Induced Absolute Configuration of Achiral Tetradentate Ligands in Metal–Organic Frameworks for Circularly Polarized Luminescence

Supramolecular Nanohelix Fabricated by Pillararene-Based Host–Guest System for Chirality Amplification, Transfer, and Circularly Polarized Luminescence in Water

The construction of circularly polarized luminescence (CPL) materials with high porosity and high rigidity is still challenging. Herein, we propose a chiral reticular chemistry strategy to prepare the homochiral porous metal-organic frameworks (MOFs) as CPL-active materials. Two pairs of enantiomeric MOFs are synthesized through the self-assembly of chiral D/L-cam (DL-camphorates) and achiral fluorescent ligand TPB (1,2,4,5-tetra(pyridin-4-yl)benzene). The glum values of Cd-CMOF-D and Cd-CMOF-L were up to 0.010 and 0.009; the high glum values could be compared to those of the partially pure multicomponent self-assembly systems obtained by the complicated process. We further trace the generation and transfer of the hierarchical chirality from chiral molecule to 3D framework, demonstrating that the CPL was dominated by the original molecular chirality rather than the global chirality of the hierarchical structure. Moreover, the single-phase white-light materials with nearly ideal CIE coordinates (0.33, 0.33) were constructed through the introduction of dye emitters into Zn-CMOF (Zn-based chiral MOF). This work provided not only an insightful view of the chirality transfer and disappearance mechanism but also an efficient method for the preparation of the highly porous CPL materials.

Circularly polarized luminescence (CPL) switches have attracted widespread attention due to their potential applications in advanced information technologies. However, the design and fabrication of solid-state multiple-responsive CPL switches remain challenging. Here, through self-assembly of chiral metal-organic frameworks (MOFs) and perovskite nanocrystals (NCs), a pair of crystalline enantiomeric (P)-(+)/(M)-(-)-EuMOF⊃MAPbX3 (MA = CH3 NH3 + , X = Cl- , Br- , I- ) adducts is prepared, where the achiral MAPbBr3 perovskite NCs embedded into chiral MOFs inherit the chirality of host MOFs by host-guest EuBr and PbO coordination bonds, which is demonstrated by synchrotron-radiation-based X-ray absorption spectroscopy. The chiral adducts show enhanced photoluminescence quantum yield (PLQY), good thermal stability of CPL in air, and photoswitchable CPL properties upon altering different UV irradiation. Based on two chiral emission centers and their different characteristics, reversible CPL switches are realized upon a diversity of external stimuli, for example, chemicals (water /CH3 NH3 Br solution) or temperatures (room temperature/high temperature). Benefiting from the extraordinary stimuli-responsive and highly reversible switchable CPL, multiple information encryptions and decryptions integrated with CPL, together with a chiroptical logic gate are successfully designed. This work opens a new avenue to generally fabricate solid-state CPL composite materials and develops new applications based on switchable CPL.

Preparing multi‐color and multi‐stimuli‐responsive circularly polarized luminescence (CPL) materials and understanding the evolution of chirality through the visualized mode is still a challenge. Here, an encapsulation engineering approach of chiral metal‐organic frameworks (MOFs) is proposed to confine guest emitters to realize multi‐color and multi‐stimuli‐responsive CPL. Based on triplet‐triplet energy transfer (TTET), white CPL and near‐infrared circularly polarized room temperature phosphorescence (NIR‐CPRTP) can be obtained by introducing the pyrene derivatives. With the introduction of the guest containing vinylpyrene group, the light‐ and thermal‐responsive CPL with the signal inversion can be realized through the reversible [2+2] cycloaddition reaction between the ligand and guest triggered by visible light/ultraviolet light or heating. Furthermore, the excitation‐dependent CPL is successfully achieved with the incorporation of excited state intramolecular proton transfer (ESIPT) molecules into nanopores. Importantly, the chirality magnification can be greatly enhanced through the chiral spatial confinement, the accurate host‐guest single crystal structures of FLT@DCF‐12 and FLT@LCF‐12 provide the visualized mode to understand the mechanism of chirality transfer, amplification and responsiveness. White LED and multiple information display and encryption are further demonstrated. This breakthrough provides a new perspective to guest‐encapsulated chiral MOFs and contributes to the construction of stimuli‐responsive CPL‐active materials.

