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

Laser flash photolysis studies on (R,S) and (S,S) diastereoisomers of the bichromophoric compounds
1−6 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 1−3 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.