Confirmation of the Structure of Trans-Cyclic Azobenzene by X-Ray Crystallography and Spectroscopic Characterization of Cyclic Azobenzene Analogs

The photoisomerization properties of a series of cyclic azobenzene (cAb) analogs, including non-substituted, Br-, CN-, carboxyl-, and amino cAb, were compared. While the trans-form of these analogs show different degrees of thermal stability, no spectral changes in the absorption wavelengths were observed for amino cAb upon exposure to purple light. The (NNMR)-N-15 chemical shifts of cAb in both cis- and trans-forms were found to show a large difference from those of aromatic azobenzene; however, the trend of difference in chemical shift is in agreement with that obtained from quantum chemical calculations. More importantly, the structure of the photoisomerization product from cis-cAb was confirmed to be, indeed, the trans-cAb by X-ray diffraction experiments. The (NMR)-N-15 data of the cAb isomers and confirmation of trans-cAb through crystal structure will be of importance for future studies in the design and applications of cAb as a photoswitch.