Electrochemically driven physical properties of solid-state materials: action mechanisms and control schemes

The various physical properties recently induced by solid-state electrochemical reactions must be comprehensively understood, and their mechanisms of action should be elucidated. Reversible changes in conductivity, magnetism, and colour have been achieved by combining the redox reactions of d metal ions and organic materials, as well as the molecular and crystal structures of solids. This review describes the electrochemically driven physical properties of conductors, magnetic materials, and electrochromic materials using various electrochemical devices.

The electrochemical modulation of single molecule fluorescence

Recently it has been shown that electrochemistry, instead of using high intensity lasers, can be used to modulate the intensity of emission of fluorophores and even switch fluorophores between their ON and OFF states as required for single molecule localisation microscopy. This modulation of fluorescence does not necessarily correlate with direct oxidation and reduction of the dyes. Questions arise from this unexpected observation related to what is the electrochemistry that occurs, what are the important variables in switching fluorophores electrochemically and what range of dyes can be modulated with electrochemistry. Herein we seek to answer some of these questions. We demonstrate how to effectively modulate the fluorescence intensity of organic dye-labelled cell samples on an indium tin oxide surface using electrochemistry with redox-active mediators present in an oxygen scavenger buffer. We showed the electrochemical fluorescence modulation is sensitive to the applied potential and the excitation laser intensity, indicating the possibility of coupled photochemical and electrochemical reactions occurring. We also compared the electrochemical fluorescence modulation of representative oxazine, rhodamine, and cyanine dyes using ATTO 655, Alexa Fluor 488, and Alexa Fluor 647. Different dyes with distinctly different structural cores show fluorescence modulation to different extents. The electrochemical fluorescence modulation will be applicable in fluorescence imaging techniques as well as biosensing.

Electrofluorochromism based on the valence change of europium complexes in electrochemical devices with Prussian blue as the counter electrode

The electrofluorochromism of Eu complexes based on the valence change between Eu3+ and Eu2+ is demonstrated in a two-electrode electrochemical device consisting of Prussian blue (PB) as the counter electrode. This study aims to improve the electrofluorochromic (EFC) performance of luminescence switching between Eu3+ and Eu2+ by enhancing the electrochemical reactivity of the EFC device. By introducing a PB film as a counter electrode in a two-electrode device, the redox reaction of Eu3+/2+ is promoted because of charge compensation by the counter PB film. The increase in the reaction charge enables faster changes in the photoluminescence from Eu3+ to Eu2+ and an increase in the blue luminescence intensity from the Eu2+ state. This approach achieves a lowered driving voltage, accelerates the electrochemical redox reaction of the Eu complex, and enhances the reversibility of the valence change of the Eu ion.

Biomimetic Exploration and Reflection on Switchable Coordination and Narrow-Band Electrofluorochromic Devices

Electrofluorochromic (EFC) materials and devices with controllable fluorescence properties show great application potential in advanced anticounterfeiting, information storage and display. However, the low color purity caused by the broad emission spectra and underperforming switching time of the existing EFC materials limit their application. Through biomimetic exploration and the study of reversible electrochemical responsive coordination reactions, boron-nitrogen embedded polyaromatics (B,N-PAHs) with narrow-band emission and high color purity have been successfully integrated into EFC systems, which also help to better understand the role of boron in biological activity. The EFC device achieve good performance containing quenching efficiency greater than 90% within short switching time (ton: 0.6 s, toff: 2.4 s), and nearly no performance change after 200 cycles test. Three primary color (red, green, and blue) EFC devices are successfully prepared. In addition, new phenomena are obtained and discussed in this biomimetic exploration of related boron reactions. The success and harvest of this exploration are expected to provide new ideas for optimizing properties and broadening applications of EFC materials. Moreover, it may provide ideas and reference significance for further exploring and understanding the function of boron compounds in biological systems.

An electrochemically responsive B-O dynamic bond to switch photoluminescence of boron-nitrogen-doped polyaromatics

Boron-doped polycyclic aromatic hydrocarbons exhibit excellent optical properties, and regulating their photophysical processes is a powerful strategy to understand the luminescence mechanism and develop new materials and applications. Herein, an electrochemically responsive B-O dynamic coordination bond is proposed, and used to regulate the photophysical processes of boron-nitrogen-doped polyaromatic hydrocarbons. The formation of the B-O coordination bond under a suitable voltage is confirmed by experiments and theoretical calculations, and B-O coordination bond can be broken back to the initial state under opposite voltage. The whole process is accompanied by reversible changes in photophysical properties. Further, electrofluorochromic devices are successfully prepared based on the above electrochemically responsive coordination bond. The success and harvest of this exploration are beneficial to understand the luminescence mechanism of boron-nitrogen-doped polyaromatic hydrocarbons, and provide ideas for design of dynamic covalent bonds and broaden material types and applications.

