Structural Tracking of a Bimolecular Reaction in Solution by Time‐Resolved X‐Ray Scattering

Every photochemical reaction starts with an electronically excited state and ends with a ground-state molecule. This is also true in bimolecular photoreactions, in which the excited molecule collides with a ground-state reactant. The collision complex may follow a potential energy surface directly to the primary ground-state product. The reaction may also proceed on an excited-state surface and reach a stable minimum configuration. This excited complex—an exciplex—is anticipated to precede the primary product of many types of bimolecular photoreactions, most prominently in photoinduced electron-transfer reactions. [1] The presence of an exciplex is easily demonstrated if it emits a characteristic emission. However, this situation is the exception rather than the rule, and exciplexes are generally not easily detected. An alternative to emission spectroscopy is time-resolved absorption spectroscopy based on laser methods. Unfortunately, neither type of spectroscopy provides direct structural information. The present study presents time-resolved X-ray scattering in aqueous solution as a new means of directly obtaining a model-independent structure of this kind of elusive intermediate.