In Situ Observation of Photoswitching by NMR Spectroscopy: A Photochemical Analogue to the Exchange Spectroscopy Experiment.

We present 1D and 2D NMR experiments that provide in situ insights into photoinduced isomerizations. Irradiation during the mixing period of an exchange spectroscopy (EXSY) experiment leads to characteristic cross peaks in 2D spectra. The phototriggered exchange of magnetization occurring in photoswitchable (Z)- and (E)-isomers of three selected azo compounds provides information on the dynamic E/Z equilibria. We report the dependence of the diagonal-to-cross-peak ratio on concentration, light intensity, and mixing time. In analogy to exchange spectroscopy, this ratio mirrors the efficiency of light induced molecular transformations. Furthermore, we present a time-saving 1D version and a combined light/phase cycle scheme for enhanced detectability of photoinduced changes in the spectrum. This insight into light-induced structural changes is highly suited to study macromolecules, in which photoswitchable units trigger conformational changes.