Excited state reversal in copper iodide clusters enables 100% exciton radiation

The non-radiative metallic core (MC) centered lowest excited states of most ligand-stabilized metallic clusters commonly quench radiative but high-lying ligand-centered excited states, e.g. intra-ligand charge transfer (LCT), which is one of the key issues limiting efficiencies of electroluminescent (EL) clusters. Herein, we realize the desired excited state reversal in a cubic [PXZDBFDP]₂Cu₄I₄ (PXZDBFDP = 10-(4,6-bis(diphenylphosphino)dibenzo[b,d]furan-2-yl)-10H-phenoxazine) modified with strongly electron-donating phenoxazine (PXZ) to strengthen donor-acceptor (D-A) interactions and enhance LCT. Consequently, its thoroughly LCT-featured first singlet (S₁) and triplet (T₁) excited states are energetically lower than its Cu₄I₄-involved excited states. This case not only increases excited-state utilization through energy transfer from non-radiative MC to radiative LCT states, but also leads to balanced dual emission of thermally activated delayed fluorescence (TADF, 52%) and phosphorescence (PH, 48%) respectively from singlet and triplet LCT states. Therefore, compared to another congener [PhPXZDBFDP]₂Cu₄I₄ (PhPXZDBFDP = 10-(4,6-bis(diphenylphosphino)dibenzo[b,d]furan-2-yl)-10H-phenylphenoxazine) with a D-π-A ligand and the normal low-lying MC states, [PXZDBFDP]₂Cu₄I₄ achieves sevenfold increased photoluminescence quantum yield of ~90%, and the 13-fold increased maximum EL external quantum efficiency of 35.5%, which is the record-high value for EL homo-copper clusters. These results demonstrate the feasibility of accurate excited-state optimization for clusters through ligand engineering.