Abstract
Triplet ketones are known to oxidize biological substrates which can lead to damage of several biomolecules such as amino acids, nucleosides and DNA. As part of our systematic study on the interaction between carbonyl compounds and phenols, the triplet reactivity of thiochromanone (1) towards substituted phenols, in acetonitrile, was investigated employing the laser flash photolysis technique. The quenching rate constants ranged from (1.1 ± 0.1) × 10(8) L mol-1 s-1 (4-cyanophenol) to (5.8 ± 1.0) × 10(9) L mol-1 s-1 (hydroquinone). A Hammett plot for the reaction of triplet 1 with phenols containing polar substituents resulted in a reaction constant ρ = -0.90. This negative value observed for the reaction constant ρ is in accord with a mechanism in which the hydrogen transfer from phenols to the triplet carbonyl involves a coupled electron/proton transfer.
Highlights
In the last few years the phenolic hydrogen abstraction by carbonyl compounds in the triplet excited state has been extensively studied employing experimental and theoretical methods.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] The mechanism for this process is 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]
In the present work we show results concerning the photochemical reactivity of the triplet excited sate of thiochromanone (1) towards phenol and its derivatives containing polar substituents, employing the laser flash photolysis technique
Laser excitation (l = 355 nm) of a deoxygenated solution of thiochromanone (1), in acetonitrile, resulted in the formation of a transient showing absorption bands at λmax = 420, 480, 540 and 580 nm (Figure 1A) and a lifetime of 1.6 μs (Figure 1B). This transient was readily quenched by b-carotene, a well-known triplet acceptor (ET = 19 kcal mol-1; Fisc=0),[35,36] for which we obtained a quenching rate constant, in acetonitrile, of (1.1 ± 0.1) × 1010 L mol-1 cm-1
Summary
In the last few years the phenolic hydrogen abstraction by carbonyl compounds in the triplet excited state has been extensively studied employing experimental and theoretical methods.[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] The mechanism for this process is 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]. Phenolic Hydrogen Abstraction by the Triplet Excited State of Thiochromanone
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