Unraveling intramolecular charge transfer coordinate by optical polarization dependent coherent vibrational spectroscopy

Coherent vibrational wavepackets (CVWs) observed in femtosecond time-resolved spectroscopy provide direct access to excited-state structural dynamics and have been widely used to investigate ultrafast photoinduced processes. Their mechanistic utility, however, has been largely limited to reactions occurring within the vibrational coherence time. Here, we demonstrate that polarization-dependent CVW analysis can be extended to weakly reactive, non-ultrafast processes and used to identify vibrations associated with the reaction coordinate. As a model system, the solvation-driven intramolecular charge-transfer (ICT) dynamics of thioflavin T are investigated. In 1-propanol, ICT proceeds more slowly than vibrational dephasing, allowing latent reactivity to remain encoded in the polarization dependence of the CVW. Two vibrational modes exhibit distinct polarization dependence in the stimulated-emission band of the intermediate state, and their assignment as ICT-related modes is supported by quantum-chemical calculations. These results demonstrate that polarization-dependent CVW analysis provides a practical route to identifying reactive vibrations beyond the conventional ultrafast limit and expands the mechanistic scope of coherent vibrational spectroscopy for a broader range of photoinduced dynamics.