Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P)

S-Shaped Double Helicene Diimides: Synthesis, Self-Assembly, and Mechanofluorochromism

Herein we present a synthesis of an S-shaped double helicene with fused imide moieties, achieving a contorted aromatic diimide (DHDI) with good fluorescence properties in both solution and the solid state. DHDI demonstrates distinct mechanofluorochromism from yellow to green emission under grinding of its crystalline powder.

Simultaneous construction of axial and planar chirality by gold/TY-Phos-catalyzed asymmetric hydroarylation

The simultaneous construction of two different chiralities via a simple operation poses considerable challenge. Herein a cationic gold-catalyzed asymmetric hydroarylation of ortho-alkynylaryl ferrocenes derivatives is developed, which enable the simultaneous construction of axial and planar chirality. The here identified TY-Phos derived gold complex is responsible for the high yield, good diastereoselectivity (>20:1 dr), high enantioselectivities (up to 99% ee) and mild conditions. The catalyst system also shows potential application in the synthesis of chiral biaryl compounds. The cause of high enantioselectivity of this hydroarylation is investigated with density functional theory caculation.

The simultaneous construction of two different types of chiralities is challenging. Here, the authors report a cationic gold-catalyzed asymmetric hydroarylation of ortho-alkynylaryl ferrocene derivatives, which enabled the simultaneous construction of axial and planar chirality.

Stereospecific synthesis of chiral P-containing polyaromatics based on 7-membered P-rings

We present the stereospecific synthesis of helicenoid-based phosphepines (7-membered P-rings) as well as chiral P-containing polycyclic aromatic hydrocarbons. In these systems, an axial to central chirality transfer takes place from the BINAP moiety to the P-atom. The impact of the molecular design on the structure, the (chir)optical (including circularly polarized luminescence) and redox properties are investigated.

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)₂²⁺ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)₂]²⁺ (11a,c, L = 6a,c) or [Ru₂L'(bipy)₄]⁴⁺ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.

Transformation from triple helicene to double helicene embedding adjacent stereogenic carbon atoms and axial stereogenicity

Transformation of triple helicene (TH) to double helicene (DH) with adjacent stereogenic carbon atoms and axial stereogenicity was achieved by the unexpected conjugate addition to the central aromatic ring of TH-1. We also studied the boundary at which different reactivities to addition reactions occur in some helicenes with different π-extension.

Heterohelicenes through 1,3-Dipolar Cycloaddition of Sydnones with Arynes: Synthesis, Origins of Selectivity, and Application to pH-Triggered Chiroptical Switch with CPL Sign Reversal

Regioselective access to heterohelicenes through the 1,3-dipolar cycloaddition of sydnones with arynes is described. Novel access to sydnones and poly(hetero)aromatic aryne precursors allowed the introduction of chemical diversity over multiple positions of the helical scaffolds. The origins of the unconventional regioselectivity during the cycloaddition steps was systematically investigated using density functional theory (DFT) calculations, unveiling the key features that control this reactivity, namely, face-to-face (π···π) or edge-to-face (C-H···π) interactions, primary orbital interactions and distortion from coplanarity in the transition structures (TSs) of the transformation. From the library of 24 derivatives synthesized, a pyridyl containing derivative displayed reversible, red-shifted, pH-triggered chiroptical switching properties, with CPL-sign reversal. It is found that protonation of the helicene causes a change of the angle between the electric and magnetic dipole moments related to the S₁ → S₀ transition, resulting in this rare case of reversible CPL sign inversion upon application of an external stimulus.

Synthesis and Properties of Spiro-double Sila[7]helicene: The LUMO Spiro-conjugation

A double helicene with a spiro-Si linker (4) was synthesized by four successive nucleophilic substitutions on SiCl₄ . Its (P,P), (M,M) and (P,M) isomers were isolated and characterized by single crystal X-ray analysis. Due to the central spirosilabi[fluorene] moiety, the two helicene units in 4 are symmetrically and nearly perpendicularly arranged. (P,P)-4 and (M,M)-4 exhibit unique optical properties attributable to the LUMO spiro-conjugation between the two sila[7]helicene units.

