Helicenes with Four Helical Turns: Dimerization of [13]Helicenes to [27]Helicenoids

The promise of enhanced circularly polarized luminescence (CPL) from organic molecules has inspired synthetic efforts to prepare elongated multi-layer helicenes. It has proven particularly challenging to synthesize and isolate enantiomerically pure multi-turn helicenes, and consequently, clear guidelines for the molecular design of helicenes to achieve large CPL output remain elusive. We explore the oxidative dimerization of a 2-naphthol-annulated hetero[13]helicene, and observe the formation of two types of structurally distinct hetero[27]helicenes, both with four helical turns, form. With CuIICl(OH)-TMEDA as the oxidant, a symmetric dimer (bi[13]) formed, joining two helicenes with the same helicity in their keto tautomeric forms. Using CuII(OTf)₂ as the oxidant led to an unsymmetrical [27]helicenoid composed by an (M)- and a (P)-enantiomer of the [13]helicene through an unusual coupling between the 1- and 3-positions of the 2-naphthol units. Structural characterization was achieved by NMR spectroscopy and single-crystal (X-ray or electron) diffraction analysis. The enantiomers of this [27]helicenoid were analyzed by electronic circular dichroism (ECD) and CPL measurements. The spectroscopic data were corroborated by DFT calculations, and the intense CPL output is preserved despite the presence of an (M)- and a (P)-helicene in the same molecule. These [27]helicenoids are the first isolated structures that feature helical π-systems with four helical turns.

C3-symmetric Triple Thia/Sulfone[6]Helicene with Dual Emissions

Four novel triple thia/sulfone[6]helicenes with C3 symmetry were obtained via Scholl reaction and sulfonation. Their helical structures were confirmed by X-ray crystal analysis. These helicenes show dual emission of fluorescence and phosphorescence and enantiomers BTT[6]H exhibit novel dual emission of circularly polarized luminescence (CPL) or circularly polarized phosphorescence (CPP) at 77 K.

Double [8]Helicene Featuring a Dibenzo[g,p]chrysene Core: Synthesis and Chiroptical Response

Double [8]helicene 1, featuring a dibenzo[g,p]chrysene core, was synthesized via the Scholl reaction, and its structure was unambiguously confirmed by single-crystal X-ray diffraction analysis of its dicationic salt [1-Cl]2+·(SbCl6-)2. The compound exhibits red fluorescence with an emission maximum at 618 nm (λem) and a quantum yield of 16.2%, highlighting its potential in optoelectronic applications. Furthermore, circular dichroism (CD) and circularly polarized luminescence (CPL) measurements reveal notable chiroptical activity, with absorption and emission dissymmetry factors of |gabs| = 5.11 × 10-3 and |glum| = 7.1 × 10-4, respectively.

Advances in chiral luminescent liquid crystals (CLLCs): from molecular design to applications

Research on circularly polarized luminescent (CPL) materials has evolved into a hot research topic because of their potential application prospects in the optoelectronics and chiroptical fields. Achieving a high glum value and high quantum efficiency is essential and challenging in CPL research. To date, various material design strategies, such as chiral organic small molecules, CPL polymers, chiral lanthanide complexes, chiral liquid crystals and supramolecular self-assembly, have been proposed to achieve a CPL emitter with a high glum value. Among them, chiral luminescent liquid crystals (CLLCs) are recognized as a key approach for achieving CPL materials with a high glum factor owing to their exceptional optical properties and flexibility. In this review, we focused on the various synthesis methods employed for developing CLLCs, their properties and their potential applications. The synthesis section discusses various approaches employed to design chiral luminescent liquid crystals, including (i) doping systems for incorporating chiral dopants into achiral liquid crystalline hosts and (ii) nondoping methods for preparing AIE active chiral luminescent liquid crystalline materials. The section on properties highlights how chirality influences the optical, electronic and structural characteristics of CLLCs. Finally, we discuss the diverse applications of CLLCs from photonics and chiral switching to optoelectronic devices and beyond. This review provides new insights into recent research developments and future opportunities in this booming research field. We anticipate that this review could offer a clear picture of the interesting properties of chiral luminescent liquid crystal materials and inspire more researchers to work in this potential area.

Frontiers in Bioinspired Polymer-Based Helical Nanofibers from Electrospinning

Helices are among the most significant structures in nature, representing an emerging group of materials distinguished by their unique helical geometry. Recently, helical nanofibers have attracted considerable attention due to their exceptional structural characteristics and versatile applications in various fields, including tissue engineering, biomedicine, nanotechnology, and chiral materials. Therefore, developing methods to fabricate biomimetic helical fibers on demand, which can exhibit a diverse range of physical properties and forms, is of great interest across multiple disciplines. Despite the significant interest in helical fibrous materials, the fabrication of such complex structures at the micro- or nanoscale level remains a major challenge. Electrospinning offers a simple and versatile technique for producing micro- and nanofibers in various helical shapes. This review systematically summarizes and classifies the state-of-the-art advancements in electrospun helical nanofibers into four categories based on their forming mechanisms: viscoelastic asymmetric contraction, bending instability motion, jet-induced buckling response, and rotary winding molding. Additionally, the recent applications of these helical nanofibrous materials in areas such as environmental remediation, interactive textiles, and biomedical engineering are also summarized. Furthermore, the current challenges and future perspectives in the field are put forward. We anticipate that the insights provided will contribute to the rational design of advanced artificial helical materials, thereby enhancing their practical applications in the future.

