Photophysics and photochemistry of a lignin-like quinoid dimer, 4,4 '-dimethoxybiphenyl-2,5,2 ',5 '-bisquin in relation to color alteration of woody materials exposed to daylight

4,4'-Dimethoxybiphenyl-2,5,2',5'-bisquinone 1 is thought to be issued, in w ood 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 me chanism of the conversion 1 --> 2 has been studied by laser flash photolysi s and by quantum yield measurements in several solvents at various temperat ures. The reaction proceeds via the triplet state of 1 and a cyclized X tra nsient species which rearranges to 2 by a sequence of deprotonation-protona tion reactions, quite efficiently in polar or acidified solutions but with a low yield in nonpolar solvents. As temperature increases, the quantum yie ld increases in polar solutions but decreases in nonpolar solvents. This un usual temperature dependence is explained by activation energy barriers on the cyclization (3)1* --> X and reopening X --> 1 elementary reactions appr oximate to 35.5 and 58.5 kJ/mol respectively, and by a conversion X --> 2 m uch faster in polar solvents than in nonpolar ones. The zwitterionic or bir adicaloid structure of X is discussed on the basis of semiempirical calcula tions'(AM1 or PM3) and of spectroscopic and kinetic measurements. Under las er flash photolysis conditions, a special mechanism involving a second-orde r rearrangement of X was found to be very efficient in any type of solvents .