Chasing Turns and Twists: Unraveling the One-Step Synthesis, Intricate Pathways, and Structural Revelations of N-Aryl Aza-quasi[8]circulenes.

An efficient one-step synthesis of N-Aryl aza-quasi[8]circulenes is reported starting from bis(biaryl)carbazoles. The intermediacy of N-aryl aza[7]helicene is investigated, and the Scholl oxidative cyclization route is invoked here to overcome the large strain during the formation of N-aryl aza-quasi[8]circulenes from N-aryl aza[7]helicene. Notably, this transformation occurs without the need for directing groups and proceeds from a more helical to a less helical pathway. Both the N-aryl aza[7]helicene and N-aryl aza-quasi[8]circulene are confirmed by single crystal X-ray structural analysis. The enantiomers of N-aryl aza[7]helicene are separated by chiral HPLC and analyzed by circular dichroism spectroscopy to investigate their chiroptical properties. However, N-aryl aza-quasi[8]circulene racemizes rapidly. The radical cations generated from aza-quasi[8]circulene through chemical oxidation exhibit broad absorption in the near-IR region and air stability up to 24 h. Optical and electrochemical studies with aza[7]helicene and aza-quasi[8]circulene derivatives highlight their potential in organic electronic devices.

On-Surface Synthesis of Helicene Oligomers.

We report on-surface synthesis of heterochiral 1D heptahelicene oligomers after deposition of a racemic heptahelicene monomer on an Au(111) surface followed by Ullmann coupling under ultrahigh vacuum conditions. Structure, chirality and mode of adsorption of the resulting dimers to octamers are inferred from the scanning probe microscopy and theoretical calculations.

Aromaticity in Semi-Condensed Figure-Eight Molecules.

Electron delocalization and aromaticity was comparatively evaluated in recently synthesized figure-eight molecules made of two condensed U-shaped polycyclic aromatic hydrocarbon moieties connected either by two single bonds or by two para-phenylene groups. The selected examples include molecules that incorporate eight-membered and sixteen-membered rings, as well as a doubly [5]helicene-bridged (1,4)cyclophane. We probe whether some electron delocalization could occur through the stereogenic single bonds in these molecules: Is aromaticity purely (semi-)local, or possibly also global in these molecules? It was concluded that the situation can go from a purely (semi-)local character when the dihedral angle at the connecting single bonds is large, such as in biphenyl, to a predominantly (semi-)local character with a minor global contribution when the dihedral angle is small, such as in the para-phenylene connectors of the [5] helicene-bridged cyclophane.

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.

Helically Chiral π-Expanded Azocines Through Regioselective Beckmann Rearrangement and Their Charged States.

We report a synthetic approach to π-expanded [6]helicenes incorporating tropone and azocine units in combination with a 5-membered ring, which exhibit intriguing structural, electronic, and chiroptical properties. The regioselective Beckmann rearrangement allows the isolation of helical scaffolds containing 8-membered lactam, azocine, and amine units. As shown by X-ray crystallographic analysis, the incorporation of tropone or azocine units leads to highly distorted [6]helicene moieties, with distinct packing motifs in the solid state. The compounds exhibit promising optoelectronic properties with considerable photoluminescence quantum yields and tunable emission wavelengths depending on the relative position of the nitrogen center within the polycyclic framework. Separation of the enantiomers by chiral high-performance liquid chromatography (HPLC) allowed characterization of their chiroptical properties by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The azocine compounds feature manifold redox chemistry, allowing for the characterization of the corresponding radical anions and cations as well as the dications and dianions, with near-infrared (NIR) absorption bands extending beyond 3000 nm. Detailed theoretical studies provided insights into the aromaticity evolution upon reduction and oxidation, suggesting that the steric strain prevents the azocine unit from undergoing aromatization, while the indene moiety dominates the observed redox chemistry.

Hexabenzoheptacene: A Longitudinally Multihelicene Nanocarbon with Local Aromaticity and Enhanced Stability.

