Y is NH2, COOH, PO3 H, PS02 H, or ##STR3## and R is a linking group including up to 10 heteroatoms and having a total of from 0 to 20 carbon atoms and heteroatoms arranged in a straight or branched chain and containing up to two ring structures with fluorescein or a derivative of fluorescein with fluorescein or a derivative of fluorescein.