Circularly Polarized Luminescence Chirality Inversion and Dual Anticounterfeiting Labels Based on Fluorescent Cholesteric Liquid Crystal Particles

The development of materials with circularly polarized luminescence (CPL) properties is a promising but challenging frontier in advanced materials science. Modulating the chiral properties of chiral polymers has also been a focus of research. Studies have been conducted to control the ground-state chirality of chiral polymers by adjusting the concentration of the chiral dopant. However, the chirality inversion of CPL of fluorescent liquid crystal particles by chiral dopant concentration has not been reported. Here, we report the preparation of fluorescent cholesteric liquid crystal (FCLC) particles that display polarizable structural color and CPL, demonstrating how varying the chiral dopant amount can reverse the CPL direction, leading to systems where the rotation directions of polarizable structural color and CPL either align or differ. This study confirmed the critical role played by the formation of the twist grain boundary phase in inducing the inversion of the ground-state chirality of FCLC particles and, subsequently, triggering the inversion process of CPL chirality. Furthermore, it leverages chiral structural color and fluorescence of FCLC particles to develop a sophisticated dual verification system. This system, utilizing both circularly polarized light and fluorescence, offers enhanced anticounterfeiting protection for high-value items.

Temperature-Induced Sign Inversion of Circularly Polarized Luminescence of Binaphthyl-Bridged Tetrathiapyrenophanes

D2-symmetric (R)-binaphthyl-bridged pyrenophanes containing thioether bonds were synthesized. The pyrenophanes exhibited the temperature-induced sign inversion of circularly polarized luminescence (CPL) while maintaining the emission wavelength and reversibility. The Δglum value reached 0.02, and the FL quenching by heat was negligible. The sign inversion of CPL originates from the inversion of intramolecular excimer chirality associated with excitation dynamics. The two pyrenes form a kinetically trapped left-handed twist excimer at low temperatures, while they form a thermodynamically favored right-handed twist excimer at high temperatures. The thioether linkers can impart flexibility suitable for the inversion of chirality of the excimers.

Helicity Control of Circularly Polarized Luminescence from Aromatic Conjugated Copolymers and Their Mixture Using Reversibly Photoinvertible Chiral Liquid Crystals

We synthesized cyclic chiral compounds [(R)/(S)-D2s] by linking a photoresponsive bisbenzothienylethene (BTE) moiety with an axially chiral binaphthyl moiety. Chiral nematic liquid crystals (N*-LCs) were prepared by adding chiral compounds as dopants to host N-LCs. These N*-LCs exhibited reversible chirality inversion upon photoisomerization between the open and closed forms of the BTE moiety. Here, the mechanism underlying chirality inversion in photoresponsive N*-LCs was investigated by comparing the helical twisting powers (HTPs) of (R)-D2s with those of analogous compounds. It was found that the helical inversion of N*-LCs containing (R)-D2s is governed by a delicate balance between two types of opposite helicity, i.e., the right-handed helicity of the inherently chiral binaphthyl moiety and the left-handed helicity of the BTE moiety bearing intramolecularly induced chirality. Namely, (R)-D2s induced chirality of the BTE moiety, which is attributed to intramolecular chirality transfer from the axially chiral binaphthyl moiety to the BTE moiety. Thus, (R)-D2s are chiral compounds with double chirality consisting of an intrinsically chiral moiety and an intramolecularly induced chiral moiety. Photocontrol of the helical senses and reversible photoinversion of the N*-LCs are achieved by utilizing UV and visible light irradiation and the steric effects of the substituents at the binaphthyl rings in (R)-D2s. In addition, photocontrol of the induced circularly polarized luminescence (CPL) was achieved using the photoinvertible N*-LC. The achiral aromatic conjugated copolymers that exhibited red, green, and blue fluorescence were dissolved and mixed in the present N*-LC, and they exhibited left- and right-handed white CPL with large dissymmetry factors (|glum|) ranging from 0.2 to 1.0. The CPLs were reversibly photoswitched due to photoisomerization between the open and PSS forms of the chiral compounds through UV and visible light irradiation.

