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

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).

本文报道了两种实现三环金属化铱(III)材料高效蓝至近紫外发射的方法，这些材料与先前报道的复合物 fac-Ir(ppz)3（ppz = 1-苯基吡唑基-N,C2'）相关。第一种方法涉及将 ppz 配体的苯基替换为 9,9-二甲基-2-荧光基团，即 fac-tris(1-[(9,9-二甲基-2-荧光基)]吡唑基-N,C2')铱(III)，简称 fac-Ir(flz)3。晶体结构分析显示，fac-Ir(flz)3 和 fac-Ir(ppz)3 在铱中心周围的配位环境相似。fac-Ir(flz)3 的吸收和发射光谱相较于 fac-Ir(ppz)3 发生了红移。在室温下，fac-Ir(flz)3 复合物表现出高效的蓝色光致发光（PL，λmax = 480 nm，φPL = 0.38）。通过寿命和量子效率确定其辐射和非辐射速率（分别为 1.0 × 10^4 s^-1 和 2.0 × 10^4 s^-1）。第二种方法利用 N-杂环卡宾（NHC）配体形成三环金属化铱复合物。两种不同的 NHC 配体复合物，即铱三(1-苯基-3-甲基咪唑啉-2-基)和铱三(1-苯基-3-甲基苯并咪唑啉-2-基)，分别简称为 Ir(pmi)3 和 Ir(pmb)3，均以面型和经线型异构体形式被分离出来。Ir(pmb)3 的 fac-和 mer-异构体的晶体结构与其对应的 Ir(ppz)3 异构体相比表明，咪唑基-卡宾配体具有比吡唑基更强的跨影响，因此赋予更大的配位场强度。fac-Ir(pmi)3 和 fac-Ir(pmb)3 复合物在室温下均表现出强烈的金属至配体电荷转移吸收转变（λ = 270−350 nm）并在近紫外区域（E0-0 = 380 nm）磷光，分别具有 φPL 值为 0.02 和 0.04。fac-Ir(pmi)3 和 fac-Ir(pmb)3 的辐射衰减率（分别为 5 × 10^4 s^-1 和 18 × 10^4 s^-1）略高于 fac-Ir(flz)3，但非辐射速率快两个数量级（即 (2−4) × 10^6 s^-1）。