Dye molecules containing two or more polymethine structural units, or at least one polymethine and one polyene structural unit, generally exhibit coupling effects that determine the spectroscopic behavior and other physicochemical properties of the substance. The coupling concept can be used inter alia to fit the quinone dyestuffs, the indigoids, and the indanthrenes into a general poymethine color system and to perdict new dye structures.
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Dähne et al. (1966) studied this question.
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