Electrochromic Molecules Research Articles

Heterocycle- and Amine-Free Electrochromic and Electrofluorochromic Molecules for Energy-Saving See-Through Smart Windows and Displays.

Electrochromic (EC) smart windows are an elegant alternative to dusty curtains, blinds, and traditional dimming devices. The EC energy storage smart windows and displays received remarkable attention in the optoelectronic industry as they hold promise for high energy efficiency, low power consumption, reversibility, and swift response to stimuli. However, achieving these properties remains challenging. Moreover, most EC molecules do not exhibit electrofluorochromism, which is highly essential for smart displays because its EC property can modulate the solar heat entering the building, and its electrofluorochromic (EFC) aspects can create lighting during the night. In this work, a structure-property relationship is utilized to develop new electrochromes that can store the injected charge, and these molecules indeed exhibit electrofluorochromism. The compounds are synthesized from tetrabenzofluorene with two aromatic acceptor units, and avoids the use of widely studied heterocycles and amine derivatives. The electrochromes switches from yellow to dark hue in solution, solid, and gel state. The compounds display exceptional electrochemical stability and reversibility in 1000 cycles and capacity retention of 93-100 % in 300 charging-discharging cycles. The proof-of-concept device fabrication of the self-dimming EC smart window presented here demonstrates that it can furnish visual comfort, modulate transmitted light and glare, and reduce energy usage.

Nonconventional Symmetric Double-Side Electrochromic Devices Employing a Nafion Conductive Layer to Unlock Superior Durability.

In this work, we present a methodology to create an effective novel double-sided symmetric architecture of solid-state electrochromic devices. This principally new nonconventional configuration provides access to novel electrochromic systems that could be applicable for the creation of smart double-side signage, smart boards, nonemissive displays, and other smart interactive devices that change their color upon application of a voltage. The proposed configuration is based on the assembly of two identical electrochromic materials facing each other through an opaque optical separator. As a proof of concept, we use an electrochromic material based on bis(4'-(pyridin-4-yl)-2,2':6',2″-terpyridine) iron complex, covalently immobilized on screen-printed surface-extended ITO support. The symmetric configuration allows for a drastic enhancement of the overall stability of the device due to both attenuation of the counter electrode polarization and minimization of electrolyte decomposition. A nontransparent ion-permeable separator, in turn, allows observing the color change of only one of the electrodes by cutting off the optical contribution of the electrode located behind it. Further functionalization of the electrochromic material with a thin layer of Nafion is a beneficial strategy to significantly boost up long-term durability of the devices. Applying a layer of Nafion to the electrochromic material results in an increase in ionic conductivity within the device and ensures better retention of electrochromic molecules on the surface, thus minimizing device decomposition during long-term electrochemical cycling. An electrochromic device that bears Nafion-functionalized electrodes can operate (i) in the dual-side mode, where both sides demonstrate effective electrochromic performance; or (ii) in a one-side manner, where only one side of the device changes color. Notably, when operating in the one-side mode, the device withstands 70,000 cycles, after which the performance of the device can be resumed by simply turning the device to the other side (via switching the polarity of the electrodes).

Synthesis and Functional Control of Near-infrared Electrochromic Molecules Based on Redox-active Para-quinodimethane Scaffolds

Synthesis and Functional Control of Near-infrared Electrochromic Molecules Based on Redox-active Para-quinodimethane Scaffolds

High-durability organic electrochromic devices based on in-situ-photocurable electrochromic materials

•Organic electrochromic materials with in situ photocuring features were proposed •The electrochromic film and device showed attractive durability and calendar life •A high-resolution electrochromic photolithographic patterning was demonstrated Electrochromic materials can undergo electrochemically driven redox processes resulting in optical signal changes, and they have shown attractive application potential in smart windows, wearable electronics, electronic papers, etc. However, their research and development (R&D) are still limited due to the lack of suitable materials and processing strategies to achieve high durability. Here, in-situ-photocurable electrochromic molecules were designed and synthesized to overcome the aforementioned challenges. Compared with the existing doping mode and polymerization mode, in-situ-photocured electrochromic films and devices showed superior durability (e.g., adhesion, multi-dimension stability, and full-contrast calendar life of >120 days) and remarkable overall performance (e.g., transparency = ∼90%, optical modulation = 72.7%, and full-contrast cycle life > 21,000). Furthermore, a high-resolution electrochromic photolithography for micro-nano patterning (recognizable pattern size: ∼2 μm, the best electrochromic resolution to date) was successfully achieved. Based on this, some potential applications in multi-level information encryption and tunable secondary colors were demonstrated. Electrochromic materials can undergo electrochemically driven redox processes resulting in optical signal changes, and they have shown attractive application potential in smart windows, wearable electronics, electronic papers, etc. However, their research and development (R&D) are still limited due to the lack of suitable materials and processing strategies to achieve high durability. Here, in-situ-photocurable electrochromic molecules were designed and synthesized to overcome the aforementioned challenges. Compared with the existing doping mode and polymerization mode, in-situ-photocured electrochromic films and devices showed superior durability (e.g., adhesion, multi-dimension stability, and full-contrast calendar life of >120 days) and remarkable overall performance (e.g., transparency = ∼90%, optical modulation = 72.7%, and full-contrast cycle life > 21,000). Furthermore, a high-resolution electrochromic photolithography for micro-nano patterning (recognizable pattern size: ∼2 μm, the best electrochromic resolution to date) was successfully achieved. Based on this, some potential applications in multi-level information encryption and tunable secondary colors were demonstrated.