Electrochemically regulated luminescence of europium complexes with β-diketone in polyether matrices

This study investigates the electrochemical modulation of luminescence color, i.e., electrofluorochromism, of an Eu complex in a polyether solvent. The electrofluorochromic (EFC) reaction of the Eu complex occurred via a reversible redox reaction between Eu3+ and Eu2+. Initially, the intrinsically stable Eu3+ complex showed intense red photoluminescence (PL) induced by f-f transitions. After the electrochemical reduction of Eu3+ to Eu2+, broad blue PL was observed attributed to the d-f transitions in the Eu2+ complex. This distinct blue luminescence from the Eu2+ complex was attributed to the effective stabilization of the Eu2+ state by the polyether solvent. The dynamic EFC reaction that changes the valence state of the Eu ion can be potentially applied to novel chemical sensors, security devices, and display devices.

Reversible Electrochromic/Electrofluorochromic Dual Switching in Zn(II)-Based Metallo-Supramolecular Polymer Films

The introduction of novel materials with multifunctional chromogenic properties, such as electrochromic/electrofluorochromic (EC/EFC) properties, has recently attracted prospective interest in the development of various optoelectronic devices and smart windows. In this study, a novel Zn(II)-based metallo-supramolecular polymer (polyZn) has been developed as an ON/OFF switchable EFC application with prominent EC behavior. In this regard, the polymeric chain of polyZn was first synthesized by 1:1 complexation in a zigzag manner with Zn(II) ions at the metal center and 4,4'-[bis(2,2':6',2″-terpyridinyl)benzene]triphenylamine (LTPY-TPA) as the redox-active ditopic ligand. The polyZn exhibits excellent solubility in organic solvents and can form a very good uniform thin film on an indium tin oxide/glass substrate by spin-coating. In a neutral state, transparent polyZn exhibits a bright yellow color to the naked eye (absorption at ∼325 nm). The electroactive triphenylamine (TPA) core of LTPA-TPY, however, undergoes reversible single-electron oxidation when a positive bias of +1.6 V vs Ag/Ag+ is applied, generating radical cations (TPA ↔ TPA•+) with a significant drop in transparency (77%). A noticeable chromic shift in the hue of the film from brilliant yellow to green was observed with the appearance of a near-infrared absorption band at ∼897 nm with a tail of 1300-1600 nm. Interestingly, in addition to this EC phenomenon, the fabricated solid-state polyZn film exhibits intense, high-contrast reddish-orange photoluminescence with λem = 650 nm, which is significantly desired as a molecular probe for bioimaging. Both the TPA core and the redox-inactive Zn(II)-terpyridine core emit orange-red photoluminescence in polyZn, which is significantly quenched upon the oxidation of the film and is re-emitted at 0.0 V vs Ag/Ag+. This ON/OFF EFC transition was sustained for several cycles. This study should motivate to design and create distinctive new unique materials with combined EC/EFC behavior for the fabrication of optoelectronic devices by combining a metal-fluorescent core with a redox-active spacer.

Survey of Recent Advances in Molecular Fluorophores, Unconjugated Polymers, and Emerging Functional Materials Designed for Electrofluorochromic Use

In this review, recent advances that exploit the intrinsic emission of organic materials for reversibly modulating their intensity with applied potential are surveyed. Key design strategies that have been adopted during the past five years for developing such electrofluorochromic materials are presented, focusing on molecular fluorophores that are coupled with redox-active moieties, intrinsically electroactive molecular fluorophores, and unconjugated emissive organic polymers. The structural effects, main challenges, and strides toward addressing the limitations of emerging fluorescent materials that are electrochemically responsive are surveyed, along with how these can be adapted for their use in electrofluorochromic devices.

