Tracking Intermolecular Coherent Vibrations in Ultrafast Excited-State Proton Transfer

Ultrafast photoinduced excited-state proton transfer (ESPT) is a fundamental photoprotective mechanism in biomolecules and functional materials, yet the role of solute–solvent interactions in governing its dynamics remains incompletely understood. We report an ultrafast spectroscopic investigation of ESPT in the photobase 2-(2'-pyridyl)benzimidazole (PBI) in methanol. Femtosecond transient absorption measurements, supported by quantum chemical calculations, indicate a multistep reaction pathway consisting of (i) a solvent-to-solute proton transfer occurring within ~2 ps, followed by (ii) nonradiative relaxation to the ground state in ~30 ps, generating a vibrationally hot ensemble, and (iii) subsequent energy dissipation to the solvent bath on a ~190 ps timescale. Superimposed oscillatory features in the early-time spectra reveal coherent nuclear wavepacket motion on the S₁ potential energy surface, indicated by a characteristic phase flip in the excited-state absorption signal. Fourier analysis identifies two dominant vibrational periods (~120 fs and ~340 fs), assigned to coupled in-plane and out-of-plane intermolecular modes involving both solute and solvent molecules. The rapid dephasing indicates evolution on an anharmonic potential surface along the ESPT reaction coordinate. These results highlight the active role of coherent solvent–solute vibrations in shaping ultrafast proton transfer dynamics.