Abstract
The nanosecond laser flash photolysis technique has been used to study the reactivity of 1,2-aceanthrylenedione (1) triplet towards phenols. Using acetonitrile as solvent, the quenching rate constant k q ranged from 3.3 x 10(5) L mol-1s-1 (phenol) to 1.3 x 10(7) L mol-1s-1 (4hydroxyphenol). Phenols with electron donating substituents were more reactive. The kinetic isotope effect (kH/kD) observed for the quenching of 1 with 4-methoxyphenol was 1.5. The Hammett plot for the reaction of the triplet of 1 with phenols gave a reaction constant rho = 1.04 in acetonitrile solution. The low value for the quenching rate constant measured for this triplet with 1,3-cyclohexadiene (k q=2.0 x 10(4) L mol-1s-1) and the diffusion controlled quenching rate constant obtained for beta-carotene suggest that the energy of the lowest triplet state for 1,2-aceanthrylenedione is located between 88 and 219 kJ mol-1.
Highlights
Hydrogen atom abstraction rate constants from the triplet state of D-diketones are extremely dependent on their chemical structure
Laser Flash Photolysis Study of the Phenolic Hydrogen Abstraction characterized as the triplet state of 1.7 The triplet nature of this species was further confirmed in the present work by quenching with E-carotene (E =88 kJ mol-1),[11] for which we obtained a diffusion controlled quenching rate constant, with the concomitant formation of the E-carotene triplet at Omax=535 nm
From these quenching experiments we can estimate that the triplet energy of 1 has limits between 88 and 219 kJ mol-1, which is in full agreement with a triplet state localized on the anthracene moiety (ET=178 kJ mol-1).[11]
Summary
Hydrogen atom abstraction rate constants from the triplet state of D-diketones are extremely dependent on their chemical structure. Biacetyl, having a low-lying nS* triplet state, shows hydrogen abstraction rate constants ranging from 3.0x103 L mol-1s-1 (2-propanol) to 3.4x108 L mol-1s-1 (phenol).[1] On the other hand, 1,1,4,4tetramethyl-1,4-dihydro-2,3-naphthalendione, which has a low lying nS* triplet state, shows remarkably fast hydrogen abstraction rate constants.[2] Despite its low triplet energy (E = 230 kJ mol-1)[2] the reaction rate constant for. T the latter diketone in the presence of hydrogen donor exceeds that of triplet benzophenone.[3] Large hydrogen abstraction rate constants were found for Ketones having a low energy triplet state with SS* character are much less reactive towards aliphatic or benzylic hydrogen abstraction than those with nS* character.[5] examples of SS* ketones for which hydrogen abstraction rate constants could be measured, such as 1,1’-, 1,2’- and
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