Photochemical and photophysical behaviour of benzoylpyridines. Part I. Triplet–triplet energy transfer to biacetyl

Solvent and protonation effects on absorption and phosphorescence spectra of 2-, 3-, and 4-benzoylpyridines have been investigated. The lowest excited singlet and triplet states have been characterized as n,π* in nature; the triplet state acquires considerable π,π* character in acidic solution only. Triplet–triplet energy transfer from benzoylpyridines to biacetyl has been studied in benzene and aqueous neutral and acidic solutions. The results obtained are compared with those for benzophenone. Triplet yields and lifetimes of the three isomeric benzoylpyridines have been determined by their efficiency in sensitizing biacetyl phosphorescence in benzene solution and have been found to be somewhat lower than for benzophenone. The position of the heteroatom greatly affects the sensitizing power in aqueous solution, since the 2-isomer has been found not to sensitize at all. In acidic medium, there is a decrease in triplet lifetime of 3-benzoylpyridine, as found for benzophenone also, whilst the other isomers are inefficient sensitizers.