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Yoshiaki Kobuke

Publications and source records attributed to Yoshiaki Kobuke.

At least 19 recordsLinked to original sources
PubMed2007

Water-soluble self-assembled butadiyne-bridged bisporphyrin: a potential two-photon-absorbing photosensitizer for photodynamic therapy.

We have synthesized a novel, two-photon-absorbing photosensitizer for two-photon-absorption photodynamic therapy (2PA-PDT). The molecule is a butadiyne-bridged porphyrin dimer terminated with two water-soluble porphyrin monomers connected through Zn-imidazolyl self-assembly and covalently linked through olefin metathesis. It has an effective two-photon-absorption (2PA) cross-section value, sigma((2)), of 33 000+/-4600 GM with 5-ns pulses at 890 nm measured by using the open-aperture Z-scan technique. The compound was found to generate singlet oxygen, cytotoxic for tumor cells in photodynamic therapy (PDT), under 2PA conditions by conducting photobleaching experiments with anthracene-9,10-dipropionic acid sodium salt (ADPA).

Journal Article↗
PubMed2006

Photoinduced electron transfer and excitation energy transfer in directly linked zinc porphyrin/zinc phthalocyanine composite.

Photoinduced electron transfer (ET) and excitation energy transfer (ENT) reactions in monomer and slipped-cofacial dimer systems of a directly linked Zn porphyrin (Por)-Zn phthalocyanine (Pc) heterodyad, ZnPc-ZnPor, were investigated by means of the picosecond and femtosecond transient absorption spectroscopies. In the dimer dyad system of two heterodyads connected through the coordination bond between two imidazolyl-substituted ZnPor bearing ZnPc, ZnPc-ZnPor(D), the rapid ENT from the ZnPor to ZnPc in the subpicosecond time region was followed by photoinduced charge separation (CS) and charge recombination (CR) with time constants of 47 and 510 ps, respectively. On the other hand in the monomer dyad system, no clear charge-separated state was observed although the CS with a time constant of 200 ps and CR with < or =70 ps were estimated. These results indicated that the dimer slipped-cofacial arrangement of pair porphyrins is advantageous for the effective production of the CS state. This advantage was discussed from the viewpoint of a decrease in the reorganization energy of the dimer relative to that of the monomer system. In addition, the electrochemical measurements indicated that the strong interaction between ZnPc and ZnPor moieties also contributed to the fast CS process despite the marginal driving force for the CS process. The dimer dyad of ZnPc-ZnPor provides full advantages in efficiencies of the light harvesting and the CS state production.

Journal Article↗
PubMed2006

Efficient charge injection from the S2 photoexcited state of special-pair mimic porphyrin assemblies anchored on a titanium-modified ITO anode.

A novel surface fabrication methodology has been accomplished, aimed at efficient anodic photocurrent generation by a photoexcited porphyrin on an ITO (indium-tin oxide) electrode. The ITO electrode was submitted to a surface sol-gel process with titanium n-butoxide in order to deposit a titanium monolayer. Subsequently, porphyrins were assembled as monolayers on the titanium-treated ITO surface via phosphonate, isophthalate, and thiolate groups. Slipped-cofacial porphyrin dimers, the so-called artificial special pair at the photoreaction center, were organized through imidazolyl-to-zinc complementary coordination of imidazolylporphyrinatozinc(II) units, which were covalently immobilized by ring-closing olefin metathesis of allyl side chains. The modified surfaces were analyzed by means of X-ray photoelectron spectroscopy. Photoirradiation of the porphyrin dimer generated a large anodic photocurrent in aqueous electrolyte solution containing hydroquinone as an electron sacrificer, due to the small reorganization energy of the dimer. The use of different linker groups led to significant differences in the efficiencies of anodic photocurrent generation. The apparent flat-band potentials evaluated from the photocurrent properties at various pH values and under biased conditions imply that the band structure of the ITO electrode is modified by the anchoring species. The quantum yield for the anodic photocurrent generation by photoexcitation at the Soret band is increased to 15 %, a surprisingly high value without a redox cascade structure on the ITO electrode surface, while excitation at the Q band is not so significant. Extensive exploration of the photocurrent properties has revealed that hot injection of the photoexcited electron from the S2 level into the conduction band of the ITO electrode takes place before internal conversion to the S1* state, through the strong electronic communication of the phosphonyl anchor with the sol-gel-modified ITO surface.

