Binaphthyl–Bipyridyl Cyclic Dyads as a Chiroptical Switch

Acid/Base-Responsive Circularly Polarized Luminescence Emitters with Configurationally Stable Nitrogen Stereogenic Centers

A way to prevent the fast configurational interconversion of tertiary amines is to invoke Tröger's base analogs, which display methano- or ethano-bridged diazocine cores fused to aromatic rings. These derivatives are configurationally stable, even in acidic media when their structures bear ethylene bridges. Here, a two- to three-step synthesis is presented of methano- and ethano-bridged Tröger's base analogs with two peripheral fluorophores, i.e., anthracene, pyrene, and 9,9-dimethylfluorene units. These compounds, possessing two nitrogen stereogenic centers, exhibit good circularly polarized luminescence (CPL) dissymmetry factors (|glum| up to 1.2 × 10-3) and brightnesses (BCPL up to 26.3 M-1 cm-1), as well as excellent fluorescence quantum yields, demonstrating the Tröger´s base core to be a convenient scaffold to prepare CPL emitters upon functionalization with simple achiral fluorophores. Furthermore, the configurationally stable ethano-bridged Tröger's base analogs are employed to modulate their CPL response, generating a CPL switch through their protonation/deprotonation by consecutive additions of acid and base. The reversibility of the switching process is demonstrated for two cycles without altering the CPL performance of the molecule. It is believed that this straightforward and efficient approach to building CPL emitters employing the Tröger's base core could lead to its incorporation in CPL-based sensors and materials.

Asymmetric Olefin Isomerization via Phase-Transfer-Catalyzed [1,3]-Hydrogen Transfer for Access to Axially Chiral Furan-Benzimidazoles

An efficient catalytic asymmetric olefin isomerization of axially chiral methylene dihydrofuran-benzimidazoles via kinetic resolution is reported. Under mild phase-transfer catalysis, axially chiral furan-benzimidazole compounds and recovered methylene dihydrofuran-benzimidazoles were obtained in high ee. The combination of the kinetic resolution and TBD-catalyzed isomerization of recovered dihydrofuran-benzimidazoles provided access to both enantiomers of furan-benzimidazoles. Deuterium-labeling experiments reveal intramolecular [1,3]-H transfer mechanism. The utility of this method was demonstrated by scale-up synthesis and functionalization of the products.

Simultaneous Construction of Axial and Central Stereogenicity by Cobalt-Catalyzed Stereoconvergent Reductive Coupling of Heterobiaryl Triflates and Aldehydes

Catalytic stereoconvergent coupling of racemic heterobiaryl triflates and aldehydes promoted by a readily available chiral cobalt complex is presented. Such processes represent an unprecedented reaction pathway for cobalt catalysis that enable simultaneous construction of axial and central stereogenicity through diastereo- and enantioselective dynamic kinetic transformations and introduction of a chiral fragment onto the heterobiaryl cores without the requirement of preforming stoichiometric amounts of organometallic reagents, affording densely functionalized secondary alcohols in up to 96 % yield, >98 : 2 dr and >99.5:0.5 er. Preliminary investigations on the application of the products demonstrate their potential for serving as a new class of chiral catalysts and ligands. Mechanistic studies suggest that a dynamic kinetic stereoselective process induced by chiral cobalt catalysis is involved.

Recent development of azahelicenes showing circularly polarized luminescence

Recently, a variety of circularly polarized luminescence (CPL) dyes have been developed as next-generation chiroptical materials. Helicenes, ortho-fused aromatics, have been recognized as some of the most promising CPL dyes. Although typical carbohelicenes show CPL, weak fluorescence is often emitted in the blue region. In contrast, heteroatom-embedded helicenes (heterohelicenes) can show intense fluorescence and CPL in the visible region because heteroatoms alter the electronic states of helicene frameworks. Among various heterohelicenes, nitrogen-embedded helicenes (azahelicenes) have unique features such as facile functionalization and sensitive responses to acid/base or metal ions. Furthermore, polycyclic aromatic hydrocarbons (PAHs) containing azaborine units have been recognized as excellent luminescent materials, and the helical derivatives, B,N-embedded helicenes, have been rapidly growing recently. In this feature article, we review and summarize the synthesis and chiroptical properties of azahelicenes, which are classified into imine-type and amine-type azahelicenes and B,N-embedded helicenes. CPL switching systems of azahelicenes are also reviewed.

