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

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.