Enhanced Luminescence of Camphor-Derived Eu(III) Complex via Aggregate Formation in Methanol-Water Solution

Luminescent probes operating in aqueous environments are of great interest for biosensing and environmental analysis. Enabling the use of hydrophobic functional luminophores in aqueous media broadens the range of viable candidates for luminescent probes. In this study, photophysical properties of a camphor-derived Eu(III) complex with hydrophilic ligands, [Eu(+tfc)₃(TEGPO)₂] (tfc: 3-(trifluoroacetyl)camphor, TEGPO: tetraethylene glycol monomethyl ether-modified triphenylphosphine oxide) in methanol-water media with various water fractions (f_w) are demonstrated. The Eu(III) complex showed aggregate formation and homogeneous dispersion in high-water fractions (f_w ≥ 50 vol%). The Eu(III) aggregates exhibited red emission, even though the complex showed little emission without aggregation. The emission intensity was enhanced depending on the water fraction. The luminescent lifetime was also extended by aggregation, which was attributed to the suppression of nonradiative relaxation pathways. Analysis of the photophysical parameters of the Eu(III) complex indicated that the emission enhancement mainly occurred because of suppression of nonradiative processes upon aggregate formation. This study provides a new design strategy for luminescent probes in aqueous media using camphor-derived lanthanide complexes.