Electrochromic effect

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Iridium as an electrode material has received considerable attention in the last decade not only because of its excellent catalytic properties but also in relation to the electrochromic effect observed for anodic iridium oxide films (AIROF). Electrochromism of iridium was thought to be of technical relevance for display applications and triggered several studies of the electrochemical and optical properties of AlROFs [67, 85-88],... 

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. 

Casadio, R., Venturoli, G. and Melandri, B. A. (1988). Evaluation of the electrical capacitance in biological membranes at different phospholipid to protein ratios -a study in photosynthetic bacterial chromatophores based on electrochromic effects, Eur. Biophys. J., 16, 243-253. 

As a result, an insolnble transparent blue polymer film forms on the electrode. Electrochemical oxidation of the film in acetonitrile initiates a rapid color change from bine to pale gray, while redaction to the first or second cathodic waves causes the film to become pale green or orange, respectively. These electrochromic effects are stable and reversible when air and water are excluded, even after 30,000 rednction cycles. The material has potential uses in electrochromic or electroluminescent devices. 

Rare-earth bisphthalocyaninates, especially LuPc2, are one of the important objects of intense investigation because of their electrochemical (electrochromic effect), electrical, and optical properties. The ball-type four t-butyl-calix[4]arene bridged double decker lutetium(III) and indium(III) Pcs have also been prepared, Fig. 3 [42],... 

Fig. 4.2. (E and F) Primary electron acceptor (Aj) ubiquinone A. (E) Light-dark optical changes related to the production of radicals (maximal at 320 and 450 nm) and electrochromic effects on bacteriopheophytin (in the near infrared region) (from Ref. 288). (F) ESR spectrum of (obtained by chemical reduction) the large line broadening and shift is eliminated when Fe is removed from the RC (from Ref. 3).

The spectral characteristics of A [ 2 have been obtained both with optical and ESR spectroscopy. The optical spectrum exhibits band shifts near 450 and 680 nm, and a broad bleaching between 450 and 550 nm [57]. It has been suggested that the bleaching is due to the reduction of an Fe-S center and the shifts to electrochromic effects on nearby chlorophyll a molecules. The ESR spectrum however presents band of g value (2.08,1.90 and 1.78) uncharacteristic for a normal Fe-S protein [57]. The nature of X remains therefore still indetermined. 

Fig. 4.3. (D and E) Primary electron acceptor (A 12) electron acceptor X. (D) Light>dark spectrum of the electron acceptor X in PSI-RC, recorded at 5°C and —0.62 V — the broad band at 450-550 nm is attributed to an FeS center and the other changes to electrochromic effects (from Ref. 57). (E) ESR spectrum of X in PSI-RC, measured at 10 K (from Ref. 290).

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).

The presence of alkali metal ions is crucial for the stabilization of excess charge trapped within the nanopartides. Intercalation of metal ions within the nanoparticle thus becomes a limiting factor as the rate of transport of these ions becomes slower in thicker metal oxide films. This in turn controls the rate of coloration and recovery of the electrochromic effects. Limited efforts have also been made to employ mixed Ti02/W03 [145], WO3/V2O5 [146], and WO3/M0O3 [147] systems to enhance the efficiency of electrochromic effects. The beneficial aspect of these nanostructured semiconductor films in electrochromic devices is yet to be explored in a systematic way. 

Fig. 19.7. Dependence of electrochromic effect of WO3 particulate film on the charging potentials. (From [143]. Reprinted with permission from the American Chemical Society.)...

Bedja S. Hotchandani P. V. Kamat, Photoelectrochemistry of quantized WO3 colloids. Electron storage, electrochromic, and photo-electrochromic effects. J. Phys. Chem. 1993, 97, 11064-11070. 

Electrochromic materials are of interest for displays, smart windows, sunroofs, etc., and are characterized by the reversible change in their color upon application of light or electrical inputs. The electrochromic effect is generally associated with the ingress/issue of electrons and metal cations. 

Octacyanomolybdates and octacyanotungstates exhibit an insertion electrochemistry, which is rather similar to that of the hexacyanoferrates, [67] and the compounds show a pronounced electrochromic effect [68]. 

Because of their extensive technological applications, the anodic behavior of most of the nonnoble metals has been extensively investigated. Only a brief outline, with emphasis on the role of hydrous oxide material, is given here. The conditions necessary for the production of a thick hydrous oxide film on iron in base (repetitive cycling between -0.50 and 1.25 V at ca. 3.3 V s 1 at 25°C) have been outlined by Burke and Murphy.202 As outlined in Fig. 11, four peaks [with maxima at ca. -0.04, 0.13, 0.30, and 0.55 V (RHE)] were observed with the bare metal on the first anodic sweep, and only two (-0.05 and -0.20 V) on the subsequent cathodic sweep. On repetitive cycling the anodic peak at +0.3 V and the cathodic peak at -0.05 V became greatly enhanced and the presence of an electrochromic effect was noted. Further indication that a dispersed hydrous oxide film was produced under these conditions... 

Electrochromic materials is a remarkable and productive research area over the last three decades since it has potential applications in smart window products, e-papers, optical shutters, transmissive and reflective displays, self-darkening mirror devices, and optical memories. The electrochromic effect has been observed in metal oxides (e.g., WO3),... 

Several other techniques have also been utilized for fibn formation, including drop casting (e.g., CdSe on a Pt electrode in order to study electrochromic effects in nanocrystal films [100]), spin casting (e.g., composite films of CdSe and electrontransporting block copolymers [101]), photochemiad deposition (e.g., CdSe films from aqueous solution [102]), and dectrospray organometaUic CVD for the formation of QD composites [103]. 

Bjorklund, R., S. Andersson, S. Allenmark, and 1. Lundstrom. 1985. Electrochromic effects of conducting polymers in water and acetonitrile. Mol Cryst Liq Cryst 121 263-270. 

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