A novel boron-stereogenic fluorophore with dual-state circular polarization luminescence via a self-dispersing strategy

Molecular engineering is a reliable approach for the development of circularly polarized luminescence (CPL) materials for various applications. However, creating dual-state CPL platforms that possess chirality while achieving a delicate balance between molecular rigidity and flexibility remains a formidable challenge. In this study, a novel bisarylboron-anchored pyrrolylsalicylhydrazone (BOPSH) platform was synthesized via a facile "one-pot" condensation. These key aryl-boron substituents not only provide structural rigidity to the fluorophore, enhancing the bright emission and suppressing emission quenching from π-π stacking in solid states due to their twisting and bulky steric effects, but also generate a boron-stereogenic center and enable strong CPL by promoting intramolecular charge-transfer transitions. As a result, these BOPSHs show intense absorption and strong dual-state emissions in both solution and solid states (with Φ PL value approaching unity), emitting across the visible region with excellent chemical, photostability, and thermal stability. Meanwhile, their enantiomers display dual-state CPL performance, with luminescence dissymmetry factors (g lum) up to 9.40 × 10-3, and CP electroluminescence (EL) with a dissymmetry factor (g EL) of 3.07 × 10-3, along with excellent maximum external quantum efficiencies (η ext,max) of 5.0%, approaching the theoretical limit for fluorescent molecules. We expect our study to break new ground in the construction of chiral dual-state materials with diverse structures.

Synthesis and Properties of Thiophene-Quinone Helicenes with Different Molecular Curvature

Naphthothiophene-quinone helicenes with different molecular curvatures, i. e., NTQ, 2,3-NDTQ and 2,6-NDTQ, are synthesized in seven steps. Diels-Alder reaction between p-benzoquinone and silyl enol ether derived from the corresponding naphthothiophenes was used as a key step. Both UV-Vis and fluorescence spectroscopy, cyclic voltammetry experiments, including DFT calculation demonstrated that, in comparison to the basic thiophene-quinone system, both M-shaped (2,3-NDTQ) and S-shaped (2,6-NDTQ) structures possessed a narrower HOMO-LUMO band gap than C-shaped NTQ, implying the effect of extended conjugation. The change of molecular shape resulted in the shift in both UV-Vis and fluorescence spectra, but the strong electron withdrawing effect of benzoquinone is more dominant.

π-Extended Heli(aminoborane)s with Highly Bright Circularly Polarized Luminescence and Narrowband Emission

Helical molecular carbons (HMCs) possess high absorption/luminescence dissymmetry factors (gabs/glum) and significant luminescence quantum yield (Φlum), resulting in a high circularly polarized luminescence (CPL) brightness (BCPL), which is essential for the development of CPL materials for practical applications. Herein, we designed and synthesized a series of boron-nitrogen (BN)-doped HMCs, named π-extended heli(aminoborane)s (E[10]HAB-A, E[10]HAB-B and E[10]HAB-C), consisting of laterally π-extended [10]helicene skeleton with alternating N and B atoms at the inner rim. The aromaticity, electronic structures, and photophysical properties of E[10]HAB-A/B/C were systematically investigated through experiments and theoretical calculations. E[10]HAB-A/B/C displayed remarkable photophysical properties, including high molar extinction coefficient and bright narrowband emission. The isolated enantiomers of E[10]HAB-A/B/C exhibited intense circular dichroism (CD) and CPL, in which E[10]HAB-A shows gabs and glum values up to 0.024 and 0.017, simultaneously with high Φlum of 82 % and a narrow full width at half maximum of 16 nm. Accordingly, E[10]HAB-A exhibits a BCPL as high as 583 M-1 cm-1, which is the largest value among the reported BN-doped HMCs. Our study indicates that inner rim BN-doping and π-extension are effective strategies to achieve high Φlum and balanced glum values in HMCs.

Hexadehydro Diels-Alder/Alkynyliodanation Cascade: A Highly Regioselective Entry to Polycyclic Aromatics

We report here a cascade process integrating the hexadehydro Diels-Alder (HDDA) reaction with alkynyliodanation, enabling efficient synthesis of highly substituted aryl-λ3-iodanes. Heating a mixture of a tetrayne and an alkynylbenziodoxole induces regioselective insertion of the tetrayne-derived aryne into the alkynyl-iodine(III) bond, yielding a 1,4-dialkynyl-2-iodanyl-3-aryl(or alkyl)benzene derivative. The unique regiochemistry facilitates subsequent π-extension, allowing divergent access to polyaromatic frameworks, such as helicenes and cyclopenta[cd]pyrenes, underscoring the utility of aryne carboiodanation in complex aromatic synthesis.

Synthesis and Chiroptical Properties of a Saddle-Shaped Quadruple Helicene with a Cyclooctatetrapyrrole Core

A quadruple helicene with a cyclooctatetrapyrrole core was synthesized via a simple Ullmann reaction. Its single-crystal structure exhibits a saddle shape. The compound shows fluorescence emission with λem = 575 nm and good chiroptical properties, including a dissymmetric absorption factor (|gabs|) of 5.84 × 10-3 and a dissymmetric emission factor (|glum|) of 1.21 × 10-2.

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.