Preparation and Orthogonal Analysis for Dual-Responsive Electrochromic Polymer Dispersed Liquid Crystal Devices.

In this work, we provide a fabrication method for dual-responsive electrochromic (EC) polymer dispersed liquid crystal (PDLC) devices. The EC PDLC device was developed by combing the PDLC technique and a colored complex formed via a redox reaction without a specific EC molecule in a simple preparation method. The mesogen played dual roles in the device for scattering in the form of microdroplets and participating in the redox reactions. Orthogonal experiments were performed with the acrylate monomer concentration, the ionic salt concentration, and the cell thickness as factors to investigate the electro-optical performance for the achievement of optimized fabrication conditions. The optimized device presented four switchable states modulated by external electric fields. The light transmittance of the device was changed by an alternative current (AC) electric field while the color change was realized by a direct current (DC) electric field. Variations of mesogen and ionic salt species can modulate the color and hue of devices, which solves the disadvantage of a single color for traditional EC devices. This work lays the foundation for realizing patterned multi-colored patterned displays and anti-counterfeiting via screen printing and inkjet printing techniques.

Magenta-Blue Electrofluorochromic Device Incorporating Eu(III) Complex, Anthracene Derivative, and Viologen Molecule

Electrochemical switching of luminescence color between magenta and blue using two types of luminescent materials and electrochromic molecules was demonstrated based on the control of excited energy transfer through an electrochromic reaction. The magenta photoluminescence, due to the integration of red luminescence from the Eu(III) complex and blue fluorescence from the anthracene derivative, was reversibly modulated to a pure-blue luminescence color by an electrochemical redox reaction. Electrofluorochromism is induced by effective excited energy transfer from the Eu(III) complex to the electrochromic molecule under a redox reaction.

Conquering residual light absorption in the transmissive states of organic electrochromic materials.

In this short review, we provide an overview of our efforts in developing a family of anodically coloring electrochromic (EC) molecules that are fully transparent and colorless in the charge neutral state, and that can rapidly switch to a vibrantly colored state upon oxidation. We employ molecules with reduced conjugation lengths to center the neutral state absorption of the electrochrome in the ultraviolet, as desired for highly transparent and colorless materials. Oxidation creates radical cations that absorb light in the visible and near infrared regions of the electromagnetic spectrum, thus providing a host of accessible colors. Combining a density functional theory (DFT) computational approach fed back to the synthetic effort, target molecules are proposed, synthesized and studied, directing us to develop a complete color palette based on these high contrast ACE molecules. Utilizing pendant phosphonic acid binding substituents in concert with high surface area mesoporous indium tin oxide (ITO) electrodes, the electrochromes can be distributed throughout the oxide film, bringing high extent of light absorption and color density.

Lifetime of electrochromic optical transition cycling of ethyl viologen diperchlorate-based electrochromic devices

Electrochromic materials and devices are enabling a variety of advanced technologies. Gel-based organic electrochromic molecules such as ethyl viologen diperchlorate are attractive options due to their simple device design and low cost processing options relative to the more expensive and complex transition metal oxide films. However, electrochromic devices are subject to extensive cycling in which failure and fatigue can eventually occur. This work presents the lifetime cycling performance of ethyl viologen diperchlorate-based electrochromic devices using two different anodic compounds, hydroquinone and ferrocene, which are cycled at different voltages, 3 V and 1.2 V, respectively. Multiple devices are cycled until failure with periodic device characterization via UV–Vis spectroscopy, electrical resistance and power measurement, and transition duration measurement. Devices with hydroquinone can transition quickly. Cycle times are sim 30 s in these samples, however, these samples also typically fail before 3000 cycles. On the other hand, devices using ferrocene transition more slowly (total cycle time sim 2 min), but have superior cycling performance with all samples surviving beyond 10,000 cycles while complying with ASTM E2141-14 standard.

