Histidine Proton Shuttle-Initiated Switchable Inversion of Circularly Polarized Luminescence

Responsive Chirality: Tailoring Supramolecular Assemblies with External Stimuli as Future Platforms for Electronic/Spintronic Materials

Supramolecular chirality is the major branch of supramolecular chemistry, which not only plays important roles in biological processes but also in synthetically designed aggregated systems. To understand the complex processing of biological systems, the only way is to design supramolecular chiral ensembles that mimic natural biomolecules such as Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA), amino acids, etc. In addition, chiral systems and self-assemblies as molecular motifs with breaking spatial inversion symmetry have been regarded as key substances in electronics and spintronics as well as in fundamental chemistry and physics. Here, in this review, our major concern is understanding modulation in spatial arrangements and packing modes under the impact of any external stimuli, which results in tailoring the handedness of resulted supramolecular chiral superstructures. We, in this review, highlighted the role of external stimuli such as solvent, chemical additives, photo exposure, etc. in altering the supramolecular chirality for their future utility as "active switches" in optoelectronic and spintronic devices and applications.

Helical Star-Shaped Bottlebrush Polymers: From Controlled Synthesis to Tunable Photoluminescence and Circularly Polarized Luminescence

The controlled synthesis of star-shaped bottlebrush polymers with tunable topologies is a challenge. However, such materials may exhibit distinct photoluminescence properties. Bottlebrush polymers have polymerization-induced emission (PIE) properties due to their aggregated side chains, and aggregation-induced emission (AIE) is also a unique luminescent property. In this work, we prepared a variety of highly active alkyne Pd catalysts and polymerized poly(L/D-lactic acid) macromonomers containing polymerizable phenylisocyanide groups as end groups to obtain a variety of topologically structured bottlebrush polymers with controllable molecular weights and narrow molecular weight distributions. Bottlebrush polymers with tetraphenyl ethylene (TPE) units as the core exhibit tunable photoluminescence and circularly polarized luminescence properties. We propose that such properties are due to the unique AIE characteristics of the TPE unit combined with the PIE characteristics of the bottlebrush polymer.

Multiple Chirality Switching of a Dye-Grafted Helical Polymer Film Driven by Acid & Base

A stimuli-responsive multiple chirality switching material, which can regulate opposed chiral absorption characteristics, has great application value in the fields of optical modulation, information storage and encryption, etc. However, due to the rareness of effective functional systems and the complexity of material structures, developing this type of material remains an insurmountable challenge. Herein, a smart polymer film with multiple chirality inversion properties was fabricated efficiently based on a newly-designed acid & base-sensitive dye-grafted helical polymer. Benefited from the cooperative effects of various weak interactions (hydrogen bonds, electrostatic interaction, etc.) under the aggregated state, this polymer film exhibited a promising acid & base-driven multiple chirality inversion property containing record switchable chiral states (up to five while the solution showed three-state switching) and good reversibility. The creative exploration of such a multiple chirality switching material can not only promote the application progress of current chiroptical regulation technology, but also provide a significant guidance for the design and synthesis of future smart chiroptical switching materials and devices.

Switchable Dual Circularly Polarized Luminescence in Through-Space Conjugated Chiral Foldamers

Organic materials with switchable dual circularly polarized luminescence (CPL) are highly desired because they can not only directly radiate tunable circularly polarized light themselves but also induce CPL for guests by providing a chiral environment in self-assembled structures or serving as the hosts for energy transfer systems. However, most organic molecules only exhibit single CPL and it remains challenging to develop organic molecules with dual CPL. Herein, novel through-space conjugated chiral foldamers are constructed by attaching two biphenyl arms to the 9,10-positions of phenanthrene, and switchable dual CPL with opposite signs at different emission wavelengths are successfully realized in the foldamers containing high-polarizability substitutes (cyano, methylthiol and methylsulfonyl). The combined experimental and computational results demonstrate that the intramolecular through-space conjugation has significant contributions to stabilizing the folded conformations. Upon photoexcitation in high-polar solvents, strong interactions between the biphenyl arms substituted with cyano, methylthio or methylsulfonyl and the polar environment induce conformation transformation for the foldamers, resulting in two transformable secondary structures of opposite chirality, accounting for the dual CPL with opposite signs. These findings highlight the important influence of the secondary structures on the chiroptical property of the foldamers and pave a new avenue towards efficient and tunable dual CPL materials.

