Fluorescence enhancement by electron-withdrawing groups on beta-diketones in Eu(III)-beta-diketonato-topo ternary complexes

Europium(III) complexes with two series of beta-diketonates and trioctylphosphine oxide (topo) have been synthesized and the fluorescence properties were studied. The beta-dikatonate ligands used had the formula R(1)COCHCOR(2) (I: R(1)=2-naphthyl, R(2)=CF3, CF2CF3, CF2CF2CF3, CF2CF2CF2CF3; II: R(1)=2-phenanthryl, R(2)=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, lambda(ex,max)=339 nm, lambda(em,max)=615 nm, PF3, lambda(ex,max)=344 nm, lambda(em,max)=615 nm, PF5 - PF9, lambda(ex,max)=346 nm, lambda(em,max)=615 nm). The ultra-violet spectra, fluorescence spectra, molar absorption coefficient (epsilon), fluorescence quantum yield (phi(f)), and fluorescence lifetime (tau) of the complexes in benzene were measured. The results reveal tendencies for phi(f) and the fluorescence absolute sensitivity (A.S=epsilon phi(f)) in the following order: phi(f), 2-naphthyl > 2-phenanthryl(for an identical R(2)); C4F9 greater than or equal to C3F7 > C2F5 > CF3 (for an identical R(1)); A.S, PF9 > PF7 > NF9 approximate to NF7 > PF5 approximate to NF5 > NF3 > PF3. The H-1 NMR signal of beta-H in the beta-diketonate ligand turns out to be a good measure to estimate phi(f); since the R(2) is more electron-withdrawing, the beta-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 4.42x10(-12) M (M=mol dm(-3)).