Construction of Axially Chiral Dialdehydes via Rhodium-Catalyzed Enantioselective C-H Amidation

Achieving axially chiral biaryl dialdehydes through asymmetric catalysis remains significantly challenging due to the lack of efficient strategies. In this report, we developed a rhodium-catalyzed enantioselective C-H amidation through chiral transient directing group strategy. With this new approach, a series of axially chiral amido dialdehydes were achieved in up to 86 % yields with 99.5 : 0.5 er. Furthermore, detailed mechanistic studies indicated that both the imine formation and C-H bond cleavage steps were reversible. More interestingly, the X-ray crystallographic analysis of Int-2 showed probable C-H/π interaction between biaryl group and chiral amine moiety. This process offered a convenient route to access axially chiral dialdehyde derivatives. More broadly, it demonstrated a new tool through transient and C-H/π synergistic interactions, which would stimulate further development of asymmetric catalytic system in enantioselective C-H functionalization.

B,N-Embedded Helical Nanographenes Showing an Ion-Triggered Chiroptical Switching Function

Intramolecular oxidative aromatic coupling of 3,6-bis(m-terphenyl-2'-yl)carbazole provided a bis(m-terphenyl)-fused carbazole, while that of 3,6-bis(m-terphenyl-2'-yl)-1,8-diphenylcarbazole afforded a bis(quaterphenyl)-fused carbazole. Borylation of the latter furnished a B,N-embedded helical nanographene binding a fluoride anion via a structural change from the three-coordinate boron to the four-coordinate boron. The anionic charge derived from the fluoride anion is stabilized over the expanded π-framework, which leads to the high binding constant (Ka) of 1×105 M-1. The four-coordinate boron species was converted back to the parent three-coordinate boron species with Ag+, and the chiroptical switch between the three-coordinate boron and four-coordinate boron species has been achieved via the ion recognition with the change in the color and glum values.

Excitation-Dependent Circularly Polarized Luminescence Inversion Driven by Dichroic Competition of Achiral Dyes in Cholesteric Liquid Crystals

The development of stimuli-responsive chiral cholesteric liquid crystals (CLCs) materials holds significant potential for achieving three-dimensional (3D) anti-counterfeiting and multi-level information encryption. However, constructing phototunable CLCs systems with easy fabrication and fast response remains a great challenge. Herein, we exploit an excitation-dependent CLCs (ExD-CLCs) material by establishing dynamically photoresponsive dichroic competition between two achiral dyes: a negative dichroic dye (SP-COOH) and a positive dichroic dye (Nile Red, NR) within a CLCs medium. The ExD-CLCs exhibits a negative circularly polarized luminescence (CPL) signal (glum=-0.16) at 625 nm when excited at 365 nm. Remarkably, under excitation at 430 nm, the CPL signal is inverted, and the glum value increases to +0.26. Notably, the helical superstructure and handedness of the ExD-CLCs remain unchanged during this reversal process. The CPL signal reversal is driven by the dichroic competition between the SP-COOH dimer, which displays strong negative dichroism in its open-ring isomer form and silent negative dichroism in its closed-ring isomer form, and the NR dye, which exhibits static positive dichroism. Leveraging these excitation-dependent CPL properties, the quadruplex numerical anti-counterfeiting using ExD-CLCs is achieved.

Ultrafast Luminescence Detection with Selective Adsorption of Carbon Disulfide in a Gold(I) Metal-Organic Framework

Although a widely used and important industrial chemical, carbon disulfide (CS2) poses a number of hazards due to its volatility and toxicity. As such, the development of multifunctional materials for the selective capture and easy recognition of CS2 is one of the crucial issues. Herein, we demonstrate completely selective CS2 adsorption among trials involving H2O, alcohols, volatile organic compounds (including thiol derivatives), N2, H2, O2, CH4, CO, NO, and CO2. We also showcase its fine detection using remarkable luminescent response in a gold(I)-based metal-organic framework (MOF) of {ZnII(pz)[AuI(CN)2]2} (pz=pyrazine; 1) with a two-fold interpenetration network. Ex situ single crystal X-ray diffraction for 1 and CS2-accommodated 1 suggested that the Au ⋅⋅⋅ Au atoms are not only luminescent centers but also act as interaction sites for CS2 modulating the Au ⋅⋅⋅ Au contacts. These experiments revealed the specificity of CS2 and how changes in the CS2-induced structure. Based on the obtained structural transformation, 1 exhibited a sensitive detecting ability for CS2 with an ultrafast response time of less than 10 s. Moreover, ex situ time-resolved photoluminescence analyses developed in this work implied that CS2 varied the energetic relaxation at the excited states related to the luminescent efficiency of the resultant MOF system.

