Structural Control of Highly Efficient Thermally Activated Delayed Fluorescence in Carbene Zinc(II) Dithiolates**

Abstract

Luminescent metal complexes based on earth abundant elements are a valuable target to substitute 4d/5d transition metal complexes as triplet emitters in advanced photonic applications. Whereas CuI complexes have been thoroughly investigated in the last two decades for this purpose, no structure‐property‐relationships for efficient luminescence involving triplet excited states from ZnII complexes are established. Herein, we report on the design of monomeric carbene zinc(II) dithiolates (CZT) featuring a donor‐acceptor‐motif that leads to highly efficient thermally activated delayed fluorescence (TADF) with for ZnII compounds unprecedented radiative rate constants kTADF = 1.2×106 s‐1 at 297 K. Our high‐level DFT/MRCI calculations revealed that the relative orientation of the ligands involved in the ligand‐to‐ligand charge transfer (1/3LLCT) states is paramount to control the TADF process. Specifically, a dihedral angle of 36‐40° leads to very efficient reverse intersystem‐crossing (rISC) on the order of 109 s‐1 due to spin–orbit coupling (SOC) mediated by the sulfur atoms in combination with a small ΔES1‐T1 of ca. 56 meV (calc. 20 meV).