A Strategy for Polar Crystals with Dipolar Heterohelicenes.

Polar crystals have attracted interest for the applications to polar materials with piezo- and pyroelectricity, and second harmonic generation. Despite their potential utility for flexible polar materials, a strategy for ordering polar helicenes has remained elusive. Here, we demonstrate the creation of polar crystals with unsymmetrically substituted aza[5]helicenes tuned by substituents. The unsymmetric aza[5]helicenes have been prepared through regioselective mono-protiodesilylations. We disclose that triisopropylsilyl-substituted derivatives show 1D chain columnar packings. In particular, enantiopure crystals showed spontaneous polarization. Optical and single-crystal X-ray diffraction experiments with other derivatives, as well as theoretical calculations, revealed that the presence of triisopropylsilyl or electron-withdrawing aryl substituents is essential for forming the 1D chain columnar structure. Hirshfeld surface analyses further showed that CH-π interactions between 1D chain columns regulate the polar assembly. Finally, we determined the polarizability of the nitro derivative by ab initio calculation to be 4.53 µC/cm2 . This value corroborates the first example of a spontaneously polar crystal of helicenes. We believe that this study will contribute to the development of polar materials from organic molecules.

π-Conjugated Compounds for Molecular Materials.

Depth of a conjugated system! The chemistry of π-conjugated compounds is discussed by Guest Editors Satoru Hiroto and Jishan Wu in their Editorial for this special issue.

Synthesis of π-Functional Molecules through Oxidation of Aromatic Amines.

In this review, we focus on the synthesis of π-conjugated functional molecules by the oxidation of aromatic amines, which is one of the most effective methods for the construction of C-C, C-N, and N-N bonds between two π-conjugated molecular units, and consider their characteristics and applications. Polyanilines are the most common products of the oxidation of aromatic amines; however, azobenzenes, phenazines, and 1,1'-binaphthyl-2,2'-diamines may be produced in this manner also, depending on the reaction conditions. Recent advances in the methodology of aniline oxidation have led to the development of high-regioselectivity industrial-scale syntheses of optically or electroactive π-functional dyes containing nitrogen atoms. In particular, the regioselective fusion of π-extended aromatic amines can be used to prepare distorted π-conjugated molecules under mild reaction conditions, allowing the construction of unprecedented curved nitrogen-containing π-conjugated molecules.

Ni(II) 10-Phosphacorrole: A Porphyrin Analogue Containing Phosphorus at the Meso Position.

Ni(II) 10-Phosphacorrole, a porphyrinoid containing phosphorus at the meso position, was synthesized from a bis(α,α'-dibromodipyrrin) Ni(II) complex and a phosphine anion via the palladium-catalyzed formation of a C-C and two C-P bonds. The optoelectronic properties of Ni(II) 10-phosphacorrole can be modulated effectively by oxidation or coordination of a metal to the phosphorus center. While Ni(II) 10-phosphacorrole exhibits a distinctly aromatic character due to the cyclic conjugation of 18 π-electrons, its oxide exhibited weak antiaromaticity, which was confirmed experimentally and theoretically.

X-Shaped Cyclobutane-Linked Tetraporphyrins through a Thermal [2+2] Cycloaddition of Etheno-Fused Diporphyrins.

A nickel-mediated tandem double cyclization of ethynylene-linked dibromodiporphyrins afforded highly reactive etheno-fused diporphyrins, which undergo a thermal [2+2] cycloaddition at the fused C-C double bond to afford the cyclobutane moiety in X-shaped cyclobutane-linked tetraporphyrins. During the reaction of a Zn(II) complex precursor, the initial double cyclization product was converted into the diketodiporphyrin, which exhibits red chemiluminescence under ambient conditions.

Supramolecular assemblies of a nitrogen-embedded buckybowl dimer with C60.

A directly connected azabuckybowl dimer was synthesized via a palladium-catalysed C-H/C-Br coupling. The electron-donating nature of the pyrrolic nitrogen atoms of the azabuckybowl enabled a strong complexation with pristine C60. In the presence of two equivalents of C60, the azabuckybowl dimer formed crystals with a 1 : 2 stoichiometry. Conversely, in diluted solution, complexes with a 1 : 1 stoichiometry of the dimer and C60 were detected predominantly, and these precipitated upon increasing the concentration of C60. Scanning electron microscopy images of the precipitate showed fibre-like aggregates, indicating the formation of supramolecular assemblies with 1D chain structures. A variable-temperature 1H NMR analysis revealed that the precipitate consists of the dimer and C60 in a 1 : 1 ratio.

Azabuckybowl-Based Molecular Tweezers as C60 and C70 Receptors.

