Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

Photoinduced non-adiabatic intramolecular processes have important applications but their mechanisms are challenging to explore. Here the authors detect and assign vibrational wavepacket dynamics in a Fe carbene complex by ultrafast X-ray emission spectroscopy and X-ray scattering, resolving nuclear and electronic motion. The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)(2)](2+) photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe K alpha and K beta X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered ((MC)-M-3) excited state surface. This (MC)-M-3 state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a (MLCT)-M-3 excited state. The sensitivity of the K alpha XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.