Confinement-Activated Fluorescence Enhancement of Dyes via Polymer-Mediated Interfacial Regulation in Cellulose Nanocrystal Films.

Controlling dye emission under confinement remains challenging because of aggregation-caused quenching. Here, we report a confinement-activated mechanism for polymer-mediated regulation of interfacial and dye emission in cellulose nanocrystal (CNC)-based systems. PEG induces negligible photophysical changes in suspension but dramatically enhances fluorescence in films, increasing the quantum yield of rhodamine 6G (Rh6G) from 0.03% to 57.42% with a prolonged fluorescence lifetime. Spectroscopic and molecular dynamics analyses show that PEG regulates dye-dye and CNC-dye organization through steric and interfacial effects. Solvent-mediated interactions largely screen these perturbations in suspension, whereas solvent removal enables cooperative regulation by the rigid CNC framework and flexible PEG chains, redistributing local dimer geometries and excitonic coupling. The fluorescence enhancement extends to multiple cationic dyes but depends strongly on the molecular structure. These findings establish a molecular framework linking polymer-mediated interfacial regulation, confinement, and dye structure, providing a strategy for tuning the emission in polysaccharide-based materials.