The fluorescence of ion pairs of the fluorenyl carbanion

Absorption, fluorescence and time-resolved excitation spectra, and fluoresc ence decays have been measured for ion pairs of fluorenyl anion with alkali metal cations in tetrahydrofuran and its 2-methyl (MTHF) and 2,5-dimethyl (DMTHF) derivatives. The nature of the ground state ion pairing changes sys tematically from tight ('contact') to loose ('solvent-separated') with incr easing solvent polarity, decreasing temperature and cation size. Thermodyna mic parameters are reported for some of the ground state equilibria. In all cases fluorescence lifetimes increase on cooling, the parameters showing c onsiderable variation with cation and solvent as well as with ion pair type . The results can be understood in terms of an off-centre displacement of t he cation in excited tight pairs. With Na+, K+ and Cs+ as counterions at te mperatures below -135 degreesC in DMTHF solutions both fluorescence spectra and lifetimes change markedly in a manner consistent with the cation remai ning in a central position on excitation; presumably the viscosity is high enough to prevent its movement. In a few other cases, conversion from tight to loose pairs occurs on excitation, but this appears to be a very fast pr ocess (sub-nanosecond) as no evidence for equilibration in the excited stat e could be found on the accessible 0.5-150 ns timescale.