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

Coinage metal(I) complexes exhibiting thermally activated delayed fluorescence (TADF) have attracted worldwide attention as emitters for OLEDs. Reducing the decay times and increasing the quantum efficiency of such emitters is the current challenge in this hot field. To address this issue, a symmetry-based design strategy has been applied herein to obtain pseudo-symmetric complexes [M2-(tpbz)(NHC)2¬]2+ (M = Cu, Ag, Au) scaffolded by 1,2,4,5-tetrakis(diphenylphosphino)benzene (tpbz) and N-heterocyclic carbene (NHC) ligands. In the solid state, these 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 has been shown that the Davydov model underlying the symmetry-based design strategy significantly increases the radiative constants of the “dimers” [M2¬(tpbz)(NHC)2¬]2+ compared to the “monomers” [M(dppb)(NHC)]+ based on 1,2-bis(diphenylphosphino)benzene (dppb). The practical potential of the designed TADF emitters has been demonstrated through their application as innovative thermo- and vapor-chromic emissive inks for advanced anti-counterfeiting labels.