American Chemical Society
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Blue and Near-UV Phosphorescence from Iridium Complexes with Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands

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posted on 2005-10-31, 00:00 authored by Tissa Sajoto, Peter I. Djurovich, Arnold Tamayo, Muhammed Yousufuddin, Robert Bau, Mark E. Thompson, Russell J. Holmes, Stephen R. Forrest
Two approaches are reported to achieve efficient blue to near-UV emission from triscyclometalated iridium(III) materials related to the previously reported complex, fac-Ir(ppz)3 (ppz = 1-phenylpyrazolyl-N,C2‘). The first involves replacement of the phenyl group of the ppz ligand with a 9,9-dimethyl-2-fluorenyl group, i.e., fac-tris(1-[(9,9-dimethyl-2-fluorenyl)]pyrazolyl-N,C2‘)iridium(III), abbreviated as fac-Ir(flz)3. Crystallographic analysis reveals that both fac-Ir(flz)3 and fac-Ir(ppz)3 have a similar coordination environment around the Ir center. The absorption and emission spectra of fac-Ir(flz)3 are red shifted from those of fac-Ir(ppz)3. The fac-Ir(flz)3 complex gives blue photoluminescence (PL) with a high efficiency (λmax = 480 nm, φPL = 0.38) at room temperature. The lifetime and quantum efficiency were used to determine the radiative and nonradiative rates (1.0 × 104 and 2.0 × 104 s-1, respectively). The second approach utilizes N-heterocyclic carbene (NHC) ligands to form triscyclometalated Ir complexes. Complexes with two different NHC ligands, i.e., iridium tris(1-phenyl-3-methylimidazolin-2-ylidene-C,C2‘), abbreviated as Ir(pmi)3, and iridium tris(1-phenyl-3-methylbenzimidazolin-2-ylidene-C,C2‘), abbreviated as Ir(pmb)3, were both isolated as facial and meridianal isomers. Comparison of the crystallographic structures of the fac- and mer-isomers of Ir(pmb)3 with the corresponding Ir(ppz)3 isomers indicates that the imidazolyl-carbene ligand has a stronger trans influence than pyrazolyl and, thus, imparts a greater ligand field strength. Both fac-Ir(pmi)3 and fac-Ir(pmb)3 complexes display strong metal-to-ligand-charge-transfer absorption transitions in the UV (λ = 270−350 nm) and phosphoresce in the near-UV region (E0-0 = 380 nm) at room temperature with φPL values of 0.02 and 0.04, respectively. The radiative decay rates for fac-Ir(pmi)3 and fac-Ir(pmb)3 (5 × 104 s-1 and 18 × 104 s-1, respectively) are somewhat higher than that of fac-Ir(flz)3, but the nonradiative rates are two orders of magnitude faster (i.e., (2−4) × 106 s-1).