Substituent effects on paratropicity and diatropicity in π-extended hexapyrrolohexaazacoronene.

Research into the application of antiaromatic compounds as molecular materials is an attractive strategy in the development of electronic materials. Antiaromatic compounds have traditionally been considered to be unstable, and thus, the creation of stable antiaromatic compounds has been sought in the field of organic chemistry. Recently, some studies have been reported on the synthesis, isolation, and elucidation of the physical properties of compounds with stability and definitive antiaromatic properties. In general, antiaromatic compounds are considered to be more susceptible to substituents due to their inherently narrow HOMO-LUMO gap compared to aromatic compounds. However, there have been no studies examining substituent effects in antiaromatic compounds. In this study, we have developed a synthetic method to introduce various substituents into π-extended hexapyrrolohexaazacoronene (homoHPHAC+), one of the stable and clearly antiaromatic compounds, and investigated the substituent effects on the optical, redox, and geometrical properties and paratropicity of a series of compounds. In addition, the properties of the two-electron oxidized form, homoHPHAC3+, were investigated. Control of electronic properties by introducing substituents into antiaromatic compounds provides a new design guideline for molecular materials.

Pyrrole-Fused Azacoronene Analog with Sulfur Embedded in the Outer Periphery.

The synthesis of sulfur-embedded hexapyrrolohexaazacoronene (HPHAC) analog 2 and its corresponding desulfurized and rearranged compounds was achieved from tetrafluoroisothianaphthene. Structures of all the new π-skeletons were determined by X-ray crystallography. Comparison of the electronic spectrum of 2 with those of its derivatives revealed less involvement of the sulfur atom in π-conjugation. Similar to the reference HPHAC (1), compound 2 and its derivatives exhibited reversible oxidation behavior. The aromaticity of both neutral and dication states has been investigated through DFT calculations.

Synthesis of Peripherally Annulated Phenanthroporphyrins.

Synthesis of unusual phenanthroporphyrins was achieved by a stepwise precursor method. Precursor porphyrins fused with aryl-substituted bicyclo[2.2.2]octadiene afforded the corresponding arylbenzoporphyrins (arylBPs) by a retro Diels-Alder reaction. Unusual phenanthroporphyrins were obtained via the intramolecular Scholl reaction of arylBPs. X-ray crystallographic analysis revealed a distorted, helical porphyrin plane. Red-shifted absorptions of tetraphenanthroporphyrin are observed at ca. 580 nm for the B band and at 700-900 nm for the Q bands. Analysis of magnetic circular dichroism spectra and time-dependent density functional theory calculations was used to explain the optical properties and electronic structures.

Tropo(thio)ne-embedded homoHPHACs: does the tropylium cation induce global antiaromaticity in expanded hexapyrrolohexaazacoronene?

Tropo(thio)ne-embedded homoHPHACs and their dications were synthesised by an electrophilic annulation of secoHPHAC and successive oxidation. 13C NMR spectra of the dications represented global 22π homoaromaticity via homoconjugation, while alkylation of a thiocarbonyl-bridged homoHPHAC produced a 24π antiaromatic monocation.

Synthesis, Properties, and Packing Structures of Wing-Shaped N-Doped Nanographene in Various Oxidation States.

A rigid wing-shaped bicyclo[2.2.2]octadiene-fused bis-hexapyrrolohexaazacoronene (HPHAC) is synthesized, and subsequent chemical oxidation affords a stable biradical dication and an aromatic tetracation. The physicochemical properties and single-crystal structures in various oxidation states are characterized. The face-to-face π-stacked dimeric structures are observed in the neutral and dicationic states. The HPHAC flakes can act as aromatic walls in a tetracation state, producing enlarged induced magnetic shielding space through the superimposition effect.

Synthesis of Non- or Antiaromatic Dicarbaamethyrin: [24]Diazulihexaphyrin(0.1.0.0.1.0).

A "3+3" condensation reaction of 1,3-di(2-pyrrolyl)azulene with aryl aldehyde followed by an oxidative aromatization afforded diazuliamethyrin, [24]diazulihexaphyrin(0.1.0.0.1.0). X-ray diffraction analysis revealed a relatively planar structure of diprotonated diazuliamethyrin with a mean plane deviation of 0.37 Å. A 24π non- or antiaromatic character was confirmed by 1H NMR, absorption, MCD spectra, and TD-DFT calculations that included the NICS values and ACID plots.

Cyclo[9]pyrrole: Selective Synthesis of [34]Nonaphyrin(0.0.0.0.0.0.0.0.0).

Cyclo[9]pyrrole, a ring-expanded porphyrin without meso-bridges and having an odd number of pyrroles, was synthesized via the oxidative coupling of 2,2':5',2″-terpyrrole. X-ray crystallography showed a C2-like symmetry with a large root-mean-square deviation. The optical properties and electronic structures were analyzed using magnetic circular dichroism spectroscopy and time-dependent density functional theory calculations.

