Photochemistry of methoxyhydroquinone and methoxy-p-benzoquinone in solution related to the photoyellowing of the lignocellulosics

The photoreactivity of methoxy-p-benzoquinone (MQ) and methoxyhydroquinone (MHQ) in dilute solution (10(-4)-10(-3) M) was, studied using continuous ir radiation and laser flash photolysis (LFP). The quinone irradiated in degas sed tetrahydrofuran (THF) gives MHQ and an adduct with the solvent. Only th e formation of hydroquinone is observed in ethanol, and hydroxylation is ev idenced in waters 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 MH Q is irradiated selectively in degassed THF, the formation of MQ and of the bis-hydroquinone, 4,4'-dimethoxy-2,5,2',5'-tetrahydroxybiphenyl, are obser ved. The, dimer is oxidized photochemically or thermally into the mono- or bis-quinones, the process being, accelerated in alkaline medium. The format ion of the dimers is strongly favored by the contiguous presence of quinone and hydroquinone. When MHQ is selectively irradiated in the presence of tr ans-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 comp ound such as 1,4-dimethoxybenzene. By contrast, no strong quenching of (EtC )-Et-3* by MHQ was observed. It is proposed that the photochemistry of the couple MQ/MHQ is governed by the formation of encounter complex between eit her (3)MQ* and MHQ or (3)MHQ* and MQ. Consequently, the fast part of the ph otoyellowing of lignocellulosics does not appear to involve the couple MHQ/ MQ or MHQ/etherified coniferaldehyde, but more likely a combination of oxid ation of the hydroquinone by ground-state oxygen and photohydration of the formed quinone from its triplet state, giving inter-alia more colored omicr on -quinonoid type molecules.