The invention concerns a transition metal coordination complex of general formula (I) {[M(X)]C(H2O)n}p wherein: M represents an element belonging to the lanthanide group and L represents a decadentate chromophore ligand of general formula (II). R1, R2, R3 and R4 independently represent hydrogen or an alkyl or aryl radical. A1, A2, A3 and A4 independently represent a structure of general formula (III). U1, U2 and U3 independently represent C or N, and R5, R6 and R7 independently represent hydrogen, an alkyl or aryl radical. Y represents C, O, S, P or N, and m is an integer representing free valences of Y. R8 represents independently hydrogen, an alkyl or aryl radical. X represents a counter-ion, and n represents hydrating water molecules. p represents monomers. H2O represents the hydrating water molecules. The invention also concerns a method for preparing such a ligand by reaction between a 6-chloromethylpyridine-2-carboxylate ethyl ester and ethylenediamine in organic medium.
