An ethyne-bridged Ni²⁺ porphyrin-[26]hexaphyrin-Ni²⁺ porphyrin triad (4) has been prepared and shown to adopt a dumbbell-like conformation despite the two bulky porphyrinylethynyl groups at the hinge positions of the hexaphyrin. The ¹H NMR and UV/vis/NIR spectra of 4 revealed that the aromatic nature of the [26]hexaphyrin unit was significantly mitigated via conjugative interaction with the porphyrinylethynyl substituents. Thermal transannular cyclization of 4 proceeded smoothly to give vinylene-bridged triad 12, in which the π-systems of the Ni²⁺ porphyrins and [26]hexaphyrin are not conjugated and the aromatic character of the latter is enhanced.

β,β-(1,4-Dithiino)subporphyrin dimers 7-syn and 7-anti were synthesized by the nucleophilic aromatic substitution reaction of 2-bromo-3-(4-methoxyphenylsulfonyl)subporphyrin 4 with 2,3-dimercaptosubporphyrin 5 under basic conditions followed by axial arylation. Additions of C₆₀ or C₇₀ to a dilute solution of 7-anti (ca. 10⁻⁶ m) in toluene did not cause appreciable UV/Vis spectral changes, while similar additions to a concentrated solution (ca. 10⁻³ m) resulted in precipitation of complexes. In contrast, dimer 7-syn captured C₆₀ and C₇₀ in different complexation stoichiometries in toluene; a 1:1 manner and a 2:1 manner, respectively, with large association constants; K_a=(1.9±0.2)×10⁶ m⁻¹ for C₆₀@7-syn, and K₁=(1.6±0.5)×10⁶ and K₂=(1.8±0.9)×10⁵ m⁻¹ for C₇₀@(7-syn)₂. These association constants are the largest for fullerenes-capture by bowl-shaped molecules reported so far. The structures of C₆₀@7-anti, C₇₀@7-anti, C₆₀@7-syn, and C₇₀@7-syn have been determined by single-crystal X-ray diffraction analysis.

Boron arylations of B-(methoxo)triphenylsubporphyrin have been developed with a combined use of ArZnI⋅LiCl and trimethylsilyl chloride. Aryl zinc reagents bearing bromo, cyano, amide, and ester groups can be employed for the B-arylation reaction to provide the corresponding B-arylated subporphyrins in moderate yields. Postmodifications of B-arylated subporphyrins have been demonstrated without loss of the B−C bond. These modifications include conversion of the cyano group into a benzoyl group with PhMgBr, hydrolysis of the ester group to give B-(4-carboxyphenyl)subporphyrin, and Pd-catalyzed Suzuki–Miyaura coupling of the 4-bromophenyl group to give a 1,4-phenylene-bridged subporphyrin–Zn^II porphyrin hybrid that displays intramolecular excitation energy transfer from the subporphyrin to the porphyrin. The newly synthesized B-arylated subporphyrins have been fully characterized by NMR, UV/Vis absorption and fluorescence spectroscopies, mass spectrometry, electrochemical measurements, and X-ray diffraction analysis.

The aromaticity reversal in the lowest triplet state (T₁) of a comparable set of Hückel/Möbius aromatic metalated expanded porphyrins was explored by optical spectroscopy and quantum calculations. In the absorption spectra, the T₁ states of the Möbius aromatic species showed broad, weak, and ill-defined spectral features with small extinction coefficients, which is in line with typical antiaromatic expanded porphyrins. In combination with quantum calculations, these results indicate that the Möbius aromatic nature of the S₀ state is reversed to Möbius antiaromaticity in the T₁ state. This is the first experimental observation of aromaticity reversal in the T₁ state of Möbius aromatic molecules.

The aromaticity reversal in the lowest triplet state (T₁) of a comparable set of Hückel/Möbius aromatic metalated expanded porphyrins was explored by optical spectroscopy and quantum calculations. In the absorption spectra, the T₁ states of the Möbius aromatic species showed broad, weak, and ill-defined spectral features with small extinction coefficients, which is in line with typical antiaromatic expanded porphyrins. In combination with quantum calculations, these results indicate that the Möbius aromatic nature of the S₀ state is reversed to Möbius antiaromaticity in the T₁ state. This is the first experimental observation of aromaticity reversal in the T₁ state of Möbius aromatic molecules.

β-to-β 2,5-Pyrrolylene linked cyclic porphyrin oligomers have been synthesized by Suzuki–Miyaura coupling of 2,5-diborylpyrrole and 3,7-dibromoporphyrin. The cyclic porphyrin oligomers exhibit roughly coplanar structures, strong excitonic coupling, small electrochemical HOMO–LUMO gaps, and ultrafast excitation energy transfer between the neighboring porphyrins via the pyrrolylene bridge.

A practical synthesis of cis-A₂B-type 5,10,15-triarylcorroles has been developed that involves [2+2]-type acid-catalyzed condensation of 5-aryldipyrromethane and 5-(pentafluorophenyl)dipyrromethane-1-carbinol and subsequent 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) induced oxidative cyclization. All the corroles thus synthesized were structurally well-characterized, and their optical and electrochemical properties were also studied.

The metalation of meso-tetrakis(pentafluorophenyl)-substituted [26]rubyrin has been explored with Group 9 metal salts (Rh^I, Co^II, Ir^III), affording a Hückel aromatic [26]rubyrin–bis-Rh^I complex with a highly curved gable-like structure, a Hückel antiaromatic [24]rubyrin–bis-Co^II complex that displays intramolecular antiferromagnetic coupling between the two Co^II ions (J=−4.5 cm⁻¹), and two Cp*-capped Ir^III complexes; in one, the iridium metal sits on the [26]rubyrin frame with two IrN bonds, whereas the other has an additional IrC bond, although both Ir^III complexes display moderate aromatic character. This work demonstrates characteristic metalation abilities of this [26]rubyrin toward Group 9 metals.

