Photomodulation of fluoride ion binding through anion-π interactions using a photoswitchable azobenzene system

The discovery of photoswitchable azobenzene-systems that undergo trans-to-cis photoisomerisation was a milestone in supramolecular chemistry. Such photoswitches have possible applications in data storage, stimuli responsive delivery systems, and molecular machines due to fast and selective switching. However, the light induced cis isomer of azobenzene is rather unstable and reverts thermally and photochemically to the thermodynamically stable trans configuration. We report, for the first time, controlled photoswitching of an azo-naphthalenediimide (azo-NDI) which can be achieved upon binding of fluoride ions through anion-π interaction. This NDI-F-NDI "sandwich" stabilises the cis configuration through the generation of an NDI(•-) radical anion, and a dianionic, NDI(2-) species that becomes unusually stable in the cis form. The sandwiched cis form reverts to the trans form only upon decomplexation of F(-). A model pollutant was successfully degraded using the photogenerated NDI-F-NDI sandwich. This opens a wide range of applications in molecular and supramolecular nanotechnology.