Reaction pathways involved in the quenching of the photoactivated aromaticketones xanthone and 1-azaxanthone by polyalkylbenzenes

The reactions of the photoexcited aromatic ketones, xanthone and 1-azaxanth one, with polyalkylbenzene donors yields the corresponding ketyl radicals a s detected by nanosecond laser flash photolysis. On the basis of formation of these photoreduced products, the quenching of the photoexcited species i s expected to occur either by a one-step hydrogen abstraction from the dono r, electron transfer followed by proton transfer from the donor, or by form ation of a charge-transfer type encounter complex prior to hydrogen atom tr ansfer. The reactions of triplet xanthone and triplet 1-azaxanthone with po lyalkylbenzene donors in acetonitrile were investigated to probe the effect of the nature of the triplet state and the redox properties on the relativ e importance of each quenching pathway. Determination of bimolecular rate c onstants, as well as analysis of kinetic isotope effects and ketyl radical yields, suggests that for both xanthone and 1-azaxanthone the quenching pro cess is dominated by formation of charge-transfer encounter complexes betwe en excited-state aromatic ketone acceptor and ground-state polyalkylbenzene donor. The reactivites of the xanthone pi,pi* triplet and 1-azaxanthone n, pi* triplet toward these donors is shown to be governed by their reduction potentials, with their electronic configuration being unimportant to the ki netics of encounter complex formation. The only exception to this is found when sterically encumbered polyalkylbenzene donors are employed. Results wi th these compounds suggest that pi,pi* and n,pi* states form encounter comp lexes of different structure which affects their ability to react with hind ered donors. Additionally, product yields with all of the donors are contro lled by both the extent of charge transfer within encounter complexes and t he encounter complex structure.