Left- and right-circularly polarized light-sensing based on colored and mechano-responsive chiral nematic liquid crystals

Circularly polarized luminescence in quantum dot-based materials

Quantum dots (QDs) have emerged as fantastic luminescent nanomaterials with significant potential due to their unique photoluminescence properties. With the rapid development of circularly polarized luminescence (CPL) materials, many researchers have associated QDs with the CPL property, resulting in numerous novel CPL-active QD-containing materials in recent years. The present work reviews the latest advances in CPL-active QD-based materials, which are classified based on the types of QDs, including perovskite QDs, carbon dots, and colloidal semiconductor QDs. The applications of CPL-active QD-based materials in biological, optoelectronic, and anti-counterfeiting fields are also discussed. Additionally, the current challenges and future perspectives in this field are summarized. This review article is expected to stimulate more unprecedented achievements based on CPL-active QD-based materials, thus further promoting their future practical applications.

Stimuli-responsive luminescence from polar cyano/isocyano-derived luminophores via structural tailoring and self-assembly

Polar cyano fragments and their isomeric isocyano counterparts have attracted great attention as stimuli-responsive luminescent materials in a wide range of fields including organic light-emitting diode devices, chemical fluorescent sensors, photoelectric semiconductors, anti-counterfeit products, etc., mainly because of their typical electron-deficient activity, noncovalent recognition ability, and variable coordination capacity. The electron-deficient and polar nature of these blocks have significant effects on the properties of the cyano/isocyano-based luminophore materials, especially concerning their condensed state-dependent electronic structures. Among them, donor-acceptor (D-A) derived unimolecular and co-assembled luminophores have attracted more attention because their large delocalized structures and noncovalent interaction recognition sites can rebuild the electronic transfer character in the aggregative state, thus endowing them with outstanding stimuli-responsive luminescent behavior via intermolecular and intramolecular charge transfer in polytropic morphologies. In this perspective paper, we give a brief introduction on stimuli-responsive organic and coordinated luminophores and the documented typical design concepts and applications in recent years. It is expected that this perspective article will not only summarize the recent developments of polar cyano/isocyano-derived luminophores and their coordination compounds via structural tailoring and self-assembly but also throw light on the future of the design of more sophisticated stimuli-responsive architectures and their versatile properties.

Introduction of a Chiral Biphenanthrene-Diol Unit to Achieve Circularly Polarized Thermally Activated Delayed Fluorescence

Circularly polarized luminescence (CPL) materials are promising candidates for future display technology. However, such highly efficient emitters suffer from the issues of difficult chiral separation and low photoluminescence quantum yield (PLQY). In this work, the chiral 4,4'-biphenanthrene-3,3'-diol (BIPOL) unit was introduced into a thermally activated delayed fluorescence (TADF) framework for the first time. We presented two series of enantiomers, R/S-o-DCzBPNCN and R/S-p-DCzBPNCN, and the synthesis of enantiopure BIPOL can be prepared via normal column chromatography. Notably, o-DCzBPNCN showed narrow singlet-triplet gap of 0.05 eV, efficient TADF, and high PLQYs of 82 % in doped films. In addition, R/S-o-DCzBPNCN exhibited high luminescence dissymmetry factor (gPL ) values of -1.94×10-2 /+1.91×10-2 in doped films. The strategy of BIPOL introduction offers a new approach to organic emitters with stereospecific synthesis, TADF, and chiroptical properties.

Circularly Polarized Light Emission from Nonchiral Perovskites Incorporated into Nanoporous Cholesteric Polymer Templates

Chiral perovskites have garnered significant attention, owing to their chiroptical properties and emerging applications. Current fabrication methods often involve complex chemical synthesis routes. Herein, an alternative approach for introducing chirality into nonchiral hybrid organic-inorganic perovskites (HOIPs) using nanotemplates composed of cholesteric polymeric networks is proposed. This method eliminates the need for additional molecular design. In this process, HOIP precursors are incorporated into a porous cholesteric polymer film, and two-dimensional (2D) HOIPs grow inside the nanopores. Circularly polarized light emission (CPLE) was observed even though the selective reflection band of the cholesteric polymer films containing a representative HOIP deviated from the emission wavelength of the 2D HOIP. This effect was confirmed by the induced circular dichroism (CD) observed in the absorbance band of the HOIP. The observed CPLE and CD are attributed to the chirality induced by the template in the originally nonchiral 2D HOIP. Additionally, the developed 2D HOIP exhibited a long exciton lifetime and good stability under harsh conditions. These findings provide valuable insights into the development and design of innovative optoelectronic materials.

