イオン液体の電子構造

Abstract

This article shows our recent study on electronic structure of several typical ionic liquids. Ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy and soft x-ray emission spectroscopy (SXES) were used for elucidating the electronic structure of ionic liquids based on methylbutylimidazolium ([Cn mim]+) cation. Non-resonant SXES spectra measured above N, O, and F 1s edges selectively probed the partial density of states of the cation and anion. They give a clear evidence that the highest occupied molecular orbital of the [C4 mim]+ cation contributes to the topmost occupied states of the ionic liquids [C4 mim]+PF6−. Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C4 mim]+PF6− is solely determined by the [C4 mim]+ cation, in contrast to usual ionic crystals like NaCl and NaBr. The ionic liquids composed of fluoride anions PF6− and BF4− have anomalous electronic structure. Such basic knowledge about the electronic structure of ionic liquid must be helpful for new molecular design of ionic liquids.