Charge-Transfer Electronic Absorption Spectra of 1-EthylpyridiniumCation and Halogen Anion Pairs in Dichloromethane and as Neat IonicLiquids.

The charge-transfer (CT) absorptionbands of ion pairs composedof 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 showsclear splitting because of spin–orbit interaction in the excitedstate. The CT transition energy of an Epy+X–ion pair in a dichloromethane solution is related to electron affinityof 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 usingthe measured absorbance, and the ion-pair concentration was estimatedon the basis of electroconductivity. Structures of Epy+X–ion pairs were also evaluated on the basis ofboth quantum-chemical calculations and NMR spectroscopy. In addition,in the absorption spectrum measured for neat EpyI liquid, a broadband 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. [ABSTRACT FROM AUTHOR]