Dianthracenylazatrioxa[8]circulene: Synthesis,Characterization and Application in OLEDs

A soluble, green-blue fluorescent, π-extended azatrioxa[8]circulene was synthesized by oxidative condensation of a 3,6-dihydroxycarbazole and 1,4-anthraquinone by using benzofuran scaffolding. This is the first circulene to incorporate anthracene within its carbon framework. Solvent-dependent fluorescence and bright green electroluminescence accompanied by excimer emission are the key optical properties of this material. The presence of sliding π-stacked columns in the single crystal of dianthracenylazatrioxa[8]circulene is found to cause a very high electron-hopping rate, thus making this material a promising n-type organic semiconductor with an electron mobility predicted to be around 2.26 cm² V⁻¹ s⁻¹. The best organic light-emitting diode (OLED) device based on the dianthracenylazatrioxa[8]circulene fluorescent emitter has a brightness of around 16 000 Cd m⁻² and an external quantum efficiency of 3.3 %. Quantum dot-based OLEDs were fabricated by using dianthracenylazatrioxa[8]circulene as a host matrix material.     

Anti-Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro-diazatrioxa[9]circulene with a Proton Placed Directly above the Central Ring

We present a high-yielding intramolecular oxidative coupling within a diazadioxa[10]helicene to give a dihydro-diazatrioxa[9]circulene. This is the first [n]circulene containing more than eight ortho-annulated rings (n>8). The single-crystal X-ray structure reveals a tight columnar packing, with a proton from a pendant naphthalene moiety centred directly above the central nine-membered ring. This distinct environment induces a significant magnetic deshielding effect on that particular proton as determined by ¹H NMR spectroscopy. The origin of the deshielding effect was investigated computationally in terms of the NICS values. It is established that the deshielding effect originates from an induced paratropic ring current from the seven aromatic rings of the [9]circulene structure, and is not due to the nine-membered ring being antiaromatic. UV/Vis spectroscopy reveals more efficient conjugation in the prepared diazatrioxa[9]circulene compared to the parent helical azaoxa[10]helicenes, and DFT calculations, including energy levels, confirm the experimental observations.     

Per‐Substituted [8]Circulene and Its Non‐Planar Fragments: Synthesis,Structural Analysis,and Properties

The syntheses, structures, and physical properties of a full series of benzannulated tetraphenylenes are reported. The palladium‐catalyzed annulation of tetraiodo‐substituted 2,3,6,7,10,11,14,15‐octamethyltetraphenylene with insufficient di(4‐anisyl)ethyne yielded a mixture of per‐substituted [8]circulene and its non‐planar fragments, including mono‐, para‐di‐, ortho‐di‐, and triannulated products. Their structures were unambiguously verified by X‐ray crystallography. Successive benzannulations significantly affect the molecular geometries, dynamic behaviors, and physical properties of the compounds. In this series of compounds, [8]circulene is the most strained one, as reflected by the significant deplanarization of the phenanthrene moieties (ca. 63° in the bay region) and the fact that it has the highest strain energy (120.6 kcal mol^?1). The dynamic behaviors of these compounds were examined both experimentally and theoretically. The ring flipping of per‐substituted [8]circulene is confirmed to proceed through pseudorotation with a barrier of around 21 kcal mol^?1, whereas its non‐planar fragments require much more energy for the ring inversion. The photophysical and electrochemical properties of the investigated compounds depend strongly on the extent of efficient π conjugation. The successive benzannulations red‐shift both the absorption and the emission bands, and reduce the first oxidation potential.     

Anti‐Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro‐diazatrioxa[9]circulene with a Proton Placed Directly above the Central Ring

We present a high‐yielding intramolecular oxidative coupling within a diazadioxa[10]helicene to give a dihydro‐diazatrioxa[9]circulene. This is the first [n]circulene containing more than eight ortho‐annulated rings (n>8). The single‐crystal X‐ray structure reveals a tight columnar packing, with a proton from a pendant naphthalene moiety centred directly above the central nine‐membered ring. This distinct environment induces a significant magnetic deshielding effect on that particular proton as determined by ¹H NMR spectroscopy. The origin of the deshielding effect was investigated computationally in terms of the NICS values. It is established that the deshielding effect originates from an induced paratropic ring current from the seven aromatic rings of the [9]circulene structure, and is not due to the nine‐membered ring being antiaromatic. UV/Vis spectroscopy reveals more efficient conjugation in the prepared diazatrioxa[9]circulene compared to the parent helical azaoxa[10]helicenes, and DFT calculations, including energy levels, confirm the experimental observations.     

