Luminescent Iridium(III) Complexes with N∧C∧N-Coordinated Terdentate Ligands: Dual Tuning of the Emission Energy and Application to Organic Light-Emitting Devices

A family of complexes (1a-3a and 1b-3b) was prepared, having the structure Ir((NCN)-C-boolean AND-N-boolean AND)((NC)-C-boolean AND)Cl. Here, (NCN)-C-boolean AND-N-boolean AND represents a terdentate, cyclometallating ligand derived from 1,3-di(2-pyridyl)benzene incorporating CH3 (1a,b), F (2a,b), or CF3 (3a,b) substituents at the 4 and 6 positions of the benzene ring, and (NC)-C-boolean AND is 2-phenylpyridine (series a) or 2-(2,4-difluorophenyl)pyridine (series b). The complexes are formed using a stepwise procedure that relies on the initial introduction of the terdentate ligand to form a dichloro-bridged iridium dimer, followed by cleavage with the (NC)-C-boolean AND ligand. H-1 NMR spectroscopy reveals that the isomer that is exclusively formed in each case is that in which the pyridyl ring of the (NC)-C-boolean AND ligand is trans to the cyclometallating aryl ring of the (NCN)-C-boolean AND-N-boolean AND ligand. This conclusion is unequivocally confirmed by X-ray diffraction analysis for two of the complexes (1b and 3a). All of the complexes are highly luminescent in degassed solution at room temperature, emitting in the green (1a,b), blue-green (2a,b), and orange-red (3a,b) regions. The bidentate ligand offers independent fine-tuning of the emission energy: for each pair, the "b" complex is blue-shifted relative to the analogous "a" complex. These trends in the excited-state energies are rationalized in terms of the relative magnitudes of the effects of the substituents on the highest occupied and lowest unoccupied orbitals, convincingly supported by time-dependent density functional theory (TD-DFT) calculations. Luminescence quantum yields are high, up to 0.7 in solution and close to unity in a PMMA matrix for the green-emitting complexes. Organic light emitting devices (OLEDs) employing this family of complexes as phosphorescent emitters have been prepared. They display high efficiencies, at least comparable, and in some cases superior, to similar devices using the well-known tris-bidentate complexes such as fac-Ir(ppy)(3). The combination of terdentate and bidentate ligands is seen to offer a versatile approach to tuning of the photophysical properties of iridium-based emitters for such applications.