Direct Tracking Excited-State Intramolecular Charge Redistribution of Acceptor-Donor-Acceptor Molecule by Means of Femtosecond Stimulated Raman Spectroscopy

Symmetric quadrupolar molecules generally exhibit apolar ground states and dipolar excited states in a polar environment, which is explained by the excited state evolution from initial charge delocalization over all molecules to localization on one branch of the molecules after a femtosecond pulse excitation. However, direct observation of excited-state charge redistribution (delocalization/localization) is hardly accessible. Here, the intramolecular charge delocalization/ localization character of a newly synthesized acceptor-donor-acceptor molecule (ADA) has been intensively investigated by femtosecond stimulated Raman scattering (FSRS) together with femtosecond transient absorption (fs-TA) spectroscopy. By tracking the excited state Raman spectra of the specific alkynyl (-C C-) bonds at each branch of ADA, we found that the nature of the relaxed S-1 state is strongly governed by solvent polarity: symmetric delocalized intramolecular charge transfer (ICT) characters occurred in apolar solvent, whereas the asymmetric localized ICT characters appeared in polar solvent because of solvation. The solvation dynamics of ADA extracted from fs-TA is consistent with the time constants obtained by FSRS, but the FSRS clearly tracks the excited state intramolecular charge transfer delocalization/localization.

Automated summary

In this study, the intramolecular charge delocalization/localization of a newly synthesized acceptor-donor-acceptor molecule was intensively investigated using femtosecond stimulated Raman scattering and femtosecond transient absorption spectroscopy. The nature of the relaxed S-1 state was found to be strongly influenced by solvent polarity, with symmetric delocalized intramolecular charge transfer characters occurring in apolar solvent and asymmetric localized ICT characters appearing in polar solvent due to solvation. The solvation dynamics extracted from fs-TA were consistent with the time constants obtained by FSRS, but FSRS clearly tracked the excited state intramolecular charge transfer delocalization/localization.