Quenching of Triplet States of Aromatic Ketones by Indole and 3‐methyl Indole in Benzene Solution. Evidence for Hydrogen‐atom Abstraction, for Quenching Due to Favourable Charge Transfer Interaction and for the Lack of Electronic Energy Transfer

Quenching of the triplet states of the aromatic ketones (KCO), benzophenone, acetophenone and xanthone, by indole and 3‐methyl indole gives rise to the neutral radicals resulting from hydrogenatom transfer with variable efficiency (40–100%). Thus in addition to the reaction, 3KCO*+RH →KCOH +R. some other quenching path or paths occur. There is no evidence for any triplet energy transfer even when this is energetically favourable, and it is suggested therefore that quenching to give ground state species following favourable charge‐transfer interactions accounts for the proportion of quenching without reaction. The spectra of the indole radicals, R., were determined and the kinetics of their decay in aerated and deaerated solution were investigated and reaction schemes proposed to explain these observations.