The carbon-iodine bond cleavage and isomerization of iodoform visualized with femtosecond X-ray liquidography.

Iodoform (CHI) has garnered significant attention for its unique ability to induce photo-cyclopropanation of olefins by releasing an iodine radical through C-I bond cleavage. However, the detailed mechanism underlying CHI photodissociation is still not fully understood. Here, we elucidate the ultrafast structural dynamics of CHI upon photoexcitation using femtosecond time-resolved X-ray liquidography (fs-TRXL) at an X-ray free-electron laser facility. The fs-TRXL data was decomposed into the isotropic and anisotropic data. The isotropic data reveal that the formation of CHI and I radicals upon photolysis precedes the emergence of -CHI-I. After a short induction period, two competing geminate recombination pathways of CHI and I radicals take place: one pathway leads to the recovery of CHI, while the other results in the formation of -CHI-I. Additionally, the anisotropic data show how the transient anisotropic distribution of both the species formed upon photoexcitation and the ground-state species depleted upon photoexcitation decays through rotational dephasing. Furthermore, the observed structural dynamics of CHI has distinctive differences with that of BiI, which can be attributed to differences in their central moieties, CH and Bi. Our findings provide insights into the photoinduced reaction dynamics of CHI, enhancing the understanding of its role in photochemical reactions.