Bathochromism polarity increases upon

The ion pair status is not predictable only on the basis of the cation used. For highly delocalizable carbanions like 9-fluorenyl in THF, the CIP fraction was found to increase rapidly in the order Li < Na < K < Cs [22] and the caesium salt was found to be the most bathochromic in the series. The degree of association of carbanion ion pairs in non polar solvents is highly dependent upon the extent of charge delocalization [23]. With highly delocalizable anions SSIP will be formed with Li cation. A completely different sequence of ion pairs was shown in the case of naphtholate anions the CIP fraction increases in the order K < Na < Li [24], These anions are thus to be considered as more localized anions in order to agree with the preceding observations. 

The solvent effects on the absorption spectra of ion pairs were studied by many authors and the direction of the observed shift depends on the change (increase or decrease) of dipole moment upon the electronic transition [25]. Generally a bathochromic shift is observed with an increase of solvent polarity. When going from a polar solvent to a less polar one, the association in the ground state increases more strongly than in the excited state this may be understood if the ion pair switches progressively from SSIP to CIP status. Observations of this type were often made, together with cation effects, as for instance in the case of alkali phenolates and enolates [7], fluorenyl and other carbanion salts [22] or even for aromatic radical anions [26, 27],... 

---