Norihito Fukui

Oxidative Fusion Reactions of meso‐(Diarylamino)porphyrins

In recent years, meso-aminoporphyrins have received increasing attention owing to their potential for vastly improving the economic and environmental validity of dye-sensitized solar cells (DSSC). As a prime example, Gr tzel and co-workers have recently reported a co-sensitized DSSC device incorporating a meso-aminoporphyrin dye that is capable of a power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight conditions. Beside this, meso-aminoporphyrins have been explored as scaffolds for realizing novel nitrogen-connected dimers, mixed-valent cation radicals, inter-valence charge-transfer absorption, photo-induced reductive meso–meso dimerization, and light-harvesting supramolecular aggregates. Although the amino groups of meso-(diarylamino)porphyrins raise the porphyrin HOMO level, electronic perturbation is only moderate because the diarylamino groups twist out of the bulky porphyrin plane, minimizing conjugation. 7] Our continued interest in conjugated porphyrinoids led us to envision the fusion of a meso-diarylamino group onto the periphery of porphyrins as the expected fused porphyrin products would be interesting in their own right, because they would be structurally similar to azagraphene-type porphyrinoids. To the best of our knowledge, there are no reports of oxidative fusion of meso-aminoporphyrins, which would enable effective pexpansion by coplanarization of the peripheral nitrogen atom lone pair and porphyrin p-electronic system. meso-Phenoxazinoporphyrin 2a (Scheme 1)was prepared in 93 % yield using a Buchwald–Hartwig amination 11] of meso-bromo Ni-porphyrin 1Ni with phenoxazine using PdPEPPSI-IPent as the catalyst. By following previously reported procedures, 13] oxidative fusion of 2 a was attempted by subjecting 2a to 20 equivalents of FeCl3 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) for 1.5 hours in CH2Cl2/MeNO2 at room temperature. This reaction afforded an unexpected fused porphyrin 3 a as a sole product in 48 % yield, in which the two 3,5-di-tertbutylphenyl groups had fused to the porphyrin periphery and not the meso-phenoxazine moiety. Although we examined numerous fusion reactions of 2a by changing the oxidants, solvents, and temperature, we could not find reaction conditions that gave us the desired phenoxazine-fused porphyrins. The ESI mass spectrum of 3a showed a parent ion peak at m/z 1107.5374 (calcd for C74H75N5NiO, m/z 1107.5320 [M]), supporting that four protons were removed as a result of oxidation. The H NMR spectrum of 3a in CDCl3 shows a singlet signal owing to the b-protons adjacent to the fused positions at 7.78 ppm. The signals related to the phenoxazine moiety do not show particularly pronounced changes. X-ray crystallographic analysis unambiguously determined the structure of 3a (Figure 1). Interestingly, the fusion reaction took place regioselectively at the bpositions, apart from the phenoxazine moiety. The UV/Vis absorption spectrum of 3a displays broad, red-shifted bands reaching out to around 880 nm, which is similar to that of closely related fused porphyrins that are not functionalized with diarylamino moieties (Figure 2). 14] The electrochemical properties of 2a and 3 a have been studied by cyclic voltammetry (see Supporting Information, Figures S40,S42). The fused porphyrin 3a displays characteristic negative shifts of Eox2 and positive shifts of Ered1, while Eox1 is only slightly affected. As a result, the electrochemical HOMO–LUMO gap of 3a (1.80 eV) is smaller than that of 2a (2.14 eV). Scheme 1. Formulas of porphyrins 1–6. Ar = 3,5-di-tert-butylphenyl.

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.

Synthesis of 7,8‐Dehydropurpurin Dimers and Their Conversion into Conformationally Constrained β‐to‐β Vinylene‐Bridged Porphyrin Dimers

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.

Porphyrin Arch-Tapes: Synthesis, Contorted Structures, and Full Conjugation

Porphyrin tapes possessing meso-meso β-β β-β triple direct linkages have been targets of extensive studies because of their fully conjugated characteristic π-electronic networks. In this paper, we report porphyrin arch-tapes that bear additional carbonyl group(s) or methylene group(s) inserted between one of the β-β linkage(s) of the porphyrin tapes. The carbonyl-inserted porphyrin arch-tapes were efficiently synthesized by double fusion reactions of β-to-β carbonyl-bridged porphyrin oligomers with DDQ and Sc(OTf)3, and were converted to the methylene-bridged porphyrin arch-tapes via Luche reduction with NaBH4 and CeCl3 followed by ionic hydrogenation with HBF4·OEt2 and BH3·NEt3. While the conventional porphyrin tapes display rigid and planar structures and low solubilities, these porphyrin arch-tapes show remarkably contorted structures, flexible conformations, and improved solubilities because of the presence of the incorporated seven-membered ring(s). Interestingly, the methylene-inserted arch-tapes exhibited conjugative electronic interactions that were comparable to those of porphyrin tapes probably owing to through-space interaction in the contorted conformations. The carbonyl-inserted arch-tapes displayed distinctly larger conjugative interactions owing to an active involvement of the carbonyl group(s) in the electronic conjugation. A similar trend was observed in the nonlinear optical properties, as evidenced by their two-photon absorption cross sections. Furthermore, as a benefit of the contorted structures, these porphyrin arch-tapes can catch C60 fullerene effectively. Naturally, the electron-rich methylene-bridged arch-tapes exhibited larger association constants than the electron-deficient carbonyl-bridged arch-tapes. Among these arch-tapes, a methylene-bridged syn-Ni(II) porphyrin trimer recorded the largest association constant of (1.5 ± 0.4) × 107 M-1 in toluene at 25 °C.

