| Abstract
| - A series of pyrazole and N-heterocyclic carbene-based tris-cyclometalated Ir(III) complexes have been synthesized and their electrochemical and photophysical properties are reported. The complexes luminesce in the blue and near-UV of the spectrum, both in fluid solution and as neat solids at 298 K. These complexes are candidate luminescent dopants in blue organic light emitting devices (OLEDs).
- 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 involvesreplacement 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 emissionspectra 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 efficiencywere used to determine the radiative and nonradiative rates (1.0 × 104 and 2.0 × 104 s-1, respectively). Thesecond approach utilizes N-heterocyclic carbene (NHC) ligands to form triscyclometalated Ir complexes. Complexeswith 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 isolatedas facial and meridianal isomers. Comparison of the crystallographic structures of the fac- and mer-isomers ofIr(pmb)3 with the corresponding Ir(ppz)3 isomers indicates that the imidazolyl-carbene ligand has a stronger transinfluence than pyrazolyl and, thus, imparts a greater ligand field strength. Both fac-Ir(pmi)3 and fac-Ir(pmb)3 complexesdisplay strong metal-to-ligand-charge-transfer absorption transitions in the UV (λ = 270−350 nm) and phosphorescein the near-UV region (E0-0 = 380 nm) at room temperature with φPL values of 0.02 and 0.04, respectively. Theradiative decay rates for fac-Ir(pmi)3 and fac-Ir(pmb)3 (5 × 104 s-1 and 18 × 104 s-1, respectively) are somewhathigher than that of fac-Ir(flz)3, but the nonradiative rates are two orders of magnitude faster (i.e., (2−4) × 106 s-1).
|
