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Stereodifferentiation in the Decay of Triplets and Biradicals Involved in Intramolecular Hydrogen Transfer from Phenols or Indoles to π,π* Aromatic Ketones

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posted on 23.01.2004, 00:00 by Julia Pérez-Prieto, Agustín Lahoz, Francisco Boscá, Ramón Martinez-Mañez, Miguel A. Miranda
Laser flash photolysis studies on (R,S) and (S,S) diastereoisomers of the bichromophoric compounds 16 have been used to investigate the possible chiral discrimination in the quenching of triplet excited ketones, resulting in formal hydrogen abstraction. Deuterium isotopic effects show that triplet deactivation in these bichromophores is dominated by hydrogen atom transfer. A remarkable stereodifferentiation is found in the intramolecular quenching of the ketone triplets of 13 and 5 by the phenolic or indolic moieties, either in methanol or acetonitrile as solvents. This indicates the existence of specific structural requirements for hydrogen transfer. On the other hand, the lifetimes of the generated biradicals show large solvent dependence; solvation appears to slow their reversion to the starting ketone. The considerable stereodifferentiation observed for the biradical lifetimes suggests that the kinetics of biradical decay is faster when the approach of the two radical termini becomes easier.