Evolving Fluorophores into Circularly Polarized Luminophores with a Chiral Naphthalene Tetramer: Proposal of Excimer Chirality Rule for Circularly Polarized Luminescence

Controlling Circularly Polarized Luminescence Enabled by Chirality and Energy Transfer from Optimized Chiral Molecules to Quantum Dots

Chirality transfer/amplification and energy transfer in artificial assembly systems are longstanding challenges inspired by biological systems. In particular, controlling intercomponent interactions to engineer CPL-active materials in multicomponent chiral coassemblies remains difficult. Here, we report the simultaneous realization of chirality and energy transfer in self-assembled hybrid systems comprising quantum dots with high photoluminescence quantum yield and organic molecules featuring aggregation-induced emission (AIE). Three chiral AIE molecules based on the tetraphenylethylene core, differing in the number and/or position of alkyl chains, were designed and synthesized. These structural variations led to distinct self-assembled morphologies, including helical fibrils, fibril bundles, and ribbons. Co-assembly with luminescent CdSe/CdS nanorods yielded organic-inorganic hybrid nanocomposites, where chirality transfer from the organic molecules to the nanorods strongly depended on the molecular structure. In parallel, efficient energy transfer from chiral donors to the nanorod acceptors was observed across all systems. Notably, the synergistic action of both chirality and energy transfer enabled the construction of CPL-active materials with high luminescence asymmetry factors. This work presents a strategy for designing chiroptical systems with enhanced chiroptical performance.

Nanoaggregation-Enhanced and Inverted Circularly Polarized Luminescence in Isomeric Schiff Base Bis(boron difluoride) Complexes

To elucidate the emergence and amplification of circularly polarized luminescence (CPL) in chiral molecules and their assemblies, we designed positionally isomeric chiral V-shaped Schiff base ligands and their corresponding bis(boron difluoride) complexes. The ligands were synthesized by condensing chiral cyclohexanediamine with either 1-hydroxy-2-naphthaldehyde or 2-hydroxy-1-naphthaldehyde, followed by complexation with boron difluoride (BF2) to yield CNB1 and CNB2, respectively. While the parent Schiff bases exhibited no CPL, BF2 coordination induced strong CPL signals in solution. Remarkably, upon aggregation in a mixed solvent system, CNB2 displayed enhanced and inverted CPL, whereas its isomer CNB1 showed attenuated emission. Through comprehensive characterization and singlecrystal analysis, we attribute this divergent behavior to distinct molecular packing modes: CNB2 forms tightly stacked aggregates with efficient π─π interactions, while CNB1 adopts a less ordered arrangement. This study establishes a clear correlation between subtle structural modifications, supramolecular packing, and CPL performance, offering a rational strategy for tailoring chiroptical properties through precise molecular design and controlled self-assembly.

Hierarchical chirality conversion switched by biaxial halogen bonding

Precise control over chirality across hierarchical levels of peptides enriches the toolbox of peptide-based chiroptical materials. In this work, we report a controlled conversion from folded molecular chirality to supramolecular chirality, which can be effectively switched by biaxial halogen bonds. Amino acid segments were conjugated to the bipyridine core as a β-turn to facilitate the formation of parallel β-sheet arrays, which enables intramolecular chirality transfer to the appended pyrenes. This allows for efficient excimer emission and boosted circularly polarized luminescence with a dissymmetry factor of up to 10-2. The folding structures are self-assembled into nanohelices where supramolecular chirality is dominated by emerged heterochirality in response to the solvent medium. The introduction of hypervalent iodine(iii) species binds to the peptide via strong biaxial I⋯N halogen bonds that transform well-ordered helices into amorphous nanoparticles. The biaxial halogen bonding complexation rearranges the assembly with extremely high efficacy, suppressing the supramolecular chirality and recovering the folded molecular chirality, when the chiroptical activities are rationally manipulated. This work unveils the dynamic conversion of artificially folded peptides, which will inspire the design of protocols for peptide- or protein-based chiroptical materials.

Rigidly Locked Pyrene Excimers in Planar Chiral Pyrenophanes for Intense and Stable Circularly Polarized Photoluminescence and Electrochemiluminescence

Aiming at the construction of novel platforms with excellent performances in both circularly polarized photoluminescence (CP-PL) and electrochemiluminescence (CP-ECL), a new family of pyrenophanes with rigidly locked pyrene dimers and varied bridges has been designed and synthesized. Attributed to densely packed pyrene excimers, the resultant pyrenophanes revealed tunable bridge-dependent emission behaviors, as investigated by femtosecond time-resolved transient absorption spectroscopy. More importantly, all these planar chiral pyrenophanes display strong CP-PL with large dissymmetry factor (gPL) values up to 0.034, and such excellent performances remain stable in a wide range of solvents, temperatures, and concentrations. Attractively, the highest gECL values up to 0.014 reported so far has been achieved for the CP-ECL emissions of the chiral pyrenophanes. Attributed to their unique structural features and outstanding CPL performances, these novel pyrenophanes could serve as promising platforms for both fundamental investigations on pyrene excimers and practical applications such as chiral sensing.

Contrasting Mechanochromic Luminescence of Enantiopure and Racemic Pyrenylprolinamides: Elucidating Solid-State Excimer Orientation by Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) and mechanochromic luminescence (MCL) have independently made substantial progress in recent years. However, the exploration of MCL in solid-state CPL materials, which holds practical significance, is still in its infancy. Herein, we report the MCL properties of readily accessible chiral pyrenylprolinamides bearing tert-butoxycarbonyl (Boc) or 2,2,2-trichloroethoxycarbonyl (Troc) groups. Enantiopure crystals of the Boc derivative display a greater MCL wavelength shift than racemic crystals, while the Troc derivative exhibit the opposite trend. Most notably, the enantiopure crystals show mechanochromic CPL. Unlike in previous examples, where CPL is quenched upon amorphization, robust CPL spectra were observed even in the amorphous states. By applying the excimer chirality rule, we have, for the first time, acquired insights into the excited-state structures within mechanically generated amorphous states. These findings offer a novel design strategy for developing mechanochromic CPL materials, paving the way for the future advancements in this emerging field.

Enantioselective Synthesis of Atropisomeric Tri-Axis Naphthalenes via Diels-Alder Reaction and Dehydrative Aromatization of Isobenzofurans

Atropisomers with multiple stereogenic axes have attracted much attention due to their increasing significance in the fields of natural products, chiral materials, and drug discoveries. However, the catalytic stereoselective construction of axially chiral ring scaffolds with more than two axes on a single benzene ring remains a challenging task. Herein, we present an efficient method for synthesizing triaxially chiral polysubstituted naphthalene scaffolds via sequential Ni(II)-catalyzed Diels-Alder reaction of isobenzofurans and TfOH-promoted dehydrative aromatization reaction. Using 1,3-biarylisobenzofurans and β-aryl-substituted α,β-unsaturated N-acyl pyrazoles as modular reaction partners, a series of naphthalenes with 1,3,4-triaxes were synthesized with excellent enantioselectivities and diastereoselectivities. Furthermore, by attaching two pyrene chromophores to this novel triaxially chiral ring scaffold, a circularly polarized luminescence (CPL)-active dye exhibiting a remarkable luminescence dissymmetry factor (glum=-0.019) and high fluorescence quantum efficiency (ØFL=0.29) was obtained, highlighting the potential applications of atropisomers with multiple stereogenic axes in the design of chiroptical organic materials.

