Abstract
Ultrafast photoinduced excited-state proton transfer (ESPT) plays a crucial role in protecting biomolecules and functional materials from photodamage. However, the influence of solute-solvent interactions on ESPT dynamics remains under active investigation. Here, we present an ultrafast spectroscopic study of ESPT in the photobase 2-(2´-pyridyl)benzimidazole (PBI) in methanol. Ultrafast absorption spectroscopy, supported by quantum chemical calculations, reveals three distinct kinetic steps: (1) a 2.2 ps solvent-to-solute proton transfer, (2) subsequent nonradiative relaxation to the ground state within 31 ps, producing a vibrationally hot ensemble with substantial excess kinetic energy, and (3) equilibration as this energy dissipates into the surrounding solvent bath over 186 ps. Femtosecond-resolved dynamics exhibit oscillatory signals indicative of coherent wavepacket motion on the S1 potential energy surface. A phase flip in the excited-state absorption maximum confirms this assignment. Fourier analysis resolves two dominant periods (∼117 fs and ∼340 fs), corresponding to in-plane and out-of-plane vibrational modes coupled between PBI and the hydrogen-bonded methanol molecule. The rapid dephasing ( < 300 fs) suggests that the nuclear wavefunction evolves on an anharmonic potential energy surface while traversing the ESPT reaction coordinate.
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The data supporting this study are available within the main text and the Supplementary Information. All relevant files are available from the corresponding authors upon reasonable request.
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Acknowledgements
This work was supported by the U.S. Department of Energy, Office of Science Early Career Research Program (DOE ECRP), Office of Basic Energy Sciences, Atomic, Molecular, and Optical Sciences Program, under award number DE-SC0026316. Y.M. was further supported by San Diego State University startup funds and the CSUBIOTECH New Investigator Grant. We acknowledge the computational resources at San Diego Supercomputer Center (SDSC Expanse) through allocation CHE240089 from the ACCESS program.
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H.Y. and Y.M. supervised the project. R.J. and H.Y. conceived the project. R.J. conducted the experimental measurements and analyzed the data. Y.M. performed the theoretical calculations. All authors contributed to the analysis and interpretation of the results and wrote the manuscript.
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H.Y. is an Editorial Board Member for Communications Chemistry, but was not involved in the editorial review of, or the decision to publish this article. All other authors declare no competing interests.
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Jarupula, R., Mao, Y. & Yong, H. Ultrafast solvent-to-solute proton transfer mediated by intermolecular coherent vibrations. Commun Chem (2026). https://doi.org/10.1038/s42004-026-01917-8
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DOI: https://doi.org/10.1038/s42004-026-01917-8
