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

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.