Recyclable soft photonic crystal film with overall improved circularly polarized luminescence.

Existing circularly polarized luminescence materials can hardly satisfy the requirements of both large luminescence dissymmetry factor and high luminescent quantum yield, which hinders their practical applications. Here, we present a soft photonic crystal film embedded with chiral nanopores that possesses excellent circularly polarized luminescence performance with a high luminescence dissymmetry factor as well as a large luminescent quantum yield when loaded with various luminescent dyes. Benefitting from the retention of chiral nanopores imprinted from a chiral liquid crystal arrangement, the chiral soft photonic crystal film can not only endow dyes with chiral properties, but also effectively avoid severe aggregation of guest dye molecules. More importantly, the soft photonic crystal film can be recycled many times by loading and eluting guest dye molecules while retaining good stability as well as circularly polarized luminescence performance, enabling various applications, including smart windows, multi-color circularly polarized luminescence and anticounterfeiting.

Enhanced chiroptic properties of nanocomposites of achiral plasmonic nanoparticles decorated with chiral dye-loaded micelles.

The development of circularly polarized luminescence (CPL)-active materials with both large luminescence dissymmetry factor (glum) and high emission efficiency continues to be a major challenge. Here, we present an approach to improve the overall CPL performance by integrating triplet-triplet annihilation-based photon upconversion (TTA-UC) with localized surface plasmon resonance. Dye-loaded chiral micelles possessing TTA-UC ability are designed and attached on the surface of achiral gold nanorods (AuNRs). The longitudinal and transversal resonance peaks of AuNRs overlap with the absorption and emission of dye-loaded chiral micelles, respectively. Typically, 43-fold amplification of glum value accompanied by 3-fold enhancement of upconversion are obtained simultaneously when Au@Ag nanorods are employed in the composites. More importantly, transient absorption spectra reveal a fast accumulation of spin-polarized triplet excitons in the composites. Therefore, the enhancement of chirality-induced spin polarization should be in charge of the amplification of glum value. Our design strategy suggests that combining plasmonic nanomaterials with chiral organic materials could aid in the development of chiroptical nanomaterials.

Regulating the excited state chirality to fabricate high-performance-solid-state circularly polarized luminescence materials.

Developing solid-state materials and greatly improving the luminescence dissymmetry factors (g lum) are the key issues for the future oriented practical application in the field of circularly polarized luminescence (CPL). However, most of the solid-state CPL-active materials suffer from aggregation caused emission quenching and relatively small g lum values, which intensively restrict the development and application. In this work, high-performance CPL-active solid-state materials were achieved by regulating the excited state chirality of a series of bi-pyrene based chiral emitters. Due to the reversible mechanochromic luminescence under external stimuli, their excited state chirality can also be switched. It was found that the pristine amorphous powder possessed weak but obvious chiroptical properties because of the inherently chiral structures. Mechanical grinding could switch the fluorescence color and eliminate the CPL activity. Subsequently, by carrying out solvent fumigation, instant crystallization with well-defined microcrystal formation occurred, which could activate the CPL emission. Due to the chiral supramolecular arrangement of chromophores in the crystalline state, the resulting excimer emission in microcrystals showed chirality amplification not only in the excited state but also in the ground state. These findings not only provide a new method to fabricate high-performance CPL-active solid-state materials, but also clarify the chirality origin of pyrene-excimer-based chiral luminophores in various states which showed the importance of CPL as a probe of excited state chirality.

Chiral Luminescent Liquid Crystal with Multi-State-Reversibility: Breakthrough in Advanced Anti-Counterfeiting Materials.

Creating a security material that carries distinct information in reflective color, fluorescence, and chiroptical property will enhance anti-counterfeiting levels to deter counterfeits ranging from currencies to pharmaceuticals, but is proven extremely challenging. In this work, an advanced anti-counterfeiting material, with three-state of each mode reversibly converted into multi-mode materials including reflective color, fluorescence, and circularly polarized luminescence signal, is constructed by loading photofluorochromic spiropyran (SP) and zinc ion (Zn2+ ) into the chiral liquid crystal. Under UV irradiation, the complexes of SP and Zn2+ will be transformed into merocyanine (MC) and MC-Zn2+ , while the energy transfer occurs from MC-Zn2+ to MC. Upon heating, MC is easy to recover to SP, while the MC-Zn2+ remains unchanged. The MC and MC-Zn2+ can be transformed into the SP and Zn2+ under visible light irradiation. The three states of each mode can reversibly convert. Furthermore, the reflective color or fluorescence of each state shows different intensities under left- and right-handed circular polarized filters, enabling easy distinguishing by naked eyes. The advanced anti-counterfeiting method with multi-state of each mode for multi-mode encryption information output will provide a new concept for designing and fabricating multi-mode anti-counterfeiting materials, improving the security level for practical application.

