Nanometals: Identifying the onset of metallic relaxation dynamics in monolayer-protected gold clusters using femtosecond spectroscopy

Electronic relaxation dynamics were studied for a series of gold monolayer-protected clusters (MPCs) whose sizes ranged from 1.5 to 2.4 nm. Au₉₆(mMBA)₄₂, Au₁₀₂(pMBA)₄₄, Au₁₁₅(pMBA)₄₉, Au₁₁₇(mMBA)₅₀, Au₁₄₄(pMBA)₆₀, Au₂₅₀(pMBA)₉₈, and Au₄₅₉(pMBA)₁₇₀ (pMBA = para-mercaptobenzoic acid; mMBA = meta-mercaptobenzoic acid) were selected for study because they bridged the expected transition from nonmetallic to metallic electron behavior. Excitation-pulse-energy-dependent measurements confirmed Au₁₄₄(pMBA)₆₀ (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron-phonon coupling constant of (1.63 ± 0.25) × 10¹⁶ W m^-3 K^-1. Smaller, nonmetallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantum-confined electronic states. Volume-dependent electronic relaxation dynamics for ≤1.8 nm MPCs were observed and attributed to a combination of large energy differences between electronic states and phonon frequencies and spatial separation of photoexcited electrons and holes. Evidence for the latter was obtained by substituting mMBA for pMBA as a passivating ligand, which resulted in a 4-fold increase in the relaxation rate constant.