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

Abstract

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 ...