We report the synthesis of a longitudinally helical molecular nanocarbon, hexabenzoheptacene (HBH), along with its dimethylated derivative (HBH-Me), which are composed of six benzene rings periodically benzannulated to both zigzag edges of a heptacene core. This benzannulation pattern endows the resulting nanocarbons with a helical heptacene core and local aromaticity, imparting enhanced solubility and stability to the system. The chiral HBH-Me adopts a more highly twisted conformation with an end-to-end twist angle of 95°, enabling the separation of the enantiomers. Both HBH and HBH-Me can be facilely oxidized into their corresponding dications, which exhibit enhanced planarity and aromaticity upon loss of electrons. Notably, both longitudinally helical nanocarbons readily promote solid state packing into two-dimensional (2D) arrangement. Single-crystal microbelts of HBH-Me show hole mobility up to 0.62 cm2 V-1 s-1, illustrating the promising potential of these longitudinally helical molecules for organic electronic devices.

A Double Twisted Nanographene with a Contorted Pyrene Core.

The mature synthetic methodologies enable us to rationally design and produce chiral nanographenes (NGs), most of which consist of multiple helical motifs. However, inherent chirality originating from twisted geometry has just emerged to be employed in chiral NGs. Herein, we report a red-emissive chiral NG constituted of orthogonally arranged two-fold twisted π-skeletons at a contorted pyrene core which contributes to optical transitions of S0→S1 and vice versa. The thus-obtained NG exhibited a robustness on its redox properties through 2e- uptake/release. The chemical oxidation generated stable radical cation whose absorption covers near-infrared I and II regions. Overall, the contorted pyrene core governs electronic nature of the chiral NG. The twist operation on NGs would be, therefore, a design strategy to alter conventional chirality induction on NGs.

Crystalline, Porous Helicene Covalent Organic Frameworks.

Helicenes are a class of fascinating chiral helical molecules with rich chemistry developed continuously over the past 100 years. Their helical, conjugated, and twisted structures make them attractive for constructing molecular systems. However, studies over the past century are mainly focused on synthesizing helicenes with increased numbers of aromatic rings and complex heterostructures, while research on inorganic, organic, and polymeric helicene materials is still embryonic. Herein, we report the first examples of helicene covalent organic frameworks, i.e., [7]Helicene sp2 c-COF-1, by condensing [7]Helicene dialdehyde with trimethyl triazine via the C=C bond formation reaction under solvothermal conditions. The resultant [7]Helicene sp2 c-COF-1 exhibits prominent X-ray diffraction peaks and assumes a highly ordered 2D lattice structure originated from the twisted configuration of [7]Helicene unit. The C=C linked [7]Helicene sp2 c-COF-1 materials exhibited extended π conjugation and broadly tuned their absorption, emission, redox activity, photoconductivity, and light-emitting activity, demonstrating rich multifunctionalities and great potentials in developing various applications. This work opens a way to a new family of COFs as well as helicene materials, enabling the exploration of unprecedented π architectures and properties.

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties.

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

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.

Facile Synthesis of Helicene-Containing Arenes for Optical Limiting Devices.

A helicene-containing arene and its linear analogue have been successfully synthesized and characterized, where the single-crystal X-ray diffraction analysis indicates that the former can arrange in an offset packing style with a π-π overlap. The introduction of pentagon-rings into the parent skeletons in the resulting compounds can boost the stability, and such helicene-containing molecule possesses higher solubility in organic solvent than the linear analogue. The structural difference has significantly influenced the optical limiting performance. The former in solution and in doped gel glass presents higher optical limiting response towards 532 nm laser than the latter. This study can enrich the functionalization of helicene, which can possess a positive effect in terms of nonlinear optical property.

Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity.