Construction of Axially Chiral Tetra-ortho-Substituted Biaryls by Palladium-Catalyzed Asymmetric Suzuki-Miyaura Coupling

Axial chiral biaryl skeletons are widely found in biologically active molecules, catalysts and chiral functional materials. However, highly catalytic stereoselective synthesis of tetra-ortho-substituted biaryls remains a challenging task. In this paper, we describe an efficient approach for construction of axially tetra-ortho-substituted biaryls via Suzuki-Miyaura coupling in the presence of a chiral monophosphate ligand developed by ourselves. This method provides a variety of products in good enantioselectivities and good yields (up to 90.7 : 9.3 er and 82 % yield) under mild conditions. Further control experiments indicate that the chiral side chain is crucial to the enantioselectivity.

Chiral Polar Bifunctional Polyimide Enantiomers for Asymmetric Photo- and Piezo-Catalysis

Chiral catalysts for asymmetric catalysis represent a crucial research focus in chemistry and materials science. However, a few cases about chiral-dependent photocatalysts primarily focus on plasmonic noble metals. Particularly, developing chiral nano-catalysts that can be driven by mechanical energy remains in the blank stage. Herein, organic polymer-based enantiomers, chiral polar polyimide (PI) microspheric nano-assembly are synthesized as novel bifunctional catalysts for asymmetric photocatalysis and piezocatalysis. The PI catalyst enantiomers present enantioselectivity towards left- and right-circularly polarized light, demonstrating chiral-dependent H2O2 photoproduction. Interestingly, enantioselectivity of the catalyst reverses under irradiation of different bands, presenting tunability in the interaction between chiral catalysts and circularly polarized light. For the first time, enantioselective piezocatalytic behavior is demonstrated by the chiral polar PI catalysts. They show remarkable chiral preference for asymmetric Diels-Alder reaction and enantioselective conversion of tyrosine substrates under ultrasonic vibration. The findings provide a new perspective into exploring metal-free chiral catalysts and their asymmetric catalysis applications across multiple energy forms.

Chiral Co-assembly Based on a Stimuli-Responsive Polymer towards Amplified Full-Color Circularly Polarized Luminescence

Stimuli-responsive circularly polarized luminescence (CPL) materials have been attaching wide attention in the field of optical information storage and encryption, while still facing the challenge of the realization of high luminescence dissymmetry factors (glum). This work presents a pair of stimuli-responsive chiral co-assemblies P7R3 and P7S3 by combining polymer PFIQ containing iso-quinoline units with chiral inducers. The obtained chiral co-assemblies can reversibly undergo significant modification in CPL behavior under trifluoroacetic acid (TFA) fumigation and annealing treatment, with the |glum| values exhibiting a reversible shift between 0.2 and 0.3. Moreover, the chiral co-assemblies before TFA fumigating can effectively induce achiral emitters to generate intense full-color CPL signals through CPL energy transfer (CPL-ET), with the corresponding |glum| values larger than 0.2. Moreover, information encryption and decryption as well as a multi-level logic gates application are achieved by leveraging the reversible stimuli-responsive CPL activity of the chiral co-assembly. This work provides a new perspective for the construction of stimuli-responsive chiral luminescent materials with large |glum| values and the activation of CPL behavior in achiral emitters.

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.

Organocatalytic Atroposelective Synthesis of Axially Chiral Indolyl Ketosulfoxonium Ylides

Despite recent advancements in the catalytic generation of axial chirality, reports on non-biaryl atropisomers remain limited because of the stringent steric requirements necessary to establish effective rotational brakes. Herein, we present a novel class of monoaryl atropisomers, indolyl ketosulfoxonium ylides, and describe an organocatalytic protocol for their synthesis. We discovered that a chiral phosphoric acid (CPA) serves as an effective catalyst for the highly enantioselective iodination of ortho-aminophenylethynyl sulfoxonium ylides. Under the optimized reaction conditions, a strong preference for the intended iodination process over the competing protonation was observed. Subsequently, intramolecular amide cyclization enabled the formation of sterically congested indole fragments. Furthermore, the synthetic utility of the products was demonstrated by showcasing versatile transformations into other chiral scaffolds with complete retention of optical purity.