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Chirality Transfer from Chiral Mesoporous Silica to Perovskite CsPbBr ₃ Nanocrystals: The Role of Chiral Confinement

Delayed luminescence (DF), including phosphorescence and thermally activated delayed fluorescence (TADF), and circularly polarized luminescence (CPL) exhibit common and broad application prospects in optoelectronic displays, biological imaging, and encryption. Thus, the combination of delayed luminescence and circularly polarized luminescence is attracting increasing attention. The encapsulation of guest emitters in various host matrices to form host-guest systems has been demonstrated to be an appealing strategy to further enhance and/or modulate their delayed luminescence and circularly polarized luminescence. Compared with conventional liquid crystals, polymers, and supramolecular matrices, porous crystalline frameworks (PCFs) including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolites and hydrogen-bonded organic frameworks (HOFs) can not only overcome shortcomings such as flexibility and disorder but also achieve the ordered encapsulation of guests and long-term stability of chiral structures, providing new promising host platforms for the development of DF and CPL. In this review, we provide a comprehensive and critical summary of the recent progress in host-guest photochemistry via the encapsulation engineering of guest emitters in PCFs, particularly focusing on delayed luminescence and circularly polarized luminescence. Initially, the general principle of phosphorescence, TADF and CPL, the combination of DF and CPL, and energy transfer processes between host and guests are introduced. Subsequently, we comprehensively discuss the critical factors affecting the encapsulation engineering of guest emitters in PCFs, such as pore structures, the confinement effect, charge and energy transfer between the host and guest, conformational dynamics, and aggregation model of guest emitters. Thereafter, we summarize the effective methods for the preparation of host-guest systems, especially single-crystal-to-single-crystal (SC-SC) transformation and epitaxial growth, which are distinct from conventional methods based on amorphous materials. Then, the recent advancements in host-guest systems based on PCFs for delayed luminescence and circularly polarized luminescence are highlighted. Finally, we present our personal insights into the challenges and future opportunities in this promising field.

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 (glum). 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.

Chiral carbon dots (CDs) with circularly polarized luminescence (CPL) are one of the most dynamic areas of modern science. However, the design, preparation, and ambiguity mechanism of solid‐state CPL‐active CDs remains a formidable challenge. Herein, for the first time, CDs with customized chiroptical activities in the solid state, especially CPL, are transcribed from chiral metal–organic framework (CMOFs) via a bottle‐around‐ship strategy. Within these CMOFs⊃CDs assemblies, CDs inherited the chirality of the host CMOFs through host–guest interactions, which is revealed by density functional theory (DFT) simulations and experimental results, and amplified the luminescence dissymmetry factor (glum) by effective artificial chiral light‐harvesting systems. Impressively, CMOFs⊃CDs in pairs generated color‐tunable CPL and white CPL with chromaticity coordinates of (0.32, 0.32). Furthermore, benefiting from excellent processability, as luminescent coatings and 3D printing inks, a white circularly polarized light‐emitting diode, and an extended 3D model “light bulb” featuring white CPL are successfully fabricated, respectively. This strategy paves a new avenue for the synthesis and advanced application of solid‐state CPL‐active CDs‐based materials.

The fabrication of circularly polarized luminescence (CPL) active materials by self‐assembly is still in its challenge. In this work, a family of homochiral metal–organic frameworks (MOFs) and metal–organic cages (MOCs) are constructed by solvothermal subcomponent self‐assembly. These MOFs feature an eta topology with trifold helical chains, while the MOCs adopt a cubic cage structure. The chiral ligands show two distinct types of conformations: “opened” and “closed” in MOFs and MOCs, respectively. Although homochiral MOFs and MOCs show similar spectra of circular dichroism and photoluminescence with similar quantum yields and lifetimes, the MOFs yield clear CPL signals and the CPL of MOCs are silent. The turn‐on CPL in MOFs achieved by tuning the conformation of ligands and controlling self‐assembly provides a new approach for development of CPL‐active MOF materials.

Optical Properties and Applications of Crystalline Materials

Host‐guest chemistry of chiral metal‐organic frameworks (MOFs) has endowed them with circularly polarized luminescence (CPL), it is still limited for MOFs to systematically tune full‐color CPL emissions and sizes. This work directionally assembles the chiral ligands, metal sites and organic dyes to prepare a series of crystalline enantiomeric D/L‐Cd/Zn‐n MOFs (n=1~5, representing the adding amount of dyes), where D/L‐Cd/Zn with the formula of Cd 2 (D/L–Cam) 2 (TPyPE) and Zn 2 (D/L–Cam) 2 (TPyPE) (D/L‐Cam=D/L‐camphoric acid, TPyPE=4,4’,4’’,4’’’‐(1,2‐henediidenetetra‐4,1‐phenylene)tetrakis[pyridine]) were used as the chiral platforms. The framework‐dye‐enabled emission and through‐space chirality transfer facilitate D/L‐Cd/Zn‐n bright full‐color CPL activity. The ideal yellow CPL of D‐Cd‐5 and D‐Zn‐4 , with |g lum | as 4.9 × 10 −3 and 1.3×10 −3 and relatively high photoluminescence quantum yield of 40.79 % and 45.40 %, are further assembled into a white CPL light‐emitting diode. The crystal sizes of D/L‐Cd/Zn‐n were found to be strongly correlated to the types and additional amounts of organic dyes, that the positive organic dyes allow for the preparation of > 7 mm bulks and negative dyes account for sub‐20 μm particles. This work opens a new avenue to fabricate full‐color emissive CPL composites and provides a potentially universal method for controlling the size of optical platforms.

