Geometrical Effects on Intramolecular Quenching of Aromatic Ketone (π,π*) Triplets by Remote Phenolic Hydrogen Abstraction

The photochemistry of a series of alkoxyacetophenone derivatives bearing remote phenolic groups has been studied using laser flash photolysis techniques. The compounds are structured with a p- or m- phenolic moiety attached via a m- or p -oxyethyl linkage to the carbonyl chromophore, and each have a lowest triplet state of π,π* configuration. The corresponding methoxy-substituted compounds have also been examined. The triplet lifetimes of the phenolic ketones vary with the positions of attachment ( meta or para ) of the oxyethyl spacer to the carbonyl and phenolic moieties, indicating a very strong dependence of the rate of intramolecular H-abstraction on geometric factors. For example, the para,para‘-linked phenolic ketone has a lifetime τ T ≈ 12 ns in dry MeCN solution at room temperature due to rapid intramolecular H-abstraction, while the meta,meta‘-derivative exhibits a lifetime τ T ≥ 11.5 μs at infinite dilution and no detectable intramolecular reactivity. The presence of as little as 0.03 M water in the solvent leads to a significant increase in triplet decay rate in all cases, in contrast to its retarding effect on the rate of bimolecular phenolic H-abstraction in model compounds. Semiempirical (PM3) calculations have been carried out to determine the optimum conformation for abstraction in each molecule as a function of substitution pattern. The variation in the rate constants for intramolecular H-abstraction throughout the series is consistent with a mechanism involving coupled electron/proton transfer within a hydrogen-bonded triplet exciplex, the overall rate of which depends critically on orbital overlap factors between the aromatic rings in the ketone and the phenol.