ESR-spectroscopy in ionic liquids: Dynamic linebroadening effects caused by electron-self exchange reactions within the methylviologene redox couple

Well-resolved ESR-spectra of the methylviologene radical cation (MV+MV+) are recorded in room-temperature liquid ions as solvents. The temperature dependences of the ESR-coupling constants are similar to those measured in classical organic solvents. Electron-self exchange rates are reported for the methylviologene redox couple (MV++/MV+MV++/MV+) in 1-butyl-3-methylimidazolium hexafluorophosphate (bmim+PF6−), 1-butlyl-3-methylimidazolium fluoroborate (bmim+BF4−) and 1-ethyl-3-imidazolium ethylsulfate (emim+O3SOEt−) within a temperature range of 350 K ≤ T ≤ 460 K. The diffusion controlled rate constants observed vary between 8.2 × 107 M−1 s−1 and 1.2 × 109 M−1 s−1. From temperature-dependent measurements the activation energies obtained range from 27.4 kJ/mol in emim+O3SOEt− to 42.1 kJ/mol in bmim+PF6−, respectively.