Toward Large Dissymmetry Factor of Circularly Polarized Luminescence in Donor-Acceptor Hybrid Systems.

Chiroptical materials with circularly polarized luminescence (CPL) activity have aroused a lot of interest. One essential factor for evaluating the features of CPL-active materials is the dissymmetry factor (glum), which represents the circular polarization of emitted light. Essentially, for the practical application of CPL, the most important issue is to greatly improve the glum value. Recently, benefiting from the flexible and efficient design in hybrid donor-acceptor systems, more and more examples involving glum value amplification have been reported. In this Perspective, we highlight the proposed mechanism for the generation and amplification of CPL in these hybrid systems. We also present the corresponding design principles and potential pitfalls in experimental processes. We hope that this Perspective can shed light on the development of highly efficient CPL-active materials.

Halogen Bonded Chiral Emitters: Generation of Chiral Fractal Architecture with Amplified Circularly Polarized Luminescence.

Self-assembled chiroptical materials have attracted considerable attention due to their great applications in wide fields. During the chiral self-assembly, it remains unknown how achiral molecules can affect the assembly process and their final chiroptical performance. Herein, we report an achiral molecule directed chiral self-assembly via halogen bonds, exhibiting not only an unprecedented chiral fractal architecture but also significantly amplified circularly polarized luminescence (CPL). Two axially chiral emitters with halogen bond sites co-assemble with an achiral 1,4-diiodotetrafluorobenzene (F4 DIB) and well-ordered chiral fractal structures with asymmetry amplification are obtained. The enhancement of the dissymmetry factors of the assemblies was up to 0.051 and 0.011, which was approximately 100 folds than those of the corresponding molecules. It was found that both the design of the chiral emitter and the highly directional halogen bond played an important role in hierarchically chirality transfer from chiral emitters to the micrometer scale chiral fractal morphology and amplified dissymmetry factors. We hope that this strategy can give a further insight into the fabrication of structurally unique featured highly efficient chiroptical materials.

Steering Triplet-Triplet Annihilation Upconversion through Enantioselective Self-Assembly in a Supramolecular Gel.

Research on chiral selection and recognition not only is of fundamental importance in resolving the origin of biological homochirality, but also is instructive in the fabrication of controlled molecular organization in supramolecular systems to modulate their chirality-related functional properties. Here we report an enantioselective assembly process between a chiral energy donor and two enantiomeric energy acceptors, which further results in chirality-controlled energy transfer and enantioselective triplet-triplet annihilation upconversion (TTA-UC). It is found that the chiral energy donor Pd(II) octaethylporphyrin derivative PdOEP-LG12 (RD) can selectively coassemble with the chiral energy acceptor LGAn (RA) with the same chiral scaffold but tends to form segregation with the energy acceptor DGAn (SA) with the opposite chiral scaffold in a thermodynamic equilibrium state. Thus, the coassembly of RA/RD shows more effective triplet-triplet energy transfer (TTET) and stronger upconverted luminescence and upconverted circularly polarized luminescence in comparison to the segregation of SA/RD. The establishment of such an enantioselective TTA-UC system highlights the applications of chirality-regulated triplet fusion in optoelectronic materials.

Photofluorochromic water-dispersible nanoparticles for single-photon-absorption upconversion cell imaging.

Photofluorochromic diarylethene (DAE) molecules have been widely investigated due to their excellent fatigue resistance and thermal stability. However, the poor water solubility of DAEs limits their biological applications to some extent. Herein, we reported two kinds of water-dispersible DAE nanoparticles (DAEI-NPs and DAEB-NPs), in which DAE molecules were stabilized by the amphiphilic polymer DSPE-mPEG2000 using the nanoprecipitation approach. The fabricated nanoparticles retain well-controlled luminescence and fluorescence photoswitching properties in aqueous solution, which could be reversibly switched on and off under the alternating irradiation of ultraviolet (UV) and visible light. In addition, the closed-ring isomers of DAEB-NPs performed hot-band-absorption-based photon upconversion when excited by a 593.5 nm laser. Bearing excellent photophysical properties and low cytotoxicity, DAEB-NPs were applicable for upconversion cell imaging without high-excitation power density and free from oxygen removal. Additionally, the imaging process could be switched on by regulating the photofluorochromic nanoparticles.