Controlling Sign and Magnitude of Circularly Polarized Luminescence of Axially Chiral Schiff-Base Boron Difluoride Complexes Bearing Polyethylene Glycol Chains

The synthesis, structure, and circularly polarized luminescence (CPL) properties of axially chiral boron difluoride complexes are described. A series of optically pure bis (boron difluoride) complexes were prepared in 5 steps from commercially available (S)- or (R)-BINOL as starting materials. The complexes were found to exhibit similar yellow photoluminescence in solution, regardless of the type of substituents on the nitrogen atoms. Notably, for the complex with polyethylene glycol chains, homogeneous and transparent films were formed by the drop-cast method, showing intense CPL with a different sign from that of the solution state. Density functional theory (DFT) calculations were performed to understand their photophysical behavior including configuration-dependent CPL properties.

Divergent Synthesis of Chiroptical Molecular Switches Based on 1,2-Diaxial Atropisomers

The development of chiroptical molecular switches for chiral sensing, data communication, optical displays, chiral logic gates, and asymmetric catalysis is currently a vibrant frontier of science and technology. Herein, we report a practical artificial dynamic system based on a 1,2-diaxial atropisomer. Organocatalytic parallel kinetic resolution allows the divergent synthesis of two sets of stereoisomers with vicinal C-C and N-N axes from the same racemic single-axis substrates. By simply varying the configuration of the single catalyst, all four stereoisomers are accessible. The successive conduction of covalent unlocking/locking and thermal-isomerization processes enables sequential switching between all four atropisomeric states with electronic circular dichroism signal reversal, providing an example of multistate chiroptical molecular switches.

Concentration-Switchable Assembly of Carbon Dots for Circularly Polarized Luminescent Amplification in Chiral Logic Gates and Deep-Red Light-Emitting Diodes

Stimuli-responsive circularly polarized luminescent (CPL) materials are expected to find widespread application in advanced information technologies, such as 3D displays, multilevel encryption, and chiral optical devices. Here, using R-/S-α-phenylethylamine and 3,4,9,10-perylenetetracarboxylic dianhydride as precursors, chiral carbon dots (Ch-CDs) exhibiting bright concentration-dependent luminescence are synthesized, demonstrating reversible responses in both their morphologies and emission spectra. By adjusting Ch-CD concentration, the switchable wavelength is extended over 180 nm (539-720 nm), with the maximum quantum efficiency reaching 100%. Meanwhile, upon increasing Ch-CD concentration, the emission wavelength red-shifts, while the chirality of the assembled nanoribbons is synchronously amplified, ultimately achieving CPL at 709 nm and a maximum luminescence asymmetry factor of 2.18 × 10-2. These values represent the longest wavelength and the largest glum reported for CDs. Considering the remarkable optical properties of the synthesized Ch-CDs, multilevel chiral logic gates are designed, and their potential practical applications are demonstrated in multilevel anti-counterfeiting encryption, flexible electronic printing, and solid-state CPL. Furthermore, deep-red chiral electroluminescence light-emitting diodes (EL-LEDs) are prepared using these Ch-CDs, achieving an external quantum efficiency of 1.98%, which is the highest value reported to date for CDs in deep-red EL-LEDs, and the first report of chiral electronic devices based on CDs.

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.

A Halogen-Bonded Fluorescent Molecular Photoswitch: Transition from 3D Cubic Lattice to 1D Helical Superstructure for Polarization Inversion of Circularly Polarized Luminescence

The fabrication of materials that can switch between circularly polarized luminescence (CPL) signals is both essential and challenging. Here, two new halogen-bonded fluorescent molecular photoswitches, namely, HB-switch 1 and HB-switch 2, containing α-cyano-substituted diarylethene compounds with different end groups were developed. Upon exposure to specific UV or visible light wavelengths, they exhibited controllable and reversible Z/E photoisomerization. When these switches were integrated into blue-phase liquid crystals (BPLCs), the temperature range of BP significantly expanded. Notably, the BP system incorporating HB-switch 1 exclusively achieved reversible polarization inversion of CPL signals under irradiation with specific UV/Visible light and during cooling/heating. The photo/thermal dual-response behavior of the CPL signals can be attributed to the phase transition from a high-symmetry 3D BP Icubic lattice to a low-symmetry 1D helical superstructure induced by the Z/E photoisomerization of HB-switch 1 and temperature changes. This study underscores the significance of employing halogen-bond assembly strategies to design materials with switchable CPL signals, opening new possibilities for CPL-active systems.