Compressing a Non-Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene

This work describes a synthetic approach where a non-planar aromatic heterocyclic [7]helicene is compressed to yield a hetero[8]circulene containing an inner antiaromatic cyclooctatetraene (COT) core. This [8]circulene consists of four benzene rings and four heterocyclic rings, and it is the first heterocyclic [8]circulene containing three different heteroatoms. The synthetic pathway proceeds via a the flattened dehydro-hetero[7]helicene, which is partially a helicene and partially a circulene: it is non-planar and helically chiral as helicenes, and contains a COT motif like [8]circulenes. The antiaromaticity of the COT core is confirmed by nucleus independent chemical shift (NICS) calculations. The planarization from a helically π-conjugated [7]helicene to a fully planar heterocyclic [8]circulene significantly alters the spectroscopic properties of the molecules. Post-functionalization of the [7]helicenes and the [8]circulenes by oxygenation of the thiophene rings to the corresponding thiophene-sulfones allows an almost complete fluorescence emission coverage of the visible region of the optical spectrum (400–700 nm).     

Highly Stable Radical Cations of N,N’-Diarylated Tetrabenzotetraaza[8]circulene

N,N’-Diarylated tetrabenzotetraaza[8]circulenes 3 a and 3 b were synthesized in good yields by a reaction sequence involving oxidation of tetrabenzodiazadithia[8]circulene 5-Oct and S_NAr reaction with aniline derivatives. The obtained aza[8]circulenes 3 a and 3 b were easily oxidized to give their radical cations 3 a⁺ and 3 b⁺ , which are highly stable under ambient conditions. X-ray diffraction analysis of radical cation 3 a⁺ showed a face-to-face dimer arrangement with an interplanar separation of 3.320 Å. The spin density of 3 a⁺ was calculated to be delocalized over the whole circulene π-systems with spin–spin exchange integral (J=−144 cm⁻¹) in the dimeric part. These radical cations displayed far red-shifted absorption bands reaching to 2000 nm. Thus this study has proved the hetero[8]circulene scaffold to be a new entry of promising electronics and spin materials.     

A Twisted Nanographene Consisting of 96 Carbon Atoms

Herein we report synthesis, structure and properties of a new type of twisted nanographene, which contains an [8]circulene moiety in a polycyclic framework of 96 sp² carbon atoms. The key steps in this synthesis are the Diels–Alder reaction of a macrocyclic diyne and the subsequent Scholl reaction forming the [8]circulene moiety. Two incompletely cyclized products were isolated from the Scholl reaction, providing insight into the cyclization of the strained octagon. This nanographene is twisted along two directions with end‐to‐end twists of 142.4° and 140.2° as revealed by X‐ray crystallography, and is flexible at room temperature as found from the computational and experimental studies.     

Design of a new axially chiral molecule by conformational fixation: 2,6-diphospha- and 2,6-diarsaspiro[3.3]heptanes

We have designed a new axially chiral diphosphine that shows conformational rigidity: 2,6-diphosphaspiro[3.3]heptane 2a. This molecule has axial asymmetry due to puckering of the two phosphetane ring systems. Although its nitrogen congener 2,6-diazaspiro[3.3]heptane 1a is easily racemized, 2,6-diphosphaspiro[3.3]heptane 2a does not racemize at ambient temperature because the lone pairs on the phosphorus atoms have a strong s-character, which results in a high barrier to inversion at the phosphorus, that is, a high barrier to racemization. We also calculated the properties of the diarsa congener 3a and predicted that it also exhibits axial chirality and that it can be separated into enantiomers at ambient temperature.     

