Coupler, also known as "color coupler", is an organic dye intermediate that can be used as a color photographic material to form a color screen. The dye coupler is present in color film and photographic paper, primarily in color photography. After exposing the photosensitive material, the color developer reacts with the silver halide to form oxidation products. The oxidation product then reacts with color developer and coupler to obtain the corresponding color image.
In general, there are three basic couplers: yellow, magenta and cyan basic couplers. Couplers can be divided into many kinds according to the colors of dyes. According to application, they can be divided into antithesis and outside types. They can also be divided into oil-soluble type, water-soluble type and polymer type. Nowadays, water-soluble coupler has been gradually replaced by oil-soluble coupler, which is more vivid and saturated. They are mainly used to make color film and color filmstrip.
Industrial use: Dyeing coupler technology has made considerable progress since the beginning of modern color photography. Particularly since the 1980s, major film and paper manufacturers have continuously improved the stability of image dyes by improving couplers, thereby improving the archival properties of images in newer color papers and films. In general, dye couplers for paper are more valued than that for film, but some modern films use coupler variants originally designed for paper to improve image permanence.
Analytical chemistry: For the purpose of improving the visible method of γ-rays detection, conditions of various acids have been examined for the color development of a fluoran-based black color former. It is suggested that the protons, which may be produced from acid generators by γ-ray radiation, can be properly detected by the color development of the black color former containing proper kinds and concentrations of bases.
Figure 1. Fluoran-based black coupler
1. Hojo M, Ueda T, Inoue A, et al. Interaction of a practical fluoran-based black color former with possible color developers, various acids and magnesium ions, in acetonitrile[J]. Journal of Molecular Liquids, 2009, 148(2-3): 109-113.