Tracking polar solvation dynamics of a photoexcited organic chromophore with ultrafast X-ray scattering

Solvation plays a key role in many photochemical and biological processes. Yet, direct atomic-scale observation of solvent reorganization around photoexcited molecules containing only light atoms has so far remained elusive. Here, we use time-resolved X-ray scattering at an X-ray free-electron laser to track with a ~ 120 fs time resolution the photoinduced ultrafast rearrangement of polar acetonitrile solvent molecules around an organic hemithioindigo chromophore. The experiments reveal that the solvation shell reorganizes with a ~ 0.3 ps time constant driven by photoinduced charge transfer in the chromophore, and then reequilibrates over ~ 3 ps as the excited state population returns to the ground state. These results demonstrate the viability of using ultrashort X-ray pulses to directly visualize the motion of light atoms in solution, paving the way for atomic-scale understanding and control of photoinduced processes in biological and synthetic photoactive organic molecules.