Rhodium-Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene-Like Molecules

The enantioselective synthesis of aza[6] and [7]helicene-like molecules have been achieved by the cationic rhodium(I)/axially chiral biaryl bisphosphine complex-catalyzed intramolecular [2+2+2] cycloaddition of cyanodiynes. This protocol was successfully applied to the diastereo- and enantioselective synthesis of an S-shaped double aza[6]helicene-like molecule with a high ee value of 89 %. Although no epimerization and racemization were observed in the double carbo[6]helicene-like molecule at 80 °C, epimerization and racemization of the double aza[6]helicene-like molecule proceeded at 80 °C. This double aza[6]helicene-like molecule showed good fluorescent quantum yields and chiroptical responses under both neutral and acidic conditions.

Triple Oxa[7]helicene with Circularly Polarized Luminescence: Enhancing the Dissymmetry Factors via Helicene Subunit Multiplication

The development of multiple heterohelicenes with a high luminescence dissymmetry factor (g_lum) is an appealing yet challenging task. Herein, we disclose the synthesis of a structurally unusual furan-based triple oxa[7]helicene, which represents the first reported triple hetero[7]helicene, via [2+2+2] cyclotrimerization/intramolecular dehydrogenative annulation. Compared to the reported double oxa[7]helicene, the triple oxa[7]helicene exhibits an improved g_lum value of 1.8 × 10^-3, exemplifying the potential of the helicene subunit multiplication approach to enhance the g_lum of heterohelicenes.

Rhodium-Catalyzed Enantioselective Synthesis of Highly Fluorescent and CPL-Active Dispiroindeno[2,1-c]fluorenes

The enantioselective synthesis of chiral [7]-helical dispirodihydro[2,1-c]indenofluorenes (DSF-IFs) was achieved for the first time in good yields with high er values (er up to 99 : 1). The crucial step of the whole reaction sequence was the enantioselective intramolecular [2+2+2] cycloaddition of tethered triynediols to indenofluorenediols, which was catalyzed by a Rh/SEGPHOS® complex. Further transformations led to the corresponding DSF-IFs. The prepared helically chiral DSF-IFs combine circularly polarized luminescence (CPL) activity (g_lum =∼10^-3 ) with exceptionally high fluorescence quantum yields (up to Φ_lum =0.97).

Circularly Polarized Fluorescent Helicene-Boranils: Synthesis, Photophysical and Chiroptical Properties

Mono- and di-boranil-substituted helicenes were prepared by BF₂ -borylation of the corresponding anils, readily synthesized by condensation of 2-amino- and 2,15-diamino-helicenes with 4-(diethylamino)salicylaldehyde. After enantiomeric resolution using HPLC, their chiroptical properties including circularly polarized fluorescence in solution and in PMMA films were investigated and rationalized with the help of NMR, X-ray and quantum-chemical calculations.

Synthesis and Properties of Partially Saturated Fluorenyl-Derived [n]Helicenes Featuring an Overcrowded Alkene

The straightforward, multigram-scale synthesis of the partially saturated H₆ -fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H₆ -fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature.

Oxidative cyclo-rearrangement of helicenes into chiral nanographenes

Nanographenes are emerging as a distinctive class of functional materials for electronic and optical devices. It is of remarkable significance to enrich the precise synthetic chemistry for these molecules. Herein, we develop a facile strategy to recompose helicenes into chiral nanographenes through a unique oxidative cyclo-rearrangement reaction. Helicenes with 7~9 ortho-fused aromatic rings are firstly oxidized and cyclized, and subsequently rearranged into nanographenes with an unsymmetrical helicoid shape through sequential 1,2-migrations. Such skeletal reconstruction is virtually driven by the gradual release of the strain of the highly distorted helicene skeleton. Importantly, the chirality of the helicene precursor can be integrally inherited by the resulting nanographene. Thus, a series of chiral nanographenes are prepared from a variety of carbohelicenes and heterohelicenes. Moreover, such cyclo-rearrangement reaction can be sequentially or simultaneously associated with conventional oxidative cyclization reactions to ulteriorly enrich the geometry diversity of nanographenes, aiming at innovative properties.