Photocontrolled chiroptical switch based on the self-assembly of azobenzene-bridged bis-tryptophan enantiomers

Chirality dynamic tuning plays fundamental roles in chemistry, material science and biological system. Herein, a pair of azobenzene-bridged bis-tryptophan enantiomers (Azo-di-d/l-Trp) were designed and synthesized via simple reactions. With the fuel of glucono-δ-lactone (GdL), releasing protons during its hydrolysis, the alkaline solution of Azo-di-d/l-Trp gradually self-assembled into contrast chiral helical structures and displayed magnitude and mirror image of circular dichroism (CD) signals. While the chiral helices converted to CD silent nanoparticles when the azobenzene moiety isomerized from trans- to cis-form under UV irradiation. More importantly, this chiroptical switch, displaying reversible interconversion between chiral amplification and silent, can be smartly controlled via photoirradiation at various wavelengths.

Full-Color Circularly Polarized Luminescence of Supramolecular Polymers with Handedness Inversion Regulated by Anion and Temperature

Constructing full-color circularly polarized luminescence (CPL) materials with switchable handedness in the solid state is an appealing yet considerably challenging task, especially for supramolecular polymer films assembled from homochiral monomers. Herein, supramolecular polymers with full-color CPL and inverted handedness are realized through the coassembly of a homochiral cholesterol derivative (PVPCC), metal ions (Zn2+), and achiral fluorescent dyes. The obtained coassembled systems show anion-directed supramolecular chirality inversion by exchanging the anions of NO3-, ClO4-, BF4-, and Cl-. For instance, the negative CD and right-handed CPL are detected in the PVPCC/Zn(NO3)2 aggregates, which convert into positive CD and left-handed CPL after introducing Cl-, corresponding to the transformation from nanorods to nanofibers. Furthermore, the tunable CPL color and handedness inversion of the coassembly system of PVPCC/Zn(NO3)2 and achiral fluorescent dyes can be established by alternately changing the assembling temperature of 298 and 273 K. Importantly, the full-color CPL polymeric materials are then constructed by doping the PVPCC/Zn(NO3)2/dyes complexes into poly(methyl methacrylate) (PMMA) film, which maintains the handedness inversion and shows the enhanced CPL performance. The work not only deepens the understanding of chirality inversion in supramolecular chemistry but also helps to construct full-color CPL materials with switchable handedness from homochiral building blocks in materials science.

Reversible Inversion of Circularly Polarized Luminescence in a Coassembly Supramolecular Structure with Achiral Sulforhodamine B Dyes

The dynamic control of circularly polarized luminescence (CPL) has far-reaching significance in optoelectronics, information storage, and data encryption. Herein, we reported the reversible inversion of CPL in a coassembly supramolecular system consisting of chiral molecules L4, which contain two positively charged viologen units, and achiral ionic surfactant sodium dodecyl sulfate (SDS) by introducing achiral sulforhodamine B (SRB) dye molecules. The chirality of CPL in the coassemblies can be efficiently regulated and inverted by simply adjusting the amount of SRB. A series of experimental characterization, including optical spectroscopy, electron microscope, ¹H NMR, and X-ray scattering measurements, suggested that SRB could coassemble with L4/SDS to establish a new stable L4/SDS/SRB supramolecular structure through electrostatic interactions. Moreover, the negative-sign CPL could revert to the positive-sign CPL if titanium dioxide (TiO₂) nanoparticles were used to decompose SRB molecules. The evolution of the CPL inversion process could be cycled at least 5 times without a significant decline in CPL signals when SRB was refueled to the system. Our results provide a facile approach to dynamically regulating the handedness of CPL in a multiple-component supramolecular system via achiral species.