Multi-Light-Responsive Upconversion-and-Downshifting-Based Circularly Polarized Luminescent Switches in Chiral Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have attracted tremendous attention for several novel applications. However, functional MOFs with light-responsive circularly polarized luminescence (CPL) are not examined in detail. Therefore, a dual CPL switch exhibiting both upconversion (UC) and downshifting (DS) CPL in the solid state is constructed by loading a luminescent diarylethene derivative (DAEC) and UC nanoparticles (UCNPs) into chiral MOFs. The chiral MOF⊃DAEC composites exhibit both photoswitchable luminescence and DS-CPL properties under alternating UV and visible light irradiation. Additionally, a reversible UC-CPL switch is realized using near-infrared (NIR) and visible light irradiation by introducing energy-level-matched UCNPs and DAEC into the chiral MOFs. The dissymmetry factor (glum ) of UC-CPL is noted to be significantly amplified through energy transfer compared to that of DS-CPL, which indicates that the information on circular polarization can be manipulated by altering the incident light. A chiroptical logic circuit with a 2D information output is designed with UV, visible, and NIR light as inputs by setting a rational threshold.

Switching Photon Upconversion by Using Photofluorochromic Annihilator with Low-Lying Triplet.

Photon upconversion based on triplet-triplet annihilation (TTA-UC) has attracted great attention due to its remarkable features including the high upconversion quantum yield, low threshold, and flexible combination of sensitizer and annihilator. Endowing TTA-UC with responsiveness will offer additional application dimensions; however, it is a challenge to develop annihilators with responsive features in the excited triplet state. Here we demonstrate the synthesis and photophysical behaviors of photofluorochromic annihilators derived from fluorescent diarylethenes. A series of turn-on mode fluorescent diarylethenes based on 1,2-bis(2-ethyl-1-benzothiophen-1,1-dioxide-3-yl)perfuorocyclopentene were synthesized, and their photochromism and photofluorochromism behaviors were thoroughly investigated. When sensitized by near-infrared ruthenium phthalocyanine, TTA-UC could be observed under excitation of 730 nm, accompanied by upconverted emission ranging from 500 to 700 nm. Because of the photoresponsive properties of the annihilators, TTA-UC can be switched between "on" and "off" by alternating irradiation of ultraviolet and visible light.

Improving the Overall Properties of Circularly Polarized Luminescent Materials Through Arene-Perfluoroarene Interactions.

A major trade-off in the field of circularly polarized luminescence (CPL) of pure organic materials is that the large luminescence dissymmetry factor (glum ) usually gives rise to the suppression of luminescence efficiency (ΦPL ). Here, a supramolecular self-assembled system, driven by arene-perfluoroarene (AP) interactions of chiral polycyclic aromatic hydrocarbons (PAHs) and octafluoronaphthalene (OFN), is reported to provide a solution to this problem. Two kinds of chiral PAHs based on pyrene and anthracene could co-assemble with OFN in hybrid solvents to form long-range-ordered AP assemblies. The detailed process of AP interaction driving self-assembly was verified by morphological measurements and fluorescence spectra. The AP assemblies exhibited chirality amplification not only in the excited state but also in the ground state. In addition, the AP assemblies showed an enhanced luminescence efficiency compared with the individual chiral PAHs due to the energy-barrier effect of OFN. The present strategy based on AP interactions could be applied to boost the development of highly efficient CPL-active materials.

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 (glum), real application examples have rarely been reported. Here, we present a complex system, which show intense circularly polarized ultraviolet luminescence (CPUVL) with large glum 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 glum 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.

New Perspectives to Trigger and Modulate Circularly Polarized Luminescence of Complex and Aggregated Systems: Energy Transfer, Photon Upconversion, Charge Transfer, and Organic Radical.

