o-Fluoroazobenzenes as Readily Synthesized Photoswitches Offering Nearly Quantitative Two-Way Isomerization with Visible Light

Azobenzene functionalized with ortho-fluorine atoms has a lower energy of the n-orbital of the Z-isomer, resulting in a separation of the E and Z isomers' n→π* absorption bands. Introducing para-substituents allows for further tuning of the absorption spectra of o-fluoroazobenzenes. In particular, electron-withdrawing ester groups give rise to a 50 nm separation of the n→π* transitions. Green and blue light can therefore be used to induce E→Z and Z→E isomerizations, respectively. The o-fluoroazobenzene scaffold is readily synthesized and can be inserted into larger structures via its aryl termini. These new azobenzene derivatives can be switched in both ways with high photoconversions, and their Z-isomers display a remarkably long thermal half-life.