Abstract: A method of changing the fluorescent properties of a complex, the method comprising: providing a first complex comprising a multidentate ligand that is coordinated to a lanthanide ion, wherein the lanthanide is selected from europium and terbium and the multidentate ligand has at least one optionally substituted phthalimide group coordinated to the lanthanide ion, contacting the complex with an aqueous liquid medium under appropriate conditions, such that at least one phthalimide group is hydrolysed to a phthalamate group, to form a second complex. Complexes and compounds are also disclosed.