Excited Singlet and Triplet pK Values of Xanthone in Aqueous Solution

Abstract

Xanthone, which Ikis a ground state pK of -4.1, is shown to have a pK^j) value of +0.96 by fluorescence intensity measurements and a pK(7\) of +3.0 by triplet-triplet absorption measurements. The unusual order pK(7\) >pK(5i) >pK(5’0) is also reproduced in the Forster cycle values calculated from the absorption, fluorescence, and phosphorescence maxima, though the numbers thus obtained for pK(Ti) and pKGS^) arc 2 to 3 units more positive. Forster’s Cycle applied to the triplettriplet 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 So and S1.