Simulated Solvation Dynamics of a Photoexcited Organic Molecule and the Associated Time-Resolved X-ray Scattering Signal

Abstract

Solvation shell reorganization is a key component of the dynamics after photoexcitation of a solvated molecule, especially for charge-transfer excitation in polar solvents. These structural dynamics can be directly observed with time-resolved X-ray solution scattering (TR-XSS) experiments. However, disentangling the scattering signal of solvation-shell reorganization can be challenging, especially for systems containing only light atoms. Here, nonequilibrium molecular dynamics simulations of a hemithioindigo chromophore in acetonitrile are used to investigate the solvation dynamics upon photoinduced charge transfer as well as vibrational excitation and cooling, and to characterize the corresponding time-resolved X-ray scattering signals. It is found that charge transfer induces rapid reorientation of the first solvation shell (0.15-0.2 ps) followed by slower reorientational and solvent redistribution within and between shells (1-2 ps). An analysis of the volume around the solute excluded to the solvent reveals an initial contraction of the solvation shell, followed by transient rebound within the first 0.4 ps, and then slower contraction. Instantaneous vibrational excitation expands the excluded volume, reaching a maximum near 0.4 ps, followed by contraction during cooling with biexponential relaxation. Low-q features in both the calculated solute-solvent and solvent-solvent scattering signals reflect rearrangements within the first solvation shell and redistribution across shells, while higher-q solute-solvent features more closely follow local first-shell restructuring. These results show that TR-XSS can provide information on both local solvent rearrangements and redistribution across solvation shells and demonstrate that interpreting the scattering signals requires accounting for changes in solvent packing induced by vibrational heating and cooling of the solute.