Self-Assembled Charge-Transfer Chiral π-Materials: Stimuli-Responsive Circularly Polarized Luminescence and Chiroptical Photothermic Effects

Despite significant achievements in the field of chiroptical organic materials, the full utilization of both the excited state and ground state chiroptical properties in a single supramolecular system is still rarely disclosed. Here, we report that the rational combination of the charge-transfer (CT) interaction with the spacer effect and controlled protonation of π-histidine leads to chiroptical organic π-materials with both circularly polarized luminescence (CPL) and the supramolecular chirality-directed chiroptical photothermic effect. Three pyrene-conjugated histidine derivatives with varied acyl linkers (PyHis, PyC1His, and PyC3His) were designed to coassemble with electron-deficient 1,2,4,5-tetracyanobenzene (TCNB), leading to the formation of supramolecular CT complexes with intense orange to red CPL depending on the linker length. The linker length also affected the protonation-induced CPL responsiveness of the corresponding CT assemblies. Upon protonation of the histidine moiety, PyC3His/TCNB CT assemblies exhibited an inverted CPL signal, while PyHis/TCNB pairs gave quenched CPL due to the disassembly. The protonation-controlled PyC3His/TCNB CT assemblies at varied pH values showed different chiroptical photothermic effects (CPEs) for the same incident chiral light despite the molecular chirality of PyC3His remaining unchanged, supporting an interesting supramolecular chirality-directed photothermic effect.

Chiral Inversion and Recovery of Supramolecular Luminescent Copper Nanocluster Hydrogels Triggered by Polyethyleneimine and Polyoxometalates

Construction of controllable chiroptical supramolecular luminescence systems is of great significance for developing intelligent chiral luminescence materials with precise and effective regulation and understanding chirality-switching phenomena in biological systems, which has attracted extensive attention. Because chiral metal nanoclusters (NCs) can provide facilities for the study of nanoscale chiral effects, in this study, we select chiral glutathione-stabilized copper NCs (G-SH-Cu NCs) to construct a supramolecular luminescent hydrogel with achiral branched polyethyleneimine (PEI) and polyoxometalates [Na₉(EuW₁₀O₃₆)·32H₂O, denoted as EuW₁₀]. Thus, a chiral property precise controlled system was constructed by self-assembly. Interestingly, the addition of PEI to G-SH-Cu NC solution induced the formation of luminescent hydrogels with chiral inversion, while further addition of EuW₁₀ not only enhanced the luminescence of the hydrogel but also recovered the chiroptical properties. The chiral inversion behavior is possibly ascribed to the hydrogen bond interaction/electrostatic interaction between G-SH-Cu NCs and PEI in the chiral inversion process, while the competition of hydrogen bonding interaction (between G-SH-Cu NCs and PEI) and electrostatic interaction (between PEI and EuW₁₀) was accountable for the chiral recovery process. Manipulation of chirality inversion in the metal NC-containing coassemblies is rare, while this work establishes a feasible strategy to modulate the chiral inversion behavior of Cu NCs, which not only produces new physicochemical properties of metal NCs through synergistic behavior but also offers a feasible way to realize the potential application of chiroptical materials.

Two Cholesterol-Containing Pyrene Derivatives: Subtle Spacer Difference, Diverse Stimuli-Responsive Luminescence, Chirality, and Self-Assembly Behaviors

Two chiral molecules 1 and 2 were designed and synthesized with a pyrene moiety directly linked to a chiral cholesterol moiety and connected through a methylene spacer, respectively. Influence of the spacer on their stimuli-responsive luminescence, chirality, and self-assembly behaviors was systematically investigated. Molecules 1 and 2 had similar aggregation-induced emission enhancement (AIEE) in solution, because of carrying the same fluorescence moiety. Both molecules displayed mechanochromism (MC) property but with different color contrast, whereas only 2 showed mechanoluminescence (ML) activity. When doping in liquid crystal molecule 5CB, both molecules induced the formation of chiral nematic liquid crystals (N*-LCs) with strong circularly polarized luminescence (CPL). Molecule 2 induced single handedness signal, irrespective of doping ratios, while 1-doped N*-LCs showed an inversion of CPL signal from negative to positive upon the increase of doping ratios. Molecules 1 and 2 also self-assembled into different coassemblies with 5CB. Their distinct behaviors were attributed to the influence of the methylene spacer, which caused different molecular conformation and steric bulkiness; accordingly, it changed intermolecular interactions and molecular packing of the two molecules and led to diverse chirality and luminescence. This work provided important model molecules to better understand the molecular structure-property relationship and guide the design of novel functional molecules.