A Photo- and Thermo-Driven Azoarene-Based Circularly Polarized Luminescence Molecular Switch in a Liquid Crystal Host

The development of chiral optical active materials with switchable circularly polarized luminescence (CPL) signals remains a challenge. Here an azoarene-based circularly polarized luminescence molecular switch, (S, R, S)-switch 1 and (R, R, R)-switch 2, are designed and prepared with an (R)-binaphthyl azo group as a chiral photosensitive moiety and two (S)- or (R)-binaphthyl fluorescent molecules with opposite or the same handedness as chiral fluorescent moieties. Both switches exhibit reversible trans/cis isomerization when irradiated by 365 nm UV light and 520 nm green light in solvent and liquid crystal (LC) media. In contrast with the control (R, R, R)-switch 2, when switch 1 is doped into nematic LCs, polarization inversion and switching-off of the CPL signals are achieved in the resultant helical superstructure upon irradiation with 365 nm UV and 520 nm green light, respectively. Meanwhile, the fluorescence intensity of the system is basically unchanged during this switching process. In particular, these variations of the CPL signals could be recovered after heating, realizing the true sense of CPL reversible switching. Taking advantage of the unique CPL switching, the proof-of-concept for "a dual-optical information encryption system" based on the above CPL active material is demonstrated.

Polymer-Stabilized Liquid Crystal Films Containing Dithienyldicyanoethene-Based Chiral Photoswitch: Multi-Modulation for Environment-Adaptative Smart Windows

A polymer-stabilized liquid crystal (PSLC)-based environment-adaptative smart window with multi-modulations is demonstrated. This PSLC system contains a right-handed dithienyldicyanoethene-based chiral photoswitch and a chiral dopant, S811, with opposite handedness, of which the reversible cis-trans photoisomerization of the switch can drive self-shading of the smart window under UV light stimulus because of the transition from nematic phase to cholesteric one. With the assistance of solar heat, the opacity of the smart window can be deepened because the heat promotes the isomerization conversion rate of the switch. This switch has no thermal relaxation at room temperature, therefore, the smart window exhibits dual stabilization: transparent state (cis-isomer) and opaque state (trans-isomer). Moreover, the incident intensity of sunlight can be regulated by an electric field, which allows the smart window to adapt to some specific situations. Such an energy-saving device can be used in buildings and vehicles to control indoor temperature and adapt to the required ambiance.

Liquid Crystal Assembly for Ultra-dissymmetric Circularly Polarized Luminescence and Beyond

Circularly polarized luminescence (CPL) is attracting much interest because it can carry extensive optical information. CPL shows left- or right-handedness and can be regarded as part of high-level visual perception to supply an extra dimension of information with regard to regular light. A key to meeting the needs for practical applications is to develop the emerging field of ultra-dissymmetric CPL. Chiral liquid crystal (LC) assemblies─otherwise referred to as cholesteric liquid crystals (CLCs)─are essentially organized helical superstructures with a highly ordered one-dimensional orientation, and distinctly superior to regular helical supramolecules. CLCs can achieve a perfect equilibrium of molecular short-range interaction and long-range orientational order, enabling molecule-scale chirality on a helical pitch and observable scale. LC assembly could be an ideal strategy for amplifying chirality, making it accessible to ultra-dissymmetric CPL. Herein, we focused on some basic but important issues regarding CPL: (i) How can CPL be created from chiral dyes? (ii) Is the chirality of luminescent dyes an essential factor for the generation of CPL? That is, can all chiral dyes emit CPL and vice versa? (iii) How can CPL be transferred within intermolecular systems, and what principles of CPL transmission should be followed? Given these queries and our work, in this Perspective we discuss the generation, transmission, and modulation of CPL with chiral LC assembly, aiming to design and build up novel chiroptical materials. Recent applications of CPL-active LC microstructures in three-dimensional displays, circularly polarized lasers, and asymmetric catalysis are also discussed.

