Unveiling the Mechanism of CPL in Blue Phase Liquid Crystal Elastomers

Researchers from the Beijing University of Chemical Technology and Southeast University have demonstrated a novel solid-state CPL-active material by incorporating Quantum Dots (QD) into the blue phase. In the future, this finds application in optical coding. The findings were published in the journal Light: Science & Applications.

The prospective uses of circularly polarized luminescence (CPL) materials in various industries, including optical storage, molecular sensors, and information encryption, have garnered considerable interest.

Thus far, the use of helical superstructured Cholesteric Liquid Crystals (CLCs) has shown to be an efficient medium for increasing glum value. Nevertheless, CPL materials made from small molecule CLCs are frequently restricted to LC cells, which limits their usefulness in specific situations.

CLC polymer films exhibit restricted reactivity towards external stimuli because of their solid-state frozen helical architecture. Meanwhile, enhanced glum values of CPL are typically obtained by matching the system's emission band and reflection band, whether in CLC polymers or small-molecule CLCs. This necessitates precise modulation of the amount of chiral agent added to the system.

Visualized full-color CPL with the largest glum absolute value of up to 0.74 is achieved by doping red, green, and blue QDs emitters, respectively. The CD signals of BPLCE and CLCE are similar, while their CPL signals are opposite, suggesting that the mechanisms triggering CPL signals in BPLCEs and CLCEs are different. Specifically, LCP is transmitted while right-handed CLCEs selectively reflect RCP.

The excited right-handed CPL is reflected when the photonic bandgap of the CLCEs partially or fully matches the emission spectrum of the luminescent molecules. Only the left-handed CPL is transmitted at the same moment. As a result, left-handed CPL signals were produced by right-handed CLCEs. On the other hand, CPL signals in BPLCEs are not induced by selective reflection.

BPLCEs are widely known for having a strongly chiral environment and a highly ordered 3D structure. Following their introduction into the BPLCE mixture, the QDs work with the molecules to construct supramolecular three-dimensional structures through self-assembly. Therefore, even in the absence of any matching between PBGs and QD emission bands, right-handed BPLCEs produce a right-handed CPL signal and a higher glum value.

In the fully polymerized network, the sample shows good thermal durability, retaining significant reflectance and fluorescence signals up to 80 °C. As a result of a flexible crosslinker, the sample shows remarkable stretching properties. These researchers examined how mechanical force stimulation affected the sample's CPL signal.

Scientists explained, “When the sample undergoes uniaxial mechanical stretching, its lattice experiences longitudinal extension, leading to the disruption of its chiral structure. As a consequence, there is a noticeable change in the CPL signal, transitioning from being observable to becoming undetectable.”

“The disappearance of the CPL signal induced by mechanical force is temporary. When the external force is removed, QD-BPLCE automatically returns to its initial state, and the CPL signal reappears. In our work, by activating the dynamic disulfide bonds within the QD-BPLCE, lattice changes induced by stretching can be fixed, ultimately leading to the permanent extinction of CPL signals,” researchers added.

The researchers concluded, “This study demonstrates the potential of developing CPL functional materials through photonic structures of BPLCEs, suggesting the advancement of BPLCEs-based CPL-active materials for optical coding and information storage applications.”

Journal Reference:

Li, S., et al. (2024) When quantum dots meet blue phase liquid crystal elastomers: visualized full-color and mechanically-switchable circularly polarized luminescence. Light, Science & Applications. doi.org/10.1038/s41377-024-01479-1