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 ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}\mathrm{P}{\mathrm{F}}_6^{-}$ and ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}{\mathrm{OTf}}^{-}$ , where ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}$ stands for methylbutylimidazolium cation and ${\mathrm{OTf}}^{-}$ 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 ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}$ cation contributes to the topmost occupied states of the ionic liquids ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}\mathrm{P}{\mathrm{F}}_6^{-}$ , while both cationic and anionic MOs contribute for the case of ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}\mathrm{O}{\mathrm{Tf}}^{-}$ . Resonant soft x-ray emission spectra at the N $1s$ edge of these ionic liquids revealed that the energy gap of ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}\mathrm{P}{\mathrm{F}}_6^{-}$ is solely determined by the ${\left[{\mathrm{C}}_4\mathrm{mim}\right]}^{+}$ 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.