Photo-triggered full-color circularly polarized luminescence based on photonic capsules for multilevel information encryption

Materials with phototunable full-color circularly polarized luminescence (CPL) have a large storage density, high-security level, and enormous prospects in the field of information encryption and decryption. In this work, device-friendly solid films with color tunability are prepared by constructing Förster resonance energy transfer (FRET) platforms with chiral donors and achiral molecular switches in liquid crystal photonic capsules (LCPCs). These LCPCs exhibit photoswitchable CPL from initial blue emission to RGB trichromatic signals under UV irradiation due to the synergistic effect of energy and chirality transfer and show strong time dependence because of the different FRET efficiencies at each time node. Based on these phototunable CPL and time response characteristics, the concept of multilevel data encryption by using LCPC films is demonstrated.

Light-triggered Modulation of Supramolecular Chirality

Dynamically controlling the supramolecular chirality is of great significance in development of functional chiral materials, which is thus essential for the specific function implementation. As an external energy input, light is remote and accurate for modulating chiral assemblies. In non-polarized light control, some photochemically reactive units (e. g., azobenzene, ɑ-cyanostilbene, spiropyran, anthracene) or photo-induced directionally rotating molecular motors were designed to drive chiral transfer or amplification. Besides, photoexcitation induced assembly based physical approach was also explored recently to regulate supramolecular chirality beyond photochemical reactions. In addition, circularly polarized light was applied to induce asymmetric arrangement of organic molecules and asymmetric photochemical synthesis of inorganic metallic nanostructures, in which both wavelength and handedness of circularly polarized light have effects on the induced supramolecular chirality. Although light-triggered chiral assemblies have been widely applied in photoelectric materials, biomedical fields, soft actuator, chiral catalysis and chiral sensing, there is a lack of systematic review on this topic. In this review, we summarized the recent studies and perspectives in the constructions and applications of light-responsive chiral assembled systems, aiming to provide better knowledge for the development of multifunctional chiral nanomaterials.

Cascade energy transfer augmented circular polarization in photofluorochromic cholesteric texture

Circularly polarized luminescence (CPL)-active light-harvesting systems consisting of a light-responsive donor (R-1), mediator (Nile red), and terminal acceptor (Cyanine 5) are constructed in cholesteric liquid crystals. A dynamically tunable CPL dissymmetry factor and energy transfer modes, are achieved via the closed-ring and open-ring conversion between R-1-O and R-1-C.

Programming and Dynamic Control of the Circular Polarization of Luminescence from an Achiral Fluorescent Dye in a Liquid Crystal Host by Molecular Motors

Circular polarized light is utilized in communication and display technologies and a major challenge is to develop systems that can be switched between left and right circular polarized luminescence with high degrees of polarization and enable multiple addressable stable states. Luminescent dyes in Liquid Crystal (LC) cholesteric phases are attractive systems to generate, amplify and modulate circularly polarized luminescence (CPL). In the present study, we employ light-driven molecular motors as photo-controlled chiral dopants in LCs to switch the handedness of the LC and the circular polarization of luminescence from an achiral dye embedded in the mesogenic material. Tuning of the color of the CPL and the retention time of the photoprogrammed helicity is demonstrated making use of a variety of motors and dyes. The flexibility offered by the design based on inherently chiral unidirectional rotary motors provides full control over CPL non-invasively by light, opening possibilities for pixilated displays with externally addressable polarization.

Amplifying Molecular Scale Rotary Motion: The Marriage of Overcrowded Alkene Molecular Motor with Liquid Crystals

Design and fabrication of macroscopic functional devices by molecular engineering is an emerging and effective strategy in exploration of advanced materials. Photoresponsive overcrowded alkene-based molecular motor (OAMM) is considered as one of the most promising molecular machines due to the unique rotary motion driven by light with high temporal and spatial precision. Amplifying the molecular rotary motions into macroscopic behaviors of photodirected systems links the molecular dynamics with macroscopic motions of materials, providing new opportunities to design novel materials and devices with a bottom-up strategy. In this review, recent developments of the light-responsive liquid crystal system triggered by OAMM will be summarized. The mechanism of amplification effect of liquid crystal matrix will be introduced first. Then progress of the OAMM-driven liquid crystal materials will be described including light-controlled photonic crystals, texture-tunable liquid crystal coating and microspheres, photoactuated soft robots, and dynamic optical devices. It is hoped that this review provides inspirations in design and exploration of light-driven soft matters and novel functional materials from molecular engineering to structural modification.