A Single-Enantiomer Emitter Enabled Superstructural Helix Inversion for Upconverting and Downshifting Luminescence with Bidirectional Circular Polarization

The helical twisting tendency of liquid crystals (LCs) is generally governed by the inherent configuration of the chiral emitter. Here, we introduce the multistage inversion of supramolecular chirality as well as circularly polarized luminescence (CPL) by manipulating the ratio of single enantiomeric emitters (R-PCP) to LC monomers (5CB). Increasing the content of R-PCP from 1 wt % to 3 wt % inverted the helix of LCs from left-handed to right-handed, accompanying a CPL sign changed from positive to negative. The biaxiality of chiral emitters, as well as the steric effect of chiral-chiral and chiral-achiral interaction, were identified as the reasons for helical sense inversion. Due to the strong helical twisting power, 4 wt % R-PCP drove the photonic band gap (PBG) of chiral LCs to match up with their emission range, leading to an inversion of the CPL again with a high dissymmetry factor (≈1.2). Directly adjusting the PBG using chiral emitters is seldom achieved in cholesteric LCs. On this basis, an achiral sensitizer PtTPBP was assembled into the helical superstructure. The generation of triplet-triplet annihilation-induced upconverted CPL from R-PCP and the downshifting CPL from PtTPBP with opposite rotation was achieved in a single chiral LC system by tuning the position of the PBG.

Circularly Polarized Light Responsive Materials: Design Strategies and Applications

Circularly polarized light (CPL) with the end of optical vector traveling along circumferential trajectory shows left- and right-handedness, which transmits chiral information to materials via complicated CPL-matter interactions. Materials with circular dichroism respond to CPL illumination selectively with differential outputs that can be used to design novel photodetectors. Racemic or achiral compounds under CPL go through photodestruction, photoresolution, and asymmetric synthesis pathways to generate enantiomeric bias and optical activity. By this strategy, helical polymers and chiral inorganic plasmonic nanostructures are synthesized directly, and their intramolecular folding and subsequent self-assembly are photomodulable as well. In the aggregated state of self-assembly and liquid crystal phase, helical sense of the dynamic molecular packing is sensitive to enantiomeric bias brought by CPL, enabling the chiral amplification to supramolecular scale. In this review, the application-guided design strategies of CPL-responsive materials are aimed to be systematically summarized and discussed. Asymmetric synthesis, resolution, and property-modulation of small organic compounds, polymers, inorganic nanoparticles, supramolecular assemblies and liquid crystals are highlighted based on the important developments during the last decades. Besides, applications of light-matter interactions including CPL detection and biomedical applications are also referred.

Cellulose as an Eco-Friendly and Sustainable Material for Optical Anticounterfeiting Applications: An Up-to-Date Appraisal

The falsification of documents, currency, pharmaceuticals, branded goods, clothing, food products, and packaging leads to severe consequences. Counterfeited products can not only pose health risks to consumers but also cause substantial economic losses that can negatively impact the global markets. Unfortunately, most anticounterfeiting strategies are easily duplicated due to rapid technological advancements. Therefore, innovative and cost-effective antiforgery techniques that can offer superior multilevel security features are continuously sought after. Due to the ever-growing global awareness of environmental pollution, renewable and eco-friendly native biopolymers are garnering wide attention in anticounterfeiting applications. This review highlights the potential use of cellulose-based eco-friendly materials to combat the counterfeiting of goods. The initial section of the review focuses on the structure, properties, and chemical modifications of cellulose as a sustainable biomaterial. Further, the topical developments reported on cellulose and nanocellulose-based materials used as fluorescent security inks, films, and papers for achieving protection against counterfeiting are presented. The studies suggest the convenient use of celluose and modified cellulose materials for promising optical antiforgery applications. Furthermore, the scope for future research developments is also discussed based on the current critical challenges in the fabrication of cellulose-based materials and their anticounterfeit applications.

