This method has been used to develop bistable electrochromic displays. The unconventional exploration of the bistable design principle may provide a new idea for the realization of bistable electrochromic devices.

Green revolution in electronic displays expected to ease energy and health crises

The technological revolution of long-awaited energy-saving and vision-friendly displays represented by bistable display technology is coming. Here we discuss methods, challenges, and opportunities for implementing bistable displays in terms of molecular design, device structure, further expansion, and required criteria, hopefully benefiting the light-related community.

A multicolour bistable electronic shelf label based on intramolecular proton-coupled electron transfer

Bistable electrochromic materials have been explored as a viable alternative to reduce energy consumption in display applications. However, the development of ideal bistable electrochromic displays (especially multicolour displays) remains challenging due to the intrinsic limitations associated with existing electrochromic processes. Here, a bistable electrochromic device with good overall performance-including bistability (>52 h), reversibility (>12,000 cycles), colouration efficiency (≥1,240 cm² C^-1) and transmittance change (70%) with fast switching (≤1.5 s)-was designed and developed based on concerted intramolecular proton-coupled electron transfer. This approach was used to develop black, magenta, yellow and blue displays as well as a multicolour bistable electrochromic shelf label. The design principles derived from this unconventional exploration of concerted intramolecular proton-coupled electron transfer may also be useful in different optoelectronic applications.

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 cm²C⁻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.

For electrochromic materials to reach their full potential for high efficiency bistable displays, technical challenges related to their underlying mechanism must be addressed. Here, the authors, through intelligent molecular design, report a solid bistable device with state-of-the-art performance.

Transparent to Black Electrochromism-The "Holy Grail" of Organic Optoelectronics

In the rapidly developing field of conjugated polymer science, the attribute of electrochromism these materials exhibit provides for a multitude of innovative application opportunities. Featuring low electric potential driven colour change, complemented by favourable mechanical and processing properties, an array of non-emissive electrochromic device (ECD) applications lays open ahead of them. Building up from the simplest two-colour cell, multielectrochromic arrangements are being devised, taking advantage of new electrochromic materials emerging at a fast pace. The ultimate device goal encompasses full control over the intensity and spectrum of passing light, including the two extremes of complete and null transmittance. With numerous electrochromic device architectures being explored and their operating parameters constantly ameliorated to pursue this target, a summary and overview of developments in the field is presented. Discussing the attributes of reported electrochromic systems, key research points and challenges are identified, providing an outlook for this exciting topic of polymer material science.