Heuristic Global Optimization in Chemical Compound Space.

We report developments in combinatorial optimization under constraints in chemical space. We considered random functions, which serve as a baseline to measure performance, and constrained optimizations over two databases of electrochromic molecules (∼5000 and 1012, respectively). These problems were optimized using sequential heuristic next-neighbor search (HNNS) (introduced in Elward and Rinderspacher Phys. Chem. Chem. Phys. 2015, 17, 24322 and Elward and Rinderspacher Mol. Syst. Des. Eng. 2018, 3, 485) and kernel-based efficient global optimization (EGO) with two reordering strategies. In addition to the average ordering method introduced with sequential HNNS, a new reordering based on a locally separable linear model is formulated and applied. Presented is the analysis of periodic kernels for EGO: the modified Dirichlet kernels D̃n(x) = ∑in[cos(2πix) - (-1)i]/i, the minimalist kernel KN(x) = cos(πx/N)2/2 - δx/2, and an analogue to the popular Gaussian kernel, GNσ(x) = e-|xmodN|2/(2σ2), where n is the order of the Dirichlet kernel, N is the number of choices on a combinatorial site, and σ is a broadening factor. We find that reordering is pivotal for high optimization efficiency, in both the average and worst-case scenarios. The new linear-estimation ordering paradigm is superior in the chemistry context. Furthermore, with judicious use of hyperparameters and algorithmic choices, EGO outperforms HNNS. The global optimum for the small chemical problem is found with >98% likelihood for all global search methods employing a linear reordering heuristic for organizing the search space.

Organic electrochromic molecules: synthesis, properties, applications and impact

Abstract Organic electronics are at the forefront of modern research, with goals of creating more efficient and environmentally benign devices. Organic molecules can achieve this as they typically result in materials that are solution-processable and less toxic than their transition-metal counterparts. Electrochromic molecules have unique color changing properties upon passing an electrical current making them highly sought after for colored displays, dimming mirrors, and smart windows. Part of my PhD work was devoted to developing a new class of electrochromic molecules, the phosphoryl-bridged viologens, with more favorable redox properties and new color changing modes. In order to understand the necessity and potential of new electrochromes, it is important to assess the history of the field and its future. In addition to designing new molecules for electrochromic devices, it is equally important to design a multifunctional species with a bright, competitive future across various organic electronic applications.

Detailed Structural Analyses of Nanofibrillated Bacterial Cellulose and Its Application as Binder Material for a Display Device.

Nanofibrillated bacterial cellulose (NFBC) is produced by culturing a cellulose-producing bacterium under agitated aerobic conditions in a carboxymethylcellulose (CMC)-supplemented medium. Detailed structural analyses revealed that NFBC fiber widths varied with the degree of substitution of the CMC used, and zeta potential values decreased with the increment of CMC concentration in the medium. Transmission electron microscopy observation after immunostaining demonstrated that CMC molecules were present on the NFBC microfibril surfaces. We tested NFBC for utility as a binder for a display device that uses electrochromic (EC) material. Introduction of a quaternary ammonium group into the EC molecules enhanced their interactions with the negatively charged NFBC microfibrils. A casting process homogeneously adsorbed the EC molecules onto the surface of a transparent electrode with NFBC. A homogeneous color change was successfully observed upon applying an electric field, suggesting that NFBC could be used as a binder material for uniform surface adsorption.

Recent Advances with Terpyridine-Based Monolayer Electrochromic Materials

Electrochromic (EC) materials are a class of “smart” materials that change their optical properties in response to an applied potential. This optical change can be the development, loss, or variation in the color of the material. EC materials have recently become a research focus because of their high commercial applicability in “smart” windows, antiglare mirrors, and low-power displays. Most current EC materials are based on thick layers of EC molecules or EC molecules/ conductive polymer blends on the electrodes. The device properties depends strongly on the overall composition and distribution of the EC species in the polymer structure. We have developed an EC systems that contain as little as a monolayer of the EC molecules deposited onto a surface-enhanced conductive support [1]. The process involves the covalent attachment of various terpyridine-based ligand to the surface of a nanostructured conductive indium − tin oxide (ITO) screen-printed support by a simple submerging of the support into an aqueous solution of L. Subsequent reaction of with metal ions (e.g. Fe, Ru) leads to the formation the monolayer of the redox-active metal complex covalently bound to the ITO support. In addition, isolated and well-defined terpyridine- based metal complexes (Fe, Os,Co) were simply deposited on ITO screen-printed surface via conventional chlorobenzylsiloxane-based template. Our fabrication approach utilizes the inter-particle porosity of the support and requires as low as a monolayer of EC active molecule, giving significant molecular economy when compared with traditional polymer-based EC devices. This presentation will feature an overview of our synthetic methods, including how we can alter the color of the films by changing either the metal center of the ligand itself [2]. Spectro-electrochemical studies enabled us to correlation optical density with redox site density. Furthermore, detailed electrochemical studies enabled the determination of the kinetic rate constants for electron transfer for these films [3], which were considerably higher than other approaches in the literature. These novel light-reflective EC materials demonstrate a high color difference, significant durability, fast switching speed, excellent stability and high EC reversibility.

