Reprinted from the Canadian Journal of Chemistry, 74, 263-275 (1996).

© National Research Council, 1996.

The Photochemistry 3,3',4,4'-Tetramethoxy- and 4-Hydroxy-3,3',4'-trimethoxystilbene - Models for Stilbene Chromophores in Peroxide-Bleached, High Yield Wood Pulps

William J. Leigh,* T. Johnathan Lewis, Vincent Lin, and J. Alberto Postigo

Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1

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 solvolysis have been determined by ferrioxalate actinometry. Direct irradiation of trans-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-tbutyldimethylsiloxy-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.