Colour formers

Colour formation reactions of this type are utilised in carbonless copy paper, which is based on the principle of colour formation on the copy as a result of pressure of writing or typing in the master sheet. In such systems, the underside of the master sheet contains the colour former, for example compound 243, encased in microcapsules, which are tiny spheres with a hard polymer outer shell. Pressure on the master sheet breaks the microcapsules and allows the colour former to come into contact with an acidic reagent coated on the copy sheet, thus causing an irreversible colour formation reaction. 

Colour formers such as compounds 243 and 245 are not inherently thermochromic. For example, they melt without any change in colour. However, they may be used to generate colour thermally, either irreversibly or reversibly, as composite materials. In thermally sensitive paper, the colour former and an acidic developer, usually a phenol, are dispersed as insoluble particles in a layer of film-forming material. When brought into contact with a thermal head at around 80-120 °C, the composite... 

Xanthene dyes are used as colour formers. These so-called fluorans usually contain amino groups sited para and meta to the central carbon atom. Such a substitution pattern gives rise to broad absorption bands and leads to almost black colour production the lactone 6.200 is a typical example. This xanthene derivative finds use in direct thermal printing [36]. The chemistry of fluoran leuco dyes has been reviewed [78]. 

The pigment is coloured in the solid form of the material because in this state the colour former interacts with the developer, possibly via an ion-pair complex. Melting of the composite interferes with this interaction, leading to a negative thermochromic effect and a loss of colour. This is shown schematically in Figure 1.19. Possible mechanisms for the colonr change have been examined. ... 

Closely related to the phthalides, leucotriarylmethanes are converted into the coloured form by hydride abstraction, chemical and photooxidation and in some cases by acids. A class of pH sensitive lencotriarylmethanes that have fonnd nse in dark blue colour formers are (1.77). They are readily synthesised from Michler s Hydrol (1.73) by reaction with aromatic or heteroaromatic amines in the presence of an acid catalyst. 

X = NEtj, R = H, = NH Ar). The blacks that are used as commercial colour formers, (1.82a and 1.82b), are synthesised by reacting 4-alkoxydiphenylamines (1.79) with the keto acids (1.80) in sulfuric acid to give the intermediate phthahdes (1.81), which are converted into the lluorans by reaction with sodium hydroxide (Figure 1.25). ... 

Figure 1.25 Synthetic route to black fluoran colour formers.

Many heterocyclic compounds have been made as colour couplers. The role of the heterocyclic residue can vary. It can actually be the colour former or an auxiliary substituent in the coupler to provide fine control over the desired features or it can be a leaving group in a two-equivalent coupler. One advantage of using two-equivalent couplers is that less silver is required which permits the coating of thinner layers which in turn gives sharper images. 

Spiropyrans have also been used as pressure-sensitive colour formers. 

Several examples of anthraquinone and azo dyes containing a 1,3,4-oxadiazole ring have been reported although less frequently in recent years. The use of 2-alkyl-5-aryl-1,3,4-oxadiazoles as photographic colour formers has been described. 

Copper mandelates react with pho ene and then pyridine to give 1,3-dioxoIan-2,4-diones, and application of this reaction to the pentafluoroKxnnpound [CeFe -C(0H)Me-C08]2Cu yields the substituted dioxolan (127) (62%). Compounds of this type give derivatives of the parent acid when they are treated with amines or alcohols, and may be polymerized to polyesters details of these reactions are awaited. Pentafluorobenzoic acid is converted into pentafluoroiodobenzene by treatment with iodine in the presence of a peroxide initiator, and certain penta-fluorobenzamido-derivatives of 4-fluorophenol, e.g. (128), prepared by conventional multistage syntheses from the parent phenol, are claimed to be useful as colour-formers in photographic emulsions. i... 

The most widely-used industrial thermochromic system is the referred to as the leuco dye type. The term leuco describes a dye that can acquire two forms, one of which is colourless. This micro-encapsulated composite system relies on colour formation from the interaction of three materials a colour former (the leuco dye), an acidic developer and a low-melting, non-volatile hydrophobic solvent, a set of ingredients similar to those used in thermally-sensitive paper as described above. However, in the leuco dye thermochromic systems, it is curious that, even though similar chemistry is involved, the composite material is coloured at low temperatures and decolourises as the temperature is raised. To explain this effect, a mechanism illustrated in simplified form in Scheme 11.4 has been proposed. At temperatures below the melting point of the solvent, the system is heterogeneous with the colour former existing as the coloured... 

Dye-manufacturers have traditionally sought outlets for their products other than in textiles in order to reduce their direct dependence on the vagaries of the textile industry as far as possible. These outlets include paper dyeing, mass coloration of plastics and polymers, leather dyeing, fur, hair and feather dyeing. The manufacture of colour-formers for so-called carbonless copying systems is an example of a modern non-textile application. 

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