Molecular Dynamics Computer Simulation Applied to Non-Photochemical Hole-Burning Processes - Resorufin in Glycerol.

Exploratory molecular dynamics calculations have been carried out to clarify molecular processes involved in nonphotochemical hole-burning (NPHB) experiments. The system chosen for study is the resorufin sodium salt in low-temperature glycerol glass. Our model employs flexible chromophore and solvent molecules. Ground and excited states for the chromophore possess fixed (but rather different) atomic charges assigned by a Huckel calculation. The system is repeatedly carried through a sequence involving preparation of a OK glass, chromophore excitation and subsequent dynamics on the excited-state surface, and finally de-excitation and dynamics on the ground-state surface. Hole burning events are frequently encountered and are identified by shifts in ground-state-system inherent structures (potential energy minima). We find that hole burning is associated with a variety of changes in the solvent hydrogen bond network produced by chromophore- excitation charge redistribution, and typically entails large spectral shifts. That such shifts are usually to the red, while experiments find blue shifts, suggests the need to incorporate charge transfer processes in future modelling for NPHB.