Enhanced triplet formation via ligand tuning: superatomic Ag₁₅Cu₁₂ nanocluster sensitizers for high-performance NIR...

Triplet-triplet annihilation upconversion (TTA-UC) offers a transformative approach for harvesting near-infrared (NIR) photons under non-coherent light excitation. However, achieving high efficiency beyond 900 nm is fundamentally challenged by the scarcity of sensitizers that combine strong NIR absorption with efficient triplet generation, as rapid non-radiative decay typically dominates in the low-energy regime. Here, we report highly efficient NIR-responsive sensitizers based on ligand-protected superatomic alloy nanoclusters (NCs), Ag₁₅Cu₁₂(CHT)₁₈R₃ [Ag15Cu12R; R = methylcarboxylate (Me) or naphthalenecarboxylate (Nap)]. These 6e^- closed-shell superatoms exhibit broadband absorption extending to 1000 nm and possess exceptionally long-lived triplet states (>10 µs). Remarkably, the substitution of the Me moieties with the bulkier Nap markedly increases the intersystem crossing (ISC) quantum yield from 0.28 to 0.57. Photophysical investigations and buried volume analysis on the ligand environment reveal that this improvement stems from a slightly accelerated ISC rate and a substantial suppression of the competitive internal conversion pathway via the ligand steric effect rigidifying the coordination environment. Consequently, the Ag15Cu12Nap/rubrene system achieves a record-level NIR-to-visible UC quantum yield of 13.1% (Φ_UCg = 14.4%) with a large anti-Stokes shift of 0.88 eV under 936 nm excitation. These findings establish Ag15Cu12R NCs as a tunable platform for high-performance TTA-UC in the deep NIR region.