Potential Dependence of Electronic Transition Spectra of Interfacial Ionic Liquids Studied by Newly Developed Electrochemical Attenuated Total Reflectance Spectroscopy.

Recently, ionic liquids at the electrode/ionic liquid interface have been intensively studied because they are promising as novel alternatives to traditional electrolyte solutions that are both safe and functional. In this study, we constructed an attenuated total-reflectance spectroscopic system that operates under electrochemical conditions in order to investigate the electronic states of ionic liquids near the electrode surface. Upon application of voltage to an ionic liquid consisting of imidazolium cations and iodide anions, electronic transition spectra in the 150-450 nm range varied. In particular, absorbance due to charge transfer from the anion to the cation drastically increased at positive potentials. The extent of spectral change and contact area between the electrode and the ionic liquid were positively correlated, and thus spectral variations reflected the behavior of the interfacial ionic liquid on the electrode. In addition to potential dependence, time dependence and hysteresis were also investigated. The newly developed system can be applied not only for ionic liquids but foreseeably also for various electrochemical materials such as organic semiconductors.