Photodegradation of substituted stilbene compounds: what colors aging paper yellow?

Photodegradation of lignin is one of the major postprocessing problems in paper production, as this renders yellowing of the paper and reduced paper quality. In this study, we have explored the photochemical properties of substituted stilbene derivatives believed to be key chromophores in the photodegradation of lignin derived from cinnamyl alcohol. In particular, the present work focuses on the computation of UV/vis electronic absorption spectra for different methoxylated stilbenes and their proposed photodegradation products. All calculations were performed using the time-dependent formalism of density functional theory (TD-DFT) and the B3LYP hybrid functional. It is concluded that the methodology employed is capable of reproducing not only the overall spectra, but also subtle features owing to the effects of different substitution patterns. For the strongly absorbing first excited singlet state (HOMO --> LUMO excitation) of the methoxylated stilbenes, the calculated transition energies are, albeit somewhat fortuitously, in excellent agreement with experimental data. The light-induced yellowing indirectly caused by the presence of stilbenes can be rationalized in terms of the absorption spectra of the resulting photodegraded o-quinones, for which distinct transitions in the 420-500 nm region of the visible spectrum lacking prior to degradation are observed.