Tetrazo[1,2-b]indazoles: Straightforward Access to Nitrogen-Rich Polyaromatics from s-Tetrazines

The straightforward access to a new class of aza-polyaromatics is reported. Starting from readily available fluorinated s-tetrazine, a cyclization process with azide leads to the formation of an unprecedented tetrazo[1,2-b]indazole or a bis-tetrazo[1,2-b]indazole (cis and trans conformers). Based on the new nitrogen core, further N-directed palladium-catalyzed ortho-C-H bond functionalization allows the introduction of halides or acetates. The physicochemical properties of these compounds were studied by a joint experimental/theoretical approach. The tetrazo[1,2-b]indazoles display solid-state π-stacking, low reduction potential, absorption in the visible range up to the near-infrared, and intense fluorescence, depending on the molecular structure.

Closed Bipolar Electrode Array for Optical Reporting Reaction-Coupled Electrochemical Sensing and Imaging

This review centers on a closed bipolar electrode (BPE) array using an electro-fluorochromism (EFC) or electro-chemiluminescence (ECL) reaction as the reporting reaction. Electrochemical signals at one pole of the closed BPE array can be transduced into the EFC or ECL signals at the opposite pole. Therefore, the current signal of a redox reaction can be easily detected and imaged by monitoring the luminescence signal. Recent developments in closed BPE array-based EFC and ECL sensing and imaging are summarized and discussed in detail. Finally, we consider the challenges and opportunities for improving the spatial resolution of closed BPE array-based electrochemical imaging, and emphasize the important application of this technique to the imaging of cellular activities at the single-cell level.

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.

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.

Triggering near-infrared luminescence of vanadyl phthalocyanine by charging

Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state-dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser-induced fluorescence (LIF) spectra of samples comprising both the mass-selected neutral molecule (VOPc⋅, a stable radical) and its cation produced upon electron ionization (EI) isolated in 5 K neon matrices. Ionization of the essentially non-emissive VOPc⋅ forms a high-spin diradical cation (VOPc+.. ) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR-emitting electro-optic devices.

Vibrationally Resolved Absorption and Luminescence Spectra of Mass-Selected Free-Base and Zinc Phthalocyanine Radical Cations Isolated in Solid Ne

We report the first vibrationally well-resolved absorption and laser-induced fluorescence spectra of the radical cations of free-base phthalocyanine (H₂Pc⁺) and zinc phthalocyanine (ZnPc⁺) isolated in 5 K neon matrices and compare them to the spectral properties of the corresponding neutrals. The samples were generated by low-energy deposition of the mass-selected ions. The spectra are also discussed in terms of time-dependent density functional theory calculations and compared with recently reported scanning tunneling microscopy-induced single-molecule luminescence of the same species adsorbed on NaCl-covered Au(111) or Ag(111) single crystal supports.

Manipulating Electroluminochromism Behavior of Viologen-Substituted Iridium(III) Complexes through Ligand Engineering for Information Display and Encryption

Electrically controlling photoluminescence has attracted great research interest and offers many opportunities for technological developments. Electroluminochromic materials undergo redox reactions under low-voltage stimuli to achieve reversible luminescence switching. Till now, photoluminescence switching of a single molecule caused by electrical stimuli is restricted to intensity response because the redox-active moieties are good electron donors or acceptors and electrical stimuli can regulate the photoinduced electron-transfer and affect the luminescence intensity. In this work, the manipulation of the electroluminochromism behavior of a series of viologen-substituted iridium(III) complexes through the regulation of ligand orbital energy levels and electronic communication between the viologen pendants and the iridium(III) complex core is reported. Electrochemical redox reactions reversibly modulate either the luminescence quenching effect or the push-pull electronic effect of the viologen substituents, achieving multicolor "on-off" luminescence response toward electrical stimuli and luminescence manipulation between two emissive states with different wavelengths and lifetimes. To illustrate the promising applications of these electroluminochromic materials, recording and displaying luminescence information under electrical stimuli are demonstrated. Information encryption is realized by letting the electroluminochromism occur in the near-infrared region or in the time domain. Near-infrared camera or time-resolved luminescence analysis can be used to help read the invisible information.

Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

UV to NIR multistate electrochromism and electrofluorochromism in dibenzophenazine-arylamine derivatives

An intriguing case of intramolecular and intervalence charge transfer-driven multistate electrochromism and electrofluorochromism in dibenzophenazin-(phenyl)methanone and arylamine-based redox-active donor-acceptor-donor molecules was elucidated. Tunable absorption from UV to NIR and on-off switching of fluorescence in a single-component all-organic molecular material by a subtle variation of electric potentials were demonstrated.

