Eiji Tsurumaki

Planar subporphyrin borenium cations.

Subporphyrin borenium cations with a carborane counterion have been prepared by treatment of B-methoxy subporphyrins with the silylium reagent Et(3)Si(CH(6)B(11)Br(6)). In contrast to the distinctly domed subphthalocyanine borenium cation, a nearly planar structure with sp(2) hybridized boron is found in the X-ray structure of the triphenylsubporphyrin borenium cation. The cations exhibit absorption and fluorescence spectra that are quite similar to those of B-methoxy subporphyrins. B-phenyl subporphyrins were prepared in good yield by reaction of subporphyrin borenium cations with phenyllithium.

Rhodium-Catalyzed Asymmetric Ring-Opening Alkynylation of Azabenzonorbornadienes

Asymmetric ring-opening alkynylation of meso-azabenzonorbornadienes with (triisopropylsilyl)acetylene giving 2-alkynyl-1-aminodihydronaphthalenes took place in high yields with high enantioselectivity in the presence of a rhodium/(R)-DTBM-segphos catalyst.

Versatile Photophysical Properties of meso‐Aryl‐Substituted Subporphyrins: Dipolar and Octupolar Charge‐Transfer Interactions

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) cm(4) 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.

Peripheral Hexabromination, Hexaphenylation, and Hexaethynylation of meso-Aryl-Substituted Subporphyrins

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.

Subporphyrins with an Axial BC Bond

Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial groups thus introduced are not conjugated with the subporphyrin core. This situation leads to effective fluorescence quenching of subporphyrins when the axial group is strongly electron donating such as 4-dimethylaminophenyl and ferrocenyl groups.

Synthesis and Properties of Boron(III)-Coordinated Subbacteriochlorins

Subbacteriochlorins, which were prepared via hydrogenation of subporphyrins with Raney nickel, are modestly aromatic due to 14π-diazaannulenic circuit and exhibit characteristic blue-shifted Soret-like bands, intensified fluorescence spectra, and high oxidation potentials.

Negative differential resistance by molecular resonant tunneling between neutral tribenzosubporphine anchored to a Au(111) surface and tribenzosubporphine cation adsorbed on to a tungsten tip.

Tribenzosubporphyrins are boron(III)-chelated triangular bowl-shaped ring-contracted porphyrins that possess a 14π-aromatic circuit. Their flat molecular shapes and discrete molecular orbital diagrams make them ideal for observation by scanning tunneling microscopy (STM). Expanding their applications toward single molecule-based devices requires a fundamental knowledge of single molecular conductance between tribenzosubporphines and the STM metal tip. We utilized a tungsten (W) STM tip to investigate the electronic properties of B-(5-mercaptopentoxy)tribenzosubporphine 1 at the single molecular level. B-(5-mercaptopentoxy)-tribenzosubporphine 1 was anchored to the Au(111) surface via reaction with 1-heptanethiol linkers that were preorganized as a self-assembled monolayer (C7S SAM) on the Au(111) substrate. This arrangement ensured that 1 was electronically decoupled from the metal surface. Differential conductance (dI/dV - V) measurements with the bare W tip exhibited a broad gap region of low conductance and three distinct responses at 2.4,-1.3, and -2.1 V. Bias-voltage-dependent STM imaging of 1 at 65 K displayed a triangle shape at -2.1 < V < -1.3 V and a circle shape at V < -2.1 V, reflecting its HOMO and HOMO-1, respectively. In addition, different conductance behaviors were reproducibly observed, which has been ascribed to the adsorption of a tribenzosubporphine-cation on the W tip. When using a W tip doped with preadsorbed tribenzosubporphine-cation, negative differential resistance (NDR) phenomena were clearly observed in a reproducible manner with a peak-to-valley ratio of 2.6, a value confirmed by spatial mapping conductance measurements. Collectively, the observed NDR phenomena have been attributed to effective molecular resonant tunneling between a neutral tribenzosubporphine anchored to the metal surface and a tribenzosubporphine cation adsorbed on a W tip.

Subporphyrinato Boron(III) Hydrides

Subporphyrinato boron(III) hydrides were prepared by reduction of subporphyrinato boron(III) methoxides with diisobutylaluminum hydride (DIBAL-H) in good yields. The authenticity of the B-H bond has been unambiguously confirmed by a (1)H NMR signal that appears as a broad quartet at -2.27 ppm with a large coupling constant with the central (11)B, characteristic B-H infrared stretching frequencies, and single crystal X-ray diffraction analysis. Red shifts in the corresponding absorption and fluorescence profiles are accounted for in terms of the electron-donating nature of the B-hydride. The hydridic character of subporphyrinato boron(III) hydrides has been demonstrated by the production of H2 via reaction with water or HCl, and controlled reductions of aromatic aldehydes and imines in the presence of a catalytic amount of Ph3C[B(C6F5)4].

β,β‐Diborylated Subporphyrinato Boron(III) Complexes as Useful Synthetic Precursors

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.

Synthesis of peripherally nitrated, aminated, and arylaminated subporphyrins.

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)2·5H2O 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-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.