Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

Abstract

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor-acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.

Ultrafast Dynamics of a Nucleobase Analogue Illuminated by a Short Intense X-ray Free Electron Laser Pulse

“…For selected applications, these qualities offer important advantages over conventional x-ray sources. Time-resolved spectroscopy on the time scale of femtoseconds to picoseconds allows us to monitor, for the first time, atoms and electrons in action [4][5][6][7][8][9][10][11][12][13][14]. For structural analysis, the extreme intensity focused into a couple of μm 2 transforms single-shot diffraction imaging of biomolecules and nanosize objects from a remote goal into a tangible reality [15][16][17][18][19].…”

X-ray absorption spectroscopy using a self-seeded soft X-ray free-electron laser

“…Prominent examples in the soft x-ray regime are 3d transition metal L-edge and ligand K-edge XAS, with their high sensitivity to oxidation and spin states, and the potential to extract the order of orbital energy levels, the symmetry and the covalency of the studied complexes [14][15][16][17][18][19]. These methods have been successfully applied to date at synchrotrons, and they are being established at XFELs to investigate ultrafast dynamics in chemical reactions on surfaces [20][21][22][23], in liquids [3,4,24,25] and gases [26].…”

Probing spin–vibronic dynamics using femtosecond X-ray spectroscopy

“…Here, one should bear in mind that the strength of X-ray spectroscopy is providing a local interpretation around the absorbing atom. Consequently, its use will be most valuable in disentangling the dynamics of complex systems which may exhibit important optically dark states and which can give a large number of broad overlapping bands, obvious examples in this direction are multi-nuclear metal complexes 15 or nanoparticles 62 .…”