Dimerization↗
PubMed2006

Assemblies of supramolecular porphyrin dimers in pentagonal and hexagonal arrays exhibiting light-harvesting antenna function.

Porphyrin-based supramolecular macrocyclic arrays were synthesized as mimics of photosynthetic light-harvesting (LH) antennae. Pentameric and hexameric macrocyclic porphyrin arrays EP5 and EP6 were constructed by complementary coordination of m-bis(ethynylene)phenylene-linked zinc-imidazolylporphyrin Zn-EP-Zn. The proton NMR spectra of noncovalently linked N-EP5 and N-EP6 indicate fast rotation of the porphyrin moieties along the ethyne axis. These macrocycles were covalently linked and identified as C-EP5 (6832 Da) and C-EP6 (8199 Da) by mass spectrometry. Fluorescence quantum yields of C-EP2 (10.0%), C-EP5 (10.1%), and C-EP6 (11.0%), even larger than that of the unit coordination dimer C-EP1 (9.3%), were significantly increased from those of the series without the ethynylene linkage. The order of increasing fluorescence quantum yields was parallel to that of decreasing fluorescence lifetimes (C-EP1 (1.65 ns), C-EP2 (1.45 ns), C-EP5 (1.42 ns), and C-EP6 (1.38 ns)), indicating that the radiative decay rate kF increased relative to the other decay rates with an increase in the number of ring components. Based on the exciton-exciton annihilation and anisotropy depolarization times, the excitation energy hopping (EEH) times in these macrocyclic systems were obtained as 21 ps for C-EP5 and 12.8 ps for C-EP6. EEH times depend strongly on the orientation factor of the component transition dipoles in the macrocyclic arrays. The hexagonal macrocyclic array with an orientation of better transition dipole coupling resulted in faster EEH time compared to the pentagonal one.

Journal Article↗
PubMed2006

Water-soluble bis(imidazolylporphyrin) self-assemblies with large two-photon absorption cross sections as potential agents for photodynamic therapy.

Two water-soluble porphyrin self-assemblies consisting of bisacetylene- and monoacetylene-linked conjugated bis(imidazolylporphyrin) have been synthesized. Two-photon absorption (2PA) cross section values in water were determined as 7500 GM for bisacetylene- and 7900 GM for monoacetylene-linked bisporphyrin by femtosecond open-aperture Z-scan measurement. These values were almost the same as those for the similar structure reported previously in CHCl(3). Therefore, the structure is suggested to be similar in CHCl(3) and aqueous solutions. Both compounds were found to efficiently generate singlet oxygen upon one-photon irradiation in a manner similar to protoporphyrin, as demonstrated by the time-resolved luminescence measurement at the characteristic band of 1270 nm. Photocytotoxicities for HeLa cancer cells were examined and found to be as high as those of hematoporphyrin, demonstrating that these newly prepared compounds are potential candidates as 2PA-photodynamic therapy agents.

Acetylene↗
PubMed2005

Two-photon absorption properties of self-assemblies of butadiyne-linked bis(imidazolylporphyrin).

Supramolecular porphyrin self-assemblies have been prepared from butadiyne-linked bis(imidazolylporphyrin) by complementary coordination of imidazole to zinc, and their two-photon absorption (2PA) and higher-order nonlinear absorption properties were investigated over femtosecond time scales using an open-aperture Z-scan method. The self-assembled porphyrin dimer of the conjugated monozinc bisporphyrin 7D was shown to have a large 2PA cross section (7.6 x 10(3) GM, where 1 GM = 10(-50) cm(4) s molecule(-1) photon(-1)) at 887 nm. By comparison of this result with that for a meso-meso-linked porphyrin array without the butadiyne connection (3.7 x 10(2) GM at 964 nm), it was demonstrated that the predominant factor in this significant enhancement of the cross section was the expansion of porphyrin-porphyrin pi-conjugation. Self-coordination and monozinc metalation were also found to be contributing factors. Furthermore, a novel self-assembled porphyrin polymer 8P consisting of a biszinc complex with a mean molecular weight of M(n) = 1.5 x 10(5) Da was shown to exhibit an extraordinarily large two-photon absorption cross section (4.4 x 10(5) GM at 873 nm). Nanosecond Z-scan experiments for 7D and 8P were also undertaken and resulted in the measurement of large effective 2PA cross sections, including the excited-state absorption (2.1 x 10(5) GM for 7D and 2.2 x 10(7) GM for 8P, respectively). Finally, three-photon absorption was observed by femtosecond Z-scan experiments at 1188 nm (7.1 x 10(-89) m(6) s(2)) and 1282 nm (1.8 x 10(-89) m(6) s(2)), an observation which is the first of its kind in porphyrin chemistry.

