Charge-Transfer Electronic Absorption Spectra of 1‑Ethylpyridinium Cation and Halogen Anion Pairs in Dichloromethane and as Neat Ionic Liquids

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.