N-heterocyclic carbene-stabilized Cu9 clusters with combined thermally activated delayed fluorescence and phosphorescence

Abstract

Metal clusters with both phosphorescence and thermally activated delayed fluorescence (TADF) at room temperature, allowing for efficient light emission and a short lifetime, are of particular interest. However, such clusters remain rare due to the challenge of achieving comparable and effective transitions T₁ → S₁ → S₀ and T₁ → S₀ simultaneously. Herein, we report two copper clusters, [Cu₉{(py)₂im}₂(PhCC)₄(CF₃CO₂)₄](PF₆) (Cu₉-Ph) and [Cu₉{(py)₂im}₂(^tBuCC)₄(CF₃CO₂)₄](PF₆) (Cu₉-TBA), for which the ambient-temperature emission stems from both TADF from the S₁ state and direct phosphorescence from the T₁ state in thermal equilibrium. At 300 K, about 69% of the emission intensity of Cu₉-TBA originates from the delayed fluorescence of the S₁ state, and an additional 31% is contributed by the T₁ → S₀ emission. In comparison, Cu₉-Ph exhibits nearly equal contributions from delayed fluorescence and phosphorescence at room temperature. As a result, the overall decay time is significantly shorter than that observed in the TADF-only situation of these clusters (e.g., reduced by 49% for Cu₉-Ph). The combined harvesting mechanism arises from the simultaneous presence of a small ΔE(S₁ − T₁) gap and relatively strong spin–orbit coupling induced by the multi-metal centers. The combined use of both decay processes makes the low-cost Cu(I) clusters promising for applications in organic light-emitting diodes.