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

Electronic relaxation dynamics of Au-25(PET)(18)(-1) and Au-25(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.