Aggregation-induced emission: the origin of lignin fluorescence

Aggregation-induced emission (AIE) characteristics of lignin, which has the intrinsic aggregation behavior, are detected and studied for the first time. A positive correlation between the growth multiple of fluorescence intensity (l(1:9)/l(10:0)) and the sulfonation degree was found in the water-ethanol system. The AIE phenomenon and mechanism were further studied by the addition of cetyltrimethyl ammonium bromide (CTAB) owing to the electrostatic interaction with lignosulfonate. It is well known that lignin contains carbonyl groups, stilbene (Ar-C-alpha=C-beta) and alpha-carbonyl (Ar-C-alpha=O) building blocks. We deduce that cluster of the carbonyl groups and restriction of intramolecular rotation (RIR) effects together contribute to the AIE activity of lignin as lignin does not exhibit blue emission based on its limited conjugated structure. Our results provide a new prospective to understand the fluorescence in lignin and explore novel potential application for the AIE activity of lignin.