Chiral functional materials with circularly polarized luminescence (CPL) have risen rapidly in recent years because of their fascinating characteristics and potential applications in various research fields. CPL refers to the differential spontaneous emission of left (L)- and right (R)-handed circularly polarized light upon photon or electron excitation. Generally, an outstanding CPL-active material needs to possess a high luminescence dissymmetry factor (glum) (defined as 2(IL - IR)/(IL + IR) where I is the emission intensity), which is between -2 and +2. Although the exciting development in CPL-active materials was achieved, the modulation of CPL signs is still a challenge. For small organic systems, a relatively small glum value, one of the key parameters of CPL, limits their practical applications. Searching for efficient approaches for amplifying glum is important. Therefore, over the past decades, besides optimizing the structure of small molecules, many other strategies to obtain efficient CPL-active materials have been developed. For instance, self-assembly has been well demonstrated as an effective approach to amplify the supramolecular chirality as well as the glum values. On the other hand, chiral liquid crystals (CLCs), which are capable of selective reflection of left- and right-handed circularly polarized light, also to serve as a host matrix for endowing guest emitters with CPL activity and high glum values. However, self-assembly focuses on modulating the conformation and spatial arrangement of chiral emitters. And the CPL of a luminophore-doped CLC matrix depends on the helix pitch and band gap positions. Lately, novel photophysical approaches to modulate CPL signs have gradually emerged.In this Account, we discuss the recent progress of excited-state-regulation involved CPL-active materials. The emergence, amplification, and inversion of CPL can be adjusted through regulation of the excited state of chiral emitters. For example, Förster resonance energy transfer (FRET) can amplify the glum values of chiral energy acceptors in chiral supramolecular assemblies. By combining the concepts of photon upconversion and CPL, high-energy upconverted circularly polarized emission was achieved under excitation of low-energy light, accompanied by an amplified glum. In addition, the organic systems with unpaired electrons, i.e., charge transfer (CT) system and open-shell π-radical, show favorable CPL properties, which can be flexibly tuned with an applied magnetic field. It should be noted that these photophysical process are associated with the excited state of chiral emitters. So far, while the main focus is on the regulation of the molecular and supramolecular nanostructures, direct regulation of the excited state of the chiral system will serve as a new platform to understand and regulate the CPL activity and will be helpful to develop smart chiroptical materials.

Dual-Mode Induction of Tunable Circularly Polarized Luminescence from Chiral Metal-Organic Frameworks.

The general approach for fabricating solid-state materials showing circularly polarized luminescence (CPL) is still in its challenge. In this work, chiral metal-organic frameworks (MOFs) with full-color and white-color circularly polarized light emission are firstly achieved through a host-guest emitter-loading strategy. Chiral zeolitic imidazolate frameworks (ZIFs, a class of MOFs) are fabricated by a facile and simple mixed-ligand coassembly pathway. Meantime, achiral dyes, quantum dots (QDs), and upconversion nanoparticles (UCNPs) are easily loaded into the chiral ZIFs during the synthetic process. Size-matched dyes can be solely encapsulated into the chiral cages of ZIF, resulting in induced CPL and enhanced luminescence efficiency in solid-state ZIF⊃dye composites. Large-sized QDs, after embedding into the gap of the ZIF particles, also exhibited intense CPL activity. Furthermore, through modulating the blending ratio of colored dyes or QDs in chiral ZIFs, white light-emitting ZIFs with circular polarization could be constructed in a solid state. In addition, through loading rare earth element-based upconversion nanoparticles (UCNPs) into chiral ZIFs, upconverted CPL (UC-CPL) could be achieved with a high dissymmetry factor (g lum). Thus, various achiral luminophores were endowed with CPL upon coupling with chiral ZIFs, which significantly deepened and enlarged the research scope of the chiroptical materials in a solid state.

Circularly Polarized Luminescence in Nanoassemblies: Generation, Amplification, and Application.

Currently, the development of circularly polarized luminescent (CPL) materials has drawn extensive attention due to the numerous potential applications in optical data storage, displays, backlights in 3D displays, and so on. While the fabrication of CPL-active materials generally requires chiral luminescent molecules, the introduction of the "self-assembly" concept offers a new perspective in obtaining the CPL-active materials. Following this approach, various self-assembled materials, including organic-, inorganic-, and hybrid systems can be endowed with CPL properties. Benefiting from the advantages of self-assembly, not only chiral molecules, but also achiral species, as well as inorganic nanoparticles have potential to be self-assembled into chiral nanoassemblies showing CPL activity. In addition, the dissymmetry factor, an important parameter of CPL materials, can be enhanced through various pathways of self-assembly. Here, the present status and progress of self-assembled nanomaterials with CPL activity are reviewed. An overview of the key factors in regulating chiral emission materials at the supramolecular level will largely boost their application in multidisciplinary fields.

