Lanthanide complexes singlet -► triplet intersystem crossing

Figure 2.18 Schematic representation of photophysical processes in lanthanide(III) complexes (antenna effect). A = absorption, F = fluorescence, P = phosphorescence, L = lanthanide-centred luminescence, ISC = intersystem crossing, ET = energy transfer S = singlet, T = triplet. Full vertical lines radiative transitions dotted vertical lines nonradiative transitions...

It has been shown in the case of /i-diketonate complexes of lanthanides that the excited singlet state of the ligand goes to the ligand triplet by intersystem crossing and then energy is transferred from the ligand triplet to the metal ion manifold. Finally, the emission occurs from the excited state of the metal. 

Fig. 7.1 Simplified energy diagram of the lanthanide organic complex system. Abs. absorption, Fluor, fluorescence, Phosph. phosphorescence, EM lanthanide (Ln ) ion emission, ISC intersystem crossing, ET energy transfer, S singlet, T triplet. Non-rad. nonradiative transitions (Reproduced from Ref. [18] by permission of the Royal Society of Chemistry)...

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