Cross-Transition-Dipole Stacking of Conjugated Organic Molecules: Structure, Exciton Behavior and Optoelectronic Property

Organic conjugated molecules have gained widespread application as organic semiconductors due to their unique optoelectronic properties. The rigidity of these large conjugated structures facilitates strong intermolecular interactions, which significantly influence their properties in the solid state through various molecular arrangements. The study of the relationship among molecular arrangement, exciton behavior, and optoelectronic properties is an eternal research topic. Cross-dipole stacking is a specific molecular arrangement that demonstrates unique characteristics and has received continuous attention over the past decades. This mini-review will first discuss the unique exciton behaviors in cross-dipole stacking based on exciton models, including weak exciton coupling and suppression of Förster resonance energy transfer. These exciton behaviors, determined by molecular stacking arrangements, are fundamental to the optoelectronic properties of cross-dipole stacking systems. Next, we will introduce well-defined cross-dipole systems and summarize their design principles from a molecular structure perspective. Finally, we will present the specific optoelectronic properties of cross-dipole stacking systems and their outstanding performance, such as high solid-state luminescence, good charge carrier mobility, and significant CD/CPL. Through this mini-review, we hope to enhance the understanding of cross-dipole stacking, contributing to the construction of such systems, the exploration of excited-state behaviors, and the discovery of high-performance materials.

White circularly polarized luminescence from a dual-component emitter induced by FRET between tetraphenylene and PDI derivatives

A chiral agent, TPE-ASP, incorporating aspartic acid as the chiral source and tetraphenylene derivatives as chromophores, was designed and synthesized. The chiral agent was self-assembled into regular spherical nanoparticles with a maximum luminescence asymmetry factor of |2.41 × 10-2| at 460 nm which is attributed to TPE-ASP. These nanoparticles can be co-assembled with a perylenediimide (PDI) derivative through electrostatic interactions, enabling the successful construction of a chiral light-harvesting system (C-LHS). The maximum Förster resonance energy transfer (FRET) efficiency (ΦET) of 94.7% was achieved at the optimal molar ratio of TPE-ASP to PDI. Fortunately, multicolour circularly polarized luminescence (CPL), spanning from blue to red, was successfully achieved with a two-component co-assembly system, and bright white CPL with CIE coordinates of (0.33, 0.32) was also obtained. Meanwhile, the average glum is |7.1 × 10-3| in the wavelength range of 400-700 nm. This discovery demonstrates the potential for spectral regulation through the two-component co-assembly strategy. It is significant for developing CPL devices with white light emission via the FRET process and also expands the functional range of chiral light-harvesting systems.

Microbe-assisted fabrication of circularly polarized luminescent bacterial cellulosic hybrids

The fabrications of circularly polarized luminescent (CPL) material are mainly based on the chemical and physical strategies. Controlled biosynthesis of CPL-active materials is beset with difficulties due to the lack of bioactive luminescent precursors and bio-reactors. Enlighted by microbe-assisted asymmetric biosynthesis, herein, we show the in situ bacterial fermentation of Komagataeibacter sucrofermentants to fabricate a series of bacterial cellulosic biofilms with CPL of green, orange, red, and near-infrared colors. This process can trigger CPL emission for CPL-silent glycosylated luminophores and amplify the glum of weak CPL-active luminophores up to a 10-2 scale. To confirm glycosidic bonds formation during the bacterial copolymerization process, we develop an assay utilizing the cellulase-catalyzed biodegradation of BC hybrids. More importantly, we achieve the information encryption and Fe3+ dual-channel detection based on hybrid bacterial cellulosic biofilms. Therefore, this study not only provides another vision for CPL materials preparation but also broadens the application of bacterial cellulosic hybrids.

[5]Helicene Based π-Conjugated Macrocycles with Persistent Figure-Eight and Möbius Shapes: Efficient Synthesis, Chiral Resolution and Bright Circularly Polarized Luminescence

π-Conjugated chiral shape-persistent molecular nanocarbons hold great potential as chiroptical materials, though their synthesis remains a considerable challenge. Here, we present a simple approach using Suzuki coupling of a [5]helicene building block with various aromatic units, enabling the one-pot synthesis of a series of chiral macrocycles with persistent figure-eight and Möbius shapes. Single-crystal structures of 7 compounds were solved, and 22 enantiomers were separated by preparative chiral HPLC. A notable pyrene-bridged figure-eight macrocycle, with its rigid, fully π-conjugated and overcrowded structure, exhibited pure excimer emission and outstanding circularly polarized luminescence (CPL) properties, including a large dissymmetric factor (|glum|=3.8×10-2) and significant CPL brightness (BCPL=710.5 M-1cm-1). This method provides a versatile synthetic platform for producing various chiral D2-symmetric figure-eight macrocycles and singly or triply twisted Möbius macrocycles with C2 and D3 symmetry, offering tunable chiroptical properties for CPL applications.

[(2-Dimesitylboryl)phenyl]ethynyl-Substituted [2.2]Paracyclophane Exhibiting Circularly Polarized Luminescence in Both Solution and Solid-State

Developing a new type of circularly polarized luminescent active small organic molecule that combines high fluorescence quantum yield and luminescence dissymmetric factor in both solution and solid state is highly challenging but promising. In this context, we designed and synthesized a unique triarylborane-based [2.2]paracyclophane derivative, m-BPhANPh2-Cp, in which an electron-accepting [(2-dimesitylboryl)phenyl]ethynyl group and an electron-donating N,N-diphenylamino group are introduced into two different benzene rings of [2.2]paracyclophane. Owing to the electronic effect of these two substituents, this compound can display charge-transfer emission with large Stokes shifts (∆υ = 4.23 - 8.20 × 103 cm-1) and fair quantum yields (ΦF = 0.15 - 0.37) in solutions. In addition, this compound can emit strong blue fluorescence in the solid state with quantum yields that are even much higher than in solution (ΦF up to 0.64 in powder and spin-coated film). Moreover, the enantiomeric forms of m-BPhANPh2-Cp can show strong CPL signals in both dilute solution and solid state with |glum|s up to 9.6 × 10-3 and 5.4 × 10-3, respectively. Thus, it is possible to achieve tunable CPL from blue to yellow in solution with high BCPLs ranging from 56.7 to 26.6 M-1 cm-1 and intense blue CPL combing high ΦF and |glum| in the solid state.