Cholesteric Liquid Crystalline Polyether with Broad Tunable Circularly Polarized Luminescence

Strong circularly polarized luminescence (CPL) with high purity and broad tunability was achieved in a new type of polyether-based cholesteric liquid crystalline (CLC) copolymers comprising chiral cholesteryl, nematic mesogens, and cross-linkable moieties. The phase boundary diagram of the copolymers was constructed, wherein the CLC phase in a wide composition and temperature window down to room temperature was achieved. Furthermore, reflection colors across the infrared and visible light regions can be continuously tuned by altering composition or temperature, which can be further fixed in the flexible CLC elastomer by photo-cross-linking. Introducing achiral dyes in the CLC thin films can generate strong CPL with distinct handedness and high dissymmetry factors (g_lum). Particularly, the left-handed full-color CPL is obtained by selective circularly polarized scattering in the spectral region outside the band gap of the CLC thin film, and the right-handed CPL with g_lum up to -1.05 is achieved within the band gap of the CLC thin film following the selective circularly polarized reflection mechanism. This type of CPL active material is expected to have potential applications in liquid crystal display and photonics.

Bio-Organic Chiral Nematic Materials with Adaptive Light Emission and On-Demand Handedness

Real-time active control of the handedness of circularly polarized light emission requires sophisticated manufacturing and structural reconfigurations of inorganic optical components that can rarely be achieved in traditional passive optical structures. Here, robust and flexible emissive optically-doped biophotonic materials that facilitate the dynamic optical activity are reported. These optically active bio-enabled materials with a chiral nematic-like organization of cellulose nanocrystals with intercalated organic dye generated strong circularly polarized photoluminescence with a high asymmetric factor. Reversible phase-shifting of the photochromic molecules intercalated into chiral nematic organization enables alternating circularly polarized light emission with on-demand handedness. Real-time alternating handedness can be triggered by either remote light illumination or changes in the acidic environment. This unique dynamic chiro-optical behavior presents an efficient way to design emissive bio-derived materials for dynamic programmable active photonic materials for optical communication, optical coding, visual protection, and visual adaptation.

Sequentially amplified circularly polarized ultraviolet luminescence for enantioselective photopolymerization

Chiral optical materials based on circularly polarized luminescence (CPL) have emerged rapidly due to their feasible applications in diverse fields of research. However, limited to the small luminescence dissymmetry factor (g_lum), real application examples have rarely been reported. Here, we present a complex system, which show intense circularly polarized ultraviolet luminescence (CPUVL) with large g_lum value, enabling a chiral UV light triggered enantioselective polymerization. By integrating sensitized triplet-triplet annihilation upconversion and CPL, both visible-to-UV upconversion emission and upconverted circularly polarized ultraviolet luminescence (UC-CPUVL) were obtained in the systems, built of chiral annihilator R(S)-4,12-biphenyl[2,2]paracyclophane (R-/S-TP), and a thermally activated delayed fluorescence (TADF) sensitizer. After dispersing this upconversion system into room-temperature nematic liquid crystal, induced chiral nematic liquid crystal could significantly amplify the g_lum value (0.19) of UC-CPUVL. Further, the UC-CPUVL emission has been used to trigger the enantioselective photopolymerization of diacetylene. This work paves the way for the further development of functional application of CPL active materials.

Orientation Control of Helical Nanofilament Phase and Its Chiroptical Applications

Chiral liquid crystal phases show fascinating structural and optical properties due to their inherent helical characteristics. Among the various chiral liquid crystal phases, the helical nanofilament phase, made of achiral bent-shaped molecules, has been of keen research interest due to its unusual polar and chiral properties. This review is intended to introduce the recent progress in orientation control and its application to the helical nanofilament phase, which includes topographic confinement, photoalignment, and chiroptical applications such as photonic crystal and chirality sensor.

Electric-Field-Regulated Energy Transfer in Chiral Liquid Crystals for Enhancing Upconverted Circularly Polarized Luminescence through Steering the Photonic Bandgap

Circularly polarized luminescent materials with high dissymmetry factor (g_lum ) have been attracting increasing attention due to their distinctive photonic properties. In this work, by incorporating upconversion nanoparticles (UCNPs) and CsPbBr₃ perovskite nanocrystals (PKNCs) into a chiral nematic liquid crystal (N*LC), enhanced upconverted circularly polarized luminescence (UC-CPL) based on a radiative energy transfer (RET) process from UCNPs to CsPbBr₃ PKNCs is successfully implemented. By locating the emission peak of CsPbBr₃ PKNCs at the center of the photonic bandgap of N*LC, the maximum g_lum value of UC-CPL can be amplified to an extremely large value of 1.1. Meanwhile, upconverted emission of UCNPs can be significantly enhanced due to the band edge enhancement effect of the N*LC, subsequently enhancing the emission of the CsPbBr₃ PKNCs through the RET process. In addition, an applied electric field can switch the upconverted emission of the UCNPs, as well as the RET process, enabling an electric-field-controlled UC-CPL switch.

