Circularly polarized luminescence (CPL) materials hold significant promise in diverse fields, including molecular sensing, information encryption, and optical storage. The luminescence dissymmetry factor (glum) quantifies CPL, ranging from +2 (ideal LCP) to -2 (ideal RCP). Achieving high glum values is crucial for practical applications. Cholesteric liquid crystals (CLCs), with their helical superstructure, have proven effective in amplifying glum. CLCs also exhibit stimulus-responsiveness, enabling the development of responsive CPL materials. Previous approaches involved incorporating achiral or chiral emitters into small-molecule or chiral LCs to create CLC-based CPL materials. However, small-molecule CLC-based CPL materials are often limited to LC cells, hindering their broader application. Liquid crystal polymers (LCPs) offer advantages over small-molecule LCs, including better processability, thermal stability, and film-forming properties. Chiral LCPs are considered promising CPL materials. While some progress has been made using aggregation-induced emission (AIE) or polyether-based CLCs, these materials often lack significant responsiveness to external stimuli. Furthermore, achieving high glum values typically requires precise matching between the emission band and the reflection band, demanding precise control over chiral agent concentration. Blue phase liquid crystals (BPLCs), with their unique 3D lattice structure, are responsive to various external stimuli and have a narrower photonic bandgap (PBG) and chiral environment which may be beneficial for improving glum value in the CPL system. BPLCEs, with their elastomeric polymer networks and flexible deformability, are particularly promising candidates for solid-state CPL-active materials. This research focuses on creating solid-state CPL-active materials with full-color and mechanically-switchable characteristics using reconfigurable BPLCEs.

The existing literature highlights the significant interest in CPL materials and their applications. Research has explored various approaches to enhance glum, focusing primarily on CLCs and the importance of matching emission and reflection bands. The use of chiral dopants in liquid crystals has been widely studied, but limitations remain in terms of achieving high glum values and external stimuli responsiveness. Small molecule LCs have been used, but their limitations in processability and thermal stability have driven research towards LCPs. Studies have shown success in incorporating AIE-active molecules and designing polyether-based CLCs copolymers to enhance fluorescence and CPL intensity. However, these approaches frequently result in limited responsiveness to external stimuli. The unique properties of blue phase liquid crystals, particularly their responsiveness to external stimuli and potential for creating elastomeric materials, provide a promising avenue for overcoming these limitations.

Freestanding QD-BPLCE films were fabricated using a one-step in situ photopolymerization reaction between acrylate monomers and dithiol. The chemical structures of the LC monomers (RM82 and RM105), chiral dopant (LC756), thiol crosslinker (BDMT), disulfide diacrylate crosslinker (DSDA), photoinitiator (I-651), and oil-soluble QDs are detailed in the Supplementary Information. Precursor mixtures were infiltrated into LC cells and cooled to form the BPI phase. The samples were then exposed to 365 nm UV light for curing. Five types of QD-BPLCEs with varying reflection and fluorescence colors were prepared by adjusting chiral agent content and QD types. The distribution of QDs within the BPI phase was characterized using various techniques including optical microscopy (POM), Kossel diagrams, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The CPL signal was measured using a CPL-200 spectrometer. The effects of QD content and film thickness on the CPL signal were investigated. The mechanical response of the CPL emission was studied by applying controlled stretching to the films and analyzing changes in the reflection spectrum and Kossel patterns. A G-CLCE film was prepared for comparison to understand the mechanism of CPL signal generation in BPLCEs versus CLCEs. Circular dichroism (CD) spectroscopy was used to characterize the chiral properties of the materials.

The study successfully demonstrated visualized full-color CPL by doping red, green, and blue QDs into BPLCEs. Importantly, the BPLCEs generated a CPL signal opposite to that observed in CLCEs, despite similar CD signals. This observation indicates that the CPL generation mechanism in BPLCEs is distinct from that in CLCEs and is independent of PBGs. High glum values (around 0.7) were achieved across all three colors, highlighting the effectiveness of the BPLCE approach. The 3D cubic superstructure of the BPLCEs plays a crucial role in inducing the strong CPL signal. The CPL signal could be reversibly switched on and off by mechanically stretching the BPLCE films. This mechanical switching results in a color change and a change in CPL signal, demonstrating the tunable nature of the material. The changes induced by stretching can be permanently fixed using dynamic disulfide bonds in the BPLCE network. The QD-BPLCE films demonstrated excellent potential for anti-counterfeiting and information encryption applications.

The findings challenge the conventional understanding of CPL generation in chiral liquid crystal systems. The observation that high glum values can be achieved in BPLCEs independently of PBG matching significantly advances the design and development of high-performance CPL materials. The mechanical tunability and fixability of the CPL signal open up possibilities for advanced applications requiring dynamic control over light polarization. The successful demonstration of anti-counterfeiting and information encryption capabilities underscore the practical potential of QD-BPLCEs. Further research could explore the optimization of QD incorporation for enhanced glum values and the investigation of other stimuli-responsive mechanisms for CPL control.

This study successfully fabricated full-color, mechanically switchable CPL-active materials using QD-doped BPLCEs. The unique chiral 3D cubic superstructure of the BPLCEs leads to strong CPL signals independent of PBG matching. The reversible mechanical switching of CPL signals and the ability to fix these states offer significant potential for diverse applications. Future work could focus on exploring broader applications, optimizing material properties, and investigating other external stimuli for CPL control.

The study focused on a specific set of QDs and BPLCE formulations. Further investigation is needed to assess the generality of the findings with different QDs and polymers. The long-term stability and durability of the mechanically switched states under various environmental conditions also require further investigation. The scalability and cost-effectiveness of the fabrication process for large-scale applications should also be considered.