Browsing by Author "Ireland, J. F."

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    Excited Singlet and Triplet pK Values of Xanthone in Aqueous Solution
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (6), 1972) Ireland, J. F.; Wyatt, P. A. H.
    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.
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    Similar Excited State pK Behaviour of Xanthone and the Benzophenones
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1973, 69 (1), 1973) Ireland, J. F.; Wyatt, P. A. H.
    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.