Photophysical and ion-ion energy transfer studies of heteroleptic complexes containing europium and terbium trivalent ions using trifluoroacetylacetonate and hexafluoroacetylacetonate as main ligands, triphenylphosphine oxide as ancillary ligand and trifluoroacetate or [(diphenylphosphoryl)(ethyl or butyl)](diphenyl)phosphine oxide as bridged ligands Documento