Ultrafast Charge Transfer Dynamics of Thioflavin T Probed by Time-Resolved Raman Spectroscopy

Thioflavin T (ThT) is a commonly used dye that probes amyloid fibrils associated with Alzheimer's and Parkinson's diseases. Binding to amyloid fibrils significantly enhances ThT fluorescence, while hindering the formation of the twisted intramolecular charge transfer (ICT) state in the dimethylaniline group. On the other hand, ThT fluorescence is strongly quenched in aqueous and aliphatic alcohol solutions with efficient ICT in the excited states. No direct experimental evidence for the structural changes of ThT with ICT has been reported yet. In this work, we report the structural changes of ThT during ICT by time-resolved Raman spectroscopic methods, which include the bend of the benzothiazole and the twist of the dimethylaniline group. Both femtosecond stimulated Raman spectroscopy and impulsive stimulated Raman spectroscopy suggest that the ICT coordinate of ThT can be strongly coupled with the low-frequency deformation modes. Time-dependent density functional theory calculations and the evaluated vibrational reorganization energies between the ground and excited states strongly support our experimental results based on time-resolved vibrational spectroscopy.