B-Methoxy β-(4-methoxyphenylsulfinyl)subporphyrin and B-phenyl β-(4-methoxyphenylsulfinyl)subporphyrin were synthesized by oxidation of the corresponding β-sulfanylsubporphyrins with m-chloroperbenzoic acid and were separated into diastereomers, respectively. B-Methoxy subporphyrin diastereomers were interconverted to each other in methanol or ethanol, whereas such interconversion was not observed for B-phenyl subporphyrin diastereomers even at high temperature. Diastereomeric interconversions of B-methoxy subporphyrins were dramatically accelerated by addition of trifluoroacetic acid. These results suggest that the diastereomeric interconversions of B-methoxy subporphyrins, namely, their bowl inversions, proceed via a mechanism involving protonation-induced generation of subporphyrin borenium cations followed by nucleophilic attacks by alcohols.

Herein, the synthesis of Möbius aromatic [28]hexaphyrin silicon complexes bearing various Si-substituents from reactions of [28]hexaphyrin 1 with suitable silicon sources in the presence of a base is reported. Si-substituents newly introduced are vinyl (4), phenyl (6), hydroxy (7), and hydride groups (8). X-ray crystallographic analysis of complexes 6 and 7 has shown trigonal bipyramidal penta-coordinated silicon atoms, which are favorable for the hexaphyrin ligands to take on smoothly twisted Möbius conformations. N-fused Si-vinyl complex 5 was also isolated and structurally well characterized. Through these studies, the Si-substituent effect has been shown to be relatively small. Reaction of 1 with HSiCl₃ in CH₂Cl₂ gave 1,16-dihydrohexaphyrin bis-chlorosilicon complex 9, probably through intramolecular hydride transfer from the silane to the pyrrolic α-carbon.

Metal porphyrins assemble to form a supramolecular architecture with a characteristic structure and characteristic properties and functions upon complexation with appropriate ligands. However, there are few applications of these assembly processes to the construction of polymeric porphyrin arrays with useful functionalities. In this study, we found that meso–meso-linked Zn^II porphyrin arrays underwent zipperlike dimerization upon complexation with di(pyrid-3-yl)acetylene (DPA) in chloroform to form discrete double-stranded porphyrin ladders. Similarly, the assembly of poly(zinc(II) porphyrinylene) with DPA gave a thermoresponsive gel, whose three-dimensional network structure was so strong that a macroporous xerogel film was obtained.

Metal porphyrins assemble to form a supramolecular architecture with a characteristic structure and characteristic properties and functions upon complexation with appropriate ligands. However, there are few applications of these assembly processes to the construction of polymeric porphyrin arrays with useful functionalities. In this study, we found that meso–meso-linked Zn^II porphyrin arrays underwent zipperlike dimerization upon complexation with di(pyrid-3-yl)acetylene (DPA) in chloroform to form discrete double-stranded porphyrin ladders. Similarly, the assembly of poly(zinc(II) porphyrinylene) with DPA gave a thermoresponsive gel, whose three-dimensional network structure was so strong that a macroporous xerogel film was obtained.

meso–meso-Linked porphyrin arrays and meso-aryl-substituted expanded porphyrins have continuously fueled my imagination for many years. In this account, my expertise in chemical research is retrospectively summarized with a particular focus on how these two novel categories of porphyrinoids were found by our group. As part of our photosynthetic model studies in collaboration with Prof. N. Mataga, the energy-gap dependence of intramolecular charge separation was examined by exploring the photoexcited dynamics of 1,4-phenylene-bridged hybrid porphyrin dimers. This study required electron-deficient porphyrins in the dimers that could serve as an electron-accepting unit towards an octaalkyl-substituted Zn(II) porphyrin donor. To this end, we employed meso-nitrated porphyrins and meso-pentafluorophenyl porphyrins. Efforts to prepare these electron-deficient porphyrins allowed us to serendipitously find both a meso–meso-linked porphyrin dimer and a series of meso-pentafluorophenyl-substituted expanded porphyrins. The meso–meso-linked Zn(II) porphyrin dimer was found as a byproduct in the nitration of 5,10-diaryl Zn(II) porphyrin with AgNO₂ but became a major product in the reaction with AgPF₆. This finding opened up a new path to directly linked porphyrin oligomers. The series of meso-pentafluorophenyl-substituted expanded porphyrins were prepared via BF₃·OEt₂-catalyzed condensation of pyrrole and pentafluorobenzaldehyde when the reaction was run at tenfold-higher substrate concentrations, as compared to the optimal conditions for the synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin. These expanded porphyrins have been shown to have attractive attributes such as flexible structures, versatile electronic states, multi-metal coordination, anion sensing, and large nonlinear optical properties. While these studies were mostly curiosity-driven, some of our work covers rather more general interests: how linearly connected molecules can be rationally synthesized and isolated in a pure and discrete form, how large π-conjugation can be realized to allow for very low energy electronic transitions, and how easily Möbius aromatic and antiaromatic molecules can be prepared.

Peripherally hexachlorinated meso-triphenyl subporphyrin 4 was prepared by chlorination of meso-triphenyl subporphyrin 1 with N-chlorosuccinimide and was effectively transformed to hexasulfanylated subporphyrins 5–8 via nucleophilic aromatic substitution (S_NAr) reactions with the corresponding thiols under basic conditions. The structures of 5–8 have been all well characterized by single-crystal X-ray analysis. ¹H NMR studies indicated that the meso-phenyl substituents undergo restricted rotation for 5–8, while the β-sulfanyl substituents are conformationally flexible in 5, 6, and 8, and are strictly regulated to an anti-conformation in 7. Judging from the absorption spectra, the oxidation and reduction potentials, and the DFT calculations, the substituent effects decrease in the order of 5>6>7>8. Subporphyrin 8 effectively captures C₆₀ in a 1:1 manner in [D₈]toluene solution.

Ni^II metalation of [42]nonaphyrin gave two Hückel aromatic [42]nonaphyrin Ni^II complexes that possess different doubly twisted figure-eight conformations. While these complexes are conformers, the activation barriers for conformational interconversion were found to be exceptionally high. However, one-way isomerization from the higher-energy conformer to the lower-energy conformer can be induced upon treatment with trifluoroacetic acid (TFA). This conformational isomerization has been suggested to proceed via a protonation-induced caterpillar-like conformational rotation of the metal-free hexaphyrin-like segment by examining the similar isomerization of an A₃B₆-type [42]nonaphyrin Ni^II complex. Rh^I metalation of the lower-energy [42]nonaphyrin Ni^II complex afforded a [42]nonaphyrin Ni^II-Rh^I hybrid complex, which was oxidized with p-chloranil to produce a [40]nonaphyrin Ni^II-Rh^I hybrid complex as the largest doubly twisted Hückel antiaromatic molecule to date.

