Upconverted Chiral Emission in Hybrid Photonic Nanomaterials: Toward Amplified Circularly Polarized Luminescence with Tunable Chirality

Circularly polarized luminescence (CPL), an emergent technique to study the excited-state chirality in molecules and materials, is finding potential applications in the field of data encryption, anticounterfeiting, and photonic technologies. In this regard, upconverted chiral luminescence holds great promise due to the inherent advantages offered by the upconversion nanophosphors (UCNPs), which include excellent photochemical stability, a large anti-Stokes shift, and high penetration depth. However, the generation of upconverted circularly polarized luminescence (UC-CPL) has remained a great challenge. Herein, we demonstrate a facile approach for the synthesis of chiral luminescent self-standing photonic films that display UC-CPL with intense luminescence dissymmetry. Cellulose nanocrystals (CNCs) that exhibit lyotropic liquid-crystalline properties provide a robust platform as a chiral host, whereas a set of lanthanide-based UCNPs that emit different colors upon excitation using a near-IR laser act as achiral guests. While the chiral emission color could be tuned by controlling the nature of the activator/sensitizer in UCNPs, the sign of CPL is modulated through physical and chemical methods such as varied sonication time and electrolyte addition. The judicious choice of the electrolyte and duration of ultrasonication assisted in regulation of the helical sense of the nematic phase of CNCs, thereby influencing the selective reflection properties of the film. The approach adopted herein for the fabrication of materials exhibiting intense UC-CPL holds the potential to be expanded to a variety of nanosystems that can ultimately find application in display technology and circularly polarized light-emitting devices