Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands

Charged
cyclometalated (C∧N) iridium­(III) complexes
with carbene-based ancillary ligands are a promising family of deep-blue
phosphorescent compounds. Their emission properties are controlled
primarily by the main C∧N ligands, in contrast to
the classical design of charged complexes where N∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine,
are generally used for this purpose. Herein we report two series of
charged iridium complexes with various carbene-based ancillary ligands.
In the first series the C∧N ligand is 2-phenylpyridine,
whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One
bis-carbene (:C∧C:) and four different pyridine–carbene
(N∧C:) chelators are used as bidentate ancillary
ligands in each series. Synthesis, X-ray crystal structures, and photophysical
and electrochemical properties of the two series of complexes are
described. At room temperature, the :C∧C: complexes
show much larger photoluminescence quantum yields (ΦPL) of ca. 30%, compared to the N∧C: analogues (around
1%). On the contrary, all of the investigated complexes are bright
emitters in the solid state both at room temperature (1% poly­(methyl
methacrylate) matrix, ΦPL 30–60%) and at 77
K. Density functional theory calculations are used to rationalize
the differences in the photophysical behavior observed upon change
of the ancillary ligands. The N∧C:-type complexes
possess a low-lying triplet metal-centered (3MC) state
mainly deactivating the excited state through nonradiative processes;
in contrast, no such state is present for the :C∧C: analogues. This finding is supported by temperature-dependent
excited-state lifetime measurements made on representative N∧C: and :C∧C: complexes.

带电荷的环金属化（C∧N）铱（III）配合物，配以碳烯基辅助配体，构成了一类具有深蓝色磷光特性的化合物家族，极具开发潜力。其发射性质主要由主C∧N配体控制，与传统的带电荷配合物设计形成鲜明对比，在后者中，通常采用具有低能π*轨道的N∧N辅助配体，如2,2'-联吡啶，以实现此目的。在本研究中，我们报道了两系列带电荷的铱配合物，分别含有不同的碳烯基辅助配体。在第一系列中，C∧N配体为2-苯基吡啶；而在第二系列中，则为2-(2,4-二氟苯基)-吡啶。每一系列中均使用了两种双碳烯(:C∧C:)和四种不同的吡啶-碳烯(N∧C:)螯合剂作为双齿辅助配体。本文详细描述了这两系列配合物的合成、X射线晶体结构、光物理和电化学性质。在室温下，:C∧C:配合物的光致发光量子产率(ΦPL)高达约30%，相较于N∧C:类似物（约1%）显著增加。相反，所有研究中的配合物在固态下均表现出明亮的发射特性，无论是在室温（1%聚甲基丙烯酸甲酯基质，ΦPL 30–60%）还是在77K时。通过密度泛函理论计算，我们解释了辅助配体变化引起的光物理行为差异。N∧C:-型配合物具有一个低能级的金属中心三重态(3MC)状态，主要通过非辐射过程使激发态失活；而:C∧C:类似物则不存在此类状态。这一发现得到了代表性N∧C:和:C∧C:配合物温度依赖性激发态寿命测量的支持。