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

Soft x-ray emission spectroscopy was used for elucidating the electronic structure of ionic liquids C(4)mimPF(6)(-) and C(4)mimOTf(-), 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)mimPF(6)(-), while both cationic and anionic MOs contribute for the case of C(4)mimOTf(-). Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of C(4)mimPF(6)(-) 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.