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.

New Design Paradigm for Color Control in Anodically Coloring Electrochromic Molecules.

A new paradigm is established for the design of conjugated anodically coloring electrochromic molecules. It is shown that through crossconjugation the electronic energy levels of the radical cation state may be controllably tuned independent of the neutral state. It is shown how cross-conjugation can be used to tune the radical cation state independent of the neutral state. Manipulating the oscillator strengths of radical cation transitions allows for tuning of the color by shifting the λmax of the low-energy absorption by over 400 nm. The neutral states of these molecules are UV absorbing, providing solutions that are colorless with L*a*b* values of 100, 0, 0. They are oxidized to vibrantly colored radical cations with absorptions that span the visible spectrum, creating green, yellow, and red chromophores. These molecules are then mixed to create transmissive, colorless blends that switch to opaque black solutions.

Electrofluorochromism at the single-molecule level.

The interplay between the oxidation state and the optical properties of molecules is important for applications in displays, sensors, and molecular-based memories. The fundamental mechanisms occurring at the level of a single molecule have been difficult to probe. We used a scanning tunneling microscope (STM) to characterize and control the fluorescence of a single zinc-phthalocyanine radical cation adsorbed on a sodium chloride-covered gold (111) sample. The neutral and oxidized states of the molecule were identified on the basis of their fluorescence spectra, which revealed very different emission energies and vibronic fingerprints. The emission of the charged molecule was controlled by tuning the thickness of the insulator and the plasmons localized at the apex of the STM tip. In addition, subnanometric variations of the tip position were used to investigate the charging and electroluminescence mechanisms.

Changing emission with charge states

Surface Chemistry The fluorescence of electrochromic molecules changes with their charge state. Doppagne et al. studied the optical emission of single zinc-phthalocyanine molecules excited by electron injection from a scanning tunneling microscope tip. The molecules were adsorbed on salt layers grown on a gold surface, so that the cationic and neutral molecules could both be observed. The primary emission shifted to lower energy for the cation, and, in addition, vibrational side bands were observed. Science , this issue p. [251][1] [1]: /lookup/doi/10.1126/science.aat1603

(Invited) Electrochemically Controllable Emission and Coloration Using Luminescent Europium(III) Complex and Electrochromic Viologen Derivatives

