Symmetrical versus asymmetrical molecular configuration in metal-assisted-through-space charge transfer TADF emitters

Abstract

The synthesis of a novel imidazolidine type N-heterocyclic carbene (NHC) Cu (I) complex with asymmetrically attached phenylsulfonyl- acceptor group is presented. The asymmetrical ligand was used for the preparation two Cu(I) carbene-metal-amide (CMA) complexes with carbazolide (complex 1) or 1,8-dimethylcarbazolide (complex 2) as the amide donors. Obtained complexes exhibit efficient thermally activated delayed fluorescence (TADF) with luminescence quantum yields (QY) reaching 0.80 in PMMA matrix. Metal promoted through-space charge transfer approach in the emitter design lowers the ΔΕ_ST gap and the small spin-orbit coupling (SOC) provided by metal atom enables high radiative rates (k_r = 2.21×10⁵ s^-1 for complex 2). The photophysical properties of the asymmetrical (1-2) and previously reported symmetrical (3-6) molecular designs are compared. Obtained results suggest a closely similar photophysical behavior for both the asymmetrical and symmetrical CMA complexes.