Role of exciton delocalization in chiroptical properties of benzothiadiazole carbon nanohoops

Development of chiral organic materials with a strong chiroptical response is crucial to advance technologies based on circularly polarized luminescence, enantioselective sensing, or unique optical signatures in anti-counterfeiting. The progress in the field is hampered by the lack of structure-property relationships that would help designing new chiral molecules. Here, we address this challenge by synthesis and investigation of two chiral macrocycles that integrate in their structure a pseudo-meta [2.2]paracyclophane with planar chirality and a highly fluorescent benzothiadiazole. Both compounds display remarkably red-shifted fluorescence with high quantum yields and large Stokes shifts. They differ in the extent of π-electron conjugation that allowed, for the first time, systematic examination of the effect of exciton delocalization on the absorption and luminescence of circularly polarized light. By a combination of steady-state spectroscopy and quantum chemical calculations, we constructed a unique structure-property relationship offering critical insights that will aid and abet the development of robust design guidelines for materials with strong electronic circular dichroism or circularly polarized luminescence of exceptional brightness.

Lighting Up Bispyrene-Functionalized Chiral Molecular Muscles with Switchable Circularly Polarized Excimer Emissions

Aiming at the further extension of the application scope of traditional molecular muscles, a novel bispyrene-functionalized chiral molecular [c2]daisy chain was designed and synthesized. Taking advantage of the unique dimeric interlocked structure of molecular [c2]daisy chain, the resultant chiral molecular muscle emits strong circularly polarized luminescence (CPL) attributed to the pyrene excimer with a high dissymmetry factor (glum) value of 0.010. More importantly, along with the solvent- or anion- induced motions of the chiral molecular muscle, the precise regulation of the pyrene stacking within its skeleton results in the switching towards either "inversed" state with sign inversion and larger glum values or "down" state with maintained handedness and smaller glum values, making it a novel multistate CPL switch. As the first example of chiral molecular muscle-based CPL switch, this proof-of-concept study not only successfully widens the application scopes of molecular muscles, but also provides a promising platform for the construction of novel smart chiral luminescent materials for practical applications.

Inversion of circularly polarized luminescence by electric current flow during transition

The development of chiral compounds exhibiting circularly polarized luminescence (CPL) has advanced remarkably in recent years. Designing CPL-active compounds requires an understanding of the electric transition dipole moment (μ) and the magnetic transition dipole moment (m) in the excited state. However, while the direction and magnitude of μ can, to some extent, be visually inferred from chemical structures, m remains elusive, posing challenges for direct predictions based on structural information. This study utilized binaphthol, a prominent chiral scaffold, and achieved CPL-sign inversion by strategically varying the substitution positions of phenylethynyl (PE) groups on the binaphthyl backbone, while maintaining consistent axial chirality. Theoretical investigation revealed that the substitution position of PE groups significantly affects the orientation of m in the excited state, leading to CPL-sign inversion. Furthermore, we propose that this CPL-sign inversion results from a reversal in the rotation of instantaneous current flow during the S1 → S0 transition, which in turn alters the orientation of m. The current flow can be predicted from the chemical structure, allowing anticipation of the properties of m and, consequently, the characteristics of CPL. This insight provides a new perspective in designing CPL-active compounds, particularly for C2-symmetric molecules where the S1 → S0 transition predominantly involves LUMO → HOMO transitions. If μ represents the directionality of electron movement during transitions, i.e., the "difference" in electron locations before and after transitions, then m could be represented as the "path" of electron movement based on the current flow during the transition.

Wide-range tunable circularly polarized luminescence in triphenylamine supramolecular polymers via charge-transfer complexation

Circularly polarized luminescence materials with broad color tunability are highly valuable for applications in 3D display and photonic technologies. Here we show that incorporating intermolecular charge-transfer complexation into chiral supramolecular polymers is an efficient strategy to achieve this objective. Adjusting the charge-transfer strength between triphenylamine donors and naphthalenemonoimide acceptors enables tunable circularly polarized luminescence signals across the visible light spectrum. This includes blue-colored emission for the supramolecular donor polymers, as well as green, yellow, orange and red-colored emission for supramolecular donor-acceptor polymers. The donor-acceptor packing modes are further influenced by the presence or absence of acetylene linkages on the triphenylamine donors, resulting in ground- or excited-state charge transfer with varying luminescent lifetimes. Additionally, white-light circularly polarized luminescence is achieved by encapsulating blue- and orange-emitting species into surfactant-based micelles in a compartmentalized manner. Overall, manipulating charge-transfer complexation in supramolecular polymers provides an effective approach to wide-range tunable circularly polarized luminescence materials.

Tunable circularly polarized luminescence behaviors caused by the structural symmetry of achiral pyrene-based emitters in chiral co-assembled systems

The regulation of circularly polarized luminescence (CPL) behavior is of great significance for practical applications. Herein, we deliberately designed three achiral pyrene derivatives (Py-1, Py-2, and Py-3) with different butoxy-phenyl substituents and the chiral binaphthyl-based inducer (R/S-B) with anchored dihedral angle to construct chiral co-assemblies, and explored their induced CPL behaviors. Interestingly, the resulting co-assemblies demonstrate tunable CPL emission behaviors caused by the structural symmetry effect of achiral pyrene-based emitters during the chiral co-assembly process. And in spin-coated films, the dissymmetry factor (gem) values were 9.1 × 10-3 for (R/S-B)1-(Py-1)10, 5.6 × 10-2 for (R/S-B)1-(Py-2)7, and 8.6 × 10-4 for (R/S-B)1-(Py-3)1, respectively. The strongest CPL emission (|gem| = 5.6 × 10-2, λem = 423 nm, QY = 34.8 %) was detected on (R/S-B)1-(Py-2)7 due to the formation of regular and ordered helical nanofibers through the strong π-π stacking interaction between the R/S-B and the achiral Py-2 emitter. The strategy presented here provides a creative approach for progressively regulating CPL emission behaviors in the chiral co-assembly process.

Achiral Solvent Inversed Helical Pathway and Cosolvent Controlled Excited-State "Majority Rule" in Enantiomeric Dansulfonamide Assemblies

Achiral solvents are commonly utilized to induce the self-assembly of chiral molecules. This study demonstrates that achiral solvents can trigger helicity inversion in the assemblies of dansyl amphiphiles and control the excited-state "majority rule" in assemblies composed of pure enantiomers, through variation of the cosolvent ratio. Specifically, enantiomers of dansyl amphiphiles self-assemble into helical structures with opposite handedness in methanol (MeOH) and acetonitrile (MeCN), together with inversed circular dichroism and circularly polarized luminescence (CPL) signals. When a mixture of MeOH and MeCN is employed, the achiral cosolvents collectively affect the CPL of the assemblies in a way similar to that of "mixed enantiomers". The dominant cosolvent governs the CPL signal. As the cosolvent composition shifts from pure MeCN to MeOH, the CPL signals undergo a significant inversion and amplification, with two maxima observed at ≈20% MeOH and 20% MeCN. This study deepens the comprehension of how achiral solvents modulate helical nanostructures and their excited-state chiroptical properties.