A dual-mode display (DMD) device which can operate in both emissive and reflective modes, having advantages of both modes, is considerable promise as a next generation display. Using the dual-mode display, we can choose either reflective or emissive mode as the situation demands. However, only a few studies have been reported on the DMD devices because it should include two different mechanisms in one device. For creation of novel materials enabling electrochemical modulation of both emission and coloration, we focused on the concept of “emission control based on EC reactions”. By combination of luminescent material and EC material, the energy transfer between the two materials can be controlled by EC reactions. When the EC molecule is colorless state, the energy gap between ground state and excited state of the EC molecule is larger than that of the luminescent material. Therefore, under the photo- excitation of the luminescent material, the transfer of excitation energy from luminescent material to EC material is not allowed, leading to bright emission from the luminescent material, i.e. “emissive mode”. When the EC molecule is colored, on the other hand, the energy gap of the EC molecule became smaller than that of luminescent material. In this state, the excitation energy is expected to transfer to the colored EC molecule from luminescent material. Since the energy transfer will quench the luminescence, only coloration of EC molecule can be obtained without photoluminescence, i.e. “reflective mode”. Here, we have tried to control coloration as well as emission based on electrical stimuli by combination of photoluminescent and EC materials to achieve a novel DMD material. In this paper, the electrochemical switching systems enabling the control of the both modes were investigated. Furthermore, a prototype DMD device with emissive and coloration based on electrochromism were fabricated. We first demonstrated both luminescence and coloration control by using a composite material containing a luminescent Eu(III) complexes and electrochromic viologen derivatives as the luminescent and coloration materials, respectively. The coloration of the composite material was controlled by the electrochromic reaction of viologen derivatives. Modulation of red emission from Eu(III) complex was also achieved by the electrochromism of viologen derivatives via intermolecular energy transfers from the excited states of Eu(III) ions to colored viologens. The 2-electrode cell containing the luminescent complexes and electrochromic molecules functioned as an electrochromic device when bias voltage was applied and as a light-emitting device if excitation lights are irradiated. When bias voltage was not applied to the cell, the outer appearance of the cell was clear transparent, the strong red emission from Eu(III) complex was observed under UV excitation. This can be considered as ‘emission on’ state. On the other hand, after application of voltage ca. 2 V, the cell changed from clear transparent state to colored state by electrochromic reaction of the viologen derivatives, resulting in ‘refection on’ state. As the coloration of the cell, red emission of the Eu(III) complex was completely quenched under colored state. In order to achieve a rapid fluorescence modulation response, it is necessary to immobilize both luminescent and electrochemically active molecules on the same electrode because response times of modulation of both emission and coloration are dependent on the response time of electrochromism and energy transfer efficiency. We finally tried to fabricate photo-electro functional modified electrode incorporating the Eu(III) complex and viologen derivative on the electrode. The novel modified electrode was fabricated by covalently- connecting the Eu(III) complex and viologen derivatives on porous TiO2 electrode through carboxyl groups or phosphate groups. The immobilizing of these materials on porous TiO2 electrode is expected to achieve absorption and fluorescence intensity, because large surface area of the TiO2 film allows the increase of adsorption amount of the materials, leading to high emission contrast of modulation. The adsorption amount of the Eu(III) complex and viologen derivative were analyzed by FT-IR and EDX measurements. The modified electrode showed strong red photoluminescence from the Eu(III) complex at transparent state. The EC reaction of the viologen derivative was also observed with rapid response of few seconds by applying -0.8 V. The photoluminescence from Eu(III) complex was successfully modulated by EC reaction of the viologen derivatives within few seconds.

In situ measurements of electrode potentials of anode and cathode in organic electrochromic devices

The in situ electrode potentials of anodes and cathodes in electrochromic (EC) devices were investigated. The solution-based 2-electrode EC device was fabricated with electrolyte solutions containing anodic and cathodic EC molecules. The electrochemical reaction in a 2-electrode EC device was controlled by the electrode reaction when electroactive molecules in smaller concentrations were used. In addition, the electrode showing less electrochemical activity in the EC device was subjected to larger overpotential under the application of a driving voltage, leading to the promotion of the redox reaction at the electrode. The results showed that the electrode potentials of the device were spontaneously regulated to facilitate both electrode reactions. It is very important to know the potential at each electrode in order to understand the reaction in detail in the 2-electrode EC device.

A greener electrochromic liquid crystal based on ionic liquid electrolytes

ABSTRACTAn electrochromic liquid crystal (ECLC) material composed of only liquid crystal (LC) and ionic liquid (IL) was developed. The LC containing the substituted diphenylacetylene serves as electrochromic (EC) material to realise transmittance and colour change under the direct current (DC) field, while the IL with the designable cation and anion served as electrolyte. Herein, a series of IL electrolytes was screened to investigate how IL tunes the electro-optic performance of the ECLC cell. By testing the electrochemistry window of ILs in EC cells, IL with the [NTf2]− anion shows adequate electrochemical stability when the EC material undergoes oxidation and reduction. The electro-optic performance of ECLC containing 1-ethoxy-4-[2-(4-pentylphenyl) ethynyl]-benzene (PEB) and IL was then evaluated by UV-vis spectrometry under the control of an electrochemical work station. Compared with other PEB-IL, PEB-[Bmim][NTf2] with [Bmim][NTf2] electrolyte shows a satisfactory transmittance at low operating voltage. Furthermore, Pd NPs in situ formed in [Bmim][NTf2] reduced the EC potential and improved the light scattering of the ECLC cell. In this work, we also designed a bifunctional device based on polymer dispersed liquid crystal (PDLC) that hosts electrochromic guest molecules, and analysed the electro-optical and electrochromic properties of LC electrolyte mixtures, in order to gain control of the incident daylight and glare in building and automotive applications.

The Effects of Immobilization of Electrochromic Molecules on the Electrode for Coloration Properties in the Device Using Phthalate Derivatives and Nickel Oxide

The Effects of Immobilization of Electrochromic Molecules on the Electrode for Coloration Properties in the Device Using Phthalate Derivatives and Nickel Oxide