What is the key factor affecting the cathodoluminescence intensity of europium complexes?

In order to substitute well-studied but hard-to-deposit inorganic cathodoluminescent (CL) compounds with coordination compounds and organic ligands, it is primarily important to establish CL correlation with photoluminescence and increase CL intensity. In this paper, homo- and mixed-ligand 1,4-terephthalates and naphthalene-2,6-carboxylates of europium were studied, including a novel complex (Eu(nda)(BPhen)(HO)) whose crystal structure was determined. They were found to display luminescence of various intensities under excitation by fast electrons, which is characteristic of Eu.