Access to distal biaxial atropisomers by iridium catalyzed asymmetric C-H alkylation

Distal biaxial atropisomers are typical structures in chiral catalysts and ligands and offer a wide variety of applications in biology and materials technology, but the development of efficient synthesis of these valuable scaffolds is still in great demand. Herein, we describe a highly efficient iridium catalyzed asymmetric C-H alkylation reaction that provides a range of new distal biaxial atropisomers with excellent yields (up to 99%) and stereoselectivity (up to 99% ee and essentially one isomer). Based on this unprecedented strategy, a polycyclic skeleton with five successive chiral centers as well as C-C and C-N (or N-N) two distal chiral axes was created successfully in mild circumstances. In addition, the optically pure products bearing fluorophores show circular polarized luminescence (CPL) properties, being potential candidate materials for CPL applications.

Copper-catalysed asymmetric annulation of yne-allylic esters with amines to access axially chiral arylpyrroles

The construction of atropisomers with 1,2-diaxes, while maintaining high enantiocontrol, presents a significant challenge due to the dynamic nature of steric hindrance at ortho-aryl substituents. Although various catalytic asymmetric methods have been developed for accessing axially chiral arylpyrroles, the synthesis of axially chiral arylpyrroles with 1,2-diaxes in a catalytic asymmetric manner has remained rare. Herein, the authors report the synthesis of diverse axially chiral arylpyrroles with 1,2-diaxes, and C-C and C-N axes through copper-catalysed asymmetirc [4 + 1] annulation of yne-allylic esters with arylamines via a remote stereocontrol strategy. This approach provides facile access to a broad range of heterobiaryl atropisomers (67 examples) in excellent enantioselectivities, each bearing one or two C-C/C-N axes, demonstrating its versatility and efficiency. The utility of this methodology is further highlighted by the transformation of the product into chiral phosphine ligand, and chiral thioureas for the use in asymmetric catalysis.

Synthesis and Optical Properties of Binaphthyl Derivatives with Comprehensive Introduction of Phenylethynyl Groups

In this study, compounds with phenylethynyl (PE) groups introduced at all of the possible positions of the methylene-bridged structure of the 1,1'-bi-2-naphthol backbone (3-PE to 8-PE) were synthesized. Compounds with four or six phenylethynyl groups (3,6-PE, 4,6-PE, 5,6-PE, 6,7-PE, and 3,4,6-PE) were also synthesized. The key reaction for the synthesis of these compounds was the Sonogashira reaction using halogen scaffolds. The new transformation methods include (1) selective bromination of the 5-position of the binaphthyl skeleton and (2) bromination of the 6-position and then iodination of the 4-position, followed by the Sonogashira reaction of iodine at the 4-position and lithiation and protonation of bromine at the 6-position. The optical properties of the compounds were evaluated. The extension of the π system greatly differed depending on the position of the phenylethynyl group. 4-PE, 4,6-PE, and 3,4,6-PE, in which the phenylethynyl groups were introduced in the extended direction of the naphthalene linkage axis, showed longer absorption and emission wavelengths and higher fluorescence quantum yields than the other compounds. In circularly polarized luminescence measurements, 7-PE showed a relatively large glum value, an interesting finding that reverses the sense.

Interfacial self-assembly of a chiral pyrene exciplex into a superhelix with enhanced circularly polarized luminescence

We found that the interfacially confined self-assembly of pyrene and phenanthrene glutamides can form strong exciplexes and amorphous superhelices, which show intensity-enhanced and sign-inverted CPL activity with improved quantum yield compared to a pyrene excimer. This work unveils the predominant role of supramolecular nanostructures over molecular configurations on CPL performance.

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Chiral perovskite-CdSe/ZnS QDs composites with high circularly polarized luminescence performance achieved through additive-solvent engineering

Chiral perovskite materials are being extensively studied as one of the most promising candidates for circularly polarized luminescence (CPL)-related applications. Balancing chirality and photoluminescence (PL) properties is of great importance for enhancing the value of the dissymmetry factor (glum), and a higher glum value indicates better CPL. Chiral perovskite/quantum dot (QD) composites emerge as an effective strategy for overcoming the dilemma that achieving strong chirality and PL in chiral perovskite while at the same time achieving high glum in this composite is very crucial. Here, we choose diphenyl sulfoxide (DPSO) as an additive in the precursor solution of chiral perovskite to regulate the lattice distortion. How structural variation affects the chiral optoelectronic properties of the chiral perovskite has been further investigated. We find that chiral perovskite/CdSe-ZnS QD composites with strong CPL have been achieved, and the calculated maximum |glum| of the composites increased over one order of magnitude after solvent-additive modulation (1.55 × 10-3 for R-DMF/QDs, 1.58 × 10-2 for R-NMP-DPSO/QDs, -2.63 × 10-3 for S-DMF/QDs, and -2.65 × 10-2 for S-NMP-DPSO/QDs), even at room temperature. Our findings suggest that solvent-additive modulation can effectively regulate the lattice distortion of chiral perovskite, enhancing the value of glum for chiral perovskite/CdSe-ZnS QD composites.

Circularly polarised luminescence from intramolecular excimer emission of bis-1,8-naphthalimide derivatives

Chiral excimers exhibit unique photophysical behaviour. However, further molecular design is required along with systematic studies on the effect of spacer groups and solvent polarity. In this study, we prepared four circularly polarised luminescence (CPL)-active molecules that exhibit intramolecular excimer emission. Bis-1,8-naphthalimide (bNI) derivatives D-LybNI, L-LybNI, D-LyMebNI, and L-LyMebNI were prepared with chiral backbones and alkyl linkages between the NI rings with chain lengths of five carbon atoms, suitable for excimer fluorescence. The fluorescence properties were investigated experimentally and theoretically using density functional theory. The molecules exhibited intramolecular excimer fluorescence in polar organic solvents. Mirror-image circular dichroism and CPL spectra were obtained for the D and L forms. D- and L-LyMebNI exhibited relatively large luminescence dissymmetry factors (|glum|) in acetonitrile of 1.9 × 10-3 and 1.6 × 10-3, respectively. Thus, this study demonstrates chiral bNI derivatives with simple synthesis procedures that emit intramolecular excimer fluorescence and have effective CPL properties. These molecules are promising for developing organic molecular systems with bright, highly polarised emission.

