Anomalous electronic structure of ionic liquids determined by soft x-ray emission spectroscopy: Contributions from the cations and anions to the occupied electronic structure

摘要

Soft x-ray emission spectroscopy was used for elucidating the electronic structure of ionic liquids [C(4)mim]+PF6- and [C(4)mim]+OTf-, where [C(4)mim](+) stands for methylbutylimidazolium cation and OTf- for the trifluoromethanesulfonate anion. Nonresonant spectra measured above N, O, and F 1s edges selectively probed the molecular orbitals (MOs) of the cation and anions. They give a clear evidence that the highest occupied molecular orbital of the [C(4)mim](+) cation contributes to the topmost occupied states of the ionic liquids [C(4)mim]+PF6-, while both cationic and anionic MOs contribute for the case of [C(4)mim]+OTf-. Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C(4)mim]+PF6- is solely determined by the [C(4)mim](+) cation, in contrast to usual ionic crystals. The ionic liquids form a new class of the ionic materials from the viewpoint of the electronic structure.