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

Abstract

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic β-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 β-O-4 segments containing γ-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin β-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 β-O-4 motifs with γ-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.

Multi-site isomerization of synergistically regulated stimuli-responsive AIE materials toward multi-level decryption

“…26 In contrast, aggregation-induced emission (AIE) represents the opposite behavior: luminogens with AIE effect (AIEgens) are non-emissive in the isolated state but can emit bright light in the aggregate state. 27–29 Accordingly, AIEgens are preferable for developing “intelligent” stimuli-responsive optical materials, promoting the continuous exploration of AIE-based materials during the past two decades. 16,30–32…”

Sustainable production of carbazole-based BioAIEgens from lignin major motifs

“…As an alternative approach, our recent work achieved a one-pot transformation of lignin β-O-4 models reacting with 3-alkenylated indole to generate carbazoles catalyzed by vanadium associated with tridentate Schiff-based ligands (Scheme 1b). 47 Nonetheless, this method necessitates the use of organovanadium complexes with intricated ligands and prefunctionalized 3-alkenylated indole derivatives, requiring significant synthetic efforts upfront. Thus, there is a pressing need to explore feasible synthesis routes for multi-substituted carbazole derivatives in an atom-economical and sustainable manner, catalyzed by non-precious metals and commercially available starting materials.…”

Solvent-triggered directional lignin valorization towards monomeric acetals or lignin polyols

“…3 Research has shown that the β-O-4 motifs are the most abundant and active linkages in lignin, making them crucial for downstream transformations. 4,5 By incorporating stabilization functions into designed solvent systems, the structure and activity of lignin can be effectively regulated, 6 thereby facilitating the extraction of high yields of lignin monomers or novel lignin streams.…”