Synthesis and Structural Data of Tetrabenzo[8]circulene

In 1976, the first attempted synthesis of the saddle‐shaped molecule [8]circulene was reported. The next 37 years produced no advancement towards the construction of this complicated molecule. But remarkably, over the last six months, a flurry of progress has been made with two groups reporting independent and strikingly different strategies for the synthesis of [8]circulene derivatives. Herein, we present a third synthetic method, in which we target tetrabenzo[8]circulene. Our approach employs a Diels–Alder reaction and a palladium‐catalyzed arylation reaction as the key steps. Despite calculations describing the instability of [8]circulene, coupled with the reported instability of synthesized derivatives of the parent molecule, the addition of four fused benzenoid rings around the periphery of the molecule provides a highly stable structure. This increased stability over the parent [8]circulene was predicted by using Clar’s theory of aromatic sextets and is a result of the compound becoming fully benzenoid upon incorporation of these additional rings. The synthesized compound exhibits remarkable stability under ambient conditions—even at elevated temperatures—with no signs of decomposition over several months. The solid‐state structure of this compound is significantly twisted compared to the calculated structure primarily as a result of crystal‐packing forces in the solid state. Despite this contortion from the lowest‐energy structure, a range of structural data is presented confirming the presence of localized aromaticity in this large polycyclic aromatic hydrocarbon.     

Photochemistry of ethenobenzocycloheptenones : A Di-π-methane rearrangement of β,γ-unsaturated aryl ketones

Benzotropones 6 react with dienophiles 7 to give endo-adducts 8 as shown by PMR. Unlike similar ketones which usually undergo 1,3-acyl shifts or oxadi-π-methane rearrangements, these ethenobenzocycloheptenones 8, on direct or sensitized irradiation, lead to the di-π-methane rearrangement products 1H-benzo[f]cycloprop[cd]-indenones 9. The structures of the latter were elucidated by the use of Eu(FOD)₃ and X-ray diffraction. Prolonged irradiation of tetrahydrofluorenone 19 a potential 1,3-acyl shift product of ethenobenzocycloheptenones, gave only dienyl aldehyde 21 in low conversion. These results suggest that the chemical pathway chosen in these reactions is dependent on geometrical and electronic factors.     

Quadruply BN-Fused Tetrathia[8]circulenes with Flexible Frameworks: Synthesis,Structures and Properties

Quadruply BN-fused tetrathia[8]circulenes were synthesized through four-fold electrophilic borylation. The single-crystal X-ray diffraction analysis revealed that the BN-fused tetrathia[8]circulene with peripheral phenyl groups exhibits crystal polymorphism, in which the circulene core adopts both planar and saddle conformations in the solid state. The experimental and theoretical studies revealed that the weaker aromaticity of azaborine compared with benzene renders the flexibility of the BN-fused tetrathia[8]circulenes.     

Heterocirculenes as a new class of organic semiconductors

We report a fabrication of field-effect transistors using the new organic semiconductors octathio[8]circulene and tetrathiotetraseleno[8]circulene . The maximum hole mobility of 9 x 10(-3) cm(2) V(-1) s(-1) is, most likely, limited by one-dimensional growth of and in thin films.     

Synthesis and phototransformations of novel styryl-substituted furo-benzobicyclo[3.2.1]octadiene derivatives

Novel cis- and trans-(o-H/Me/vinyl) substituted styryl furo-benzobicyclo[3.2.1]octadiene derivatives (7a,b, 8) were prepared and transformed to the novel naphthofuran derivatives of benzobicyclo[3.2.1]octadiene (6a,b) and novel phenanthrene-benzobicyclo[3.2.1]octadiene derivative (11) by photochemical electrocyclic ring closure in the presence of iodine and by intramolecular photoinduced [4+2] cycloaddition, respectively. These novel annelated bicyclo[3.2.1]octadiene derivatives (6a,b, 11) are especially interesting for their rigid methano-bridged junction of two aromatic units at defined geometrical arrangement and thereby as potentials for molecular clips.     

