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

Abstract

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 Au 38 (SC 6 H 13 ) 24 monolayerprotected 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.

Energy-Conserving and Thermally Corrected Neglect of Back-Reaction Approximation Method for Nonadiabatic Molecular Dynamics

“…Nonadiabatic molecular dynamics (NA-MD) is a widely adopted family of computational methods to model quantum dynamics of excited states in various systems. − The NA-MD simulations have found their use in describing various kinds of processes in many molecular and condensed-matter systems: modeling nonradiative electron–hole recombination and “hot” carrier relaxation in quantum dots and molecular clusters, − nanotubes, , plasmonic systems, or exotic states of matter; modeling photoinduced isomerization and reactive processes in various molecular systems; − and modeling charge transfer and charge carrier trapping processes in 2D materials, − interfaces, ,− ,, organic solids, , and pristine and defect-containing bulk semiconductors. − …”