Daiki Shimizu

meso‐Hydroxysubporphyrins: A Cyclic Trimeric Assembly and a Stable meso‐Oxy Radical

Treatment of meso-chlorosubporphyrin with potassium hydroxide in DMSO followed by aqueous work up and recrystallization gave a cyclic trimer consisting of meso-hydroxysubporphyrin units linked between the central boron atoms and meso-hydroxy groups. Solutions of this trimer are nonfluorescent, but become fluorescent when exposed to acid or base, since hydrolytic cleavage of the axial B-O bonds generates the meso-hydroxysubporphyrin monomer or its oxyanion. Ring cleavage of the trimer was also effected by reaction with phenylmagnesium bromide to produce meso-hydroxy-B-phenyl subporphyrin, which can be quantitatively oxidized with PbO2 to furnish a subporphyrin meso-oxy radical as a remarkably stable species as a result of spin delocalization over almost the entire molecule.

Triarylporphyrin meso-Oxy Radicals: Remarkable Chemical Stabilities and Oxidation to Oxophlorin π-Cations

5-Hydroxy-10,15,20-triarylporphyrin (oxophlorin) and its Ni(II) and Zn(II) complexes were oxidized with PbO2 to give the corresponding porphyrin meso-oxy radicals as remarkably stable species. These radicals were fully characterized with X-ray diffraction analysis, UV/vis/NIR absorption and ESR spectroscopies, magnetic susceptibility measurement, electrochemical studies, and theoretical calculations. Free-base radical and its Ni(II) complex have been shown to exist as a monoradical in solution, while the Zn(II) complex exists in an equilibrium between monomer (doublet monoradical) and dimer (a non-Kekulé singlet biradicaloid) with a dimerization constant of KD = 3.0 × 10(5) M(-1) in noncoordinating CH2Cl2 but becomes a pyridine-coordinated monoradical upon addition of pyridine. Variable temperature magnetic susceptibility measurements of these radicals revealed different magnetic interactions in the solid-states, which has been interpreted in terms of their different packing structures in a microscopic sense. These radicals undergo one-electron oxidation and reduction in a reversible manner within narrow potential windows of 0.57-0.82 V. Finally, one-electron oxidation of Ni(II) and Zn(II) porphyrin meso-oxy radicals with tris(4-bromophenyl)aminium hexachloroantimonate furnished oxophlorin π-cations, which displayed nonaromatic closed-shell character, NIR absorption, and significant double bond character of the C-O bond.

Nucleophilic Aromatic Substitution Reactions of meso‐Bromosubporphyrin: Synthesis of a Thiopyrane‐Fused Subporphyrin

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

Highly planar diarylamine-fused porphyrins and their remarkably stable radical cations

Oxidative fusion reactions of meso-phenoxazino Ni(II) porphyrin were found to be temperature dependent, giving rise to either a doubly phenylene-fused product at room temperature or a singly phenoxazine-fused product at 70 °C. The latter was further oxidized to a doubly phenoxazine-fused Ni(II) porphyrin, which was subsequently converted to the corresponding free base porphyrin and Zn(II) porphyrin. Compared to previously reported diphenylamine-fused porphyrins that displayed a molecular twist, doubly phenoxazine-fused porphyrins exhibited distinctly different properties owing to their highly planar structures, such as larger fluorescence quantum yields, formation of an offset face-to-face dimer both in solution and the solid state, and the generation of a mixed-valence π-radical cation dimer upon electrochemical oxidation. One-electron oxidation of the phenoxazine-fused Ni(II) porphyrin with Magic Blue gave the corresponding radical cation, which was certainly stable and could be isolated by separation over a silica gel column but slowly chlorinated at the reactive β-positions in the solid state. This finding led to us to examine β,β′-dichlorinated phenoxazine-fused and diphenylamine-fused Ni(II) porphyrins, which, upon treatment with Magic Blue, provided remarkably stable radical cations to an unprecedented level. It is actually possible to purify these radical cations by silica gel chromatography, and they can be stored for over 6 months without any sign of deterioration. Moreover, they exhibited no degradation even after the CH2Cl2 solution was washed with water. However, subtle structural differences (planar versus partly twisted) led to different crystal packing structures and solid-state magnetic properties.

