Red light, green light! Emission through three-photon upconversion processes from polymeric organic–lanthanide complexes (lanthanide, Ln=Tb, Eu) was observed upon excitation of the tripodal organic ligand at 845 nm. Multiphoton absorption by the ligand followed by a transfer of energy to Ln (see picture; S=singlet state, T=triplet state) gives rise to characteristic Eu (red) or Tb (green) emission.
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Wong et al. (2004) studied this question.
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