Coupler compounds

Hydroxyl-substituted naphthimidazoles, benzothiazoles, benzimidazoles, benzotriazoles, benzoselenazoles and benzoxathiolones may be regarded as substituted phenolic couplers. Compound (121) is a specific example of the latter type which yields intense brown images (51USP2547843). In the absence of hydroxyl groups, alkyl-substituted derivatives of imidazoles or thiazoles may be couplers in their own right by virtue of their active alkyl substituent. 

In most color photographic products, organic compounds such as couplers or redox dye releasers are added to the melted emulsions before coating. These compounds are essential to the development reactions that produce the dye molecules composing color images. 

The Fuji CopiArt monochrome proofing system is based on the photogeneration of color from leuco dyes or diazo-coupling (35). CopiArt includes both positive and negative working systems (Fig. 6). For the positive working system, a diazo compound (6) reacts with a coupler (7) as shown. 

Fig. 6. CopiArt proofing system (a) stmcture of positive working system, where A = organic base, = coupler, Q = microcapsule, and = diazo compound and (b) stmcture of negative working system, where A = photoinitiator, = leuco dye, Q — microcapsule, and...

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

In mordant dyes, phenols, naphthols, and enolizable carbonyl compounds, such as pyrazolones, are generally the couplers. As a rule, 2 1 metal complexes are formed ia the afterchroming process. A typical example of a mordant dye is Eriochrome Black T (18b) which is made from the important dyestuff iatermediate nitro-l,2,4-acid, 4-amiQO-3-hydroxy-7-nitro-l-naphthalenesulfonic acid [6259-63-8]. Eriochrome Red B [3618-63-1] (49) (Cl Mordant Red 7 Cl 18760) (1, 2,4-acid — l-phenyl-3-methyl-5-pyrazolone) is another example. The equiUbrium of the two tautomeric forms depends on the nature of the solvent. 

L/(mol-s) (39,40). QDI is also attacked by hydroxide ion (eq. 4) to produce a quinone monoimine (QMI), itself an oxidized developer derived from /)-aminopheno1. Such compounds can further react with coupler, albeit at a slower rate than QDI, to form a dye and were cited in the seminal patent as color developers (32). However, the dyes derived from this deaminated developer have different hues from the QDI dyes, and these hues are pH-dependent as a consequence of the phenoHc group contributed by the developer. Although the deamination reaction to produce QMI is fast, the rate constant is 10 to 10 L/(mol-s) (40—42), its effect is somewhat offset by the redox reaction of the QMI with the reduced developer, present in large excess, to regenerate the desired QDI. The primary net effect of the deamination reaction is to enlarge the resulting dye cloud (43). 

The second widely used class of yeHow couplers is the pi val oyl a ceta n il i des (7) and related compounds bearing a fully substituted carbon adjacent to the keto group. The dyes from these couplers tend to show significantly improved light stabHity and so these couplers have been widely adopted for use in color papers as weH as many projection materials. In general, the dyes have more narrow bandwidths and less unwanted green absorptions (67). 

Used industrially as a chemical intermediate in the manufacture of couplers for color photography, as a photo polymerization agent for vinyl compounds, as a solvent, and as an enzyme inactivator in biological research. 

Diazo A family of reprographic processes (including Diazotype), based on the coupling of diazonium compounds with dye couplers to form colored compounds. Exposure of the dia-zonium compounds to near-ultraviolet radiation destroys them, so illuminated areas do not develop color. 

These dyes are invariably monoazo compounds with the reactive system attached to the diazo component, owing to the ready availability of monosulphonated phenylenediamine intermediates. Pyrazolone couplers are most commonly used, as in structure 7.82 (where Z is the reactive grouping), and this is particularly the case for greenish yellow vinylsulphone dyes. Catalytic wet fading by phthalocyanine or triphenodioxazine blues is a characteristic weakness of azopyrazolone yellows (section 3.3.4). Pyridones (7.83), barbituric acid (7.84) and acetoacetarylide (7.85 Ar = aryl) coupling components are also represented in this sector, with the same type of diazo component to carry the reactive function. 

Attempts to produce dyes other than azamethines and indoanilines as images have been reported. The compound (45) with a naphtholic coupler gives a blue azo dye (46) (57GEP963297, 57GEP1000689) and the magenta azine dye (48) is formed from the p- phenylenediamine (47) and phenol. The formation of this dye requires six equivalents of silver halide (53MI11400). 

As incorporated couplers, the compounds must be rendered non-wandering. This property is normally conferred by a substituent such as a long chain hydrocarbon residue. Also, because the couplers will be retained in the layers where dye formation does not take place, they must be colourless and not give stain on exposure to light, heat and moisture. Incorporated couplers must not affect the sensitivity of the emulsion layers in which they are coated. 

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. 

The naphthol (60) is a representative of a class of compounds termed development inhibitor release couplers. Phenylmercaptotetrazole is released from this coupler during dye formation. The thiol can then inhibit development within the layer in which it is formed thereby causing a beneficial effect which is displayed in the form of a sharper image. Where the thiol wanders into an adjacent layer and causes inhibition of development some correction for the unwanted absorption of the image dye can be achieved (69MI11401). 

The patent literature describes many compounds capable of taking part in azo-coupling, but relatively few have attained practical importance. They fall essentially into three classes aromatic hydrocarbons containing hydroxyl substituents, compounds containing active methylene groups, and heterocyclic couplers. 

Couplers may be hydroxy or amine derivatives of aromatic compounds, such as benzene, naphthalene or anthracene. Phenol, naptbol, aniline, cresol. paraaminophenol and dimethylpuraphenylenediumine are examples. With these compounds coupling takes place with the hydrogen atom which is in the ortho or para position ut the hydroxy or amino group on the coupler. 

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