Longitudinal Extension of Double π-Helix Enables Near-Infrared Amplified Dissymmetry and Chiroptical Response

Near-infrared (NIR) circularly polarized light absorbing or emitting holds great promise for highly sensitive and precise bioimaging, biosensing, and photodetectors. Aiming at designing NIR chiral molecular systems with amplified dissymmetry and robust chiroptical response, herein, we present a series of double π-helical dimers with longitudinally extended π-entwined substructures via Ullmann or Yamamoto homocoupling reactions. Circular dichroism (CD) spectra revealed an approximate linear bathochromic shift with the rising number of naphthalene subunits, indicating a red to NIR chiroptical response. Particularly, the terrylene diimide-entwined dimers exhibited the strongest CD intensities, with the maximal |Δε| reaching up to 393 M-1 cm-1 at 666 nm for th-TDI[2]; and a record-high chiroptical response (|ΔΔε|) between the neutral and dianionic species of 520 M-1 cm-1 at 833 nm for th-TDI[2]Cl was achieved upon further reduction to its dianionic state. Time-dependent density functional theory (TDDFT) calculations suggested that the pronounced intensification of the CD spectra originated from a simultaneous enhancement of both electric (μ) and magnetic (m) transition dipole moments, ultimately leading to an overall increase in the rotatory strength (R). Notably, the circularly polarized luminescence (CPL) brightness (BCPL) reached 77 M-1 cm-1 for th-TDI[2]Cl, among the highest values reported for NIR-CPL emitters. Furthermore, all chiral dianions exhibited excellent air stability under ambient conditions with half-life times of up to 10 days in N-methylpyrrolidone (NMP), which is significant for future biological applications and chiroptic switches.

Linker-dependent control of the chiroptical properties of polymethylene-vaulted trans-bis[(β-iminomethyl)naphthoxy]platinum(II) complexes

The effects of polymethylene bridges on the chiroptical properties of trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms were examined both experimentally and theoretically using newly designed planar chiral Pt analogues (1) having three-dimensional superstructures. A series of optically pure polymethylene-vaulted Pt complexes (R)- and (S)-1 were synthesized and characterized with regard to the chiroptical behaviour of the trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms. These complexes were found to exhibit structure-dependent chiroptical characteristics in solution, such that the absolute values of specific rotation, the circular dichroism dissymmetry factor (gabs) and the circularly polarized luminescence dissymmetry factor (glum) all increased upon shortening the polymethylene bridges. Density functional theory and time dependent density functional theory calculations were used to analyse vaulted and non-vaulted complexes, which demonstrated that the present linker-dependent chiroptical properties resulted from constraint-induced changes in the square planar Pt coordination centres rather than from chiral distortion along the coordination platforms.

Achiral substituent- and stoichiometry-controlled inversion of supramolecular chirality and circularly polarized luminescence in ternary co-assemblies

Handedness inversion of supramolecular chirality and circularly polarized luminescence (CPL) in assembled systems containing more than two components with higher complexity is of prominent importance to simulate biological multicomponent species and design advanced chiral materials, but it remains a considerable challenge. Herein, we have successfully developed ternary co-assembly systems based on aromatic amino acids, vinylnaphthalene derivatives and 1,2,4,5-tetracyanobenzene with effective chirality transfer. Notably, the handedness of supramolecular chirality and CPL can be readily inverted by changing the residues of amino acids, the substituents of achiral vinylnaphthalene derivatives, or by adjusting the stoichiometric ratio. The hydrogen bonds, charge transfer interactions, and steric hindrance are proved to be the crucial factors for the chirality inversion. This flexible control over chirality not only offers insights into developing multicomponent chiral materials with desirable handedness from simple molecular building blocks, but also is of practical value for use in chiroptics, chiral sensing, and photoelectric devices.

Recent Advances of Chiral Isolated and Small Organic Molecules: Structure and Properties for Circularly Polarized Luminescence

This paper explores recent advancements in the field of circularly polarized luminescence (CPL) exhibited by small and isolated organic molecules. The development and application of small CPL molecule are systematically reviewed through eight different chiral skeleton sections. Investigating the intricate interplay between molecular structure and CPL properties, the paper aims at providing and enlighting novel strategies for CPL-based applications.

Supramolecular Chirality Achieved by Assembly of Small π-Molecules of Octahydrobinaphtols with Axial Chirality

5,5',6,6',7,7',8,8'-Octahydro-1,1'-bi-2-naphthol (hbNaph) is an axially chiral molecule consisting of a smaller π-electronic system than that for 1,1'-bi-2-naphthol (BINOL). The absorption and circular dichroism (CD) bands of hbNaph appear in a shorter wavelength region below 310 nm, compared to those of BINOL, and its fluorescence is in the invisible UV region. However, increasing the concentration of hbNaph in solution up to 0.1 M results in its absorption edge gradually extending to longer wavelength, with a shoulder around 330 nm, and finally increasing to about 450 nm. At the same time, blue fluorescence is clearly observed, as well as a new CD band with the sign of the Cotton signals reversed from those obtained for dilute solutions. These results suggest that, at high concentrations, hbNaph forms chiral aggregates, in which π-electrons are delocalized over multiple molecules. To further understand how molecular axial chirality is transformed to supramolecular chirality, we attempted to construct aggregate models by simulating CD spectra using a time-dependent density functional theory. The only reasonable model obtained was that involving the counterclockwise R-enantiomer forming a clockwise helix, while the clockwise S-enantiomer forms a counterclockwise helix. We conclude, however, that, for such helixes, the most plausible model is densely packed and forms when the dihedral angle between the two phenol rings of hbNaph is acute, at around 75°, which reproduces the aggregate-induced CD sign inversion.

Unraveling the Remarkable Influence of Substituents on the Emission Variation and Circularly Polarized Luminescence of Dinuclear Aluminum Triple-Stranded Helicates

This study explored the development of functional dyes using aluminum, focusing on aluminum-based dinuclear triple-stranded helicates, and examined the effects of substituent variations on their structural and optical properties. Key findings revealed that the modification of methyl groups to the pyrrole positions significantly extended the conjugation system, resulting in a red shift in the absorption and emission spectra. Conversely, the modification of methyl groups at the methine positions due to steric hindrances increased the torsion angle of the ligands, leading to a blue shift in the absorption and emission spectra. A common feature across all complexes was that in the excited state, one of the three ligands underwent significant structural relaxation. This led to a pronounced Stokes shift and minimal spectra overlap with high photoluminescence behaviors. Moreover, our research extended to the optical resolution of the newly synthesized complexes by analyzing the chiroptical properties of the resulting enantiomers, including their circular dichroism and circularly polarized luminescence. These insights offer valuable contributions to the design and application of novel aluminum-based functional dyes, potentially influencing a range of fields, from materials science to optoelectronics.

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

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

Controllable π-π coupling of intramolecular dimer models in aggregated states

π-π coupling as a common interaction plays a key role in emissions, transport and mechanical properties of organic materials. However, the precise control of π-π coupling is still challenging owing to the possible interference from other intermolecular interactions in the aggregated state, usually resulting in uncontrollable emission properties. Herein, with the rational construction of intramolecular dimer models and crystal engineering, π-π coupling can be subtly modulated by conformation variation with balanced π-π and π-solvent interactions and visualized by green-to-blue emission switching. Moreover, it can rapidly respond to temperature, pressure and mechanical force, affording a facile way to modulate π-π coupling in situ. This work contributes to a deeper understanding of the internal mechanism of molecular motions in aggregated states.

