Systematic Investigation of Solvent-Dependent Aggregation-Induced Emission in Alkane-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 alkane-bridged lignin (ASL)—were synthesized via a two-step sulfomethylation/alkyl-bridging strategy. Fourier Transform Infrared (FTIR) spectroscopy, molecular structural characterization, 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 in 90% ethanol, ASL exhibited superior hydrophobicity and 44% higher fluorescence intensity (8500 vs. 5900 a.u.) 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. 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. AFM imaging demonstrated that alkyl bridging suppressed aggregation-caused quenching (ACQ) and facilitated ordered nanostructures (hollow spheres in 50% ethanol, irregular sheets in 70% ethanol). ASL possesses high quantum yield, excellent biocompatibility, and high renewability. This work establishes molecular engineering (alkyl 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.