The digital era's information explosion necessitates advanced information security measures. While QR codes offer basic encryption, more sophisticated techniques are needed. Fluorescence-switchable and time-responsive afterglow materials have improved 3D code capabilities. However, circularly polarized luminescence (CPL) offers an additional dimension (polarization state), enhancing security. Previous work has explored double-layer patterns and CPL spectra for information encryption. Luminescent cholesteric liquid crystals (CLCs), with their self-organized helical superstructures and stimuli-responsive nature, are promising for generating tunable CPL. Recent advancements include wavelength-controllable CPL-active laser arrays and photochemical dual-responsive CPL materials. However, multilevel security encryption using CLC-based CPL materials remains under-explored, hampered by limitations in multiple responses, precise local tuning, high g_lum values, and ease of operation. This research aims to address these challenges by developing phototunable CLC systems with fluorogenic properties and good processability.

The existing literature extensively covers the use of fluorescence-switchable materials and time-responsive afterglow materials for enhancing information security, particularly in the context of upgrading QR codes to three-dimensional (3D) codes. Several studies have investigated the potential of circularly polarized luminescence (CPL) for information encryption, exploring techniques such as double-layer patterns under UV irradiation and CPL spectral analysis for authentication. Research on luminescent cholesteric liquid crystals (CLCs) has shown their promise in generating tunable CPL due to their self-organized helical superstructures and stimuli-responsive properties. While some progress has been made in creating wavelength-controllable CPL-active laser arrays and multidimensional security labels using photochemical dual-responsive CPL materials, the development of multilevel security encryption technology based on CLC-active materials for practical applications remains limited due to challenges in achieving multiple responses, precise local tuning, high g_lum values, and easy operation.

The researchers constructed a FRET platform in CLCs using a chiral fluorescent molecule, (S)-CNB (as donor), and achiral molecular switches, DG and DR (as acceptors). The synthesis of these molecules was confirmed via NMR and HR-MS. The chiral induction ability of (S)-CNB was measured using the Grandjean-Cano method, determining its helical twisting power (HTP). The photocyclization/cycloreversion properties of DG and DR under UV and visible light were investigated using NMR spectroscopy. Binary ((S)-CNB/DG, (S)-LC-D) and ternary ((S)-CNB/DG/DR, (S)-LC-T) FRET systems were built in CLC helical superstructures. Phototunable CPL and fluorescence were observed, with color changes attributed to FRET processes. Time-resolved fluorescence lifetime measurements quantified radiative decay and FRET efficiency. Liquid crystal photonic capsules (LCPCs) were fabricated via interfacial polymerization, encapsulating CLCs to improve processability. LCPC films were prepared by spin-coating LCPC-PVA solutions. The phototunable fluorescence and CPL performance of these films were characterized, and CIE coordinates were determined. Full-color CPL and fluorescence were achieved by blending different LCPCs. Finally, multilevel information encryption was demonstrated using these LCPC films, integrating fluorescence, CPL, full-color emission, and time response characteristics. The encryption involved different combinations of LCPCs and leveraging UV light exposure time, color changes and polarization states to encode and decode information.

The study successfully synthesized and characterized chiral and achiral fluorophores for use in a FRET system within a liquid crystal matrix. The (S)-CNB molecule exhibited strong right-handed CPL, while the achiral switches, DG and DR, demonstrated reversible photoisomerization with distinct spectral changes. The integration of these components into binary (LC-D) and ternary (LC-T) systems within the CLC helical structure resulted in phototunable CPL with color changes from blue to green (LC-D) and red (LC-T) upon UV irradiation. The FRET efficiency was high, exceeding 80% for LC-D and 90% for LC-T. The encapsulation of the CLC-FRET system into LCPCs enabled the creation of processable films that retained the phototunable CPL properties. These films exhibited full-color emission by blending different LCPCs, allowing for the generation of various colors, including white light. The time-dependent behavior of the CPL emission further enhanced the complexity and security of the encryption scheme. The researchers demonstrated 2D, 3D and 4D encryption methods, integrating fluorescence, CPL, full-color emission, and time-dependent responses for multi-level information security.

The successful generation of phototunable full-color CPL using the FRET system within LCPCs demonstrates a significant advancement in information encryption technology. The integration of multiple encryption parameters, including color, polarization, and time dependence, significantly enhances the security level compared to traditional methods. The use of LCPCs addresses the processability challenges associated with liquid crystal-based systems, making the technology more practical for real-world applications. The findings have implications for data storage and anti-counterfeiting applications, opening new avenues for secure information protection.

This research successfully developed device-friendly solid films with phototunable full-color CPL based on LCPCs. The FRET platforms within the LCPCs enabled photoswitchable trichromatic CPL emissions. The LCPCs showed good processability and strong time-dependent CPL, creating a robust platform for 4D data encryption. Future research could focus on exploring new fluorophores with improved CPL properties and expanding the range of achievable colors and encryption levels.

While the study demonstrates a promising approach to information encryption, limitations exist. The g_lum values obtained from the LCPCs were slightly lower than those from LC cells due to the imperfect planar orientation of helical superstructures within the microcapsules. Further optimization of the LCPC fabrication process could potentially improve these values. The long-term stability of the photochromic switches under repeated cycling needs further investigation for practical applications.