Reversible halogenation of cyanine dyes

The reaction of bromine or iodine with cyanine dyes results in the formation of an equilibrium mixture of the dye and the halogen used, their 1: 1 addition product, the mesohalogen derivative of the dye, the 1: 1 addition product of the latter and the halogen used, the protonated dye and its dimeric or polymeric forms. The equilibrium constants of the different reactions are presented. The original dyes are easily regenerated from the halogenated products if other halogen acceptors are present in the system. The processes described can be of importance in the chromatic sensitization of photographic materials.