Luminescence and Electron Dynamics in Atomically Precise Nanoclusters with Eight Superatomic Electrons

The [Au-25(SR)(18)](-) and [Au-13(dppe)(5)Cl-2](3+) [dppe = 1,2-bis-(diphenylphosphino)ethane] nanoclusters both possess a 13-atom icosahedral core with 8 delocalized superatomic electrons (8e), but their emission properties and time-resolved electron dynamics differ significantly. In this work, experimental photoluminescence and photoluminescence decay measurements are combined with time-dependent density functional theory calculations of radiative and nonradiative decay properties and lifetimes to elucidate the similarities and differences in the emission of these two nanoclusters with similar cores. In this work, the photodynamic properties of [Au-13(dppe)(5)Cl-2](3+) are elucidated theoretically for the first time. [Au-13(dppe)(5)Cl-2](3+) exhibits a single strong emission peak compared to the weaker bimodal luminescence of [Au-25(SR)(18)](-) (modeled here as [Au-25(SH)(18)](-)). The strongly emissive state is found to arise from deexcitation out of the S1 state, similar to what is seen for [Au-25(SH)(18)](-). Both theory and experiment exhibit microsecond lifetimes for this state. Transient absorption measurements and theoretical calculations demonstrate that the excited-state lifetimes for higher excited states are typically less than 1 ps. The decay times for the higher excited states of [Au-13(dppe)(5)Cl-2](3+) and its model compound [Au-13(pe)(5)Cl-2](3+) [pe = 1,2-bis(phosphino)ethane] are observed to be shorter than the lifetimes of the corresponding states of [Au-25(SR)(18)](-); this occurs because the energy gap separating degenerate sets of unoccupied orbitals is only similar to 0.2 eV in [Au-13(dppe)(5)Cl-2](3+) compared to a similar to 0.6 eV energy gap in [Au-25(SH)(18)](-).