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

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 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 τT ≈ 12 ns in dry MeCN solution at room temperature due to rapid intramolecular H-abstraction, while the meta,meta‘-derivative exhibits a lifetime τT ≥ 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.

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

“…Similar conclusions apparently also hold for the quenching of other triplet sensitizers like biacetyl [16] and flavin derivatives [17,18] by anisole, which prompted researchers to use anisole (derivatives) as low-reactivity reference systems for the chemistry of phenols. [19][20][21] While our own results for some BP derivatives are in accord with the charge-transfer mechanism in ACN solutions, we show herein that the reactivity of triplet excited BP towards anisole is dramatically dependent on the nature of the solvent. In particular, the reactivity of this system is dramatically enhanced in protic solvents and even reaches the reactivity of the well-documented BP/phenol system.…”

Time-Resolved EPR Investigation of the Photo-initiated Intramolecular Antioxidant Reaction of Vitamin K−Vitamin E Linked Molecule

“…A recent photochemical study of molecules linking phenol with aromatic ketones showed that the intramolecular hydrogen bond between phenol and the excited triplet ketone induced an electron transfer followed by proton transfer. 45 In this mechanism, the contribution of the tunneling effect should be negligibly small.…”

Photoinduced oxidation of sea salt halides by aromatic ketones: a source of halogenated radicals

“…The implications of the presence of such light-absorbing organics and the possible photochemical reactions are not yet well understood in the sea surface microlayer. But clearly, the presence of such photoactive compounds at the air/sea interfaces could give rise to photosensitized processes (Becker et al, 1995;Canonica et al, 2006Canonica et al, , 2000Canonica et al, , 2005Das et al, 1981;Ivanov et al, 2005;Lathioor and Leigh, 2001;Lathioor et al, 1999;Leigh et al, 1996) which may also affect chemistry in the marine boundary layer (MBL) (Reeser et al, 2008). Previously, we showed that chlorophyll may be a photochemical source of halogenated radicals or the driving force for ozone deposition at low wind speed (Clifford et al, 2008).…”