Nanographenes are emerging as a distinctive class of functional materials for electronic and optical devices. Here, the authors develop a facile strategy to recompose helicenes into a variety of chiral nanographenes through an oxidative cyclo-rearrangement reaction.

Flavin-Helicene Amphiphilic Hybrids: Synthesis, Characterization, and Preparation of Surface-Supported Films

This work reports on the preparation and structural characterization of flavo[7]helicene 1 (flavin-[7]helicene conjugate), which was subsequently characterized at the molecular level in either an aqueous environment or an organic phase, at the supramolecular level in the form of polymeric layers, and also embedded in a lipidic mesophase environment to study the resulting properties of such a hybrid relative to its parent molecules. The flavin benzo[g]pteridin-2,4-dione (isoalloxazine) was selected for conjugation because of its photoactivity and reversible redox behavior. Compound 1 was prepared from 2-nitroso[6]helicene and 6-methylamino-3-methyluracil, and characterized using common structural and spectroscopic tools: circular dichroism (CD), circularly polarized luminescence (CPL) spectroscopy, cyclic voltammetry (CV), and DFT quantum calculations. In addition, a methodology that allows the loading of 1 enantiomers into an internally nanostructured lipid (1-monoolein) matrix was developed.

Synthesis, Structure, and Chiroptical Properties of Indolo- and Pyridopyrrolo-Carbazole-Based C2 -Symmetric Azahelicenes

Treatment of 11,12-bis(1,1'-biphenyl-3-yl or 6-phenylpyridin-2-yl)-substituted 11,12-dihydro-indolo[2,3-a]carbazole with an oxidizing system of Pd(II)/Ag(I) induced effective double dehydrogenative cyclization to afford the corresponding π-extended azahelicenes. The optical resolutions were readily achieved by a preparative chiral HPLC. It was found that the pyridopyrrolo-carbazole-based azahelicene that contains four nitrogen atoms exhibits ca. 6 times larger dissymmetry factors both in circularly dichroism (CD) and circularly polarized luminescence (CPL), |g_CD | and |g_CPL | values being 1.1×10^-2 and 4.4×10^-3 , respectively, as compared with the parent indolocarbazole-based azahelicene. Theoretical calculations at the RI-CC2 level were employed to rationalize the observed enhanced chiroptical responses. The (chir)optical properties of the former helicene was further tuned by a protonation leading to remarkable red-shift with a considerable enhancement of the |g_CPL | value.

Modular Synthesis of Pentagonal and Hexagonal Ring-Fused NBN-Phenalenes Leading to an Excited-State Aromatization-Induced Structural Planarization Molecular Library

