Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of Au₂₅(SR)₁₈^– Monolayer-Protected Clusters

Electronic relaxation dynamics of Au₂₅(PET)₁₈^–1 and Au₂₅(PET*)₁₈^–1 monolayer-protected clusters (MPCs) were examined using femtosecond time-resolved transient absorption spectroscopy (fsTA). The use of two different excitation wavelengths (400 and 800 nm) allowed for quantification of state-resolved and ligand-dependent carrier dynamics for gold MPCs. Specifically, one-photon 400 nm (3.1 eV) and two-photon 800 nm (1.55 eV) interband excitations promoted electrons from the MPC ligand band into gold superatom d states. Following rapid internal conversion, carriers generated by interband excitation exhibited picosecond relaxation dynamics that depended upon both ligand structure and the dielectric of the dispersing medium. These solvent- and ligand-dependent effects were attributed to charge-transfer processes mediated by the manifold of ligand-based states. In contrast, one-photon intraband (gold sp–sp) excitation by 800 nm light resulted in solvent- and ligand-independent relaxation dynamics. The observed solvent independences of these data were attributed to internal relaxation via superatom p and d states localized to the MPC core. Effectively, these core-based transitions were screened from dielectric influences of the dispersing medium by the MPC gold–thiolate protecting units. Additionally, a low frequency (2.4 THz) modulation of TA signal amplitude was detected following intraband excitation. The 2.4 THz mode was consistent with Au–Au expansion in the MPC core. Based on these data, we conclude that intraband relaxation among the MPC Superatom states is mediated by low-frequency vibrations of the gold core. Structure-specific and state-resolved descriptions of MPC electron dynamics are necessary for integration of metal clusters as functional components in photonic materials.