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

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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.