Highly Luminescent Chiral Double π-Helical Nanoribbons

Unveiling the mechanism behind chirality propagation and dissymmetry amplification at the molecular level is of significance for the development of chiral systems with comprehensively outstanding chiroptical performances. Herein, we have presented a straightforward Cu-mediated Ullmann homocoupling approach to synthesize perylene diimide-entwined double π-helical nanoribbons encompassing dimer, trimer, and tetramer while producing homochiral or heterochiral linking of chiral centers. A significant dissymmetry amplification was achieved, with absorption dissymmetry factors (|gabs|) increasing from 0.009 to 0.017 and further to 0.019, and luminescence dissymmetry factors (|glum|) rising from 0.007 to 0.013 and eventually to 0.015 for homochiral double π-helical oligomers. The disparity of magnetic transition dipole moment (m) densities in homochiral and heterochiral tetramers by time-dependent density functional theory calculations confirmed that homochiral oligomerization can maximize the total m, which is favorable for achieving ever-increasing g factors. Notably, these double π-helices exhibited exceptional photoluminescence quantum yields (ΦPL) ranging from 83 to 95%. The circularly polarized luminescence brightness (BCPL) eventually reached a remarkable 575 M-1 cm-1 for the homochiral tetramer, which is among the highest values reported for chiral small molecules. This kind of linearly extended double π-helices offers a platform for a comprehensive understanding of the mechanism behind chirality propagation and dissymmetry amplification.

Structurally Diverse Pyrene-decorated Planar Chiral [2,2]Paracyclophanes with Tunable Circularly Polarized Luminescence between Monomer and Excimer

We reported the synthesis of a series of structurally diverse CPL-active molecules, in which pyrene units were installed to chiral pm/po-[2,2]PCP scaffolds either with or without a triple bond spacer for pm/po-PCP-P1 and pm/po-PCP-P2, respectively. The X-ray crystallographic analyses revealed that these pyrene-based [2,2]PCP derivatives exhibited diverse structures and crystal packings in the solid phases. The pyrene-based [2,2]PCP derivatives exhibit various (chir)optical properties in organic solutions, depending on their respective structures. In a mixture of dioxane and water, pm/po-PCP-P1 emit green excimer fluorescence, whereas pm/po-PCP-P2 emit blue one. The chiroptical investigation demonstrated that Rp-pm-PCP-P1 and Rp-pm-PCP-P2 exhibited completely opposite CD and CPL signals even they possess the same chiral Rp-[2,2]PCP core. The same argument also holds for other chiral pyrene-based [2,2]PCP derivatives. The theoretical calculation revealed that these unusual phenomena were attributed to different orientation between transition electric dipole moments and the magnetic dipole moments originating from the presence or absence of a triple bond spacer. These pyrene-based [2,2]PCP derivatives display various colours and fluorescence emissions in the solid state and PMMA films, possibly due to the different packings as observed in the crystal structure. Moreover, these compounds also can interact with perylene diimide through π-π interactions, leading to near-white fluorescence.

NHC-Catalyzed Chemo- and Enantioselective Reaction between Aldehydes and Enals for Access to Axially Chiral Arylaldehydes

A chiral carbene-catalyzed chemo- and enantioselective reaction with racemic biaryl aldehydes and α-bromoenals is developed for access to axially chiral 2-arylbenzaldehydes through atroposelective dynamic kinetic resolution (DKR) processes. This atroposelective DKR strategy can tolerate a broad scope of substrates with diverse functionalities. The axially chiral 2-aryl benzaldehyde products generally afford moderate to good yields and enantioselectivities. The axially chiral molecules afforded from the current approach are variable through simple transformations to afford axially chiral functional molecules with excellent optical purities.

Vibrational circular dichroism spectroscopy in the C-D, XY, and XYZ stretching region

Vibrational circular dichroism (VCD) spectroscopy is a powerful technique for structural analysis of chiral molecules, but information available from VCD spectra of large molecular systems can be limited by severe overlap of vibrational bands. While common chiral molecules do not absorb in the 1900-2400 cm-1 region, observation of VCD signals in this spectrally-isolated region is possible for molecules containing C-D, XY, and XYZ chromophores. Thus, a strategic introduction of these chromophores to a target molecule may produce VCD signals informative for molecular structures. VCD spectroscopy in the 1900-2400 cm-1 region is a rather unexplored research field and its basic properties remain to be investigated. This perspective article discusses insight obtained so far on the usefulness and physicochemical aspects of VCD spectroscopy in this region with briefly summarizing previous experimental VCD studies including classic examples as well as our recent results. We show that anharmonic effects such as overtones and combination bands often complicate VCD patterns. On the other hand, some molecules exhibit characteristic VCD signals that can be well interpreted by harmonic DFT spectral calculations for structural analysis. This article also discusses several examples of the use of this region for studying solute-solvent interactions and for VCD signal augmentation.

Processable circularly polarized luminescence material enables flexible stereoscopic 3D imaging

Endowing three-dimensional (3D) displays with flexibility drives innovation in the next-generation wearable and smart electronic technology. Printing circularly polarized luminescence (CPL) materials on stretchable panels gives the chance to build desired flexible stereoscopic displays: CPL provides unusual optical rotation characteristics to achieve the considerable contrast ratio and wide viewing angle. However, the lack of printable, intense circularly polarized optical materials suitable for flexible processing hinders the implementation of flexible 3D devices. Here, we report a controllable and macroscopic production of printable CPL-active photonic paints using a designed confining helical co-assembly strategy, achieving a maximum luminescence dissymmetry factor (glum) value of 1.6. We print customized graphics and meter-long luminous coatings with these paints on a range of substates such as polypropylene, cotton fabric, and polyester fabric. We then demonstrate a flexible textile 3D display panel with two printed sets of pixel arrays based on the orthogonal CPL emission, which lays an efficient framework for future intelligent displays and clothing.

Chiroptical Amplification of Induced Circularly Polarized Luminescence in Nucleotide-Templated Supramolecular Polymer

Efficient circularly polarized luminescence (CPL) from purely organic molecules holds great promise for applications in displays, sensing, and bioimaging. However, achieving high dissymmetry values (glum ) from organic chromophores remains a significant challenge. Herein, we present a bioinspired approach using adenosine triphosphate (ATP)-triggered supramolecular polymerization of a naphthalene diimide-derived monomer (ANSG) to induce CPL with a remarkable glum value of 1.1×10-2 . The ANSG molecules undergo a templated, chiral self-assembly through a cooperative growth mechanism in the presence of ATP, resulting in scrolled nanotubes with aggregation-induced enhanced emission (AIEE) and induced CPL. Furthermore, we demonstrate the concept of chiroptical amplification of induced CPL by efficiently amplifying asymmetry using a mixture of chiral ATP and achiral pyrophosphate. This innovative approach opens numerous opportunities in the emerging field of circularly polarized luminescence.

