Polyelectrochromism

The LB technique is amenable to the fabrication of ECDs as demonstrated by the report of a thin-film display based on bis(phthalocyaninato)praseodymium(III).75 The electrochromic electrode in the display was fabricated by deposition of multilayers (10-20 layers, r+00-200 A) of the complex onto ITO-coated glass (7 x4cm2) slides. The display exhibited blue-green-yellow-red polyelectrochromicity over a potential range of —2 to +2V. After 105 cycles no significant... 

The structures and redox properties of these complexes have been extensively reviewed 170,171 of interest here is the presence of an intense NIR transition in the neutral and mono-anionic forms, but not the dianionic forms, i.e., the complexes are polyelectrochromic. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. A large number of substituted dithiolene ligands has been prepared and used to prepare complexes of Ni, Pd, and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. 1,400 nm and extinction coefficients up to ca. 40,000 dm3 mol-1 cm-1 (see refs. 170,171 for an extensive listing). 

Synthesis of novel polyelectrochromic materials with the new structural motif utilizing stabilized carbocations has been demonstrated by several examples. The Wurster type violene-cyanine hybrid systems (22z+ and 232+) exhibit the presumed two color changes. The new cyanine-cyanine hybrid system (24 ) with a cyanine unit at the one terminus exhibits three color changes. 

Electrochromic materials of interest can be classified according to three types of color change (1) from a bleached state (transparent) to a colored state, (2) between two colored states, and (3) between several colored states, if more than two redox states are accessible. This behavior is called polyelectrochromic... 

Electrochromism is the reversible change in optical properties of a material caused by redox reactions. The redox reactions can be initiated when the material is placed on the surface of an electrode. When the electro-chromic material is capable of showing several colors, it is addressed as polyelectrochromic. 

Polyaniline is one of the earliest studied conjugated polymers and remains as one of the most researched, especially in the field of electrochromics. While potentially quite useful as a stable and cost-effective conductor, it is a bit unfortunate that substituted polyanilines exhibit little change in the electrochromic properties as a function of substituent identity and position. Substituted and unsubstituted polyanilines have the potentially useful property of being polyelectrochromic in thin film form, but structural modification has not been shown to increase the range of colors available. 

Another approach is the formation of composites to obtain polyelectrochromism. Some thiophene-based systems were studied by Irwin et al. and showed a multi-color electrochromism having three or... 

The colour palette of electrochromism is very large, thanks to the large number of electrochromic materials. There are many possibilities for extending the colour palette of an electrochromic stmcture the superposition of several electrochromic materials, each on different layers [65] the juxtaposition of several electrochromic materials on the same layer [59] the choice of a natural polyelectrochromic material the mixture of an electrochromic material with a classic dye the mixture of two... 

Hong S-F, Hwang S-C, Chen L-C. Deposition-order-dependent polyelectrochromic and redox behaviors of the polyaniline—prussian blue bilayer. Electrochim Acta 2008 53(21) 6215-27. 

As a consequence of the strong interrelationship between PANI s optical properties and its redox chemistry, PANl possesses polyelectrochromic abilities. In strongly acidic electrolytes, PANl that is doped with small-molecule acids can stably and reversibly switch between its green ES and transparent LB forms at higher pH, PANI s ES state is dedoped to EB and the material s electroactivity is lost. Transitions from ES to the purple PB form of PANl involve the loss of protons and thus their electrochemistry is pH dependent. In the acidic media required to stabilize switching between ES and LB, however, the oxidation potential required to access PB is beyond the PANI s degradation... 

Where more than two redox states are electrochemically accessible in a given electrolyte solution, the electrochromic material may exhibit several colors and be termed polyelectrochromic, a frequent property of electrochromic pol5uners (2). 

Of the numerous conducting polymers based on substituted anilines that have been investigated, those with alkyl substituents have received much attention. Poly(o-toluidine) and poly(m-toluidine) films have been found to offer enhanced stability of polyelectrochromic response in comparison with polyaniline... 

With the ability of stabilizing both cations and anions due to its polarized structure, azulene can in principle be used as a building block for the construction of redox-active molecules, such as polyelectrochromic materials, which respond to different potentials with a variety of colors. In this context, Ito and coworkers synthesized a series of 6-azulenyl-substituted benzenes (4-7) [9-11] and 6-azulenylethynyl-substituted benzenes (8-11) [12], as shown in Figure 4.3, and investigated their redox behaviors. 

Chemical species that can be electrochemically switched between different colours are said to be electrochromic. Electrochromism results from the generation of different visible-region electronic absorption bands on switching between redox states. The colour change is commonly between a transparent ( bleached ) state and a coloured state, or between two coloured states. In cases where more than two redox states are electrochemically available, the electrochromic material may exhibit several colours and be termed polyelectrochromic. Likely applications of electrochromic materials include their use in controllable light-reflective or... 

Five-colour polyelectrochromicity, by application of an outer Nafion layer (with subsequent electrostatic incorporation of the methyl viologen system) to an inner layer of PB, is possible (Figure 19). The transparent/purple viologen dication/radical cation electrochromicity operates in the potential region where the PB is in its (reduced) transparent state the bilayer electrode system thus exhibits yellow/green/ blue/transparent/purple colours. 

---