Structure and ultrafast dynamics of tri-nuclear Ag-/Tl-Pt₂POP₄ complexes in solution

The energetics and dynamics of ion assembly in solution has broad influence in nanomaterials and inorganic synthesis. To investigate the fundamental processes involved, we present a time-resolved x-ray solution scattering (TR-XSS) study of the trinuclear silver and thallium complexes of the diplatinum ion PtPOP [Pt₂(H₂P₂O₅) ${}_4^{4-}$ ] in aqueous solution. These complexes, their structural properties, and their electronic structure are not well understood and afford a unique opportunity to study the metal-metal bond formation that influences molecular and material assembly in solution. We present model-independent analysis of the observed dynamics as well as an analysis incorporating time-resolved structural refinements of key bond lengths with <100 fs time resolution. We find that upon photoexcitation, the Pt atoms contract $\sim$ 0.25 Å toward the center of both the Ag- and the Tl-PtPOP complexes, as previously observed for the PtPOP anion. For the AgPtPOP system, an ultrafast Ag-Pt bond expansion of $\sim$ 0.2 Å is observed, whereas in contrast, the TlPtPOP system exhibits a Tl-Pt bond contraction of $\sim$ 0.3 Å upon photoexcitation. For both complexes, the change in electronic state leads to coherent ("wave-packet") oscillations along the metal-Pt coordinates. Based on these structural dynamics, we propose an electronic structure model that describes the metal-metal bonding behavior in both the ground and excited state for both complexes.