Electronic Relaxation Dynamics of the Au₄₂(SC₈H₉)₃₂ Cluster Nanorod Studied Using 2D Electronic Spectroscopy

The rod-like Au₄₂(SC₈H₉)₃₂ monolayer-protected cluster (MPC) was studied using two-dimensional electronic spectroscopy (2DES). This study combined analysis of the excitation power, cross-peak specific maps, and time-dependent 2DES signals that resulted from excitation of a longitudinal electronic resonance at 13 500 cm^-1. The Au₄₂(SC₈H₉)₃₂ longitudinal resonance is of interest due to the exceptional photothermal efficiency of this MPC. A traditional plasmonic gold nanorod is used throughout as a point of comparison. The excitation power study and 2DES results obtained from excitation of the longitudinal resonance were distinct from plasmonic excitations, thus implicating it as an exitonic system. The time-dependent signal amplitudes of 2DES cross-peaks showed that the longitudinal mode consisted of multiple electronic fine structure states that internally converted in a state-to-state manner. Taken together, these results point to a manifold of nondegenerate electronic states, rather than a collective plasmon resonance, that comprise the longitudinal mode excitation and dynamics of Au₄₂(SC₈H₉)₃₂.