Although polycyclic aromatic hydrocarbons (PAHs) with a nitrogen-boron-nitrogen (NBN) moiety have recently attracted tremendous interest due to their intriguing electronic and optoelectronic properties, all of the NBN-fused π-systems reported to date are called NBN-dibenzophenalenes and were synthesized by electrophilic aromatic substitution. The synthesis of NBN-phenalenes remains challenging, and transition-metal catalysis has never been utilized to construct NBN-embedded π-scaffolds. Herein, a palladium-catalyzed cyclization/bicyclization strategy was developed for the synthesis of diverse pentagonal and hexagonal ring-fused NBN-phenalenes and half-NBN-phenalenes. All of the NBN-embedded π-scaffolds presented in our paper are fluorescent in both solution and the solid state. Further investigations showed that the five-membered NBN rings exhibit the properties of traditional luminogens, while those with a six-membered NBN ring generally undergo photoinduced structural planarization (PISP) and exhibit different colors and quantum yields of fluorescence with different concentrations in solution. Time-resolved spectroscopy and TD-DFT calculations revealed that excited-state aromatization is the driving force for PISP in hexagonal ring-fused NBN-π systems, leading to the formation of excimers. Notably, the scope of PISP compounds is still quite limited, and PISP has never been observed in NBN-π systems before. These hexagonal ring-fused NBN-π systems constitute a novel PISP molecular library and appear to be a new class of aggregation-induced excimer emission (AIEE) materials. Finally, the AIEE behavior of these six-membered NBN rings was applied to the detection of nitro explosives, achieving excellent sensitivity. In general, this work provides a new viewpoint for synthesizing NBN-fused π-systems and understanding the excited-state motion of luminogens.

Chiral Near-Infrared Fluorophores by Self-Promoted Oxidative Coupling of Cationic Helicenes with Amines/Enamines

In one pot, tertiary alkyl amines are oxidized to enamines by cationic dioxa[6]helicene, which further reacts as electrophile and oxidant to form mono or bis donor-π-acceptor coupling products. This original and convergent synthetic approach provides a strong extension of conjugation yielding chromophores that absorb intensively in far-red or NIR domains (λ_max up to 791 nm) and fluoresce in the NIR as well (λ_max up to 887 nm). Intense ECD properties around 790 nm with a |Δϵ| value up to 60 M^-1  cm^-1 are observed.

Toward Zigzag-edged Helical Nanographene Based on [7]Helicene

Due to their unique chemical and physical properties, zigzag-edged nanographenes have attracted increasing interest in recent years. Herein, a novel zigzag-edged nanographene (6) containing a [7]helicene subunit was designed and synthesized. However, because of the high reactivities of zigzag edges, compound 1 with a diketone structure was obtained owing to the oxidation of 6. The helical carbon skeleton of 1 is unambiguously revealed by single-crystal X-ray crystallography analysis. The photophysical properties of the precursor and helical diketone 1 are studied by UV-vis absorption spectroscopy. The electrochemical property of 1 is investigated by cyclic voltammetry, which was further studied by density functional theory (DFT) calculations (ΔE_g ^Cal =2.94 eV). The work reported here not only represents the synthesis of an unprecedented [7]helicene-embedded nanographene, but also provides the possibility for the synthesis of helical nanographenes with rich zigzag edges.

Triggering G-Quadruplex Conformation Switching with [7]Helicenes

The dynamic interplay between two types of chiral structures; fully conjugated racemic hetero[7]helicenes and DNA strands prone to fold into G-quadruplex structures is described. Both the [7]helicenes and the G-quadruplex DNA structures exist in more than one conformation in solution. We show that the structures interact with and stabilise each other, mutually amplifying and stabilising certain conformations at increased temperatures. The [7]helicene ligands L1 and L2 stabilise the parallel conformation of k-ras significantly, whereas hybrid (K⁺ ) and antiparallel (Na⁺ ) h-telo G-quadruplexes are stabilised upon conformational switching into altered G-quadruplex conformations. Both L1 and L2 induce parallel G-quadruplexes from hybrid structures (K⁺ ) and L1 induces hybrid G-quadruplexes from antiparallel conformations (Na⁺ ). Enantioselective binding of one helicene enantiomer is observed for helicene ligand L2, and VTCD melting experiments are used to estimate the racemisation barrier of the helicene.