Invited for the cover of this issue are Takanori Soya and Atsuhiro Osuka at Kyoto University. The image depicts Group 9 metal (Co, Rh, and Ir) complexes of meso-aryl-substituted rubyrin and a meteorite approaching to the atmosphere. A large amount of Iridium is often contained in meteorites. Read the full text of the article at 10.1002/chem.201501080.

[26]Hexaphyrin(1.1.1.1.1.1) bearing two 5-formyl-2-pyrrolyl groups at the 5- and 20-positions was prepared by cross-condensation of 5,10-bis(pentafluorophenyl)-substituted tripyrrane with 2,5-diformylpyrrole as an effective binuclear metal-coordinating ligand, owing to the two hemiporphyrin-like NNNN pockets. In fact, metalation of this hexaphyrin with Zn^II, Cu^II, and Pd^II salts proceed smoothly at room temperature to give the corresponding bismetal complexes that displayed remarkably redshifted absorption spectra reaching deep into near infrared region. These redshifted absorption bands are ascribed, through electrochemical investigations and DFT calculations, to two structural motifs: the N-metalopyrrole substructure that elevates the HOMO level due to the electron-donating property and the two coordinated metal ions that serve as Lewis acids to lower the LUMO level.

Iridium-catalyzed borylation of B-aryl meso-free subporphyrinato boron(III) complexes (hereinafter referred to simply as subporphyrins) with bis(pinacolato)diboron gave 2,13-diborylated subporphyrins regioselectively, which served as promising synthetic precursors for 2,13-diarylated subporphyrins and doubly β-to-β 1,3-butadiyne-bridged subporphyrin dimers. 2,13-Diarylated subporphyrins display perturbed absorption spectra, depending upon the β-aryl substituents. Doubly 1,3-butadiyne-bridged syn and anti subporphyrin dimers thus prepared exhibit differently altered absorption spectra with split Soret-like bands, which have been accounted for in terms of exciton coupling.

Iridium-catalyzed borylation of B-aryl meso-free subporphyrinato boron(III) complexes (hereinafter referred to simply as subporphyrins) with bis(pinacolato)diboron gave 2,13-diborylated subporphyrins regioselectively, which served as promising synthetic precursors for 2,13-diarylated subporphyrins and doubly β-to-β 1,3-butadiyne-bridged subporphyrin dimers. 2,13-Diarylated subporphyrins display perturbed absorption spectra, depending upon the β-aryl substituents. Doubly 1,3-butadiyne-bridged syn and anti subporphyrin dimers thus prepared exhibit differently altered absorption spectra with split Soret-like bands, which have been accounted for in terms of exciton coupling.

The retro-Diels–Alder reaction has been used for the synthesis of an opp-dibenzo[26]hexaphyrin and an opp-dibenzo[28]hexaphyrin from the corresponding opp-bis(bicyclo[2.2.2]octadiene)-fused hexaphyrins. The opp-dibenzo[26]hexaphyrin has a rectangular conformation regardless of solvent polarity, whereas the conformational changes of [28]hexaphyrins are dependent on the solvent polarity, that is, figure-of-eight conformations in nonpolar solvents, and aromatic, twisted Möbius conformations in polar solvents. Comparison of the optical properties of bis-rhodium(I) complexes of annulated and non-annulated hexaphyrins revealed that opp-dibenzo annulation causes modest but distinct spectral red shifts.

Iodine–magnesium exchange between iodoporphyrins and iPrMgCl·LiCl proceeded successfully without decomposition of the porphyrin core. The resulting porphyrinyl Grignard reagents are nucleophilic enough to react with various carbonyl compounds, such as aldehydes, ketones, and amides. Furthermore, the porphyrinyl Grignard reagents underwent transmetalation to afford porphyrinyl copper and zinc species of mild and unique reactivity. These could be engaged in 1,4-addition and Negishi coupling, respectively.

ABC-Type subporphyrins 5a–5h, which bear three different meso-aryl substituents, were rationally synthesized by condensation of AB-type tripyrranes and aroyl chlorides. ABC-Type subporphyrins 5i and 5j were synthesized by Pd-catalyzed amination reaction of 4-bromophenyl subporphyrins 5e and 5h, respectively. Comparative studies on these ABC-type subporphyrins with A₃-type subporphyrins and A₂B-type subporphyrins revealed that substituent effects are mostly simple superpositions of individual substituents but cooperative effects are recognized for subporphyrins which bear both electron-donating and electron-withdrawing substituents. Despite extensive attempts, enantiomeric separation of B-methoxy ABC-type subporphyrins was unsuccessful, whereas B-phenylated ABC-type subporphyrins were separated to pure enantiomers. Separated enantiomers showed weak but distinct CD signals reflecting the chiral clockwise/anticlockwise arrangements of the meso-aryl substituents. These results suggest facile racemization through S_N1-type heterolysis of the B–OMe bond in solution.

7,8-Dehydropurpurin has attracted much attention owing to the dual 18π- and 20π-electron circuits in its macrocyclic conjugation. The two-fold Pd-catalyzed [3+2] annulation of meso-bromoporphyrin with 1,4-diphenylbutadiyne furnished 7,8-dehydropurpurin dimers. The 8^a,8^a-linked dimer displays a red-shifted and enhanced absorption band in the NIR region and a small electrochemical HOMO–LUMO band gap as a consequence of efficient conjugation between the two coplanar 7,8-dehydropurpurin units. Treatment of this dimer with N-bromosuccinimide in chloroform and ethanol gave β-to-β vinylene-bridged porphyrin dimers. Owing to the highly constrained conformations, these dimers exhibit perturbed absorption spectra, small Stokes shifts, and high fluorescence quantum yields.