The synthesis of [15]annulenone 4,7:10,13-dioxides and aromatic 4,7:10,13-dioxido[15]annulenium cations : Novel 14]gp ‘odd’ annulene systems

2,15 - Dimethoxycarbonyl - [15]annulenone 4,7:10,13 - dioxide (14) has been prepared by the condensation of cis -α,β -bis(5 - formyl - 2 - furyl)ethy]ene (13) with dimethyl acetonedicarboxylate. Treatment of 14 with conc H₂SO₄ led to 4,7:10,13-dioxido[15]annulenone 2,15-dicarboxylic acid anhydride (17), which was subsequently converted to the corresponding dicarboxylic acid (18) by dilute KOH. Decarboxylation of 18 gave rise to two isomeric [15]annulenone 4,7:10,13 - dioxides, i.e., the tri-cis isomer (7) and the mono - trans - di - cis isomer (8).Regarding to the ring current effects, the proton chemical shifts of these [15]annulenones were compared with those of a reference model, 4,7:10,13 - dioxido - cyclopentadecaheptaene (2.4.6.8.10.12.14) (3). Both of the parent [15]annulenones (7 and 8) have been interpreted as nondiatropic, while the anhydride (17) has been shown to be diatropic, sustaining an induced diamagnetic ring current. The enforced planarity and symmetrical geometry of the anhydride have been discussed. As expected, when 7, 8, 14, 17 and 18 were dissolved in CF₃COOH or conc H₂SO₄, completely delocalized [15]annulenium cations were produced, all of which proved to be diatropic. Three possible geometrical isomers of these 14π cations were established experimentally.     

Chair-boat equilibria in bicyclo[3.3.1]nonane and some 3- and 3,7-substituted derivatives: Thermodynamic parameters and geometry of the conformers as obtained by molecular mechanics

ΔG-Values for conformational equilibria in 3,7-substituted bicyclo[3.3.1]nonanes have been obtained by means of epimerisation experiments (cc α bc) and by variable temperature ¹³C NMR (bc α bb). The results of these experiments fit well with those of molecular mechanics using the Schleyer force field. In bicyclo[3.3.1]nonane and 3β-substituted derivatives the cc conformation predominates; however, the bulky 3β-substituent t-butyl, is found to have a destabilizing effect. A 3α-substituent forces the substituted wing into the boat conformation. For the 3α,7α-substituted derivatives the conformational preferences depend on the size of the substituents: for 3α-methyl-7α-t-butylbicyclo[3.3.1]nonane the cb and t-bb conformers are of approximately equal enthalpy. The geometries, obtained by the calculations, show that the conformers of bicyclo[3.3.1]nonane (cc, bc and t-bb) are all distinctly flattened. The boat wings of bc conformers are not twisted to any extent. The t-bb is the most stable bb conformation. The influence of substitution at positions 3 or 7 is discussed in detail; in general, a bulky substituent such as t-butyl, affects the geometry of both wings of the ring system. The calculated geometries are in good agreement with the conclusions of previous ¹H NMR investigations.     

Facile Synthesis of Azahelicenes and Diaza[8]circulenes through the Intramolecular Scholl Reaction

Carbazole-based aza[7]helicenes and hetero[9]helicenes were successfully obtained via the intramolecular Scholl reaction of 3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-bis(naphthylphenyl)-appended carbazole gave a triple helicene via an unexpected simultaneous double aryl rearrangement. DFT calculations suggested that the rearrangement proceeded via an arenium cation intermediate. In addition, the reaction of methoxy-appended substrate gave an azahepta[8]circulene via the concurrent C−C bond formation. These helical dyes showed circularly polarized luminescence. The azahepta[8]circulene was further transformed into deeply saddle-distorted dibenzodiaza[8]circulenes as the first example of its solution-based synthesis and unambiguous structural determination.     

