Fluorescence Enhancement by Electron-Withdrawing Groups on β-Diketones in Eu(III)-β-diketonato-topo Ternary Complexes

Abstract

Europium(III) complexes with two series of β-diketonates and trioctylphosphine oxide (topo) have been synthesized and the fluorescence properties were studied. The β-dikatonate ligands used had the formula R₁COCHCOR₂ (I: R₁=2- naphthyl, R₂=CF₃, CF₂CF₃, CF₂CF₂CF₃, CF₂CF₂CF₂CF₃; II: R₁=2-phenanthryl, R₂=CF₃, CF₂CF₃, CF₂CF₂CF₃, CF₂CF₂CF₂CF₃, which are designated as NF₃, NF₅, NF₇, NF₉, PF₃, PF₅, PF₇ and PF₉, respectively). These complexes having the composition of [Eu(L)₃(topo)₂] (L=NF₃-NF₉, PF₃) and [Eu(L) ₃(topo)_1.5] (L=PF₅-PF₉) gave very strong fluorescence emission when excited by UV light (complexes of NF₃-NF₉, λ_{ex, max}=339nm, λ_{em, max}=615nm, PF₃, λ_{ex, max}=344nm, λ_{em, max}=615nm, PF₅-PF₉, λ_{ex, max}=346nm, λ_{em, max}=615nm). The ultra-violet spectra, fluorescence spectra, molar absorption coefficient (ε), fluorescence quantum yield (φ_f), and fluorescence lifetime (τ) of the complexes in benzene were measured. The results reveal tendencies for φ_f and the fluorescence absolute sensitivity (A.S=εφ_f) in the following order: φ_f, 2-naphthyl>2-phenanthryl(for an identical R₂); C₄F₉≥C₃F₇>C₂F₅>CF₃(for an identical R₁); A.S, PF₉>PF₇>NF₉≈NF₇>PF₅≈NF₅>NF₃>PF₃. The ¹H NMR signal of β-H in the β-diketonate ligand turns out to be a good measure to estimate φ_f; since the R₂ is more electron-withdrawing, the β-H chemical shift goes to a higher field and the fluorescence of the complex becomes more enhanced. The detection limit of the complex [Eu(L)₃(topo)_1.5] (L=PF₇) is 4.42×10^-12M (M=mol dm^-3).