Electronic Properties of Cyano Ionic Liquids: A Valence Band Photoemission Study

The valence band spectra of three cyano-ionic liquids based on 1-ethyl-3-methylimidazolium (Im2,1+) paired with thiocyanate (SCN-), dicyanamide (N(CN)2-), and tricyanomethanide (C(CN)3-) have been measured using ultraviolet and X-ray photoemission spectroscopy. Experimental spectra are compared to their corresponding density of states, weighted by photoemission cross sections, calculated for clusters of ions pairs of increasing size. Thus, this study bridges single ion approaches to 3D periodical DFT studies and enables the exploration of the different aspects of electronic structure establishment in ILs. Even for a relatively small cluster size, the relative energy of cation and anions states shifts by an amount that corresponds closely to that expected from the Madelung energy of a bulk IL, and the photoemission cross section-weighted DOS spectra are in good agreement with the measured valence bands. Trends in the relative energy and ionic character of the frontier orbitals across this series of cyano-ILs are discussed.