Geometric and solvent effects on intramolecular phenolic hydrogen abstraction by carbonyl n, π* and π, π* triplets

The photochemistry of a series of alkoxyacetophenone, -benzophenone, and -indanone derivatives, which contain a remote phenolic group linked to the ketone by a para,para'- or meta,meta'-oxyethyl spacer, has been studied in acetonitrile and dichloromethane solutions using laser flash photolysis techniques. The corresponding methoxy-substituted compounds and, in the case of the alkoxyindanones, derivatives bearing just a remote phenyl substituent, have also been examined. The triplet lifetimes of the phenolic compounds are determined by the rates of intramolecular abstraction of the remote phenolic hydrogen, and depend on the solvent, the geometry of attachment and the configuration of the lowest triplet state. In contrast to the large (>500-fold) difference in lifetime of the para,para'- and meta,meta'-alkoxyacetophenone derivatives, both of which have lowest π,π* triplet states, smaller differences are observed for the alkoxyindanone (lowest charge transfer triplet, ~twofold difference) and alkoxybenzophe...