Pushing the ligand field to the limit – Cyclometalated complexes of d8-configured metal ions as efficient triplet emitters

廉价高效的发光金属有机络合物对于光催化剂、光伏器件和有机发光二极管(OLED)的发展至关重要。到目前为止，大多数商业应用都依赖于Ru(II)、Os(II)或Ir(III)的配合物，但铂(II)和Au(III)的配合物是有趣的替代品。该项目旨在从这些已建立的复合体转向其他d8构型的衍生品。显然，Ag(III)、Pd(II)和Ni(II)的配合物具有挑战性，特别是考虑到自旋-轨道耦合的减少和配体-场分裂的降低。然而，考虑到镍或银的丰度明显较高，价格较低，它们构成了各种应用的有吸引力的候选者。因此，我们将实现量身定制的三齿和四齿配体作为具有0、1、2或3个负电荷和环金属化位置的螯合发光单元，以增强配体的场分裂，同时避免以金属为中心的解离态的布居。具有三齿配体的D8-配合物具有一个开放的配位侧翼的优势，可用于插入辅助配体以完成正方形平面环境；它们通过进一步增强配体的场强来微调吸收和发射能量，特别是光致发光量子产率，以减缓无辐射失活速率。这里提出的一个全新的概念将涉及开发基于重元素的σ供体作为辅助配体-E(芳基)3(E=Ge，Sn或Pb)或Pn(芳基)3(Pn=Sb，As或Bi)：这将使我们能够增强如果使用更轻(更便宜)的金属中心所需的自旋-轨道耦合，以促进辐射路径。在此背景下，我们还将研究依赖于较轻金属的热激活延迟荧光(TADF)过程的潜力。我们将结合独特的协同方法，涉及我们互补的专业领域，从电化学表征、稳态和时间分辨光致发光光谱(PL，从6 K到380K)，到有机金属合成，在电子设计和解决激发态动力学的复杂(TD)密度泛函计算的指导下。因此，我们将评估激发态的性质和涉及的跃迁，而从头计算将为新的有机金属配位化合物的合理设计提供预测指导。在第一个资助期内巩固的已经富有成效的合作将得到扩大，包括有机金属络合物的化学合成(Strassert+Klein)、光物理和光谱学(Strassert)、(光谱)电化学(Klein)、量子化学模拟(Doltsinis)以及与SPP内的其他合作伙伴正在进行的和新的合作。

The availability of cheap and efficient luminescent organometallic complexes is crucial for the development of photocatalysts, photovoltaic devices and organic light-emitting diodes (OLEDs). To date, most commercial applications rely on Ru(II), Os(II) or Ir(III) complexes, but Pt(II) and Au(III) complexes have shown to be interesting alternatives. This project aims to shift from these established complexes to other d8-configured derivatives. It is clear that Ag(III), Pd(II) and Ni(II) complexes are challenging, particularly in view of the reduced spin-orbit coupling and lower ligand-field splitting. However, they constitute appealing candidates for diverse applications considering the markedly higher abundance and lower price of Ni or Ag. Hence, we will implement tailored tri- and tetradentate ligands as chelating luminophoric units with 0, 1, 2 or 3 negative charges and cyclometalation sites to enhance the ligand field splitting while avoiding population of dissociative metal-centred states. d8-complexes with tridentate ligands offer the advantage of one open coordination flank that is available for the insertion of an ancillary ligand to complete the square-planar environment; they fine-tune absorption and emission energies and particularly the photoluminescence quantum yields by further enhancing the ligand field strength to slow down radiationless deactivation rates. A fundamentally new concept proposed herein will involve exploitation of heavy-element-based σ-donors as ancillary ligands of the type –E(aryl)3 (E = Ge, Sn or Pb) or Pn(aryl)3 (Pn = Sb, As, or Bi): This will allow us to enhance the spin-orbit coupling required if lighter (cheaper) metal centres are used, in order to boost radiative pathways. In this context, we will also study the potential of thermally-activated delayed fluorescence (TADF) processes relying on the lighter metals. We will combine a uniquely synergetic approach involving our complementary expertise fields spanning from electrochemical characterisation, steady-state and time-resolved photoluminescence spectroscopies (PL, from 6 to 380 K), to organometallic synthesis guided by in silico design and sophisticated (TD)DFT calculations addressing excited state dynamics. Hence, we will assess the character of the excited states and the involved transitions, while ab initio calculations will give predictive guidelines for the rational design of new organometallic coordination compounds. The already productive cooperation consolidated within the first funding period will be expanded and includes chemical synthesis of organometallic complexes (Strassert + Klein), photophysics and spectroscopy (Strassert), (spectro)electrochemistry (Klein), quantum-chemical simulations (Doltsinis) as well as ongoing and new collaborations with further partners within the SPP.