Laser Flash Photolysis of 1,2-Diketopyracene and a Theoretical Study of the Phenolic Hydrogen Abstraction by the Triplet State of Cyclic α-Diketones

Abstract

Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.

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

“…This mechanism, shown in Scheme 1, involves an initial electron transfer from the phenol to the excited ketone, through an intermediate exciplex, followed by an ultrafast proton transfer, which ultimately results in the formation of the radical pair ketyl/ phenoxyl. [1][2][3][4][5][6][7][8] In support to this mechanism, Figure 4 shows a representative spectrum obtained after laser irradiation of a deoxygenated solution of thiochromanone (1) acetonitrile. In this spectrum the absorptions relative to triplet 1 were substituted for new ones in the 340-440 nm region, which can be associated to a mixture of the ketyl radical 2 derived from the thiochromanone triplet, as well as to the 4-methoxyphenoxyl radical, which shows absorption at 405 nm 41 (Scheme 1).…”

“…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. 4,16 Experimentally, this coupled electron/proton transfer mechanism ultimately results in the formation of the ketyl-aryloxyl radical pair.…”

“…[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.…”

“…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. 4,16 Experimentally, this coupled electron/proton transfer mechanism ultimately results in the formation of the ketyl-aryloxyl radical pair.Triplet ketones are known to oxidize biological substrates such as amino acids, nucleosides and DNA. Due to this, topical or systemic administration of drugs containing the carbonyl chromophore can lead to sever sideeffects upon exposure to sunlight.…”

Estudo da reatividade do estado excitado triplete de 1,4-diaza-9-fluorenonas frente a doadores de hidrogênio e de elétron

“…37 Assim, o mecanismo sugerido no processo de abstração de hidrogênio fenólico por carbonilas triplete deve envolver a formação inicial de um exciplexo triplete, seguida por uma transferência acoplada de elétron/próton, levando finalmente aos produtos finais, ou seja, o par de radicais fenoxila/ cetila (Esquema 2). [38][39][40][41][42][43][44][45][46][47] Supressores nos quais o processo de desativação do estado excitado triplete se passa através um processo de transferência de elétron, como 1,4- Figura 6 mostra um espectro representativo para a formação do ânion radical derivado de 1,4-diaza-benz[b]-9-fluorenona (2), o qual mostra absorções máximas a 440 e 540 nm. Por outro lado, a espectroscopia de transientes mostrou que a fotólise de 1,4-diaza-9-fluorenona (1) em presença de DABCO, em acetonitrila, resultou na formação do ânion radical correspondente, o qual apresenta absorções máximas a 460 e a 540 nm.…”