Atomically Precise Au 42 Nanorods with Longitudinal Excitons for an Intense Photothermal Effect.

Metallic-state gold nanorods are well known to exhibit strong longitudinal plasmon excitations in the near-infrared region (NIR) suitable for photothermal conversion. However, when the size decreases below ∼2 nm, Au nanostructures become nonmetallic, and whether the longitudinal excitation in plasmonic nanorods can be inherited is unknown. Here, we report atomically precise rod-shaped Au 42 (SCH 2 Ph) 32 with a hexagonal-close-packed Au 20 kernel of aspect ratio as high as 6.2, which exhibits an intense absorption at 815 nm with a high molar absorption coefficient of 1.4 × 10 5 M -1 cm -1 . Compared to other rod-shaped nanoclusters, Au 42 possesses a much more effective photothermal conversion with a large temperature increase of ∼27 °C within 5 min (λ ex = 808 nm, 1 W cm -2 ) at an ultralow concentration of 50 μg mL -1 in toluene. Density functional theory calculations show that the NIR transition is mainly along the long axis of the Au 20 kernel in Au 42 , i.e., a longitudinal excitonic oscillation, akin to the longitudinal plasmon in metallic-state nanorods. Transient absorption spectroscopy reveals that the fast decay in Au 42 is similar to that of shorter-aspect-ratio nanorods but is followed by an additional slow decay with a long lifetime of 2400 ns for the Au 42 nanorod. This work provides the first case that an intense longitudinal excitation is obtained in molecular-like nanorods, which can be used as photothermal converters and hold potential in biomedical therapy, photoacoustic imaging, and photocatalysis.