A compound represented by the following structural formula: ** Formula ** or a salt thereof, in which: X1 and X2 are independently selected from the group consisting of C (CH2K1) (CH2K2), O, S and Se； K1 and K2 are independently selected from the group consisting of H or a C1-C20 alkyl group, a C1-C20 aliphatic group and a C1-C20 aliphatic group substituted with -OR *, N (R *) 2 or -SR * ； or K1 and K2 together form part of a substituted or unsubstituted heterocyclic or carbocyclic ring； each of Y1 and Y2 is independently a ring condensed with benzo, a ring condensed with naphtha or a ring condensed with pyrido； n1 is 1, 2 or 3； R2, R11 and R12 are independently H, F, Br, Cl, C1-C6 alkyl, C1-C6 alkoxy, aryloxy, a nitrogen-containing heterocyclic ring, a nitrogen-containing heteroaromatic ring, a sulphonate, an iminium ion or any of two adjacent R12 and R11 substituents or R2 and R1 substituents, when taken in combination, form a substituted or unsubstituted 4, 5 or 6-membered carbocyclic ring, a substituted or unsubstituted unsubstituted carbocyclic ring or a substituted or substituted carbocyclic aryl ring or unsubstituted, in which each of the carbocyclic rings is independently optionally substituted one or more times with C1-C6 alkyl, halogen or OR * or Sir *； R1 and R13 are (CH2) xCH3, when x is an integer selected from 0 to 6； or R1 and R13 are independently (CH2) nSO3 - or (CH2) nSO3H when n is an integer selected from 2 to 6； R3, R4 and R5 are independently selected from the group consisting of H, carboxylate, carboxylic acid, carboxylic ester, amine, amide, sulfonamide, hydroxyl, alkoxy, a sulfonic acid moiety and a sulfonate moiety； wherein at least two of the groups R1, R3, R4, R5 or R13 contain a sulfonic acid or sulfonate group； wherein the remainder -SO2NR6-Q-CHR7- is: (a) -SO2N (CH3) -CH2- or (b) is of the formula: ** Formula ** and W is absent or is selected from the group consisting in -SO2NR6-Q-CHR7-, -O-, -COO- and -CONH-； h>； = 0-10； k>； = 0 o