Panchromatic Fluorescence Emission from Thienosquaraines Dyes: White Light Electrofluorochromic Devices

Electrofluorochromic devices (EFCDs) that allow the modulation of the light emitted by electroactive fluorophores are very attractive in the research field of optoelectronics. Here, the electrofluorochromic behaviour of a series of squaraine dyes was studied for the first time. In solutions, all compounds are photoluminescent with maxima located in the range 665-690 nm, characterized by quantum yields ranging from 30% to 4.1%. Squaraines were incorporated in a polymer gel used as an active layer in all-in-one gel switchable EFCDs. An aggregation induced quenching occurs in the gel phase, causing a significant decrease in the emission quantum yield in the device. However, the squaraines containing the thieno groups (thienosquaraines, TSQs) show a panchromatic emission and their electrofluorochromism allows the tuning of the fluorescence intensity from 500 nm to the near infrared. Indeed, the application of a potential difference to the device induces a reversible quenching of their emission that is significantly higher and occurs at shorter switching times for TSQs-based devices compared to the reference squaraine dye (DIBSQ). Interestingly, the TSQs fluorescence spectral profile becomes more structured under voltage, and this could be explained by the shift of the aggregates/monomer equilibrium toward the monomeric species, due to electrochemical oxidation, which causes the disassembling of aggregates. This effect may be used to modulate the colour of the fluorescence light emitted by a device and paves the way for conceiving new electrofluorochromic materials based on this mechanism.

Thermoresponsive multicolor-emissive materials based on solid lipid nanoparticles

Despite the recent advances in the field of thermofluorochromism, the fabrication of thermoresponsive multicolor-emissive materials in a simple, low-cost and versatile manner still remains a challenge. Herein we accomplish this goal by expanding the concept of matrix-induced thermofluorochromism, where a sudden two-state variation of dyes' emission is promoted by the solid-liquid transition of a surrounding phase change material (e.g., paraffins). We demonstrate that this behavior can be transferred to the nanoscale by the synthesis of dye-loaded solid lipid nanoparticles, different types of which can then be combined into a single platform to obtain multicolor thermofluorochromism using a single type of emitter. Because of the reduced dimensions of these particles, they can be utilized to prepare transparent nanocomposites and inkjet-printed patterns showing complex thermoresponsive luminescence signals and applications ranging from smart displays to thermal sensing and high-security anti-counterfeiting.

Stimuli-Responsive AIEgens

The unique advantages and the exciting application prospects of AIEgens have triggered booming developments in this area in recent years. Among them, stimuli-responsive AIEgens have received particular attention and impressive progress, and they have been demonstrated to show tremendous potential in many fields from physical chemistry to materials science and to biology and medicine. Here, the recent achievements of stimuli-responsive AIEgens in terms of seven most representative types of stimuli including force, light, polarity, temperature, electricity, ion, and pH, are summarized. Based on typical examples, it is illustrated how each type of systems realize the desired stimuli-responsive performance for various applications. The key work principles behind them are ultimately deciphered and figured out to offer new insights and guidelines for the design and engineering of the next-generation stimuli-responsive luminescent materials for more broad applications.

Simultaneously achieving high capacity storage and multilevel anti-counterfeiting using electrochromic and electrofluorochromic dual-functional AIE polymers

With the advent of the big data era, information storage and security are becoming increasingly important. However, high capacity information storage and multilevel anti-counterfeiting are typically difficult to achieve simultaneously. To address this challenge, herein, two electrochromic and electrofluorochromic dual-functional polymers with aggregation-induced emission (AIE) characteristics were rationally designed and facilely prepared. Upon applying voltages, the absorption and fluorescence spectra of the AIE polymers can undergo reversible changes, accompanied by variation of their color and emission. By utilizing the controllable characteristics of the polymers, dual-mode display devices were fabricated via a simple spraying technique. More interestingly, a four-dimensional color code device was constructed by adding color change multiplexing to the two-dimensional space, thereby achieving high capacity information storage. Moreover, the color code device can also be applied in the multilevel anti-counterfeiting area. The encrypted information can be dynamically converted under different voltages. Thus, the AIE polymers show great promise for applications in multidimensional information storage and dynamic anti-counterfeiting, and the design strategy may provide a new avenue for advanced information storage and high security technology.

