Xanthone, triplet state energies

The results described above represent the first example of the FR mechanism (Scheme 1). Semiempirical calculations on this molecule showed that the intersystem crossing to the excited triplet state is favored The reaction cannot be sensitized by xanthone because the triplet state of 3,4-diphenyl-1,2,5-oxadiazole is lower than that of xanthone. The cleavage of the triplet state to the biradical is favored, considering the relative energy of this intermediate (Fig. 23) (OOOUPl). 

Sensitisation by a high-energy donor, such as triplet xanthone, can populate both the Qi and the D, states by energy transfer (Figure 10.4). This results in both the photosolvation reaction and phosphorescence emission. 

The excited states involved in the mechanisms of the photochemical transformations of the 2-alkylidenecyclo-butanones were elucidated without special difficulty (vide supra). These a, B-unsaturated ketones undergo only one of the reactions characteristic of cyclobutanones (i.e., ring expansion). In addition, the triplet energy of the enone chromophore is low enough that this excited state may be efficiently and selectively populated by standard carbonyl triplet sensitizers (e.g., acetophenone, xanthone, and benzophenone), thereby demonstrating that ring expansion occurs via the 2-alkylidenecyclobutanone state, while the isomerization process (i.e., [81]J[82]) occurs via T. ... 

BF, and SOgMe in (142), the cleavage is sensitized by xanthone but evidence is presented which indicates that for the former four compounds exciplex formation occurs rather than triplet energy transfer. A useful semi quantitative scale of photofugacities of the substituent groups in the excited singlet state is given. 

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