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Four members of a new class of cycloborylated hexa-, octa-, and deca-helicenes (1 a-d) have been prepared in enantiopure form, along with two cycloplatinated deca-helicenes (1 d', 1 d 1 ), further extending the family of cycloplatinated hexa- and octa-helicenes reported previously. The azabora[n]helicenes display intense electronic circular dichroism and large optical rotations; the dependence of the optical activity on the size of the helix (n=6, 8, 10) and the number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (nonpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives (1 a 1 -d 1 )., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Synthetic options for conversion of indeno[2,1‐c]fluorene‐5,8‐diones into larger polyaromatic compounds with extended π‐conjugated systems were explored. The crucial two‐fold transformation of the five‐membered ketone moieties of indeno[2,1‐c]fluorene‐5,8‐diones was based either on a one‐carbon homologation or on a ring‐expansion reaction which eventually enabled syntheses of compounds possessing substituted tribenzo[a,e,l]indeno[1,2,3‐hi]acephenanthrylenes and [5]helicenes scaffolds. The former was prepared by using a reaction sequence based on Ramirez olefination, intramolecular Heck reaction, and intermolecular Suzuki‐Miyaura cross‐coupling, whereas the latter utilized a sequence comprising Büchner−Curtius−Schlotterbeck reaction and intermolecular Suzuki‐Miyaura cross‐coupling. Scholl reaction of selected [5]helicenes yielded planarized polyaromatic compounds with the tribenzo[a,e,l]indeno[1,2,3‐hi]acephenanthrylene scaffold. Where possible, structures of products were confirmed by X‐ray diffraction analyses. Photophysical properties were determined as well. [ABSTRACT FROM AUTHOR]

The combination of different distortion motifs in nanographenes and polycyclic conjugated hydrocarbons has been the focus of significant attention during the last decade. The continual discovery of multiple carbon allotropes has fueled the field. A plethora of research groups around the globe are engaged in developing methods for the preparation of discrete graphene or fullerene fragments and new carbon allotropes. The goal is to fully understand the structure–property relationships in these systems. Herein, we present our journey and contributions to the field, from the development of a synthetic methodology towards functionalized heptagon-containing hexa- peri -hexabenzocoronenes through the preparation of saddle-helix hybrid nanographenes, the study of heptagon-containing cumulenic and open-shell systems, nanographenes embedded with higher order carbocycles, and the study of their electronic properties on-surface and by single-molecule conductance. 1 Introduction 2 The Finding of a Cornerstone Scaffold 3 Saddle-Helix Hybrid Nanographenes 4 Cumulenic and Diradicaloid Systems 5 Supramolecular Studies 6 Octagon- and Nonagon-Embedded Nanographenes 7 Molecular Electronics and On-Surface Studies 8 Outlook [ABSTRACT FROM AUTHOR]

We investigate the relative stability of increasingly large helicenes at the CCSD(T) level via the high-level G4(MP2) thermochemical protocol. The relative energies of [n]helicenes (n = 4–9) are obtained via the following reaction: [n]helicene + benzene → [n + 1]helicene + ethene. This reaction conserves the number of sp2-hybridized carbons, the number of aromatic rings, and the helical structures on the two sides of the reaction. We show that the reaction energy converges to an asymptotic value of ΔH298 = + 22.4 kJ/mol for increasingly large helicenes. For comparison, for [n]acenes, the same reaction converges to a much higher asymptotic reaction enthalpy of ΔH298 = + 56.8 kJ/mol. This difference between the two asymptotic reaction enthalpies sheds light on the relative thermodynamic stability of increasingly large helicenes. We proceed to use the G4(MP2) reaction energies to evaluate the performance of dispersion-corrected density functional theory (DFT) and semiempirical molecular orbital (SMO) methods for the relative energies of [n]helicenes. Nearly all DFT methods perform poorly with root-mean-square deviations (RMSDs) above 10 kJ/mol. The best-performing DFT method, BLYP-D4, attains an RMSD = 5.2 kJ/mol. Surprisingly, the advanced SMO methods, XTB and PM7, outperform the DFT methods and result in RMSDs of 3.0 and 3.1 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

