Couplers in colour photography—chemistry and function Part 2

Abstract

Part 2 deals with magenta and cyan couplers and—in addition—white couplers and scavengers, centred about the years between 1960 and 2000 as in Part 1.Magenta couplers have been a domain of heteroaromatic compounds. This fundamental situation has not changed, except for the fact that the predominance of pyrazolone type magenta couplers has been broken. Today, 3-acylamino- and 3-anilinopyrazolones are still used, but mainly in colour negative film, and pyrazolotriazoles of the pyrazolo[5,1-c](1,2,4)triazole and pyrazolo[1,5-b](1,2,4)triazole classes have taken a larger share, despite the fact that suitable syntheses had to be explored for both classes.In colour negative (CN) paper, where the lower reactivity level of pyrazolotriazoles does not really bother, the inherently clean absorption and high tinctorial strength of the azomethine dyes from pyrazolotriazoles plays an important role. While the stability level of azomethine dyes from 3-anilinopyrazolones was considerably improved between 1960 and 1980, it does not compare favourably to the stability achieved today by azomethine dyes from pyrazolotriazoles. This applies equally to stain resistance, light fading and dark fading.Within the anilinopyrazolone series, valuable two-equivalent couplers have been found by the introduction of arylthio groups as leaving groups. Within the acylaminopyrazolones this has been achieved by the introduction of pyrazoles as leaving groups. The chemical problems encountered in these—and in comparable, but less successful—transformations must not be underestimated. Within the pyrazolotriazole classes, chlorine presents itself as a trouble-free and readily introduced leaving group.Throughout the years, phenols and naphthols have been used as cyan couplers. For both classes of cyan couplers a strong need for improvement in dye stability was recognized. In CN film, the cyan dyes from 1-hydroxy-2-naphthoic amides showed low resistance to reduction by exhausted bleaches and were replaced by 2-phenylureido-5-acylaminophenols or 5-amino-substituted 1-hydroxy-2-naphthoic amides. In CN paper, 2-acylamino-4,6-dichloro-5-methylphenol type cyan couplers were replaced by 2-acylamino-4,6-dichloro-5-ethylphenols which show high stability to light fading. Thus, improved dark fading resistance of the cyan dye in colour paper has turned out to be a major objective of continued chemical research.Research on, among other options, the field of polyazapentalenes, a large group of bicyclic acidic heteroaromatic compounds, comprising also the well-known pyrazolotriazoles, has yielded some classes of new heterocyclic cyan couplers giving cyan dyes of outstanding tinctorial strength and absorption, of which the pyrrolo[1,2-b](1,2,4-triazoles) are the most prominent examples. The dyes from presently known pyrrolotriazole couplers are stable to dark fading, but they need some improvement in light stability. An entirely different approach to solving the dark stability problem without any sacrifice in hue and light stability has been found within the class of 2,5-diacylaminophenols, by introduction of a sulphone-modified acyl group.White couplers and scavengers are expected to function without additional problems, and in most cases do. Nevertheless, the compounds under discussion are interesting under theoretical aspects. True white couplers, e.g. of the 4-methylpyrazolone class, show incomplete coupling and do not fix the developer residue permanently: it can be shown to be split off by oxidation. Scavengers of the 2,4-disulphonamidophenol class and also of the 2,5-disulphonamidophenol class do not show the coupling reaction of a two-equivalent coupler; instead they undergo electron transfer to an oxidized colour developer.