Helicene-Fluorescein Hybrids: A Reversible Base-Triggered (Chir)optical Switch with Sign Inversion of Circularly Polarized Luminescence

Novel circularly-polarized-luminescence (CPL) materials were designed and synthesized by integrating the intrinsic chirality of helicene with the acid/base-responsive properties of fluorescein. The synthesized helicene-fluorescein hybrids exhibit reversible switching between blue fluorescence under neutral conditions and red fluorescence under basic conditions. Furthermore, these hybrid compounds demonstrate unique chiroptical switching behavior with a rare base-triggered CPL sign inversion, where (P)-isomers show a negative CPL signal around 450 nm under neutral conditions and a positive signal around 620 nm under basic conditions, with three-fold enhanced dissymmetry factors (|glum|) under basic conditions. The structural basis for this switching was elucidated through TD-DFT calculations, revealing distinct angles between magnetic and electric transition dipole moments in closed versus open forms. Live-cell imaging experiments using HeLa cells revealed that the hybrid compounds exhibit intracellular red emission with minimal cytotoxicity, thus promising potential as chiral fluorescent probes for biological applications.

Flexible and rigid "chirally distorted" π-systems: binaphthyl conjugates as organic CPL-active chromophores

The inclusion of high-performance dyes into chiral π-conjugated systems is an effective strategy for activating significant chiroptical properties. We report the preparation and characterization of configurationally stable, axially-chiral π-conjugated systems in which acridone or 2,5-diarylamino-terephthalate has been fused into the chiral scaffold of a 1,1'-binaphthyl moiety. The high-yielding synthesis afforded π-conjugated systems with characteristics essentially matching those of the parent dyes while introducing detectable CPL activity in solution. In the acridone conjugate, good fluorescence is maintained in solution, but in the solid state, the distortion introduced by the binaphthyl system does not substantially help in restoring emissive properties; the flexibility and the emissive properties of the 2,5-diphenylamino-terephthalate chromophore are maintained in the conjugate. The new chiral chromophoric systems show absorption in the UV-vis domain, with good fluorescence properties in the visible range (quantum yields up to 23% and glum values up to 4 × 10-4).

Selective and Divergent Synthesis of Naphthalene- and Phenanthrene-Fused Azahelicenes by Turning Rearrangement On or Off

The Scholl reaction has been used to synthesize a variety of polycyclic aromatic hydrocarbons, where 1,2-aryl shifts have sometimes occurred to yield unique rearrangement products. However, such 1,2-aryl shifts are often uncontrollable, and the selective and divergent synthesis with or without rearrangement is desired. Here, we achieved the control of the rearrangement in the Scholl reaction of carbazoles by changing the N-substituents. The Scholl reaction of 3,6-bis{2-(2-naphthyl)phenyl}carbazoles and 3,6-bis{2-(9-phenanthrenyl)phenyl}carbazoles with an N-benzyl group gave multiple azahelicenes via double rearrangement, while those with an N-benzoyl group gave aza[9]helicene and quadruple [4]helicene in the former and latter cases, respectively. The reaction mechanisms on the divergent reaction pathways were investigated by DFT calculations, which well supported the experimental results.

Structures, Chiroptical Properties, and Unexpectedly Facile Helical Inversion of Highly Elongated Anthracene-Fused Expanded Helicenes

Helical fused anthracenes were elongated by fusing additional aromatic units at both ends to yield novel expanded helicenes. Compounds [5]HA2N and [7]HA consisting of 19 and 21 benzene rings, respectively, were synthesized by fourfold cycloisomerization of the corresponding terminal alkyne precursors. The helical structures were confirmed by X-ray crystallographic analysis, where the aromatic frameworks stacked effectively with the helical turn numbers exceeding two. The enantiomers of the two compounds were resolved by chiral HPLC. Whereas [5]HA2N readily underwent enantiomerization at room temperature at the barrier to enantiomerization of 91 kJ mol-1, the barrier was enhanced to 99 kJ mol-1 for the long analog [7]HA. The enantiomers of [7]HA exhibited strong responses in the circular dichroism (CD) and circularly polarized luminescence (CPL) spectra, as scaled by dissymmetry factors |gabs|=0.034 and |glum|=0.012. Theoretical calculations by the r2SCAN-3c method suggested stepwise mechanisms for the enantiomerization via helical inversion with acceptable barrier heights. The unexpectedly flexible nature of the aromatic frameworks of [5]HA2N and [7]HA was discussed on the basis of the proposed mechanism.

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.

Circularly Polarized Organic Light-Emitting Diode Based on Device Functional Layer Materials

Circularly polarized light-emitting devices have found extensive application prospects in 3D displays and optoelectronic information. Among them, circularly polarized organic light-emitting diodes (CP-OLED), as a rising star of circularly polarized light-emitting devices, achieved good research results. However, the preparation of CP-OLED with a high electroluminescence asymmetry factor and high external quantum efficiency is a hot and difficult research topic. At present, the approaches for achieving circularly polarized electroluminescence via CP-OLED are: 1) Using chiral materials as luminescent materials, 2) Utilizing chiral functional materials. This review summarizes recent methodologies used for manufacturing CP-OLED. It focuses on the construction strategies and applications of chiral functional materials (chiral host materials, chiral hole transport materials, and chiral electron transport materials) in CP-OLED. While challenges such as complex chiral design and material interactions persist, advancements in material design and device architecture propel CP-OLED forward. These developments promise to elevate CP-OLED as a focal point in optoelectronic research, facilitating high-performance, circularly polarized luminescence (CPL)-capable devices for practical applications.

