Structurally modified indocyanine green dyes. Modification of the polyene linker

We have prepared a series of indocyanine green dicarboxylic acid derivatives (9a,b, 4, 10, 13) with modified polyene linkers in an attempt to increase the structural rigidity of the polyene linker and thereby the fluorescent yield. Incorporation of five- and six-membered rings into the polyene system led to lower florescent yield for 9a,b, 4 and 10, but shortening the chain by two carbon atoms led to an increase in fluorescent yield for 13.