Excited singlet and triplet pK values of xanthone in aqueous solution

Abstract

Xanthone, which has a ground state pK of –4.1, is shown to have a pK(S₁) value of +0.96 by fluorescence intensity measurements and a pK(T₁) of +3.0 by triplet-triplet absorption measurements. The unusual order pK(T₁) >pK(S₁) >pK(S₀) is also reproduced in the Förster cycle values calculated from the absorption, fluorescence, and phosphorescence maxima, though the numbers thus obtained for pK(T₁) and pK(S₁) are 2 to 3 units more positive. Förster's Cycle applied to the triplet-triplet absorption spectra of the protonated and unprotonated species indicates that pK(T₂) is –0.7, so that in basic strength the second triplet state lies between S₀ and S₁.