Electrochemical Fluorescence Switch of Organic Fluorescent or Fluorogenic Molecules

This short review is aimed at emphasizing the most prominent recent works devoted to the fluorescence modulation of organic fluorescent or fluorogenic molecules by electrochemistry. This still expanding research field not only addresses the smart uses of known molecules or the design of new ones, but also investigates the development of instrumentation providing time- and space-resolved information at the molecular level. Important considerations including fluorescent/fluorogenic probes, reversible/irreversible fluorescence switch, direct/indirect fluorescence modulation, or environment properties are especially scrutinized in recent works dealing with bioanalysis perspectives.

State of the Art in the Preparation and Properties of Molecular Monomeric s-Heptazines: Syntheses, Characteristics, and Functional Applications

This review gives an account on the fast expanding field of monomeric (or molecular) heptazines, at the exclusion of their various polymeric forms, often referred to as carbon nitrides. While examples of monomeric heptazines were extremely limited until the beginning of this century, the field has started expanding quickly since then, as has the number of reports on polymeric materials, though previous reviews did not separate these fields. We provide here a detailed report on the synthetic procedures for molecular heptazines. We also extensively report on the different achievements realized from these new molecules, in the fields of physical chemistry, spectroscopy, materials preparation, (photo)catalysis, and devices. After a comprehensive summary and discussion on heptazines syntheses and characteristics, we show that starting from well-defined molecules allows a versatility of approaches and a wide tunability of the expected properties. It comes out that the field of monomeric heptazines is now emerging and possibly heading toward maturity, while diverging from the one of polymeric carbon nitrides. It is likely that this area of research will quickly surge to the forefront of the search for active organic molecules, with special attention to the domains of catalysis and organic-based functional materials and devices.

Reversibly Switchable Fluorescent Molecular Systems Based on Metallacarborane-Perylenediimide Conjugates

Icosahedral metallacarboranes are θ-shaped anionic molecules in which two icosahedra share one vertex that is a metal center. The most remarkable of these compounds is the anionic cobalt-based metallacarborane [Co(C₂ B₉ H₁₁ )₂ ]^- , whose oxidation-reduction processes occur via an outer sphere electron process. This, along with its low density negative charge, makes [Co(C₂ B₉ H₁₁ )₂ ]^- very appealing to participate in electron-transfer processes. In this work, [Co(C₂ B₉ H₁₁ )₂ ]^- is tethered to a perylenediimide dye to produce the first examples of switchable luminescent molecules and materials based on metallacarboranes. In particular, the electronic communication of [Co(C₂ B₉ H₁₁ )₂ ]^- with the appended chromophore unit in these compounds can be regulated upon application of redox stimuli, which allows the reversible modulation of the emitted fluorescence. As such, they behave as electrochemically-controlled fluorescent molecular switches in solution, which surpass the performance of previous systems based on conjugates of perylendiimides with ferrocene. Remarkably, they can form gels by treatment with appropriate mixtures of organic solvents, which result from the self-assembly of the cobaltabisdicarbollide-perylendiimide conjugates into 1D nanostructures. The interplay between dye π-stacking and metallacarborane electronic and steric interactions ultimately governs the supramolecular arrangement in these materials, which for one of the compounds prepared allows preserving the luminescent behavior in the gel state.

Spectroelectrochemistry: A Powerful Tool for Studying Fundamental Properties and Emerging Applications of Solid-State Materials Including Metal–Organic Frameworks

Spectroelectrochemistry (SEC) encompasses a broad suite of electroanalytical techniques where electrochemistry is coupled with various spectroscopic methods. This powerful and versatile array of methods is characterised as in situ, where a fundamental property is measured in real time as the redox state is varied through an applied voltage. SEC has a long and rich history and has proved highly valuable for discerning mechanistic aspects of redox reactions that underpin the function of biological, chemical, and physical systems in the solid and solution states, as well as in thin films and even in single molecules. This perspective article highlights the state of the art in solid-state SEC (ultraviolet–visible–near-infrared, infrared, Raman, photoluminescence, electron paramagnetic resonance, and X-ray absorption spectroscopy) relevant to interrogating solid state materials, particularly those in the burgeoning field of metal–organic frameworks (MOFs). Emphasis is on developments in the field over the past 10 years and prospects for application of SEC techniques to probing fundamental aspects of MOFs and MOF-derived materials, along with their emerging applications in next-generation technologies for energy storage and transformation. Along with informing the already expert practitioner of SEC, this article provides some guidance for researchers interested in entering the field.