Enantioconvergent synthesis of axially chiral amides enabled by Pd-catalyzed dynamic kinetic asymmetric aminocarbonylation

Atropisomeric biaryls bearing carbonyl groups have attracted increasing attention due to their prevalence in diverse bioactive molecules and crucial role as efficient organo-catalysts or ligands in asymmetric transformations. However, their preparation often involves tedious multiple steps, and the direct synthesis via asymmetric carbonylation has scarcely been investigated. Herein, we report an efficient palladium-catalyzed enantioconvergent aminocarbonylation of racemic heterobiaryl triflates with amines via dynamic kinetic asymmetric transformation (DyKAT). This protocol features a broad substrate scope and excellent compatibility for rapid construction of axially chiral amides in good to high yields with excellent enantioselectivities. Detailed mechanistic investigations discover that the base can impede the intramolecular hydrogen bond-assisted axis rotation of the products, thus allowing for the success to achieve high enantioselectivity. Moreover, the achieved axially chiral heterobiaryl amides can be directly utilized as N,N,N-pincer ligands in copper-catalyzed enantioselective formation of C(sp3)-N and C(sp3)-P bonds.

Enantioselective Synthesis of Heteroatom-Linked Non-Biaryl Atropisomers

Atropisomers hold significant fascination, not only for their prevalence in natural compounds but also for their biological importance and wide-ranging applications as chiral materials, ligands, and organocatalysts. While biaryl and heterobiaryl atropisomers are commonly studied, the enantioselective synthesis of less abundant heteroatom-linked non-biaryl atropisomers presents a formidable challenge in modern organic synthesis. Unlike classical atropisomers, these molecules allow rotation around two bonds, resulting in low barriers to enantiomerization through concerted bond rotations. In recent years the discovery of new configurationally stable rare non-biaryl scaffolds such as aryl amines, aryl ethers and aryl sulfones as well as innovative methodologies to control their configuration have been disclosed in the literature and constitute the topic of this minireview.

Functionalized Chiral Materials for Use in Chiral Sensors

Chirality represents a fundamental attribute within living systems and is a pervasive phenomenon in the natural world. The identification and analysis of chiral materials within natural environments and biological systems hold paramount importance in clinical, chemical, and biological sciences. Within chiral analysis, there is a burgeoning focus on developing chiral sensors exhibiting exceptional selectivity, sensitivity, and stability, marking it as a forefront area of research. In the past decade (2013-2023), approximately 1990 papers concerning the application of various chiral materials in chiral sensors have been published. Biological materials and nanomaterials have important applications in the development of chiral sensors, which accounting for 26.67% and 45.24% of the material-related applications in these sensors, respectively; moreover, the development of chiral nanomaterials is closely related to the development of portable and stable chiral sensors. Natural chiral materials, utilized as selective recognition units, are combined with carriers characterized by good physical and chemical properties through functionalization to form various functional chiral materials, which improve the recognition efficiency of chiral sensors. In this article, from the perspective of biological materials, polymer materials, nanomaterials, and other functional chiral materials, the applications of chiral sensors are summarized and the research prospects of chiral sensors are discussed.

Chiral α-Aminophosphonates as Ligands in Copper-Catalyzed Asymmetric Oxidative Coupling of 2-Naphthols

Chiral α-aminophosphonates with adjacent carbon and phosphonate stereogenic centers have been employed as ligands in the copper-catalyzed oxidative coupling of 2-naphthols, resulting in the production of chiral BINOLs in favorable yields and moderate to good enantiomeric excess. This represents the first application of chiral P-based ligands to enable such a transformation. The synthesis of these chiral α-aminophosphonate ligands offers a significant advantage over approaches that typically necessitate elaborate synthetic processes for chiral ligand production.

Recent advances in circularly polarized luminescence of planar chiral organic compounds

Circularly polarized luminescence (CPL), as an important chiroptical phenomenon, can not only directly characterize excited-state structural information about chiroptical materials but also has great application prospects in 3D optical displays, information storage, biological probes, CPL lasers and so forth. Recently, chiral organic small molecules with CPL have attracted a lot of research interest because of their excellent luminescence efficiency, clear molecular structures, unique flexibility and easy functionalization. Planar chiral organic compounds make up an important class of chiral organic small molecular materials and often have rigid macrocyclic skeletons, which have important research value in the field of chiral supramolecular chemistry (e.g., chiral self-assembly and chiral host-guest chemistry). Therefore, research into planar chiral organic compounds has become a hotspot for CPL. It is time to summarize the recent developments in CPL-active compounds based on planar chirality. In this feature article, we summarize various types of CPL-active compounds based on planar chirality. Meanwhile, we overview recent research in the field of planar chiral CPL-active compounds in terms of optoelectronic devices, asymmetric catalysis, and chiroptical sensing. Finally, we discuss their future research prospects in the field of CPL-active materials. We hope that this review will be helpful to research work related to planar chiral luminescent materials and promote the development of chiral macrocyclic chemistry.

