Chiral Carbene Zinc(II) Dithiolates: Efficient TADF Emitters Showing Circularly Polarized Luminescence

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 Cu(I) 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 Zn(II) compounds unprecedented radiative rate constants k(TADF) = 1.2x10^6 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 (1/3^LLCT) 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 10^9 s^-1 due to spin–orbit coupling (SOC) mediated by the sulfur atoms in combination with a small E(S1-T1) of ca. 56 meV (calc. 20 meV). In addition, the chiral carbene ligand leads to the generation of circularly polarized luminescence (CPL) with high dissymmetry values g(lum) of up to 3.3x10^-2 in polystyrene (PS).