A compound of formula wherein R 1 is hydrogen, lower alkyl, halogen, cyano, trifluoromethyl or azido; R 2 is a linker consisting of a straight or branched chain alkylene group with 1 to 300 carbon atoms, wherein optionally (a) one or more carbon atoms are replaced by oxygen; (b) one or more carbon atoms are replaced by nitrogen carrying a hydrogen atom, and an adjacent carbon atom is substituted by oxo, representing an amide function -NH-CO-; (c) one or more carbon atoms are replaced by oxygen, and an adjacent carbon atom is substituted by oxo, representing an ester function -O-CO-; (d) the bond between two adjacent carbon atoms is a double or a triple bond, representing a function -CH=CH- or -C≡C-; (e) one or more carbon atoms are replaced by a phenylene, a saturated or unsaturated cycloalkylene, a saturated or unsaturated bicycloalkylene, a bridging heteroaromatic or a bridging saturated or unsaturated heterocyclyl group; (f) two adjacent carbon atoms are replaced by a disulfide linkage -S-S-; or a combination of two or more alkylene and/or modified alkylene groups as defined under (a) to (f) hereinbefore, optionally containing substituents; and L is one or a plurality of same or different labels selected from a fluorophore, a chromophore, a magnetic label, a radioactive label, an affinity label, a molecule which is capable of crosslinking to other molecules, a molecule which is capable of generating hydroxyl radicals upon exposure to H 2 O 2 and ascorbate, a molecule which is capable of generating reactive radicals upon irradiation with light, a molecule attachable or covalently attached to a solid support, a nucleic acid or a derivative thereof capable of undergoing base-pairing with its complementary strand, a lipid or other hydrophobic molecule with membrane-inserting properties, methotrexate, and a cell membrane transport enhancer group. wherein R 1 is hydrogen, lower alkyl, halogen, cyano, trifluoromethyl or azido; R 2 is a linker consisting of a straight or branched chain alkylene group with 1 to 300 carbon atoms, wherein optionally (a) one or more carbon atoms are replaced by oxygen; (b) one or more carbon atoms are replaced by nitrogen carrying a hydrogen atom, and an adjacent carbon atom is substituted by oxo, representing an amide function -NH-CO-; (c) one or more carbon atoms are replaced by oxygen, and an adjacent carbon atom is substituted by oxo, representing an ester function -O-CO-; (d) the bond between two adjacent carbon atoms is a double or a triple bond, representing a function -CH=CH- or -C≡C-; (e) one or more carbon atoms are replaced by a phenylene, a saturated or unsaturated cycloalkylene, a saturated or unsaturated bicycloalkylene, a bridging heteroaromatic or a bridging saturated or unsaturated heterocyclyl group; (f) two adjacent carbon atoms are replaced by a disulfide linkage -S-S-; (a) one or more carbon atoms are replaced by oxygen; (b) one or more carbon atoms are replaced by nitrogen carrying a hydrogen atom, and an adjacent carbon atom is substituted by oxo, representing an amide function -NH-CO-; (c) one or more carbon atoms are replaced by oxygen, and an adjacent carbon atom is substituted by oxo, representing an ester function -O-CO-; (d) the bond between two adjacent carbon atoms is a double or a triple bond, representing a function -CH=CH- or -C≡C-; (e) one or more carbon atoms are replaced by a phenylene, a saturated or unsaturated cycloalkylene, a saturated or unsaturated bicycloalkylene, a bridging heteroaromatic or a bridging saturated or unsaturated heterocyclyl group; (f) two adjacent carbon atoms are replaced by a disulfide linkage -S-S-; or a combination of two or more alkylene and/or modified alkylene groups as defined under (a) to (f) hereinbefore, optionally containing substituents; and L is one or a plurality of same or different labels selected from a fluorophore, a chromophore, a magnetic label, a radioactive label, an affinity label, a molecule which is capable of crosslinking to other molecules, a molecule which is capable of generating hydroxyl radicals upon exposure to H 2 O 2 and ascorbate, a molecule which is capable of generating reactive radicals upon irradiation with light, a molecule attachable or covalently attached to a solid support, a nucleic acid or a derivative thereof capable of undergoing base-pairing with its complementary strand, a lipid or other hydrophobic molecule with membrane-inserting properties, methotrexate, and a cell membrane transport enhancer group.