Patent ID: 7753967

Claim:
A process for converting a perylene-3,4:9,10-tetracarboxylic diimide of formula I to a fluorescent dye, wherein R 1 and R 2 are each unbranched, branched or cyclic C 1 -C 8 -alkyl, said process comprising: a) dissolving said perylene-3,4:9,10-tetracarboxylic diimide in sulfuric acid as a solvent, said solvent having a molecular volume of ≦230 Å 3 , at a first temperature of from 20 to 60° C. to obtain a solution, b) cooling said solution or said suspension obtained in step a) by a process consisting of b1), b1) cooling said solution obtained in step a) to a second temperature at or below the crystallization temperature of said perylene-3,4:9,10-tetracarboxlyic diimide of formula I and, adding water or dilute aqueous solutions of the solvent until the first crystals form, wherein further crystallization continues in said solution at said second temperature after the first crystals form, c) isolating said one or more solvate crystals formed in step b) and d) then removing said solvent from said one or more solvate crystals.