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.

A Carbazole-Centered Expanded Helicene Stabilized with Hexabenzocoronene (HBC) Units

The synthesis and stabilization of heteroatom-doped nanocarbon molecules, such as carbazole-containing (super)helicenes, present significant challenges due to the complexities involved in maintaining structural integrity and electronic functionality. In this study, we successfully synthesized a carbazole-centered expanded tris-hexabenzo[7]helicene (1) via a facile FeCl3-mediated Scholl coupling reaction. 1 exhibits both chemical and chiral stability and demonstrates fluorescence at 628 nm with a quantum yield of 0.40. Additionally, the enantiomers of 1 display pronounced chiroptical properties, including a distinct circular dichroism (CD) signal spanning from 300 to 600 nm. The absorption dissymmetry factor (|gabs|) is determined to be 2.98×10-3, while the circularly polarized luminescence brightness (BCPL) is measured as 32.50 M-1 cm-1.

[(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.

Asymmetric Synthesis of Azahelicenes via CPA-Catalyzed Kinetic Resolution

The azahelicenes are structurally fascinating and practically useful chiral scaffolds, but their synthesis, especially in a catalytically asymmetric manner, is rather challenging. Herein, we report a CPA-catalyzed transfer hydrogenation process, which enables a rapid kinetic resolution of aza[6]helicenes. The established strategy provides facile access to enantioenriched aza[6]helicenes and tetrahydro[6]helicenes from easily available starting materials. A gram-scale reaction and facile conversion of the helical products into a promising chiral Lewis base catalyst, a chiroptical switch material, and monophosphine ligands further highlight the potential application of this protocol.

Adding multiple electrons to helicenes: how they respond?

Helicenes of increasing dimensions and complexity have recently burst into the scene due to their unique structures coupled with interesting chiral, optical, and conducting properties. The helicene-related research has quickly progressed from fundamental curiosity to a diverse range of applications in organic catalysis, optoelectronic devices, chiroptical switches, sensors, and energy storage. The in-depth understanding of electron accepting properties of helicenes should further advance their materials chemistry applications, however, previous reports only relied on spectrocopic and electrochemical studies, while their structural changes weren't extensively discussed. Therefore, we initiated a broad investigation of chemical reduction behaviour of helicenes ranging in size and properties coupled with X-ray diffraction characterization of the reduced products. The responses of helicenes with different structures to the stepwise electron addition were investigated using a combination of X-ray crystallography, spectroscopic methods, and calculations. This study revealed topology- and charge-dependent consequences of chemical reduction ranging from reversible geometry perturbation to irreversible core transformation and site-specific reactivity of helicenes in addition to original alkali metal coordination patterns. This overview is focused on the crystallographically confirmed examples stemming from chemical reduction reactions of different helicenes with alkali metals. The opened discussion should stimulate further exploration of reactivity and complexation of novel π-expanded and heteroatom-doped helicenes based on the revealed structure-property correlations, thus advancing their applications as intriguing new materials.

Donor-Acceptor-Donor Dyads with Electron-Rich π-Extended Azahelicenes to Panchromatic Absorbing Dyes

Panchromatic dyes have been highly useful in the realm of optical devices. Here, we report that panchromatic dyes with heterohelicenes have been successfully synthesized using a donor-acceptor strategy. Our synthesis resulted in the creation of π-extended aza[5]helicene oligomers with butadiyne linkages, which displayed bathochromically shifted absorption and emission spectra. The solvent-dependent optical measurements revealed the intramolecular charge transfer characteristic of these molecules, and theoretical calculations described the biased molecular orbitals on the azahelicene units that generated the charge-transfer characteristic. Encouraged by these results, we also prepared donor-acceptor-donor dyads using azahelicenes and dimide derivatives, resulting in panchromatic absorbing characteristics covering the range from 250 nm to 800 nm. Theoretical calculations showed the presence of mixed charge-transfer transitions and localized transitions on the azahelicene units, which led to a broad light-absorbing property covering the near IR region. Additionally, we conducted measurements of circular dichroism and circularly polarized luminescence for the obtained products. The g-values were reduced by oligomerization, indicating that the lowest energy transitions were allowed in nature.

Twist along Central C-C Bonds in a Series of Fully π-Fused Propellanes

Without stereogenic carbon centers, organic molecules can be chiral when they have large energy barriers for conformational inversion. In this work, conformational behaviors are investigated for a series of tricyclic propellane skeletons with increasing 6-membered-ring peripheral moieties fused with aromatic rings. According to theoretical calculations, trinaphtho[3.3.3]propellane has three vertical naphthalene rings like triptycene shape without torsion along the central C-C single bond. On the other hand, hexabenzo[4.4.4]propellane shows hexaphenylethane-like ca. 60° twist along the bond with large activation energy of 64 kcal mol-1 for twist inversion because of the high congestion caused by three 6-membered-ring loops. Indeed, the [4.4.4]propellane gives a stable pair of chiroptical enantiomers toward heating at 146 °C. By contrast, a hybrid [4.3.3]propellane exhibits fast interconversion between two twisted conformations even at -80 °C.