Reductive metalation of [44]decaphyrin with [Pd₂(dba)₃] provided a Hückel aromatic [46]decaphyrin Pd^II complex, which was readily oxidized upon treatment with DDQ to produce a Hückel antiaromatic [44]decaphyrin Pd^II complex. In CH₂Cl₂ solution the latter complex underwent slow tautomerization to a Möbius aromatic [44]decaphyrin Pd^II complex which exists as a mixture of conformers in dynamic equilibrium. To the best of our knowledge, these three Pd^II complexes represent the largest Hückel aromatic, Hückel antiaromatic, and Möbius aromatic complexes to date.

meso-Bromosubporphyrin undergoes nucleophilic aromatic substitution (S_NAr) reactions with arylamines, diarylamines, phenols, ethanol, thiophenols, and n-butanethiol in the presence of suitable bases to provide the corresponding substitution products. The S_NAr reactions also proceed well with pyrrole, indole, and carbazole to provide substitution products in moderate to good yields. Finally, the S_NAr reaction with 2-bromothiophenol and subsequent intramolecular peripheral arylation reaction affords a thiopyrane-fused subporphyrin.

Protonation of meso-aryl [28]hexaphyrins(1.1.1.1.1.1) triggered conformational changes. Whereas protonation with trifluoroacetic acid led to the formation of monoprotonated Möbius aromatic species, protonation with methanesulfonic acid led to the formation of diprotonated triangular antiaromatic species. A peripherally hexaphenylated [28]hexaphyrin was rationally designed and prepared to undergo diprotonation to favorably afford a triangular-shaped antiaromatic species.

Direct β-to-β linked branched and cyclic porphyrin trimers and pentamers have been synthesized by the Suzuki–Miyaura coupling of β-borylporphyrins and β-bromoporphyrins. The cyclic porphyrin trimer, the smallest directly linked cyclic porphyrin wheel to date, and its twined pentamer, exhibit small electrochemical HOMO–LUMO gaps, broad nonsplit Soret bands, and red-shifted Q-bands, thus indicating large electronic interactions between the constituent porphyrin units.

A cross-conjugated hexaphyrin that carries two meso-oxacyclohexadienylidenyl (OCH) groups 9 was synthesized from the condensation of 5,10-bis(pentafluorophenyl)tripyrrane with 3,5-di-tert-butyl-4-hydroxybenzaldehyde. The reduction of 9 with NaBH₄ afforded the Möbius aromatic [28]hexaphyrin 10. Bis-rhodium complex 11, prepared from the reaction of 10 with [{RhCl(CO)₂}₂], displays strong Hückel antiaromatic character because of the 28 π electrons that occupy the conjugated circuit on the enforced planar structure. The oxidation of 11 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) yielded complexes 12 and 13 depending upon the reaction conditions. Both 12 and 13 are planar owing to bis-rhodium metalation. Although complex 12 bears two meso-OCH groups at the long sides and is quinonoidal and nonaromatic in nature, complex 13 bears 3,5-di-tert-butyl-4-hydroxyphenyl and OCH groups and exhibits a moderate diatropic ring current despite its cross-conjugated electronic circuit. The diatropic ring current increases upon increasing the solvent polarity, most likely due to an increased contribution of an aromatic zwitterionic resonance hybrid.

Incorporation of Si^IV into an expanded porphyrin has been achieved for the first time. Treatment of [28]hexaphyrin 1 with CH₃SiCl₃ and N,N-diisopropylethylamine gave Si^IV complex 2 and its N-fused product 4 that both have Möbius aromatic nature. In both complexes, the coordinated Si atom is satisfied in a typical trigonal bipyramidal coordination. Si^IV incorporation induces conformational rigidification and redshifted absorption profiles due to σ–π conjugation between the Si atom and hexaphyrin macrocycle. Tamao–Fleming oxidation of 2 with H₂O₂ gave β-hydroxy [28]hexaphyrin 5, which exists as a ruffled rectangular shape in the solid state, yet it has been revealed to exist predominantly as a twisted Möbius aromatic conformer in CH₂Cl₂.

A₂B-type B-methoxy subporphyrins 3 a–g and B-phenyl subporphyrins 7 a–c,e,g bearing meso-(2-substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable-temperature (VT) ¹H NMR spectroscopy. In these subporphyrins, the rotation of meso-aryl substituents is hindered by a rationally installed 2-substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH^≠ and ΔS^≠ in ΔG^≠. 2-Methyl and 2-ethyl groups of the meso-aryl substituents in subporphyrins 3 e, 3 f, and 7 e induce larger rotational barriers than 2-alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4-dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B-phenyl subporphyrins than for B-methoxy subporphyrins indicate a negligible contribution of S_N1-type heterolysis in the rotation of meso-aryl substituents.

3,7-Dibromo-10,15,20,25,30-pentakis(pentafluorophenyl)hexaphyrin (3) and 3,7-bis(triisopropylsilyl)-10,15,20,25,30-pentakis(pentafluorophenyl)hexaphyrin (4) were synthesized by a stepwise route, in which prefunctionalized dipyrromethane Sn⁴⁺ complexes serve as key precursors. Hexaphyrins 3 and 4 have rectangular structures with an unsubstituted meso position on the shorter side. These hexaphyrins are stable in air.

A 1,3-phenylene-bridged hexameric Zn^II porphyrin wheel was synthesized by a Suzuki–Miyaura coupling reaction through a one-pot or a stepwise route. The hexameric wheel structure was revealed by using X-ray diffraction analysis. The porphyrin wheel exhibits a split Soret band due to effective exciton coupling and displays efficient excitation energy transfer along the wheel. Measurements of fluorescence anisotropy decay and pump-power-dependent decay reveal a rapid excitation energy hopping along the wheel with a rate of 1.4 ps.