Aromatic Metamorphosis of Dibenzofurans into Triphenylenes Starting with Nickel-Catalyzed Ring-Opening C–O Arylation

A new class of aromatic metamorphosis has been developed in which dibenzofurans were converted into triphenylenes. This transformation is composed of three successive operations: (1) nickel-catalyzed ring-opening C-O bond arylation with arylmagnesium bromides, (2) trifluoromethanesulfonylation (triflation) of the resulting hydroxy moiety with Tf2O, and (3) palladium-catalyzed or photoinduced ring closure. In the last ring-closing step, the photoinduced process has proven to be more productive than the palladium-catalyzed one. By employing π-extended dinaphthofuran as the substrate, dorsally benzo-fused [5]helicene was obtained in a satisfactory yield.

meso–meso‐Linked Diarylamine‐Fused Porphyrin Dimers

A meso-meso-linked diphenylamine-fused porphyrin dimer and its methoxy-substituted analogue were synthesized from a meso-meso-linked porphyrin dimer by a reaction sequence involving Ir-catalyzed β-selective borylation, iodination, meso-chlorination, and SN Ar reactions with diarylamines followed by electron-transfer-mediated intramolecular double C-H/C-I coupling. While these dimers commonly display characteristic split Soret bands and small oxidation potentials, they produced different products upon oxidation with tris(4-bromophenyl)aminium hexachloroantimonate. Namely, the diphenylamine-fused porphyrin dimer was converted into a dicationic closed-shell quinonoidal dimer, while the methoxy-substituted dimer gave a meso-meso, β-β doubly linked porphyrin dimer.

Pictet–Spengler Synthesis of Quinoline‐Fused Porphyrins and Phenanthroline‐Fused Diporphyrins

Doubly and quadruply quinoline-fused porphyrins were effectively synthesized through a reaction sequence consisting of Suzuki-Miyaura coupling of β-borylated porphyrins with 2-iodoaniline and subsequent Pictet-Spengler cyclization. These quinoline-fused porphyrins display red-shifted absorption bands and higher electron-accepting abilities. This synthetic protocol also allowed the synthesis of phenanthroline-fused porphyrin dimers, which bound either a NiII or ZnII cation. The resultant metal complexes displayed further red shifted absorption spectra and molecular twists to effect an almost perpendicular arrangement of the two porphyrins.

Metalation Control of Open-Shell Character in meso-meso Linked Porphyrin meso-Oxy Radical Dimers

Control of open-shell character of meso-meso linked porphyrin meso-oxy radical dimers has been demonstrated by core metalation. Namely, NiII -porphyrin dimer 6Ni exhibits a clear 1 H NMR spectrum and a distorted but rather coplanar quinonoidal structure consisting of two ruffled porphyrin rings, in accordance with the previous report. Freebase dimer 6H2 shows a similar quinonoidal structure in the solid state but displays slightly broader and temperature-dependent 1 H NMR spectra, indicating a partial diradical character in solution that increases at high temperature. In sharp contrast, bis-imidazole-coordinated ZnII -porphyrin dimer 6ZnIm2 exhibits a perpendicular structure consisting of two planar ZnII -porphyrins and has been characterized as a distinct open-shell diradical on the basis of its non-observable 1 H NMR signals, a clear ESR signal, and a characteristic absorption spectrum reaching about 1700 nm. Despite the distinct diradical character, 6ZnIm2 is an extremely stable molecule.

meso-to-meso PtII-bridged NiII-porphyrin dimers

The synthesis and characterization of meso-to-meso PtII-bridged NiII-porphyrin dimers 5–7 are reported herein. A boron–platinum exchange reaction of meso-pinacolatoborylporphyrin 8 with Pt(cod)Cl2 afforded the cod-coordinated meso-to-meso PtII-linked cis-dimer 5, which was subsequently converted to the 1,3-bis(diphenylphosphino)propane (DPPP)-coordinated cis-dimer 6 and the PPh3-coordinated trans-dimer 7 upon treatment with DPPP and PPh3, respectively. On the other hand, a reaction of 5 with tri-tert-butylphosphine induced reductive elimination to give the meso–meso directly linked diporphyrin 9. Comparative studies of these PtII-bridged NiII-porphyrin dimers were conducted by UV/Vis absorption spectroscopy, electrochemical studies, and theoretical calculations. In the course of these studies, we found that cis-dimers 5 and 6 underwent an unprecedented reductive elimination to give the meso–meso linked dimer 9 upon chemical or electrochemical one-electron oxidation, while trans-dimer 7 was stable under such conditions.

Diphenylphosphine-Oxide-Fused and Diphenylphosphine-Fused Porphyrins: Synthesis, Tunable Electronic Properties, and Formation of Cofacial Dimers

Diphenylphosphine-oxide-fused NiII porphyrin 8 was synthesized from 3,5,7-trichloroporphyrin 5 via a reaction sequence of nucleophilic aromatic substitution with lithium diphenylphosphide, oxidation with H2 O2 , and palladium-catalyzed intramolecular cyclization. Reduction of 8 with HSiCl3 gave diphenylphosphine-fused NiII porphyrin 9. The embedded P=O and P moieties serve as a strong electron-accepting and electron-donating group to perturb the optical and electrochemical properties of the NiII porphyrin. NiII porphyrin 9 is diamagnetic with a low-spin NiII center in solution but becomes paramagnetic with a five-coordinated NiII center with high-spin (S=1) state in the solid state. Diphenylphosphine-oxide-fused ZnII porphyrin 10 was also synthesized and shown to form a face-to-face dimer with mutual O-Zn bonds in the crystal and in nonpolar and moderately polar solvents. The dimerization of 10 in CDCl3 has been revealed to be an entropy-driven process with a large entropy gain (ΔSD =207 J K-1  mol-1 ).