Intramolecular alkyne aromatization is a powerful tool that enables the synthesis of nonplanar polycyclic aromatic hydrocarbons. Herein, an unexpected intramolecular alkyne aromatization via a trifluoroacetic acid‐promoted cyclization is described, in which two structural isomers, expanded [9]helicene (1) and π‐extended double [4]helicene (2) are obtained. A possible rearrangement mechanism is proposed to account for the formation of 1. The geometric and optoelectronic properties of these two nonplanar molecular nanocarbons are comprehensively investigated by single‐crystal X‐ray, UV–vis absorption, photoluminescence spectra, and cyclic voltammetry analysis. These two structural isomers exhibit wide energy gaps with similar energy levels, which further apply them as molecular floating gate in organic field‐effect transistor nonvolatile memory (OFET‐NVM) devices. The nonplanar geometry of 1 and 2 shows a remarkable effect on charge‐trapping behaviors in OFET‐NVMs; the 1‐based device displays a more than threefold wider memory window (MW, 44.5 V) than that of the 2‐based device (14.2 V), and a large charge‐trapping density of 1.08 × 1013 cm−2. The distinct different charge‐trapping behavior is likely attributed to the different molecular geometries, resulting in different molecular‐packing modes. It is revealed in this study that controlling the geometry of molecular nanocarbons is a new strategy for application in organic electronics. [ABSTRACT FROM AUTHOR]

The research interest in azulene‐embedded polycyclic aromatic hydrocarbons (PAHs) has significantly increased recently, but the lack of efficient synthetic strategies impedes the investigation of their structure‐property relationships and further opto‐electronic applications. Here we report a modular synthetic strategy towards diverse azulene‐embedded PAHs by a tandem Suzuki coupling and base‐promoted Knoevenagel‐type condensation with good yields and great structural versatility, including non‐alternant thiophene‐rich PAHs, butterfly‐ or Z‐shaped PAHs bearing two azulene units, and the first example of a two‐azulene‐embedded double [5]helicene. The structural topology, aromaticity and photophysical properties were investigated by NMR, X‐ray crystallography analysis and UV/Vis absorption spectroscopy assisted by DFT calculations. This strategy provides a new platform for rapidly synthesizing unexplored non‐alternant PAHs or even graphene nanoribbons with multiple azulene units. [ABSTRACT FROM AUTHOR]

We herein present the synthetic approach to a new antiaromatic double aza[7]helicene C that features NN‐embedded polycyclic aromatic hydrocarbons (PAHs). This heteroatom‐doped helicene showed a rarely obtained long‐wavelength emission and far‐red circularly polarized luminescence (CPL) in the solid state. These optical and chiroptical properties could be ascribed to both the NN‐PAH core structure and the further extension through angular ring fusions. Such a unique electronic structure also culminated in facile chemical oxidations of neutral C to the positively charged chiral radical (C⋅+) and dication species (C2+). Interestingly, DFT computations revealed that the pyridazine central core showed an antiaromaticity‐to‐aromaticity switching, in contrast to the inversed transition for the helical periphery in cationic states. The reported approaches are anticipated to lead to the development of further redox‐active chiral systems for potential applications in chiroptoelectronics, spintronics as well as fluorescent bioimaging. [ABSTRACT FROM AUTHOR]

Double helicenes are appealing chiral frameworks. Their π‐extension is desirable to achieve (chir)optical response in the visible and near‐infrared (NIR) region, but access to higher double [n]helicenes (n≥8) has remained challenging. Herein, we report an unprecedented π‐extended double [9]helicene (D9H), unambiguously revealing its structure by single‐crystal X‐ray diffraction. D9H shows remarkable NIR emission from 750 to 1100 nm with a high photoluminescence quantum yield of 18 %. In addition, optically pure D9H exhibits panchromatic circular dichroism with a notable dissymmetry factor (gCD) of 0.019 at 590 nm, which is among the highest in the visible region for reported helicenes. [ABSTRACT FROM AUTHOR]

The polarizability of polycyclic aromatic hydrocarbons (PAHs) is an important property that relates to their abundance in natural environments. To assess the differences in the mean polarizability of planar and non-planar polycyclic aromatic hydrocarbons (PAHs), we computationally studied the PAH series of circulenes (kekulene C48H24, [13]circulene C52H26, and septulene C56H28), a number of the nearest helicenes, and their "expanded" isomers. We mean under "nearest" a close number of aromatic rings: 11 (C46H26), 12 (C50H28), 13 (C54H30), and 14 (C58H32). For these PAHs, we performed the quantum chemical calculations of thermodynamic and polarizability parameters with the PBE/3ζ density functional theory method, which is widely used in the theoretical chemistry of fullerenes and PAHs. The calculated mean polarizabilities (in Å3) ranged from 80.1 for [11]helicene to 135.5 for septulene, and while the circulenes and expanded helicenes had similar values, the mean polarizability of the normal helicenes was markedly lower. In all four pairs of helical PAHs, the expanded helicene was energetically considerably more favorable than its standard helicene isomer. Herewith, the ratio of their polarizabilities was equal to 1.3. [ABSTRACT FROM AUTHOR]

