Aminated and amidated structures introduced by ethylenediamine pretreatment endow lignin with bright fluorescence

Abstract

As a natural photoluminescent (PL) polymer, lignin has enormous potential in the fabrication of sustainable and environmentally friendly luminescent materials for various applications such as bio-imaging and chemical sensing. However, lignin exhibits weak PL emission due to its limited conjugated structures, which has largely restricted its further applications in PL techniques. In the present study, a one-step ethylenediamine (EDA) pretreatment strategy was designed to directly fractionate lignin from biomass and endowed with electron-rich N atoms or electron-donating subunits. The fractionated lignin showed an increase in its PL intensity by 6.6 times in an aqueous solution. The results show that the introduction of electron-donating N-containing groups (–NH₂, –NH–, –CN–, and –CONH–) into lignin could red-shift the wavelength through decreasing energy gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and increase the efficiency of clusteroluminescence (CL) simultaneously. This aminated lignin solution shows two-stage concentration-dependent PL, excitation-dependent luminescence effect, multiple emission peaks, and red-shift fluorescence emission, which is solid evidence for the presence of clusterization-triggered emission (CTE). Aggregation of N-containing groups contributes to the electron (n → π*) transition and inter-and/or intra-molecular electron delocalization. It also strengthens the through-space interaction between N and O, stabilizes the structure from non-radiative rotation, and prevents self-quenching. Additionally, due to the presence of numerous basic groups, the fluorescence behaviors of aminated lignin have good pH responsiveness. This amination and amidation approach provides a novel route for broadening the scope of lignin utilization in the field of organic fluorescence materials and helps gain further mechanistic insight into CL.