UV photochemistry of solvated bromoform probed by liquid phase ultrafast electron diffraction*

Studying photoinduced molecular dynamics in the gas phase provides insights into transient molecular structures and their evolution without environmental perturbations. However, many chemical reactions occur exclusively in the liquid phase, where intermolecular interactions may play a critical role. Investigating liquid-phase photochemistry is thus crucial for understanding these processes in realistic environments. Bromoform (CHBr₃), a major precursor to atmospheric bromine that contributes to catalytic ozone destruction, is primarily produced in the solvated phase by marine macroalgae before being released into the atmosphere. In the gas phase, recent ultrafast electron diffraction (UED) studies reveal two primary relaxation pathways: direct dissociation to release a bromine atom and isomerization to form iso-bromoform [1]. Here, we extend these studies to solvated systems by employing liquid-phase UED (LUED) with vacuum-compatible sub-micron thick liquid sheet jets to investigate the effects of a solvent environment on the UV-excited dynamics of bromoform. Previous studies suggest that the solvent cage surrounding excited bromoform introduces a slower isomerization mechanism on the order of a few ps, in addition to the sub-200 fs direct isomerization pathway observed in the gas phase. Using a 2M solution of CHBr₃ in 1-propanol, we have collected LUED data to examine these solvent-mediated effects in detail. Interactions with the solvent are expected to open additional reaction pathways, potentially offering insights into the complex interplay between inter- and intramolecular interactions in liquid-phase bromoform photochemistry.

[1] L. Hoffmann et al., J. Am. Chem. Soc. 2024, 146, 28070−28079.