Evolution from the Plasmon to Exciton State in Atomically Precise Gold Nanoparticles

The evolution from the metallic (or plasmonic) to molecular state in metal nanoparticles constitutes a central question in nanoscience research because of its importance in revealing the origin of metallic bonding and offering fundamental insights into the birth of surface plasmon resonance. Here, we utilize the atomically precise gold nanoparticles protected by thiolate ranging in size from 1 nm to 3.5 nm (including Au$_{\mathrm{25}}$, Au$_{\mathrm{38}}$, Au$_{\mathrm{144}}$, Au$_{\mathrm{333}}$, Au$_{\mathrm{\sim 520}}$, Au$_{\mathrm{\sim 940}})$ and investigate the grand transition from metallic to molecular state by femtosecond transient absorption spectroscopy, as well as the impact of the transition on catalytic reactions. By directly probing the electron-phonon coupling of the gold nanoparticles, we have mapped out that the transition occurs between 2.3 nm (Au$_{\mathrm{333}})$ and 1.7 nm (Au$_{\mathrm{144}})$. This study paves the way for future exploitation of the grand transition and its impact on the physicochemical properties of metal nanoparticles, in particular the applications in energy transfer and utilization.