Photoluminescence-enhanced cholesteric films: Coassembling copper nanoclusters with cellulose nanocrystals.

Iridescent and luminescent composite films were fabricated through a coassembly strategy, in which glutathione-stabilized copper nanoclusters (GSH-CuNCs) were incorporated into chiral nematic structures of a cellulose nanocrystal (CNC) film. Through variations in the helical pitch, these composite films exhibited broadband reflection. The fluorescence emission spectrum of the composite film exhibited peaks at 439 and 600 nm, corresponding to crystallization-induced emission from CNCs and assembly-induced emission from CuNCs. The enhanced luminescence and prolonged lifetime of the composite film were attributed to the confinement effect of solid layers and attendant intermolecular interactions. By tuning the reaction time, temperature, and pH of the solution, the emission color and intensity of the CuNCs could be changed. At appropriate GSH and Cu2+ concentrations, the chiral organization of GSH-CuNCs enabled the composite CNC film to exhibit right-handed chiral fluorescence with an asymmetry factor of -0.16. Luminescent composite films were employed to fabricate LEDs with custom colors and patterns.