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.