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.

Radially π-Extended Pyrrole-Fused Azacoronene: A Series of Crystal Structures of HPHAC with Various Oxidation States.

Radially π-extended hexapyrrolohexaazacoronene (HPHAC 2) was synthesized in two steps with 2,5-di-tert-butyl-acenaphtho[1,2-c]pyrrole and hexafluorobenzene. Like its parent HPHAC 1a, 2 exhibited reversible oxidation behavior. Crystal structures of not only the neutral and dicationic compounds but also the radical cation were all revealed. While analyzing its peculiar structure, the formation of a 1:1 complex with C60 was observed in solution by NMR and absorption analyses. The spectroscopic analyses of 2 and 1a were performed based on magnetic circular dichroism and theoretical calculations.

Synthesis and Properties of NitroHPHAC: The First Example of Substitution Reaction on HPHAC.

Hexapyrrolohexaazacoronene (HPHAC) is one of the N-containing polycyclic aromatic hydrocarbons in which six pyrroles are fused circularly around a benzene. Despite the recent development of HPHAC analogues, there is no report on direct introduction of functional groups into the HPHAC skeleton. This work reports the first example of nitration reaction of decaethylHPHAC. The structures of nitrodecaethylHPHAC including neutral and two oxidized species (radical cation and dication), intramolecular charge transfer (ICT) character, and global aromaticity of the dication are discussed.

Synthesis and Aromaticity of Benzene-Fused Doubly N-Confused Porphyrins.

A bicyclo[2.2.2]octadiene(BCOD)-fused bis(dipyrromethane) derivative was reacted with methylal in the presence of trifluoroacetic acid followed by oxidation with chloranil to give a doubly N-confused phlorin derivative, which did not undergo the retro-Diels-Alder reaction extruding an ethylene molecule on heating. In contrast, a 7-tert-butoxybicyclo[2.2.1]heptadiene(7-BuOBCHD)-fused bis(dipyrromethane) yielded benzene-ring-fused doubly N-confused porphyrins by following similar reaction sequences.

Synthesis and Isolation of Antiaromatic Expanded Azacoronene via Intramolecular Vilsmeier-Type Reaction.

An antiaromatic cation of the expanded hexapyrrolohexaazacoronene (HPHAC) 1+ was synthesized by a Vilsmeier-type reaction of the partially unfused HPHAC 2. X-ray diffraction analysis revealed the formation of a seven-membered ring with a methyne linkage between the pyrrole moieties. Although 1+ is a monocation, upfield shifts of the peripheral ethyl protons were clearly observed in the 1H NMR spectra, indicating 24π antiaromaticity. Global antiaromaticity was also supported by nucleus-independent chemical shift and anisotropy of the induced current density calculations. Cation 1+ displayed two reversible oxidations and one irreversible reduction in the cyclic voltammetry measurements. Treatment of 1+ with NOSbF6 gave aromatic trication 13+ with 22π-electron conjugation.

Synthesis and Redox Properties of Pyrrole- and Azulene-Fused Azacoronene.

Synthesis of an azacoronene, in which both pyrrole and azulene moieties are circularly fused, was achieved just in three steps. This new azacoronene exhibited multistep reversible oxidations under electrochemical and chemical conditions. Formation of an aromatic 22π-electron conjugation and a tropylium cation (6π-electron conjugation) in the dicationic state was revealed by the single-crystal X-ray crystallographic analysis as well as the nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.

Synthesis, Structures, and Properties of Core-Expanded Azacoronene Analogue: A Twisted π-System with Two N-Doped Heptagons.

A core-expanded, pyrrole-fused azacoronene analogue containing two unusual N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped nonplanar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure was unambiguously confirmed by single-crystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich π-system showed four reversible oxidation peaks. Despite the nonplanar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.

Benzene-fused bis(acenaphthoBODIPY)s, stable near-infrared-selective dyes.

Benzene-fused bis(acenaphthoBODIPY)s prepared by retro-Diels-Alder reaction of bicyclo[2.2.2]octadiene-fused precursors showed strong absorption bands in the near-infrared region and very weak absorptions in the visible region.

Template Synthesis of Decaphyrin without Meso-Bridges: Cyclo[10]pyrrole.

An acenaphthylene-fused cyclo[10]pyrrole 1b was selectively synthesized via an oxidative coupling reaction of the corresponding 2,2'-bipyrrole with the appropriate dianion template, croconate anion. The structure of 1b as the isolated largest cyclo[n]pyrrole was elucidated by X-ray crystallographic analysis. The absorption spectrum exhibited a markedly red-shifted, intensified L band at 1982 nm, which was interpreted by application of Michl's perimeter and Gouterman's 4-orbital models, supported by magnetic circular dichroism (MCD) data and theoretical calculations.