One-Step Simultaneous Synthesis of Circularly Polarized Luminescent Multiple Helicenes Using a Chrysene Framework

A series of multiple helicenes was simultaneously synthesized in one step by intramolecular cyclization of a single chrysene derivative containing two 2-[(4-alkoxyphenyl)ethynyl]phenyl units accompanied by rearrangements of the aryl pendants. The electrophile-induced double cyclization with or without aryl migrations proceeded efficiently under acidic conditions to afford annulative π-extension of the chrysene units and produced quadruple (QH-2), triple (TH-2), and double (DH-2) helicenes containing [4]- and/or [5]helicene frameworks with dynamic and/or static helicene chirality in one step. Three multiple helicenes' structures were determined by X-ray crystallography and/or density functional theory calculations. The multiple TH-2 and DH-2 helicenes were separated into enantiomers because of the stable one and two [5]helicene moieties, respectively, and showed intense circular dichroism and circularly polarized luminescence. Although QH-2, which comprises four [4]helicene subunits, was not resolved into enantiomers, the TH-2 enantiomers were further separated into a pair of diastereomers at low temperature resulting from their substituted [4]helicene chirality.

Achieving high circularly polarized luminescence with push-pull helicenic systems: from rationalized design to top-emission CP-OLED applications

While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g lum, hardly exceeds 10-2 at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push-pull systems based on carbo[6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric (μe ) and magnetic (μ m ) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g lum values, i.e. up to 3-4 × 10-2. Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g El of around 8 × 10-3. These results bring about further molecular design guidelines to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures.

Helicene synthesis by Brønsted acid-catalyzed cycloaromatization in HFIP [(CF3)2CHOH]

A facile synthesis of carbo- and heterohelicenes was achieved via tandem cycloaromatization of bisacetal precursors, which were readily prepared through C–C bond formation by Suzuki–Miyaura coupling. This cyclization was efficiently realized by a catalytic amount of trifluoromethanesulfonic acid (TfOH) in a cation-stabilizing solvent, 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), which readily allowed gram-scale syntheses of higher-order helicenes, double helical helicenes, and heterohelicenes.

Modular Synthesis of Organoboron Helically Chiral Compounds: Cutouts from Extended Helices

Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All-ortho-fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φfl ) of 18-24 % in solution, green or yellow solid-state emission (Φfl up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12×10-2 . Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φfl of up to 47 % in CH2 Cl2 and 25 % in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than that of H1. Single-crystal X-ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices.

Twofold C-H Activation Enables Synthesis of a Diazacoronene-Type Fluorophore with Near Infrared Emission Through Isosteric Replacement

The synthesis and photophysical properties of a soluble amide-embedded coronene is reported. The key step in this synthesis is the twofold C-H activation of diazaperylene by a rhodium(III)Cp* catalyst. This unprecedented structural motif shows intense fluorescence in the near infrared region with a small Stokes shift and a distinct vibronic structure, which exhibits a slight extent of negative solvatochromism. Comparison of this compound with some relevant compounds revealed the importance of the amide incorporation in the peripheral concave region including an angular position to retain high aromaticity reflecting that of parent coronene. Treatment of this compound with Lewis acid B(C₆ F₅ )₃ formed a bis-adduct, which exhibited enhanced aromaticity as a consequence of the increased double bond character of the amide C-N bonds.

Elucidation of Diverse Solid-State Packing in a Family of Electron-Deficient Expanded Helicenes via Microcrystal Electron Diffraction (MicroED)*

Solid-state packing plays a defining role in the properties of a molecular organic material, but it is difficult to elucidate in the absence of single crystals that are suitable for X-ray diffraction. Herein, we demonstrate the coupling of divergent synthesis with microcrystal electron diffraction (MicroED) for rapid assessment of solid-state packing motifs, using a class of chiral nanocarbons-expanded helicenes-as a proof of concept. Two highly selective oxidative dearomatizations of a readily accessible helicene provided a divergent route to four electron-deficient analogues containing quinone or quinoxaline units. Crystallization efforts consistently yielded microcrystals that were unsuitable for single-crystal X-ray diffraction, but ideal for MicroED. This technique facilitated the elucidation of solid-state structures of all five compounds with <1.1 Å resolution. The otherwise-inaccessible data revealed a range of notable packing behaviors, including four different space groups, homochirality in a crystal for a helicene with an extremely low enantiomerization barrier, and nanometer scale cavities.