We designed and synthesized molecular tweezers consisting of nitrogen-embedded buckybowl subunits. The judicious choice of the covalent linkers modulated their binding strength with C60 or C70 in solution. Titration studies by optical and 1H NMR analyses revealed a 1:1 composition of the resulting complexes. X-ray diffraction analysis elucidated their solid-state structures, in which two azabuckybowl units surround one fullerene molecule. The large association constants stabilize the complexes toward redox reactions and the purification process on silica-gel column chromatography. The linker enabled tuning of the cavity size for binding of fullerenes, achieving complementary fullerene hosts for C60 and C70: the carbazole-bridged dimer preferentially associates with C70 over C60, while the phenanthrene-bridged dimer interacts with C60 more strongly than C70. Electrochemical analysis in combination with density functional theory calculations indicated the existence of intermolecular charge-transfer interactions between the buckybowl units and the fullerenes. Nonlinear optical measurements showed that the two-photon absorption cross sections of the molecular tweezers are enhanced upon association with fullerenes.

Reversible σ-Bond Formation in Bowl-Shaped π-Radical Cations: The Effects of Curved and Planar Structures.

The reversible formation of σ-bonds between organic radicals has been widely investigated. However, reports on the formation of σ-dimers from delocalized π-radical cations are scarce. Herein, we report the reversible σ-dimerization behavior of a bowl-shaped π-radical cation generated from a nitrogen-embedded buckybowl, both in the crystalline state and in solution. The detailed structure of the σ-dimer in the crystalline state was determined by a single-crystal X-ray diffraction analysis. The monomeric radical cation exists predominantly in solution at room temperature, while dimerization of the radical cations occurs through carbon-carbon σ-bond formation upon reducing the temperature. 1H NMR and optical spectroscopy measurements confirmed the formation of a σ-dimer at low temperature. Comparative studies with a similar yet planar π-conjugated system suggested that the curved structure of the bowl-shaped π-radical cation facilitates the σ-dimerization at one of the internal sp2-hybridized carbon atoms. This trend was also observed for the nucleophilic addition reaction of methanol to the π-radical cations. The methoxylation reaction proceeded only for the curved π-radical cation. Theoretical calculations revealed that the large relief of structural strain at the α-carbon atom during the dimerization or nucleophilic addition reactions accelerated the bond formation at the internal carbon atom of the curved radical cation.

Acid-Mediated Migration of Bromide in an Antiaromatic Porphyrinoid: Preparation of Two Regioisomeric Ni(II) Bromonorcorroles.

The regioselective halogenation of porphyrinoids is critical for their selective functionalization, which enables the fine tuning of their electronic and optical properties. Here we report the synthesis of two regioisomeric Ni(II) bromonorcorroles on the basis of the acid-induced migration of the bromo substituent. Treatment of Ni(II) dimesitylnorcorrole with N-bromosuccinimide (NBS) selectively afforded Ni(II) 3-bromonorcorrole, which was further converted into Ni(II) 2-bromonorcorrole upon treatment with hydrogen bromide. In addition, the reaction of Ni(II) dimesitylnorcorrole with an excess amount of NBS afforded the octabrominated product. The reaction mechanism of the bromination reaction of Ni(II) dimesitylnorcorrole was investigated by theoretical calculations.

Shaping Antiaromatic π-Systems by Metalation: Synthesis of a Bowl-Shaped Antiaromatic Palladium Norcorrole.

The synthesis of a bowl-shaped antiaromatic molecule was achieved through the deformation of a planar antiaromatic porphyrinic π-conjugation system by insertion of palladium into the small cavity of a metal-free norcorrole. The bowl-to-bowl inversion dynamics of the antiaromatic Pd-coordinated norcorrole was determined by variable-temperature 1 H NMR spectroscopy. The metal-free norcorrole was prepared from acid-induced demetalation of a copper norcorrole, which was obtained from the intramolecular coupling of a bis(diiododipyrrin) copper complex with copper thiophenecarboxylate.

π-Extended Dihydrophenazines with Three-State NIR Electrochromism Involving Large Conformational Changes.

π-Extended dihydrophenazines were successfully prepared by oxidation of 2-(N-arylamino)anthracenes. Their roof-type conformations were revealed by X-ray diffraction analysis, and the analysis of the optical properties indicated the presence of intramolecular charge-transfer processes. Upon chemical oxidation, the electronic absorption dramatically changed in a two-step fashion. The electron spin resonance (ESR) analysis revealed that, depending on the amount of oxidant added, either a paramagnetic radical cation or a diamagnetic dication was generated. The NMR analysis revealed a conformational change upon oxidation, which was supported by theoretical calculations. A three-state electrochromic behavior was observed during the electrochemical oxidation and reduction cycles, showing sequential switching between visible and near-infrared (NIR) absorption properties upon application of electrochemical stimuli.

10-Silacorroles Exhibiting Near-Infrared Absorption and Emission.

10-Silacorroles were obtained from the Pd-catalyzed silylative cyclization of a bis(α,α'-dibromodipyrrin) NiII precursor with dihydrosilanes. These 10-silacorroles show substantially red-shifted absorption bands relative to those of normal porphyrins and isocorroles. Notably, the corresponding free base and ZnII  10-silacorroles exhibit emissions in the NIR region. Theoretical calculations on these 10-silacorroles revealed the presence of σ*-π* conjugation between the silyl group and the tetrapyrrole π system, which significantly lowers their LUMO energy levels.

Synthesis of bright red-emissive dicyanoetheno-bridged hexa-peri-hexabenzocoronene dimers.

