Controlled self-assembly and luminescence characteristics of Eu(III) complexes in binary aqueous/organic media

Luminescence of sodium tetrakis(naphthoyl trifluoroacetonato) europium(III) (Na[Eu(nta)4]) in binary aqueous-ethanol media is quenched continuously with an increase in the water content, which is ascribed to commonly observed relaxation of photoexcited lanthanide complexes through vibrational coupling with coordinating water. Meanwhile, replacement of sodium ion with an ammonium amphiphile 1 gives a lipid complex 1[Eu(nta)4] which shows distinct changes: its luminescence quantum yield Φ is remarkably increased to ~0.6 above the water content of ~60 vol. %. This unusual enhancement in luminescence intensity occurs in response to self-assembly of 1[Eu(nta)4] into nanoparticles. The lipid counterions provide a hydrophobic atmosphere inside nanoparticles, and they simultaneously form monolayers on the nanoparticle surface that enhance dispersion stability. The size of nanoparticles is tunable depending on the volume fraction of water in the binary media. The lipid-assisted self-assembly of lanthanide complexes provides a unique means to fabricate luminescent nanomaterials, and this approach will be widely applied to fabricate functional coordination nanomaterials.