Influences of condensation and conjugations in lignin derivatives on photo-chemical behaviors

Abstract

Photochemical behaviors of native lignin derivatives such as sulfric lignin (SL), dioxane lignin (DL), alkaline lignin (AL), lignosulfonate (LS) and lignophenol (LP) were qualitatively investigated using UV spectroscopy and Fluorescence spectroscopy. All lignin derivatives were prepared directly from softwood (Hinoki cypress, Chamaecyparis obtusa) and hardwood (Birch, Betula maximowicziana) with different degrees of conjugation in structures produced through isolation processes. Though all lignin derivatives showed broad absorption peaks between 260-350 nm in UV spectra, it was difficult to estimate influences of conjugations because of broad peaks. On the other hands, fluorescence spectra demonstrated different peaks from 300 nm to 600 nm with several shoulder peaks due to different relaxation processes according to wood species and for different chemical structures of lignin. Moreover, the strongest fluorescence of SL in THF at 310 nm and 380 nm implied relatively parallel conformations of aromatic rings fixed in complicated 3-dimensional structures. LS and LP showed different peaks between wood species. On the other hands, relatively small fluorescence of other lignins also implied conjugations positioned randomly in lignin macromolecular chains. As a result, it was found that different peaks in emission spectra were reflected by not only different energy levels of conjugations around aromatic rings but conformation of aromatic conjugation groups.