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Nagao Kobayashi

Publications and source records attributed to Nagao Kobayashi.

At least 19 recordsLinked to original sources
PubMed2004

A concept for controlling singlet oxygen (1 Delta g) yields using nitroxide radicals: phthalocyaninatosilicon covalently linked to nitroxide radicals.

In this study, we have investigated the singlet oxygen ((1)Delta(g)) generation mechanism using phthalocyaninatosilicon (SiPc) covalently linked to nitroxide radicals (NRs), and we succeeded in increasing the singlet oxygen quantum yield (Phi(Delta)) by linking the NRs. This originates from both an increase in the triplet quantum yield and excited-state lifetimes long enough to utilize photochemical reactions. Because the electron exchange interactions with paramagnetic species were known to result only in very fast excited-state relaxation, leading to a decrease in photochemical reaction yields, this increase in Phi(Delta) is an unusual and precious example for increasing photochemical reaction yields by electron exchange interactions with paramagnetic species. In addition, our experiments and theoretical analyses show that the spin-selective energy transfer rate constant is not influenced by linking the NRs and can be evaluated by the product of spin-statistical factors and matrix elements between the initial and final states.

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PubMed2004

Synthesis and spectroscopic and electrochemical studies of novel benzo- or 2,3-naphtho-fused tetraazachlorins, bacteriochlorins, and isobacteriochlorins.

Benzene- or 2,3-naphthalene-ring-expanded tetraazachlorins (TACs), tetraazabacteriochlorins (TABCs), and tetraazaisobacteriochlorins (TAiBCs) have been synthesized by using tetramethylsuccinonitrile as a source of hydrogenated sites. The derived compounds were characterized by using NMR spectroscopy, X-ray crystallography, electronic and magnetic circular dichroism (MCD) spectroscopy, and electrochemical and spectroelectrochemical methods. X-ray analysis revealed that the benzene-fused TAiBC deviates slightly from planarity at the hydrogenated sites as a result of the presence of sp(3) carbons, which prefer a nonplanar tetrahedral conformation. The spectral data were analyzed by using a band deconvolution technique. In the electronic absorption spectra of TAC and TABC species, the Q band splits into two intense components and smaller splittings were observed for the 2,3-naphthalene-fused derivatives relative to the benzo-fused species. In contrast, in the case of TAiBCs, the Q band splitting was apparently not observed in absorption spectra, as expected from the C(2v) molecular symmetry. However, MCD signals of the Q band in TAiBCs showed Faraday B terms, implying that the accidental degeneracy of the LUMO and LUMO+1 was broken even for adjacently ring-fused species. Relative molecular orbital energies were estimated by using cyclic and differential pulse voltammetry. The first reduction potentials were close for TACs and TABCs, although those of TAiBCs shifted to more negative potentials. In contrast, although TABCs and TAiBCs exhibited similar first oxidation potentials, those of TACs appeared at more positive potentials. These properties were reproduced and rationalized by molecular orbital and configuration interaction calculations within the framework of the ZINDO/S Hamiltonian. DFT-level frequency calculations have succeeded in reproducing the IR spectra for low-symmetry tetraazaporphyrin (TAP) derivatives for the first time. The relationship between structures and spectral features is discussed.

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PubMed2003

A phthalocyanine dendrimer capable of forming spherical micelles.

A novel and intrinsically spherical micelle has been prepared by utilizing a silicon phthalocyanine ((WG3)SiPc) that has a thin hydrophobic alkyl chain and a bulky hydrophilic poly(aryl ether) dendrimer with terminal carboxyl groups, as its two axial ligands. Gel-permeation chromatography and cryo-transmission electron microscopic experiments indicate that (WG3)SiPcs self-assemble to form a spherical micelle at very low concentrations in aqueous solution. Depending on the pH of the aqueous phase, (WG3)SiPc shuttles between aqueous and organic phases. In the presence of hydrophobic molecules, this transfer is accompanied by the inclusion of these (guest) molecules, indicating that the micelle acts as a molecular capsule with a nanospace surrounded by functional phthalocyanine planes.

