A compound having the following structure: or a stereoisomer, pharmaceutically acceptable salt or tautomer thereof, wherein: x 1, x 2, x 3, x 4, x 5 and x 6 are, at each occurrence, independently an integer from 0 to 6; A is at each occurrence, independently or a moiety comprising one or more, fused, aryl or heteroaryl ring system; M is, at each occurrence, independently a biologically active moiety, a prodrug of a biologically active moiety, or a fluorescent dye, provided at least one occurrence of M is not a fluorescent dye; L is a physiologically cleavable linker; R 1 is, at each occurrence, independently H, alkyl or alkoxy; R 2 and R 3 are each independently H, OH, SH, alkyl, alkoxy, alkylether, -OP(=R a)(R b)R c, Q, L', wherein: R a is O or S; R b is OH, SH, O -, S -, OR d or SR d; R c is OH, SH, O -, S -, OR d, SR d, alkyl, alkoxy, alkylether, alkoxyalkylether, phosphate, thiophosphate, phosphoalkyl, thiophosphoalkyl, phosphoalkylether or thiophosphoalkylether; and R d is a counter ion; R 4 is, at each occurrence, independently OH, SH, O -, S -, OR d or SR d; R 5 is, at each occurrence, independently oxo, thioxo or absent; Q is, at each occurrence, independently a moiety comprising a reactive group capable of forming a covalent bond with an analyte molecule, a solid support or a complementary reactive group Q'; L' is, at each occurrence, independently a linker comprising a covalent bond to Q, a targeting moiety, a linker comprising a covalent bond to a targeting moiety, a a solid support or solid support residue, a linker comprising a covalent bond to a solid support or solid support residue or a linker comprising a covalent bond to a further compound of structure; m is, at each occurrence, independently an integer of zero or greater, provided that at least one occurrence of m is an integer of one or greater; and n is an integer of one or greater. x 1, x 2, x 3, x 4, x 5 and x 6 are, at each occurrence, independently an integer from 0 to 6; A is at each occurrence, independently or a moiety comprising one or more, fused, aryl or heteroaryl ring system; M is, at each occurrence, independently a biologically active moiety, a prodrug of a biologically active moiety, or a fluorescent dye, provided at least one occurrence of M is not a fluorescent dye; L is a physiologically cleavable linker; R 1 is, at each occurrence, independently H, alkyl or alkoxy; R 2 and R 3 are each independently H, OH, SH, alkyl, alkoxy, alkylether, -OP(=R a)(R b)R c, Q, L', wherein: R a is O or S; R b is OH, SH, O -, S -, OR d or SR d; R c is OH, SH, O -, S -, OR d, SR d, alkyl, alkoxy, alkylether, alkoxyalkylether, phosphate, thiophosphate, phosphoalkyl, thiophosphoalkyl, phosphoalkylether or thiophosphoalkylether; and R d is a counter ion; R 4 is, at each occurrence, independently OH, SH, O -, S -, OR d or SR d; R 5 is, at each occurrence, independently oxo, thioxo or absent; Q is, at each occurrence, independently a moiety comprising a reactive group capable of forming a covalent bond with an analyte molecule, a solid support or a complementary reactive group Q'; L' is, at each occurrence, independently a linker comprising a covalent bond to Q, a targeting moiety, a linker comprising a covalent bond to a targeting moiety, a a solid support or solid support residue, a linker comprising a covalent bond to a solid support or solid support residue or a linker comprising a covalent bond to a further compound of structure; m is, at each occurrence, independently an integer of zero or greater, provided that at least one occurrence of m is an integer of one or greater; and n is an integer of one or greater.