A suspension of N-(6'-hydroxyhexyl)1,4-diaminoanthraquinone-2,3-dicarboximide (10.0 parts) in pyridine (200.0 parts) was heated to 90° C. under nitrogen and maintained at this temperature for 1 hour or until complete dissolution of the dye. The reaction mixture was cooled to 40° C. Caproyl chloride (6.9 parts) was then added dropwise at a rate such that the temperature did not exceed 50° C. The reaction mixture was cooled to room temperature. After 3 hours, ethanol (20.0 parts) was added dropwise. The mixture was cooled in ice for 3 hours. The dark blue solid obtained was separated by filtration and washed with methanol (100 parts). After drying in high vacuo for 6 hours at 45° C., there was obtained N-(6'-n-octanoylhexyl)1,4-diaminoanthraquinone-2,3-dicarboximide (11.5 parts, 88% from N-(6'-hydroxyhexyl)1,4-diaminoanthraquinone-2,3-dicarboximide) (Formula 6); m.p.: 157°-158° C.; lambdamax 680 nm.
