Electronic polymers electrochromic

Through the hybrid incorporation and copolymerization of various conjugated units, the electronic and electrochromic properties of the resulting polymers can be varied to cover colors across the entire visible spectrum. In this manner, the advantages of different conjugated vmits (i.e., low oxidation potential or solubilizing substituents) can be utilized while playing on the interaction of the comonomers (i.e., with the donor-acceptor systems). 

Types of Electrochromic Materials. Electrochromic materials are of three basic types (2). In a given electrolyte solution, type I materials are soluble in both the reduced and oxidized (redox) states. Type II materials are soluble in one redox state, but form a solid film on the surface of an electrode following electron transfer. Electrochromic polymers are examples of type III materials, where both redox states are solids, generally studied as thin films on electrode surfaces. For types II and III, once the redox state has been switched, no further charge injection is needed to retain the new electrochromic state, and such systems are said to have optical memory . In contrast, for type I electrochromic materials, diffusion of the soluble electrochemically-generated product material away from the electrode occurs and it is necessary to keep current flowing until the whole solution has been electrolyzed. 

SESHADRI, V., PADILLA, J., BIRCAN, H., RADMARD, B., DRAPER, R., et al., Optimization, preparation, and electrical short evaluation for 30 cm active area dual conjugated polymer electrochromic windows. Organic Electronics, 2007, 8,367-81. 

Polymers, large molecules made up of smaller molecules in a repeating pattern, are used for many electrochromic materials. Conjugating polymers, which have alternating single and double bonds, are particularly suitable. Figure B shows the electrochemical oxidation of the conjugated polymer, polythiophene. Oxidation (in which electrons are removed) produces a semiconductive polymer. The neutral (unoxidized) polythiophene is red in color, whereas the semiconductive polythiophene (oxidized) is blue. In their neutral... 

The conformational mobility of a chromophoric main-chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or electrochromisms. It is widely reported that o-conjugating polysilanes exhibit these phenomena remarkably clearly.34 However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational properties of the main chain, electronic state, and (chir)optical characteristics. In 1996, we reported that in various polysilanes in tetrahydrofuran (THF) at 30°C, the main-chain peak intensity per silicon repeat unit, e (Si repeat unit)-1 dm3 cm-1, increases exponentially as the viscosity index, a, increases.41 Although conventional viscometric measurements often requires a wide range of low-dispersity molecular-weight polymer samples, a size exclusion chromatography (SEC) machine equipped with a viscometric detector can afford... 

Electronically conducting polymers also have an important role to play as electrodes in electrochromic devices. This is described in Chapter 9. 

The examples of polyacetylenes whose main chain is directly bonded to heteroaromatic rings (e.g., silole, carbazole, imidazole, tetrathiafulvalene, ferrocene) are increasing in number. Such polymers are usually obtained by one of catalysts (W, Mo, and Rh). The formed polymers are expected to display interesting (opto)electronic properties such as electrochromism, cyclic voltammetry, electroluminescence, and so on. 

Further research on the substitution of the thiophene 3-position with phenyl groups containing electron-withdrawing or electron-donating groups (such as methyl, methoxy, fluoro, chloro, bromo, trifluoromethyl, sulfoxy) in the para position have lead to polymers with unique features [57]. The electron-withdraw-ing groups allow the formation of a radical anion and thus stabilize the n-doped state. As a result, such conducting polymers can be reversibly oxidized and reduced and electrochromic devices can be built with identical anode and cathode materials [58]. 

Steady-state photoinduced electrochromism was achieved in organic solutions [34,35], microcrystals [36,37], LB films [7,8,38,39], and polymer films [40- 48] which was due only to the photoinduced electron transfer reaction via the excited state of specific IPCT complexes [49,50], The photochemical coloring and the thermal fading due to the reverse electron transfer were highly reversible in a deaerated atmosphere in all systems [7,8,34 18], The lifetime of colored (blue)... 

The conformational mobility of a chromophoric main chain polymer is often connected to its electronic structure. Therefore, changes in the UV-visible absorption spectra and/or chiroptical properties are spectroscopically observable as thermo-, solvato-, piezo-, or/and electrochromisms. It is widely reported that a-conjugating polysilanes exhibit these phenomena remarkably clearly [54], However, their structural origins were controversial until recently, since limited information was available on the correlation between the conformational prop-... 

Modified TiC>2 surfaces have also found application in the design of fast elec-trochromic devices. The influence of the substrate on the behavior of interfacial assemblies is well illustrated in this book. However, it is important to realize that the electrochromic behavior observed for modified TiC>2 surfaces was not expected. The oxidation and reduction of attached electrochromic dyes are not mediated by the semiconductor itself but by an electron-hopping process, not unlike that observed for redox polymers, where the electrochemical reaction is controlled by the underlying indium-tin oxide (ITO) contact. These developments show that devices based on interfacial assemblies are a realistic target and that further work in this area is worthwhile. 

In a few cases, polysilanes are chromotropic in the presence of an electric field (electrochromism). This was first shown for the copolymer (CF3CI BCtBSiMejn-co-t/i-PrSiMe)m, n m = 45 5558. In an electric field of 108 Vm 1the electronic absorption band for this polymer intensified by 50% and shifted from 294 to 299 nm. The changes are reversible when the field is removed. This is apparently the first example of electric field dependence of the absorption for any polymer, unaccompanied by electrochemical oxidation or reduction. The structural change accompanying this chromotropism is not understood. Other polysilanes with polar side groups may also show electrochromic behavior, but have not yet been studied. 

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