Evaluation of luminescent properties of a novel europium complex for solution-phase microenvironmental probing

Photofunctional lanthanide coordination compounds are attractive optical probes owing to their sharp line-like emissions and long-lived excited states. Herein, we report the synthesis and biological photophysical evaluation of a novel heavily fluorinated europium(III) complex, [Eu(F7NB)3]·H2O (F7NB = heptadecafluorononanoate), designed as an environment-sensitive luminescent reporter for protein-rich biological media. Steady-state and time-resolved luminescence studies were performed in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with fetal bovine serum (FBS) and in HeLa cell cultures. Increasing protein content produced a progressive decrease in the hypersensitive Eu(III) 5D0 → 7F2 emission band at 612 nm, while medium autofluorescence at 440 nm remained unchanged. Simultaneously, the luminescence lifetime increased from 412 μs in aqueous solution to 520 μs in cell-free protein-containing media, indicating reduced non-radiative deactivation. Matrix-isolated experiments confirmed that hydrophobic interactions between the fluorinated ligand shell and serum albumin binding pockets displace coordinated water molecules from the Eu(III) inner coordination sphere, thereby suppressing O–H vibrational quenching. In contrast, cellular internalization through endocytic pathways resulted in partial emission quenching and a shorter lifetime of 340 μs, reflecting intracellular microenvironmental effects and competing deactivation pathways. The unique fluorinated ligand framework simultaneously minimizes high-energy C–H oscillators and provides a hydrophobic fluorine shield that promotes selective protein interactions. These findings establish [Eu(F7NB)3]·H2O as a robust luminescent coordination probe for monitoring protein binding, cellular uptake, and microenvironmental changes in complex biological systems.