Influence of the metal ion on the two-photon absorption properties of lanthanide complexes including near-IR emitters

The synthesis of tris(2-thenoyltrifluoroacetonate)lanthanide(III) complexes featuring a diethylaminostyryl-2,2'-bipyridine coligand was achieved for lanthanum; the near-infrared (NIR) emitters neodymium, erbium, and ytterbium; and the transition-metal yttrium. The photophysical properties were thoroughly studied, and it was demonstrated that the conjugated bipyridine ligand acts as a good antenna for the sensitization of the NIR emitters. The two-photon absorption (TPA) properties of all five complexes were investigated by using both two-photon excited fluorescence and the Z-scan method. We demonstrate that the nature of the rare earth ion has almost no influence on the TPA properties centered on the conjugated bipyridyl ligand. Finally, we show that Yb(III) is sensitized by a two-photon antenna effect, and that Nd(III) is mostly sensitized by a one-photon process involving direct excitation of forbidden f-f transitions.