| Abstract: | CONCLUSION. The results presented above show clearly that a protic environment is necessary for degradation to occur from the triplet state of I. According to our proposed mechanism, C-O bond cleavage (and therefore degradation) becomes a viable reaction pathway from the triplet state of I because hydrogen bonding provides steric hindrance to -phenyl quenching, a competing, non-product forming pathway. In mechanical pulp fibres, the energy barriers to p-phenyl quenching are enormous, because structures such as I are bonded to the rest of the solid lignin polymer. Thus, under the conditions encountered in paper, bond cleavage probably occurs with high efficiency, from both singlet and triplet excited states. A recent electron paramagnetic resonance study (73) has shown that I also degrades by a photoreduction pathway in the presence of alcohols or water, through the intermediacy of the corresponding ketyl radical. Since corresponding ketyl radicals and triplet states often have overlapping absorption spectra, it is important to emphasize that the transient observed in our optical flash photolysis experiments is the triplet state (not the ketyl radical), the key evidence being that both the yield and the lifetime of the transient are decreased by triplet quenchers. An important question or future research should be to determine the relative yields of photoreduction and C-O cleavage to the photodegradation of I. Quantitative assessment of the importance of hydrogen bonding, and the determination of other possible effects of the protic environment on the reaction mechanism, must await the development of an efficient radical scavenger which does not at the same time quench the triplet state. |
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