m-Terphenyl-based lanthanide complexes functionalized with a triphenylene antenna chromophore ((Ln)1) exhibit sensitized visible and near-infrared emission upon photoexcitation of the triphenylene antenna at 310 nm. Luminescence lifetime measurements of the (Eu)1 and (Tb)1 complexes in methanol-h 1 and methanol-d 1 revealed that one methanol molecule is coordinated to the lanthanide ion, indicating that all eight donor atoms provided by the ligand are involved in the encapsulation of the lanthanide ion. The luminescence lifetimes of the near-IR-emitting complexes (Er)1, (Nd)1, and (Yb)1 in DMSO-h 6 and DMSO-d 6 are in the microsecond range, and are dominated by nonradiative deactivation of the luminescent state. The processes preceding the lanthanide luminescence in the sensitization process have been studied in detail. The complexed lanthanide ion reduces the antenna fluorescence and increases the intersystem crossing rate via an external heavy atom effect. The subsequent energy-transfer process was found to take place via the antenna triplet state in all complexes. Luminescence quantum yield measurements and transient absorption spectroscopy indicated that in solution two conformational isomers of the complexes exist: one in which no energy transfer takes place, and one in which the energy transfer does take place, resulting in the lanthanide luminescence. The intramolecular energy-transfer rate is higher in the (Eu)1 and (Tb)1 complexes than in the near-infraredemitting complexes. In methanol the energy-transfer rate is 3.8 × 10 7 s -1 for (Eu)1 and (Tb)1. In DMSO-d 6 the intramolecular energy-transfer rate is higher in the (Nd)1 complex (1.3 × 10 7 s -1 ) than in the (Er)1 (3.8 × 10 6 s -1 ) and (Yb)1 (4.9 × 10 6 s -1 ) complexes.

A series of ternary Eu 3+ complexes are presented consisting of a polydentate m-terphenyl-based Eu 3+ complex (Eu)1 and different antenna chromophores possessing lanthanide(III) ion coordinating properties. The series of investigated antenna chromophores consist of 1,10-phenanthroline, tetraazatriphenylene, and three β-diketonates, namely dibenzoylmethane, benzoyltrifluoroacetylacetonate, and hexafluoroacetylacetonate. As a result of the synergistic complexation of Eu 3+ by the polydentate ligand and the bidentate antenna, the distance between the antenna and lanthanide ion has been minimized and the Eu 3+ ion has been shielded completely from the solvent. These are two important requirements to obtain efficiently emitting lanthanide-(III) complexes. The formation of the ternary complexes and their photophysical properties, in particular the population of the Eu 3+ excited states and the efficiency of the sensitization process, have been studied in detail. Based on these measurements, it can be concluded that the aforementioned strategy of synergistic complexation has indeed led to the construction of efficiently emitting Eu 3+ complexes. The β-diketonate ternary Eu 3+ complexes combine a high stability (K ) 3.8 ( 0.2 × 10 7 M -1 ) with high overall luminescence quantum yields of up to 0.29. The energy transfer from the sensitizer to the Eu 3+ is exclusively to the 5 D 1 level, from which the 5 D 0 level is populated.

Hexa-deutero dimethylsulfoxide (DMSO-d6) solutions of terphenyl-based Nd3+, Yb3+, and Er3+ complexes functionalized with a triphenylene antenna chromophore exhibit room temperature near-infrared luminescence at wavelengths of interest for the optical telecommunication network (∼1330 and ∼1550 nm). The sensitizing process takes place through the triplet state of triphenylene as can be concluded from the oxygen dependence of the sensitized luminescence. A significant fraction of the excited triphenylene triplet state is quenched by oxygen, instead of contributing to the population of the luminescent state of the lanthanide ion. The luminescence lifetimes of the triphenylene-functionalized lanthanide complexes ((2)Ln) are in the range of microseconds with a lifetime of 18.6 μs for (2)Yb, 3.4 μs for (2)Er, and 2.5 μs for (2)Nd in DMSO-d6. These luminescence lifetimes seem almost completely dominated by the vibrational quenching by the organic groups in the polydentate ligand and solvent molecules, which leads to low overall luminescence quantum yields.

Near-infrared emissive lanthanide complexes were synthesized with covalently attached sensitizers that absorb in the visible. This functionalization was designed such that the sensitizer is in close proximity to the lanthanide ion, which is a prerequisite for efficient energy transfer from the excited sensitizer to the lanthanide ion. The sensitizers used were fluorescein, eosin, and erythrosin, which were linked via a β-alanine spacer to the polydentate chelate. The sensitizers were chosen because they absorb visible light and are structurally very similar, but the intrinsic intersystem crossing quantum yields of the sensitizers vary significantly, because of the presence of the heavy atoms (bromine in eosin and iodine in erythrosin). It was expected that an intrinsic high intersystem crossing would be beneficial in the sensitization process, because energy transfer occurs through the triplet state of sensitizers. However, because of the enhanced intersystem crossing of the sensitizers by the nearby heavy and paramagnetic lanthanide ions, these intrinsic differences were largely diminished. It was even found that fluorescein acts as a more efficient sensitizer for the NIR emission than eosin and erythrosin. The donating triplet state of fluorescein is higher in energy than that of eosin and erythrosin, resulting in less energy back transfer and therefore in a higher efficiency of sensitized emission. This and considerations of selection rules for energy transfer to the lanthanide ions made it possible to distinguish the 4 F 9/2 level of Nd 3+ as the main acceptor channel for energy transfer. In the Er 3+ complexes, the enhancement in intersystem crossing was lower in the eosin and erythrosin complexes than in the fluorescein complex, which was concluded from the remaining complex fluorescence. Furthermore, it is tentatively concluded that additional pathways other than those allowed in Dexter energy transfer play a role in the sensitization of Er 3+ . In the Yb 3+ complexes, the higher efficiency of sensitization by fluorescein is due to the enhanced intersystem crossing that is larger in the fluorescein complex than in the eosin or erythrosin complex.

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