Shape-switchable azo-macrocycles

The synthesis of four shape-switchable macrocycles comprising different peripheral substituents is described. The macrocycles 1-4 consist of m-terphenyl semicircles interlinked by two azo joints, These macrocycles were assembled from nitro-functionahzed 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 osmometry, because mass spectrometry failed in the cases of 2 and 3. The E -< Z photoisomerization reactions were analysed by UV/Vis spectroscopy complemented by H-1 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)