Photochemistry of Methoxyhydroquinone and Methoxy-p-benzoquinone in Solution Related to the Photoyellowing of the Lignocellulosics¶

Abstract

The photoreactivity of methoxy-p-benzoquinone (MQ) and methoxyhydroquinone (MHQ) in dilute solution (10(-4)-10(-3) M) was studied using continuous irradiation and laser flash photolysis (LFP). The quinone irradiated in degassed tetrahydrofuran (THF) gives MHQ and an adduct with the solvent. Only the formation of hydroquinone is observed in ethanol, and hydroxylation is evidenced in water, whereas the compound is stable in CCl4. The bis-quinone, 4,4'-dimethoxybiphenyl-2,5,2',5'-bisquinone, and the dibenzofurane-quinone, 8-hydroxy-3,7-dimethoxydibenzofuran-1,4-quinone, are formed in the presence of MHQ, whereas the reactivity is low with ethylconiferyl alcohol. When MHQ is irradiated selectively in degassed THF, the formation of MQ and of the bis-hydroquinone, 4,4'-dimethoxy-2,5,2',5'-tetrahydroxy-biphenyl, are observed. The dimer is oxidized photochemically or thermally into the mono- or bis-quinones, the process being accelerated in alkaline medium. The formation of the dimers is strongly favored by the contiguous presence of quinone and hydroquinone. When MHQ is selectively irradiated in the presence of transethylconiferaldehyde (EtC), quinone formation and isomerization of EtC are observed. LFP experiments, performed with a selective excitation of MQ, indicate that the triplet state of the quinone is strongly quenched by MHQ to conduce to a semiquinone radical. The interaction between 3MQ* and MQ is mainly driven by an electron transfer process according to the similar value of the quenching rate constant found with another electron donor compound such as 1,4-dimethoxybenzene. By contrast, no strong quenching of 3EtC* by MHQ was observed. It is proposed that the photochemistry of the couple MQ/MHQ is governed by the formation of encounter complex between either 3MQ* and MHQ or 3MHQ* and MQ. Consequently, the fast part of the photoyellowing of lignocellulosics does not appear to involve the couple MHQ/MQ or MHQ/etherified coniferaldehyde, but more likely a combination of oxidation of the hydroquinone by ground-state oxygen and photohydration of the formed quinone from its triplet state, giving inter-alia more colored o-quinonoid type molecules.

2‐Methoxyhydroquinone from Vanillin for Aqueous Redox‐Flow Batteries

“…This dimer was formed either by reaction of MQ with MHQ or by partial oxidation of the MHQMHQ. Although the formation of 3,7-dimethoxydibenzo[b,d]furan-1,4,8-triol (DMBMQ) has been described, [25,27] we could not detect it in our experiments. As the persistence of the MHQ/MQ system is of utmost importance for its use in redox flow batteries, we explored the decomposition mechanism in more detail.…”

“…MQ degraded in alkaline media within seconds, and photoproducts were already observed after a few milliseconds. [25,26] In solution, MHQ readily dimerizes to give the insoluble dimer 4,4'-dimethoxy-2,5,2',5'-tetrahydroxy-biphenyl (MHQMHQ), which was characterized by X-ray diffraction analysis (Supporting Information, Figure S6, Tables S2-S8). In the mother liquor, we identified another side product (2',5'dihydroxy-4,4'-dimethoxy-[1,1'-biphenyl]-2,5-dione, MQMHQ) by 1 H-NMR spectroscopy ( Figure S7, †ESI).…”

2‐Methoxyhydroquinone from Vanillin for Aqueous Redox‐Flow Batteries

“…This dimer was formed either by reaction of MQ with MHQ or by partial oxidation of the MHQMHQ. Although the formation of 3,7‐dimethoxydibenzo[ b,d ]furan‐1,4,8‐triol (DMBMQ) has been described, [25, 27] we could not detect it in our experiments. As the persistence of the MHQ/MQ system is of utmost importance for its use in redox flow batteries, we explored the decomposition mechanism in more detail.…”

“…MHQ has not been considered so far for any electrochemical applications due to its instability under UV light and its sensitivity to acidic and alkaline conditions. MQ degraded in alkaline media within seconds, and photoproducts were already observed after a few milliseconds [25, 26] …”