Chiroptical Response Control of Planar and Axially Chiral Polymethylene-Vaulted Platinum(II) Complexes Bearing 1,1'-Binaphthyl Frameworks

In this study, the synthesis, structure, and chiroptical response control of planar chiral polymethylene-vaulted trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing axially chiral 1,1'-binaphthyl ligands are described. A series of enantiopure polymethylene (n = 4-10)-vaulted complexes were prepared in 6 steps using commercially available (R)- or (S)-BINOL as the starting material without an optical resolution process. The trans-coordination and three-dimensional vaulted structures of the platinum complexes were elucidated from X-ray diffraction (XRD) studies. The complexes were found to show structural dependence of chiroptical responses in the dilute solution state such that the absolute values of [α]D, dissymmetry factors gabs in circular dichroism (CD), and glum in circularly polarized luminescence (CPL) increased upon shortening the length of the polymethylene bridges. The enhanced chiroptical responses were theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and the results are discussed in terms of the molecular structures and transition dipole moments of the ground states. The structural dependence of the chiroptical responses was ascribed to the distortion of the coordination platforms caused by restriction of the vaulting methylene linkers.

Small Molecules Mediated the Chirality Transfer in Self-Assembled Nanocomposites with Strong Circularly Polarized Luminescence

Manipulation of the chirality at all scales has a cross-disciplinary importance and may address key challenges at the heart of physical sciences. One critical question in this field is how the chirality of one entity can be transferred to the asymmetry of another entity. Here, we find that small molecules play a crucial role in the chirality transfer from chiral organic molecules to CdSe/CdS nanorods, where the handedness of the nanorod assemblies either agrees or disagrees with that of the molecular assemblies, leading to the positive or inverse chirality transfer. The assembling mode of nanorods on the molecular assemblies, where the nanorods are either lying or standing, is closely associated with the handedness of the nanorod assemblies, resulting in opposite chirality. Furthermore, we have found that circularly polarized emission from chiral assemblies of nanorods is dependent on molecular additives. The promoted luminescence dissymmetry factor (glum) of the nanocomposites with a high value of ∼0.3 could be attained under optimal conditions.

Stepwise Solution-Interfacial Nanoarchitectonics for Assembled Film with Full-Color and White-Light Circularly Polarized Luminescence

The fabrication of chiral thin films with tunable circularly polarized luminescence (CPL) colors is important in developing chiroptical materials but remains challenging due to the lack of assembly-initiated chiral film formation methodology. Here, by adopting a combined solution aggregation and interfacial assembly strategy, we report the fabrication of chiral film materials with full-color and white-light CPL. A biquinoline glutamic acid ester (abbreviated as BQGE) shows a typical aggregation-induced emission property with blue CPL after solution aggregation. Subsequent interfacial assembly of these solution aggregates on a solid substrate leads to the formation of a CPL active film consisting of nanobelt structures. Since the BQGE molecule has a coordination site, the CPL emission of an individual BQGE film can be extended from blue to green emission upon coordination with a zinc ion, accompanied by morphology transition from nanobelts to nanofibers. Further extension to red-color CPL is successfully achieved by coassembly with an achiral acceptor dye. Interestingly, the proper combination of coordination ratio and acceptor loading ratio provides bright white-light CPL emission from the BQGE/Zn²⁺/PDA triad composite film. This work provides a new approach to fabricating chiroptical film materials with controlled microscopic morphology and tunable CPL properties.

Two-ring chirality generated by the alignment of two achiral phenylacetylene macrocycles

When two achiral rings are bound mechanically, a chiral source is generated in the assembly. The chiroptical properties could be modulated according to the relative occupation of each ring in the assembly. In fact, we have found that two isomeric assemblies (1 and 2) show unique properties in each assembly with two achiral rings of phenylacetylene macrocycle (PAM). When considering the difference in the chiroptical properties of these two isomeric assemblies (6PAM × 2), no comparison was available based on no activity of the achiral component element itself (6PAM). In this work, we synthesized a two-ring chiral analog (4) by the ring-fusion of two 6PAMs to an 11PAM, and examined the chiroptical properties of 4, since the single helix was imparted as a chiral source. By comparison of the chiroptical properties (molar circular dichroism and molar optical rotation) of 1 and 2 to those of 4, we demonstrated that the disparity was related to the alignment of the two achiral rings.

Visible Helicity Induction and Memory in Polyallene toward Circularly Polarized Luminescence, Helicity Discrimination, and Enantiomer Separation

Inspired by biological helices (e.g., DNA), artificial helical polymers have attracted intense attention. However, precise synthesis of one-handed helices from achiral materials remains a formidable challenge. Herein, a series of achiral poly(biphenyl allene)s with controlled molar mass and low dispersity were prepared and induced into one-handed helices using chiral amines and alcohols. The induced one-handed helix was simultaneously memorized, even after the chiral inducer was removed. The switchable induction processes were visible to naked eye; the achiral polymers exhibited blue emission (irradiated at 365 nm), whereas the induced one-handed helices exhibited cyan emission with clear circularly polarized luminescence. The induced helices formed stable gels in various solvents with helicity discrimination ability: the same-handed helix gels were self-healing, whereas the gels of opposite-handed helicity were self-sorted. Moreover, the induced helices could separate enantiomers via enantioselective crystallization with high efficiency and switchable enantioselectivity.

Solvent Modulation of Chiral Perovskite Films Enables High Circularly Polarized Luminescence Performance from Chiral Perovskite/Quantum Dot Composites

Materials with circularly polarized luminescence (CPL) activity are promising in many chiroptoelectronics fields, such as for biological probes, asymmetric photosynthesis, information storage, spintronic devices, and so on. Promoting the value of the dissymmetry factor (glum) for the CPL-active materials based on chiral perovskite draws increasing attention since a higher glum value indicates better CPL. In this work, we find that, after being treated with a facile solvent modulation strategy, the chirality of 2D chiral perovskite films has been enhanced a lot, which we attribute to an increased lattice distortion degree. By forming chiral perovskite/quantum dot (QD) composites, the CPL-active material is successfully obtained. The calculated maximum |glum| of these composites increased over 4 times after solvent modulation treatment (1.53 × 10-3 for the pristine sample of R-DMF and 6.91 × 10-3 for R-NMP) at room temperature. Moreover, the enhancement of the CPL intensity is ascribed to two aspects: one is the generation and transportation of spin-polarized charge carriers from chiral perovskite films to combine in the QD layer, and the other is the solvent modulation strategy to enlarge the lattice distortion of chiral perovskite films. This facile route provides an effective way to construct CPL-active materials. More importantly, this kind of composite material (chiral perovskite film/QD layer) can be easily applied for fabricating circularly polarized light-emitting diode devices for electroluminescence.

