Superatom spin-state dynamics of structurally precise metal monolayer-protected clusters (MPCs)

Electronic spin-state dynamics were studied for a series of Au₂₅(SC₈H₉)₁₈ ^q and Au₂₄Pd(SC₈H₉)₁₈ monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = -1, 0, +1. These clusters were chosen for study because Au₂₅(SC₈H₉)₁₈ ^-1 is a closed-shell superatomic cluster, but Au₂₅(SC₈H₉)₁₈ ⁰ is an open-shell (7-electron) system; Au₂₅(SC₈H₉)₁₈ ⁺¹ and PdAu₂₄(SC₈H₉)₁₈ ⁰ are isoelectronic (6-electron) closed-shell systems. Carrier dynamics for electronic fine structure spin states were isolated using femtosecond time-resolved circularly polarized transient-absorption spectroscopy (fs-CPTA). Excitation energies of 1.82 eV and 1.97 eV were chosen for these measurements on Au₂₅(SC₈H₉)₁₈ ⁰ in order to achieve resonance matching with electronic fine structure transitions within the superatomic P- and D-orbital manifolds; 1.82-eV excited an unpaired P_z electron to D states, whereas 1.97-eV was resonant with transitions between filled P_x and P_y subshells and higher-energy D orbitals. fs-CPTA measurements revealed multiple spin-polarized transient signals for neutral (open shell) Au₂₅(SC₈H₉)₁₈, following 1.82-eV excitation, which persisted for several picoseconds; time constants of 5.03 ± 0.38 ps and 2.36 ± 0.59 ps were measured using 2.43 and 2.14 eV probes, respectively. Polarization-dependent fs-CPTA measurements of PdAu₂₄(SC₈H₉)₁₈ clusters exhibit no spin-conversion dynamics, similar to the isoelectronic Au₂₅(SC₈H₉)₁₈ ⁺¹ counterpart. These observations of cluster-specific dynamics resulted from spin-polarized superatom P to D excitation, via an unpaired P_z electron of the open-shell seven-electron Au₂₅(SC₈H₉)₁₈ MPC. These results suggest that MPCs may serve as structurally well-defined prototypes for understanding spin and quantum state dynamics in nanoscale metal systems.