Dual Enhancement of Phosphorescence and Circularly Polarized Luminescence through Entropically Driven Self-Assembly of a Platinum(II) Complex

Addressing the dual enhancement of circular polarization (glum) and luminescence quantum yield (QY) in circularly polarized luminescence (CPL) systems poses a significant challenge. In this study, we present an innovative strategy utilizing the entropically driven self-assembly of amphiphilic phosphorescent platinum(II) complexes (L-Pt) with tetraethylene glycol chains, resulting in unique temperature dependencies. The entropically driven self-assembly of L-Pt leads to a synergistic improvement in phosphorescence emission efficiency (QY was amplified from 15 % at 25 °C to 53 % at 60 °C) and chirality, both in the ground state and the excited state (glum value has been magnified from 0.04×10-2 to 0.06) with increasing temperature. Notably, we observed reversible modulation of phosphorescence and chirality observed over at least 10 cycles through successive heating and cooling, highlighting the intelligent control of luminescence and chiroptical properties by regulating intermolecular interactions among neighboring L-Pt molecules. Importantly, the QY and glum of the L-Pt assembly in solid state were measured as 69 % and 0.16 respectively, representing relatively high values compared to most self-assembled CPL systems. This study marks the pioneering demonstration of dual thermo-enhancement of phosphorescence and CPL and provides valuable insights into the thermal effects on high-temperature and switchable CPL materials.

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.

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.

Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene

4,4'-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4'-biazulene confirmed its axial chirality. The enantiomers of 4,4'-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'-biazulene is comparable to that of 1,1'-binaphthyl.

Design and Catalytic Asymmetric Synthesis of Furan-Indole Compounds Bearing both Axial and Central Chirality

In the chemistry community, catalytic asymmetric synthesis of furan-based compounds bearing both axial and central chirality has proven to be a significant but challenging issue owing to the importance and difficulty in constructing such frameworks. In this work, we have realized the first catalytic asymmetric synthesis of five-five-membered furan-based compounds bearing both axial and central chirality via organocatalytic asymmetric (2+4) annulation of achiral furan-indoles with 2,3-indolyldimethanols with uncommon regioselectivity. By this strategy, furan-indole compounds bearing both axial and central chirality were synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities. Moreover, theoretical calculations were conducted to provide an in-depth understanding of the reaction pathway, activation mode, and the origin of the selectivity.

Cobalt-Catalyzed Asymmetric Reductive Alkenylation and Arylation of Heterobiaryl Tosylates: Kinetic Resolution or Dynamic Kinetic Resolution?

Herein, we report a cobalt-catalyzed atroposelective reductive cross-coupling of racemic heterobiaryl tosylates with a C(sp2)-X type electrophile. Both aryl and alkenyl halides are competent precursors for this reaction, providing a variety of heterobiaryls as the products in a highly enantioselective manner with high functionality tolerance. The related asymmetric arylation and alkenylation are discovered to proceed with divergent mechanisms. The reaction pathway changes from kinetic resolution (KR) when alkenyl bromides and aryl iodides bearing strong electron-withdrawing substitution on the para-position are employed as the starting materials to an enantioconvergent transformation via dynamic KR of configurationally labile cobaltacycles when relatively electron-rich aryl iodides are used. The change of the reaction mechanisms turns out to arise from the relative rates of two competing elementary steps, which are the epimerization of the cyclic Co(I) intermediates and their trapping by the coupling electrophiles of the C(sp2)-type via oxidative addition.

The Effect of Intramolecular Hydrogen Bonds on the Rotational Barriers of the Biaryl C-C Axis

Axially chiral compounds are attracting more attention recently. Although hydrogen bonds are reported as a vital weak force that influences the properties of compounds, the effect of intramolecular hydrogen bonds on the atropisomerization of the Caryl -Caryl single bonds has not yet been well quantitatively investigated. Here, a series of axially chiral biaryl compounds were synthesized to study the effect of hydrogen bonds on the rotational barriers of the biaryl C-C axis. Experimental studies demonstrated that the rotational barrier of hydrogen bonding biaryl 9 was significantly lower (46.7 kJ mol-1 ) than biaryl 10 without hydrogen bonds. Furthermore, theoretical studies revealed that the intramolecular hydrogen bond stabilized the transition state (TS) of tri-ortho-substituted biaryl 9, relieving the steric repulsion in the TS. We believe that this study will provide chemists with a deeper understanding of the atropisomerization process of axially chiral biaryl compounds.

