Role of Solvent and Effect of Substituent on Azobenzene Isomerization by Using Room-Temperature Ionic Liquids as Reaction Media

The effects of a <i>para</i> substituent, as the electron-donating −OCH<sub>3</sub> and −OtBu groups and the electron-withdrawing −Br and −F atoms, on azobenzene isomerization have been investigated in a series of imidazolium ionic liquids (BMIM PF<sub>6</sub>, BMIM BF<sub>4</sub>, BMIM Tf<sub>2</sub>N, EMIM Tf<sub>2</sub>N, BM<sub>2</sub>IM Tf<sub>2</sub>N, and HMIM Tf<sub>2</sub>N). The thermal <i>cis</i>–<i>trans</i> conversion tends to be improved in the presence of the substituent, as pointed out by the first-order rate constants measured at 25 °C. Both the rotation and the inversion mechanisms occur in BMIM Tf<sub>2</sub>N, EMIM Tf<sub>2</sub>N, and HMIM Tf<sub>2</sub>N, as highlighted by typical V-shape Hammett plots, but only rotation takes place in BMIM PF<sub>6</sub>, BMIM BF<sub>4</sub>, and BM<sub>2</sub>IM Tf<sub>2</sub>N. The possible interactions between the cation and the anion of the solvent and both the isomers of the azobenzene derivatives have been studied by small-wide-angle X-ray scattering (SWAXS). The calculated <i>cis</i> population in the photostationary state and the hardness parameter η of the <i>trans</i> isomer show that azobenzene and F-azobenzene are the less reactive molecules for the <i>trans–cis</i> conversion in all the investigated ionic liquids.