Tungsten blue charge

Figure 8.1 Beer s law-type plot of change in optical absorbance against charge density q for the cell WO3 polymer electrolyte Prussian Blue. Reprinted from Inaba, H Iwaka, M., Nakase, K., Yasukawa, H., Seo, I. and Oyama, N., Electrochromic display device of tungsten trioxide and Prussian Blue films using polymer gel electrolyte of methacrylate , Electrochim. Acta, 40, 227-232 (1995), Copyright 1995, with permission from Elsevier Science.

Mechanistically the colour is formed by an optical charge-transfer between metal centres in the solid-state lattice, e.g. in tungsten trioxide this involves partial reduction of the pale yellow W" to the blue state. This reduction requires partial insertion of a balancing cation, as shown schematically in Figure 1.32, where M is usually lithium or hydrogen. 

The second study involved the local reduction or oxidation of an elec-trochromic material, tungsten oxide (33). This system is of particular interest since it has been the first system where the local injection of charge could be followed by an instantaneous change of color, making it possible to follow the process with an optical microscope. The stable form of tungsten oxide is W03, which is colorless and poorly conductive. The reduction of W03 [Eq. (5)] results in the formation of deep blue tungsten bronze that is substantially more conducting. 

So, H. and Pope, M. (1972). Origin of Some Charge-Transfer Spectra. Oxo Compounds of Vanadium, Molybdenum, Tungsten, and Niobium Including Heteropoly Anions and Heteropoly Blues,/no/j. Chem., 11, pp. 1441—1443. 

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