Alkynes↗
PubMed2005

Surface-grafted multiporphyrin arrays as light-harvesting antennae to amplify photocurrent generation.

Organized multiporphyrin arrays were developed on the conductive surface by a novel coordination-directed molecular architecture aiming at efficient photoelectric conversion. The basic strategy employs the mutual coordination of two imidazolylporphyrinatozinc(II) units to form a cofacial dimer. Thus, meso,meso-linked bis(imidazolylporphyrinatozinc) (Zn2(ImP)2) was organized onto imidazolylporphyrinatozinc on the gold substrate as a self-assembled monolayer. The organized Zn2(ImP)2 bearing allyl side chains was covalently linked by ring-closing olefin metathesis catalyzed with Grubbs catalyst. Alternating coordination/metathesis reactions allow the stepwise accumulation of multiporphyrin arrays on the gold electrode. A successive increase in absorption over a wide wavelength range occurred after each accumulation step of Zn2(ImP)2 on the gold electrode, and cathodic photocurrent generation was enhanced in the aqueous electrolyte system, containing viologen as an electron carrier. The significant increase of the photocurrent indicates that the multiporphyrin array works as a "light-harvesting antenna" on the gold electrode.

Journal Article↗
PubMed2005

A giant supramolecular light-harvesting antenna-acceptor composite.

A giant light-harvesting antenna-acceptor composite was constructed by heterodimerization of imidazolylmanganese(III)porphyrin to molecular terminals of the zinc porphyrin array composed of meso-meso linked bis(imidazolylzincporphyrin). Fluorescence quenching titration indicated that the terminal imidazolylmanganase(III)porphyrin quenched excited zinc porphyrin separated by a large number of intervening porphyrins and that the meso-meso linked bis(imidazolylzincporphyrin) array was an efficient light-harvesting antenna.

Journal Article↗
PubMed2005

Single molecule visualization of coordination-assembled porphyrin macrocycles reinforced with covalent linkings.

Coordination-assembled porphyrin macrocycles reinforced with covalent bondings were deposited on a metal surface by a pulse injection method, and their scanning tunneling microscopy (STM) images were recorded under ultrahigh vacuum conditions at liquid nitrogen temperature. The decamer ring consisting of 30 porphyrins gave clear circular STM images with hollow structure, whereas that without covalent linking did not give clear circular images, showing that covalent linking of the coordination pairs by ring-closing metathesis reaction was effective to reinforce the supramolecular structure on a metal surface. This strategy will be applicable to a variety of supramolecular assemblies.

Journal Article↗
PubMed2005

Excitation-energy migration in self-assembled cyclic zinc(II)-porphyrin arrays: a close mimicry of a natural light-harvesting system.

The excitation-energy-hopping (EEH) times within two-dimensional cyclic zinc(II)-porphyrin arrays 5 and 6, which were prepared by intermolecular coordination and ring-closing metathesis reaction of olefins, were deduced by modeling the EEH process based on the anisotropy depolarization as well as the exciton-exciton annihilation dynamics. Assuming the number of energy-hopping sites N = 5 and 6, the two different experimental observables, that is, anisotropy depolarization and exciton-excition annihilation times, consistently give the EEH times of 8.0 +/- 0.5 and 5.3 +/- 0.6 ps through the 1,3-phenylene linkages of 5 and 6, respectively. Accordingly, the self-assembled cyclic porphyrin arrays have proven to be well-defined two-dimensional models for natural light-harvesting complexes.

Absorption↗
PubMed2005

Hexameric and pentameric slipped-cofacial dimers: toward an artificial light-harvesting complex.

