Luminescent Iridium(III) Complexes with N∧C∧N-Coordinated Terdentate Ligands: Dual Tuning of the Emission Energy and Application to Organic Light-Emitting Devices

A family of complexes (1a-3a and 1b-3b) was prepared, having the structure Ir­(N∧C∧N)­(N∧C)­Cl. Here, N∧C∧N represents a terdentate, cyclometallating ligand derived from 1,3-di­(2-pyridyl)­benzene incorporating CH3 (1a,b), F (2a,b), or CF3 (3a,b) substituents at the 4 and 6 positions of the benzene ring, and N∧C is 2-phenylpyridine (series a) or 2-(2,4-difluorophenyl)­pyridine (series b). The complexes are formed using a stepwise procedure that relies on the initial introduction of the terdentate ligand to form a dichloro-bridged iridium dimer, followed by cleavage with the N∧C ligand. 1H NMR spectroscopy reveals that the isomer that is exclusively formed in each case is that in which the pyridyl ring of the N∧C ligand is trans to the cyclometallating aryl ring of the N∧C∧N ligand. This conclusion is unequivocally confirmed by X-ray diffraction analysis for two of the complexes (1b and 3a). All of the complexes are highly luminescent in degassed solution at room temperature, emitting in the green (1a,b), blue-green (2a,b), and orange-red (3a,b) regions. The bidentate ligand offers independent fine-tuning of the emission energy: for each pair, the “b” complex is blue-shifted relative to the analogous “a” complex. These trends in the excited-state energies are rationalized in terms of the relative magnitudes of the effects of the substituents on the highest occupied and lowest unoccupied orbitals, convincingly supported by time-dependent density functional theory (TD-DFT) calculations. Luminescence quantum yields are high, up to 0.7 in solution and close to unity in a PMMA matrix for the green-emitting complexes. Organic light emitting devices (OLEDs) employing this family of complexes as phosphorescent emitters have been prepared. They display high efficiencies, at least comparable, and in some cases superior, to similar devices using the well-known tris-bidentate complexes such as fac-Ir­(ppy)3. The combination of terdentate and bidentate ligands is seen to offer a versatile approach to tuning of the photophysical properties of iridium-based emitters for such applications.

本研究合成了一系列结构为Ir(N∧C∧N)(N∧C)Cl的配合物（编号1a~3a及1b~3b）。其中，N∧C∧N代表由1,3-二(2-吡啶基)苯衍生得到的三齿环金属化配体，该配体在苯环的4位与6位带有甲基（1a、1b）、氟原子（2a、2b）或三氟甲基（3a、3b）取代基；而N∧C则为双齿配体，其中系列a对应2-苯基吡啶，系列b对应2-(2,4-二氟苯基)吡啶。该系列配合物采用分步法制备：首先引入三齿配体，生成二氯桥联铱二聚体，随后通过与N∧C配体的裂解反应得到目标配合物。通过氢核磁共振（¹H NMR）光谱表征可知，每组反应仅生成单一异构体，即双齿配体N∧C的吡啶环与三齿配体N∧C∧N的环金属化芳环处于反式位置的异构体。该结论通过对两种配合物（1b与3a）的X射线衍射分析得到了明确验证。所有配合物在室温脱气溶液中均表现出强发光特性：1a、1b发射绿光，2a、2b发射蓝绿光，3a、3b发射橙红光。双齿配体可独立对发射能量进行精细调控：每一组对应配合物中，编号为“b”的配合物相较于同系列“a”配合物均发生蓝移。激发态能量的上述变化规律可通过取代基对最高占据分子轨道与最低未占据分子轨道的相对影响程度得到合理解释，且该解释得到了含时密度泛函理论（TD-DFT）计算结果的有力支撑。该系列配合物的发光量子产率较高：绿光发射配合物在溶液中的量子产率最高可达0.7，在聚甲基丙烯酸甲酯（PMMA）基质中的量子产率接近1。以该系列配合物作为磷光发光体的有机发光二极管（OLED）已被成功制备。此类器件的效率至少可与采用经典三双齿配合物（如面式三(2-苯基吡啶)合铱fac-Ir(ppy)3）的同类器件相媲美，部分器件的效率甚至更优。三齿配体与双齿配体的组合策略为调控此类应用中铱基发光材料的光物理性质提供了一种通用且灵活的途径。