Asymmetric systematic synthesis, structures, and (chir)optical properties of a series of dihetero[8]helicenes

A series of dihetero[8]helicenes have been systematically synthesized in enantiomerically enriched forms by utilizing the characteristic transformations of the organosulfur functionality. The synthetic route begins with assembling a ternaphthyl common synthetic intermediate from 2-naphthol and bissulfinylnaphthalene through an extended Pummerer reaction followed by facile multi-gram-scale resolution. The subsequent cyclization reactions into dioxa- and dithia[8]helicenes take place with excellent axial-to-helical chirality conversion. Dithia[8]helicene is further transformed into the nitrogen and the carbon analogs by replacing the two endocyclic sulfur atoms via SNAr-based skeletal reconstruction. The efficient systematic synthesis has enabled comprehensive evaluation of physical properties, which has clarified the effect of the endocyclic atoms on their structures and (chir)optical properties as well as the unexpected conformational stability of the common helical framework.

Synthesis of azahelicenes through Mallory reaction of imine precursors: corannulene substrates provide an exception to the rule in oxidative photocyclizations of diarylethenes

Typically, the synthesis of phenanthrene-based polycyclic aromatic hydrocarbons relies on the Mallory reaction. In this approach, stilbene (PhCH[double bond, length as m-dash]CHPh)-based precursors undergo an oxidative photocyclization reaction to join the two adjacent aromatic rings into an extended aromatic structure. However, if one C[double bond, length as m-dash]C carbon atom is replaced by a nitrogen atom (C[double bond, length as m-dash]N), the synthesis becomes practically infeasible. Here, we show the very first examples of a successful Mallory reaction on stilbene-like imine precursors involving the molecularly curved corannulene nucleus. The isolated yields exceed 90% and the resulting single and double aza[4]helicenes exhibit adjustable high affinity for electrons.

Tuning Transition Electric and Magnetic Dipole Moments: [7]Helicenes Showing Intense Circularly Polarized Luminescence

Helicenes are promising candidates for chiral optoelectronic materials because of their helically twisted π-conjugated system. However, the emission intensity of unsubstituted helicenes is very weak (Φ_f < 0.05) due to a small oscillator strength for the S₁ → S₀ transition. In this work, we investigated the substitution position of the [7]helicene framework so that the S₁ → S₀ transition has a large transition magnetic dipole moment (TMDM) and is partially symmetry-allowed. A [7]helicene derivative thus designed showed a large fluorescence emission rate (k_f = 0.02 ns^-1) and a large TMDM for the S₁ → S₀ transition (|m| = 2.37 × 10^-20 erg·Gauss^-1), which are more than 10 times greater than those of unsubstituted [7]helicene (k_f = 0.001 ns^-1, |m| = 0.045 × 10^-20 erg·Gauss^-1). As a result, we achieved the [7]helicene derivative whose dissymmetry factor of CPL and fluorescence quantum yield were both high (|g_CPL| = 1.3 × 10^-2, Φ_f = 0.17) in the solution phase.

En Route Towards the Control of Luminescent, Optically-Active 3D Architectures

π-Extended systems are key components for the development of future organic electronic technologies. While conceiving molecules with improved properties is fundamental for the evolution of materials science, keeping control over the 3D arrangement of molecules represents an ever-expanding challenge. Herein, a synthetic protocol to replace carbon atoms of π-systems by dissymmetric phosphorus atoms is reported; in particular, it allowed for conceiving new fused phosphapyrene derivatives with improved properties. The presence of dissymmetric phosphorus atoms precluded the formation of excimers. X-ray diffraction revealed that, meanwhile, strong intermolecular interactions are taking place in the solid state. The phosphapyrenes photoluminesce in the visible region with high quantum yields; importantly, they are CD-active. In addition, the unique non-planar features of phosphorus atoms allowed for the control of the 3D arrangement of molecules, rendering lemniscate-like structures. Based on our discoveries, we envisage the possibility to construct higher-order, chiral 3D architectures from larger phosphorus-containing π-systems.

