compounds of the invention are ions of transition metals such as lanthanum europium scandium indium terbium gandolinium cerium cobalt and gallium ions it was preferred to use a lanthanum ion due to its ability to fluoresce when complexed with the diureadopyridine preparation of the chelates of the ureadopyridines and the metal ions is effected in the conventional manner for the preparation of chelation compounds in practice this can be accomplished by contacting the pyridine with an appropriate metal ion salt in an inert liquid medium as shown in the examples the diureadopyridine and a metal salt was mixed in the an inert liquid such as methanol and stirred at room temperature the 2 6 diureadopyridines of this invention being receptive to chelation may be advantageously utilized in any of the several known techniques involving fluorescence generated by incident light or with x ray radiation as indicated above the 2 6 diureadopyridines of the present invention complex with metal ions preferably transition metal ions forming tridentate pentadentate or heptadentate chelates depending on the nature of the r group chelates of lanthanum exhibit fluorescence both with incident and x ray radiation the novel compounds of this invention are intended for use in a variety of systems which utilize the change in electronic properties of a compound due to the exposure of such compound to incident light or x ray radiation and wherein such.