Electrochromic Systems

Up to now, a wide variety of EC materials have been developed, which can be classified into five distinct categories, namely, transition-metal oxides (especially tungsten oxide); inorganic coordination complexes (Prussian blue and phthalocyanines);¹¹⁵ organic molecules (4,4‟-bipyridylium salts and quinone, in solution and as polymer films); conjugated polymers (side-chain-substituted polymers or main-chain fully conjugated polymers);98, 99, 116

and arylamine-based polymers (polyimide types, polyamide types and epoxy types).^117-119 Five different categories mentioned above have been developed for the electrochromic materials shown as following.

1.2.1-1 Transition-metal Oxides Tungsten Trioxide (WO3)

Many transition metal oxide films can be electrochemically switched to a non-stoichiometric redox state which has an intense electronic absorption band due to optical intervalence charge transfer.^{98, 100} A typical and most widely studied example is the tungsten trioxide (WO₃) system, since its electrochromism was first reported in 1969.¹⁰⁹ Tungsten oxide has a nearly cubic structure which may be simply described as an “empty-perovskite” type formed by WO6 octahedra that share corners. The empty space inside the cubes is considerable and this provides the availability of a large number of interstitial sites where the guest ions can be inserted. Tungsten trioxide, with all tungsten sites as oxidation state W^VI, is a transparent thin film. On electrochemical reduction, W^V sites are generated to give the electrochromic (blue coloration to the film) effect. Although, there is still controversy about the detailed coloration mechanism, it is generally accepted that the injection and extraction of electrons and metal cations (Li⁺, H⁺, etc.) play an important role. WO3 is a cathodically ion insertion material. The blue coloration in the

thin film of WO3 can be erased by the electrochemical oxidation. In the case of Li⁺ cations the electrochemical reaction can be written as Scheme 1.5.

Scheme 1.5 Iridium Oxide (IrO2)

In the case of tungsten trioxide, the more intensely absorbing redox state is produced on reduction (cathodic ion-insertion). In contrast, Group VIII metal oxides become coloured on electrochemical oxidation (anodic ion-insertion); as in the case of hydrated iridium oxide (strictly iridium hydroxide). The mechanism of colouration is uncertain, with both proton extraction and anion insertion routes being proposed. The first is based on cation loss (as shown in Scheme 1.6),^{120, 121} while the second is based on anion insertion (as shown Scheme in 1.7).¹²² Irrespective of whether the mechanism is hydroxyl insertion or proton extraction, Ir(OH)₃ is the bleached form of the oxide and the coloured form is IrO2.^123-126

Scheme 1.6

made by Diesbach in Berlin in 1704,¹²⁷ is extensive used as pigment in the formulation of paints, lacquers and printing inks. Since 1978, Neff first reported the electrochromic properties Prussian blue [PB, iron(III) hexacyanoferrate(II)].¹²⁶ PB becomes a famous electrochromism now because of its multiple changes in color, general formulation of PB is M‟k[M”(CN)6]_l (M‟ and M” are transition metals with different oxidation number).^{115, 128} When the PB went through an electrochemical reduction process, the color could be changed from blue to colorless (as shown in Scheme 1.8). Moreover, due to an oxidation, the color of PB would be changed into yellow (as shown in Scheme 1.9).¹²⁹

Scheme 1.8

[Fe^IIIFe^II(CN)₆] + e [Fe^IIFe^II(CN)₆]

2-blue yellow

Scheme 1.9 Phthalocyanines¹⁰²

Phthalocyanines (Scheme 1.10), similar to the electroactive system of Prussian blue, are tetraazatetrabenzo derivatives of porphyrins with highly delocalized p electron systems. Metallophthalocyanines are important industrial pigments used primarily in inks and for colouring plastics and metal surfaces. Since the first report of the polyelectrochromism of thin films of lutetium bis(phthalocyanine) Lu(Pc)₂ in 1970, numerous metal phthalocyanines have been investigated for their electrochromic properties. Mechanical problems, such as film fracture and/or loss of adhesion to the electrode substrate, arise from anion ingress/egress during color switching. Despite such difficulties, Lu(Pc)2-based electrochromic displays with good reversibility, fast

[Fe^IIIFe^II(CN)₆] [Fe^IIIFe^III(CN)₆] + e

blue colorless

response times, and little degradation over > 5 × 10⁶ cycles have been described.¹³⁰

Scheme 1.10 Metallophthalocyanine (Pc) 1.2.1-3 Organic Molecules

Viologens¹³¹

The viologen (Scheme 1.11) are diquaternary derivatives of 4,4‟-biohenyl. The name comes from the fact that this class of compounds is easily reduced to the radical monocation, which is intensely blue colored. Of the three common viologen redox states (Scheme 1.11), the dication is the most stable and is colorless. When the dication gets an electron and the color will switch to blue or yellow. In addition, as the substituted group changes, such as increasing of the carbon chain length or conjugated growth, it will also change its color (Table 1.1).^132-134 Viologens are investigated for use on EC systems because of their ability to change color reversibly many times upon reduction and oxidation.

