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

Benjamin, Helen; Liang, Jie; Liu, Yu; Geng, Yun; Liu, Xingman; Zhu, Dongxia; Batsanov, Andrei S.; Bryce, Martin R.

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.

Citation

Benjamin, H., Liang, J., Liu, Y., Geng, Y., Liu, X., Zhu, D., …Bryce, M. R. (2017). Color Tuning of Efficient Electroluminescence in the Blue and Green Regions Using Heteroleptic Iridium Complexes with 2-Phenoxyoxazole Ancillary Ligands. Organometallics, 36(9), 1810-1821. https://doi.org/10.1021/acs.organomet.7b00161

• Journal Article Type: Article
• Online Publication Date: Apr 20, 2017
• Publication Date: May 8, 2017
• Deposit Date: May 2, 2017
• Publicly Available Date: Apr 20, 2018
• Journal: Organometallics
• Print ISSN: 0276-7333
• Electronic ISSN: 1520-6041
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 36
• Issue: 9
• Pages: 1810-1821
• DOI: https://doi.org/10.1021/acs.organomet.7b00161
• Public URL: https://durham-repository.worktribe.com/output/1359765