Photochemistry of lignin model compounds on solid supports

Abstract

The photochemistry of several substituted α-(aryloxy)acetoveratrones that are models for chromophores in lignin has been studied using a combination of laser flash photolysis and product studies in solution and on silica, Na-X zeolite, and cellulose. The lifetimes of the triplet ketones vary substantially with the electron-donating ability of the substituents in the α-aryloxy ring, with values ranging from 30 ns for the 4-methoxy derivative to 5.5 µs for the 4-cyano ketone in acetonitrile. The triplet ketones are considerably longer lived on a silica surface than in solution and do not decay with first-order kinetics; however, they are all shorter lived than triplet acetoveratrone under the same conditions, indicating that β-phenyl quenching and (or) β-cleavage dominate their decay. The same triplets show quite different kinetic behavior in Na-X or cellulose and there is no indication in these systems for the importance of either β-phenyl quenching or β-cleavage in determining the rates of triplet decay. Product studies for α-guaiacoxyacetoveratrone indicate that β-cleavage from the singlet excited state is responsible for product formation; in contrast to solution results, the phenacyl–phenoxyl radical pairs react exclusively by geminate recombination to give rearranged ketones resulting from ortho and para ring coupling of the two radicals. Product formation is very inefficient for this ketone in cellulose, indicating either a low singlet reactivity or efficient geminate recombination of the initial radical pair to regenerate starting material.