Construction and Circularly Polarized Luminescence of Thiophene-Based Multiple Helicenes

Thiophene-based monohelicene (TS[7]H), triple helicenes (TT[7]H), and hexapole helicenes (TH[7]H) were synthesized via oxidative photocyclization and cascade Suzuki/intramolecular cyclization as the crucial steps. The enantiomers of TS[7]H, TT[7]H-2, and TH[7]H exhibited circularly polarized luminescence (CPL), and the luminescence dissymmetry factors (glum) gradually increased from -5.1 × 10-4 to -2.0 × 10-3 with an increase in multiplicity from TS[7]H to TH[7]H. In addition, TS[7]H, TT[7]H, and TH[7]H displayed a second-level long afterglow at 77 K.

Induced Circularly Polarized Luminescence From 0D Quantum Dots by 2D Chiral Nanosheets

Materials with circularly polarized luminescence (CPL) exhibit great application potential in biological scenes such as cell imaging, optical probes, etc. However, most developed materials are non-aqueous and toxic, which seriously restricts their compatibility with the life systems. Thus, it is necessary to explore a water-based CPL system with high biocompatibility so that to promote the biologic application process. Herein, a facile and efficient route to achieve the CPL properties of a functional aqueous solution is demonstrated by the combination of 0D quantum dots (QDs) and 2D chiral nanosheets. Benefited by the specific absorption ability of nanosheets for left/right-handed CPL, the QDs adsorbed onto the surface of nanosheets through hydrogen bond interactions showed apparent CPL features. In addition, this system has a good extensibility as the CPL property can be effectively regulated by changing the kind of emissive QDs. More importantly, this water-based nano-composite with facile fabrication process (one-step mixing) is suitable for the real applications, which is undoubtedly beneficial for the further progress of functional CPL materials.

Ultra-Narrowband Circularly Polarized Luminescence from Multiple 1,4-Azaborine-Embedded Helical Nanographenes

In this manuscript we present a strategy to achieve ultranarrowband circularly polarized luminescence (CPL) from multiple 1,4-azaborine-embedded helical nanographenes. The impact of number and position of boron and nitrogen atoms in the rigid core of the molecule on optical properties─including absorption and emission maxima, photoluminescence quantum yield, Stokes shift, excited singlet-triplet energy gap and full width at half-maximum (fwhm) for CPL and fluorescence─was investigated. The molecules reported here exhibits ultranarrowband fluorescence (fwhm 16-17.5 nm in toluene) and CPL (fwhm 18-19 nm in toluene). To the best of our knowledge, this is among the narrowest CPL for any organic molecule reported to date. Quantum chemical calculations, including computed CPL spectra involving vibronic contributions, provide valuable insights for future molecular design aimed at achieving narrowband CPL.

Pd-Catalyzed Dual C-H Activation/Cyclization: Convergent and Divergent Synthesis of 1-Azahelicenes

Herein, we report a convergent synthesis of 1-azahelicenes using easily available quinoline derivatives and cyclic diaryliodonium salts as starting materials. This reaction undergoes a palladium-catalyzed dual C-H activation/cyclization process to give facile access to a wide range of 1-aza[5]helicenes and 1-aza[6]helicenes with abundant functional groups (including F, Cl, Br, I, CF3, SeR, SR, and heteroaryl) in moderate to excellent yields, thereby providing new opportunities to fine-tune the properties of the helicene backbone. In addition, the obtained products could be further transformed into helicene-based Lewis base catalysts and redox switch materials easily. Notably, one selected 1-azahelicene shined bright yellow light by aggregation. These features enlarge the chemical space of 1-azahelicenes and inspire further utilization in other areas of research.

Tether-entangled conjugated helices

A new design concept, tether-entangled conjugated helices (TECHs), is introduced for helical polyaromatic molecules. TECHs consist of a linear polyaromatic ladder backbone and periodically entangling tethers with the same planar chirality. By limiting the length of tether, all tethers synchronously bend and twist the backbone with the same manner, and change it into a helical ribbon with a determinate helical chirality. The 3D helical features are customizable via modular synthesis by using two types of synthons, the planar chiral tethering unit (C 2 symmetry) and the docking unit (C 2h symmetry), and no post chiral resolution is needed. Moreover, TECHs possess persistent chiral properties due to the covalent locking of helical configuration by tethers. Concave-type and convex-type oligomeric TECHs are prepared as a proof-of-concept. Unconventional double-helix π-dimers are observed in the single crystals of concave-type TECHs. Theoretical studies indicate the smaller binding energies in double-helix π-dimers than conventional planar π-dimers. A concentration-depend emission is found for concave-type TECHs, probably due to the formation of double-helix π-dimers in the excited state. All TECHs show strong circularly polarized luminescence (CPL) with dissymmetric factors (|g lum|) generally over 10-3. Among them, the (P)-T4-tBu shows the highest |g lum| of 1.0 × 10-2 and a high CPL brightness of 316 M-1 cm-1.

Narrowband Blue Circularly Polarized Luminescence Emitter Based on BN-Doped Benzo[6]helicene with Stimuli-Responsive Properties

Boron-doped helicenes, known for their unique electronic and photophysical properties, are of great interest for numerous applications. This research introduces two new azabora[6]helicenes, H[6]BN1 and H[6]BN2, synthesized through an efficient method. These molecules have boron and nitrogen atoms in opposing positions, enhancing their distinctive attributes. Both helicenes show excellent emission properties, with H[6]BN1 and H[6]BN2 exhibiting narrowband blue fluorescence and circularly polarized luminescence (CPL), achieving glum values of 4~5×10-4 which is beneficial for chiroptical applications. The addition of a donor group, 3, 6-di-tert-butyl-9H-carbazole, in H[6]BN2 improves luminescence, likely due to enhanced molecular orbital overlap and electron delocalization. H[6]BN1's needle-like single crystals exhibit mechanochromism, changing luminescent color from yellow to green under mechanical stress, which is promising for stimulus-responsive materials. In conclusion, this study presents a novel class of BN[6]helicenes with superior chiroptical properties. Their combination of electronic features and mechanochromism makes them ideal for advanced chiroptical materials, expanding the potential of helicene-based compounds and offering new directions for the synthesis of molecules with specific chiroptical characteristics.

