The fluorescence of ion pairs of the fluorenyl carbanion

Abstract

Absorption, fluorescence and time-resolved excitation spectra, and fluorescence 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 systematically from tight (‘contact’) to loose (‘solvent-separated’) with increasing solvent polarity, decreasing temperature and cation size. Thermodynamic parameters are reported for some of the ground state equilibria. In all cases fluorescence lifetimes increase on cooling, the parameters showing considerable 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 the cation in excited tight pairs. With Na⁺, K⁺ and Cs⁺ as counterions at temperatures below − 135°C in DMTHF solutions both fluorescence spectra and lifetimes change markedly in a manner consistent with the cation remaining 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 process (sub-nanosecond) as no evidence for equilibration in the excited state could be found on the accessible 0.5–150 ns timescale.