We have discovered a dual (i. e., soft and hard) Lewis acidity of alumina that enables rapid one‐pot π‐extension through the activation of terminal alkynes followed by C−F activation. The tandem reaction introduces an acenaphthene fragment – an essential moiety of geodesic polyarenes. This reaction provides quick access to elusive non‐alternant polyarenes such as π‐extended buckybowls and helicenes through three‐point annulation of the 1‐(2‐ethynyl‐6‐fluorophenyl)naphthalene moiety. The versatility of the developed method was demonstrated by the synthesis of unprecedented structural fragments of elusive geodesic graphene nanoribbons. [ABSTRACT FROM AUTHOR]

Azulene is a non‐benzenoid aromatic building block with unique chemical structure and physicochemical properties. By using the "bottom‐up" synthetic strategy, we synthesized three azulene‐embedded [n]helicenes ([n]AzHs, n=5, 6 and 7), in which one terminal azulene subunit was fused with n‐2 benzene rings. P‐ and M‐enantiomers were observed in the packing diagrams of [5]‐, and [6]AzHs. However, P‐ and M‐[7]AzHs could be isolated by recrystallization of the racemic mixture. These [n]AzHs were endowed with new properties through the azulene moiety such as low‐lying first electric state (S1), small optical energy gap and anti‐Kasha emission. [6]‐, and [7]AzHs exhibit strong chiroptical responses with high absorption dissymmetry factor (gabs) maxima of about 0.02, which is among the highest |gabs| values of helicenes in the visible range. These azulene‐embedded [n]helicenes contribute to the non‐benzenoid helicene library and allow the structure–property relationships to be better understood. [ABSTRACT FROM AUTHOR]

Hybrid polyaromatic hydrocarbons (PAHs) consisting of helicene and acene domains, referred to as [7]heli‐D‐acenes, are introduced as scaffolds to generate enantiopure twisted acenes (heli‐twistacenes) by a torque, lock, and propagate (TLP) approach. Computational methods with and without dispersion corrections were used to explore the structural and electronic features of these PAHs and to explore the possible formation of twistomers that might complicate reaction mixtures. Syntheses of unsubstituted and disubstituted members of the [7]heli‐D‐acene series confirmed the viability of the TLP approach, and together with the computational results, provided proof‐of‐concept of this new approach as a viable means to generate enantiopure twisted‐acenes. The X‐ray structures, absorption, fluorescence, phosphorescence, and CD spectra of these first generation heli‐acenes are compared to the structure and photophysical properties of pentacene and [7]helicene. A high barrier for the enantio‐enriched M enantiomer of 19,24‐dicyano[7]heli‐D‐anthracene verified its configurational stability at room temperature. [ABSTRACT FROM AUTHOR]

A novel synthetic strategy was developed for the construction of difficult‐to‐access structurally constrained boron‐doped polycyclic aromatic hydrocarbons (sc‐B‐PAHs) via a cascade reaction from the readily available ortho‐aryl‐substituted diarylalkynes. This domino process involves borylative cyclization, 1,4‐boron migration and successive two‐fold electrophilic borylation. Two types of sc‐B‐PAHs bearing B‐doped [4]helicene (1 a‐1 i) or BN‐doped [4]helicene (1 n‐1 t) and double [4]helicene (1 u‐1 v) are constructed by this cascade reaction. Remarkably, this synthetic strategy is characterized by modest yields (20–50 %) and broad substrate scope (18 examples) with versatile functional group tolerance. The resultant sc‐B‐PAHs show good stability under ambient conditions and are thoroughly investigated by X‐ray crystallography, UV/Vis absorption and fluorescence spectroscopy, and cyclic voltammetry. Interestingly enough, BN‐doped [4]helicene 1 o forms a unique alternating π‐stacked dimer of enantiomers within a helical columnar superstructure, while BN‐doped double [4]helicene 1 u establishes an unprecedented π‐stacked trimeric sandwich structure with a rare 2D lamellar π‐stacking. The synthetic approach reported herein represents a powerful tool for the rapid generation of novel sc‐B‐PAHs, which are highly attractive for the elucidation of the structure‐property relationship and for potential optoelectronic applications. [ABSTRACT FROM AUTHOR]

