Photoinduced Hydrogen Abstraction from Phenols by Aromatic Ketones. A New Mechanism for Hydrogen Abstraction by Carbonyl n,π* and π,π* Triplets

Abstract

Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl n,π* and π,π* triplets is derived from the results for a series of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical. The biradicals have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecular quenching of the parent methoxy-substituted ketones by p-cresol. Deviation from this trend is observed for the alkoxyindanone derivative, where an in-plane approach of the phenolic hydrogen to the carbonyl n-orbital is not possible. The trends in the rate constants for bimolecular quenching of a series of substituted benzophenones by p-cresol indicate that for n,π* triplet abstractions, the quenching mechanism is different for electron donor-substituted and electron acceptor-substituted ketones. For π,π* triplets and donor-substituted (n,π* triplet) benzophenones, abstraction is proposed to occur by a mechanism involving the intermediacy of a hydrogen-bonded exciplex, which yields the corresponding radicals by sequential electron- and proton-transfer. The rate constant for quenching by this mechanism thus depends mainly on the basicity of the ketone triplet state and the oxidation potential of the phenol.

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

“…Excited triplet states of aromatic ketones are known to be strong oxidants; this has been studied with different electron donating compounds such as phenol (Canonica et al, 2006(Canonica et al, , 2000Leigh et al, 1996) and oxygen (Mehrdad et al, 2002a, b). Immediately after the laser pulse a new spectrum, displayed as solid symbols in Fig.…”

“…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).…”

Functionalization of polymeric surfaces by simple photoactivation of CH bonds

“…36 The results showed that formaldehyde, acetone, and cyclohexanone have a high grafting efficiency, especially acetone which gives the highest grafting efficiency. The reaction of phenols as efficient inhibitors implies two facts: the H atoms in PhOH have a high reactivity to readily be abstracted [37][38][39] and the formed PhOÁradicals have a strong ability to scavenge carbon-centered chain radicals. 40 In this work, acetone as photoinitiator and different phenols [including 4-hydroxylbenzene sulfonic acid (HBSA), p-aminophenol, tyramine, and 4-hydroxythiophenol and tyrosine] with a para substituent XR (denoted XRPhOH) as the reactants were proposed to incorporate a broad range of functional X groups with an R spacer onto the polymeric surface as illustrated in Scheme 1.…”

Phenolic hydrogen abstraction by the triplet excited state of thiochromanone: a laser flash photolysis study

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The mechanism for this process is now well established and involves the initial formation of a hydrogen-bonded exciplex in the triplet excited state, followed by electron transfer and, subsequently, an ultrafast proton transfer. 4,9-16 DFT calculations clearly showed that for the triplet complex (exciplex) ketone-phenol the hydrogen transfer is predominantly occurring.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The mechanism for this process is now well established and involves the initial formation of a hydrogen-bonded exciplex in the triplet excited state, followed by electron transfer and, subsequently, an ultrafast proton transfer. 4,[9][10][11][12][13][14][15][16] DFT calculations clearly showed that for the triplet complex (exciplex) ketone-phenol the hydrogen transfer is predominantly occurring. 8 The very fast quenching rate constants of triplet carbonyls towards phenols is consequence of both the low reduction potential of the former as well as the low oxidation potential of the later.…”