Shape-Switchable Azo-Macrocycles

The synthesis of four shape-switchable macrocycles comprising different peripheral substituents is described. The macro- cycles 1-4 consist of m-terphenyl semicircles interlinked by two azo joints. These macrocycles were assembled from ni- tro-functionalized m-terphenyl moieties through reductive dimerization. The semicircles were assembled through Suzuki cross-coupling reactions. The molecular weights of the macrocycles were determined by vapour pressure osmome- try, because mass spectrometry failed in the cases of 2 and 3. The E - Z photoisomerization reactions were analysed by UV/Vis spectroscopy complemented by JH NMR studies. A very slow thermal back-reaction indicated considerable stabilization of the Z isomer. The reduced efficiency of the thermal back-reaction probably arises from the reduced degree of freedom due to the mechanical interlinking of the two azo groups. The photostationary state consisted of all-Z (85%) and all-E isomers (15 %). The E - Z transformation induced by irradiation displayed simple exponential kinetics, which indicates pairwise switching of the two azo groups in a macrocycle, at least on the timescale under investigation.