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Photochemistry of Methoxyhydroquinone and Methoxy‐ p ‐benzoquinone in Solution Related to the Photoyellowing of the Lignocellulosics ¶
Author(s) -
BéarnaisBarbry Stéphane,
Bonneau Roland,
Castellan Alain
Publication year - 2001
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2001)0740542pomamp2.0.co2
Subject(s) - chemistry , benzoquinone , photochemistry , biochemistry
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 CCl 4 . 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′‐tetrahydroxybiphenyl, 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 trans ‐ethylconiferaldehyde (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 3 MQ* 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 3 EtC* 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 3 MQ* and MHQ or 3 MHQ* 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.