Large Azobenzene Macrocycles : Formation and Detection by NMR and MS Methods

Azobenzene macrocycles are widely investigated as potential drug delivery systems and as part of molecular machines due to their photo-responsive properties. Herein, we detect the formation of a series of new azobenzene macrocycles that feature up to eight switchable repeating units. High-resolution mass spectrometry and ion mobility (IM) mass spectrometry experiments and 1H and diffusion-ordered spectroscopy (DOSY) NMR are used to detect the presence of the macrocycles that contain 10 to 40 aromatic rings in the gas phase and solution, respectively. The responsiveness of the Z-to-E photo-switching of the smallest of the macrocycles, exhibiting 2 azobenzene units and in total 10 aromatic rings, separated with column chromatography, is studied with irradiation experiments, using both UV-Vis light and thermal excitation and relaxation, and monitoring the sample with UV-Vis absorption and 1H NMR spectroscopy. DFT calculations are used to understand the conformation of this isolated species in solution.