Stable Subporphyrin meso‐Aminyl Radicals without Resonance Stabilization by a Neighboring Heteroatom

Most aminyl radicals studied so far are resonance-stabilized by neighboring heteroatoms, and those without such stabilization are usually short-lived. We report herein that subporphyrin meso-2,4,6-trichlorophenylaminyl radicals and a bis(5-subporphyrinyl)aminyl radical are fairly stable under ambient conditions without such stabilization. The subporphyrin meso-2,4,6-trichlorophenylaminyl radical crystal structure displays a characteristically short Cmeso -N bond and a perpendicular arrangement of the meso-arylamino group. The stabilities of these radicals have been ascribed to extensive spin delocalization over the subporphyrin π-electronic network as well as steric protection around the aminyl radical center.

Synthesis of meso-heteroatom-substituted subporphyrins

meso-aryloxy-, meso-ethoxy-, meso-arylsulfanyl-, meso-butylsulfanyl-, and meso-phosphoryl-substituted subporphyrins were efficiently synthesized by Pd-catalyzed substitution reactions of meso-bromosubporphyrin with heteroatom-nucleophiles.

A Benzene‐1,3,5‐Triaminyl Radical Fused with ZnII‐Porphyrins: Remarkable Stability and a High‐Spin Quartet Ground State

A benzene-1,3,5-triaminyl radical fused with three ZnII -porphyrins was synthesized through a three-fold oxidative fusion reaction of 1,3,5-tris(ZnII -porphyrinylamino)benzene followed by oxidation with PbO2 as key steps. This triaminyl radical has been shown to possess a quartet ground state with a doublet-quartet energy gap of 3.1 kJ mol-1 by superconducting quantum interference device (SQUID) studies. Despite its high-spin nature, this triradical is remarkably stable, which allows its separation and recrystallization under ambient conditions. Moreover, this triradical can be stored as a solid for more than one year without serious deterioration. The high stability of the triradical is attributed to effective spin delocalization over the porphyrin segments and steric protection at the nitrogen centers and the porphyrin meso positions.

Stable NiII Porphyrin meso-Oxy Radical with a Quartet Ground State

10,15,20-Tris(pentafluorophenyl)-substituted NiII -porphyrin meso-oxy radical bearing two coordinating pyridines was synthesized as a stable radical with a quartet ground state (S=3/2). X-ray structural analysis revealed that the NiII porphyrin moiety is fairly planar and the Ni-N bond lengths are considerably longer, indicating the high-spin state of the NiII center. The radical exhibited a quartet ground state, indicating the ferromagnetic interaction between the high-spin NiII center (S=1) and the porphyrin meso-oxy radical (S=1/2).

Effect of bulky meso-substituents on photoinduced twisted intramolecular charge transfer processes in meso-diarylamino subporphyrins

Photoexcited-state dynamics of meso-diarylamino subporphyrins 4–6 in cyclohexane, toluene, and acetonitrile have been studied by steady-state/time-resolved absorption and fluorescence experiments and quantum calculations. While 4 emits fluorescence from the locally excited state regardless of solvent polarity, the fluorescence of 5 and 6 are solvent-polarity dependent. The observed efficient fluorescent quenching of 5 and 6 has been ascribed to twisted intramolecular charge transfer (TICT), in which the two N–Cipso bonds in 5 and the Cmeso–N bond in 6 are twisted to facilitate the intramolecular electron transfer. In 2-methyltetrahydrofuran (2-MeTHF), the fluorescence of 5 and 6 are both almost completely quenched at 297 K, but restored at 77 K (below the melting point of 2-MeTHF) because of the frozen molecular twisting. Furthermore, electrochemical studies also revealed the charge separation processes of 4–6 are thermally unfavorable in nonpolar toluene and cyclohexane unless their structures change contrary to the observed efficient fluorescence quenching. These observations support the TICT mechanism. In addition, the formation of TICT state is affected by steric effect as the size of the meso-substituents increases.

Diarylamine-Fused Subporphyrins: Proof of Twisted Intramolecular Charge Transfer (TICT) Mechanism

Diarylamine-fused subporphyrins 1 a and 1 b were synthesized from β,β-diiodo-meso-chloro subporphyrin 2 through a one-pot procedure involving nucleophilic aromatic substitution and SRN 1-type intramolecular fusion reaction as the first example of meso-nitrogen-embedded subporphyrin. While non-fused counterparts 3 b and 4 b display effective fluorescence quenching due to twisted intramolecular charge transfer (TICT) in CH2 Cl2 at room temperature, 1 b emits fluorescence with ΦF =0.18 under the same conditions, because conformational twisting is not allowed due to the fused structure. In addition, the oxidative titration with tris(4-bromophenyl)ammoniumyl hexachloroantimonate gave cation radical 7 for 1 a and cation radical and dication for 1 b. Actually, cation radical 7 was isolated through separation over silica gel column but was found to slowly decompose in concentrated solutions.