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)

Synthesis, Structural Characterization and Phosphorescence Properties of Trigonal Zn(II) Carbene Complexes

The sterically demanding N-heterocyclic carbene ITr (N,N\u27-bis(triphenylmethyl)imidazolylidene) was employed for the preparation of novel trigonal zinc(II) complexes of the type [ZnX2(ITr)] (X = Cl (1), Br (2), I (3)), for which the low coordination mode was confirmed in both solution and the solid state. Because of the atypical coordination geometry, the reactivity of 1-3 was studied in detail, using partial or exhaustive halide exchange and halide abstraction reactions to access [ZnLCl(ITr)] (L = carbazolate (4), 3,6-di-tert-butyl-carbazolate (5), phenoxazine (6) and phenothiazine (7)), [Zn(bdt)(ITr)] (bdt = benzene-1,2-dithiolate) (8) and cationic [Zn(2-X)(ITr)]2[BPhF4]2 (X = Cl (9), Br (10), I (11)), all of which were isolated and structurally characterized. Importantly, for all complexes 4-11, the trigonal coordination environment of the ZnII ion is maintained, demonstrating a highly stabilizing effect due to the steric demand of the ITr ligand, which protects the metal center from further ligand association. In addition, complexes 1-3 and 8-11 show long-lived luminescence from triplet excited states in the solid state at room temperature according to our photophysical studies. Our quantum chemical DFT/MRCI calculations reveal that the phosphorescence of 8 originates from a locally excited triplet state on the bdt ligand. They further suggest that the phenyl substituents of ITr are photochemically not innocent but can coordinate to the electron deficient metal center of this trigonal complex in the excited state