Ion-pair formation of the carbanions of xanthene and thioxanthene, studied by their absorption and fluorescence spectra

Abstract

The ion-pair formation of the carbanions of xanthene (XH^–) and thioxanthene (TxH^–) with alkali metal cations has been investigated by optical absorption and emission spectroscopy. The absorption spectra depend on solvent, counter ion and temperature in the same way as has been found for other carbanions. The XH^– fluorescence spectrum exhibits an unexpected dependence on temperature; from TxH^– no fluorescence has been observed.

The absorption spectra have been analysed by deconvolution into Gaussian curves and are interpreted with the aid of the results of π-electron SCF-MO calculations. The enthalpies and entropies of the conversion of the contact ion pairs into the solvent-separated ion pairs of both carbanions have been calculated from the spectral data at different temperatures. The excited state acidity of XH₂ has been determined from the absorption and fluorescence spectra. The experimental results are discussed in connection with those of some related systems.