1-(Triisopropylsilyl)-3,4-dichloropyrrole and 1-(triisopropylsilyl)-3,4-difluoropyrrole were conveniently prepared from the corresponding 3,4-dibromopyrrole by lithiation followed by halogenation. 2,3,17,18-Tetrahalogeno [26]- and [28]hexaphyrins have been prepared by condensation of 3,4-dihalopyrroles and a dipyrromethane-dicarbinol. 2,3,17,18-Tetrahalogenated hexaphyrins display variable structural and electronic properties depending upon the halogen atom and the number of π-electrons. Tetrabromo[28]hexaphyrin and tetrachloro[28]hexaphyrin were further reduced with excess NaBH₄ to furnish meso-reduced hexaphyrins as the first example of phlorin-type meso-aryl-substituted hexaphyrins.

A meso-bromidoplatiniobis(triphenylphosphine) η¹-organometallic porphyrin monomer was prepared by the oxidative addition of meso-bromoZnDPP (DPP=dianion of 5,15-diphenylporphyrin) to a platinum(0) species. The meso–meso directly linked dimeric porphyrin (5) was prepared from this monomer by silver(I)-promoted oxidative coupling and planarized to give a triply linked dizinc(II) porphyrin dimer (8). Acidic demetallation of 8 afforded the bis(free base) 9. Dimer 5 was demetallated then remetallated with nickel(II) to give the dinickel(II) analogue 10, the X-ray crystal structure of which showed a twisted molecule with ruffled, orthogonal NiDPP rings, terminated by square-planar trans-[Pt(PPh₃)₂Br] units. New compounds were fully characterized spectroscopically, and the fused diporphyrin exhibited a broad, low-energy, near-IR electronic absorption band near 1100 nm. Electrochemical measurements of this series indicate that the organometallic fragment is a strong electron donor towards the porphyrin ring. The triply linked organometallic diporphyrin has a substantially lowered first one-electron oxidation potential (−0.35 V versus the ferrocene/ferrocenium couple (Fc/Fc⁺)) and a narrow HOMO–LUMO gap of 0.96 V. Solutions prepared for NMR spectroscopy slowly decompose with degradation of the signals, which is attributed to partial oxidation to the cation radical. This paramagnetic species can be reduced in situ by hydrazine to restore the NMR spectrum to its former appearance. The combined influence of the two [Pt(PPh₃)₂Br] electron-donating substituents is sufficient to make dimer 5 too aerobically unstable to allow further elaboration.

2-Nitro-5,10,15-tri(4-tert-butylphenyl)subporphyrin 2 was prepared by the nitration of 5,10,15-tri(4-tert- butylphenyl)subporphyrin 1a with five equivalents of Cu(NO₃)₂⋅5 H₂O in a mixed EtOAc/Ac₂O solution and was reduced into 2-amino-5,10,15-tri(4-tert-butylphenyl)subporphyrin 3. Bromination of 5,10,15-triphenylsubporphyrin 1b with 1.5 equivalents of N-bromosuccinimide (NBS) gave 2-bromo-5,10,15-triphenylsubporphyrin, which was converted into various 2-arylamino-5,10,15-triphenylsubporphyrins (4a, 4b, 4c, 4d) and 2-benzamido-5,10,15-triphenylsubporphyrin 5 through Pd-catalyzed cross-coupling reactions. These molecules constitute the first examples of mono-β-substituted subporphyrins. These subporphyrins exhibit significantly perturbed optical and electrochemical properties, which reflect a large influence of the peripherally attached substituents on the electronic networks of subporphyrins.

Peripherally hexaarylated subporphyrazines (SubPzs) have been prepared through a Pd-catalyzed, CuTC-mediated coupling of a hexaethylsulfanylated subporphyrazine with arylboronic acids. The introduced aryl substituents strongly influence the electronic properties of the subporphyrazine through effective conjugative interaction. Aryl rings endowed with π-electron-donating groups at the para positions produce a remarkable perturbation of the electron density of the SubPz macrocycle. This is reflected through significant redshifts of the SubPz CT and Q-bands, together with increase of the molar absorptivity of the former, with respect to those exhibited by the hexaphenyl-SubPz 2 a. Moreover, the trend in the first SubPz reduction potentials correlates with the Hammett constants (σ_p) corresponding to the para substituents of the aryl. The domed, extended SubPz π-system self-assembles in the solid state to form a dimeric capsule that houses a solvent molecule.

1,4-Phenylene-bridged [26]hexaphyrin dimer 3 was synthesized by the condensation of 1,4-phenylene-bridged tetrapyrrole carbinol 7 with tetrapyrromethane 8. The dimeric structure of 3 was confirmed by X-ray analysis. ¹H NMR spectroscopy revealed that 3 and its [28]hexaphyrin congeners 4 exist as conformational mixtures in solution. Such conformational dynamics were found to be frozen for their Rh^I complexes 9, which were prepared by Rh^I metalation of 4. Complexes 9 were quantitatively oxidized by MnO₂ to [26]hexaphyrn dimers 10 without conformational change. The structure of 10LL-dl was determined by X-ray analysis to confirm the caterpillar-type conformational isomerization. The Rh^I complexes 9 constitute rare examples of covalently linked, antiaromatic porphyrinoid dimers.

meso-Hexakis(pentafluorophenyl)-substituted neutral hexaphyrin with a 26π-electronic circuit can be regarded as a real homolog of porphyrin with an 18π-electronic circuit with respect to a quite flat molecular structure and strong aromaticity. We have investigated additional aromaticity enhancement of meso-hexakis(pentafluorophenyl)[26]hexaphyrin(1.1.1.1.1.1) by deprotonation of the inner NH groups in the macrocyclic molecular cavity to try to induce further structural planarization. Deprotonated mono- and dianions of [26]hexaphyrin display sharp B-like bands, remarkably strong fluorescence, and long-lived singlet and triplet excited-states, which indicate enhanced aromaticity. Structural, spectroscopic, and computational studies have revealed that deprotonation induces structural deformations, which lead to a change in the main conjugated π-electronic circuit and cause enhanced aromaticity.

