Journal of the American Chemical Society 121, 11984-11992 (1999).

1999 American Chemical Society

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

Edward C. Lathioor, William J. Leigh* and Michael J. St. Pierre

Contribution from the Department of Chemistry, McMaster University, Hamilton, ON  Canada L8S 4M1

Abstract: 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 para- or meta-phenolic moiety attached via a meta- or para-oxyethyl linkage to the carbonyl chromophore, and each have a lowest triplet state of p,p* 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 tT ~ 12-ns in dry MeCN solution at room temperature due to rapid intramolecular H-abstraction, while the meta,meta’-derivative exhibits a lifetime tT 11.5-ms 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. Semi-empirical (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, where the overall rate constant for reaction depends critically on the ability of the system to allow orbital overlap between the aromatic rings in the ketone and the phenol.

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