There is provided rare earth metal complexes of formula (I): wherein M is a cation, RE is a rare earth metal; X is a counter anion; and R1, R2, R3, and R4 are each independently selected from hydrogen, alkyl of 1-8 carbon atoms, aryl, halo, and alkoxy. Also provided is a composition of europium bis(2,2′-bipyridine-N,N′)-trinitrate with a diketonate dopant. The disclosure also provides processes for preparing the rare earth metal complexes. A mark comprising the rare earth metal complexes, and a method of applying the mark, are disclosed. The rare earth metal complexes may be used in printing systems, and for security applications.