5,10,15-Tris(pentafluorophenyl)tetrapyrromethane was efficiently prepared through a route involving stepwise diaroylation of 5-pentafluorophenyldipyrromethane. A₂B₆-type [36]octaphyrins were prepared by the cross condensation of the tetrapyrromethane with aryl aldehydes in moderate yields. A₂B₆-type [36]octaphyrins bearing 2,4,6-trifluorophenyl, 2,6-dichlorophenyl, and phenyl substituents underwent Cu^II-metalation-induced fragmentation to give two molecules of AB₃-type Cu^II porphyrins. A₂B₆-type [36]octaphyrin bearing 3-thienyl substituents underwent thermal N-thienyl fusion reactions to provide a modestly aromatic [38]octaphyrin, which, upon treatment with MnO₂, underwent further N-thienyl fusion and subsequent oxidation to give a nonaromatic doubly N-thienyl fused [36]octaphyrin.

BODIPY–subporphyrin hybrids bridged by a 1,4-biphenylene or 1,4-diphenylethynylene spacer were synthesized either by the palladium-catalyzed Suzuki–Miyaura reaction or the Sonogashira reaction. Their structural and optical properties were examined with respect to the bridge and BODIPY structures. In all cases, intramolecular excitationenergy transfer from the subporphyrin core to the BODIPY peripheries is efficient. Depending on the presence or absence of β-methyl groups adjacent to the meso position of the BODIPY subunit, the fluorescence is either increased or decreased. It was also found that the electronic interaction between the subporphyrin core and the meso-(1,4-phenylene) substituents causes spectral changes for the subporphyrin part, which is not observed for porphyrin counterparts.

Die Chemie von expandierten Porphyrinen, den höheren Homologen der Porphyrine, wurde im Laufe der letzten drei Jahrzehnte ausgiebig untersucht. Expandierte Porphyrine weisen Strukturen, elektronische Eigenschaften, Reaktivitäten und ein Koordinationsverhalten auf, die sich gänzlich von denen der Porphyrine unterscheiden. Im Laufe dieser Untersuchungen wurde immer deutlicher, dass expandierte Porphyrine im Hinblick auf ihre Aromatizität und die Koordination an mehrere Metallionen oder als funktionelle Farbstoffe, nichtlinear-optische Materialien, Ionenrezeptoren, stabile organische Radikale und vieles mehr äußerst attraktiv sind. Kontinuierlich tauchen expandierte Porphyrine mit überraschenden neuen Strukturen und Eigenschaften auf. Letzteres wird durch die einfache Synthese Möbius-aromatischer und sogar antiaromatischer Systeme mit verdrillten Molekülstrukturen besonders deutlich. Hier werden Fortschritte auf dem Gebiet der expandierten Porphyrine seit dem Erscheinen des Aufsatzes von Sessler und Seidel im Jahr 2003 dargelegt.

Hybrid porphyrin tapes 3 and 4, consisting of a mixture of 3,5-di-tert-butylphenyl-substituted donor-type Zn^II–porphyrins and pentafluorophenyl-substituted acceptor-type Zn^II–porphyrins, were prepared by a synthetic route involving cross-condensation reaction of a Ni^II–porphyrinyldipyrromethane and pentafluorophenyldipyrromethane with pentafluorobenzaldehyde followed by appropriate demetalation, remetalation, and oxidative ring-closure reaction. The Ni^II-substituted porphyrin tapes 5 (Ni-Zn-Ni) and 6 (Ni-H₂-Ni) were also prepared through similar routes. The hybrid porphyrin tapes 3 and 4 are more soluble and more stable than normal porphyrin tapes 1 and 2 consisting of only donor-type Zn^II–porphyrins. The solid-state and crystal packing structures of 3, 4, and 5 were elucidated by single-crystal X-ray diffraction analysis. Singly meso–meso-linked hybrid porphyrin arrays 12 and 14 exhibit redox potentials that roughly correspond to each constituent porphyrin segments, while the redox potentials of the hybrid porphyrin tapes 3 and 4 are positively shifted as a whole. The two-photon absorption (TPA) values of 1–6 were measured by using a wavelength-scanning open aperture Z-scan method and found to be 1900, 21 000, 2200, 27 000, 24 000, and 26 000 GM, respectively. These results illustrate an important effect of elongation of π-electron conjugation for the enhancement of TPA values. The hybrid porphyrin tapes show slightly larger TPA values than the parent ones.

Single-molecule photophysical properties of two families of linear porphyrin arrays have been investigated by single-molecule fluorescence detection techniques. Butadiyne-linked arrays (Z_NB) with extensive π-conjugation perform as photostable one-quantum systems. This demonstration has been suggested by the long-lasting initial emissive state and subsequent discrete one-step photobleaching in the fluorescence intensity trajectories (FITs). As in the behavior of a one-quantum system, Z_NB shows anti-bunching data in the coincidence measurements. On the other hand, in directly-linked arrays (Z_N) with strong dipole coupling, each porphyrin moiety keeps individual character in photobleaching dynamics. The stepwise photobleachings in the FITs account for this explanation. Most of the FITs of Z_N do not carry momentary cessation of fluorescence emission, which has been explained by the strongly bound electron-hole pair of Frenkel exciton that suppresses charge transfer between the molecule and surrounding polymers. These results give insight into the influences of interchromophorinc interactions between porphyrin moieties in the multiporphyrin arrays on their fluorescence dynamics at the single-molecule level.

We have investigated conformational switching dynamics of meso-heptakis(pentafluorophenyl) [32]heptaphyrin(1.1.1.1.1.1.1) in various solvents using steady-state, time-resolved, and temperature-dependent spectroscopy. Absorption and fluorescence spectra of [32]heptaphyrin are quite sensitive to solvent environments. In nonpolar toluene, the antiaromatic figure-of-eight conformation is dominant, as seen in the X-ray crystallography, based on broad and weak absorption bands without any fluorescence and moderate paratropic ring current. On the other hand, a well-resolved sharp absorption spectrum, strong fluorescence, and diatropic ring current in the ¹H NMR spectrum in slightly polar THF indicate that most of [32]heptaphyrin molecules take significantly distorted Möbius conformation with aromatic character. By using transient absorption (TA) spectroscopy, the lowest singlet excited-state lifetimes have been revealed to decay biexponentially with the time constants of 5 and 65 ps for figure-of-eight and Möbius conformations, respectively. Based on these results along with vertical excitation energy calculations, we are able to assign two conformers as Hückel antiaromatic and Möbius aromatic species, respectively; it shoulf be noted that the aromaticity/antiaromaticity does not change with temperature variation. Interestingly, in moderately polar solvent, ethyl ether, we find out that these two conformational isomers coexist with a dynamic equilibrium, as revealed by excitation-wavelength-dependent TA, temperature-dependent absorption and ¹H NMR spectra. Through our findings, we have demonstrated that the conformational switching dynamics between Hückel antiaromatic and Möbius aromatic conformers in [32]heptaphyrin(1.1.1.1.1.1.1) are strongly affected by solvent medium as well as temperature.

