Molecular-dynamics computer simulation applied to nonphotochemical 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 H\"uckel calculation. The system is repeatedly carried through a sequence involving preparation of a 0 K glass, chromophore excitation and subsequent dynamics on the excited-state surface, finally deexcitation 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 modeling for NPHB.