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PubMed2003

Effect of peripheral substitution on the electronic absorption and fluorescence spectra of metal-free and zinc phthalocyanines.

The effect of substituents on the position and intensity of the electronic absorption and fluorescence spectra of phthalocyanines (Pcs) was examined for 35 Pc compounds. When electron-releasing groups are bound to four alpha-benzo positions of the Pc skeleton, the B and Q bands shift to longer wavelength. Relative to this shift, the effect of introducing the same electron-releasing groups at the other four alpha positions amounts to about 1.6-2.0. Although the effect is not always clearly seen, introduction of electron-releasing groups in the beta-benzo positions of the Pc skeleton generally shifts the Q band to shorter wavelength. The effect of electron-withdrawing groups is exactly the opposite with respect to the alpha and beta positions. These effects can be reasonably explained by considering the magnitude of the atomic orbital coefficients of the carbon atoms derived from molecular orbital (MO) calculations. In addition, the following intriguing phenomena were observed in the experiments, although not all were explained theoretically: 1) the splitting of the Q band of metal-free Pcs decreases with increasing wavelength of the Q band, 2) the ring currents of Pcs with Q bands at longer wavelength are generally smaller, and 3) the absorption coefficients of the Q band of Pc compounds with 16-electron-releasing substituents are larger than those of the corresponding tetra- and octasubstituted Pcs by several tens of percent. 4) Our PPP calculations suggested that the absorption coefficient of the Q band of Pcs with more strongly electron releasing substituents is larger. 5) The second HOMO of the Pcs with the Q band at longer wavelength has b(1u) symmetry, as opposed to the a(2u) symmetry of normal Pcs. 6) Pcs showing S1 emission maxima at wavelengths longer than about 740 nm generally have quantum yields of less than 0.1.

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PubMed2003

An optically-active subphthalocyanine dimer.

A subphthalocyanine (SubPc) dimer linked by racemic 1,1'-binaphth-2,2'-dioxy (BINOL), 1, has been synthesized and characterized by 1- and 2-D 1H NMR, high resolution mass spectrometry (FAB), X-ray crystallography and electronic absorption spectroscopy. Additionally, the dimer derived from optically-active (R)-(+)-BINOL, 2, has been made and studied using CD and MCD spectroscopies.

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PubMed2003

Selective ligation to sterically isolated metallophthalocyanines.

A new class of sterically hindered phthalocyanine has been synthesized and characterized. Pentaphenylbenzene units were introduced at the periphery of each phthalocyanine to yield a sterically protected metal center. The attachment of the bulky oligophenylbenzene units resulted in the complete isolation of individual MPc molecules. The pockets around a cobalt center affect selectivity in the ligation of pyridines that have different sizes and shapes.

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PubMed2002

Direct EPR observations of both oxidized and reduced porphyrin dimers in the lowest excited states: a novel analysis of a homobiradical with a strong antiferromagnetic interaction.

The oxidized and reduced forms of bis(tetra-tert-butyltetraazaporphinato)lutetium (III) (Lu(TAP)2) have been examined by time-resolved EPR (TREPR) spectroscopy, showing both the first EPR investigation of a homobiradical with a strong antiferromagnetic interaction and usefulness of TREPR for investigating excited-state properties of different oxidation states.

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PubMed2002

Electronic structure of reduced symmetry peripheral fused-ring-substituted phthalocyanines.

