One‐End‐Truncated Au 52 and Au 70 Quantum Rods: High‐Quantum‐Yield NIR‐II Emitters With Aspect‐Ratio‐Dominated Photophysics

ABSTRACT Precise control over absorption/emission in NIR‐II (1000–1700 nm) is essential for advancing photonic technologies. Atomically precise gold quantum rods (QRs) provide a structurally well‐defined platform for fine‐tuning NIR‐II anisotropic excitonic behavior at the atomic level. In this study, we report Au 52 (PET) 38 and Au 70 (PET) 50 (PET = 2‐phenylethanethiolate) as members of a distinct branch of Au QRs with one‐end‐truncated kernel topology. The Au 52 and Au 70 exhibit NIR‐II absorption at 1032 and 1275 nm and nanosecond fluorescence at 1100 and 1360 nm, respectively. Their high quantum yields (20.9% for Au 52 and 14.8% for Au 70 ) together with high brightness (5.6 × 10 4 M −1 cm −1 for Au 52 and a similar order of magnitude for Au 70 ) substantially exceed those of most molecular NIR‐II dyes, highlighting their promise as efficient inorganic NIR‐II fluorophores. Comparative analysis across two structurally distinct Au QR families, including one‐end‐truncated and double‐pointed rods, reveals that the longitudinal absorption and emission peak wavelengths, as well as the fluorescence linewidths, follow a common aspect‐ratio‐dependent scaling relation, suggesting length‐dependent modulation of the ground‐ and excited‐state electronic structure. These findings uncover a hierarchical interplay between rod elongation and kernel topology in defining NIR‐II excitonic behavior and expand the structural design space of atomically precise QRs.