Circularly polarized luminescence in molecular recognition systems: Recent achievements

Circularly polarized luminescence (CPL) dyes are recognized to be new generation materials and have been actively developed. Molecular recognition systems provide nice approaches to novel CPL materials, such as stimuli-responsive switches and chemical sensing materials. CPL may be induced simply by mixing chiral or achiral, luminescent or nonluminescent host and guest; there are several combinations. Molecular recognition can potentially save time and effort to construct well-ordered chiral structures with noncovalent attractive interactions as compared with the multi-step synthesis of covalently bonded dyes. It is a challenging subject to engage molecular recognition events with CPL, and it is important and interesting to see how it is achieved. In fact, simple molecular recognition systems can even enable the fine adjustment of CPL performance and detailed conformational/configurational analysis of the excited state. Here we overview the recent achievements of simple host-guest complexes capable of exhibiting CPL, summarizing concisely the host/guest structures, CPL intensities, and characteristics.

Dibenzotropylium-Capped Orthogonal Geometry Enabling Isolation and Examination of a Series of Hydrocarbons with Multiple 14π-Aromatic Units

A series of six dications composed of pure hydrocarbons with one to six non-substituted 9,10-anthrylene units end-capped with two dibenzotropyliums were designed and synthesized to elucidate the electronic properties of huge oligo(9,10-anthrylene) backbones. Their structures were successfully determined by X-ray analyses even in the case of eight planar 14π-electron units, revealing that all dications adopt almost orthogonally twisted structures between neighboring units. Spectroscopic and voltammetric analyses show that neither the significant overlap of orbitals nor the delocalization of electrons between 14π-electron units occurs due to the orthogonally twisted geometry even in solution. As a result, sequential oxidation processes were observed with the reversible formation of multivalent cations with the release of the same number of electrons as the number of anthrylene units. Upon two-electron reduction, a closed-shell butterfly-shaped form was obtained from the dication containing one anthrylene unit, whereas open-shell twisted biradicals were isolated as stable entities in the cases of derivatives containing three to six anthrylene units. Notably, from the derivative with two anthrylene units, a metastable open-shell isomer was obtained quantitatively and underwent slow thermal conversion to the most stable closed-shell isomer (E_a = 23.1 kcal mol^-1). There is a drastic change in oxidation potentials between two neutral species (ΔE = 1.32 V in CH₂Cl₂). Since the present dications were regenerated upon oxidation of the isolated reduction products, these systems may contribute to the development of advanced response systems capable of switching color, magnetic properties, and oxidative properties by using a "cation-capped orthogonal geometry".

Atroposelective Synthesis of Triaryl α-Pyranones with 1,2-Diaxes by N-Heterocyclic Carbene Organocatalysis

Atropisomers bearing multiple stereogenic axes are of increasing importance to the field of material science, pharmaceuticals, and catalysis. However, the atroposelective construction of multi-axis atropisomers remains rare and challenging, due to the intrinsical difficulties in the stereo-control of the multiple stereogenic axes. Herein, we demonstrate a single-step construction of a new class of 1,2-diaxially chiral triaryl α-pyranones by an N-heterocyclic carbene organocatalytic asymmetric [3+3] annulation of well-designed alkynyl acylazolium precursors and enolizable sterically hindered 2-aryl ketones. The protocol features broad substrate scope (>50 examples), excellent stereo-control (most cases >20 : 1 dr, up to 99.5 : 0.5 er), and potentially useful synthetic applications. The success of this reaction relies on the rational design of structurally matched reaction partners and the careful selection of the asymmetric catalytic system. DFT calculations have also been performed to discover and rationalize the origin of the high stereoselectivity of this reaction.

External environment sensitive circularly polarized luminescence properties of a chiral boron difluoride complex

A chiral Schiff-base boron difluoride complex bearing a diethylamino group was synthesized. Its photophysical properties were investigated and compared with those of its non-substituted analogue. The complex was found to exhibit solvatofluorochromism with bluish-white emission in moderately polar solvents and intense blue emission in nonpolar solvent. Circularly polarized luminescence (CPL) properties were also examined and it was found that the absolute value of the luminescence dissymmetry factor (g lum) increases significantly in the KBr-dispersed pellet state compared to the solution state. Notably, CPL intensity of the complex enhanced approximately three times upon addition of CH3SO3H in CH2Cl2. Density functional theory (DFT) calculations were conducted to further understand the photophysical properties.

Cyclic arrays of five pyrenes on one rim of a planar chiral pillar[5]arene

Spatial arrangement of multiple planar chromophores is an emerging strategy for molecule-based chiroptical materials via easy and systematic synthesis. We attached five pyrene planes to a chiral macrocycle, pillar[5]arene, producing a set of chiroptical molecules in which pyrene-derived absorption and emission were endowed with dissymmetry by effective transfer of chiral information. The chiroptical response was dependent on linker structures and substituted patterns because of variable interactions between pyrene units. One of these hybrids showed larger dissymmetry factor and response wavelength (g lum = 7.0 × 10-3 at ca. 547 nm) than reported pillar[5]arene-based molecules using the pillar[5]arene cores as parts of photo-responsive π-conjugated units.

Stereomutation and chiroptical bias in the kinetically controlled supramolecular polymerization of cyano-luminogens

The synthesis of two pairs of enantiomeric cyano-luminogens 1 and 2, in which the central chromophore is a p-phenylene or a 2,5-dithienylbenzene moiety, respectively, is described and their supramolecular polymerization under kinetic and thermodynamic control investigated. Compounds 1 and 2 form supramolecular polymers by quadruple H-bonding arrays between the amide groups and the π-stacking of the central aromatic moieties. In addition, the peripheral benzamide units are able to form intramolecularly H-bonded pseudocycles that behave as metastable monomer M* thus affording kinetically and thermodynamically controlled aggregated species AggI and AggII. The chiroptical and emissive features of compounds 1 and 2 strongly depend on the aggregation state and the nature of the central aromatic unit. Compounds 1 exhibit a bisignated dichroic response of different intensity but with similar sign for both AggI1 and AggII1 species, which suggests the formation of helical aggregates. In fact, these helical supramolecular polymers can be visualized by AFM imaging. Furthermore, both AggI and AggII species formed by the self-assembly of compounds 1 show CPL (circularly polarized light) activity of opposite sign depending on the aggregation state. Thienyl-derivatives 2 display dissimilar chiroptical, morphological and emissive characteristics for the corresponding kinetically and thermodynamically controlled aggregated species AggI and AggII in comparison to those registered for compounds 1. Thus, a stereomutation phenomenon is observed in the AggI2 → AggII2 conversion. In addition, AggI2 is arranged into nanoparticles that evolve to helical aggregates to afford AggII2. The dissimilar chiroptical and morphological features of AggI2 and AggII2 are also appreciated in the emissive properties. Thus, whilst AggI2 experiences a clear AIE (aggregation induced emission) process and CPL activity, the thermodynamically controlled AggII2 undergoes an ACQ (aggregation caused quenching) process in which the CPL activity is cancelled.