Synergistic Complexation of Eu³⁺by a Polydentate Ligand and a Bidentate Antenna to Obtain Ternary Complexes with High Luminescence Quantum Yields

Abstract

A series of ternary Eu3+ complexes are presented consisting of a polydentate m-terphenyl-based Eu3+ 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 Eu3+ by the polydentate ligand and the bidentate antenna, the distance between the antenna and lanthanide ion has been minimized and the Eu3+ 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 Eu3+ 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 Eu3+ complexes. The β-diketonate ternary Eu3+ complexes combine a high stability (K = 3.8 ± 0.2 × 107 M-1) with high overall luminescence quantum yields of up to 0.29. The energy transfer from the sensitizer to the Eu3+ is exclusively to the 5D1 level, from which the 5D0 level is populated.