Symmetry-Dependent Dynamics in Au₃₈(SC₆H₁₃)₂₄ Revealed by Polarization-Dependent Two-Dimensional Electronic Spectroscopy

Structurally precise metal nanoclusters exhibit complex electronic relaxation dynamics that differ from those of plasmonic nanoparticles. Here, we show the importance of the electronic state symmetry in determining the relaxation pathways of gold nanoclusters. Polarization-dependent two-dimensional (2D) electronic spectroscopy (p-2DES) was used to map state-specific relaxation dynamics in Au₃₈(SC₆H₁₃)₂₄ monolayer-protected clusters (MPCs). A positive-amplitude differential signal at 1.96 eV excitation and detection energies that spanned 1.91-2.05 eV was resolved in the p-2DES cross-peak specific spectra. The positive-amplitude signal stood out from the otherwise broad transient bleach signals due to its unique time-dependent amplitude, anisotropy, and transition orientation angles. Density functional theory (DFT) calculations and selection rules indicated that the positive-amplitude signal in S_cp spectra feature most likely corresponds to a dark E-symmetry state localized to the MPC Au-S ligand band. Time-dependent analysis of p-2DES-detected cross-peaks at different excitation-detection energies indicated that the dark state is indirectly populated through relaxation of the initially excited bright states and relaxes through a pathway that is uniquely accessed by 1.96 eV excitation. These results reveal the surprising influence of excited state symmetry on carrier relaxation for metal colloids in the range of 1-2 nm, where electronic state densities are high. These insights are made possible by the combined use of p-2DES and DFT, which made state assignments possible. State-selective dynamics may provide a strategy for controlling the energy flow in metal nanoclusters.