Electrochromic indicators

Intensive investigations of REM phthalocyanines have led to the foundation of several useful properties of them. These complexes are offered for utilazation as electrodes for electrochemical concentration and isolation of iodine from dilute water solutions [123, 124]. Changing of color of REM phthalocyanines in electric field became the basis for elaboration and preparation of rare earth electrochromic indicators [145, 146] and light filters [122]. 

Despite the considerable progress made in the few years in which anodic insertion/extraction films have been known, neither film compositions, film properties, nor electrochemical reactions are sufficientiy well characterized. There have been disagreements, as indicated for h-IrO and h-NiO in Table 2, as to whether is being extracted or OH inserted during coloration. The general problem is best illustrated by the important example of Pmssian blue. Early work (47—50) resulted in two different sets of equations for electrochromic reduction ... 

Quantitative analysis of the XPS data indicates a constant O/Ir ratio of close to 3 [34] and an OH/O ratio of 2 for a potential of 0.9 V and of 1/2 at 1.25 Vsce. These results, together with electrochemical data substantiate the reaction mechanism given in Fig. 26 for the electrochromic effect and for 02 evolution. 

Figure 7.6 Mirrors (a) an ordinary car driver s mirror reflects the lights of a following car, which can dazzle the driver (b) in an electrochromic mirror, a layer of optically absorbing chemical is electro-generated in front of the reflector layer, thereby decreasing the scope for dazzle. The width of the arrows indicates the relative light intensity...

In some contexts, an indicator (dye) such as this is said to be electrochromic, that is, the potential determines the colour (the name comes from the Greek word chromos, meaning colour ). A few representative redox indicator systems are shown in Table 4.1. 

Take apart the word electrochromic to indicate its meaning. 

By combining these two equations, we can say that if a material is electrochromic and the electrolysis is performed within a constant volume of solution, then the (faradaic) charge is proportional to the optical absorbance. This relationship of Abs a 2 is illustrated in Figure 8.1, where the absorbance, Abs, of the electrochromic colour (as y ) is seen to increase linearly as the charge increases (as x ). The linearity of the graph indicates that both Q and Abs relate to the same... 

Electrochromism The electrochemical generation of colour in accompaniment with a redox reaction, e.g. as displayed by a redox indicator. 

Redox indicator A chemical that changes colour (is electrochromic ) when undergoing redox change since the change occurs at a specific potential, the substance is able to function as an indicator. 

Interest has developed in electrochromic light transmission modulators, which are called smart windows , for control of temperature and lighting in buildings and automobiles. A cross section of an electrochromic light transmission modulator is shown in Fig. 11.31 (Rauh and Cogan, 1988). The two electrochromic elements of the structure are designated ECl and EC2, and are sandwiched between two thin film, optically transparent, electrodes of ITO and separated by an electrolyte. The ECl layer should colour when a negative potential is applied and the EC2 layer should either colour under positive potentials or remain in a transparent state. This is indicated by the chemical reactions ... 

The PFg" salts of [Ru(bpy)2(110)] and [Ru(110)3] and analogous complexes containing 4,4 -bis(substituted) ferrocenyl ligands (110 ), have been synthesized and characterized the tris(chelate) complexes are either poorly soluble or insoluble. Electropolymerization of [Ru(110 )3][PF6]2 produces an electrochromic film. The complex [Ru(bpy)2(lll)] undergoes electropolymerization on Pt and glassy carbon electrodes, although the related complex [Ru(bpy)2(112)] does not. Electrochemical and spectroscopic properties of the films indicate that they form by both head-to-tail and tail-to-tail monomer coupling. ... 

The chemistry and applications of the colour change grouping, containing all the well-known isms of chromic phenomena, namely photochromism, thermo-chromism, ionochromism, electrochromism and solvatochromism, as well as the lesser-known ones such as tribochromism and vapochromism, are covered in Chapter 1. These chromic phenomena impinge on our everyday life, e.g. in photo-chromic spectacle lens, thermochromic temperature indicators, fax paper, smart windows and mirrors and in visual displays. 

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). °... 

Fuel cell polymer battery photoelectric cell capacitor Storage element liquid crystal display device electrochromic display device electrochemiluminescence device photoelectric transducer Biosensor ion sensor detector in HPLC and FIA gas sensor voltam-metric indicator electrode reference electrode... 

The formation of colored radical cations (and accompanying radical anions) and the possibility of oxidation (or reduction) by an electrochemical process indicates the possibility of erasable photo-electrochromic devices. 

The electrochromism spectrum of the visible absorption region of [Ru(bpy)3] + recorded in a PVA polymer film exhibits a second derivative pattern (Figure 9) indicative of an excitation with a substantial A/x. A symmetric excitation of all three ligands leads to no excited state dipole moment. Electrochromism spectra of mono-immine complexes were also found to be similar to the tris-immine parent complex." These results were taken as evidence for localization in the symmetric complexes. 

The electrochromic shift of the carotenoids is usually calibrated with K-diffusion potential in the presence of valinomycin. One problem is that the shifts observed in respiring chromatophores (where the proton electrochemical potential is predominantly in the form of a membrane potential) are much larger than those induced by the calibrating diffusion potential, so that an extensive extrapolation is required. Thus, the carotenoids in illuminated chromatophores may indicate a membrane potential in excess of 300 mV, whereas the distribution of CNS, an electrically permeant anion, in the same system only indicates 140 mV [32]. The extent of this discrepancy, and the uncertainty as to whether the carotenoids see the bulk-phase potential, or only the local electrical field within the membrane, limits the confidence with which carotenoids may be used for quantitative as opposed to qualitative potential measurements. 

Fig. 4.4. (D and E) Primary electron acceptor (An 2) plastoquinone (or quencher Q). (D) Optical spectrum of the light induced plastosemiquinone anion (O), compared to the spectrum of PQ semi-quinone in non-aqueous solvent ( ). The additional spectral shifts at 545 and 685 nm are attributed to electrochromic effects on pheophytin (from Ref. 85). (E) ESR spectrum of A7, j reduced by light (a) or by dithionite (b) in Ch/amydomonas PSIl particles (from Ref. 87). (F) Secondary electron acceptor (An ) plastoquinone. (F) Flash-induced optical changes due to the reduction of the secondary electron acceptor plastoquinone the spectra oscillate in a dampened sequence following subsequent flashes indicating the production of semiquinone (1st and 3rd flash) and quinol species (2nd and 4th) (from Ref. 103).

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