A compound represented by the following structural formula: or a salt thereof, wherein: X 1 and X 2 are independently selected from the group consisting of C(CH 2 K 1)(CH 2 K 2), O, S and Se; K 1 and K 2 are independently selected from the group consisting of H or a C 1 -C 20 alkyl group, a C 1 -C 20 aliphatic group and a C 1 -C 20 aliphatic group substituted with -OR*, N(R*) 2 or -SR*; or K 1 and K 2 together are part of a substituted or unsubstituted carbocyclic, or heterocyclic ring; Y 1 and Y 2 are each independently a benzo-condensed ring, a naphtha-condensed ring or a pyrido-condensed ring; n 1 is 1, 2, or 3; R 2, R 11 and R 12 are independently H, F, Br, Cl, C 1- C 6 alkyl, C 1 -C 6 alkoxy, aryloxy, a nitrogen-containing heterocyclic ring, a nitrogen-containing heteroaromatic ring, a sulfonate, an iminium ion, or any two adjacent R 12 and R 11 substituents or R 2 and R 1 substituents, when taken in combination, form a 4-, 5-, or 6-membered substituted or unsubstituted carbocyclic ring, substituted or unsubstituted non-aromatic carbocyclic ring or a substituted or unsubstituted carbocyclic aryl ring, wherein the carbocyclic rings are each independently optionally substituted one or more times by C 1 -C 6 alkyl, halogen, or OR* or Sir*; R 1 and R 13 are (CH 2) x CH 3, when x is an integer selected from 0 to 6; or R 1 and R 13 are independently (CH 2) n SO 3 - or (CH 2) n SO 3 H when n is an integer selected from 2 to 6; R 3, R 4 and R 5 are independently selected from the group consisting of H, carboxylate, carboxylic acid, carboxylic ester, amine, amide, sulfonamide, hydroxyl, alkoxyl, a sulphonic acid moiety and a sulphonate moiety; wherein at least two of the groups R 1, R 3, R 4, R 5 or R 13 contain a sulfonic acid or a sulfonate group; wherein the moiety -SO 2 NR 6 -Q-CHR 7 - is: (a) -SO 2 N(CH 3)-CH 2 - or (b) is of formula: and W is absent or is selected from the group consisting of -SO 2 NR 6 -Q-CHR 7 -, -O-,-COO-, and -CONH-; h = 0-10; k = 0 or 1; d = 0-6; m = 0-6; and p = 0-6; Z is selected from the group consisting of -COCl, -CONHNH 2, substituted and unsubstituted N-hydroxysuccinimido esters, nitro- or fluoro- phenol esters, -NCS, -CHO,-COCH 2 I, phosphoramidite and maleimide group; and each R* is independently -H or Cl-20 alkyl. X 1 and X 2 are independently selected from the group consisting of C(CH 2 K 1)(CH 2 K 2), O, S and Se; K 1 and K 2 are independently selected from the group consisting of H or a C 1 -C 20 alkyl group, a C 1 -C 20 aliphatic group and a C 1 -C 20 aliphatic group substituted with -OR*, N(R*) 2 or -SR*; or K 1 and K 2 together are part of a substituted or unsubstituted carbocyclic, or heterocyclic ring; Y 1 and Y 2 are each independently a benzo-condensed ring, a naphtha-condensed ring or a pyrido-condensed ring; n 1 is 1, 2, or 3; R 2, R 11 and R 12 are independently H, F, Br, Cl, C 1- C 6 alkyl, C 1 -C 6 alkoxy, aryloxy, a nitrogen-containing heterocyclic ring, a nitrogen-containing heteroaromatic ring, a sulfonate, an iminium ion, or any two adjacent R 12 and R 11 substituents or R 2 and R 1 substituents, when taken in combination, form a 4-, 5-, or 6-membered substituted or unsubstituted carbocyclic ring, substituted or unsubstituted non-aromatic carbocyclic ring or a substituted or unsubstituted carbocyclic aryl ring, wherein the carbocyclic rings are each independently optionally substituted one or more times by C 1 -C 6 alkyl, halogen, or OR* or Sir*; R 1 and R 13 are (CH 2) x CH 3, when x is an integer selected from 0 to 6; or R 1 and R 13 are independently (CH 2) n SO 3 - or (CH 2) n SO 3 H when n is an integer selected from 2 to 6; R 3, R 4 and R 5 are independently selected from the group consisting of H, carboxylate, carboxylic acid, carboxylic ester, amine, amide, sulfonamide, hydroxyl, alkoxyl, a sulphonic acid moiety and a sulphonate moiety; wherein at least two of the groups R 1, R 3, R 4, R 5 or R 13 contain a sulfonic acid or a sulfonate group; wherein the moiety -SO 2 NR 6 -Q-CHR 7 - is: (a) -SO 2 N(CH 3)-CH 2 - or (b) is of formula: and W is absent or is selected from the group consisting of -SO 2 NR 6 -Q-CHR 7 -, -O-,-COO-, and -CONH-; h = 0-10; k = 0 or 1; d = 0-6; m = 0-6; and p = 0-6; Z is selected from the group consisting of -COCl, -CONHNH 2, substituted and unsubstituted N-hydroxysuccinimido esters, nitro- or fluoro- phenol esters, -NCS, -CHO,-COCH 2 I, phosphoramidite and maleimide group; and each R* is independently -H or Cl-20 alkyl.