# Ultrafast solvent-to-solute proton transfer mediated by intermolecular coherent vibrations

## TL;DR

This study reveals how proton transfer in a photobase occurs rapidly and is influenced by vibrations between the molecule and solvent.

## Contribution

The paper identifies coherent vibrations between a photobase and methanol that mediate ultrafast proton transfer.

## Key findings

- A 2.2 ps solvent-to-solute proton transfer was observed in the photobase PBI in methanol.
- Coherent vibrations with periods of ~117 fs and ~340 fs were linked to the ESPT process.
- Energy equilibration occurred over 186 ps as excess energy dissipated into the solvent.

## 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.

Ultrafast photoinduced excited-state proton transfer (ESPT) plays a crucial role in protecting biomolecules and functional materials from photodamage, but how solute-solvent interactions influence ESPT dynamics is not fully understood. Here, the authors use ultrafast absorption spectroscopy supported by quantum chemical calculations to study ESPT in a photobase, 2- (2’-pyridyl)benzimidazole, in methanol, tracking the complete reaction pathway and reporting early-time intermolecular coherent vibrations that may mediate the ESPT process.