W-Shaped π-Extended Double Undecabenzo[7]helicene

A chiral W-shaped fully π-extended double [7]helicene (ED7H) has been synthesized and fully characterized. It displays fluorescence emission (λem = 636 nm) with a quantum yield (Φf) of 0.10. In comparison to its X-shaped and monomict π-extended [7]helicene analogues, enantiopure W-shaped ED7H exhibited superior chiral optical characteristics, including distinct circular dichroism signals from 400 to 650 nm, a good dissymmetric emission factor |glum| of 4 × 10-3, and a circularly polarized luminescence brightness value BCPL of 42 M-1 cm-1.

Impacts of heteroatom substitution on the excited state dynamics of π-extended helicenes

Benzo-annulated aza[9]helicene ([9]AH) and thia[9]helicene ([9]TH) were prepared as novel π-extended heterohelicenes. [9]TH showed a quite short fluorescence lifetime of ∼0.3 ns and intense phosphorescence at low temperature that were attributed to its larger spin-orbit coupling and faster intersystem crossing between pseudo-degenerate S1/2 and triplet states.

Cyclic Azahelicene Dimers Showing Bright Circularly Polarized Luminescence and Selective Fluoride Recognition

Although helicenes are promising molecules, the synthetic difficulty and tediousness have often been problems, and only small amounts of optically pure helicenes have been obtained by using chiral HPLC in most cases. Herein, aza[7]helicenes or closed-aza[7]helicenes with (1R)-menthyl substituents were selectively synthesized via the intramolecular Scholl reaction, and the diastereomeric pairs were separated by silica gel column chromatography. The optically pure helicenes were further transformed into the corresponding cyclic dimers, and the chiroptical properties were investigated. The rigid π-frameworks of the dimers led to the high molar extinction coefficients and fluorescence quantum yields, while the twisted helicene moieties induced clear Cotton effects and CPL in the visible region, and the high CPL brightness (BCPL) was achieved. Furthermore, the cyclic dimers were found to have the macrocyclic cavity with the two NH groups suitable for the selective binding of a fluoride anion, which induced significantly redshifted fluorescence and CPL in the red region.

Unusually Short H⋅⋅⋅H Contacts in Intramolecularly Cyclized Helically Fused Anthracenes

The intramolecular coupling of dichloro-substituted helically fused anthracenes using the Yamamoto coupling yielded cyclized products with sterically congested molecular structures. The X-ray analysis and DFT calculations showed that the aromatic framework adopted a nonplanar structure with a twisted conformation about the newly formed single bond, which acts as a chiral axis. Interestingly, the X-ray structure obtained through the Hirshfeld atom refinement revealed short interatomic distances between the inner hydrogen atoms (1.648-1.692 Å), much shorter than the sum of their van der Waals radii. Owing to these unusually short contacts, the 1H NMR spectrum exhibited a significant deshielding (12.5 ppm) and a large nuclear Overhauser effect (44 %). Additionally, the IR spectrum displayed a high-frequency shift of the C-H stretching vibration. These observations, along with the noncovalent interaction plot indicative of a characteristic steric environment, strongly support the presence of steric hindrance. Moreover, dynamic NMR measurement of the mesityl-substituted derivative yielded a barrier to helical inversion of 84 kJ mol-1. The optical properties and crystal packing of the cyclized products are also reported.

Synthesis of Aza[n]helicenes up to n = 19: Hydrogen-Bond-Assisted Solubility and Benzannulation Strategy

Synthetic challenges toward anomalous structures and electronic states often involve handling problems such as insolubility in common organic solvents and oxidative degradation under aerobic conditions. We designed benzo-annulated aza[n]helicenes, which benefit from both the suppressed elevation of highest occupied molecular orbital (HOMO) energies and high solubility due to hydrogen bonding with solvent molecules to overcome these challenges. This strategy enabled the synthesis of six new aza[n]helicenes ([n]AHs) of different lengths (n = 9-19) from acyclic precursors via one-pot intramolecular oxidative fusion reactions. The structures of all of the synthesized aza[n]helicenes were determined by X-ray diffraction (XRD) analysis, and their electrochemical potentials were measured by cyclic voltammetry. Among the synthesized aza[n]helicenes, [17]AH and [19]AH are the first heterohelicenes with a triple-layered helix. The noncovalent interaction (NCI) plots confirm the existence of an effective π-π interaction between the layers. The absorption and fluorescence spectra red-shifted as the helical lengths increased, without any distinct saturation points. The optical resolutions of N-butylated [9]AH, [11]AH, [13]AH, and [15]AH were accomplished, and their circular dichroism (CD) and circularly polarized luminescence (CPL) were measured. Thus, the structural, (chir)optical, and electrochemical properties of the aza[n]helicenes were comprehensively analyzed.

