Abstract
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 Au42(SCH2Ph)32 with a hexagonal-close-packed Au20 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 x 105 M-1 cm-1. Compared to other rod-shaped nanoclusters, Au42 possesses a much more effective photothermal conversion with a large temperature increase of -27 degrees C within 5 min (lambda ex = 808 nm, 1 W cm-2) at an ultralow concentration of 50 mu g mL-1 in toluene. Density functional theory calculations show that the NIR transition is mainly along the long axis of the Au20 kernel in Au42, i.e., a longitudinal excitonic oscillation, akin to the longitudinal plasmon in metallic-state nanorods. Transient absorption spectroscopy reveals that the fast decay in Au42 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 Au42 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.