Multiple chirality inversion of pyridine Schiff-base cholesterol-based metal-organic supramolecular polymers

Based on a metal coordination driven co-assembly strategy, a metal-organic supramolecular polymer system of pyridine Schiff-base cholesterol and metal ions with multiple supramolecular chirality inversion was successfully achieved by the stoichiometry and exchange of metal ions (such as Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Ag⁺), as well as the solvent polarity.

Recent advances in chiral nanomaterials with unique electric and magnetic properties

Research on chiral nanomaterials (NMs) has grown radically with a rapid increase in the number of publications over the past decade. It has attracted a large number of scientists in various fields predominantly because of the emergence of unprecedented electric, optical, and magnetic properties when chirality arises in NMs. For applications, it is particularly informative and fascinating to investigate how chiral NMs interact with electromagnetic waves and magnetic fields, depending on their intrinsic composition properties, atomic distortions, and assembled structures. This review provides an overview of recent advances in chiral NMs, such as semiconducting, metallic, and magnetic nanostructures.

Turn-On Mode Circularly Polarized Luminescence in Self-Organized Cholesteric Superstructure for Active Photonic Applications

Developing circularly polarized luminescence (CPL)-active materials with a large luminescence dissymmetry factor (g_lum) or stimulus responses has evoked a lot of interest in the past few years; however, the light-controllable "on/off" CPL still remains a challenge. Here, a novel diarylethene-based chiral fluorescent photoswitch featuring "turn-on" CPL characteristic is developed, designated as (S,S)-switch 6, which can undergo reversible photocyclization/cycloreversion upon irradiation with UV and visible light. (S,S)-Switch 6 shows completely reversible "off-on-off"-responsive CPL behavior in solution. By doping (S,S)-switch 6 into nematic liquid crystals (LCs), the consequent luminescent cholesteric LCs (CLCs) exhibit a larger g_lum value enhanced 2 orders of magnitude when irradiated with UV light, which can be attributed to the highly ordered helical arrangement of CLCs. The potentials of this turn-on type CPL material for anticounterfeiting and information encryption are illustrated. Furthermore, the visualization of circularly polarized (CP) fluorescent patterns can be successfully achieved by constructing the double-layer CPL system consisting of a CP luminescent layer and a polymer cholesteric reflective layer. The proposed concept establishes a light-controlled off-on-off CPL platform that is of tremendous potential for applications in multi-informational data storage and encryption devices.

Brightening up Circularly Polarized Luminescence of Monosubstituted Polyacetylene by Conformation Control: Mechanism, Switching, and Sensing

The first example of luminescent monosubstituted polyacetylenes (mono-PAs) is presented, based on a contracted cis-cisoid polyene backbone. It has an excellent circularly polarized luminescence (CPL) performance with a high dissymmetric factor (up to the order of 10^-1 ). The luminescence stems from the helical cis-cisoid PA backbone, which is tightly fixed by the strong intramolecular hydrogen bonds, thereby reversing the energy order of excited states and enabling an emissive energy dissipation. CPL switches are facilely achieved by the solvent and temperature through reversible conformational transition. By taking advantages of fast response and high sensitivity, the thin film of mono-PAs could be used as a CPL-based probe for quantitative detection of trifluoroacetic acid with a wider linear dynamic range than those of photoluminescence and circular dichroism. This work opens a new avenue to develop novel smart CPL materials through modulating conformational transition.

Irradiation-Wavelength Directing Circularly Polarized Luminescence in Self-Organized Helical Superstructures Enabled by Hydrogen-Bonded Chiral Fluorescent Molecular Switches

Two light-driven chiral fluorescent molecular switches, (R,S,R)-switch 1 and (R,S,R)-switch 2, are prepared by means of hydrogen-bonded (H-bonded) assembly of a photoresponsive (S) chiral fluorescent molecule, respectively with a cyano substitution at different positions as an H-bond acceptor and an opposite (R) chiral molecule as an H-bond donor. The resulting two switches exhibit tunable and reversible Z/E photoisomerization irradiated with 450 nm blue and 365 nm UV light. When doped into an achiral liquid crystal, both switches are found to be able to form a CPL tunable luminescent helical superstructure. In contrast to the tunable CPL characteristics of the system incorporating switch 2, exposure of the system incorporating switch 1 to 365 nm and 450 nm radiation can lead to controllable different photostationary CPL behavior, including switching-off and polarization inversion. In addition, optical information coding is demonstrated using the system containing switch 1.

