A compound of formula I: or a pharmaceutically acceptable salt, solvate or ester thereof; wherein: any one of X 1 and X 3 is a N atom and the remaining two or one of X 1 or X 3 and X 2 are independently CR 13; Y is selected from SO 2 and SO; R 1 is selected from OCH 3, OCH 2 CH 3, O-propyl, O-butyl; R 3 and R 7 are each independently selected from OCH 3, OCH 2 CH 3, O-propyl, O-butyl, and H; R 2 is selected from NO 2, NH 2, NHMe, NHC(=O)Me, NHEt, NMe 2, NEt 2, NMeC(=O)Me, NEtC(=O)Me, NEtCO 2 Me, NHCH 2 C(=O)Et, NHCH 2 CO 2 Et, NHCH 2 C(=O)Me, NHCH 2 CO 2 H, NHCH 2 C(=O)NH 2, NHSO 2 Me, NHSO 2 CF 3, NHCH 2 CH 2 NEt 2, the one or two R 13 groups are each independently selected from H and R 10, R 10 is selected from R 8, alkyl, aryl, heteroaryl and combinations of two or more thereof and combinations with one or more R 9, or R 10 is one or more moieties R 11 linking one or more alkyl, alkoxy, aryl, heteroaryl or R 8 or R 9 groups or combinations thereof, directly or via a moiety selected from alkylene, arylene, heteroarylene or combinations thereof, wherein alkyl, aryl, heteroaryl groups or moieties thereof may be substituted with one or more groups R 12, or R 10 is selected from a group R 12; R 11 is selected from O-, N-, NH-, N=C, CO-, COO-, CON-, CONH-, SO 2 -, SO 2 N-, SO 2 NH-; R 12 is selected from halogeno, NH 2, NO 2, CN, OH, COOH, CONH 2, C(=NH)NH 2, SO 3 H, SO 2 NH 2, SO 2 CH 3, OCH 3, OCH 2 CH 3, CF 3; R 8 and R 9 comprise one or more solubilising moieties independently chosen from i) neutral hydrophilic groups, ii) ionisable organic acids, iii) ionisable organic bases, iv) chemical functions or moieties providing covalent or non-covalent attachment or binding to a solid phase or an immobile receptor and combinations thereof; R 4, R 5 and R 6 are each independently selected from H and R 10, or two of R 4 to R 6 are linked to form a cyclic ether or amine containing one or more additional oxygen or nitrogen atoms; and wherein two or more of R 3 to R 7 are independently selected from OC 1-6 alkyl. any one of X 1 and X 3 is a N atom and the remaining two or one of X 1 or X 3 and X 2 are independently CR 13; Y is selected from SO 2 and SO; R 1 is selected from OCH 3, OCH 2 CH 3, O-propyl, O-butyl; R 3 and R 7 are each independently selected from OCH 3, OCH 2 CH 3, O-propyl, O-butyl, and H; R 2 is selected from NO 2, NH 2, NHMe, NHC(=O)Me, NHEt, NMe 2, NEt 2, NMeC(=O)Me, NEtC(=O)Me, NEtCO 2 Me, NHCH 2 C(=O)Et, NHCH 2 CO 2 Et, NHCH 2 C(=O)Me, NHCH 2 CO 2 H, NHCH 2 C(=O)NH 2, NHSO 2 Me, NHSO 2 CF 3, NHCH 2 CH 2 NEt 2, the one or two R 13 groups are each independently selected from H and R 10, R 10 is selected from R 8, alkyl, aryl, heteroaryl and combinations of two or more thereof and combinations with one or more R 9, or R 10 is one or more moieties R 11 linking one or more alkyl, alkoxy, aryl, heteroaryl or R 8 or R 9 groups or combinations thereof, directly or via a moiety selected from alkylene, arylene, heteroarylene or combinations thereof, wherein alkyl, aryl, heteroaryl groups or moieties thereof may be substituted with one or more groups R 12, or R 10 is selected from a group R 12; R 11 is selected from O-, N-, NH-, N=C, CO-, COO-, CON-, CONH-, SO 2 -, SO 2 N-, SO 2 NH-; R 12 is selected from halogeno, NH 2, NO 2, CN, OH, COOH, CONH 2, C(=NH)NH 2, SO 3 H, SO 2 NH 2, SO 2 CH 3, OCH 3, OCH 2 CH 3, CF 3; R 8 and R 9 comprise one or more solubilising moieties independently chosen from i) neutral hydrophilic groups, ii) ionisable organic acids, iii) ionisable organic bases, iv) chemical functions or moieties providing covalent or non-covalent attachment or binding to a solid phase or an immobile receptor and combinations thereof; R 4, R 5 and R 6 are each independently selected from H and R 10, or two of R 4 to R 6 are linked to form a cyclic ether or amine containing one or more additional oxygen or nitrogen atoms; and wherein two or more of R 3 to R 7 are independently selected from OC 1-6 alkyl.