Transition metal complexes (TMCs) are often used as photosensitizers in artificial photosynthetic systems. However, their excited-state behaviors in terms of structural dynamics on the ultrafast time scale are complex and largely unknown, hindering the rational design of molecular complexes for converting light to fuel and electricity. A complete TMC excited-state charge transfer trajectory must include interplays of the electronic and nuclear motions before thermalization. While optical spectroscopic methods provide extensive information regarding a system’s electronic properties, such methods lack direct structural information. As a direct structural probe, X-ray solution scattering (XSS) serves as an excellent compliment to optical methods. Here, we utilize XSS measurements at the LCLS to track the structural dynamics of a series of Pt dimer complexes following excitation into the metal-metal-to-ligand charge transfer band on the femtosecond time scale. Depopulation of the Pt-Pt dσ* orbital causes transient bond formation and subsequent contraction in the Pt-Pt distance. A coherent vibrational wavepacket is observed for each complex with a frequency of ~150 cm<sup>-1</sup>, consistent with an excited-state Pt-Pt vibration, which persists to longer time delays (&gt; 1 ps). By combining these results with optical impulsive stimulated Raman spectroscopy and computational methods, we can quantify how minor alterations in the ligand structure tune the potential energy surfaces; these results present possible avenues for controlling excited-state trajectories/outcomes through judicious choice of the organic linker ligands.

Deciphering electronic-nuclear coupling following photoexcitation of a series of Pt dimer complexes using high-energy X-ray solution scattering

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ACS Fall 2022

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