Extinction coefficients of triplet-triplet transitions

A new method for determining these coefficients has been developed. The method employs pulse radiolysis for triplet excitation and, by means of triplet-triplet ( T–T ) energy transfer, leads to a comparison between the extinction coefficients of the benzophenone ketyl radical (ф 2 ĊOH) and the triplet states of several aromatic compounds whose triplet levels are lower than that of benzophenone. Since the ketyl extinction coefficient can be determined independently, the actual T–T extinction coefficients can be obtained. Values of the extinction coefficient at the absorption maximum were obtained for anthracene (57200 m -1 cm -1 ), naphthalene (22600 m -1 cm -1 ), phenanthrene (21000 m -1 cm -1 ) and 1,2-benzanthracene (25100 m -1 cm -1 ), all relative to an extinction coefficient for the benzophenone ketyl maxi­mum around 540 nm determined to be 3220 m -1 cm -1 . For several other compounds the above method for various reasons could not be applied. In such cases estimates were obtained by energy transfer experiments, leading to comparisons between their triplet absorptions and those of the compounds already determined. Estimates were thus obtained for the maximum T–T extinction coefficients of benzophenone (10300 m -1 cm -1 ), biacetyl (6400 m -1 cm -1 ), diphenyl (35400 m -1 cm -1 ), and acridine (28800 m -1 cm -1 ). During the course of these experiments estimates were also obtained for the rates of the following reactions : e - aq. + benzophenone ( k ii ═ 3·0 × 10 10 m -1 s -1 ), OH˙ + benzophenone ( k ii ═ 8·7 × 10 9 m -1 s -1 ) and hydrogen abstraction by triplet benzophenone from cyclo­hexane, to give ф 2 ĊOH ( k ii ~ 8 × 10 5 m -1 s -1 ).