Color Tuning of Efficient Electroluminescence in the Blue and Green Regions Using Heteroleptic Iridium Complexes with 2-Phenoxyoxazole Ancillary Ligands

Abstract

A rational molecular design strategy for tuning the emission color of phosphorescent complexes by functionalization of the bis(2-phenylpyridine)(2-(2′-oxyphenyl)-2-oxazoline/oxazole)iridium(III) framework is reported. Five new complexes (2–6) have been synthesized in good yields and characterized by cyclic voltammetry, absorption, and photoluminescence studies, by time-dependent density functional theory (TD-DFT) calculations, and by single-crystal X-ray diffraction studies for complexes 2, 4, and 6. An interesting feature of the complexes is that the HOMO is localized on the Ir d-orbitals and the phenoxylate part of the “ancillary” ligand, while the LUMO is located on the pyridyl ring of the ppy ligands. A few other complexes containing 2′-oxyphenyl-2-oxazoline/oxazole ancillary ligands have been reported previously; however, until now there has not been a systematic investigation into manipulating this unusual frontier orbital distribution to tune the emissive properties. It is shown that exchanging the phenylpyridine (ppy) ligand for 2,4-difluoro-ppy gives a blue shift of 21–22 nm (from 1 to 2 and from 4 to 5), and the introduction of electron-withdrawing substituents (SO2Me, CF3) onto the phenoxylate ring of the (2′-oxyphenyl)-2-oxazole ligand results in a further blue shift of 13–20 nm. Combining these functionalizations gives sky-blue emission with λmaxPL 476 and 479 nm for complexes 5 and 6 in dichloromethane solution. The solution quantum yields of all the complexes are within the range ΦPL 0.42–0.73. The observed lifetimes (τobs = 1.52–3.01 μs) and spectral profiles are indicative of phosphorescence from a mixture of ligand-centered and MLCT excited states. (TD-)DFT calculations are in close agreement with the observed photophysical and electrochemical properties of the complexes. Phosphorescent organic light-emitting diodes have been fabricated using complexes 2, 3, 5, and 6 as the emitter, doped in a 4,4′-bis(N-carbazolyl)biphenyl host, giving efficient emission in the blue-green region. Notably, complex 5 gives λmaxEL 480 nm with a maximum brightness of 26150 cd m–2.