Boost the Circularly Polarized Phosphorescence of Chiral Organometallic Platinum Complexes by Hierarchical Assembly into Fibrillar Networks

Circularly polarized luminescence (CPL)-active molecular materials have drawn increasing attention due to their promising applications for next-generation display and optoelectronic technologies. Currently, it is challenging to obtain CPL materials with both large luminescence dissymmetry factor (glum) and high quantum yield (Φ). A pair of enantiomeric N N C-type Pt(II) complexes (L/D)-1 modified with chiral Leucine methyl ester are presented herein. Though the solutions of these complexes are CPL-inactive, the spin-coated thin films of (L/D)-1 exhibit giantly-amplified circularly polarized phosphorescences with |glum| of 0.53 at 560 nm and Φair of ~50 %, as well as appealing circular dichroism (CD) signals with the maximum absorption dissymmetry factor |gabs| of 0.37-0.43 at 480 nm. This superior CPL performance benefits from the hierarchical formation of crystalline fibrillar networks upon spin coating. Comparative studies of another pair of chiral Pt(II) complexes (L/D)-2 with a symmetric N C N coordination mode suggest that the asymmetric N N C coordination of (L/D)-1 are favorable for the efficient exciton delocalization to amplify the CPL performance. Optical applications of the thin films of (L/D)-1 in CPL-contrast imaging and inducing CP light generation from achiral emitters and common light-emitting diode lamps have been successfully realized.

Resolution of 9,10-Diketo[7]helicene and Its Use in One-Step Preparation of Helicene-Based D-A-D Push-Pull Systems

Racemic 9,10-diketo[7]helicene was successfully separated into enantiomers using a reversible and stereoselective reaction with 2,2'-diamino-1,1'-binaphthalene with moderate yields but with remarkable purity (>99% de). The enantiomerically pure diketone was used as a convenient starting material for the preparation of helicene-based push-pull molecules, which incorporated aza-aryl acceptors and diarylaminophenylene donor groups in a single step. A series of six push-pull systems, along with three reference molecules without donors, were prepared and studied using UV/vis and fluorescence measurements, circular dichroism, and DFT calculations.

Synthesis of sterically congested double helicene by alkyne cycloisomerization

Alkyne cycloisomerization of 2,7,10,15-tetra(ortho-alkynylphenyl)benzo[g,p]chrysene containing bulky 4-alkoxy-2,6-dimethylphenyl groups at the alkyne terminals selectively proceeded at the sterically crowded bay-region. The obtained double helicene adopts a distorted structure with a high racemization barrier due to the intramolecular steric repulsion.

Simultaneous discovery of chiral and achiral dyes: elucidating the optical functions of helical and flag-hinged boron tetradentate complexes

The construction of novel complexes can lead to the manifestation of unexpected structures and properties, thereby making chemical exploration in experiments a potential source for novel discoveries. In this study, by reacting 6,6'-dihydrazineyl-2,2'-bipyridine with acyl chlorides and subsequently coordinating with boron trifluoride, two different boron-tetradentate ligand complexes were simultaneously generated. One of these complexes exhibited a unique structure in which tetra-BF2 moieties coordinated to all four coordination sites of the ligand molecule, forming a flag-hinged structure around the bipyridine part. The second complex featured a helical structure formed by the hybridization of two BF2 and one B-O-B moieties, representing a highly unusual form of the complex. The structures of these two boron complexes were consistently observed when various substituted acyl chlorides were employed. Furthermore, it was found that enhancing electron-donor properties could induce a redshift in emissions. Utilizing the dimethylamino group as the proton receptor promoted a yellow-to-blue fluorescence switch in the tetra-BF2 complex and an OFF/ON fluorescence in the B-O-B bridged complex upon protonation. The helical chirality observed in the latter complex resulted in stable (P)/(M)-enantiomers after optical resolution. This complex exhibited circular dichroism with a |gabs| of up to 1.2 × 10-2 and circularly polarized luminescence with a |glum| on the order of 10-3 in solution and polymer film.

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

A chiral photoswitch based on enantiospecific interconversion between binaphthyl and helicenoid skeletons

A novel chiral photoswitch composed of a binaphthyl unit and a hexafluorocyclopentene ring has been synthesized. This chiral photoswitch exhibited thermally reversible photochromism between the binaphthyl and helicenoid forms based on 6π-electrocyclization. The helicity of the binaphthyl moiety was reversed upon stereospecific photocyclization and reverted back during the thermal ring opening.

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.

Impact of helical elongation of symmetric oxa[n]helicenes on their structural, photophysical, and chiroptical characteristics

The adjustment of the main helical scaffold in helicenes is a fundamental strategy for modulating their optical features, thereby enhancing their potential for diverse applications. This work explores the influence of helical elongation (n = 5-9) on the structural, photophysical, and chiroptical features of symmetric oxa[n]helicenes. Crystal structure analyses revealed structural variations with helical extension, impacting torsion angles, helical pitch, and packing arrangements. Through theoretical investigations using density functional theory (DFT) calculations, the impact of helical extension on aromaticity, planarity distortion, and heightened chiral stability were discussed. Photophysical features were studied through spectrophotometric analysis, with insights gained through time-dependent DFT (TD-DFT) calculations. Following optical resolution via chiral high-performance liquid chromatography (HPLC), the chiroptical properties of both enantiomers of oxa[7]helicene and oxa[9]helicene were investigated. A slight variation in the main helical scaffold of oxa[n]helicenes from [7] to [9] induced an approximately three-fold increase in dissymmetry factors with the biggest values of|glum| of oxa[9]helicene (2.2 × 10-3) compared to|glum|of oxa[7]helicene (0.8 × 10-3), findings discussed and supported by TD-DFT calculations.

