Revealing the Origin of Emission Quenching on Trivalent Europium β-Diketonate Complex Nanoparticles.

Eu(III) complex nanoparticles (NPs) prepared via reprecipitation exhibit characteristic emission in aqueous solutions without amphiphilic substituents, making them promising for bioimaging applications. However, although it is well established that water efficiently quenches emissions of Eu(III) complexes, reported emission quantum yields of water-soluble Eu(III) complex NPs vary significantly among studies, and the factors governing these emission properties have yet to be fully elucidated. In this study, we systematically investigated the emission quenching mechanisms of Eu(III) β-diketonate complex NPs by time-resolved absorption and emission spectroscopies. We found that the primary pathway is the interaction between coordinated and external water molecules, which strongly enhances nonradiative deactivation. Notably, coordinated water facilitates stronger interactions with the surrounding water, further promoting quenching. Additionally, we also found that aggregation causes emission quenching in Eu(III) complexes lacking phenyl groups in the β-diketonate moiety, which have smaller molecular volumes. These findings clarify the origin of luminescence variability in Eu(III) complex NPs and provide a foundation for rational material design to enhance luminescence stability in aqueous environments.