Geometrical effects on intramolecular quenching of aromatic ketone (pi,pi*) triplets by remote phenolic hydrogen abstraction

The photochemistry of a series of alkoxyacetophenone derivatives bearing re mote phenolic groups has been studied using laser flash photolysis techniqu es. The compounds are structured with a p- or m-phenolic moiety attached vi a a m- or p-oxyethyl linkage to the carbonyl chromophore, and each have a l owest triplet state of pi,pi* configuration. The corresponding methoxy-subs tituted compounds have also been examined. The triplet lifetimes of the phe nolic ketones vary with the positions of attachment (meta or para) of the o xyethyl spacer to the carbonyl and phenolic moieties, indicating a very str ong dependence of the rate of intramolecular H-abstraction on geometric fac tors. For example, the: para,para'-linked phenolic ketone has a lifetime ta u(T) approximate to 12 ns in dry MeCN solution at room temperature due to r apid intramolecular H-abstraction, while the meta,meta'-derivative exhibits a lifetime tau(T) greater than or equal to 11.5 mu s 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 rare in ail cases, in contrast to its retarding effect on the rate of bimo lecular phenolic H-abstraction in model compounds. Semiempirical (PM3) calc ulations have been carried out to determine the optimum conformation for ab straction in each molecule as a function of substitution pattern. The varia tion in the rate constants for intramolecular H-abstraction throughout the series is consistent with a mechanism involving coupled electron/proton tra nsfer 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.