Overcrowded bistricyclic aromatic enes (BAEs) have several conformations such as twisted and anti-folded conformers, and their stereochemistry and chromism have been studied in earnest. In this study, boron-containing heteromerous BAEs having various tricyclic structures were synthesized and their photophysical properties investigated. Single-crystal X-ray analysis revealed that the introduction of a rigid fluorene unit resulted in a twisted conformer, whereas the introduction of flexible units such as thioxanthene and 9,9-dimethyl-9,10-dihydroanthracene units resulted in an anti-folded conformer. The absorption spectra of the heteromerous BAEs were dependent on the introduced tricyclic structures, suggesting the immense impact of the tricyclic structures on the electronic structures of BAEs. DFT calculations revealed the large effect of the flexibility of the tricyclic structures on the thermodynamic stability of the conformers. In addition, the boron-containing heteromerous BAEs underwent photocyclization reactions, indicating their potential application as precursors of polyaromatic hydrocarbons and helical aromatic materials.

Post-Installation of Fused Benzoheptagons at the Periphery of NiII Porphyrins: Helical Structures and Conformation-Adjustable Fullerenes Binding.

Fused-benzoheptagon-installed NiII porphyrins were synthesized by a protocol consisting of (2-formyl)arylation at the meso-position(s) of NiII porphyrins, conversion of formyl group to methoxyethene group by Wittig reaction, and final Bi(OTf)3 -catalyzed cyclization. The structures of these porphyrins have been revealed by X-ray analysis. Owing to the installed heptagon ring(s), these porphyrins show curved structures with conformational flexibility. Dimer has been shown to have a small activation barrier for inversion and to capture C60 and C70 with large association constants with adjustable conformational changes.

Sparkling Organic Phosphorescence from Fluorinated Tetrathia[7]helicenes: Synthesis and Photophysical, Electrochemical and Computational Studies.

Structure-property correlations in the thiahelicene family are often not trivial beacuse most of the functional groups present on the helical scaffold modify the conjugation size of the π-system. Selecting fluorine-containing groups to provide strong inductive effects without interacting with low-lying orbitals of the system could be the way to overcome the issue. Here we report a study on three fluorine-functionalized tetrathia[7]helicenes, highlighting interesting correlations between the position of the functional groups and the conjugated skeleton properties. Helicenes Heli-F2 and Heli-CF-F2 were prepared by photoinduced isomerization-electrocyclization (the Mallory photocyclization) of the corresponding fluorinated benzodithienyl-ethenes Alk-F2 and Alk-CF-F2, which were prepared in high yields through stereo-conservative Stille reaction. Notably these helicenes were found to display green phosphorescence around 530-550 nm, and the studies suggest an efficient spin-orbit coupling mechanism in these high-energy triplet nonplanar conjugated molecules. Both helicenes and their precursors were thoroughly characterized by means of optical and electrochemical measurements, while DFT calculations enable a rationale on their structure-property correlations to be defined.

Modulation of Chiroptical and Photophysical Properties in Helicenic Rhenium(I) Systems: The Use of an N-(Aza[6]helicenyl)-NHC Ligand.

The photophysical and chiroptical properties of a novel, chiral helicene-NHC-Re(I) complex bearing an N-(aza[6]helicenyl)-benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene-Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties.

Optical Physical Mechanisms of Helicene Carbon Nanohoop with Möbius Topology.

Optical and spectral properties of carbon nanohoop with Möbius topology is of great interest in nano-science and nano-technology. And it can be imagined that it has a lot of unexpected potential application prospects. However, theoretical calculations based on some figure-of-eight helicene carbon nanohoop with Möbius topology are still insufficient. Therefore, in this paper, we theoretically study the optical and spectral properties of figure-of-eight helicene carbon nanohoop with Möbius topology. Optical and spectral properties are analyzed with visualization method of transition density matrix and charge density difference, which reveal the unique characterization of carbon nanohoop with Möbius topology. Our results can not only deepen the understanding of the optical physical mechanisms of the nanorings with mobius carbons, but also provide deeper insight on optical properties and potential design on optical nanodevices.

Effect of Fluorine Substitution on Absorption and Emission Properties of Figure-eight-shaped [5]Helicene Dimer: A Computational Study at RI-MP2/RI-ADC(2) Level.

