Heterobimetallic iridiumIII–europiumIII complex: the role of donor energy on sensitising the EuIII ion

Abstract

In luminescent devices, the combination of red, green, and blue luminophores (RGB system) allows the creation of other colours in the electromagnetic spectrum, including the perception of white light. Among the three fundamental colours necessary to create the RGB system, normally, the red colour has received great attention compared with the others because red luminophores present lower Φ due to the energy gap law. Red emitters are easily obtained using Eu^III ion, however, although the red emitters based on the Eu^III ion have high colour purity, the Φ normally is lower compared with red emitters based on the Ir^III ion, even though the colour purity of the latter is not so high. Thus, to produce efficient red-emitting complexes, a strategy that has been developed is the synthesis of heterobimetallic d–f complexes to join the high Φ of the d-metal complexes with the high colour purity of the Eu^III ion. Herein, a novel dual bimetallic emitter was synthesised and fully characterised, the [{Ir(dfppy)₂(μ-bpdc)}₃Eu₂]Cl₃·nH₂O·mCH₃OH (dfppy is the 2-(2,4-difluorophenyl)pyridine cyclometallating ligand, and the ancillary ligand bpdc is the 2,2′bipyridine-3,3′-dicarboxylic acid). This complex was immobilised in a polymeric PMMA film presenting a high Φ value, 42.2 ± 4.2%, in the yellow spectral region. Its precursor, the novel green-emitting Ir^III complex, [Ir(dfppy)₂(bpdc)], showed 97.2 ± 9.7% of Φ after doped in PMMA matrix. Both films were used to coat a UV-LED chip to investigate the photophysical properties when these complexes are applied to create phosphor-converted LEDs. The bimetallic prototype exhibited higher radiant photostability after 18 h of operation compared with the heteroleptic Ir^III complex-doped film. For the first time, the lowest triplet energy (19 103 cm⁻¹) from an Ir^III complex that efficiently sensitises the Eu^III ion in an Ir^III–Eu^III heterobimetallic complex has been determined using voltage modulation in a UV-coated LED prototype, which can be used to engineer new pure red-emitting d–f bimetallic systems.