Influence of bidentate structure of an aryl phosphine oxide ligand on photophysical properties of its Eu^III complex

The bidentate phosphine oxide ligand 1,8-bis(diphenylphosphino) naphthalene oxide (NAPO) and its Eu^III complex 1 Eu(TTA)₃(NAPO) (TTA=2-thenoyltrifluoroacetonate) were chosen to study the effect of bidentate phosphine oxide ligand on the photophysical properties of the corresponding complex. The intramolecular energy transfer processes of 1 were studied. The investigation showed that with bidentate structure NAPO could suppress solvent-induced quenching by enforcing the ligand-ligand interaction and the rigidity of the complex. Contrasted with the monodentate triphenylphosphine oxide (TPPO), NAPO had a higher first triplet excited energy level (T ₁) and a lower first singlet excited energy level (S₁), which fitted to the corresponding excited energy levels of anion ligand TTA much more and supported more efficient singlet and triplet energy transfer in its Eu^III complex.