Torands Revisited: Metal Sequestration and Self-Assembly of Cyclo-2,9-tris-1,10-phenanthroline Hexaaza Macrocycles.

A series of novel toroidal cyclo-2,9-tris-1,10-phenanthroline macrocycles with an unusual hexaaza cavity are reported. Nickel-mediated Yamamoto aryl-aryl coupling was found to be a versatile tool for the cyclotrimerization of functionalized 1,10-phenathroline precursors. Due to the now improved processability, both liquid-crystalline behavior in the bulk phase and two-dimensional self-assembly at the molecular level could be studied, for the first time, for a torand system. The macrocycles exhibited a strong affinity for the complexation of different metal cations, as evidenced by MALDI-TOF analysis and spectroscopic methods. Experimental results were correlated to an extensive computational study of the cyclo-2,9-tris-1,10-phenanthroline cavity and its binding mode for metal cations. Due to the combination of several interesting features, toroidal macrocycles may find future applications in the field of ion and charge transport through molecular channels, as well as for chemical sensing and molecular writing in surface-confined monolayers under STM conditions.

Synthesis, structures, and photophysical properties of π-expanded oligothiophene 8-mers and their Saturn-like C60 complexes.

Two isomers of a multifunctional π-expanded macrocyclic oligothiophene 8-mer, E,E-1 and Z,Z-1, were synthesized using a McMurry coupling of a dialdehyde composed of four 2,5-thienylene and three ethynylene units under high dilution conditions. On the other hand, cyclo[8](2,5-thienylene-ethynylene) 2 was synthesized by intramolecular Sonogashira cyclization of ethynyl bromide 5. From STM measurements, both E,E-1 and Z,Z-1 formed self-assembled monolayers at the solid-liquid interface to produce porous networks, and from X-ray analyses of E,E-1 and 2, both compounds had a round shape with a honeycomb stacked structure. E,E-1 formed various fibrous polymorphs due to nanophase separation of the macrorings. E,E-1 and Z,Z-1 in solution exhibited photochromism upon irradiation with visible and UV light, respectively, and this photoisomerization was confirmed by using STM. Furthermore, amorphous films of Z,Z-1 and E,E-1 showed photoisomerization, although single crystals, fibers, and square tubes of E,E-1 remained unchanged under similar conditions. E,E-1 with a 12.5-14.7 Å inner cavity incorporated fullerene C60 in the cavity in solution and the solid state to produce a Saturn-like complex, whose structure was determined by X-ray analysis. 2 also formed a Saturn-like complex with C60 in the solid state. These Saturn-like complexes are stabilized by van der Waals interactions between the sulfur atoms of 8-mer and C60. The complexes exhibited charge-transfer interactions in the solid state. Like E,E-1, Saturn-like complex E,E-1⊃C60 formed small cube and fiber structures depending on the solvent used, whereas those of Saturn-like complex 2⊃C60 were limited due to the rigidity of the macroring of 2.

Effect of substituents on the structure, stability, and π-dimerization of dithienylpyrrole radical cations.

A series of 2,5-di(2-thienyl)-N-methylpyrrole derivatives 1a-1d with methylthio end-capping groups and electron-donating substituents at the 3-position of the thiophene rings was synthesized, and the effects of the substituents on the structure, stability, and π-dimerization ability of the radical cation were investigated using UV-vis-NIR and electron spin resonance spectra and density functional theory (DFT) calculations. Among the electron-donating methyl, methoxy, and methylthio substituents, the methoxy derivative 1c gave the most stable radical cation, which persisted in dichloromethane at room temperature under nitrogen for several hours without any apparent decomposition. In addition, 1c(•+) had the largest π-dimerization enthalpy among 1a(•+)-1d(•+). DFT calculations with the M06-2X method revealed that methyl and methylthio derivatives 1b(•+) and 1d(•+) as well as 1c(•+) adopt a cis-cis conformation, in contrast to the trans-trans conformer of unsubstituted 1a(•+), while the π-dimers of all of these compounds were shown to have a cis-cis conformation. On the basis of further detailed analyses, the preformed cis-cis conformation and the weaker intramolecular and intermolecular steric repulsions were considered to explain why 1c(•+) has the largest π-dimerization enthalpy.

Dithieno[3,4-b:3',4'-d]thiophene-annelated antiaromatic planar cyclooctatetraene with olefinic protons.

The design and synthesis of a new planar cyclooctatetraene (COT) with protons directly connected to the COT ring was attained by monoannelation with dithieno[3,4-b:3',4'-d]thiophene. The planar structure of the COT core was unambiguously confirmed by X-ray crystallography. The magnetic antiaromaticity of the COT core was found to be higher than that of the previously synthesized planar COTs with olefinic protons, according to the results of (1)H NMR and absorption spectra as well as NICS calculations.