Chirality is a very important characteristic of optically active molecules and polyaromatics with helical structures, and plays a vital role in various applications in material science. In the present work, we show the effects of fluorine substitution at various positions in a figure-8-shaped [5]helicene dimer on the ground and excited state g-factors. Calculations for the ground and excited states are performed at the MP2 and ADC(2) levels of theory, respectively. The results reveal that fluorination has a large effect on the excited state structures. The values of the excited state dissymmetry factors for the molecules with fluorinations at both ends of the figure-8 systems are smaller than that of the parent system. On the other hand, fluorinations only in the stacked-phenyl region results in an increase in the value of g cpl ${\left| {g_{{\rm{cpl}}} } \right|}$ . The perfluorinated system shows the smallest g cpl ${\left| {g_{{\rm{cpl}}} } \right|}$ .

Experimental and Theoretical Structure Elucidation of the [2 : 1] Complex Ion of Carbo[n]helicene with n=6, 7 and 8 and Ag.

Gas-phase complexes of [n]helicenes with n=6, 7 and 8 and the silver(I) cation are generated utilizing electrospray ionization mass spectrometry (ESI-MS). Besides the well-established [1 : 1] helicene/Ag+ -complex in which the helicene provides a tweezer-like surrounding for the Ag+ , there is also a [2 : 1] complex formed. Density functional theory (DFT) calculations in conjunction with energy-resolved collision-induced dissociation (ER-CID) experiments reveal that the second helicene attaches via π-π stacking to the first helicene, which is part of the pre-formed [1 : 1] tweezer complex with Ag+ . For polycyclic aromatic hydrocarbons (PAHs) of planar structure, the [2 : 1] complex with silver(I) is typically structured as an Ag+ -bound dimer in which the Ag+ would bind to both PAHs as the central metal ion (PAH-Ag+ -PAH). For helicenes, the Ag+ -bound dimer is of similar thermochemical stability as the π-π stacked dimer, however, it is kinetically inaccessible. Coronene (Cor) is investigated in comparison to the helicenes as an essentially planar PAH. In analogy to the π-π stacked dimer of the helicenes, the Cor-Ag+ -Cor-Cor complex is also observed. Competition experiments using [n]helicene mixtures reveal that the tweezer complexes of Ag+ are preferably formed with the larger helicenes, with n=6 being entirely ignored as the host for Ag+ in the presence of n=7 or 8.

DNA-induced circularly polarized luminescence of helicene racemates.

Enantiopure helicenes have been extensively investigated due to their outstanding chiroptical properties, while helicene racemates are considered as chiroptically silent. Here, we describe a facile method to produce circularly polarized luminescence (CPL) from helicene racemates via supramolecular association with DNA in aqueous solution. Racemic cationic helicene derivatives are immobilized in the grooves of commercially available double-stranded right-handed DNA, and the discrimination of left- and right-handed helicenes by chiral DNA is monitored by single molecule force spectroscopy. This subsequently leads to the generation of prominent CPL with dissymmetric factor |glum | of close to 0.01, which is approximate to enantiopure helicenes. The strategy developed in this work avoids the tedious and expensive chiral resolution process and provides a distinctive insight into the fabrication of CPL-emitting systems.

Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons.

The precision synthesis of helical bilayer nanographenes (NGs) with new topology is of substantial interest because of their exotic physicochemical properties. However, helical bilayer NGs bearing non-hexagonal rings remain synthetically challenging. Here we present the efficient synthesis of the first helical bilayer nonbenzenoid nanographene (HBNG1) from a tailor-made azulene-embedded precursor, which contains a novel [10]helicene backbone with two embedded heptagons. Single-crystal X-ray analysis reveals its highly twisted bilayer geometry with a record small interlayer distance of 3.2 Šamong the reported helical bilayer NGs. Notably, the close interlayer distance between the two layers offers intramolecular through-space conjugation as revealed by in situ spectroelectrochemistry studies together with DFT simulations. Furthermore, the chiroptical properties of the P/M enantiomers of HBNG1 are also evaluated by circular dichroism and circularly polarized luminescence.