Quantum-efficiency enhancement and mechanical responsiveness of solid-state photoluminescent flexible materials containing uniaxial cellulose nanocrystal arrays

Assembling cellulose nanocrystals (CNCs) can induce solid-state photoluminescence based on Stokes scattering. Such photoluminescent materials are free of photo-quenching and show great potential in optical applications, whereas poor flexibility of assembled CNC arrays limits their applications. Here, a binary co-assembly of 1D nanoparticles and long-chain polymers was explored to produce the uniaxial CNC arrays into transparent poly(vinyl alcohol) (PVA) membranes, which enhanced the mechanical properties, especially ductility. Besides, the CNC assembly was controlled by adjusting the mass ratio between CNCs and PVA. The result indicated that co-assembling with PVA could improve the uniaxial orientation of CNC arrays, which played a crucial role in enhancing the emission quantum efficiency (EQE). Stretching the PVA/CNC membrane could further induce an enhancement in EQE together with a blue shift in excitation wavelength. The mechanism study on stimulation-response suggested that the EQE enhancement and blue shift came from the change in uniaxial orientation degree and periodicity of the CNC assembly, respectively. Since the stimulation-responsive enhancement in EQE (from ca. 40 to 60%) can even be observed by naked eyes, we believe such cellulose-based materials can be widely used in optical sensors.