Atomistic visualization of photoinduced electron transfer process in a nonpolar solvent

Electron transfer, the process of one electron relocating from one chemical entity to another, is one of the most ubiquitous elementary reactions and plays a critical role in almost all fields of chemistry. While seminal theories such as Marcus theory have provided foundational frameworks for electron transfer, they rely on postulated atomistic pictures that have never been directly observed. For instance, Marcus theory describes electron transfer is driven by solvent reorganization, which is only verified indirectly through the observation of the Marcus inverted region. Here, we present a direct atomic-resolution measurement of nuclear motion during electron transfer in the liquid phase, achieved using mega-electron-volt liquid-phase ultrafast electron diffraction. Instead of solvent reorganization, we observe that the back-electron transfer following photoionization in neat CCl4 proceeds with a ballistic cleavage of a C–Cl bond, followed by a 1.7-Å shuttling of the chlorine nucleus between donor and acceptor, accompanied by a 0.6-Å umbrella opening on both sides. These nuclear motions facilitate an ultrafast intermolecular back-electron transfer within 340 fs. Our findings demonstrate that the atomistic details of electron transfer involve complicated multidimensional nuclear motions across donor, acceptor, and solvent molecules, which goes far beyond the framework of Marcus theory.