Conformationally Constrained Bidentate Ligands Drive Record-High NIR Quantum Yield in Cu Nanoclusters

Atom-precise copper nanoclusters (Cu NCs) with near-infrared (NIR) luminescence show promise in biomedical and optoelectronic applications, due to their cost-effectiveness, low toxicity, and tunable photophysics. However, their practical application is limited by extremely low NIR photoluminescence quantum yields (PLQYs) (<1%) at room-temperature (RT) in solution, as well as their pronounced sensitivity to oxidation. Here we present two structurally well-defined 15-nuclear Cu NCs, the monophosphine-stabilized copper-thiolate cluster [Cu₁₅(TPP)₆(PET)₁₃]²⁺ (Cu₁₅-TPP) and its diphosphine analogue [Cu₁₅(DPPB)₃(PET)₁₂H]²⁺ (Cu₁₅-DPPB), which exhibit drastically different NIR PLQYs. Single-crystal X-ray diffraction (SC-XRD) reveals that both NCs feature a comparable triple-helical Cu₉ core but distinct surface ligand arrangements. In Cu₁₅-DPPB, the diphosphine chelator DPPB adopts a cis-cis conformation to rigidify ligand shell. In contrast, the monodentate TPP ligand in Cu₁₅-TPP leads to a less rigidified ligand shell. This structural disparity enables a 186-fold enhancement in NIR PLQY for Cu₁₅-DPPB (37.2% in nondegassed solution and 46% in the solid state at RT) versus Cu₁₅-TPP (0.2% in solution), with emission maxima at ∼750 nm. The 37.2% PLQY of Cu₁₅-DPPB is the highest reported for solution-phase NIR-emitting Cu-thiolate NCs. Excited-state dynamics studies unveil that this surface rigidification accelerates intersystem crossing (ISC) to populate triplet-state with boosted radiative decay (∼157-fold higher), and suppresses the nonradiative decay (∼0.53-fold lower). These findings demonstrate that ligand conformational engineering offers a new strategy to overcome intrinsic limitations of Cu-based emitters (e.g., weak spin-orbit coupling and slow intersystem crossing), and develop high-performance solution-phase RT NIR luminescent Cu NCs.