Use of a compound conforming to general formula below: wherein: Y represents a pyridine ring and n is 1, whereby the compound conforms to formula below: wherein: R 1 represents a hydrogen or halogen atom; a 5- or 6-membered aromatic ring or a sequence of several 5- or 6-membered aromatic rings bonded to one another by a covalent bond, this ring or at least one of these rings optionally comprising one or more heteroatoms and/or substituents; or a linear or branched C 1 to C 12 hydrocarbon group optionally comprising one or more heteroatoms; and R 2 represents a carboxylic acid group; a 5- or 6-membered aromatic ring, optionally comprising one or more heteroatoms and/or substituents; or else a unit of formula below: or else Y represents a group of formula -(CH 2) p -Z where p is 1 or 2, Z is a 5- to 9-membered non-aromatic ring, which optionally comprises one or more heteroatoms and/or substituents, and n is an integer from 1 to 5; the substituent(s) being selected from the halogen atoms and the -COOR', -CHO, -OR', -SR', -SCOR', -SO 2 R', -NR'R", -CONR'R", -C(Hal) 3, -OC(Hal) 3, -C Hal and -CN groups in which R' and R" represent a hydrogen atom or a C 1 to C 3 alkyl group, while Hal represents a halogen atom; as an organic chromophore for complexing a lanthanide. Y represents a pyridine ring and n is 1, whereby the compound conforms to formula below: wherein: R 1 represents a hydrogen or halogen atom; a 5- or 6-membered aromatic ring or a sequence of several 5- or 6-membered aromatic rings bonded to one another by a covalent bond, this ring or at least one of these rings optionally comprising one or more heteroatoms and/or substituents; or a linear or branched C 1 to C 12 hydrocarbon group optionally comprising one or more heteroatoms; and R 2 represents a carboxylic acid group; a 5- or 6-membered aromatic ring, optionally comprising one or more heteroatoms and/or substituents; or else a unit of formula below: or else Y represents a group of formula -(CH 2) p -Z where p is 1 or 2, Z is a 5- to 9-membered non-aromatic ring, which optionally comprises one or more heteroatoms and/or substituents, and n is an integer from 1 to 5; the substituent(s) being selected from the halogen atoms and the -COOR', -CHO, -OR', -SR', -SCOR', -SO 2 R', -NR'R", -CONR'R", -C(Hal) 3, -OC(Hal) 3, -C Hal and -CN groups in which R' and R" represent a hydrogen atom or a C 1 to C 3 alkyl group, while Hal represents a halogen atom; R 1 represents a hydrogen or halogen atom; a 5- or 6-membered aromatic ring or a sequence of several 5- or 6-membered aromatic rings bonded to one another by a covalent bond, this ring or at least one of these rings optionally comprising one or more heteroatoms and/or substituents; or a linear or branched C 1 to C 12 hydrocarbon group optionally comprising one or more heteroatoms; and R 2 represents a carboxylic acid group; a 5- or 6-membered aromatic ring, optionally comprising one or more heteroatoms and/or substituents; or else a unit of formula below: or else Y represents a group of formula -(CH 2) p -Z where p is 1 or 2, Z is a 5- to 9-membered non-aromatic ring, which optionally comprises one or more heteroatoms and/or substituents, and n is an integer from 1 to 5; the substituent(s) being selected from the halogen atoms and the -COOR', -CHO, -OR', -SR', -SCOR', -SO 2 R', -NR'R", -CONR'R", -C(Hal) 3, -OC(Hal) 3, -C Hal and -CN groups in which R' and R" represent a hydrogen atom or a C 1 to C 3 alkyl group, while Hal represents a halogen atom;