Couplers in colour photography—chemistry and function Part 1

Abstract

Rudolf Fischer's concept of ‘chromogenic development’ of sensitized silver halide, which used only one colour developer for producing the triad of subtractive primaries yellow—magenta—cyan in the presence of dye-forming substances called ‘couplers’, marked the starting point of the lengthy evolution that has generated a huge market for professional and consumer photography. The development of the colour-forming system has led to industrial products of high performance. This will be discussed in Section 1 with regard to its general aspects, as far as the years between 1960 and 2000 are concerned, and available sources, found mainly within the patent literature, can be evaluated.

Section 2 deals with coupling and dye formation. The key step has been described as an electrophilic substitution at the coupler carbanion. Occasionally, the reaction sequence shows unexpected deviations from the theoretical scheme and leads to side products. The known methods for determining ‘coupling rates’ and ‘dye formation rates’ in microheterogeneous systems such as coupler dispersions are discussed. A comparison between measured coupling rates and ‘dye yield’ shows that the role of the coupling rate in dye formation might have been overestimated. The role of the ‘oxidized colour developer’ complex is examined and gaps in the knowledge about it are located. The benefits resulting from almost exclusive use of two-equivalent couplers are pointed out.

Section 3, discussing recent advances in the field of yellow couplers, comprises a treatment of the important coupler classes of the acylacetoanilide type, in particular pivaloylacetanilides, p-alkoxybenzoylacetanilides and cycloacylacetanilides, and of the malonanilide type. Less common classes of yellow couplers such as enaminoamides and oxazolinonecarbonamides are reviewed according to their marginal role. Leaving groups and their specific role in yellow coupler chemistry are discussed more broadly, while the importance of adequate ballasting groups is briefly touched upon.

The paper comprises three parts: Part 2 follows in this Issue while Part 3 will appear in the next Issue.

Keywords:

• additive mixing
• azomethine dyes
• ballasting groups
• benzoylacetanilides
• chromogenic development (coupling)
• chromophor
• colour developer
• colour photography
• colour separation
• couplers
• coupler dispersion
• coupler reactivity
• coupler solvents
• coupling reaction
• coupling efficiency
• coupling rate
• cyclic imides
• cycloalkanoylacetanilides
• cyclovoltammetric method
• diffusion-controlled process
• dye yield
• enaminoacetamides
• four-equivalent coupling
• four-equivalent coupler
• hetarylacetamides
• hydantoins
• imidazoles
• isoxazolones
• leaving groups
• leuco compound (dye)
• malonanilides
• oxazolidine-2,4-diones
• pivaloylacetanilides
• phenol leaving groups
• Potentiometric method
• quinone diimine (QDI)
• semiquinone diimine (SQDI)
• spectrophotometric method
• subtractive mixing
• subtractive primaries
• Texter's method
• tinctorial strength
• triazole leaving groups
• two-equivalent coupler
• yellow couplers