Switchable Circularly Polarized Luminescence in Supramolecular Gels through Photomodulated FRET

While the tremendous deal of efforts has been dedicated to the design and fabrication of materials with circularly polarized luminescence (CPL), the development of the chiroptical switch between different CPL signals is one of the important routes toward its application. Here, we prepared a supramolecular gel from the coassemblies containing a chiral gelator (9-fluoren-methoxycarbonyl-functionalized glutamate derivatives, FLG), a fluorescent molecule [(rhodamine B, RhB) or (2',7'-dichlorofluorescein sodium salt, DCF)], and a photochromic molecule [1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene, DAE], thus constructing photomodulated switchable CPL soft materials. It was found that FLG could form supramolecular gel in ethanol and self-assemble into left-handed twisted nanostructures. During the formation of a co-gel with RhB (or DCF) and DAE, the chirality of FLG could be effectively transferred to both the fluorescent and photochromic components, which induced them with chiroptical properties including CPL and circular dichroism (CD). DAE undergoes a reversible transition between the achromatous open state and the dark purple closed state in the co-gel under alternating irradiation with UV and visible light. During such a process, an intermolecular Förster resonance energy transfer (FRET) behavior from fluorescent RhB to ring-closed DAE caused the emission quenching of RhB, which led to CPL silence of RhB in the co-gel. Subsequent irradiation with visible light caused the restoration of the emission and CPL activity with the restored open state. These changes could be repeated many times upon alternate UV and visible irradiation. Therefore, a reversible CPL switch was fabricated in supramolecular gels through the photomodulated FRET process.

Not by Serendipity: Rationally Designed Reversible Temperature-Responsive Circularly Polarized Luminescence Inversion by Coupling Two Scenarios of Harata-Kodaka's Rule

Intelligent control over the handedness of circularly polarized luminescence (CPL) is of special significance in smart optoelectronics, information storage, and data encryption; however, it still remains a great challenge to rationally design a CPL material that displays reversible handedness inversion without changing the system composition. Herein, we show this comes true by coupling the two scenarios of Harata-Kodaka's rule on the same supramolecular platform of crystalline microtubes self-assembled from surfactant-cyclodextrin host-guest complexes. Upon coassembling a linear dye with its electronic transition dipole moment outside of the cavity of β-CyD, the chirality transfer from the induced chirality of SDS in the SDS@2β-CyD microtubes to the dye generates left-handed CPL at room temperature. Upon elevating temperature, the dye forms inclusion complex with β-CyD, so that right-handed CPL is induced because the polar group of the dye is outside of the cavity of β-CyD. This process is completely reversible. We envision that host-guest chemistry would be very promising in creating smart CPL inversion materials for a vast number of applications.

Near-infrared light-induced photoisomerization and photodissociation of a chiral fluorescent photoswitch in cholesteric liquid crystals assisted by upconversion nanoparticles

Upconversion-luminescence-induced reflective color switching and fluorescence tuning of a cholesteric liquid crystal (CLC) cells were investigated. The CLC system was constructed by co-doping a chiral fluorescence photoswitch, switch 5, and upconversion nanoparticles (UCNPs) into nematic LC media. Under irradiation with 980 nm NIR light, the UCNPs emit both 450 nm blue light and 365 nm UV light to induce the simultaneous Z-to-E and E-to-Z photoisomerization of switch 5. This continuous rotation-inversion movement further leads to an irreversible photoisomerization and photodissociation of dicyanodistyrylthiophene moieties in switch 5. As a result, the reflective color of the CLC cell changed from blue to red and the fluorescence intensity decreased as well when exposed to 980 nm NIR light. Finally, optically written reflective-photoluminescent dual mode CLC cells were further demonstrated.

pH-influenced handedness inversion of circularly polarized luminescence

Supramolecular co-assemblies between tolane-derived Phe–Phe dipeptides and 1,2-diaminoethane were fabricated, and CPL handedness inversion was achieved by regulating the pH value.

