An in situ colorimetric measurement study of electrochromism in the di-n-heptyl viologen system

An in situ colorimetric method, based on the CIE (Commission Internationale de l'Eclairage) system of colorimetry, has been applied to the study of the electrochromic N,N´-bis(n-heptyl)-4,4´-bipyridylium (‘di-n-heptyl viologen’) system in aqueous solution on transmissive ITO/glass substrates. On electrochemical reduction of the di-n-heptyl viologen di-cation, the purple di-n-heptyl viologen radical cation salt deposits as a film and the changes in hue and saturation have been tracked using CIE 1931 xy chromaticity coordinates. The CIELAB 1976 colour space coordinates of the purple di-n-heptyl viologen radical cation salt were L* = 76, a* = 33 and b* = –20, with a complementary wavelength of 548 nm. A sharp decrease in luminance was found on formation of the di-n-heptyl viologen radical cation salt. Colour coordinates for the reverse (oxidation) direction plots show hysteresis, implying that specific choice of colour values depends on both the potential applied and from which direction the potential is changed

In situ colorimetric and composite coloration efficiency measurements for electrochromic Prussian blue

An in situ colorimetric method, based on the CIE (Commission Internationale de l’Eclairage) system of colorimetry, has been successfully applied to the study of electrochromism in electrochemically deposited films of Prussian blue (iron(III) hexacyanoferrate(II), PB) on transmissive ITO/glass substrates for the first time. On electrochemical reduction of PB to Prussian white (iron(II) hexacyanoferrate(II), PW), sharp and reversible changes in the hue and saturation occur, as shown by the track of the CIE 1931 xy chromaticity coordinates. For PB, the CIELAB 1976 colour space coordinates were L* 5 73, a*5226 and b*5233, with a dominant wavelength calculated as 488 nm. Concurrently, as the intensely absorbing PB mixed-valence chromophore is ‘bleached’ to the transparent PW, a large increase in the relative luminance of the electrochromic film is observed. On oxidation of PB, the CIELAB 1976 colour space coordinates show the transition through intermediate green to the Prussian yellow (iron(III) hexacyanoferrate(III), PY) state (L* 5 94, a* 5 2 and b* 5 18), with a steady increase in relative luminance. To reliably compare the power requirement of PB films with other electrochromic systems, composite coloration efficiencies (CCE’s) have been calculated, using a tandem chronoabsorptometry/chronocoulometry method, as previously developed for organic polymer systems. Using 95% of the total transmittance change at lmax as reference point, coloration efficiencies, g 5 DA(lmax)/Q, were calculated as 143 and 150 cm2 C21 respectively for the PB/PW and PW/PB electrochromic transitions

Electrochromic organic and polymeric materials for display applications

An electrochromic material is one where a reversible color change takes place upon reduction (gain of electrons) or oxidation (loss of electrons), on passage of electrical current after the application of an appropriate electrode potential. In this review the general field of electrochromism is introduced, with coverage of the types, applications, and chemical classes of electrochromic materials and the experimental methods that are used in their study. The main classes of electrochromic organic and polymeric materials are then surveyed, with descriptions of representative examples based on transition metal coordination complexes, viologen systems, and conducting polymers. Examples of the application of such organic and polymeric electrochromic materials in electrochromic displays are given

Dioxypyrrole-Based Polymers via Dehalogenation Polycondensation Using Various Electrophilic Halogen Sources

A convenient and efficient deiodination polycondensation method for the synthesis of dioxypyrrole-based (XDOP) polymers is reported. <i>N</i>-Halosuccinimides, iodine, and bromine were evaluated as halogenating agents to produce 2,5-halodioxypyrroles <i>in situ</i> via halodecarboxylation of 3,4-dioxypyrrole-2,5-dicarboxylic acids, which were then polymerized at 60 °C using dichloromethane or chloroform as solvent. When iodine and <i>N</i>-iodosuccinimide were employed as electrophilic halogen sources, the methodology produced macromolecules (<i>M</i>_n = 6.2–22.9 kDa) in satisfactory yields (55–71%) for two <i>N</i>-alkyl-3,4-dialkyloxypyrrole-based monomers that were tested. This method can be employed to produce a variety of XDOP-based homopolymers and regioregular copolymers starting from discrete oligomers under relatively mild reaction conditions

Discrete Donor–Acceptor Conjugated Systems in Neutral and Oxidized States: Implications toward Molecular Design for High Contrast Electrochromics

