Construction of a tetrabenzotetrathia[8]circulene by a "fold-in" oxidative fusion reaction: synthesis and optical properties

Herein we report the first synthesis of a tetrabenzotetrathia[8]circulene by a "fold-in" type oxidative fusion reaction. Compared to the pristine tetrathia[8]circulene, the four-fold benzoannulation slightly weakened the antiaromatic character of the central COT ring. The tetrabenzotetrathia[8]circulene exhibited fluorescence at room temperature, and phosphorescence at 77 K with a phosphorescence quantum yield of 11.7%.

Synthesis, Structures and Properties of Trioxa[9]circulene and Diepoxycyclononatrinaphthalene

This article presents trioxa[9]circulene (3) as a novel member of hetero[n]circulenes. Its synthesis began with the synthesis of dimethoxydioxa[8]helicene (5) and used dimethoxydiepoxycyclononatrinaphthalene (4) as a key intermediate, despite the condensation reaction predominantly yielding a 1,4-addition byproduct. The structures and properties of 3-5 were extensively investigated using experimental and computational methods. Analysis of the crystal structures reveal elongation of the internal C-C bonds in the nine-membered ring of 3 compared to 4 and 5. Computational studies demonstrate the remarkable flexibility of trioxa[9]circulene's saddle-shaped polycyclic framework, while the other two compounds are rigid with large racemization barriers. Optically pure forms of 4 and 5 exhibit absorption and luminescence dissymmetry factors on the order of 10-2, with smaller values observed for compound 4. In the crystal structures, molecules of 3 stack to form columns with remarkable π-π overlap, and the π-π interactions of 4 exhibit short intermolecular C-to-C contacts. Consequently, the solution-processed film of 4 functioned as a p-type organic semiconductor in field effect transistors.

Synthesis of Closed-Heterohelicenes Interconvertible between Their Monomeric and Dimeric Forms

Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed-heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed-[7]helicene dimers connected at the β-position of one of the pyrrole units with remarkably elongated C-C bonds of about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light to give the corresponding monomers. Thus, interconversion between the closed-helicene monomer and dimer was achieved. The optically pure dimer was photo-dissociated into the monomers associated with a turn-on of circularly polarized luminescence (CPL).

Unraveling the Aromatic Rule of Cyclic Superatomic Molecules in π-Conjugated Compounds

The aromaticity of π-conjugated compounds has long been a confusing issue. Based on a recently emerged two-dimensional (2D) superatomic-molecule theory, a unified rule was built to decipher the aromaticity of cyclic superatomic molecules of π-conjugated compounds from the chemical bonding perspective. Herein, a series of planar [n]helicenes and [n]circulenes, composed of benzene, thiophene, or furfuran, are systemically studied and seen as superatomic molecules ◊On-2◊F2 or ◊On, where superatoms ◊F and ◊O denote π-conjugated units with 5 and 4 π electrons, respectively. The ascertained superatomic Lewis structures intuitively display aromaticity with each basic unit meeting the superatomic sextet rule of benzene, similar to classical valence bond theory, which is favored by the synthesized complex π-conjugated compounds comprising different numbers and kinds of subrings. The evolutionary trend of ring currents and chemical bonding suggests a local ribbon-like aromaticity in these π-conjugated compounds. Moreover, nonplanar helical π-conjugated compounds have the potential to evolve into spring-like periodic materials with excellent physical properties.

Synthesis of penta- and hexa(3,4-thienylene): size-dependent structural properties of cyclic oligothiophenes

Penta- and hexa(3,4-thienylene)s were synthesized as a potential precursor for thiophene-containing polyarenes, and the structures were determined via X-ray crystallography. The interconversion of thiophene rings is fast in penta(3,4-thienylene), and slow in hexa(3,4-thienylene) reflecting the activation energy for enantiomerization. Size-dependent bathochromic shifts were observed in UV-vis absorption spectra.

Electronic Materials: An Antiaromatic Propeller Made from the Four-Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105  M-1  cm-1 in CH2 Cl2 ) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Fold-in Synthesis of a Pentabenzopentaaza[10]circulene

A pentabenzopentaaza[10]circulene has been synthesized as the largest fully conjugated hetero[n]circulene via a fold-in oxidative fusion reaction of an ortho-phenylene-bridged cyclic pyrrole pentamer. This circulene takes a saddle-distorted structure with bond lengths of the central ten-membered-ring in the range of 1.455-1.493 Å. Relatively broad absorption and fluorescence spectra were observed, which reflects its flexible structure, in accordance with the low-temperature NMR spectra and theoretical calculations. The energy barrier for saddle-to-saddle interconversion was estimated to be quite small (≈3 kcal mol^-1 ).

Theoretical study on the structures, electronic properties, and aromaticity of thia[4]circulenes

The structures, electronic properties, and aromaticity of a series of thia[4]circulenes are predicted using quantum chemical calculations.