Anion-Driven Circularly Polarized Luminescence Inversion of Unsymmetrical Europium(III) Complexes for Target Identifiable Sensing

Anion-responsive sign inversion of circularly polarized luminescence (CPL) was successfully achieved by N3O6-type nona-coordinated europium(III) (Eu3+) complexes [(R)-1 and (S)-1] composed of a less-hindered unsymmetrical N3-tridentate ligand (a chiral bis(oxazoline) ligand) and three O2-chelating (β-diketonate) ligands. Here, (R)-1 exhibited a positive CPL signal (IL - IR > 0) at the 5D0 → 7F1 transition of Eu3+, which can be changed to a negative sign (i.e., IL - IR > 0 → IL - IR < 0) by the coordination of trifluoroacetic anions (CF3COO-) to the Eu3+ center. However, (R)-1 preserved the original positive CPL signal (i.e., IL - IR > 0 → IL - IR > 0) in the presence of a wide range of competing anions (Cl-, Br-, I-, BF4-, ClO4-, ReO4-, PF6-, OTf-, and SbF6-). Thus, (R)-1 acts as a smart target identifiable probe, where the CPL measurement (IL - IR) can distinguish the signals from the competing anions (i.e., IL - IR < 0 vs IL - IR > 0) and eliminate the background emission (i.e., IL - IR = 0) from the background emitter (achiral luminescent compounds). The presented approach is also promising in terms of bio-inspired optical methodology because it enables nature's developed chiral sensitivity to use circularly polarized light for object identification (i.e., IL - IR = 0 vs | IL - IR | > 0).

Signal Inversion and Amplification of Circularly Polarized Luminescence in a Poly(phenylacetylene)-Based Composite System Assisted by Achiral PMMA

Multichannel regulable circularly polarized luminescence (CPL) is fascinating because of its fundamental and application interest. There are few reports on helical sense (P-/M-helix) modulation and chiral signal amplification of polyacetylenes with the assistance of achiral polymers and further applications in precisely and conveniently regulating CPL handedness and magnitude. Herein, a helical poly(phenylacetylene)-based CPL-active system was constructed, in which CPL inversion occurred by adding achiral PMMA into a helical poly(phenylacetylene)-CHCl₃ solution. Significantly, there is almost 10 times magnification of luminescence dissymmetry factor values (|g_lum|) during this process. The above phenomena could be ascribed to the PMMA-assisted polyene backbone elongation and the formation of a more ordered helical structure for the poly(phenylacetylene)s. More interestingly, the CPL signal can be facilely inverted and switched by simply changing the thickness of the PPhAD/PMMA layer. The temperature-driven dynamic CPL handedness inversion and magnitude modulation can also be achieved. Based on the multiple regulations for CPL, logic operations were developed, and the practical application is further facilitated by designing various CPL patterns. This study establishes effective and convenient strategies to switch the handedness, magnitude, and wavelength of CPL, which may generate a breakthrough in the manufacturing of CPL-active smart materials and devices with promising application potential.

Helically Chiral Hybrid Cyclodextrin Metal-Organic Framework Exhibiting Circularly Polarized Luminescence

Three achiral polycyclic aromatic fluorophores─namely, 1-pyrenecarboxylic acid, 9-anthracenecarboxylic acid, and perylene-3,9-dicarboxylic acid─were chosen based on their desired properties before being incorporated into the construction of a K⁺-carrying gamma-cyclodextrin (γ-CD)-based metal-organic framework (CD-MOF-1) and γ-CD-containing hybrid frameworks (CD-HFs). Among these fluorophores, only the pyrene-carrying one shows significant noncovalent bonding interactions with γ-CD in solution. This fluorophore is encapsulated in a CD-HF with a trigonal superstructure instead of the common cubic CD-MOF-1 found in the case of the other two fluorophores. Single-crystal X-ray diffraction analysis of the trigonal CD-HF reveals a π-stacked chiral positioning of the pyrene-carrying fluorophore inside the (γ-CD)₂ tunnels and held uniformly around an enantiomorphous 3₂ screw axis along the c direction in the solid-state structure. This helix-like structure demonstrates an additional level of chirality over and above the point-chiral stereogenic centers of γ-CD and the axial chirality associated with the self-assembled π-stacked fluorophores. These arrangements result in specifically generated photophysical and chiroptical properties, such as the controlled emergence of circularly polarized luminescence (CPL) emission. In this manner, a complete understanding of the mechanism of chirality transfer from a chiral host (CD-HF) to an encapsulated achiral fluorophore has been achieved, an attribute which is often missing in the development of materials with CPL.

