Luminescent N^C^N Pincer Ni(II), Pd(II), and Pt(II) Complexes with a Pendant Coumarin Group: The Role of Auxiliary Ligands and Environments

Square-planar d8-configured metal complexes and their excited states play a key role in photocatalysis, sensing, and optoelectronic devices. However, metal-to-ligand charge-transfer (MLCT) excited states involving transition metals, particularly those with a 3d8 electronic configuration, present challenges due to rapid nonradiative relaxation via low-lying metal-centered (MC) states. In this work, an isoelectronic and isostructural series of cyclometalated complexes [MX(dpb)] with M = Ni(II), Pd(II), and Pt(II), dpb = 1,3-di(2-pyridyl)phenide, and auxiliary ligand X = chlorido, azido, and triazolato were studied by UV/vis absorption, steady-state, and time-resolved photoluminescence spectroscopy in solution and frozen glassy matrix at 77 K, along with DFT calculations. Consistently, the Pd(II) and Pt(II) complexes exhibited a characteristic emission from their triplet ligand-centered (3LC) excited states. In contrast, Ni(II) complexes with auxiliary chlorido, azido, and triazolatoCOOCH3,COOCH3 ligands were nonemissive at low and room temperatures, due to the presence of low-lying MC-type ligand-field excited states. On the other hand, [Ni(triazolatoCoumarin,COOCH3)(dpb)] showed phosphorescence from the T5 state in a frozen glassy matrix at 77 K, since the restrictive environment limits structural relaxation, while at room temperature, the primary emission is due to singlet LC excited states from the coumarin moiety of the free ligand.平面四配位d8构型金属配合物及其激发态在光催化、传感与光电器件领域发挥着关键作用。然而，涉及过渡金属的金属到配体电荷转移（metal-to-ligand charge-transfer, MLCT）激发态，尤其是具有3d8电子构型的体系，因经由低能金属中心（metal-centered, MC）态发生快速无辐射弛豫而面临诸多挑战。本工作针对一系列等电子且同结构的环金属化配合物[MX(dpb)]展开研究，其中M为Ni(II)、Pd(II)与Pt(II)，dpb为1,3-二(2-吡啶基)苯基负离子，辅助配体X分别为氯配位体、叠氮配位体与三唑配位体；研究手段涵盖溶液及77 K冷冻玻璃态基质中的紫外-可见吸收、稳态与时间分辨光致发光光谱，同时结合密度泛函理论（density functional theory, DFT）计算。实验结果一致表明，Pd(II)与Pt(II)配合物展现出源自其三线态配体中心（triplet ligand-centered, 3LC）激发态的特征发射。与之形成鲜明对比的是，带有氯、叠氮及带甲酯基三唑辅助配体的Ni(II)配合物在低温与室温下均无发射，这是由于其存在低能MC型配体场激发态。而配合物[Ni(triazolatoCoumarin,COOCH3)(dpb)]在77 K的冷冻玻璃态基质中展现出源自T5态的磷光，这是因为受限环境抑制了结构弛豫；而在室温下，其主要发射源自游离配体香豆素部分的单线态配体中心激发态。