Iridium(III) Complexes with Phenyl-tetrazoles as Cyclometalating Ligands

Ir­(III)
cationic complexes with cyclometalating tetrazolate ligands
were prepared for the first time, following a two-step strategy based
on (i) a silver-assisted cyclometalation reaction of a tetrazole derivative
with IrCl3 affording a bis-cyclometalated solvato-complex P ([Ir­(ptrz)2­(CH3CN)2]+, Hptrz = 2-methyl-5-phenyl-2H-tetrazole);
(ii) a substitution reaction with five neutral ancillary ligands to
get [Ir­(ptrz)2L]+, with L = 2,2′-bypiridine
(1), 4,4′-di-tert-butyl-2,2′-bipyridine
(2), 1,10-phenanthroline (3), and 2-(1-phenyl-1H-1,2,3-triazol-4-yl)­pyridine (4), and [Ir­(ptrz)2L2]+, with L = tert-butyl isocyanide (5). X-ray crystal structures of P, 2, and 3 were solved. Electrochemical
and photophysical studies, along with density functional theory calculations,
allowed a comprehensive rationalization of the electronic properties
of 1–5. In acetonitrile at 298 K,
complexes equipped with bipyridine or phenanthroline ancillary ligands
(1–3) exhibit intense and structureless
emission bands centered at around 540 nm, with metal-to-ligand and
ligand-to-ligand charge transfer (MLCT/LLCT) character; their photoluminescence
quantum yields (PLQYs) are in the range of 55–70%. By contrast,
the luminescence band of 5 is weak, structured, and blue-shifted
and is attributed to a ligand-centered (LC) triplet state of the tetrazolate
cyclometalated ligand. The PLQY of 4 is extremely low
(<0.1%) since its lowest level is a nonemissive triplet metal-centered
(3MC) state. In rigid matrix at 77 K, all of the complexes
exhibit intense luminescence. Ligands 1–3 are also strong emitters in solid matrices at room temperature
(1% poly­(methyl methacrylate) matrix and neat films), with PLQYs in
the range of 27–70%. Good quality films of 2 could
be obtained to make light-emitting electrochemical cells that emit
bright green light and exhibit a maximum luminance of 310 cd m–2. Tetrazolate cyclometalated ligands push the emission
of Ir­(III) complexes to the blue, when compared to pyrazolate or triazolate
analogues. More generally, among the cationic Ir­(III) complexes without
fluorine substituents on the cyclometalated ligands, 1–3 exhibit the highest-energy MLCT/LLCT emission
bands ever reported.

本研究首次报道了采用两步策略合成的、带有环金属化四唑配体的三价铱（Ir(III)）阳离子配合物：(i) 通过银辅助的四唑衍生物与三氯化铱（IrCl₃）的环金属化反应，得到双环金属化溶剂化配合物P，即[Ir(ptrz)₂(CH₃CN)₂]⁺，其中Hptrz代表2-甲基-5-苯基-2H-四唑；(ii) 使该配合物与五种中性辅助配体发生取代反应，得到系列目标配合物：[Ir(ptrz)₂L]⁺（L分别为2,2'-联吡啶（1）、4,4'-二叔丁基-2,2'-联吡啶（2）、1,10-菲咯啉（3）以及2-(1-苯基-1H-1,2,3-三唑-4-基)吡啶（4））与[Ir(ptrz)₂L₂]⁺（L为叔丁基异氰化物（5））。已解析出配合物P、2和3的X射线晶体结构。通过电化学与光物理研究结合密度泛函理论计算，我们对1~5的电子性质进行了全面的合理化阐释。在298 K的乙腈溶剂中，带有联吡啶或菲咯啉类辅助配体的配合物（1~3）展现出以约540 nm为中心的强而无结构的发射带，其兼具金属到配体电荷转移（MLCT）与配体到配体电荷转移（LLCT）的特征；此类配合物的光致发光量子产率（PLQYs）介于55%~70%之间。与之形成对比的是，配合物5的发射带较弱且具有精细结构，同时发生蓝移，该发射被归因于四唑环金属配体的配体定域（LC）三重态。配合物4的PLQY极低（<0.1%），因其最低激发态为非发光的金属定域（³MC）三重态。在77 K的刚性基质中，所有配合物均表现出强发光性能。配合物1~3在室温下的聚甲基丙烯酸甲酯（PMMA）基质（掺杂比例1%）及纯膜中同样具有优异的发光性能，其PLQY范围为27%~70%。我们成功制备了配合物2的高质量薄膜，以此构建的发光电化学池可发射明亮的绿光，最大亮度可达310 cd·m⁻²。相较于吡唑酸盐或三唑酸盐类类似配合物，四唑环金属配体可将三价铱配合物的发射峰蓝移。更广泛地说，在环金属配体不含氟取代基的阳离子三价铱配合物中，1~3的MLCT/LLCT发射带能量为迄今已报道的最高值。