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.

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.

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.

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.

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.

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.

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.

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.

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.

Synthesis of highly twisted and fully π-conjugated porphyrinic oligomers.

Highly twisted π-conjugated molecules have been attractive but challenging targets. We report here an efficient synthesis of highly twisted diporphyrins with 126° and 136° twist angles that involves an oxidative fusion reaction of planar aminoporphyrin precursors at room temperature. Repeated amination-oxidative fusion sequences provide a unidirectionally twisted tetramer. The twisting angle of the tetramer is 298°.

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.

Near-IR absorbing nickel(II) porphyrinoids prepared by regioselective insertion of silylenes into antiaromatic nickel(II) norcorrole.

The treatment of an antiaromatic norcorrole Ni(II) complex with a kinetically stabilized silylene provided ring-expansion products in excellent yields through the highly regio- and stereoselective insertion into the ß-ß pyrrolic CC bonds. The resultant Ni(II) porphyrinoid monoinsertion product exhibited relatively strong near-IR absorption bands due to the small HOMO-LUMO gap in spite of the disrupted cyclic π-conjugation by the silicon atom.

Energy and electron transfer from fluorescent mesostructured organosilica framework to guest dyes.

Energy and electron transfer from frameworks of nanoporous or mesostructured materials to guest species in the nanochannels have been attracting much attention because of their increasing availability for the design and construction of solid photofunctional systems, such as luminescent materials, photovoltaic devices, and photocatalysts. In the present study, energy and electron-transfer behavior of dye-doped periodic mesostructured organosilica films with different host-guest arrangements were systematically examined. Fluorescent tetraphenylpyrene (TPPy)-silica mesostructured films were used as a host donor. The location of guest perylene bisimide (PBI) dye molecules, acting as an acceptor, could be controlled on the basis of the molecular design of the PBI substituent groups. PBI dyes with bulky substituents and polar anchoring groups were located at the pore surface with low self-aggregation, which induced efficient energy or electron transfer because of the close host-guest arrangement. However, PBI dye with bulky and hydrophobic substituents was located in the center of template surfactant micelles; the fluorescence emission from the host TPPy groups was hardly quenched when the host-guest distance was longer than the critical Förster radius (ca. 4.5 nm). The relationship between the energy or electron-transfer efficiency and the location of guest species in the channels of mesostructured organosilica was first revealed by molecular design of the PBI substituents.

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.

Excess polarizability reveals exciton localization/delocalization controlled by linking positions on porphyrin rings in butadiyne-bridged porphyrin dimers.

We report direct measurements of the excess polarizability volumes of butadiyne-bridged zinc porphyrin dimers at singly beta-to-beta (1Zn) and doubly beta-to-beta (2Zn) positions using the transient dc photoconductivity (TDCP) technique. The excess polarizability volumes of the singlet exciton for 1Zn and 2Zn are 110 and 270 A(3), respectively, while those of the triplet exciton are approximately 100 A(3) for both dimers. Our measurements suggest that the singlet exciton is mainly localized on one porphyrin subunit for 1Zn, similar to the case for the porphyrin monomer. While the exciton is fully delocalized on two porphyrin subunits in the case of meso-to-meso linked dimer (3Zn), the extent of exciton localization/delocalization for doubly beta-to-beta linked dimer lies between those of singly beta-to-beta and meso-to-meso linked dimers. Electronic structure calculations show that the dramatically different extents of exciton localization/delocalization are the results of frontier orbital coefficients being small at beta positions but large at meso positions. Two butadiyne linkages between the porphyrins at beta positions (2Zn) clearly facilitate electronic communication between the two porphyrin subunits by virtue of stabilization of cumulenic charge resonance structures through enforced planarity.

Unusual interchromophoric interactions in beta,beta' directly and doubly linked corrole dimers: prohibited electronic communication and abnormal singlet ground states.

Directly and doubly beta,beta'-linked corrole dimers (DH(3)CD, DH(2)CD, and DZnCD) are excellent platforms for the investigation of intercorrole interactions because of their enforced coplanar geometries and short edge-to-edge distances. Through the use of these molecules along with the reference monomer H(3)CM and the singly beta,beta'-linked corrole dimer SH(3)CD, the intercorrole interactions have been systematically studied by density functional theory calculations, ultrafast photophysical measurements, and two-photon absorption measurements. A particular focus was placed on revealing factors that are important for the induced photophysical properties of the doubly linked corrole dimers compared with corrole monomer. In the doubly linked corrole dimers, strong molecular orbital interactions caused by the coplanar geometry and the short interchromophoric distance give rise to perturbations of the electronic states that are responsible for the red-shifted and intensified Q-like band in DH(3)CD and the broad NIR absorption bands and fast excitation-energy relaxation processes in DH(2)CD and DZnCD. On the other hand, electronic communication between corrole units is prohibited by the structurally constrained octagonal core in the center, so each constituent corrole unit in the doubly linked corrole dimers maintains an intrinsic pi-conjugation system. Consequently, the overall aromaticity of the directly linked corrole dimers can be explained in terms of a linear sum of two constituent corrole monomers, and the singlet biradical character of DH(2)CD and DZnCD can be understood in terms of two unpaired electrons (one from each constituent oxidized corrole monomer) and their appropriate interaction. In addition, the nonlinear optical properties of DH(2)CD and DZnCD with singlet biradical character have been confirmed to be significantly enhanced compared with those of closed-shell DH(3)CD. Collectively, double beta,beta'-linkages of corroles provide the coplanar geometry with a short interchromophoric distance and the strained octagonal core that play key roles in allowing the strong molecular orbital interactions and restricting the electronic communication between the two corroles, respectively.

Conformational changes of meso-aryl substituted expanded porphyrins upon protonation: effects on photophysical properties and aromaticity.

meso-Hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) and meso-hexakis(pentafluorophenyl) [38]nonaphyrin(1.1.0.1.1.0.1.1.0) have been investigated with a particular focus on their photophysical properties affected by protonation with acids using steady-state and time-resolved spectroscopic measurements along with femtosecond Z-scan method. It was found that the smaller Stokes shift and longer excited singlet/triplet state lifetimes of protonated [30]heptaphyrin and [38]nonaphyrin compared to their distorted neutral counterparts are strongly associated with the rigid and planar molecular structures. Much larger two photon absorption cross-section values of protonated [30]heptaphyrin and [38]nonaphyrin (6300 and 6040 GM) than those of their neutral forms (1350 and 1300 GM) also reflect the enhanced rigidity and planarity as well as aromaticity. In parallel with this, the nucleus-independent chemical shift (NICS) values of protonated forms exhibit large negative values, -14.3 and -11.5 ppm for [30]heptaphyrin and [38]nonaphyrin, respectively, at central positions. Thus we have demonstrated the structure-property relationships between molecular planarity, photophysical properties, and aromaticity of expanded porphyrins upon protonation based on our experimental and theoretical investigations. This study also promises a possibility of structural control of expanded porphyrins through protonation in which the molecular flexibilities of expanded porphyrins lead to distorted structures especially as the number of pyrrole rings increases.

Zwitterionic corroles: regioselective nucleophilic pyridination of a doubly linked biscorrole.

Pyridine attacks: Nucleophilic addition of pyridine derivatives to a doubly linked corrole, which is a stable singlet biradical species, occurs at the bay area with high regioselectivity to provide zwitterionic dimers (see picture; Ar = C(6)F(5)). Charge transfer between the anionic corrole and the pyridinium groups induces effective fluorescence quenching of the corrole dimer, which can be utilized for selective fluoride ion recognition.

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.

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.