Hyperconjugative Aromaticity-Based Circularly Polarized Luminescence Enhancement in Polyaurated Heterocycles

Hyperconjugative aromaticity (HA) frequently appears in metalla-aromatics, but its effect on photophysical properties remains unexplored to date. Herein, we reveal two different HA scenarios in nearly isostructural triaurated indolium and benzofuranylium compounds. The biased HAs show a discernible effect on the spatial arrangement of metal atoms and thus tailor metal parentage in frontier orbitals and the HOMO-LUMO energy gap. Theoretical calculations and structural analyses demonstrate that HA not only influences the degree of electron delocalization over the trimetalated aromatic rings but also affects π-coordination of Au(I) and intercluster aurophilic interaction. Consequently, the triaurated benzofuranylium complex shows better photoluminescence performance (quantum yield up to 49.7%) over the indolium analogue. Furthermore, four pairs of axially chiral bibenzofuran-centered trinuclear and hexanuclear gold clusters were purposefully synthesized to correlate their HA-involved structures with the chiroptical response. The triaurated benzofuranylium complexes exhibit strong circular dichroism (CD) response in solution but CPL silence even in solid film. In contrast, the hexa-aurated homologues display strong CD and intense CPL signals in both aggregated state and solid film (luminescence anisotropy factor glum up to 10-3). Their amplified chiroptical response is finally ascribed to the dominant intermolecular exciton couplings of large assemblies formed through the HA-tailored aggregation of hexanuclear compounds.

Chiral Transient Ligand Enabled Enantioselective Synthesis of Atropisomers Decorated with Unactivated Olefins via a Palladium-Catalyzed C-H Olefination

Herein, atroposelective synthesis of axially chiral biaryls with unactivated olefins by a palladium-catalyzed C-H olefination using a chiral transient directing group strategy has been disclosed. This protocol is well compatible with a variety of biaryl-2-aldehydes as well as various olefins such as allyl sulfonamides and allyl sulfones to provide the atroposelective olefinated products in synthetically useful yields with excellent enantioselectivities up to >99% ee. In addition, a wide number of axially chiral biaryl alcohols were synthesized by the simple diversification of the products in excellent enantioselectivity.

Dynamic Kinetic Resolution of Indole-Based Sulfenylated Heterobiaryls by Rhodium-Catalyzed Atroposelective Reductive Aldol Reaction

A highly enantio- and diastereoselective dynamic kinetic resolution (DKR) of configurationally labile 3-aryl indole-2-carbaldehydes is described. The DKR proceeds via a Rh-catalyzed intermolecular asymmetric reductive aldol reaction with acrylate esters, with simultaneous generation of three stereogenic elements. The strategy relies on the labilization of the stereogenic axis that takes place thanks to a transient Lewis acid-base interaction (LABI) between the formyl group and a thioether moiety strategically located at the ortho' position. The atropisomeric indole products present a high degree of functionalization and can be further converted to a series of axially chiral derivatives, thereby expanding their potential application in drug discovery and asymmetric catalysis.

Dynamic Kinetic Resolution of 2-(Quinolin-8-yl)Benzaldehydes: Atroposelective Iridium-Catalyzed Transfer Hydrogenative Allylation

An atroposelective Ir-catalyzed dynamic kinetic resolution (DKR) of 2-(quinolin-8-yl)benzaldehydes/1-naphthaldehydes by transfer hydrogenative coupling of allyl acetate is disclosed. The allylation reaction takes place with simultaneous installation of central and axial chirality, reaching high diastereoselectivities and excellent enantiomeric excesses when ortho-cyclometalated iridium-DM-BINAP is used as the catalyst. The racemization of the substrates occurs through a designed transient Lewis acid-base interaction between the quinoline nitrogen atom and the aldehyde carbonyl group.

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.

Atroposelective Synthesis of Axially Chiral Styrenes Connecting an Axially Chiral Naphthyl-indole Moiety Using Chiral Phosphoric Acid Catalysis

The organocatalytic asymmetric reactions of C2-unsubstituted racemic naphthyl-indoles with orthoalkynylnaphthols were employed to synthesize axially chiral styrenes connected to an axially chiral naphthyl-indole unit. Utilizing chiral phosphoric acid as the catalyst, these axially chiral styrenes were prepared in good yields (up to 96%) and excellent stereoselectivity (up to >99.9% ee, >20:1 dr, and >99:1 E/Z) in mild conditions. Moreover, further synthetic transformations were achieved with high yields and excellent stereocontrol.

