Bromosubstituted Norbornadienes and Their Reversible Photolytic Transformation to Quadricyclanes

Two new model systems for use within the rapidly develop- ing ultrafast time resolved x-ray scattering techniques have been prepared. Their photoisomerisation from norbornadiene to quadricyclane was found to be a suitable reaction to fol- low. Simulations of scattering patterns (not included in this report) showed that if heavy atoms are included in these molecular structures, then the transformation can be followed by transient X-ray scattering techniques. Two new bromosub- stituted norbornadienes were synthesised and characterised. Absorption spectroscopy showed that the norbornadienes are converted quantitatively to quadricyclanes under ultra- violet irradiation. Nuclear magnetic resonance (NMR) studies showed that the process was fully reversible and that the norbornadienes could be completely recovered even without addition of catalysts. Furthermore, it was shown that the formation of quadricyclane from norbornadiene was unaf- fected by triplet sensitisers. The two new model systems synthesised thus are strong candidates for use in time- resolved X-ray scattering studies in both gas and condensed phases.