Charge-Transfer Interaction of Methyl Viologen with Zeolite Framework and Dramatic Blue Shift of Methyl Viologen−Arene Charge-Transfer Band upon Increasing the Size of Alkali Metal Cation

A series of alkali metal-exchanged zeolites X and Y co-exchanged with small amounts (1 per unit cell) of methyl viologen (MV2+) were prepared. The diffuse reflectance UV−vis spectra of the dried zeolites showed broad absorption bands in the 220−320-nm region. Decomposition of the bands revealed that each absorption band consists of two Gaussian bands. Of the two, the one that appears at the higher energy region is always more intense and much broader than the other that appears at the lower energy region. The broader, higher energy band (BHEB) progressively red-shifted upon increasing the electropositivity of the alkali metal cation, while the narrower, lower energy band (NLEB) remained stationary. The BHEB was assigned as the charge-transfer (CT) band between MV2+ and the zeolite framework on the basis of the linear relationship established between the energy of the absorption maximum and Sanderson's partial charge of the framework oxygen. The remaining, stationary NLEB was consequently assigned as the absorption band of MV2+ in the zeolite media. Subsequent formation of CT complexes of MV2+ with guest arene donors such as anthracene, 1-methoxynaphthalene, and pentamethylbenzene within the zeolites showed a dramatic blue shift of MV2+−arene CT bands upon increasing the size of alkali metal cation. The possible reasons for the dramatic, cation-dependent spectral shift of MV2+−arene CT bands are discussed.