Combining non-equilibrium simulations and coarse-grained modelling allows for a fine-grained decomposition of...

In this work, we present 4000 independent non-equilibrium molecular dynamics simulations of the time-dependent Stokes shift observed for the chromophore coumarin 153 in the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. The solvent is simulated using a coarse-grained model, which radically reduces computation demand, while maintaining the anisotropy of shape and charge distribution typical for ionic liquids. Using both, the high number of simulations obtainable with a coarse-grained model as well as the additivity of the non-equilibrium observable, we can study in great detail the origins of the time-dependent Stokes shift in this idealized ionic liquid system. We confirm the previously reported dominant role of the anions and show that the time scales of the relaxation processes for cations and anions are quite different. Additionally, decomposition into contributions from solvation shells defined by Voronoi tessellation clearly demonstrates that only the first two solvation shells contribute to the total effect, while contributions from the bulk solvent almost perfectly compensate and vanish. Furthermore, this decomposition by solvation shells clearly shows that the sub-picosecond relaxation stems almost entirely from the first shell. Owing to the high statistical accuracy, the total Stokes shift can be resolved into contributions from individual solute atoms. This reveals that only 9 out of 36 atoms of coumarin 153 yield significant individual contributions. Even more, two atoms comprise 90% of the total signal.