Donor–acceptor systems are ubiquitous redox-active materials in electrochromic devices, making the study of their neutral and charged state characteristics expedient for the design of materials with improved properties. In this paper, we explore the absorption properties of the neutral and oxidized states of two dioxythiophene- and benzothiadiazole-containing penta- and hepta-heterocycles (EPBPE, EPPBPPE) having a monodisperse, well-defined π-conjugated structure, using electrochemistry, optical absorption and electron paramagnetic resonance (EPR) spectroscopy, spectroelectrochemistry, and microscopy. The molecules and their precursors were obtained via a direct (hetero)­arylation coupling strategy that exploits stoichiometric control to obtain well-defined ter- and penta-heterocycles from bifunctional heteroarenes. Both molecules show intense and narrow dual-band absorptions in the visible region, reflecting the discrete nature of their π-systems, leading to strongly colored neutral states. The electron-rich dioxythiophene units enable access to their radical cation and dication states at potentials below 5 mV and 260 mV (vs ferrocene/ferrocenium), respectively, and give rise to stability toward repeated oxidative switching (voltammetric cycling). EPR and absorption spectroscopy of their chemically and electrochemically derived oxidized states showed them to be dominated by polaronic, π-dimeric, and, in the case of EPPBPPE, bipolaronic charge carriers. These species exhibited transitions with maxima in the near-IR region, leading to highly transmissive oxidized states and promising structures for high contrast electrochromics. A polymer (Poly-EPBPE) that maintains a discrete conjugated segment along the backbone was also designed using EPBPE as the multi-ring heterocycle linked together with an aliphatic <i>n</i>-decyl chain, to obtain a mechanically robust yet solution processable material. Poly-EPBPE showed narrow optical transitions and well-resolved oxidation waves in solution that correlated strongly with the properties of EPBPE. However, strong intermolecular interactions were observed in the absorption spectroscopy and electrochemistry of its film state. The oxidized state absorption properties of Poly-EPBPE reflected these interactions, with absorption properties dominated by π-dimers and higher order aggregates, leading to irreversibility in its film spectroelectrochemistry. The coupled structural, optical, electrochemical, magnetic, and microscopic studies enabled us to propose potential resonance structures of the charge carriers in these discrete conjugated systems and inform the design of high contrast electrochromic materials

Direct Imide Formation from Thiophene Dicarboxylic Acids Gives Expanded Side-Chain Selection in Thienopyrrolediones

The synthesis of thienopyrroledione (TPD) has been updated to reduce the number of synthetic steps, remove hazardous and toxic reagents, reduce the amount of byproduct waste, and reduce the use of solvents when unnecessary. Diverse functionalization is possible, introducing 16 examples in yields from 34% to 95%. This reaction scheme was shown to be general for thiophene imides, and a more thorough exploration into side chain engineering is presented with TPD acceptors often used in organic electronic applications

Efficient Synthesis of Dithienogermole (DTG) Derivatives via Olefin Cross-Metathesis

4,4′-Bis-(4-pentenyl)-dithieno[3,2-<i>b</i>:2′,3′-<i>d</i>]germole was synthesized as a functional building block for the efficient preparation of dithienogermole (DTG) derivatives with varying alkyl chain lengths and pendant functionalities in excellent yields. These derivatives were efficiently isolated via olefin cross-metathesis followed by hydrogenation

Conjugated Polyelectrolytes as Water Processable Precursors to Aqueous Compatible Redox Active Polymers for Diverse Applications: Electrochromism, Charge Storage, and Biocompatible Organic Electronics

An organic soluble precursor polymer was prepared by direct (hetero)­arylation polymerization of 3,4-ethylenedioxythiophene (EDOT) with a solubilizing, 3,4-propylenedioxythiophene (ProDOT) derivative bearing ester-functionalized side chains. Chemical defunctionalization of the polymer, using base to hydrolyze the esters, yields a conjugated polyelectrolyte (CPE) that is readily soluble in water. This aqueous soluble CPE can then be processed using high-throughput coating methods from water-based inks. Postprocessing functionalization of the polymer film using dilute acid creates a solvent resistant film that is compatible with both organic and aqueous electrolyte systems for redox switching. The introduction of an unfunctionalized EDOT unit results in a soluble polymer that has a low oxidation potential and that is highly electroactive and pseudocapacitive in a wide voltage range (2 V in propylene carbonate-based electrolytes and 1.55 V in aqueous electrolytes) making it an attractive material for lightweight and flexible supercapacitors. Films of this copolymer demonstrate exceptionally rapid redox switching (10 V/s) and higher mass capacitance in aqueous electrolyte solutions than in organic solutions. Supercapacitors incorporating the solvent resistant copolymer exhibit symmetric charge/discharge behavior at currents of up to 20 A/g (1 s discharge) and are able to maintain >75% of the initial capacitance over 175 000 cycles using 0.5 M NaCl/water as the device electrolyte. Rapid electrochromic switching (∼0.2 s) from vibrant blue to colorless is also maintained in this salt–water electrolyte. The versatility of this polymer is further shown in a series of organic and aqueous electrolyte systems, including biologically compatible electrolytes (NaCl/water, Ringer’s solution, and human serum) and even sport drinks (Gatorade and Powerade), demonstrating the robustness of this polymer to differing ionic conditions. Based on these results, it is apparent that this polymer and similar systems have great potential in multiple electrochemical applications such as electrochromic devices, supercapacitors, and biocompatible devices

Designing a Soluble PEDOT Analogue without Surfactants or Dispersants

Copolymerization of alkoxy-functionalized 3,4-propylene­dioxy­thiophenes (ProDOTs) with unfunctionalized 3,4-ethylene­dioxy­thiophenes (EDOTs) in varying ratios using direct arylation yields a series of solution processable polymers with highly tunable optical and electronic properties. Within this series, we have identified ProDOT–EDOT₂, a copolymer containing 67% EDOT compositionally, that combines the low oxidation potential, the redox behavior, and the deep-blue neutral color that are characteristic of PEDOT with the high solubility, exceptional electrochromic contrast, and color neutrality in the oxidized state characteristic of alkoxy-functionalized PProDOTs

Efficient Synthesis of Benzo[1,2‑<i>b</i>:6,5‑<i>b</i>′]dithiophene-4,5-dione (BDTD) and Its Chemical Transformations into Precursors for π‑Conjugated Materials

A straightforward synthesis of the fused-aromatic dione benzo[1,2-<i>b</i>:6,5-<i>b</i>′]dithiophene-4,5-dione (BDTD) has been developed. This fused-aromatic dione was subjected to various chemical transformations to generate diverse molecules with potential use in π-conjugated materials for organic electronics