Toward highly efficient TADF-active Cu(i), Ag(i) and Au(i) carbene complexes using symmetry-based design strategy†

Abstract

Coinage metal(I) complexes exhibiting thermally activated delayed fluorescence (TADF) have attracted worldwide attention as emitters for OLEDs. Reducing the emission lifetime and improving the quantum efficiency of such emitters is a current challenge in this hot field. To address this issue (challenge), a symmetry-based design strategy has been applied herein to obtain pseudo-symmetric complexes [M₂(tdpb)(NHC)₂]²⁺ (M = Cu, Ag, Au) scaffolded by 1,2,4,5-tetrakis(diphenylphosphino)benzene (tdpb) and N-heterocyclic carbene (NHC) ligands. In the solid state at ambient temperature, the synthesized compounds exhibit cyan to yellow TADF of the metal-to-ligand charge transfer type with excellent quantum yields (58–89%) and short decay times (2.5–15 μs). It is shown that the symmetry-based design strategy leads to a significant increase in the radiative rate constants for the “dimers” [M₂(tdpb)(NHC)₂]²⁺ compared to the “monomers” [M(dppb)(NHC)]⁺ based on 1,2-bis(diphenylphosphino)benzene (dppb). The practical potential of the developed TADF emitters was also demonstrated through their application as innovative thermo- and vapor-chromic emission inks for advanced anti-counterfeiting labels.