Electronic Structure of Ionic Liquids Studied by UV Photoemission and Inverse Photoemission Spectroscopy
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Electronic Structure of Ionic Liquids Studied by UV Photoemission and Inverse Photoemission Spectroscopy Toshio Nishi1, Yasunori Kamizuru1, Kaname Kanai1, Yukio Ouchi1, and Kazuhiko Seki1,2 1 Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan 2 Institute for Advanced Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
ABSTRACT Electronic structures of the ionic liquids were studied by ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The sample materials contain 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n=4, 8, 10) as the cation in combination with fluorine-containing anions (tetrafluoroboronate BF4-, hexafluorophosphate PF6-). Comparing the calculated density of states with the observed spectra, we found that the ionization thresholds of these ionic liquids are determined by the highest occupied molecular orbitals (HOMO) of the cation, although the calculated HOMOs of the isolated anions are higher than that of isolated cation. The combination of the UPS and IPES results reveals that the band gaps of these ionic liquids are determined by only cation. INTRODUCTION Recently, various organic semiconductors have attracted considerable attention due to their potential use for applications to electronic devices such as organic light emitting diodes (OLEDs) or organic field effect transistors (OFETs). The electronic structure of the unoccupied states is an important factor for understanding various electronic properties and improving such devices. Many experimental studies about occupied states of organic materials have been performed by means of UV photoelectron spectroscopy (UPS). On the other hand, the observation of electronic structures of the unoccupied states has mostly been carried out using indirect methods such as UV-Visible optical absorption. Although inverse photoemission spectroscopy (IPES) gives direct information of unoccupied states its application has been hindered by the very low efficiency of the inverse photoemission process and the severe radiation damage to organic materials by incident electrons. Thus only a small number of IPES studies on organics have been reported by the groups of Koch, Kahn, Sato, and others on organic materials [1-6]. In this work, we have developed new IPES system and investigated the electronic structures of ionic liquids by UPS and IPES. Ionic liquids are salts which are in a liquid phase at room temperature, and attracting much interest because of their unique properties such as wide temperature range of the liquid phase, excellent chemical stability, low vapor pressure, wide electrochemical window, and high ionic conductivity [7-12]. Researchers in the fields of synthetic and catalytic chemistry are also exploiting their potential as green "designer solvents". Such room-temperature ionic liquids usually consist of an organic cation and an inorganic anion. Typical inorganic salts (e.g. Na+Cl-) have high melting points, but the use of flexible organic compou
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