Tracking Structural Evolution during Symmetry-Breaking Charge Separation in Quadrupolar Perylene Bisimide with Time-Resolved Impulsive Stimulated Raman Spectroscopy

Elucidating structural roles in photoinduced charge transfer is indispensable, as nuclear rearrangements are simultaneously usually involved in the dynamics. However, it is hard to evaluate whether the structural changes occur or not by using conventional time-resolved electronic spectroscopy. Here, time-resolved impulsive stimulated Raman spectroscopy is applied to record the evolution of vibrational snapshots during charge-separation dynamics of donor-acceptor-donor-type quadrupolar perylene bisimide in real time. Drastic frequency shifts were observed for several Raman bands with their population kinetics, thus symmetry-breaking charge separation accompanies significant structural changes, as supported by (TD)-DFT calculations. A comparison between time-resolved Raman spectra of the neutral S₁ state and the radical anion species shows that the spectral signatures, especially in high-frequency regions, provide important clues to bond length alternation patterns in the PBI core.