Similar Excited State pK Behaviour of Xanthone and the Benzophenones

Abstract

Benzophenone, like xanthone, shows the excited state pK order pK(T’1)>pK(S1)>pK(S'o), which is readily understood in terms of the Si-Tl splitting to be expected from the h-tt* and tt-tt* 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 Forster cycle. Transients were observed from both B and BI1+ forms by flash spectroscopy. Several substituted benzophenones behaved similarly, but the p-phenyl compound has a different pK order (due to the larger tt-tt* Sx-Ti 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.