Photoprocesses of p-Benzoquinones in Aqueous Solution

Abstract

The photochemistry of 1,4-benzoquinone (BQ) and several derivatives, for example, duroquinone, trimethyl-2,5- or 2,6-dimethyl-, and methyl-BQ in aqueous solution or mixtures with polar media, for example, acetonitrile or 2,2,2-trifluoroethanol, was studied by time-resolved UV−vis spectroscopy after pulses at 248 and 308 nm. The triplet state and the semiquinone radical (•QH/Q•-) of BQs are spectroscopically and kinetically separated intermediates. The radical yield in the absence of H-atom donors is low and significantly increased in the presence of alcohols. Efficient photoinduced charge formation, because of Q•- and H+ after H-atom transfer from 2-propanol to the triplet state, and small effects in the absence of a donor were observed by transient conductivity. The quantum yield of photodecomposition, λirr = 254 nm, is substantial for BQ, MeBQ, and Me2BQs in aqueous solution, but small for Me4BQ. To account for the efficient photoconversion of BQs into hydrobenzoquinones and 2-hydroxy-1,4-benzoquinones, a novel water-mediated reaction not involving free radicals is proposed as major step. This mechanism is consistent with the prediction that the observed triplet state is monomeric and the yield of Q•-, detected by both transient absorption and conductivity, is low for sub-millimolar BQ, MeBQ, and Me2BQs at pH 5−6. In addition, H-atom abstraction from a polar organic solvent or by self-quenching plays a role in mixtures with water or at enhanced quinone concentration, respectively.

Laser photolysis study of anthraquinone in binary mixtures ofionic liquid [bmim][PF6] and organic solvent

“…Since the 540nm band disappears while the 380 nm band remains almost unchanged in shape and intensity when the solution is purged with oxygen, the 540 nm band is assigned to − • AQ that is produced via electron transfer between the 3 AQ* and DMF (Eq.4). As shown in Eq.5, another possibility is an electron transfer from AQ to 3 AQ* [24]. Then …”

Hydrated electron (e aq − ) generation from p -benzoquinone/UV: Combined experimental and theoretical study

“…In addition, it has been well demonstrated that the formation of p-hydroquinone can be induced by the photoreactions of p-BQ (an oxidation intermediate of phenol [25]) in aqueous solution under UV irradiation at 253.7 nm [26], in which the redox cycling is involved [26]. With the formation of p-hydroquinone, it is reasonable to expect the generation of eaq  by the UV photolysis of p-BQ in aqueous solution [27].…”

“…Several studies have reported some distinct mechanisms of the UV photolysis of p-BQ in aqueous solution, which can lead to the formation of similar products, namely the hydroquinone and hydroxylated quinone [26,30−33]. These mechanisms are classified as the OH • [30−32], direct triplet [26], and quinone-quinone or electron transfer mechanisms [26,33]. These mechanisms are initiated by the same steps: the excitation of quinone to its first excited singlet state, followed by the intersystem crossing to its first triplet state [26,28−33].…”

“…In the OH • mechanism, the abstraction of hydrogen atom from water molecule by the first triplet state of quinone can induce the formation of OH • radical and semiquinone radical, and the two radicals can lead to the formation of the hydroquinone and hydroxylated quinone [30−32]. In the direct triplet mechanism, the first triplet state of quinone reacts with water molecule directly, leading to the formation of benzene-1,2,4-triol with rearrangement, and then benzene-1,2,4-triol can be oxidized by quinone, resulting in the formation of the hydroquinone and hydroxylated quinone with a molar ratio of 1 : 1 [26]. At high concentrations of quinone, the first triplet state of quinone may oxidize quinone by one electron transfer, leading to the formation of semiquinone radical anion and cation, which is designated as the electron transfer mechanism [26,33].…”

“…In the direct triplet mechanism, the first triplet state of quinone reacts with water molecule directly, leading to the formation of benzene-1,2,4-triol with rearrangement, and then benzene-1,2,4-triol can be oxidized by quinone, resulting in the formation of the hydroquinone and hydroxylated quinone with a molar ratio of 1 : 1 [26]. At high concentrations of quinone, the first triplet state of quinone may oxidize quinone by one electron transfer, leading to the formation of semiquinone radical anion and cation, which is designated as the electron transfer mechanism [26,33]. Then, the hydroquinone and hydroxylated quinone can be generated from the semiquinone radical anion and cation [26,33].…”