Stereoselective Syntheses,Structures, and Properties of Extremely Distorted Chiral Nanographenes Embedding Hextuple Helicenes

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°).     

Helical Nanographenes Containing an Azulene Unit: Synthesis,Crystal Structures,and Properties

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.     

BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis,Aromaticity, and Reactivity

BN-embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and Lewis acidic properties were investigated by X-ray crystallography, optical spectroscopy, and cyclic voltammetry. The B−N bonds show delocalized double-bond characteristics and the conjugation can be extended through the trans-orientated aromatic azaborine units. Calculations reveal the relatively lower aromaticity for the inner azaborine rings in the BN-embedded PAH oligomers. The frontier orbitals of the longer oligomers are delocalized over the inner aromatic rings. Consequently, the inner moieties of the BN-embedded PAH oligomers are more active than the outer parts. This is confirmed by a simple oxidation reaction, which has significant effects on the aromaticity and the intramolecular charge-transfer interactions.     

Emerging Bottom-Up Strategies for the Synthesis of Graphene Nanoribbons and Related Structures

The solution-phase synthesis is one of the most promising strategies for the preparation of well-defined graphene nanoribbons (GNRs) in large scale. To prepare high quality, defect-free GNRs, cycloaromatization reactions need to be very efficient, proceed without side reaction and mild enough to accommodate the presence of various functional groups. In this Minireview, we present the latest synthetic approaches for the synthesis of GNRs and related structures, including alkyne benzannulation, photochemical cyclodehydrohalogenation, Mallory and Pd- and Ni-catalyzed reactions.     

A Unified Mechanism on the Formation of Acenes,Helicenes, and Phenacenes in the Gas Phase

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.     

Cobalt-Catalyzed Regio- and Stereoselective Hydroboration of Allenes

An efficient pincer-ligand-based cobalt-complex-catalyzed allene hydroboration affording Z-allylic boronates is described. The reaction demonstrates an excellent regio- as well as Z-stereoselectivity and a wide substrate scope that tolerates many functional groups. Based on solvent-assisted electrospray ionization mass spectrometry (SAESI-MS) studies, a rationale for the cobalt-catalyzed hydroboration involving the highly selective insertion of an allene into the Co−H bond to form Z-allylic cobalt intermediates is proposed.     

Pentaindenocorannulene: Properties,Assemblies, and C60 Complex

Pentaindenocorannulene (C₅₀H₂₀ , 1 ), a deep bowl polynuclear aromatic hydrocarbon, accepts 4 electrons, crystallizes in columnar bowl‐in‐bowl assemblies and forms a nested C₆₀@ 1 ₂ complex. Spectra, structures and computations are presented.     

Synthesis,Characterization, and Functionalization of 1‐Boraphenalenes

1‐Boraphenalenes have been synthesized by reaction of BBr₃ with 1‐(aryl‐ethynyl)naphthalenes, 1‐ethynylnaphthalene, and 1‐(pent‐1‐yn‐1‐yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench‐stable products. All of these 1‐boraphenalenes have LUMOs localized on the planar C₁₂B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C₅B ring in 1‐boraphenalenes is dramatically lower than the C₆ rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1‐boraphenalenes.     

Synthesis and Optoelectronic Properties of Hexa‐peri‐hexabenzoborazinocoronene

The first rational synthesis of a BN‐doped coronene derivative in which the central benzene ring has been replaced by a borazine core is described. This includes six C−C ring‐closure steps that, through intramolecular Friedel–Crafts‐type reactions, allow the stepwise planarization of the hexaarylborazine precursor. UV/Vis absorption, emission, and electrochemical investigations show that the introduction of the central BN core induces a dramatic widening of the HOMO–LUMO gap and an enhancement of the blue‐shifted emissive properties with respect to its all‐carbon congener.     

Heptagon‐Embedded Pentacene: Synthesis,Structures, and Thin‐Film Transistors of Dibenzo[d,d′]benzo[1,2‐a:4,5‐a′]dicycloheptenes

This study presents a new class of conjugated polycyclic molecules that contain seven‐membered rings, detailing their synthesis, crystal structures and semiconductor properties. These molecules have a nearly flat C₆‐C₇‐C₆‐C₇‐C₆ polycyclic framework with a p‐quinodimethane core. With field‐effect mobilities of up to 0.76 cm² V⁻¹ s⁻¹ as measured from solution‐processed thin‐film transistors, these molecules are alternatives to the well‐studied pentacene analogues for applications in organic electronic devices.     