Dual Stimuli-Responsive High-Efficiency Circularly Polarized Luminescence from Light-Emitting Chiral Nematic Liquid Crystals

Considerable luminescence dissymmetry factor (g_lum) is vital for application implementation of circularly polarized luminescence (CPL) materials. Moreover, a dual CPL switch has promising prospects in high-security encryption and sensor devices. Herein, we designed and synthesized an emissive chiral nematic liquid crystal (N*-LC) by doping a luminescent chiral additive (NO₂-CS-C₆-Chol) into a nematic liquid crystal (5CB). The helical assembly structure produced by inducing the formation of N*-LC endows the prepared emissive N*-LC with a larger g_lum value. With the increase of the doping concentration from 1 to 10 wt %, the helical pitch (P) of N*-LC gradually decreases from 25.48 to 3.92 μm. The corresponding g_lum value increases first, reaches the maximum value (-0.38) at 6 wt %, and then decreases slightly. Further, the prepared emissive N*-LC doped with 6 wt % NO₂-CS-C₆-Chol is injected into an indium-tin oxide (ITO)-coated LC cell, to which a direct current (DC) electric field is applied. The g_lum value can be repeatedly shuttled between the "on" and "off" state by adjusting the applied voltage. Meanwhile, owing to the inherent thermal dependence of the liquid crystal phase structure, the g_lum value can also be switched between the on and off state by regulating the temperature. Therefore, an electrically controlled and thermocontrolled dual CPL switching device is successfully constructed.

Right-/left-handed helical G-quartet nanostructures with full-color and energy transfer circularly polarized luminescence

Right (R)- and left (L)-handed helical G-quartet nanostructures were synthesized for the first time simultaneously via the self-assembly of 5'-guanosine monophosphate (GMP), the helical handedness of which is well regulated by metal ions. These g-nanostructures were further applied as circularly polarized luminescence (CPL) templates to realize full-color R-/L-CPL and Förster resonance energy transfer CPL. The glum value reached 10-2, indicating their excellent template function for CPL materials design and application.

Photonic Multishells Composed of Cholesteric Liquid Crystals Designed by Controlled Phase Separation in Emulsion Drops

Cholesteric liquid crystals (CLCs), also known as chiral nematic LCs, show a photonic stopband, which is promising for various optical applications. In particular, CLCs confined in microcompartments are useful for sensing, lasing, and optical barcoding at the microscale. The integration of distinct CLCs into single microstructures can provide advanced functionality. In this work, CLC multishells with multiple stopbands are created by liquid-liquid phase separation (LLPS) in a simple yet highly controlled manner. A homogeneous ternary mixture of LC, hydrophilic liquid, and co-solvent is microfluidically emulsified to form uniform oil-in-water drops, which undergo LLPS to form onion-like drops composed of alternating CLC-rich and CLC-depleted layers. The multiplicity is controlled from one to five by adjusting the initial composition of the ternary mixture, which dictates the number of consecutive steps of LLPS. Interestingly, the concentration of the chiral dopant becomes reduced from the outermost to the innermost CLC drop due to uneven partitioning during LLPS, which results in multiple stopbands. Therefore, the photonic multishells show multiple structural colors. In addition, dye-doped multishells provide band-edge lasing at two different wavelengths. This new class of photonic multishells will provide new opportunities for advanced optical applications.

Near-infrared light-controlled circularly polarized luminescence of self-organized emissive helical superstructures assisted by upconversion nanoparticles

The reversible switching of circularly polarized luminescence in a self-organized emissive helical superstructure using 980 nm NIR excitation light with different power intensities is reported for the first time.

Supramolecular chiroptical switches

Recent progress in chiroptical switches including on/off, amplification, and inversion of the chiral signals such as ECD and CPL in supramolecular assemblies is shown.