The photochemistry of 3,3′,4,4′-tetramethoxy-and 4-hydroxy-3,3′,4′-trimethoxystilbene — models for stilbene chromophores in peroxide-bleached, high-yield wood pulps

Abstract

The photochemistry of the title compounds has been investigated in ethanol and tetrahydrofuran solution under aerobic and anaerobic conditions. Direct irradiation of trans-3,3′,4,4′-tetramethoxystilbene (trans-1) in deoxygenated ethanol leads to the rapid establishment of a photostationary state with the cis isomer, and the slower formation of the ethyl ether corresponding to addition of ethanol across the olefinic C=C bond and cyclobutane dimers. The same products are formed upon photolysis in the presence of oxygen under the same conditions but, in addition, two isomeric tetramethoxyphenanthrenes and 3, 4-dimethoxybenzaldehyde are formed. Photolysis of trans-1 in oxygenated tetrahydrofuran leads to the same products in different relative yields. Quantum yields for cis, trans photoisomerization, phenanthrene formation, and addition of ethanol have been determined by ferrioxalate actinometry. Direct irradiation of tran-4-hydroxy-3,3′,4′-trimethoxystilbene (trans-2) in ethanol solution also results in rapid cis–trans isomerization and the formation of (three) isomeric phenanthrene derivatives in photolyses carried out in the presence of oxygen, although the material balance is low. The various products of photolysis of trans-2 have been independently synthesized by desilylation of the products isolated from photolysis of trans-4-tert-butyldimethylsiloxy-3,3′,4′-trimethoxystilbene (trans-3) under similar conditions. Fluorescence-quenching experiments have been carried out to determine the relative rates of quenching of the excited singlet states of trans-1 and trans-2 by alcohols and oxygen. The formation of aldehydes is proposed to arise via reaction of superoxide ion with stilbene radical cations, which are formed by electron-transfer quenching of the stilbene excited singlet state by oxygen. Key words: stilbenes, lignin, photochemistry, photooxidation.