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The solvatochromic shift of the most intensive band of fluorenone radical anion (FN*‐) in the visible region has been examined in 10 polar formally aprotic media. It has been found that this shift depends on the solvent acceptor number (AN) and static dielectric constant (D) according to planar regression: It is suggested that the ground state of FN*‐ is relatively stabilized in a solvent with a high acceptor number and the dipole moment of the solute increases during an excitation.

Two-parameter regression including the acceptor number (AN) and permittivity (ε) of the solvent explains to some extent the solvatochromic shift of the absorption band of benzophenone radical anion (Ph2C•-O-) in the visible region. In 11 dipolar aprotic solvents characterized by ε > 10, in which the Ph2C•-O- anions are not associated with (C2H5)4N+ cations (the radicals were prepared by cathodic reduction of benzophenone in the presence of (C2H5)4N+ClO4- as electrolyte) a correlation equation has the form νmax (in cm-1) = 86.2(AN) - 19.7 · 103[(ε - 1)/(2ε + 1)] + 21.32 · 103. It reflects the relative stabilization of the ground state in a medium with a high acceptor number and an increase in the dipole moment of Ph2C•-O- during excitation.

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Cyclic voltammetric studies have been carried out on monensin anion complexes with the alkali metal cations in binary mixtures of dimethylsulfoxide with acetonitrile, acetone and hexamethylphosphoric triamide. respectively. The log Ks values obtained for the complexity constants can be well described by empirical relation log Ks = αDN + β, where DN stands for the effective donor number of a given solvent mixture and α and β mark the regression coefficients. This result is in agreement with the predominant role of cation solvation. The complexes show a sharp stability maximum for the Na+ complex in all media under investigation.