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

3,4-Alkylenedioxypyrroles: Functionalized Derivatives as Monomers for New Electron-Rich Conducting and Electroactive Polymers

New functionalized derivatives of 3,4-ethylenedioxypyrrole (EDOP, 5a) and 3,4-(1,3-propylenedioxy)pyrrole (ProDOP, 5b) as especially electron-rich monomers which yield highly electroactive and stable conducting polymers useful for a diverse set of applications have been synthesized. N-Alkylations of ProDOP were carried out to yield a variety of ProDOP derivatives having alkyl, sulfonatoalkoxy, glyme, and glyme alcohol pendant chains. Iodization of EDOP and ProDOP via iodo-decarboxylation afforded iodo-functionalized derivatives useful for subsequent aryl coupling chemistry. N-Protection and formylation of EDOP, followed by Knoevenagel condensation of the resultant 2-formyl-EDOP with aryl acetonitrile derivatives, led to 1-cyano-2-(2-(3,4-ethylenedioxypyrryl))-1-(2-thienyl)vinylene (23) (Th-CNV-EDOP) and 1-cyano-2-(2-(3,4-ethylenedioxypyrryl))-1-(2-(3,4-ethylenedioxythienyl)vinylene (26) (EDOT-CNV-EDOP). A 14-crown-4-ether 34 based dioxypyrrole was synthesized with a cavity potentially useful for lithium ion coordination and sensing in the resultant electroactive polymer. C-Alkylated ProDOPs (43a, 43b, and 43c) containing octyl, ethylhexyl, and dioctyl substituents appended to the central methylene of the propylene bridge, were prepared as monomers for potentially soluble π-conjugated polymers

Synthesis of π-Conjugated Molecules Based on 3,4-Dioxypyrroles <i>via</i> Pd-Mediated Decarboxylative Cross-Coupling

A general scheme for the synthesis of π-conjugated molecules based on 3,4-dioxypyrroles is presented. The π-conjugated molecules were synthesized via Pd-mediated decarboxylative cross-coupling using various 3,4-propylenedioxypyrrole carboxylic acids and aryl bromides, including the base-sensitive electron acceptor 4,7-dibromobenzo­[c]­[1,2,5]­thiadiazole (BTD). N-Methylpyrrolidone was used as solvent, Pd­(acac)2 was employed as the palladium source and P­(o-tol)3 as the ligand. The methodology was applied to 3,4-dioxypyrrole monoacids and 3,4-dioxypyrrole diacids to produce multi-ring π-conjugated systems containing phenyl, thiophenyl, BTD, and pyridinyl units. In general, the method has yielded a practical approach for the synthesis of 3,4-dioxypyrrole-based π-conjugated molecules in acceptable to high yields of 44–94%

Simple Interface Modification of Electroactive Polymer Film Electrodes

Understanding the role of interface properties is crucial in the search for alternative design strategies to optimize the efficiency, performance, and lifetime of both solid-state and redox active organic semiconductor devices. Recent advances have focused on controlling and tailoring interfacial effects on the morphology and molecular structure of the active film in multilayer devices triggering new developments in the area of interface engineering. Here, we demonstrate that an inorganic electrode/organic semiconductor interface modification using PEDOT:PSS as an interfacial material influences the charge and ion transport, capacitive, morphological, and color switching properties of a solution processed purple-to-clear switching electrochromic PProDOT-(CH2OEtHx)2 polymer film. We find that the barrier to charge transport from electrode to active material is lowered when adding this PEDOT:PSS film, allowing us to present a fully roll-to-roll compatible, simple, and versatile battery-type electrochromic device (ECD) design without the need for oxidizing the charge storage film, in combination with improved processing reproducibility. In addition to producing ECDs with minimal color differences compared to devices prepared in the more traditional and complicated manner, this new ECD design strategy provides competitive performance showing a consistent optical contrast of 50–55% and switching times of 2–4 s

3,4-Propylenedioxypyrrole-Based Conjugated Oligomers <i>via</i> Pd-Mediated Decarboxylative Cross Coupling

An effective decarboxylative cross-coupling involving a 3,4-dioxypyrrole is reported. Several conjugated oligomers were synthesized in high yields using various aryl bromides. No copper salt or other transmetalating agent was required. The reaction conditions employed displayed relatively low sensitivity toward the presence of water

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