title: Charge-transfer electronic absorption spectra of 1-ethylpyridinium cation and halogen anion pairs in dichloromethane and as neat ionic liquids.
abstract: The charge-transfer (CT) absorption bands of ion pairs composed of 1-ethylpyridinium (Epy(+)) and halogen anions (X(-): Cl, Br, or I) were measured in dichloromethane solutions of EpyX. The CT band of the Epy(+)I(-) ion pair shows clear splitting because of spin-orbit interaction in the excited state. The CT transition energy of an Epy(+)X(-) ion pair in a dichloromethane solution is related to electron affinity of X, which is in accordance with the Mulliken theory for CT bands. Extinction coefficients for the CT bands of the Epy(+)X(-) ion pairs in dichloromethane were determined using the measured absorbance, and the ion-pair concentration was estimated on the basis of electroconductivity. Structures of Epy(+)X(-) ion pairs were also evaluated on the basis of both quantum-chemical calculations and NMR spectroscopy. In addition, in the absorption spectrum measured for neat EpyI liquid, a broad band appeared at a longer wavelength side of the S1(ππ*) band. This new band has been assigned to the CT band of the Epy(+)I(-) ion pair formed in neat EpyI liquid.
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 H-89, Ohokayama, Meguro-ku, Tokyo 152-8551, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Ogura
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Akai
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Shibuya
authors.forename: Akio
authors.initials: A
authors.lastname: Kawai