A compound of formula, in its free acid, salt or mixed salt form wherein R 1 signifies hydrogen or SO 3 M, R 2 signifies hydrogen or SO 3 M, R 3 signifies hydrogen or SO 3 M, R 4 signifies hydrogen or SO 3 M, R 5 signifies hydrogen, unsubstituted unbranched C 1 -C 8 -alkyl or unsubstituted branched C 3 -C 8 -alkyl, or substituted unbranched C 1 -C 8 -alkyl or substituted branched C 3 -C 8 -alkyl, substituted or unsubstituted phenyl, or unsubstituted unbranched C 1 -C 12 -alkoxy, or unsubstituted branched C 3 -C 12 -alkoxy, or substituted unbranched C 1 -C 12 -alkoxy, or substituted branched C 3 -C 12 -alkoxy, or NH 2, R 6 and R 7 signify independently from each other unsubstituted unbranched C 1 -C 8 -alkyl or unsubstituted branched C 3 -C 8 -alkyl, or substituted unbranched C 1 -C 8 alkyl or substituted branched C 3 -C 8 -alkyl, or unsubstituted aryl, or substituted -(CH 2) n -aryl with n = 1, 2, 3 or 4, wherein the substituents of the substituted aryl are selected from the group of -H,; SO 3 M, wherein M signifies a counter cation as defined below unsubstituted or substituted, linear or branched C 1-6 alkyl, unsubstituted or substituted, linear or branched C 1-6 alkoxy; wherein the substituents of the substituted alkyl and alkoxy groups are selected from the group consisting of heteroatoms other than hydrogen, halogen, -CN, -NH 2 or - COOM, wherein M signifies a counter cation as defined below; or wherein the substituents in R5, R6 and R7 are selected from hydroxyl group or cyano group, or from hydroxyl group and cyano group; under the proviso that said compound of formula contains at least one anionic group, preferably SO 3 M group, wherein M is selected from hydrogen cation, alkali metal cation, alkaline earth metal cation, ammonium cation and alkylammonium cation or mixtures thereof. R 1 signifies hydrogen or SO 3 M, R 2 signifies hydrogen or SO 3 M, R 3 signifies hydrogen or SO 3 M, R 4 signifies hydrogen or SO 3 M, R 5 signifies hydrogen, unsubstituted unbranched C 1 -C 8 -alkyl or unsubstituted branched C 3 -C 8 -alkyl, or substituted unbranched C 1 -C 8 -alkyl or substituted branched C 3 -C 8 -alkyl, substituted or unsubstituted phenyl, or unsubstituted unbranched C 1 -C 12 -alkoxy, or unsubstituted branched C 3 -C 12 -alkoxy, or substituted unbranched C 1 -C 12 -alkoxy, or substituted branched C 3 -C 12 -alkoxy, or NH 2, R 6 and R 7 signify independently from each other unsubstituted unbranched C 1 -C 8 -alkyl or unsubstituted branched C 3 -C 8 -alkyl, or substituted unbranched C 1 -C 8 alkyl or substituted branched C 3 -C 8 -alkyl, or unsubstituted aryl, or substituted -(CH 2) n -aryl with n = 1, 2, 3 or 4, wherein the substituents of the substituted aryl are selected from the group of -H,; SO 3 M, wherein M signifies a counter cation as defined below unsubstituted or substituted, linear or branched C 1-6 alkyl, unsubstituted or substituted, linear or branched C 1-6 alkoxy; wherein the substituents of the substituted alkyl and alkoxy groups are selected from the group consisting of heteroatoms other than hydrogen, halogen, -CN, -NH 2 or - COOM, wherein M signifies a counter cation as defined below; or wherein the substituents in R5, R6 and R7 are selected from hydroxyl group or cyano group, or from hydroxyl group and cyano group; under the proviso that said compound of formula contains at least one anionic group, preferably SO 3 M group, wherein M is selected from hydrogen cation, alkali metal cation, alkaline earth metal cation, ammonium cation and alkylammonium cation or mixtures thereof.