Time-Resolved Probing of the Nonequilibrium Structural Solvation Dynamics by the Time-Dependent Stokes Shift

TL;DR

This paper develops a quantum theory to analyze how the structure of a polar solvent relaxes around a photoexcited molecule over time, revealing complex dynamics including bi-modal shifts and slow decay tails.

Contribution

It introduces a nonequilibrium quantum framework for time-dependent solvation in explicit solvents, capturing coupled structural and polarization dynamics.

Findings

Bi-modal Stokes shift observed in a shrinking solute.

Long-tail decay explained by slow structural variations.

Coupling between solute size change and solvent response.

Abstract

The time-dependent fluorescence Stokes shifts monitors the relaxation of the polarization of a polar solvent in the surroundings of a photoexcited solute molecule, but also the structural variation of the solute following photoexcitation and the subsequent molecular charge redistribution. Here, we formulate a simple nonequilibrium quantum theory of solvation for an explicitly time-dependent continuous solvent. The time-dependent solvent induces nonequilibrium fluctuations on the solvent dynamics which are directly reflected in different time components in the time-dependent Stokes shift. We illustrate the structural dynamics in the presence of an explicitly time-dependent solvent by the example of a dynamically shrinking solute which leads to a bi-modal Stokes shift. Interestingly, both contributions are mutually coupled. Furthermore, we can explain a prominent long-tail decay of the Stokes shift associated to slow structural dynamical variations.