Rapid Access to Nanographenes and Fused Heteroaromatics by Palladium‐Catalyzed Annulative π‐Extension Reaction of Unfunctionalized Aromatics with Diiodobiaryls

Efficient and rapid access to nanographenes and π‐extended fused heteroaromatics is important in materials science. Herein, we report a palladium‐catalyzed efficient one‐step annulative π‐extension (APEX) reaction of polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics, producing various π‐extended aromatics. In the presence of a cationic Pd complex, triflic acid, silver pivalate, and diiodobiaryls, diverse unfunctionalized PAHs and heteroaromatics were directly transformed into larger PAHs, nanographenes, and π‐extended fused heteroaromatics in a single step. In the reactions that afford [5]helicene substructures, simultaneous dehydrogenative ring closures occur at the fjord regions to form unprecedented larger nanographenes. This successive APEX reaction is notable as it stiches five aryl–aryl bonds by C−H functionalization in a single operation. Moreover, the unique molecular structures, crystal‐packing structures, photophysical properties, and frontier molecular orbitals of the thus‐formed nanographenes were elucidated.     

Lessons in Strain and Stability: Enantioselective Synthesis of (+)-[5]-Ladderanoic Acid

The synthesis of structurally complex and highly strained natural products provides unique challenges and unexpected opportunities for the development of new reactions and strategies. Herein, the synthesis of (+)-[5]-ladderanoic acid is reported. En route to the target, unusual and unexpected strain release driven transformations were uncovered. This occurrence required a drastic revision of the synthetic design that ultimately led to the development of a novel stepwise cyclobutane assembly by an allylboration/Zweifel olefination sequence.     

Photochemical Strain-Release-Driven Cyclobutylation of C(sp3)-Centered Radicals

A new photoredox-catalyzed decarboxylative radical addition approach to functionalized cyclobutanes is described. The reaction involves an unprecedented formal Giese-type addition of C(sp³)-centered radicals to highly strained bicyclo[1.1.0]butanes. The mild photoredox conditions, which make use of a readily available and bench stable phenyl sulfonyl bicyclo[1.1.0]butane, proved to be amenable to a diverse range of α-amino and α-oxy carboxylic acids, providing a concise route to 1,3-disubstituted cyclobutanes. Furthermore, kinetic studies and DFT calculations unveiled mechanistic details on bicyclo[1.1.0]butane reactivity relative to the corresponding olefin system.     

Dearomatization Approach Toward a Superbenzoquinone‐Based Diradicaloid,Tetraradicaloid, and Hexaradicaloid

Reported here is the step‐by‐step dearomatization of a highly aromatic polycyclic aromatic hydrocarbon (PAH), the hexa‐peri‐hexabenzocoronene (also called as “superbenzene”), to give a series of superbenzoquinones containing two, four, and six ketone groups. Different from traditional PAH‐based quinones, these superbenzoquinones show open‐shell multiradical character by rearomatization in the open‐shell forms as experimentally validated by X‐ray crystallographic analysis, NMR and ESR spectroscopy, and FT‐IR measurements, as well as theoretically supported by restricted active space spin‐flip calculations. These compounds exhibit structure‐ and molecular‐symmetry‐dependent optical, electrochemical, and magnetic properties.     

Twisted N‐Doped Nano‐Graphenes: Synthesis,Characterization, and Resolution

Two diastereoisomeric N‐doped nanographene derivatives have been efficiently prepared in two synthetic steps starting from an ethynylated hexaazatriphenylene building block. The first derivative adopts a D₃‐symmetrical propeller‐shaped structure with three equivalent nanographene foils. The structure of the second diastereoisomer is C₂‐symmetrical and differs from the first one by the way two peripheral nanographene foils overlap. Owing to their intertwined structures, the two N‐doped nanographenes are soluble in organic solvents and could be characterized by a combination of several analytical tools. Resolution of the D₃‐symmetrical derivative has been achieved and CD measurements revealed extremely strong Cotton effects.     

A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor–Acceptor Fulleretic Covalent-Organic Materials

The effect of donor (D)–acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for “static” changes in materials properties, an external stimulus was applied for “dynamic” control of the electronic profiles. Overall, the presented D–A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.