An improvement in the coloration properties of Ag deposition-based plasmonic EC devices by precise control of shape and density of deposited Ag nanoparticles

Ag nanoparticles exhibit various colors depending on their localized surface plasmon resonance (LSPR). Based on this phenomenon, Ag deposition-based electrochromic devices can represent various optical states in a single device such as the three primary colors (cyan, magenta, and yellow), silver mirror, black and transparent. A control of the morphology of Ag nanoparticles can lead to dramatic changes in color, as their size and shape influence the LSPR band. In this research, we focused on the diffusion rate of Ag⁺ ions when Ag nanoparticles are electrochemically deposited. Consequently, well-isolated Ag nanoparticles were obtained due to the slow growth rate by using an electrolyte with a low concentration of Ag⁺ ions, resulting in an improvement in the color quality of cyan and magenta. Additionally, spherical Ag nanoparticles were deposited in the same device by optimizing their voltage application conditions, which represented yellow and green colors. In particular, green coloration is a unique phenomenon because it can appear by the combination of two absorption peaks of LSPR. As a result of investigating the finite-difference time-domain method, it was observed that the LSPR band in the long wavelength region was originated from the effects of the connection between Ag particles.

A Single-Pixel RGB Device in a Colorful Alphanumeric Electrofluorochromic Display

With the rapid development of human society, consumer electronics have brought marvelous changes to human daily life, but they are accompanied by the much higher demand of display quality and visual experience. Therefore, ideal conversion among the three primary colors-red (R), green (G), and blue (B)-in a single pixel has been a better way to avoid the insurmountable technical barrier of subpixel technology of modern displays. Electrofluorochromic (EFC) materials capable of a novel luminescent switching, open a powerful way to design optoelectronic devices for displays and information storage etc. Colorful EFC devices, especially emitting the ideal three primary colors without subpixel technology, have been a challenge for years. Herein, a long-awaited single-pixel device with RGB color is fabricated successfully based on proton-coupled electron transfer. The RGB EFC device exhibits outstanding EFC properties, such as low turn-on voltage (+1.0 and -1.0 V), large color gamut, and good stability (500 cycles for each color). Prototypes of colorful alphanumeric displays are well demonstrated in a facile way. The success of this new exploration of single-pixel RGB EFC device not only provides the possibility of full-color emission in EFC devices, but also will widely broaden the EFC system and their applications.

1,2,4,5-Tetrakis(tetramethylguanidino)-3,6-diethynyl-benzenes: Fluorescent Probes, Redox-Active Ligands and Strong Organic Electron Donors

In this work, the change of reactivity induced by the introduction of two para-ethynyl substituents (CCSi(iPr)3 or CCH) to the organic electron-donor 1,2,4,5-tetrakis(tetramethylguanidino)-benzene is evaluated. The redox-properties and redox-state dependent fluorescence are evaluated, and dinuclear CuI and CuII complexes synthesized. The Lewis-acidic B(C6 F5 )3 substitutes the proton of the ethynyl -CCH groups to give new anionic -CCB(C6 F5 )3 - substituents, leading eventually to a novel dianionic strong electron donor in its diprotonated form. Its two-electron oxidation with dioxygen in the presence of a copper catalyst yields the first redox-active guanidine that is neutral (instead of cationic) in its oxidized form.

Solution-Processable Copolymers Based on Triphenylamine and 3,4-Ethylenedioxythiophene: Facile Synthesis and Multielectrochromism

In comparison with traditional inorganic electrochromic materials, organic polymers offer advantages such as fast switching speed, flexibility, lightweightness, low cost and nontoxicity, solution-processability, and color tunability. Herein, a series of hyper-branched copolymers are synthesized from triphenylamine and 3,4-ethylenedioxythiophene with different feed ratios via iron(III) chloride (FeCl₃ )-mediated oxidative polymerization. The resultant organic-soluble polymers are easily processable and their corresponding electrochromic devices are found to be stable with limited degradation upon 2500 cycles. In addition to their facile synthesis to achieve solution-processable polymers, studies also show that the polymers exhibit multielectrochromic properties and give rise to five colored states upon oxidative-doping by applying an external voltage between 0 and 2.0 V, providing an interesting example of polymers with unique electrochromic switching among up to five colors, from yellow at the neutral state, to pale green, pale purple, orange, and finally gray.

