Towards bioresource-based aggregation-induced emission luminogens from lignin -O-4 motifs as renewable resources

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic beta-O-4 lignin segments is of high importance to meet high value added biorefinery demands. However, executing this process remains a huge challenge due to the incompatible reaction conditions of the depolymerization of lignin beta-O-4 segments containing gamma-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin beta-O-4 moieties with 3-alkenylated indoles catalyzed by vanadium-based complexes have been successfully developed, affording a wide range of functionalized carbazoles with up to 92% yield. Experiments and density functional theory calculations suggest that the reaction pathway involves the selective cleavage of double C-O bonds/Diels-Alder cycloaddition/dehydrogenative aromatization. Photophysical investigations show that these carbazole products represent a class of BioAIEgens with twisted intramolecular charge transfer. Distinctions of emission behavior were revealed based on unique acceptor-donor-acceptor-type molecular conformations as well as molecular packings. This work features lignin beta-O-4 motifs with gamma-OH functionalities as renewable substrates, without the need to apply external oxidant/reductant systems. Here, we show a concise and sustainable route to functional carbazoles with AIE properties, building a bridge between lignin and BioAIE materials. Biorenewable feedstocks are important for developing more sustainable materials, but developing processes can be challenging due to incompatible reaction conditions. Here, the authors report the one-pot preparation of aggregation-induced emission luminogens from lignin moieties.