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.

Catalytic Dearomative [1,5]-Sigmatropic Carbon Shift of Heterole-Fused Norcaradienes Enabled Concise Helicenation.

In contrast to the locked fluxionality of norcaradienes fused by benzene, unexplored less aromatic heterole-fused norcaradienes, creatively generated by intramolecular hydroarylation of heteroaryl alkynylcyclopropanes, reserve a balancing fluxionality that permits a dearomative 1,5-sigmatropic carbon shift of norcaradienes akin to the reduced aromaticity of heterole. This "walk" shift was confirmed by the isolation of a cycloheptatriene species derived from ring-expansion of a dearomatized alkynylated heterole-fused norcaradiene. A following ester-directed ring-opening rearomatization of these dearomatized heterole-fused norcaradienes gives the products featuring migratory acylmethyls that are competent for helicenation with the neighboring (hetero)arenes via (formal) dehydrative alkenylation. Such balancing reactivity of heterole-fused norcaradienes will open up the opportunity for the development of controllable reactions of fused norcaradienes.

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.

A Helicene-Based Single-Molecule Inductor and Capacitor with Frequency-Dependent Charge-Transport Pathways.

Despite extensive studies has been explored on single-molecule switches and rectifiers, the design of single-molecule inductors has not been explored due to the experimental challenges in the investigation of frequency-dependent charge transport at the single-molecule scale. In this study, we synthesized a helicene-based helical molecular wire and carried out meticulous single-molecule conductance measurements, combined with current-voltage (IV) studies with varying frequencies using the scanning tunneling microscope break junction (STM-BJ) technique. Our results reveal the formation of a single-molecule junction and highlight the unique behavior of the molecular wire in response to different alternating current (AC) varying frequencies. The transport of charges occurs selectively either through the coiled backbone of the conjugated helical structure or vertically via π-π stacking, depending on the frequency of the applied AC. Notably, our investigation demonstrates the functionality of the wire as an inductor at low frequencies, and a capacitor at high frequencies. This work lays the foundation for a systematic approach to designing, fabricating, and implementing single-molecule logic devices such as inductors and wave filters.

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.

Corannulene-based Quintuple [6]/[7]helicenes: Well-preserved Bowl Core, Inhibited Bowl Inversion and Supramolecular Assembly with Fullerenes.

Herein, corannulene-based quintuple [6]helicenes (Q[6]H-1 andQ[6]H-2) and [7]helicene (Q[7]H) were synthesized via penta-fold Heck and Mallory reaction. Notably, Q[7]H represents the highest reported helicene based on corannulene. X-ray crystallography reveals that Q[6]H-2 adopts a propeller-shaped conformation with a well-preserved corannulene core, while Q[6]H-1 and Q[7]H exhibit quasi-propeller-shaped conformations. Upon heating, conformer Q[6]H-1 undergoes conversion to the thermodynamically more stable conformer Q[6]H-2, whereas conformer Q[7]H remains unchanged due to larger steric congestion. Racemization of the enantiomer of Q[6]H-1 and conformational conversion were observed simultaneously at elevated temperature, with DFT studies indicating a racemization barrier of 28.64 kcal·mol-1. In contrast, the racemization barrier for Q[6]H-2 was calculated to be 45.46 kcal·mol-1, indicating exceptional chiral stability. Surprisingly, the bowl inversions of Q[6]H-1 and Q[6]H-2 conformers are somewhat inhibited by the helical blades, whereas this was not observed for other possible conformers of Q[6]H. These results first demonstrated that subtle conformational variations can lead to significant changes in chiral stability and bowl inversions of multihelicenes. Due to the well-preserved corannulene core, propeller-shaped conformation and electron complementarity, Q[6]H-2 can recognize fullerenes in both solution and solid state, which is a rare instance of co-crystallization assembly between multiple helicenes and fullerenes.

Can Deep Learning Search for Exceptional Chiroptical Properties? The Halogenated [6]Helicene Case.

The relationship between chemical structure and chiroptical properties is not always clearly understood. Nowadays, efforts to develop new systems with enhanced optical properties follow the trial-error method. A large number of data would allow us to obtain more robust conclusions and guide research toward molecules with practical applications. In this sense, in this work we predict the chiroptical properties of millions of halogenated [6]helicenes in terms of the rotatory strength (R). We have used DFT calculations to randomly create derivatives including from 1 to 16 halogen atoms, that were then used as a data set to train different deep neural network models. These models allow us to i) predict the Rmax for any halogenated [6]helicene with a very low computational cost, and ii) to understand the physical reasons that favour some substitutions over others. Finally, we synthesized derivatives with higher predicted Rmax obtaining excellent correlation among the values obtained experimentally and the predicted ones.

Rational Design of Crystalline and Enantiomerically Pure Helicenes with Open-Shell Singlet Ground States.

Helicene diradical derivatives have attracted widespread attentions because of their unique magnetic and chiroptoelectronic properties, however, crystalline and enantiomerically pure forms of helicene diradicals are extremely rare. Herein, we describe the rational design and synthesis of o-quinone functionalized helicene diradicals with crystalline enantiomerical purity. Diradical dianion salt Rac-3K and its enantiomers P/M-3K were obtained by reduction of corresponding precursors Rac-3 and P/M-3 with two equivalent potassium graphite in THF in the presence of (di)benzo-18-crown-6. Neutral dioxoborocyclic helicene diradicals (Rac-3B and P/M-3B) were produced by reactions of Rac-3 or P/M-3 with chlorobis(perfluorophenyl)borane (B(C6F5)2Cl. Crystal structures of compounds Rac-3K, Rac-3B and P/M-3K were obtained by single crystal X-ray diffraction. Their open-shell singlet state ground states were confirmed by electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements and theoretical calculations. Their chiroptical properties were investigated by the electronic circular dichroism (ECD) spectroscopy. This work provides the first examples of enantiopure helicene diradical dianions and boron-containing helicene diradicals.

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.

Near-Infrared Emitting Helically Twisted Conjugated Frameworks Consisting of Alternant Donor-π-Acceptor Units and Multiple Boron Atoms.

A novel design strategy to construct bright and narrow near-infrared (NIR) emission materials with suppressed shoulder peaks can significantly enhance their performance in various applications. Herein, we have successfully synthesized a series of helically twisted D-π-A conjugated systems bridged by boron atoms, achieving bright red to near-infrared (NIR) emissions with notably narrow full-width at half-maximum (FWHM) values of 35 nm (0.08 eV) and photoluminescence quantum yield (PLQY) up to 80%. These compounds display redshifted emissions up to 753 nm in higher concentrations. Cis/trans configurational isomers of multi-boron-bridged molecule BN3 exhibit similar photophysical properties. The unique combination of boron-induced coordination-enhanced charge transfer (CE-CT) and the helically twisted conjugated framework is pivotal in achieving the redshifted, narrowband emission. X-ray crystallographic analysis of BN2 and BN3-a reveals that the extension of boron-bridged D-π-A skeletons significantly increases the distortion of the skeleton. Systematic theoretical calculations show how the boron CE-CT mechanism, in conjunction with the helical twist, leads to the narrowing of emission bands while simultaneously red-shifting them into the NIR region. This work could open new avenues for the development of advanced materials with tailored optical properties, particularly in the challenging and highly sought-after NIR spectrum.

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.

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.

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.

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.