Scheme 1.11 N

M N

N N

N N

N N

Phthalocyanine (Pc)

Table 1.6 Color of viologens based on different substituted structure¹³⁵

-R Group Number of Effective Carbon

Color of First molecules, but on one-electron reduction, form brightly-coloured stable solid films of radical anion on the electrode.^136-141

Benzoquinones (both ortho an para), 1,4-naphthoquinones, and 9,10-anthraquinones have been studied (Scheme 1.12), for example, 1 4-benzoquinone in propylene carbonate containing LiClO_4.¹³⁸ Coloured films were formed on cathode electrodes. Table 1.7 lists the electrochromic behaviors for electrochromic quinone-based systems.

In general, ortho benzoquinones are more electrochemically stable than their parar isomers, o-chloranil (3,4,5,6-tetrachorobenzoquinone) being the most stable quinone

studied, having a cycle life in excess of 10⁵ cycles. Aminoanthraquinones show a more complicated electrochemical behavior than naphthaquinone: at moderate potentials, two redox couples are exhibited during cyclic voltammetry, representing first state followed at more negative potentials by a second reduction reaction (Scheme 1.13).

Scheme 1.12 Table 1.7 Color of quinone systems^136-141

Quinone (RQ) Soild Film? Color of First Reduction State o-3,4,5,6-tetrachlorobenzoQ yes intense blue

o-3,4,5,6-tetrabromobenzoQ yes blue

p-benzoQ yes light blue

p-2,3,5,6-tetrafluorobenzoQ no yellow

p-2,3,5,6-tetrachlorobenzoQ no yellow

p-2,3-dicyano-5,6-dichlorobenzoQ no pink

5-aminonaphthoQ yes purple/blue

1-aminoanthraQ yes -

2-aminoanthraQ yes -

1,5-diaminoanthraQ yes purple

Scheme 1.13

1.2.1-4 Conducting Polymers

Widespread interest in conducting polymer is led to the award of the 2000 Nobel Prize in Chemistry to Shirakawa, Heeger and MacDiarmid.¹⁴²

Types of electroactive conducting polymers including polypyrroles, polyanilines, and polythiophenes, poly(ethylenedioxythiophene) (PEDOT), poly(propylenedioxythiophene) (PProDOT), and poly(propylenedioxypyrrole) (PProDOP) (as shown in Scheme 1.14), are latently electrochromic in thin-film form, redox switching giving rise to new optical absorption bands in accompaniment with transfer of electrons and counter anions (as shown in Figure 1.4). In their oxidized forms, conducting polymers are charge balanced, doped with counter anions (p-doping) and have a delocalized π-electron band structure, oxidative p-doping shifts the optical absorption band towards the lower energy part of the spectrum. The color change or contrast between doped and undoped forms of the polymer depends on the magnitude of the bandgap of the undoped polymer.

Among the organic electrochromic materials, conducting polymers have received more attention because of their additional advantages over inorganic compounds.

Advantages and disadvantages of conducting polymers can be summarized as following (Table 1.8):

Scheme 1.14 Some conducting polymers.

Table 1.8 The advantages and drawbacks of conducting polymer^143-145

Advantages Disadvantages

1. Outstanding coloration efficiency 1. Difficult to find a compatible counter electrode

2. Multiple color possibilities 2. Cycle life may be an issue 3. Easy to process, low cost 3. Environmental instability 4. Low driving voltage in devices

(< 1.5 or 0 V)

5. Rapid response time in device (~1 s)

S

Figure 1.4 Representative electrochromic polymers. Color swatches are representations of thin films based on measured CIE 1931 Yxy color coordinates.

Key:  0 = neutral; I = intermediate; + = oxidized; − and − − = reduced.¹⁴⁶

1.2.1-5 Arylamine-based Polymers

Aromatic amines (arylamines) are generally colorless unless they undergo some form of charge-transfer interaction with an electron-deficient acceptor species. By contrast, the product of one-electron oxidation yields a radical cation which, in organic solution, processes a brilliant color. Arylamines are thus candidate electrochromic materials.

Electron-rich triarylamines can be easily oxidized to form stable radical cations, and the oxidation process is always associated with a noticeable change of coloration.

Thus, studies of the synthesis and electrochromism of triarylamine-based polymers have been reported in the literature.^147-149

Notably, since 2005 our group has reported some solution-processable high-performance polymers (e.g., polyimide types, polyamide types, and epoxy types) utilizing the triarylamine units as an electrochromic functional moiety (Figure1.5),^96,

97, 117-119

which showed interesting color transitions with good electrochromic reversibility in the visible region or near-infrared (NIR) range, and can be differentiated on the basis of method of increasing coloring stages. And the most significant advantage is that they all exhibit transparency at the natural state.

Figure 1.5 High-performance polymers (e.g., polyimide types, polyamide types, and epoxy types) utilizing the triarylamine units as an electrochromic functional moiety.