Enthalpically and Entropically Favorable Self-Assembly: Synthesis of C4h-Symmetric Tetraazatetrathia[8]circulenes by Regioselective Introduction of Pyridine Rings

Self-assembly of π-conjugated molecules in solution generally occurs owing to either an enthalpic or an entropic gain; however, designing π-conjugated systems that simultaneously exhibit enthalpically and entropically favorable self-assembly behavior is challenging. Herein, the self-assembly behavior of tetraazatetrathia[8]circulenes is disclosed, which is driven by both enthalpy and entropy. Single-crystal X-ray diffraction analysis demonstrated that molecules of these tetraazatetrathia[8]circulenes form face-to-face stacked dimers with a 1D columnar structure owing to the circularly arranged dipole moments. Importantly, concentration- and temperature-dependent ¹H NMR spectra revealed that the formation of self-assemblies of tetraazatetrathia[8]circulenes in chloroform and methanol is favored by both enthalpic and entropic factors. The unique association behavior is due to the presence of sp²-hybridized nitrogen atoms, which weakly coordinate to the hydrogen atoms of these solvents and reduce the π-electron density of the circulene cores.     

Calix[4]quinones. Part 4: The ClO2 oxidation of calix[4]arene dialkyl ethers

Except for the special case of calix[4]arene diethyl ether 1, the chlorine dioxide oxidation of dialkyl ethers 2-5 yielded only the corresponding calix[4]diquinone dialkyl ethers 8-11. Chlorine dioxide oxidation of calix[4]arene diethyl ether 1 produced two isomeric products 6 and 7, which were stable enough to be isolated by column chromatography. However, a slow conformational interconversion between isomeric pair 6 and 7 was observed at room temperature, and the equilibrium was reached after 400 h at 18 °C with an amount of 5:3 in favor of syn-isomer.     

Synthetic and structural studies of [6]-, [7]- and [10]metacyclophanes

A new preparative sequence from 2,3-polymethylene-2-cyclopentenone 5 to 2,6-polymethylenebromobenzenes 3 (n = 6, 7, 10) and 2,6-polymethylenephenyllithiums 6 has been found. The reaction of 6 with various electrophiles produces a number of new compounds to disclose the unique reactivity of the aryl C-Li moiety surrounded by the polymethylene chain. Photolysis of 3a and 3b provides transannular products 8, 10 and 11, all arising from the proximity between the aromatic bromine and the aliphatic hydrogen intraannularly opposed to be removed as HBr. Spectrometric study gives quantitative data of the dependence of the molecular geometry upon the chain length and the aromatic substituents. The energy barriers ΔG_c^≠ of the conformational flipping are 17·4 kcal/mol (T_c 76·5°) for [6]metacyclophane (7a), 11·5 kcal/mol (T_c ?28°) for [7]metacyclophane (7b), ·8 kcal/mol for [10]metacyclophane (7c). The lower-energy process of the aliphatic chain in [6]metacyclophane derivatives is the pseudorotation with substituent-dependent barrier ΔG_c^≠ 11·1 kcal/mol (T_c ?31·5°) for 7a, 12·4 kcal/mol (T_c ?4·5°) for 3a and 12·7 kcal/mol (T_c 1·0°) for 12a. The rather large rotational barrier is attributed to the compressed structure of each system. The benzene ring distortion of the cyclophanes is deduced from the bathochromic shift of the B-band and the diamagnetic shift of the benzene proton signals in the PMR.     

Electronic structures of some cata-condensed nonalternant hydrocarbons: pentalenopentalenes,azulenoazulenes, and pentalenoazulenes

On the basis of the second-order Jahn-Teller theory and the semiempirical SCF MO method, we have examined the energetically most favorable ground-state geometrical structures with respect to C-C bond lengths of some cata-condensed nonalternant hydrocarbons consisting of five- and seven-membered rings. It is predicted that both the geometrical valence isomers of pentalenopentalenes, 1 and 2, should suffer molecular-symmetry reduction, exhibiting a marked double-bond fixation in the peripheral carbon skeleton, while in azulenoazulenes, 3 and 4, no molecular-symmetry reduction occurs. Tne electronic spectra have been calculated using the energetically favorable geometrical structures obtained by the SCF MO calculations. The theoretical values for azuleno[1, 2-b]azulene (3) are in good agreement with experimental data.