Effect of methyl substitution on the intramolecular triplet deactivation of p-methoxy-β-phenylpropiophenone

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Abstract
The photochemistry of several methyl-substituted p-methoxy-β-phenylpropiophenones (1–5) has been examined by nanosecond laser flash photolysis using 337-nm laser excitation. The ketone triplets decay by intramolecular β-aryl quenching of the carbonyl in a well-known process that is believed to involve charge transfer. Surprisingly, methyl substitution at the β-methylene group, which is not a participant in the deactiviation mechanism, causes a dramatic decrease in the triplet lifetime; the effect is attributed to different conformational preferences in ketones 1–3 and is rationalized on the basis of MMX calculations. Methyl substitution in the β-aryl ring affects the ease of oxidation of this ring and as a result induces changes in the kinetics for intramolecular charge transfer. This conclusion is further supported by electrochemical studies of ketones 1, 4, and 5. Key words: laser flash photolysis, triplet ketones, intramolecular quenching, charge transfer.

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