Similar excited state pK behaviour of xanthone and the benzophenones

Abstract

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.