Amplifying Dissymmetry Factor of Upconverted Circularly Polarized Luminescence through Chirality-Induced Spin Polarization in the Photon Upconversion Process.

A general phenomenon about upconverted circularly polarized luminescence (UC-CPL) based on triplet-triplet annihilation (TTA) was realized in an ambient environment by coupling three kinds of chiral acceptors with corresponding achiral sensitizers. All of the dissymmetry factors of UC-CPL exhibited significant amplification compared with the prompt CPL of the used chiral acceptors. Chirality-induced spin polarization during the TTA-UC process was in charge of the amplified dissymmetry factor of UC-CPL. Chirality-induced spin-polarized triplet excitons will suppress the TTA efficiency because the spin-polarized electrons go against the electron exchange within triplet excitons. However, the chirality-induced spin-polarized singlet excitons resulting from the TTA process can be promoted, enabling a large dissymmetry factor of UC-CPL.

Circularly polarized luminescence of achiral open-shell π-radicals.

Stable luminescent π-radicals with doublet emission have attracted growing attention for functional molecular materials. However, their chiroptical properties, particularly their doublet emission-based circularly polarized luminescence, have never been investigated. Here, we investigate the circularly polarized luminescence (CPL) properties of a series of achiral luminescent open-shell π-radicals through various chirality regulation approaches, including induction by a magnetic field, supramolecular coassembly and chiral liquid crystal encapsulation.

A general equilibrium approach to pricing volatility risk.

This paper provides a general equilibrium approach to pricing volatility. Existing models (e.g., ARCH/GARCH, stochastic volatility) take a statistical approach to estimating volatility, volatility indices (e.g., CBOE VIX) use a weighted combination of options, and utility based models assume a specific type of preferences. In contrast we treat volatility as an asset and price it using the general equilibrium state pricing framework. Our results show that the general equilibrium volatility method developed in this paper provides superior forecasting ability for realized volatility and serves as an effective fear gauge. We demonstrate the flexibility and generality of our approach by pricing downside risk and upside opportunity. Finally, we show that the superior forecasting ability of our approach generates significant economic value through volatility timing.

Enhanced Circularly Polarized Luminescence in Emissive Charge-Transfer Complexes.

Achieving a large dissymmetry factor (glum ) is a challenge in the field of circularly polarized luminescence (CPL). A chiral charge-transfer (CT) system consisting of chiral electron donor and achiral electron acceptor shows bright circularly polarized emission with large glum value. The chiral emissive CT complexes could be fabricated through various approaches, such as grinding, crystallization, spin coating, and gelatinization, by simply blending chiral donor and achiral acceptor. The structural synergy originating from π-π stacking and strong CT interactions resulted in the long-range ordered self-assembly, enabling the formation of supramolecular gels. Benefiting from the large magnetic dipole transition moment in the CT state, the CPL activity of CT complexes exhibited large circular polarization. Our design strategy of the chiral emissive CT complexes is expected to help the development of new molecular engineering strategies for designing highly efficient CPL-active materials.

Enhanced Circularly Polarized Luminescence from Reorganized Chiral Emitters on the Skeleton of a Zeolitic Imidazolate Framework.

A chiral zeolitic imidazolate framework (ZIF) showing circularly polarized luminescence (CPL) has been successfully constructed by blending binapthyl-derived chiral emitters with ZIF-8 rhombic dodecahedron nanoparticles. This approach solves a major trade-off in CPL-active materials: the large luminescence dissymmetry factor (glum ) always suffers from suppression of luminescence efficiency. Compared to the optical properties of chiral emitters, the obtained chiral ZIF nanomaterials showed an enhanced fluorescence efficiency while the |glum | value is significantly amplified by one order of magnitude. Additionally, enantioselective fluorescence sensing in response to α-hydroxycarboxylic acids has been enhanced in chiral ZIFs. Reorganization and conjunction of chiral emitters to the skeleton of ZIF nanoparticles can greatly improve both the luminescence quantum yield and circularly polarization, which facilitates the design of more efficient chiroptical materials.