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

The [Au₂₅(SR)₁₈]⁻ and [Au₁₃(dppe)₅Cl₂]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 [Au₁₃(dppe)₅Cl₂]³⁺ are elucidated theoretically for the first time. [Au₁₃(dppe)₅Cl₂]³⁺ exhibits a single strong emission peak compared to the weaker bimodal luminescence of [Au₂₅(SR)₁₈]⁻ (modeled here as [Au₂₅(SH)₁₈]⁻). The strongly emissive state is found to arise from deexcitation out of the S₁ state, similar to what is seen for [Au₂₅(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 [Au₁₃(dppe)₅Cl₂]³⁺ and its model compound [Au₁₃(pe)₅Cl₂]³⁺ [pe = 1,2-bis(phosphino)ethane] are observed to be shorter than the lifetimes of the corresponding states of [Au₂₅(SR)₁₈]⁻; this occurs because the energy gap separating degenerate sets of unoccupied orbitals is only ∼0.2 eV in [Au₁₃(dppe)₅Cl₂]³⁺ compared to a ∼0.6 eV energy gap in [Au₂₅(SH)₁₈]⁻.