The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a “turn‐on” europium(III) emission process with a small, water‐soluble lanthanide complex triggered by uranyl(VI) ions.

New europium(III) β-diketonate based complexes with the general formula [Eu(β-Dik)₃(NL)_x], where β-Dik = 2-thenoyltrifluoroacetonate, 4,4,4-trifluoro-1-phenyl-1,3-butanedione, 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione, NL = diphenyl sulfoxide (x = 2), bis[2-(diphenylphosphino)phenyl] ether oxide (x = 1), triphenylphosphine oxide (x = 2), 5,6-epoxy-5,6-dihydro-[1,10]phenanthroline (x = 1), are designed and synthesized. The coordination complexes are comprehensively characterized by elemental analysis, infrared and ¹H, ¹³C and ¹⁹F NMR spectroscopy. The complexes are embedded into poly(ethylene-co-vinyl acetate) (EVA), poly(methyl methacrylate) (PMMA) or poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVB) matrices. Photoluminescence behavior is investigated in detail and it exhibits the characteristic ⁵D₀ → ⁷F_0–4 emission bands with quantum yields of 55–83% in the solid state and 34–86% in the polymers. Encapsulation of CIGS solar cells with luminescent downshifting (LDS) layers results in an important improvement of external quantum efficiency (EQE) in the UV region, from 14% and up to 58% at 360 nm for the best compound. The short circuit current density (J_sc) in the range of 300–400 nm increases up to 0.77 mA cm⁻² for the best LDS which corresponds to 71% of the J_sc enhancement of an ideal downshifter. I–V measurements follow the spectral response data with an absolute increase in conversion efficiency of up to 0.8%.