Visualising electrochemical reaction layers: mediated vs. direct oxidation

Electrochemical treatments are widely used for 'clean up' in which toxic metals and organic compounds are removed using direct or mediated electrolysis. Herein we report novel studies offering proof of concept that spectrofluorometric electrochemistry can provide important mechanistic detail into these processes. A thin layer opto-electrochemical cell, with a carbon fibre (radius 3.5 μm) working electrode, is used to visualise the optical responses of the oxidative destruction of a fluorophore either directly, on an electrode, or via the indirect reaction of the analyte with an electrochemically formed species which 'mediates' the destruction. The optical responses of these two reaction mechanisms are first predicted by numerical simulation followed by experimental validation of each using two fluorescent probes, a redox inactive (in the electrochemical window) 1,3,6,8-pyrenetetrasulfonic acid and the redox-active derivative 8-hydroxypyrene-1,3,6-trisulfonic acid. In the vicinity of a carbon electrode held at different oxidative potentials, the contrast between indirect electro-destruction, chlorination, and direct oxidation is very obvious. Excellent agreement is seen between the numerically predicted fluorescence intensity profiles and experiment.

Multicolored Cathodically Coloring Electrochromism and Electrofluorochromism in Regioisomeric Star-Shaped Carbazole Dibenzofurans

In this work, a series of fluorescent cathodically coloring electrochromic (EC) small molecules o-, m-, and p-DBFDCz with 3,5-di(9H-carbazol-9-yl)benzene (DCz) linked to dibenzofuran (DBF) at different substitutional positions were synthesized and fully characterized. These compounds are electroactive and undergo quasi-reversible two-step single-electron reduction generating radical anions and dianions. The absorptions of o-, m-, and p-DBFDCz in the neutral states lie in the UV region (λ_onset ≈ 350 nm), showing high transparency, while the absorption of their reduced states can be largely tuned across the visible region through driving voltage and substitutional positions. Initially generated spectroelectrochemically radical anions show absorption in the short-wavelength region of ∼380-500 nm with weak broad absorptions at longer wavelengths. On further reduction, these bands disappear on the cost of growing intense bands from dianions at longer wavelengths of 500-700 nm with some tail absorptions in the shorter-wavelength region. This renders the colors of the EC devices based on these materials, which are changed from green to red, yellow to magenta, and light to deep blue for o-, m-, and p-DBFDCz, respectively, covering four legs of the L*a*b* color space. Besides excellent optical contrast (>90%) and high coloration efficiency (up to 504 cm² C^-1), the fluorescence observed in solution of neutral o-, m-, and p-DBFDCz can be modulated between the fluorescence and quenched states by direct electrochemical redox reactions. Both EC and electrofluorochromic (EFC) processes are reversible on cycling. This research demonstrates the feasibility of developing multifunctional EC/EFC materials with multicolored electrochromism through exploiting electrochemical properties of traditional fluorescent small molecules.

Luminescence Tunable Europium and Samarium Complexes: Reversible On/Off Switching and White-Light Emission

Single-molecule functional materials with luminescence tunable by external stimuli are of increasing interest due to their application in sensors, display devices, biomarkers, and switches. Herein, new europium and samarium complexes with ligands having triphenylamine (TPA) groups as the redox center and 2,2'-bipyridine (bpy) as the coordinating groups and diketonate (tta) as the second ligand have been constructed. The complexes show white-light emission in selected solvents for proper mixtures of the emission from Ln³⁺ ions and the ligands. Meanwhile, they exhibit reversible luminescence switching on/off properties by controlling the external potential owing to intramolecular energy transfer from the Ln³⁺ ions to the electrochemically generated radical cation of TPA^•+. Time-dependent density functional theory (TD-DFT) calculations have been performed to study the electronic spectra. The proposed intramolecular energy transfer processes have been verified by density functional theory (DFT) studies.

Dual-Switching Electrochromism and Electrofluorochromism Derived from Diphenylamine-Based Polyamides with Spirobifluorene/Pyrene as Bridged Fluorescence Units

