Systematic investigation of solvent-dependent aggregation-induced emission in alkyl chain-bridged lignin-based amphiphilic polymers

Abstract

Aggregation governs the photophysical properties of fluorophores, particularly in aggregation-induced emission (AIE) systems, where emission efficiency is contingent upon regulated molecular packing. Lignin, as the most abundant renewable natural aromatic polymer, constitutes a sustainable scaffold for the development of AIE-active materials; however, its intrinsic potential in this field remains largely underexplored. Herein, two amphiphilic lignin derivatives, sulfomethylated lignin (SL) and alkyl chain-bridged lignin (ASL), were synthesized via a two-step sulfomethylation/alkyl-bridging strategy. Fourier transform infrared (FTIR) spectroscopy and contact angle measurements verified the successful grafting of hydrophobic moieties onto the lignin backbone, which remarkably improved the material's hydrophobicity. The self-assembly behaviors and AIE responses of SL and ASL in water/ethanol mixed solvents were systematically investigated by tuning hydrophilic–hydrophobic balance, solvent polarity, and concentration. Fluorescence spectroscopy and atomic force microscopy (AFM) revealed that ASL exhibited higher fluorescence intensity than SL, attributed to enhanced formation of well-defined aggregates. Notably, both SL and ASL showed monotonically increased fluorescence with ethanol volume fraction (0–90%), confirming solvent-mediated AIE effect. Below the critical aggregation concentration (CAC), synergistic π–π stacking and hydrophobic interactions promoted luminescence; above the CAC, excessive aggregation caused light-scattering-induced quenching. ASL possesses high quantum yield, excellent biocompatibility, and high renewability. This work establishes molecular engineering (alkyl chain bridging) and solvent tuning as powerful strategies for optimizing lignin-based AIE systems, offering a green, low-cost alternative for biosensing, flexible displays, and sustainable optoelectronics.