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

The effects of a para substituent, as the electron-donating −OCH₃ 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 PF₆, BMIM BF₄, BMIM Tf₂N, EMIM Tf₂N, BM₂IM Tf₂N, and HMIM Tf₂N). The thermal cis–trans 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 Tf₂N, EMIM Tf₂N, and HMIM Tf₂N, as highlighted by typical V-shape Hammett plots, but only rotation takes place in BMIM PF₆, BMIM BF₄, and BM₂IM Tf₂N. 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 cis population in the photostationary state and the hardness parameter η of the trans isomer show that azobenzene and F-azobenzene are the less reactive molecules for the trans–cis conversion in all the investigated ionic liquids.