Electrochromic (EC)/electrofluorochromic (EFC) bifunctional materials are receiving great attention because of their promising applications in optoelectronic devices. However, the development of ideal EC/EFC bifunctional materials is still a great challenge because of the poor integration of EC/EFC performances (optical contrast, response speed, and switching stability). Herein, we reported two novel diphenylamine-based mixed valence (MV) polyamides (S-HPA and P-HPA) with spirobifluorene (2,7-positions) and pyrene (1,6-positions) as bridged fluorescence units, respectively, showing impressive cyclability and fluorescence contrast with rapid switching. Through the formation of an effective electronic coupling between the two nitrogen centers using spirobifluorene/pyrene bridges, we demonstrated that different bridges have significant effects on the thermal and electrooptical characteristics of polyamides. In addition to lower fluorescence quantum yield and glass transition temperature, the S-HPA exhibited superior cyclability (contrast change <3.4%/14% over 500/300 cycles for EC/EFC switching), higher color/fluorescence contrast (64%/304%), and faster switching time (<2.6 s), mainly owing to the shorter conjugated length and more twisted configuration of the spirobifluorene bridge. The design principle of MV polymers with fluorophore bridges proposed here will be a promising way to realize high-performance EC/EFC devices and will also provide new insights into their future development and applications.

1,2,5,6-Tetrakis(guanidino)-Naphthalenes: Electron Donors, Fluorescent Probes and Redox-Active Ligands

New redox-active 1,2,5,6-tetrakis(guanidino)-naphthalene compounds, isolable and storable in the neutral and deep-green dicationic redox states and oxidisable further in two one-electron steps to the tetracations, are reported. Protonation switches on blue fluorescence, with the fluorescence intensity (quantum yield) increasing with the degree of protonation. Reactions with N-halogenosuccinimides or N-halogenophthalimides led to a series of new redox-active halogeno- and succinimido-/phthalimido-substituted derivatives. These highly selective reactions are proposed to proceed via the tri- or tetracationic state as the intermediate. The derivatives are oxidised reversibly at slightly higher potentials than that of the unsubstituted compounds to dications and further to tri- and tetracations. The integration of redox-active ligands in the transition-metal complexes shifts the redox potentials to higher values and also allows reversible oxidation in two potentially separated one-electron steps.

Donor-Acceptor 1,2,4,5-Tetrazines Prepared by the Buchwald-Hartwig Cross-Coupling Reaction and Their Photoluminescence Turn-On Property by Inverse Electron Demand Diels-Alder Reaction

A facile efficient synthetic tool, Buchwald-Hartwig cross-coupling reaction, for the functionalization of 1,2,4,5-tetrazines is presented. Important factors affecting the Buchwald-Hartwig cross-coupling reaction have been optimized. Seven new donor-acceptor tetrazine molecules (TA1-TA7) were conveniently prepared in good to high yields (61-72%). They have been subsequently engaged in the inverse electron demand Diels-Alder (iEDDA) reaction with cyclooctyne. The photophysical and electrochemical properties of the new pyridazines have been studied. Some are fluorescent acting as turn-on probes. More importantly, two pyridazines (DA3 and DA6) exhibit room-temperature phosphorescence (RTP) properties.

A viologen-based multifunctional Eu-MOF: photo/electro-modulated chromism and luminescence

A novel viologen-based multifunctional Eu-MOF integrating photochromism, photomodulated fluorescence, and electrochromic and electroluminochromic properties was investigated.

Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates

Multicolored electrochromic and electrofluorochromic materials containing triphenylamine and benzoates were developed.

Self-Driven Multicolor Electrochromic Energy Storage Windows Powered by a "Perpetual" Rechargeable Battery

Electrochromic windows (ECWs) become an appealing concept for green buildings. However, conventional ECWs need external biases to operate causing energy consumption and are usually restricted by monotonous color. Recently, electrochromic energy storage windows (EESWs) integrating the functions of electrochromism and energy storage in one device have attracted particular attention in various fields, such as self-powered addressable displays, human-readable batteries, and most importantly energy-efficient smart windows. Herein, a color-tunable (nonemissive-red-yellow-green) self-powered EESW is initially presented utilizing Prussian blue (PB) as a controller of the fluorescent component of CdSe quantum dots. The key design feature is that without any external stimuli, the EESW can be powered by a rechargeable "perpetual" battery, which is composed of two half-cell couples of Fe/PB and Prussian white (PW)/Pt. This technique allows to achieve only by switching the connection status of the two half-cells, the fast discharging and self-charging process of the EESWs with high and sustainable charge-storage capacity. Remarkably, the fabricated self-powered EESWs exhibit quick response ("off" 7 s, "on" 50 s), large transmittance spectra contrast, and high fluorescent contrast modulation (60-86%) over a wide optical range, and great reproducibility (only 3% of the modulation ratio decreased after 30 cycles), which is comparable to ECWs powered by an electrochemical potentiostat.