The investigation of new synthetic routes towards positively charged N‐heterocycles is of high importance for the further development of medicinal chemistry, functional materials, catalysis and other areas of application. For accessing acridinium dyes, we previously reported an approach based on an aryne‐imine‐aryne coupling followed by the subsequent oxidation of the acridane intermediates. Herein, we now present an unusual reaction outcome when phenanthryne is used as aryne component. Under optimized conditions, this two‐step synthetic methodology led to the formation of a helical tetrabenzophenanthridinium derivative. Furthermore, the susceptibility of this product to photoinduced cyclodehydrogenation was observed, providing a highly fluorescent planar polycyclic aromatic hydrocarbon with a positively charged nitrogen. The photophysical and electrochemical properties of the mesityl‐phenyltetrabenzophenanthridinium tetrafluoroborate were also determined. [ABSTRACT FROM AUTHOR]

Attachment of cyclic imide groups to polycyclic aromatic hydrocarbons (PAHs) leads to fascinating electronic and luminescence properties, with rylene diimides being a representative example. The close to unity fluorescence quantum yields and electron-acceptor properties render them suitable for application in organic electronics and photovoltaics. Recent reports show that, in line with planar PAHs, the imide functionalization has also endowed helical three-dimensional PAHs with similar beneficial photophysical properties. In this article, we have summarized the state-of-the-art research developments in the field of helicene–imide hybrid functional molecules, with a particular focus on synthesis, (chir)optical and redox properties, and applications in electronics. Additionally, we have highlighted our recent work, introducing a novel family of functional chiral molecules, namely, [ n ]helicene diimides, as three-dimensional relatives of rylene diimides. [ABSTRACT FROM AUTHOR]

A new family of chiral saddle‐helix hybrid nanographenes is reported. The first hexa‐peri‐hexabenzocoronene (HBC) analogues bearing a nine‐membered carbocycle are presented. Furthermore, for the first time, π‐extended carbo[n]helicenes containing a nine‐membered ring as part of the helical moiety have been synthesized. The combination of a [5]helicene moiety and a nonagon ring in a single chiral motif induces a tremendous distortion from planarity into the nanographenic structures compared to other saddle‐helix hybrids such as heptagon‐ and octagon‐containing π‐extended carbo[5]helicenes. In fact, the interplanar angle of the two terminal rings reaches the largest angle (134.8°) of a carbohelicene reported to date, thus being by far the most twisted helicene yet prepared. Photophysical properties evaluation showed improved absorption dissymmetry factors (|gabs|=4.2×10−3) in the new family of nonagon‐containing π‐extended carbo[5]helicenes. [ABSTRACT FROM AUTHOR]

We designed a straightforward synthetic route towards a full‐fledged family of π‐extended helicenes: superhelicenes. They have two hexa‐peri‐hexabenzocoronenes (HBCs) in common that are connected via a central five‐membered ring. By means of structurally altering this 5‐membered ring, we realized a versatile library of molecular building blocks. Not only the superhelicene structure, but also their features are tuned with ease. In‐depth physico‐chemical characterizations served as a proof of concept thereof. The superhelicene enantiomers were separated, their circular dichroism was measured in preliminary studies and concluded with an enantiomeric assignment. Our work was rounded‐off by crystal structure analyses. Mixed stacks of M‐ and P‐isomers led to twisted molecular wires. Using such stacks, charge‐carrier mobilities were calculated, giving reason to expect outstanding hole transporting properties. [ABSTRACT FROM AUTHOR]

The chiral self‐assembly of trispentahelicene propellers on a gold surface has been investigated in ultrahigh vacuum by means of scanning tunneling microscopy and time‐of‐flight secondary ion mass spectrometry. The trispentahelicene propellers aggregate into mirror domains with an enantiomeric ratio of 2 : 1. Thermally induced cyclodehydrogenation leads to planarization into nanographenes, which self‐assemble into closed‐packed layers with two different azimuths. Further treatment induces in part dimerization and trimerization by intermolecular cyclodehydrogenation. [ABSTRACT FROM AUTHOR]

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 (ΔEgCal=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. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons consisting of three fused anthracene units were designed as new π‐conjugated compounds having helical structures. These expanded helicenes were synthesized by Pt‐catalyzed cycloisomerization of the corresponding ethynyl‐substituted precursors. The nonplanar and helical structure was confirmed by X‐ray analysis and DFT calculations, and the barrier to helical inversion was estimated to be 34 kJ mol−1. The enantiomers of the diphenyl derivative were successfully resolved by chiral HPLC. Enantiopure samples showed good chiroptical performance in the CD (|Δϵ| 1380 L mol−1 cm−1) and CPL (|glum| 0.013) spectra, and these values were considerably large for simple organic molecules. The unique chiroptical properties are discussed on the basis of the molecular structure and the electronic state with the aid of time‐dependent DFT calculations. [ABSTRACT FROM AUTHOR]