Stoichiometric Ratio Controlled Dimension Transition and Supramolecular Chirality Enhancement in a Two-Component Assembly System

To control the dimension of the supramolecular system was of great significance. We construct a two component self-assembly system, in which the gelator LHC18 and achiral azobenzene carboxylic acid could co-assembly and form gels. By modulating the stoichiometric ratio of the two components, not only the morphology could be transformed from 1D nanaotube to 0D nanospheres but also the supramolecualr chirality could be tuned. This work could provide some insights to the control of dimension and the supramolecular chirality in the two-component systems by simply modulating the stoichiometric ratio.

Multistimuli-Responsive and Antifreeze Aggregation-Induced Emission-Active Gels Based on CuNCs

Multistimuli-responsive fluorescent gelsbased small molecular gelator by supramolecular assembly, possessing excellent dynamic and reversible characteristic, have caused much concern. In this article, aggregation-induced emission-active fluorescence gels (AIE-gels) with chirality were developed by combining Cu nanoclusters (CuNCs) and natural amino acids, l-tryptophan (l-Trp) or d-Tryptophan (d-Trp). In DMSO/H₂O mixed solvents, CuNCs can self-assemble to form intertwined fibersbased nanoparticles with numerous pores by introducing Zn²⁺. Fibers as second networks of heteronetwork structures are characterized with the participation of l-Trp or d-Trp for cross-linking to reinforce mechanical strength and chiral regulation of gel networks. Aggregation-induced emission enhancement (AIEE) of CuNCs endows the gels with excellent fluorescent properties by introducing solvents and gelation process. The fluorescent gels exhibit sufficient fluorescence intensity (FI) at -20 °C to -80 °C and possess sensitive responsibility including gel-sol transition and fluorescence behavior for stimuli of mechanical force, heating, pH, H₂O₂, and ethylene diamine tetraacetic acid (EDTA).

Tuning Rheological Behaviors of Supramolecular Aqueous Gels via Charge Transfer Interactions

Rheological properties are critical for determining real applications of supramolecular gels in various fields. Correspondingly, the modulation of gel rheology will be very important for meeting real requirements. In this aspect, a few strategies were applied to tune the rheological behaviors of supramolecular gels, but some specific interactions like charge transfer (CT) interactions were less explored at the molecular level. Herein, we report a pyrene-containing derivative of diphenylalanine as a donor gelator and naphthalenediimide or 3,5-dinitrobenzene as matching acceptor molecules. It was found that the viscoelastic properties and strength of the original gel could be tuned through addition of different acceptor molecules to the original gel with changing the ratios of the selected acceptor molecules. As a result, storage modulus was continuously adjusted over a wide range from 190,000 to 50,000 Pa by CT interactions. Furthermore, the mechanism of the CT-induced change in rheological properties was understood and clarified through relevant techniques (e.g., UV-Vis, fluorescence, and FT-IR spectroscopy and TEM). The findings in this work would provide a novel strategy to modulate the rheological properties of supramolecular gels for adaption to broader fields of real applications.

Solvent-regulated chiral exciton coupling and CPL sign inversion of an amphiphilic glutamide-cyanostilbene

The chiral exciton couplings within a Y-shaped amphiphilic glutamide-cyanostilbene (GCS) could be significantly biased by solvent polarity and hydration effects, which led to sign inversion of both the circular dichroism and circularly polarized luminescence of the GCS assemblies.

