LIGNIN-LIKE MOLECULES - STRUCTURE AND PHOTOPHYSICS OF CRYSTALLINE ALPHA-GUAIACOXYACETOVERATRONE

Alpha-Guaiacoxyacetoveratrone (I) has been widely used as a lignin mod el in photochemical studies, and its solution photochemistry is known to resemble closely the processes which are responsible for the light- induced yellowing of lignin-rich paper products. In the solid state, I proved to be essentially photostable under lamp irradiation. Laser fl ash photolysis studies with diffuse reflectance detection reveal no de tectable intermediate triplet states and/or free radicals. A combinati on of solid-state NMR, X-ray crystallography, and fluorescence studies led to the conclusion that this unexpected stability of I in the soli d state is due to the stacking of molecules in an excimer-like arrange ment. This leads to significant excited singlet-state stabilization, w hich is also reflected in a large red-shift of the fluorescence. The c rystal structure of I reveals an asymmetric unit containing four molec ules in different conformations; solid-state C-13 NMR data on I is con sistent with the conformational differences revealed by the crystallog raphic data. It is suggested that the photostabilization that I achiev es spontaneously may lead to possible ways of improving on the stabili ty of lignin-rich products.