A new family of distorted ribbon‐shaped nanographenes was designed, synthesized, and their optical and electrochemical properties were evaluated, pointing out an unprecedented correlation between their structural characteristics and the two‐photon absorption (TPA) responses and electrochemical band gaps. Three nanographene ribbons have been prepared: a seven‐membered‐ring‐containing nanographene presenting a tropone moiety at the edge, its full‐carbon analogue, and a purely hexagonal one. We have found that the TPA cross‐sections and the electrochemical band gaps of the seven‐membered‐ring‐containing compounds are higher and lower, respectively, than those of the fully hexagonal polycyclic aromatic hydrocarbon (PAH). Interestingly, the inclusion of additional curvature has a positive effect in terms of non‐linear optical properties of those ribbons. [ABSTRACT FROM AUTHOR]

The partial fluorination of polycyclic aromatic hydrocarbons often produces a layered crystal packing, where fluorinated aromatic surfaces are stacked over nonfluorinated aromatic surfaces. Herein, we report the synthesis and crystal packing of partially fluorinated [4]helicenes with steric congestion resulting from H and F atoms in the fjord region. F6‐[4]Helicene forms head‐to‐tail columnar stacks consisting of an alternate arrangement of perfluorinated and nonfluorinated naphthalene moieties. With decreasing fluorine content, aromatic stacking switched from arene−fluoroarene (ArH−ArF) hetero‐stacking to ArH−ArH/ArF−ArF homo‐stacking with the help of intermolecular C−H⋅⋅⋅F contacts in the fjord region. As a result, head‐to‐head columnar stacks appear. Therefore, the conventional ArH−ArF stacking motif is not always applicable to Fn‐[4]helicenes with twisted π‐surfaces. [ABSTRACT FROM AUTHOR]

Three unprecedented helical nanographenes (1, 2, and 3) containing an azulene unit are synthesized. The resultant helical structures are unambiguously confirmed by X‐ray crystallographic analysis. The embedded azulene unit in 2 possesses a record‐high twisting degree (16.1°) as a result of the contiguous steric repulsion at the helical inner rim. Structural analysis in combination with theoretical calculations reveals that these helical nanographenes manifest a global aromatic structure, while the inner azulene unit exhibits weak antiaromatic character. Furthermore, UV/Vis‐spectral measurements reveal that superhelicenes 2 and 3 possess narrow energy gaps (2: 1.88 eV; 3: 2.03 eV), as corroborated by cyclic voltammetry and supported by density functional theory (DFT) calculations. The stable oxidized and reduced states of 2 and 3 are characterized by in‐situ EPR/Vis–NIR spectroelectrochemistry. Our study provides a novel synthetic strategy for helical nanographenes containing azulene units as well as their associated structures and physical properties. [ABSTRACT FROM AUTHOR]

A unified low‐temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes—polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho‐condensed arrangements of fused benzene rings—is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas‐phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer‐selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free‐radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry. [ABSTRACT FROM AUTHOR]

Chiral organic fluorescent molecules with circularly polarized luminescence (CPL) properties have attracted much attention for their potential applications in chiral optical and electronic devices. Benzo [ ghi ]perylene is one of the classical polycyclic aromatic hydrocarbons (PAHs) with good fluorescent properties and has many applications in the fields including optoelectronic devices and fluorescence probes. In this work, chiral benzo [ ghi ]perylene substituted [n]helicenes were directly obtained from chiral BINOL. The introduction of benzo [ ghi ]perylene structure greatly improved the fluorescent efficiency of the pristine helicene molecules. These benzo [ ghi ]perylene substituted [n]helicenes show strong blue fluorescent emission in organic solutions with good fluorescent quantum yields. These molecules could also exhibit moderate fluorescent emission in the solid-states. Based on the DFT calculations results, the HOMOs and LUMOs of these molecules are mainly contributed by benzo [ ghi ]perylene, which forms an orthogonal geometry with helicene moieties. The enantiomers of these molecules show opposite signals in CD and CPL spectra. The g abs and g lum values could be increased with elongation of helicene structure. Compounds with [7]helicene moiety show relatively large g abs and g lum values, which could reach up to 3.6 × 10−3 and 1.0 × 10−3, respectively. This work has provided a new strategy to develop chiral organic emitters through integration of benzo [ ghi ]perylene and helicene structures, which exhibit efficient fluorescent emission and intense CPL properties. [Display omitted] • Chiral benzo [ ghi ]perylene substituted [n]helicenes have been synthesized. • These molecules display strong blue fluorescent emission in solutions. • These molecules show strong Cotton effect in CD spectra with good g abs values. • Benzo [ ghi ]perylene substituted [6]helicenes exhibits intense CPL properties. [ABSTRACT FROM AUTHOR]