Solvatochromic Fluorescence of a GFP Chromophore-Containing Organogelator in Solutions and Organogels

Solvatofluorochromism, a solvation effect on the fluorescence color of an organic dye, is a property generally limited to fluid solutions. We demonstrate herein the concept of solid-state solvatofluorochromism by using an organogelator (1-SG), which consists of a solvatofluorochromic green fluorescence protein (GFP) chromophore (1) and a sugar gelator (SG). While 1-SG could be located in the liquid phase or in the fibrous solid matrix of the SG gel, our results show that the one in the solid matrix but near the liquid interface has superior fluorescence stability and quantum efficiency as well as solvatofluorochromicity than the one in the liquid phase. In addition, the phenomenon of fluorescence turn-on occurs when the gel is formed in protic solvents. These features have been applied to perform multicolor fluorescence patterning, chemical vapor sensing, data encryption and decryption, and real-time fluorescence cell imaging.

Fabrication of a chiral luminescent hydrogel from gold nanoclusters via molecular recognition

A novel supramolecular chiral hydrogel with enhanced emission was obtained by the co-assembly of achiral thiobarbituric acid-modified gold nanoclusters (TBA-AuNCs) with chiral histidine molecules. Chirality transfer from histidine to the supramolecular hydrogels was achieved through the π-π stacking and intermolecular H-bonding based on molecular recognition. This work gives a creative strategy for the building of chiral nanocluster-based materials.

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.

Inversing supramolecular chirality and boosting circularly polarized luminescence of pyrene moieties via a gel matrix

Alkyl-substituted l/d-glutamide derivatives (L/D-SG) were designed as gelators to fabricate host gel matrices. Pyrene-appended l/d-glutamide derivatives (Py-LG/Py-DG) were employed as guest luminophores to investigate chiral packing and emission behavior in gel matrices. It was found that Py-LG and Py-DG are prone to form P- and M-chirally packed assemblies in DMSO gels, respectively. However, the chiral packing was inversed, and CPL was boosted after Py-LG/Py-DG was embedded in the L/D-SG gel matrix. M-chiral packing together with left-handed excimer emission ((-)-CPL) was observed in the Py-LG immobilized L-SG gel matrix, while P-chiral packing together with right-handed excimer emission ((+)-CPL) was found in the Py-DG immobilized L-SG gel matrix. It is more interesting to find that the molecular chirality of the matrix gelator did not affect the supramolecular chirality of pyrene assemblies. Either l or a d-matrix gelator can inverse the supramolecular chirality of the pure gel, but did not follow the chirality of the matrix. It was found that the gel matrix converts intralayer pyrene-pyrene (Py-Py) packing in the pure pyrene gel to interlayer Py-Py packing, thus giving an opposite chirality. The study not only deepened our understanding of the supramolecular chirality transfer but also unveiled the effects of an inert gel matrix in regulating the chiroptical properties.

Circularly polarized luminescence from organic micro-/nano-structures

Circularly polarized light exhibits promising applications in future displays and photonic technologies. Circularly polarized luminescence (CPL) from chiral luminophores is an ideal approach to directly generating circularly polarized light, in which the energy loss induced by the circularly polarized filters can be reduced. Among various chiral luminophores, organic micro-/nano-structures have attracted increasing attention owing to the high quantum efficiency and luminescence dissymmetry factor. Herein, the recent progress of CPL from organic micro-/nano-structures is summarized. Firstly, the design principles of CPL-active organic micro-/nano-structures are expounded from the construction of micro-/nano-structure and the introduction of chirality. Based on these design principles, several typical organic micro-/nano-structures with CPL activity are introduced in detail, including self-assembly of small molecules, self-assembly of π-conjugated polymers, and self-assembly on micro-/nanoscale architectures. Subsequently, we discuss the external stimuli that can regulate CPL performance, including solvents, pH value, metal ions, mechanical force, and temperature. We also summarize the applications of CPL-active materials in organic light-emitting diodes, optical information processing, and chemical and biological sensing. Finally, the current challenges and prospects in this emerging field are presented. It is expected that this review will provide a guide for the design of excellent CPL-active materials.

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.

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.

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.