The chemistry of expanded porphyrins, which are higher homologues of porphyrins, has been intensively explored for the last three decades. Expanded porphyrins exhibit structures, electronic properties, coordination chemistry, and reactivities that are entirely different from those of porphyrins. Through these studies, it has become increasingly apparent that expanded porphyrins are attractive in views of aromaticity and multimetal coordination, or as functional dyes, nonlinear optical materials, ion receptors, or stable organic radicals. As such, we have continuously witnessed the emergence of expanded porphyrins that exhibit unprecedented structures and properties, as is highlighted by the facile realization of Möbius aromatic and even antiaromatic systems with twisted molecular structures. In this Review, the recent progress of the chemistry of expanded porphyrins after the seminal Review by Sessler and Seidel in 2003 is presented.

Nucleophilic substitution reactions of 5,10,15-tris(pentafluorophenyl)corrole (1) with amines were examined as a post-modification route to functional corroles. Reaction of 1 with an excess of amine nucleophiles led to exclusive formation of 5,10,15-tris(4-amino-2,3,5,6-tetrafluorophenyl)-substituted corroles. In this nucleophilic reaction, 5- and 15-pentafluorophenyl substituents were found to be more reactive than the substituent at the 10-position. This substitution reaction was applied for the preparation of ABC-type corroles and corrole-based organogels. The latter exhibited a blue-shifted Soret band and small fluorescence quantum yields in nonpolar hydrocarbon solvents due to the formation of H-type aggregates.

meso-Triazolyl-appended Zn^II–porphyrins were readily prepared by Cu^I-catalyzed 1,3-dipolar cycloaddition of benzyl azide to meso-ethynylated Zn^II–porphyrin (click chemistry). In noncoordinating CHCl₃ solvent, spontaneous assembly occurred to form tetrameric array (3)₂ from meso–meso-linked diporphyrins 3, and dodecameric porphyrin squares (4)₄ and (5)₄ from the L-shaped meso–meso-linked triporphyrins 4 and 5. The structures of these assemblies were examined by ¹H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares (4)₄ and (5)₄ were probed by small- and wide-angle X-ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation-energy migration processes in these assemblies were also investigated in detail by using both steady-state and time-resolved spectroscopic methods, which revealed efficient excited-energy transfer (EET) between the meso–meso-linked Zn^II–porphyrin units that occurred with time constants of 1.5 ps⁻¹ for (3)₂ and 8.8 ps⁻¹ for (5)₄.

Recent progress in the synthesis of covalently linked porphyrin arrays with large two-Photon absorption (TPA) cross-section values has been reviewed with a particular focus on the relation of TPA properties with molecular structures. Covalently linked porphyrin arrays continue to be important and useful for the creation of functional materials owing to their chemical robustness, fine-tuning, and easy manipulation. More importantly, the porphyrin electronic systems are quite susceptible to periphery conjugative perturbations, hence allowing facile fabrications to extensively delocalized systems. This property has been used for exploration of porphyrin-based molecular systems with large TPA values, demonstrating a general trend that enhancement in electronic interactions leads to large TPA cross-section values. As a representative example, the porphyrin tapes exhibit larger TPA values owing to the fully delocalized nature of the π-electrons. This Focus Review will help understand the structural requirements of porphyrin arrays with large TPA values, which will be useful for future applications in optical communication in the IR region.

Donor–acceptor systems based on subporphyrins with nitro and amino substituents at meta and para positions of the meso-phenyl groups were synthesized and their photophysical properties have been systematically investigated. These molecules show two types of charge-transfer interactions, that is, from center to periphery and periphery to center depending on the peripheral substitution, in which the subporphyrin moiety plays a dual role as both donor and acceptor. Based on the solvent-polarity-dependent photophysical properties, we have shown that the fluorescence emission of para isomers originates from the solvatochromic, dipolar, symmetry-broken, and relaxed excited states, whereas the non-solvatochromic fluorescence of meta isomers is of the octupolar type with false symmetry breaking. The restricted meso-(4-aminophenyl) rotation at low temperature prevents the intramolecular charge-transfer (ICT)-forming process. The two-photon absorption (TPA) cross-section values were determined by photoexcitation at 800 nm in nonpolar toluene and polar acetonitrile solvents to see the effect of ICT on the TPA processes. The large enhancement in the TPA cross-section value of approximately 3200 GM (1 GM=10⁻⁵⁰ cm⁴ s photon⁻¹) with donor–acceptor substitution has been attributed to the octupolar effect and ICT interactions. A correlation was found between the electron-donating/-withdrawing abilities of the peripheral groups and the TPA cross-section values, that is, p-aminophenyl>m-aminophenyl>nitrophenyl. The increased stability of octupolar ICT interactions in highly polar solvents enhances the TPA cross-section value by a factor of approximately 2 and 4, respectively, for p-amino- and m-nitrophenyl-substituted subporphyrins. On the other hand, the stabilization of the symmetry-broken, dipolar ICT state gives rise to a negligible impact on the TPA processes.

