Efficient visible light sensitisation of water-soluble near-infrared luminescent lanthanide complexes

Fluorexon (4',5'-bis[N,N-bis(carboxymethyl)aminomethyl]fluorescein) forms stable, water-soluble complexes with lanthanide ions. The complexes with neodymium(III), erbium(III) and ytterbium(III) display sensitised near-infrared lanthanide luminescence upon excitation of the fluorexon with visible light. The photosensitisation efficiency is very high (0.5-1.0) as a result of lanthanide-enhanced intersystem crossing in the sensitising chromophore and rapid intracomplex energy transfer. The overall luminescence quantum yields, however, are low (2 x 10(-4)-5 x 10(-3)) due to nonradiative deactivation of the excited lanthanide ion.