<p indent="0mm">Circularly polarized luminescence (CPL) materials, due to their special chiral luminescence properties, have attracted widespread attention in information encryption, asymmetric catalysis, optoelectronic devices, and biosensing. Traditional methods for preparing CPL materials often require complex and tedious synthesis processes. Besides, it is difficult to enhance the luminescence dissymmetry factor (<italic>g</italic>_lum) and meet practical applications. Constructing CPL materials by modulating the chiral physical environment of chromophores has the advantages of simple preparation and strong applicability, compatible with various chiral or achiral chromophores, and generally having large luminescence dissymmetry factor, providing the possibility of practical applications of CPL materials. Self-assembly, a bottom-up construction method, is a common strategy to change the chiral physical environment of chromophores. Chiral self-assembly refers to the phenomenon where assembly elements accumulate into asymmetric structures through one or more non-covalent interactions. There are three possible situations: (1) The chiral chromophore can form a chiral structure through self-assembly, and significantly enlarge CD and CPL; (2) for the achiral chromophore, it can generally assemble with other chiral materials to form a chiral assembly structure by asymmetric stacking arrangement and obtain CPL performance; (3) the completely achiral chromophore can also generate chirality and CPL performance by symmetry breaking during assembly. The non-covalent interactions usually have strong modularity and dynamic modulation of performance can be achieved through reasonable design, which is suitable for constructing the intelligent responsive CPL materials. Besides, thanks to the development of nanofabrication technology, the artificial metasurface, a top-down construction method, has also become a new method to construct efficient CPL materials by changing the chiral physical environment of chromophores. There are two mechanisms for CPL in metasurfaces: Firstly, the chiral electromagnetic field generated by the metasurfaces can change the spin state of emitted light, leading to spontaneous circularly polarized light emission; the second is that chiral metasurfaces can selectively absorb or reflect circularly polarized light of a certain chirality, thereby enhancing circularly polarized luminescence of another chirality. Artificial metasurfaces are typically composed of one or more layers of artificially designed nanostructures so the thickness of metasurfaces is always small. What’s more, it also has advantages in controlling material structures. Thanks to these advantages, CPL materials modulated by the chiral physical environment of chromophores have a lot of applications. However, there are still some problems in this kind of material, for example, the low <italic>g</italic>_lum factor in self-assembly and there is only a small number of examples of CPL metasurfaces. In this review, based on the construction strategy of CPL materials modulated by the chiral physical environment of chromophores, we introduce the relevant concepts of CPL, summarize the research progress of constructing high-performance CPL materials by self-assembly and metasurfaces, and the application of CPL materials in information encryption, biosensing, and asymmetric photopolymerization, and propose the current problems and the prospect.

Circularly polarized luminescence (CPL) is an important part in the research of modern luminescent materials and photoelectric devices. Usually, chiral molecules or chiral structures are the key factors to induce CPL spontaneous emission. In this study, a scale-effect model based on scalar theory was proposed to better understand the CPL signal of luminescent materials. Besides chiral structures being able to induce CPL, achiral ordered structures can also have a significant influence on CPL signals. These achiral structures are mainly reflected in the particle scale in micro-order or macro-order, i.e. the CPL signal measured under most conditions depends on the scale of the ordered medium, and does not reflect the inherent chirality of the excited state of the luminescent molecule. This kind of influence is difficult to be eliminated by simple and universal strategies in macro-measurement. At the same time, it is found that the measurement entropy of CPL detection may be the key factor to determine the isotropy and anisotropy of the CPL signal. This discovery would bring new opportunities to the research of chiral luminescent materials. This strategy can also greatly reduce the development difficulty of CPL materials and show high application potential in biomedical, photoelectric information and other fields.

Host-guest chemistry of chiral metal-organic frameworks (MOFs) has endowed them with circularly polarized luminescence (CPL), it is still limited for MOFs to systematically tune full-color CPL emissions and sizes. This work directionally assembles the chiral ligands, metal sites and organic dyes to prepare a series of crystalline enantiomeric D/L-Cd/Zn-n MOFs (n=1~5, representing the adding amount of dyes), where D/L-Cd/Zn with the formula of Cd2(D/L-Cam)2(TPyPE) and Zn2(D/L-Cam)2(TPyPE) (D/L-Cam=D/L-camphoric acid, TPyPE=4,4',4'',4'''-(1,2-henediidenetetra-4,1-phenylene)tetrakis[pyridine]) were used as the chiral platforms. The framework-dye-enabled emission and through-space chirality transfer facilitate D/L-Cd/Zn-n bright full-color CPL activity. The ideal yellow CPL of D-Cd-5 and D-Zn-4, with |glum| as 4.9 × 10-3 and 1.3×10-3 and relatively high photoluminescence quantum yield of 40.79 % and 45.40 %, are further assembled into a white CPL light-emitting diode. The crystal sizes of D/L-Cd/Zn-n were found to be strongly correlated to the types and additional amounts of organic dyes, that the positive organic dyes allow for the preparation of > 7 mm bulks and negative dyes account for sub-20 μm particles. This work opens a new avenue to fabricate full-color emissive CPL composites and provides a potentially universal method for controlling the size of optical platforms.