We report a molecular design and concept using π‐system elongation and steric effects from helicenes surrounding a triphenylene core toward stable chiral polycyclic aromatic hydrocarbons (PAHs) with a maximal π‐distortion to tackle their aromaticity, supramolecular and molecular properties. The selective syntheses, and the structural, conformational and chiroptical properties of two diastereomeric large multi‐helicenes of formula C90H48 having a triphenylene core and embedding three [5]helicene units on their inner edges and three [7]helicene units at their periphery are reported based on diastereoselective and, when applicable, enantiospecific Yamamoto‐type cyclotrimerizations of racemic or enantiopure 9,10‐dibromo[7]helicene. Both molecules have an extremely distorted triphenylene core, and one of them exhibits the largest torsion angle recorded so far for a benzene ring (twist=36.9°). [ABSTRACT FROM AUTHOR]

Herein, we have synthesized an enantiomerically pure propeller‐shaped PAH, C90H48, possessing three [7]helicene and three [5]helicene subunits. This compound can be obtained in gram quantities in a straightforward manner. The photophysical and chiroptical properties were investigated using UV/Vis absorption and emission, optical rotation and circular dichroism spectroscopy, supported by DFT calculations. The nonlinear optical properties were investigated by two‐photon absorption measurements using linearly and circularly polarized light. The extremely twisted structure and packing of the homochiral compound were investigated by single‐crystal X‐ray diffraction analysis. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) that contain both five‐ and seven‐membered rings are rare, and those where these rings are annulated to each other and build azulene units have, to date, mainly been generated in minute amounts on surfaces. Herein, a rational approach to synthesize soluble contorted PAHs containing two embedded azulene units in the bulk is presented. By stepwise detachment of tert‐butyl groups, a series of three azulene embedded PAHs with different degrees of contortion has been made to study the impact of curvature on aromaticity and conjugation. Furthermore, the azulene PAHs showed high fluorescence quantum yields in the NIR regime. [ABSTRACT FROM AUTHOR]

We report on the one‐ and two‐water clusters of [4]helicene, the smallest polycyclic aromatic hydrocarbon with a helical sense, which were captured in the gas phase using high‐resolution rotational spectroscopy. The structures of the complexes are unambiguously revealed using microwave spectra of isotopically enriched species. In the one‐water cluster, the apparent splitting pattern is consistent with a tunneling motion that encompasses an exchange of strongly and weakly bonded water hydrogens. This motion is "locked" in the two‐water cluster. The relevant intermolecular contacts, symmetry, and aromaticity effects are unveiled for the microsolvated chiral topologies. These observations entail the first glance at the structures and internal dynamics of the water binding motifs of a chiral polycyclic aromatic hydrocarbon. [ABSTRACT FROM AUTHOR]

The controlled preparation of well-defined distorted nanographenes by a bottom-up approach based on organic synthesis permits the direct establishment of unprecedented structure–property relationships in carbon nanostructures. The simultaneous incorporation of various defects in nanographenes affords highly curved structures with novel or enhanced photophysical properties. In this sense, we recently reported a fully helical and saddle-shaped nanographene ribbon containing the first undecabenzo[7]helicene unit. Both its linear and nonlinear optical properties are enhanced in comparison with those of other partially π-extended [7]helicenes. Moreover, the new superhelicene exhibits the highest emission dissymmetry factor (g lum) reported to date for a homochiral nanographene. The combination of both nonlinear and chiroptical properties in nanographenes opens up new possible future applications for those distorted nanostructures. 1 Introduction 2 Synthesis of Embedded Seven-Membered Rings 3 Combination of Defects: Seven-Membered Rings and π-Extended Helicenes 4 Conclusions and Outlook [ABSTRACT FROM AUTHOR]