[structure: see text] We prepared a zinc complex of bis(1-methylimidazolyl)-m-gable porphyrin. The automatically assembled coordinate species showed a complex mixture of wide molecular weight distributions accompanied with assemblies of specific assembly numbers. When this zinc complex was once dissociated by the addition of methanol and reorganized again by elimination of the methanol under high-dilution conditions in chloroform to facilitate intramolecular coordinate structure formation, two convergent assemblies were obtained through analysis by gel permeation chromatography. These assemblies gave round-shaped particles on solid substrates by probe microscopies (atomic force and scanning tunneling microscopies). These two components were separated by GPC and evidence from small-angle X-ray scattering measurements in solution with synchrotron radiation was consistent with hexameric and pentameric macrorings of the gable porphyrin. The porphyrin macrorings did not show any fluorescence quenching by assembly formation, and we anticipate that the macrorings of gable porphyrins represent a good model for an artificial light-harvesting complex.

Journal Article↗
PubMed2005

Coordination assembled rings of ferrocene-bridged trisporphyrin with flexible hinge-like motion: selective dimer ring formation, its transformation to larger rings, and vice versa.

Ferrocene-bridged trisporphyrin (2) was synthesized by two-steps condensation of corresponding aldehydes and dipyrromethanes, and its self-assembling behavior based on the complementary coordination motif of imidazolylporphyrinatozinc(II) was investigated in conjunction with hinge-like flexibility given by freely rotating cyclopentadienyl rings of ferrocene connector. Ferrocene-bridged trisporphyrin (2) spontaneously and exclusively generated the dimeric ring (7) upon simple zinc(II) insertion, indicating that the freely rotating hinge connector favored the smallest ring formation. Taking advantage of the unique hinge-like flexibility of ferrocene, we attempted to transform the dimer ring into a mixture of porphyrin macrocycles by reorganizing the structure cleaved once by pyridine. A series of porphyrin macrocycles from trimer to decamer can be separated into its components by preparative gel permeation chromatograms. Macrocycles obtained are kept stable in the absence of coordinating solvents. On the other hand, they were easily transformed to the dimer ring in the presence of coordinating solvents such as methanol, showing that the transformation is completely reversible and can be controlled by the choice of the solvent system. A series of porphyrin macrocycles was confirmed via covalent linking of each complementary coordination dimer pair by metathesis reaction in the presence of Grubbs's catalyst. The coordination behavior of the bidentate ligands with different spacer lengths toward the dimer ring revealed that only the bidentate ligand (15) with a spacer length that matched the facing central porphyrins was selectively accommodated inside the ring. Coordination assembled flexible rings with tunable cavities and multiple coordination sites will be used as versatile hosts for a wide variety of guest molecules.

Journal Article↗
PubMed2004

Role of the special pair in the charge-separating event in photosynthesis.

We synthesized special-pair/electron-acceptor systems consisting of a complementary slipped cofacial dimer of imidazolyl-substituted zinc porphyrin, bearing pyromellitdiimide as the electron acceptor. In the case of the dimer, the first and second oxidation potentials were split into a total of four peaks in the differential pulse voltammetry measurement. Furthermore, the shift values of the first oxidation potentials obtained by changing the solvent polarity for the dimer were almost half of those observed for the monomer. These results indicate that the radical cation is delocalized over the whole pi system of the dimer. Time-resolved transient absorption measurements revealed that, relative to the corresponding monomer, the dimer accelerated the charge separation rate, but decelerated the charge recombination rate. The smaller reorganization energy of the slipped cofacial dimer relative to that of the monomeric system demonstrates the significance of the special-pair arrangement for efficient charge separation in photosynthesis.

Electron Transport↗
PubMed2004

Supramolecular ion channels from a transmembrane bischolic acid derivative showing two discrete conductances.

Bischolic acid derivative 1 linked by m-xylylene dicarbamate at the 3,3'-position was synthesized and the single ion channel properties were examined. Compound 1 showed two clearly distinct conductances, 9.5 (type A) and 25.3 pS (type B), under 500 mM KCl symmetric salt conditions, whereas various conductances, ranging from 5-20 pS, were observed in the tetramethylether analogue reported previously. Results indicate that the replacement of four methylether groups with hydroxyls at the 7, 7', 12, and 12' positions in the bischolic acid framework influences the stability of aggregated (supramolecular) ion channel structures. Ion permeability ratios (potassium/chloride and potassium/sodium) for each type of channel were also determined.

Cell Membrane Permeability↗