The introduction of a dicyanomethyl anion group to hexa-peri-hexabenzocoronene (HBC) substantially enhanced the emission properties of HBC due to a large perturbation of its electronic structure. In addition, dicyanoetheno-bridged HBC dimers obtained from oxidation of a dicyanomethyl HBC anion exhibited bright red emission in solution and solid states. Intramolecular charge transfer interactions between the HBC units and the dicyanoethene bridge induced solvatochromic behaviour in their emission spectra. Dicyanoetheno-bridged HBC dimers exhibited cis-trans photoisomerization behaviour in the solution, affording the mixture in cis-isomer dominance in the photostationary state. Theoretical calculations revealed that the cis-isomer is more thermodynamically stable than the trans-isomer.

Synthesis and Functionalization of Porphyrins through Organometallic Methodologies.

This review focuses on the postfunctionalization of porphyrins and related compounds through catalytic and stoichiometric organometallic methodologies. The employment of organometallic reactions has become common in porphyrin synthesis. Palladium-catalyzed cross-coupling reactions are now standard techniques for constructing carbon-carbon bonds in porphyrin synthesis. In addition, iridium- or palladium-catalyzed direct C-H functionalization of porphyrins is emerging as an efficient way to install various substituents onto porphyrins. Furthermore, the copper-mediated Huisgen cycloaddition reaction has become a frequent strategy to incorporate porphyrin units into functional molecules. The use of these organometallic techniques, along with the traditional porphyrin synthesis, now allows chemists to construct a wide range of highly elaborated and complex porphyrin architectures.

Synthesis of Free-Base 10-Azacorroles.

A novel synthetic protocol has been developed for free-base 10-azacorroles. Ni(0)-mediated homocoupling of nitrogen-bridged bisdipyrrin Zn(II) complexes afforded a series of free-base 5,15-diaryl-10-azacorroles in good yields. Pd(II) and Cu(II) complexes have been prepared through metalation of free-base 10-azacorroles. Optical and electrochemical properties of 10-azacorrole metal complexes can be tuned by meso-aryl substituents and central metals. Cyclic voltammetry and theoretical calculations elucidated that the central metals of 10-azacorroles significantly affected their electronic properties.

Control of Conformation and Chirality of Nonplanar π-Conjugated Diporphyrins Using Substituents and Axial Ligands.

Nonplanar conformations of pyrazine-fused Zn(II) diporphyrins could be controlled by the choice of the meso-aryl substituents and an axial ligand on the central metals. Zn(II) diporphyrins bearing sterically demanding meso-aryl groups with ortho-substituents led to a twisted chiral D2 conformation, while an achiral C2h form was preferred in the case of aryl groups without ortho-substituents. Helical chirality induction on Zn(II) diporphyrins in the twisted conformation was achieved by controlling their handedness of the molecular twist through coordination of optically active 1-phenethylamine.

The synthesis of Ni(II) and Al(III) 10-azacorroles through coordination-induced cyclisation involving 1,2-migration.

We have synthesized Ni(II) and Al(III) 10-azacorroles through coordination-induced cyclisation of nitrogen-bridged bisdipyrrins. Al(III) 10-azacorrole displayed contrasting optoelectronic behaviours from Ni(II) 10-azacorrole, underlining the different electronic effects of the azacorrole ligand from the porphyrin.

Nitrogen-embedded buckybowl and its assembly with C60.

Curved π-conjugated molecules have attracted considerable interest because of the unique properties originating from their curved π surface. However, the synthesis of such distorted molecules requires harsh conditions, which hamper easy access to heteroatom-containing curved π systems. Here we report the synthesis of a π-extended azacorannulene with nitrogen in its centre. The oxidation of 9-aminophenanthrene provides tetrabenzocarbazole, which is converted to the azabuckybowl through palladium-catalysed intramolecular coupling. The electron-donating nature and curved π surface of the azabuckybowl enable its tight association with C60 in solution and solid states. High charge mobility is observed for the azabuckybowl/C60 assembly. This compound may be of interest in the fields of curved π systems as fullerene hosts, anisotropic π donors and precursors to nitrogen-containing nanocarbon materials.

Regioselective Nucleophilic Functionalization of Antiaromatic Nickel(II) Norcorroles.

Treatment of antiaromatic nickel(II) norcorrole with potassium cyanide provided nickel(II) 3-cyanonorcorrole with perfect regioselectivity without the help of a catalyst. The reaction of the nickel(II) norcorrole with phenol or thiophenol in the presence of a base also yielded substitution products. The antiaromatic 16π conjugation system in the norcorrole core was preserved in the functionalized products. Introduction of phenylthio groups significantly decreased the HOMO-LUMO gap and enhanced the near IR absorption property.

Isolation of a 1,4-diketone intermediate in oxidative dimerization of 2-hydroxyanthracene and its conversion to oxahelicene.

Oxidation of 2-hydroxy-9,10-dialkynylanthracenes resulted in highly regioselective dimerization to furnish metastable dearomatized 1,4-diketones rather than stable aromatic diols. The 1,4-diketones were converted to oxahelicenes, which exhibited strong fluorescence both in solution and solid state as well as chiroptical properties.