This account focuses on the synthesis and structures of symmetric multiple carbohelicenes; i.e., fully fused polycyclic aromatic hydrocarbons containing two or more symmetric helicene moieties. Synergies of the multiplexed helicene structures within a π-system generate a number of local minima and transition states between each state. Based on recent studies on multiple helicenes, a systematic molecular design for further multiplexed symmetric helicenes is proposed in the last section of this article. [ABSTRACT FROM AUTHOR]

Abstract: A three‐dimensional π‐conjugated chiral cage with six [5]helicene units (a triple helicene cage) was synthesized for the first time. Taking advantage of the Yamamoto coupling reaction, the triflate‐substituted triple [5]helicene, a strained and preorganized precursor, was dimerized to afford the target compound. Single‐crystal X‐ray diffraction analysis revealed the unique structural features of the triple helicene cage: a cage‐shaped rigid structure with outer helical grooves and an inner chiral cavity. All‐P and all‐M enantiomers were separated successfully by HPLC over a chiral column and their chiroptical properties were characterized by circular dichroism spectra. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis and structural analysis of a quintuple [6]helicene with a corannulene core is reported. The compound was synthesized from corannulene in three steps including a five‐fold intramolecular direct arylation. X‐ray crystallographic analysis revealed a C5‐symmetric propeller‐shaped structure and one‐dimensional alignment in the solid state. The enantiomers of the quintuple [6]helicene were successfully separated by HPLC, and the chirality of the two fractions was identified by CD spectroscopy. A kinetic study yielded a racemization barrier of 34.2 kcal mol−1, which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl‐to‐bowl inversion of the corannulene moiety and a step‐by‐step chiral inversion pathway for the five [6]helicene moieties. [ABSTRACT FROM AUTHOR]

Perkin condensation of chrysenyl-6-acetic acid with chrysenylene-6,12-diglyoxylic acid followed by in situ esterification gives a bismaleate, whose conjugated stilbene moieties are efficiently shielded against intermolecular condensations and undergo iodine-catalyzed oxidative photocyclization in toluene without the need for high dilution. The concentration is limited by the low solubility of the flexible bismaleate at room temperature. The so-obtained double [7]helicene crystallizes in a nonchiral meso form. It is notably more soluble than its flexible precursor because it cannot fold to optimize π-π stacking. [ABSTRACT FROM AUTHOR]

[Display omitted] Helicenes fused by ortho -annulated benzene rings are one of interesting subset of chiral polycyclic aromatic hydrocarbons (PAHs). Helicenes containing non-benzenoid rings such as pentagonal and heptagonal rings have attracted increasing attentions because of their novel electronic properties, optical characteristics, dynamic behavior, and solid-state packing. Despite the disclosure of several beautiful examples in recent years, this field remains young and challenging due to the intrinsic difficulties of the formation of non-hexagonal rings. Herein an overview of non-benzenoid carbohelicenes with respect to structures, synthesis, optical properties, and stereochemical dynamics is provided. In this review, recent reports of helicenes with non-hexagons as chiral motif are surveyed with an emphasis on synthesis, molecular configurations and intriguing optical properties. Finally, a brief conclusion of non-benzenoid helicenes is achieved and an outlook for this field is provided according to these discussions. [ABSTRACT FROM AUTHOR]

Two distinct structural elements that render π-systems nonplanar, i.e., geodesic curvature and helical motifs, have been combined into new polyarenes that contain both features. The resultant corannulene-[n]helicenes (n = 5, 6) show unique molecular dynamics in their enantiomerization processes, including inversion motions of both the bowl and the helix. Optical resolution of a corannulene-based skeletally chiral molecule was also achieved for the first time, and the influence of the bowl-motif annulation on the chiroptical properties was investigated. [ABSTRACT FROM AUTHOR]

The synthesis of an efficient precursor of the novel 1,7-naphthodiyne synthon is reported. Preliminary experiments demonstrate the usefulness of this platform for the synthesis of sterically congested polyarenes, such as helicenes and angularly fused acene derivatives. Furthermore, a novel intramolecular aryne trapping reaction is described. [ABSTRACT FROM AUTHOR]

[Display omitted] We reported the synthesis and properties of an unexpected superhelicene-like 1 in which two flat polycyclic aromatic hydrocarbons are connected via a central eight-membered ring. The structural analysis of 1 was performed by NMR spectroscopy and mass spectrometry measurements together with single-crystal X-ray analysis. The photophysical properties and aromaticity of 1 were further investigated by the absorption and emission spectroscopies in combination with theoretical calculations. [ABSTRACT FROM AUTHOR]

