Photophysics and Photochemistry of a Lignin-like Quinoid Dimer, 4,4‘-Dimethoxybiphenyl-2,5,2‘,5‘-bisquinone, in Relation to Color Alteration of Woody Materials Exposed to Daylight

Abstract

4,4‘-Dimethoxybiphenyl-2,5,2‘,5‘-bisquinone 1 is thought to be issued, in wood and lignin-rich pulps, from the photochemical coupling and oxidation of methoxy(hydro)quinone. Under irradiation in solution, 1 is transformed to a highly colored dibenzofuran derivative, 2, as unique photoproduct. The mechanism of the conversion 1 → 2 has been studied by laser flash photolysis and by quantum yield measurements in several solvents at various temperatures. The reaction proceeds via the triplet state of 1 and a cyclized X transient species which rearranges to 2 by a sequence of deprotonation−protonation reactions, quite efficiently in polar or acidified solutions but with a low yield in nonpolar solvents. As temperature increases, the quantum yield increases in polar solutions but decreases in nonpolar solvents. This unusual temperature dependence is explained by activation energy barriers on the cyclization 31* → X and reopening X → 1 elementary reactions, ≈ 35.5 and 58.5 kJ/mol respectively, and by a conversion X → 2 much faster in polar solvents than in nonpolar ones. The zwitterionic or biradicaloid structure of X is discussed on the basis of semiempirical calculations (AM1 or PM3) and of spectroscopic and kinetic measurements. Under laser flash photolysis conditions, a special mechanism involving a second-order rearrangement of X was found to be very efficient in any type of solvents.