Polymeric Electrochromes

Many aromatic ring systems including aniline, pyrroles and thiophenes form extensively conjugated, electroactive polymers, which can be oxidised or reduced between 

The other important electrochromic polymers are the polypyrroles and polythiophenes, obtained by polymerisation of the parent pyrrole and thiophene or, more importantly, their 3,4-substituted derivatives. The most widely studied of these two classes of polymers in electrochromic outlets are the poly thiophenes, which are readily synthesised by the reaction of the substituted monomer with FeClj in chloroform solution. The colour change properties of a variety of poly thiophenes in the presence of a counter-ion are shown in Table 1.14.  

One of the big drawbacks associated with the use of many conducting polymers as electrochromic materials is their low cycle life stability. To overcome this, and other electrochromic properties, many composite materials have been studied. These composites include mixtures with other optically complementary, conducting polymers and inorganic electrochromes, such as tungsten trioxide and Prussian Blue, and colour enhancing agents or redox indicators, exemplified by the inherently electrochromic indigo carmine (1.96). ° 

---

The added electron is delocalized on the monovalent radical ion to which it is reduced (3). There is no general agreement on the molecular representation of the reduced stmcture. Various other viologen compounds have been mentioned (9,12). Even a polymeric electrochromic device (15) has been made, though the penalty for polymerization is a loss in device speed. Methylviologen dichloride [1910-42-5] was dissolved in hydrated... 

Table 1. Comparison between inorganic and polymeric electrochromic materials...

Sonmez, G., Shen, C.K.E, Rubin, Y.,Wudl, E, 2004a. A red, green, and blue (RGB) polymeric electrochromic device (PECD) the dawning of the PECD era. Angewa. Chem. Int. Ed. 43,1498-1502. 

Beaujuge, P.M., Ellinger, S., Reynolds, J.R., 2008a. The donor-acceptor approach allows a black-to-transmissive switching polymeric electrochrome. Nat. Mater. 7,795-799. 

Sonmez, G., et al. 2005. A processable green polymeric electrochromic. Macromolecules 38 669. 

Sonmez, G., H. Meng, and F. Wudl. 2004. Organic polymeric electrochromic devices Polychro-mism with very high coloration efficiency. Chem Mater 16 (4) 574-580. 

Hu, H., L. Hechavarria, and J. Campos. 2003. Optical and electrical responses of polymeric electrochromic devices Effect of polyacid incorporation in polyaniline film. Solid State Ionics 161 165-172. 

These examples of electrochromic devices serve to display the breadth of their applicability to any number of systems. Polymeric electrochromics, particularly those based on thiophene and its derivatives, show promise for use in display technologies. The processability of many of these systems makes them specifically suited for large-area applications, such as smart windows, billboards or organic photovoltaic cells (solar cells), which currently suffer from large environmental and practical costs when fabricating large-area devices. 

Figure 20.12 Spectroscopy of PEDOT films showing optical changes with tones of blue colors at different potentials. Reprinted with permission from Red, Green, and Blue Colors in Polymeric Electrochromics. G. Sonmez, H. B. Sonmez, C. K. F. Shen, F. WudI, Adv. Mater., 16, 1907 (2004). Copyright Wiley-VCH Verlag GmbH.

Construction of a dual-polymer electrochromic device (type iii memory device) that has two face-to-face polymer layers in each cell producing 400 (20 x 20) different combinations of absorptions was shown by Sonmez and Sonmez [84], In this study, several examples of this 3x3 pixel dual polymeric electrochromic device composed of red poly(3-alkylthiophene), green poly(2,3-di(thien-3-yl)-5, 7-di(thien-2-yl)thieno[3,4-fc]pyrazine] and blue PEDOT polymers, which switch at different wavelengths, were presented. These distinct absorption states have led to speculation that electrochromic polymers could be used for memory storage devices and computing functionalities. 

Complexes of alkaline metal salts and the random co-polymer of ethylene oxide and epichlorohydrin, P(EO-EPI), in 84 16 proportion (Daiso), present high ionic conductivities at room temperature (20-40 mS cm at 30 °C, [H2O] < 0.01%). This co-polymer has been used in alt-polymeric electrochromic and photoelectrochemical devices, with satisfactory results. "... 

---