Superatom Spin-State Dynamics of Structurally Precise Metal Monolayer-Protected Clusters (Mpcs)

Electronic spin-state dynamics were studied for a series of Au25(SC8H9)18q 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)180 is an open-shell (7-electron) system; Au25(SC8H9)18+1 and PdAu24(SC8H9)180 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)180 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 Pz electron to D states, whereas 1.97-eV was resonant with transitions between filled Px and Py 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 Pz 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.