Ultrafast Photochemical Reaction of Propanol in Liquid Phase: Fragment-Specific Cage Structures Probed by Ultrafast Electron Diffraction

Laser-induced ionization initiates ultrafast photochemical reactions that are governed by both intramolecular structural changes and intermolecular interactions. In the liquid phase, these dynamic processes are strongly affected by hydrogen bonding and molecular confinement, leading to reaction pathways that differ fundamentally from those in the gas phase. Here, we directly observe liquid-phase photochemical reactions of 1-propanol using ultrafast electron diffraction (UED) following multi-photon ionization with 800-nm laser pulses. The measured structural dynamics reveal reaction initiation by β-scission, producing hydroxymethyl (CH₂OH) and ethyl (C₂H₅) fragments and following intermolecular structural dynamics. Complementary ab initio molecular dynamics simulations show that these structural features originate from ultrafast, fragment-specific cage restructuring driven by hydrogen-bonded solvent networks. Notably, the ethyl fragment shows higher mobility, as the hydroxymethyl is constrained by the hydrogen bonding with neighbor molecules. These results provide structural evidence of fragment-specific cage effects in liquid-phase photochemistry and show the critical role of hydrogen-bonded environments in controlling fragment mobility and reactivity on ultrafast timescales.