Retrieving photochemically active structures by time-resolved EXAFS spectroscopy

Describing the nature and structure of molecular excited states is important in order to understand their chemical reactivity and role as intermediates in photochemical reactions. The recent implementation of X-ray absorption spectroscopy in the ultrafast time domain allows studying the electronic and structural dynamics of photochemically active molecules in solutions. In this work we present the structural determination of a photoexcited diplatinum molecule, [Pt₂(P₂O ₅H₂)₄]^4-, which plays a photocatalytic role in important chemical conversions. A novel analysis of time-resolved EXAFS spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments has been performed to derive a contraction of 0.31(5) of the two Pt atoms and a ligand expansion of 0.010(6) . The former is assigned to the formation of a transient Pt-Pt bond in the excited state, while the latter indicates a concomitant weakening of the Pt-ligand coordination bonds.