Electronic Absorption Spectra of the Alkali Metal Complexes with Bipyridine

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Abstract
Electronic absorption spectra of the alkali metal complexes with 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine or 5,5′-dimethyl-2,2′-bipyridine have been observed in tetrahydrofuran solutions. Each spectrum has four major bands in the near infrared, visible and near ultraviolet regions, and is essentially due to the π–π*transitions in the negative ion (extra π electron trapping state) of bipyridine or its dimethyl derivatives coordinating to a metal ion. The band about 18000 cm−1 is the most sensitive to the metal ion species involved and assigned to a transition with the polarization parallel to the short axis of the bipyridine molecule. Each absorption band of the bipyrdine negative ion shows characteristic spectral shift upon CH3-substitutions in its peripheral positions. This CH3-substitution effect on the absorption spectra of bipyridine negative ion is quite different from that of complexes including neutral bipyridines and gives a basis of the criterion to determine whether the coordinating bipyridine in a certain bipyridine complex do exist in negative ion (π electron trapping state) or not.

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