Similar excited state pK behaviour of xanthone and the benzophenones

Benzophenone, like xanthone, shows the excited state pK order pK(T1) > pK(S1) > pK(S0), which is readily understood in terms of the S1–T1 splitting to be expected from the n–π* and π–π* states of the unprotonated and protonated forms respectively. Both fluorescence and phosphorescence occur from the protonated form at 77 K, but in fluid solutions the pK* values were not accessible to direct determination but were estimated by the Föster cycle. Transients were observed from both B and BH+ forms by flash spectroscopy. Several substituted benzophenones behaved similarly, but the p-phenyl compound has a different pK order (due to the larger π–π*S1–T1 splitting in the B form) and with the N-protonation of the p-amino compound the order is completely reversed. The pK* values for the phenolic ionization of the p-hydroxy compound are also reassessed.