Large energy gap between singlet and triplet states is no longer a problem: intermediate charge transfer state boosts overall quantum yield up to 67% in Eu3+ complexes

New complexes of europium, gadolinium and terbium trinitrates with N,N,N’,N’-tetraalkyl substituted phenanthroline diamides were synthesized. The europium complexes were found to be highly efficient in terms of luminescence properties (max quantum yield = 67%). The significant influence of the structure of the ligands on the photophysical characteristics of their complexes was demonstrated. Thus, the incorporation of various substituents (Cl, F, O, OH) into the phenanthroline core causes significant changes in the luminescent behavior of the obtained coordination compounds. We observed significant differences in the energy gap between the excited states S₁ and T₁, especially in the L2H·Eu(NO₃)₃ and L2FOH·Eu(NO₃)₃ complexes, which both demonstrated high overall quantum yields (66% and 67%, respectively). Study of the diffuse reflection spectra of terbium complexes suggested the phenomenon of charge transfer, potentially ligand-to-ligand (LLCT) or intra-ligand (ILCT), rather than ligand-to-metal charge transfer (LMCT). These results highlight the complicated relationship between ligand structure, energy transfer mechanisms and quantum yield in rare earth element complexes, shedding light on ways to optimize their luminescent properties.

Graphical abstract