Metal Ion Mediated Instant Z → E Isomerization of Azobenzene Macrocycles in the Absence of Light.

The classical photoswitch azobenzenes reversibly interconvert between the E- and the Z-isomers with light. Here, we report a pair of new macrocyclic azobenzenes characterized thoroughly by spectroscopic methods and single crystal X-ray diffraction structures, and one of the compounds displays a quantitative conversion of the E- to the Z-form. These compounds, besides their normal photoswitching behavior, display an unusual instant switching of the Z-form to the E-isomer in the presence of Cu2+ ions in the dark under 273 K. The Cu2+ complex can stay in the Z-form under constant UV radiation. However, it reverts to the E-form as soon as the exposure to the UV is ceased. The same phenomenon is also observed with Ag+ ions albeit it is a bit slower. This unusual instant switching of the azobenzene systems with metal ions prompted the detailed studies to unravel the reason behind this behavior.