Directly meso-meso, β-β, β-β triply linked porphyrin arrays are exceptional π-conjugated molecules exhibiting remarkably red-shifted absorption bands extending deeply in the IR region. In order to determine the effective conjugated length (ECL), we embarked on the synthesis of the porphyrin tapes far beyond the 12-mer, which is the longest we have prepared so far. In this study, to find the compromise between the feasibility of the meso-meso coupling reaction up to longer arrays and the sufficient solubility and chemical stability of the resultant porphyrin tapes, we prepared hybrid meso-meso linked porphyrin arrays BOn up to 24-mer, which have two different aryl groups, a 2,4,6-tris(3,5-di-tert-butylphenoxy) phenyl group (Ar¹) and a 3,5-dioctyloxy phenyl group (Ar²). All these arrays were effectively converted into the corresponding triply linked porphyrin tapes TBOn by oxidation with DDQ-Sc(OTf)₃. Importantly, the low energy Q-band-like absorption bands of TBOn are progressively red-shifted with an increase in the number of porphyrins n until 16 but the red-shift is saturated at n=16, indicating the ECL of the porphyrin tape to be around 14–16. The regularly introduced meso-aryl bulky substituents impose facial encumbrance, hence leading to the effective suppression of π–π interactions as well as improvement of the chemical stabilities of TBOn.

Capped subporphyrins 12–16 with C₃ molecular symmetry were synthesized from 5,10,15-tri(3-aminophenyl)-substituted subporphyrin 8 and tripodal trialdehydes 2–6 by Lindsey’s entropically favored macrocyclization. X-ray diffraction analysis has revealed that the concave surface of the subporphyrin core is selectively capped with a 1,3,5-substituted benzene moiety. Capped subporphyrins 15 and 16, with a five-atom arm and thus large inner cavities, exhibit solvent incorporation behavior in their crystal structures. On the other hand, subporphyrins 12 and 13 exhibit tight structures, in which the cap and subporphyrin core are found much closer with average interplanar separations of 3.56 and 3.15 Å, respectively. Variable-temperature ¹H NMR measurements revealed that subporphyrins 12, 13, and 16 undergo spiral interconversions between P and M forms depending on the arm length and the electronic nature of the cap. Of these, subporphyrin 13, with a 1,3,5-tri(alkoxycarbonyl)benzene cap strapped by three-atom arms, exhibits a considerably slow spiral interconversion with a large enthalpy change of ΔH^≠=76.4 kJ mol⁻¹ and a characteristic redshift of the Soret-like band and enhancement of the Q(0,0) band. These properties are ascribed to considerable through-space charge-transfer interactions between the electron-deficient cap and the subporphyrin core and the multiple CHπ interactions.

The cover picture shows the crystal structures and colors of meso-pentafluorophenyl-substituted expanded porphyrins. [18]Porphyrin, [26]hexaphyrin, [36]octaphyrin, [44]decaphyrin, [52]dodecaphyrin, [62]tetradecaphyrin, [72]hexadecaphyrin, and [80]octadecaphyrin were synthesized and characterized. As indicated, the absorption bands of these expanded porphyrins are redshifted as their size increases, and [80]octadecaphyrin shows its absorption maximum at 953 nm. Their solutions exhibit various colors, reflecting the macrocyclic conjugation like a rainbow. Details are discussed in the article by A. Osuka et al. on p. 1341 ff.

Acid-catalyzed condensation of meso-pentafluorophenyl dipyrromethane 1 (100 mM) and pentafluorobenzaldehyde 2 (100 mM) at 0 °C gave a series of expanded porphyrins with even number of pyrrolic subunits up to octadecaphyrin(1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1) 18 in improved yields, particularly for larger expanded porphyrins. The solid-state structure of decaphyrin(1.1.1.1.1.1.1.1.1.1) 10 was revealed by X-ray diffraction analysis to be a crescent-like conformation. The Soret-like absorption bands are increasinglyredshifted as the size of the ring increases, but such atrend becomes nearly saturated for hexadecaphyrin(1.1.1.1.1.1.1.1.1.1.1.1.1.1.1.1) 16 and 18, which exhibit bands in the near-infrared region at 939 and 953 nm, respectively.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Starting from 1,3-phenylene linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag^I-promoted coupling reactions provided linear oligomers from 2nZA up to 128ZA. Of these zigzag shaped porphyrin arrays, the Ag^I-promoted intramolecular cyclization reaction of 2 nZA (n=5, 6, 8, 9, 12, and 16) under dilute conditions gave the corresponding cyclic porphyrin wheels C2nZA (n=5, 6, 8, 9, 12, and 16), whereas large arrays 2nZA (n=24, 32, and 48) did not provide cyclic porphyrin products. These large discrete porphyrin arrays and wheels were fully characterized by means of ¹H NMR spectroscopy, MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, GPC-HPLC analysis, and the scanning tunneling microscopy (STM) technique. The STM images of C12ZA and C18ZA reveal their large circular structures. In the cyclic structures of C2nZA in solution, however, the gradual decrease in fluorescence quantum yields and fluorescence lifetimes are observed, reflecting some conformational heterogeneities. Collectively, the present work provides an important contribution to the construction of fully covalently linked large cyclic arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.

Effective peripheral fabrication methods of meso-aryl-substituted subporphyrins were explored for the first time. Hexabrominated subporphyrins 2 were prepared quantitatively from the bromination of subporphyrins 1 with bromine. Hexaphenylated subporphyrins 3 and hexaethynylated subporphyrins 4 and 5 were synthesized by Suzuki–Miyaura coupling and Stille coupling, respectively, in good yields. X-ray crystal structures of 2 b, 3 b, 4 b, and 5 a revealed preservation of the bowl-shaped bent structures with bowl depths similar to that of 1. Hexaethynylated subporphyrins exhibit large two-photon-absorption cross-sections due to effective delocalization of the conjugated network to the ethynyl substituents.

A triply linked Zn^II diporphyrin underwent site-selective cycloaddition reactions with thermally generated o-xylylene to provide a triply linked porphyrin–chlorin hybrid and a triply linked chlorin dimer in moderate yields. The former product is a symmetry-allowed [4+2] cycloadduct, while the latter is a symmetry-forbidden [4+4] cycloadduct. Oxidation of the latter product with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) provided a triply linked diporphyrin fused with a benzocyclooctatriene segment. This oxidized product and above [4+2] cycloadduct were structurally characterized by single crystal X-ray diffraction analysis. The observed site-selectivity is considered to arise from the large MO coefficients at the bay-area in the LUMO of the triply linked diporphyrin. The anomalous thermal [4+4] cycloaddition may be ascribed to the highly conjugated and quite perturbed electronic properties of triply linked Zn^II diporphyrin.