Designing Tetraoxa[8]circulenes To Serve as Hosts and Sensors

In D₄-symmetric tetraoxa[8]circulenes, alternating fused benzene and furan rings form an octagonal array. These compounds are little known despite their novel properties, which include extended planar π-conjugation and a formally antiaromatic cyclooctatetraene core. Tetraoxa[8]circulenes can be formed by acid-induced cyclocondensations of suitable quinones, but existing methods often give very low yields. In addition, π-stacking of simple tetraoxa[8]circulenes reduces solubility and limits opportunities to form homogeneous mixtures or cocrystals with other compounds. To help make tetraoxa[8]circulenes more useful, we have developed better ways to synthesize them, and we have used these methods to produce awkwardly shaped derivatives with large concave electron-rich aromatic surfaces. These compounds crystallize to form open structures that can accommodate various guests, including C₆₀. Analysis of the structures shows that the cyclooctatetraene core of the hosts exhibits surprising variations in C-C bond lengths and conjugation, which appear to be related to the gain or loss of aromaticity. This allows tetraoxa[8]circulenes to serve as sensitive probes of local molecular environment and to be used as sensors of electron-deficient species such as nitroaromatic compounds.

Aromaticity of Heterocirculenes

This review summarizes the results on the aromaticity of a series of synthesized and hypothetical neutral heterocirculene molecules and their double charged ions. The aromaticity of heterocirculenes is a direct reflection of their electronic structure responsible for the specific optoelectronic and photophysical properties. We show how the presence of a heteroatom in the outer macrocycle affects the aromaticity of hetero[8]circulenes. In addition, we also describe the change in aromaticity and strain energy for a series of the “lower” (n < 8) and “higher” (n > 8) hetero[n]circulenes. It was demonstrated that the loss of planarity with increased strain leads to an increased antiaromaticity of the lower hetero[n]circulenes, whereas higher hetero[n]circulenes demonstrate a more pronounced aromatic nature because of the small departure from planarity of each heteroarene ring in hetero[n]circulene molecule. Finally, we discuss the aromatic nature of the first examples of π-extended hetero[8]circulenes.

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.

A Tale of Two Isomers: Enhanced Antiaromaticity/Diradical Character versus Deleterious Ring-Opening of Benzofuran-fused s-Indacenes and Dicyclopenta[b,g]naphthalenes

We examine the effects of fusing two benzofurans to s-indacene (indacenodibenzofurans, IDBFs) and dicyclopenta[b,g]naphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity and diradical character of these core units. Synthesis via 3-functionalized benzofuran yields syn-IDBF and syn-IIDBF. syn-IDBF possesses a high degree of paratropicity, exceeding that of the parent hydrocarbon, which in turn results in strong diradical character for syn-IIDBF. In the case of the anti-isomers, synthesized via 2-substituted benzofurans, these effects are decreased; however, both derivatives undergo an unexpected ring-opening reaction during the final dearomatization step. All the results are compared to the benzothiophene-fused analogues and show that the increased electronegativity of oxygen in the syn-fused derivatives leads to enhancement of the antiaromatic core causing greater paratropicity. For syn-IIDBF increased diradical character results from rearomati-zation of the core naphthalene unit in order to relieve this paratropicity.

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.

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.

Synthesis and Characterization of Peralkylated Pyrrole-Fused Azacoronene

A hexapyrrolohexaazacoronene (HPHAC) with 12 less-bulky peripheral ethyl groups than its aryl-containing HPHAC counterpart was synthesized to investigate the innate character of HPHAC. X-ray diffraction analysis revealed that HPHAC had a planar structure and close packing because of CH-π interactions between the alkyl groups and the HPHAC core. Compared to the previously reported HPHAC decorated with 12 peripheral aryl groups, this electron-rich π-system exhibited reversible multistep oxidations at low potentials and easily formed mono- and dicationic salts and charge-transfer (CT) complexes with 7,7,8,8-tetracyano-p-quinodimethane. These oxidized species exhibited clear changes in the bond-length alternation of the pyrrole units in the crystal state, indicating charge and spin delocalization. The distinct upfield shift of the central carbon signal of the dication in the ¹³C NMR spectrum affirms the global aromaticity from the viewpoint of a magnetic criterion. In the UV-vis/NIR spectra, broad absorption in the NIR region was observed only for HPHAC²⁺ and not the structurally similar cyclo[6]pyrrole. Magnetic circular dichroism measurements and time-dependent density functional theory calculations revealed that the broad absorption was assigned to the CT transition from the central benzene ring to the outer pyrrole rings.

Improved Synthesis of ortho-Phenylene-bridged Cyclic Tetrapyrroles and Oxidative Fusion Reactions Toward Substituted Tetraaza[8]circulenes

Hetero[8]circulenes have emerged as novel functionalized heteronanographenes that show various promising functions such as bright fluorescence, charge transporting, and redox reactivities. One of the effective synthetic strategy is the fold-in type oxidative fusion reaction of ortho-phenylene-bridged cyclic tetrapyrroles, whose construction, however, is not well-sophisticated in terms of reproducibility and possibility for versatile derivatization. In this paper, a "reverse" coupling strategy has been developed, which enabled synthesis of opp-type low symmetric analogues of cyclic tetrapyrroles. Oxidative fusion reaction conditions to afford tetraaza[8]circulenes have also been reinvestigated and improved. Substituent effects of cyclic tetrapyrroles and tetrabenzotetraaza[8]circulenes are studied for solid-state structures and packing structures, redox potentials, and optical properties.