Histidine Proton Shuttle-Initiated Switchable Inversion of Circularly Polarized Luminescence

Switchable inversion of the sign of circularly polarized luminescence (CPL) in chiral supramolecular systems has gained remarkable interest because of its role in understanding the chirality-switching phenomena in biological systems and developing smart chiral luminescent materials. Herein, inspired by the histidine proton shuttle in natural enzymes, we synthesized a histidine π-gel (PyC₃H) and realized reversible inversion of supramolecular chirality and CPL by receiving and then transferring a proton. It was found that in the course of histidine protonation by adding an external proton source, the transcription of intrinsic molecular chirality of PyC₃H to the supramolecular level biased, achieving dynamic control over the PyC₃H gel with left-handed CPL inversed into the right-handed one. The mechanism study revealed that the supramolecular chirality and CPL inversion are mainly affected by the cooperation adjustment of hydrogen bonds and π-π stacking upon histidine protonation and deprotonation, which causes the re-orientations of pyrene chromophores. This work sets up an alternative effective method to fabricate tunable CPL-active materials while using the same chiral small molecules, which provides a new insight into developing bio-inspired switchable supramolecular materials.

Multifarious Chiral Nanoarchitectures Serving as Handed-Selective Fluorescence Filters for Generating Full-Color Circularly Polarized Luminescence

A series of full-color circularly polarized luminescence (CPL)-active materials are fabricated by judiciously combining multifarious chiral nanoarchitectures with achiral fluorescence dyes. The investigated nanoarchitectures include organic polymer nanofibers, organic-inorganic hybrid nanoflowers, and inorganic nanoflowers. The as-prepared chiral nanoarchitectures all can act as handed-selective fluorescence filters to powerfully transform unpolarized fluorescent light into circularly polarized luminescence. Also notable, no interaction is required between chiral and fluorescent components for achieving CPL emission. The present study provides a convenient and universal approach for preparing full-color CPL materials. Following the strategy, numerous chiroptical materials with CPL performance can be expected due to the abundant chiral matters and achiral fluorophores.

Color-Tunable and Stimulus-Responsive Luminescent Liquid Crystalline Polymers Fabricated by Hydrogen Bonding

Luminescent liquid crystalline polymers (LLCPs) show extensive application potentials, such as liquid crystal displays and circularly polarized luminescence. In this work, we employ a hydrogen-bonding strategy different from the traditional covalent-bonding method to fabricate LLCPs. First, the acceptor and donor of hydrogen bonding, (4,4'-dibutanoxy tetraphenylethylene)-1-pyridine (PTPEC₄) and poly(2-vinyl terephthalic acid) (PPA), respectively, are successfully synthesized. Then, mixtures with different molar ratios ( x's) of PTPEC₄ to PPA are used to prepare a series of LLCPs [denoted as PPA(PTPEC₄) _x]. The resultant LLCPs show a smectic A phase ( x ≥ 0.8), a columnar nematic phase (0.6 ≤ x ≤ 0.05), and an amorphous state ( x = 0.025), depending on the x value. Meanwhile, all polymers exhibit typical aggregation-induced emission behavior. More interestingly, with the variation of the PTPEC₄ content, the series of LLCPs show different colors, that is, the emission peak red shifts from 510 nm ( x = 1.0) to 551 nm ( x = 0.025). Furthermore, because of the reversible protonation effect of the N atom of pyridine in PTPEC₄ by the strong proton acid, PPA(PTPEC₄) _x shows reversible color transformation. This work provides a new method to construct LLCPs with different emission colors and reversible color transformation.

Optically active and photoswitchable Tröger's base analogs

Azo carrying Tröger's base analogs introduced chirality, thermochromic and phototunable properties to an achiral liquid crystal host.

Strong CPL of achiral AIE-active dyes induced by supramolecular self-assembly in chiral nematic liquid crystals (AIE-N*-LCs)

AIE-N*-LCs prepared by supramolecular self-assembly between achiral AIE-active dyes and N*-LCs can exhibit highly strong CPL signals with g_em values in the range from 0.97 to 1.42. Most importantly, their emission wavelength can be tuned by changing the AIE-active dye.