Get a better glimpse on sequential photoreactions of trisnorbornadienes with ¹⁹F NMR spectroscopy

It is shown exemplarily with a trifluorinated trisnorbornadienylbenzene that ¹⁹F NMR spectroscopy may be applied as a useful complementary method for the investigation of sequential photoreactions. The trisnorbornadiene core structure was used as it figures as promising scaffold for molecular solar thermal (MOST) energy storage. The target compound was readily synthesized by a Suzuki-Miyaura coupling reaction and examined with respect to the key properties for MOST applications. Upon direct or photosensitized irradiation, the trisnorbornadiene was transformed stepwise and almost quantitatively into the corresponding trisquadricyclane. Even though the reaction can be monitored by photometry or by in situ ¹H NMR spectroscopy, unambiguous assignment of the distinct intermediate mono- and bisquadricyclanes was not possible because of signal overlap. In contrast, this shortcoming is circumvented with in situ ¹⁹F NMR-spectroscopic analysis. Contrary to the ¹H NMR spectra, the ¹⁹F NMR spectra show significantly fewer characteristic and sufficiently separated signals that allow the unambiguous identification of all photoproducts and, thus, their detection in the course of the photoreaction.