B,N-Embedded Hetero[9]helicene Toward Highly Efficient Circularly Polarized Electroluminescence

The intrinsic helical π-conjugated skeleton makes helicenes highly promising for circularly polarized electroluminescence (CPEL). Generally, carbon helicenes undergo low external quantum efficiency (EQE), while the incorporation of a multi-resonance thermally activated delayed fluorescence (MR-TADF) BN structure has led to an improvement. However, the reported B,N-embedded helicenes all show low electroluminescence dissymmetry factors (gEL), typically around 1×10-3. Therefore, the development of B,N-embedded helicenes with both a high EQE and gEL value is crucial for achieving highly efficient CPEL. Herein, a facile approach to synthesize B,N-embedded hetero[9]helicenes, BN[9]H, is presented. BN[9]H shows a bright photoluminescence with a maximum at 578 nm with a high luminescence dissymmetry factor (|glum|) up to 5.8×10-3, attributed to its inherited MR-TADF property and intrinsic helical skeleton. Furthermore, circularly polarized OLED devices incorporating BN[9]H as an emitter show a maximum EQE of 35.5 %, a small full width at half-maximum of 48 nm, and, more importantly, a high |gEL| value of 6.2×10-3. The Q-factor (|EQE×gEL|) of CP-OLEDs is determined to be 2.2×10-3, which is the highest among helicene analogues. This work provides a new approach for the synthesis of higher helicenes and paves a new way for the construction of highly efficient CPEL materials.

Catalytic asymmetric construction of helicenes via transformation of biaryls

This review showcases a systematic overview of the available tools for the catalytic asymmetric transformation of biaryl substrates toward the construction of challenging enantioenriched helicenes and the conceptual aspects associated with each type of transformation. Depending on the properties of the biaryl and the nature of the process, several methodologies have been developed, including olefin metathesis, hydroarylation of alkynes, C-X (X = C, O, N) coupling, and C-H functionalization. Pioneering studies and an array of representative reactions are discussed to underscore the potential of these synthetic protocols.

Significance of Vibronic Coupling that Shapes Circularly Polarized Luminescence of Double Helicenes

The circularly polarized luminescence (CPL) spectra of S- and X-shaped double helicenes exhibit distinct vibrational structures and overall shape variations. In this study, we conducted an in-depth investigation into the vibronic effects influencing the CPL spectra of two double helicenes, namely DPC and DNH. Employing state-of-the-art computations utilizing the FC-HT1|VH model at the CAM-B3LYP/def2-TZVP level, we unveiled the paramount impact of Franck-Condon (FC), Herzberg-Teller (HT), and Duschinsky effects on their chiroptical responses. Our research underscores the pivotal role of structural deformations associated with the S1-to-S0 electronic transition in molding CPL spectra and wavelength-dependent dissymmetry (g) factor values, as well as the significance of HT effects in shaping and enhancing CPL responses. This extensive investigation not only advances our comprehension of the vibronic characteristics in configurationally distinct double helicenes but also offers valuable insights for the design of chiral molecules featuring controllable or finely-tunable CPL responses.

Enantiomerization of five-membered-heterocycle-embedded helicenes: A DFT study

In this work, DFT theoretical calculations were employed to investigate the enantiomerization of helicenes embedded with five-membered heterocycles. The original benzene rings in the helicene backbone were replaced by heterocycles such as furan, thiophene, pyrrole, or phosphole to create [n]helicenes with n ranging from 4 to 7. The impact of the type, position, and number of heterocycles on the enantiomerization barrier was systematically evaluated. Notably, the enantiomerization barrier was found to be significantly dependent on the rotatory angle and the position of the heterocycles, particularly for [4, 5]helicenes. With less rotatory angle of heterocycle, the enantiomerization barrier of helicenes was revealed to be lower, while when the heterocycle was close to the central part of the helicene chain, the barrier was also lower. Furthermore, the number of thiophene rings also had a marked effect on enantiomerization, showing a decrease of the barrier with more thiophene rings placed on the helicenes backbone. We expect this work would deliver new perspective on the relative studies for the helicene conformational conversion.

One-pot synthesis of azabora[6]helicene by a Schiff base forming reaction

Azabora[6]helicene as a new heterohelicene analogue was synthesized by a one-pot reaction of commercially available 2,6-diaminopyridine and benzo[c,d]indole-2(1H)-one and subsequent boron coordination. While the single-crystal X-ray diffraction analysis elucidated a helical structure in the solid state, a dynamic helicity inversion was observed in solution.

A highly fluorescent bora[6]helicene exhibiting circularly polarized light emission

Heteroatom-doped helicenes have attracted great research interest due to their inherent chirality enabling fascinating new applications. Herein we present our successful synthesis of 19c-boratribenzo[gh,jk,mn][6]helicene, the hitherto longest and first configurationally stable pristine bora[n]helicene. It displays intense orange fluorescence and circularly polarized light (CPL) emission with a high quantum yield of up to 84%. X-ray single crystal analysis reveals a highly twisted, helical shape and intriguing intermolecular stacking. Complexation with a size-complemental aza[4]helicene yielded an unprecedented hetero-chiral π-π-stacked helicene dimer.