Double helicenes have attracted increasing attention of researchers for their unique structures as well as potential application in material science. Most of the double helicene compounds are all-carbon helicenes, and nitrogen-doped double helicenes are rarely synthesized and studied. As polycyclic aromatic hydrocarbons doped with heteroatoms could show specific optoelectronic properties, it is necessary to develop new strategy for the synthesis of double azahelicenes. In this work, three double aza [5]helicene compounds have been designed and synthesized through Bischler-Napieralski cyclization. These molecules have symmetrical saddle-type conformations and P/M configurations. Moreover, these molecules could exhibit positive solvatochromic properties and have large Stokes shifts in various organic solvents. According to the DFT calculation results, the distributions of HOMOs are mainly over bilateral aromatic rings, while the LUMOs are delocalized over the central benzothiadiazole or benzopyrazine moiety. The calculation results are in consistent with the fluorescent properties of these molecules. This work provides a new strategy for the synthesis of double aza [5]helicene molecules with fluorescent properties. Double aza [5]helicene derivatives have been synthesized. These compounds have symmetrical saddle-type conformations and PM configurations. These double aza [5]helicenes have large Stokes shifts and could exhibit positive solvatochromic properties in solutions. [Display omitted] • Three double aza [5]helicene derivatives have been synthesized. • These double helicene molecules have symmetrical saddle-type conformations and P/M configurations. • These molecules have large Stokes shifts. • These molecules could exhibit positive solvatochromic properties in solutions. [ABSTRACT FROM AUTHOR]

Helicenes are polycyclic Jt-conjugated systems consisting ortho-fused aromatic rings. Since the technology of helicenes is still in its infancy, the expected environmental inputs and levels are currently unknown. However, the very promising properties of helicenes predict many potential applications in various fields. The present study investigates ecotoxic properties of [6]helicene to aquatic (Daphnia magna, Desmodesmus subspicatus, Vibrio fischeri) and soil (Folsomia Candida, Enchytraeus crypticus) organisms. In the case of the aquatic species, no significant effect was observed even at the highest nominal concentration tested of 0.63 mg/L. In the case of the soil species, the higher of the two tested concentrations of 1.7 mg/g showed an inhibition of reproduction of 51% and 39% of Enchytraeus crypticus and Folsomia Candida, respectively. The soil toxicity of [6]helicene is higher than expected from its lipophilicity, which might suggest a specific toxicity mechanism. Further research is necessary to fully understand the toxicity of helicenes. [ABSTRACT FROM AUTHOR]

Highlights: [•] NICS are introduced as a means to evaluate the ligand properties of polycyclic Cp-anions. [•] Synthetic methods to access various benzannulated Cp-derivatives are discussed. [•] Transition metals complexes of benzannulated Cp-derivatives are reviewed. [ABSTRACT FROM AUTHOR]

Es wird über die erste asymmetrische organokatalytische Synthese von Helicenen berichtet. Eine neue SPINOL-abgeleitete Phosphorsäure mit ausgedehnten π-Substituenten katalysiert die asymmetrische Synthese von Helicenen über eine enantioselektive Fischer-Indolisierung. Eine Vielzahl von Azahelicenen und Diazahelicenen konnte auf diese Weise mit guten bis sehr guten Ausbeuten und Enantioselektivitäten erhalten werden. [ABSTRACT FROM AUTHOR]

Aromatic stabilities of acenes and helicenes, which are responsible for their biological, environmental and cancerous behavior have been modeled using a newly introduced Sadhana (Sd) and A indices. The results are compared with those obtained from PI (Padmakar-Ivan) index. The regression analysis has shown that excellent results are obtained by considering acenes and helicenes as separate classes of isomeric benzenoid hydrocarbons and that A index is better index than both PI and Sd indices. [Copyright &y& Elsevier]

Generalized helicenes are defined. A theoretical analysis is presented for the intramolecular dehydrogenation of helicenes, bridging adjacent rungs of the helix with the formation of 4-, 5-, and/or 6-membered rings, yielding helical tubulenes to which Euler's formula for polyhedra no longer applies as such constructions are derived from Möbius strips and correspond to nonplanar graphs (i.e., have crossing lines when drawn in two dimensions). Intermolecular dehydrogenation of two parallel-superimposed polycyclic aromatic hydrocarbons with formation of similar rings leads to convex polyhedral carbon cages. Carbon helical nanotubes and some carbon cages that are different from fullerenes are also discussed. [ABSTRACT FROM AUTHOR]