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.

Atroposelective Synthesis of 2,2'-Bis(arylamino)-1,1'-biaryls by Oxidative Iron(III)- and Phosphoric Acid-Catalyzed C-C Coupling of Diarylamines

We describe an iron-catalyzed asymmetric oxidative C-C coupling of diarylamines which proceeds at room temperature with air as final oxidant. Using hexadecafluorophthalocyanine-iron(II) as catalyst in the presence of catalytic amounts of an axially chiral biaryl phosphoric acid, the resulting chiral 2,2'-diamino-1,1'-biaryls are obtained in up to 90 % ee as confirmed by chiral HPLC. A detailed mechanism has been proposed with a radical cation-chiral phosphate ion pair as key intermediate leading to the observed asymmetric induction.

A pillar[5]arene-based planar chiral charge-transfer dye with enhanced circularly polarized luminescence and multiple responsive chiroptical changes

The fabrication of circularly polarized luminescent (CPL) organic dyes based on macrocyclic architecture has become an importantly studied topic in recent years because it is of great importance to both chiral science and supramolecular chemistry, where pillar[n]arenes are emerging as a promising class of planar chiral macrocyclic hosts for CPL. We herein synthesized an unusual planar chiral charge-transfer dye (P5BB) by covalent coupling of triarylborane (Ar3B) as an electron acceptor to parent pillar[5]arene as an electron donor. The intramolecular charge transfer (ICT) nature of P5BB not only caused a thermally responsive emission but also boosted the luminescence dissymmetry factor (g lum). Interestingly, the specific binding of fluoride ions changed the photophysical properties of P5BB, including absorption, fluorescence, circular dichroism (CD), and CPL, which could be exploited as an optical probe for multi-channel detection of fluoride ions. Furthermore, the chiroptical changes were observed upon addition of 1,4-dibromobutane as an achiral guest.

Circularly polarized electroluminescence from a single-crystal organic microcavity light-emitting diode based on photonic spin-orbit interactions

Circularly polarized (CP) electroluminescence from organic light-emitting diodes (OLEDs) has aroused considerable attention for their potential in future display and photonic technologies. The development of CP-OLEDs relies largely on chiral-emitters, which not only remain rare owing to difficulties in design and synthesis but also limit the performance of electroluminescence. When the polarization (pseudospin) degrees of freedom of a photon interact with its orbital angular momentum, photonic spin-orbit interaction (SOI) emerges such as Rashba-Dresselhaus (RD) effect. Here, we demonstrate a chiral-emitter-free microcavity CP-OLED with a high dissymmetry factor (g_EL) and high luminance by embedding a thin two-dimensional organic single crystal (2D-OSC) between two silver layers which serve as two metallic mirrors forming a microcavity and meanwhile also as two electrodes in an OLED architecture. In the presence of the RD effect, the SOIs in the birefringent 2D-OSC microcavity result in a controllable spin-splitting with CP dispersions. Thanks to the high emission efficiency and high carrier mobility of the OSC, chiral-emitter-free CP-OLEDs have been demonstrated exhibiting a high g_EL of 1.1 and a maximum luminance of about 60000 cd/m², which places our device among the best performing CP-OLEDs. This strategy opens an avenue for practical applications towards on-chip microcavity CP-OLEDs.

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.

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.

Aggregation-Induced Emission and Circularly Polarized Luminescence Duality in Tetracationic Binaphthyl-Based Cyclophanes

Here, we report an approach to the synthesis of highly charged enantiopure cyclophanes by the insertion of axially chiral enantiomeric binaphthyl fluorophores into the constitutions of pyridinium-based macrocycles. Remarkably, these fluorescent tetracationic cyclophanes exhibit a significant AIE compared to their neutral optically active binaphthyl precursors. A combination of theoretical calculations and time-resolved spectroscopy reveal that the AIE originates from limited torsional vibrations associated with the axes of chirality present in the chiral enantiomeric binaphthyl units and the fine-tuning of their electronic landscape when incorporated within the cyclophane structure. Furthermore, these highly charged enantiopure cyclophanes display CPL responses both in solution and in the aggregated state. This unique duality of AIE and CPL in these tetracationic cyclophanes is destined to be of major importance in future development of photonic devices and bio-applications.

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).