Absorption and emission spectra of difluorenyl-calcium and -strontium in tetrahydrofuran and in tetrahydropyran

Abstract

Difluorenylcalcium (Fl₂Ca) has been studied by absorption and emission spectrometry in tetrahydrofuran (THF) and in tetrahydropyran (THP). The absorption spectrum shows the presence of both contact and solvent-separated ion-pairs. The solvation of Fl₂Ca in THF is probably a stepwise process. In THP only contact ion-pairs are found. The emission spectrum in THF is independent of the excitation wavelength and of the concentration, and the relative intensities within the emission band do not change in the range of +20 to –40 °C. The excitation spectrum shows only one maximum, at 373 nm.

The emission may be caused by an excited-state cation solvent relaxation process and appears to occur from a separated ion pair in the ion triplet. The emission spectrum of Fl₂Sr was examined in order to compare the characteristics of the Ba, Sr, and Ca salts; it depends on the excitation wavelength. The excitation spectrum also shows one maximum. The emission is probably due to a mechanism similar to that for Fl₂Ca.