Access to benzo-fused aza[7]helicene via unexpected indolization of alkyne-amine

An efficient base-mediated indolization of alkyne-amine for the synthesis of quinolone-fused indole was developed and further applied to synthesize di- and triaza[7]helicenes bearing two and three pentagons, respectively.

Extended enantiopure ortho-phenylene ethylene (o-OPE)-based helical systems as scaffolds for supramolecular architectures: a study of chiroptical response and its connection to the CISS effect

Versatile enantiopure helical systems are described and are of interest owing to their intense chiroptical responses, their attractive architecture for metallosupramolecular chemistry and CISS effect.

Triskelion-shaped iridium-helicene NHC complex

Triskelion-shaped cycloiridiated complexes with three N-[6]helicenyl-NHC ligands were prepared with uncommon diastereoselectivities and their configurations were assigned using NMR analyses. They show strong ECD and OR, and yellow CP phosphorescence.

Luminescent dysprosium single-molecule magnets made from designed chiral BINOL-derived bisphosphate ligands

The nature of the aromatic system and the number of chiral centers of the BINOL derived bisphosphate ligands influence both the chiroptical properties and single-molecule magnet behavior of a series of dysprosium-based chains.

Solid-state versus solution investigation of a luminescent chiral BINOL-derived bisphosphate single-molecule magnet

The enantiopure coordination polymer [Dy(hfac)₃((S/R)-L)]_n ([(S/R)-1]n) involving a BINOL-derived bisphosphate ligand (S/R)-L is investigated both in solution and solid-state.

Schiff-base [4]helicene Zn(II) complexes as chiral emitters

The controlled preparation of chiral emissive transition metal complexes is fundamental in the field of circularly polarized luminescence (CPL) active molecular materials. For this purpose, enantiopure Zn(ii) complexes 1 and 2 based on a tetradentate salen ligand surrounded by [4]helicene moieties, together with their racemic counterpart 3, have been herein synthesized. Chirality is primarily brought about by chiral 1,2-cyclohexane-diamines. Alternatively, achiral complex 4 based on ortho-phenylene-diamine has been prepared as well. Single crystal X-ray diffraction analyses have been performed on helicenic intermediates 8 and 9 and complexes 1 and 4. Complexes 1 and 4 display the typical tetradentate O,N,N,O coordination around Zn(ii) characteristic of salen ligands, and bear two [4]helicene moieties. The zinc complexes are luminescent in the visible range around 560 nm at room temperature in aerated solutions with the QY reaching ca. 15% for a luminescence lifetime of 5.5 ns. The optical activities of these complexes have been assessed by CD and CPL, and compared to DFT calculations.

Carbo[n]helicenes Restricted to Enantiomerize: An Insight into the Design Process of Configurationally Stable Functional Chiral PAHs

The most important stereodynamic feature of carbo[n]helicenes is the interconversion of their enantiomers. The Gibbs activation energy (ΔG^≠(T)) of this process, which determines the rate of enantiomerization, dictates the configurational stability of [n]helicenes. High values of ΔG^≠(T) are required for applications of functional chiral molecules incorporating [n]helicenes or helicene substructures. This minireview provides an overview of the mechanism, recent developments, and factors affecting the enantiomerization of [n]helicenes, which will accelerate the design process of configurationally stable functional chiral molecules based on helicene substructures. Additionally, this minireview addresses the misconception and irregularities in the recent literature on how the terms “racemization” and “enantiomerization” are used as well as how the activation parameters are calculated for [n]helicenes and related compounds.