Reduced symmetry phthalocyanines are finding use in an increasing number of industrial applications. A detailed understanding of the electronic structure of the pi-system will greatly facilitate the design of new complexes, which fit the specifications required in many of these emerging high technology fields. NMR, electronic absorption, magnetic circular dichroism (MCD), and fluorescence emission and excitation spectra have been recorded for five generic metal phthalocyanine (MPc) derivatives in which additional benzene rings are fused either radially or obliquely onto at least one of the four peripheral benzo groups. The spectroscopy of four radially substituted compounds, zinc mononaphthotribenzotetraazaporphyrine (Zn3B1N), zinc monobenzotrinaphthotetraazaporphyrine (Zn1B3N), and two cis and trans zinc dibenzodinaphthotetraazaporphyrine (Zn2B2N) isomers, is compared to that of the obliquely fused structural isomer of Zn3B1N (Zn3BoN) and the D(4)(h)() symmetry parent compounds, ZnPc and zinc naphthalocyanine (ZnNc). The selection of Zn(II) as the central metal eliminates the possibility of charge transfer between the metal and ring. None of the complexes studied contain any sigma-bonded peripheral substituents. (1)H NMR signals of the seven compounds are assigned on the basis of the coupling patterns, integrated proton numbers, and decoupling experiments. The SIMPFIT program was used to perform spectral band deconvolution analyses of absorption and MCD spectra. ZINDO molecular orbital calculations are described, and the optical spectra are assigned on the basis of the MO models that have been developed previously to account for the spectral properties of metal porphyrin (MP(-2)) and metal phthalocyanine (MPc(-2)) complexes.

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PubMed2002

Adjacent versus opposite type di-aromatic ring-fused phthalocyanine derivatives: synthesis, spectroscopy, electrochemistry, and molecular orbital calculations.

A series of adjacent and opposite type di-aromatic ring-fused phthalocyanines (Pc's) of varying size have been prepared and characterized spectroscopically and electrochemically, and most of their properties have been reasonably reproduced by molecular orbital (MO) calculations. The adjacent isomers alone were obtained preferentially by using a diphthalonitrile unit linked via a short aryl chain. The main results are summarized as follows. (i) The Q-band shifts to longer wavelength and its intensity increases, but the degree of change decreases, with increasing molecular size. On the bases of the experiments and MO calculations, setting the size of the effect of benzene directly fused to the tetraazaporphyrin (TAP) skeleton at unity, the effect of the second and third benzene units is roughly about 0.75-0.80 and 0.48 +/- 0.06, respectively. As a result of this, among compounds having an isomeric pi-system, the Q-band of a D(4h) type species lies at longer wavelength than those of adjacently and oppositely di-aromatic ring-fused species. (ii) The Q-band of adjacently substituted species does not split appreciably, while that of the oppositely substituted species splits substantially, the extent having a parallel relationship with the ratio of long to short axes in the molecule. In general, the larger the ratio, the larger the splitting. (iii) The Q-band of oppositely dibenzo-fused and bis(dialkyl)-substituted TAP does not show explicit splitting because of the large coefficients of the carbons substituted with alkyl groups in the MOs. (iv) Interestingly, the first oxidation in adjacently and oppositely dibenzo-fused CoTAP occurs at the cobalt and ligand, respectively, although they are isomers to each other.

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PubMed2002

Mono-aromatic ring-fused versus adjacently di-aromatic ring-fused tetraazaporphyrins: regioselective synthesis and their spectroscopic and electrochemical properties.

A method which preferentially produces adjacently di-aromatic ring-substituted tetraazaporphyrins (TAPs) has been developed, and their electrochemical and spectroscopic properties have been studied and compared with those of the corresponding series of mono-aromatic ring-fused TAPs. Mono-aromatic ring-fused TAPs show a split Q-band, and the splitting energy increases with increasing size of the aromatic ring. In addition, for the split Q-bands, the relative intensity of the band at longer wavelength decreases with increasing molecular size of the fused aromatics, compared with the shorter wavelength band. In the di-aromatic ring-fused TAPs, this kind of splitting is not seen, and only a shift of the band is observed. The intensity and band position of the split or unsplit Q-bands are quantitatively evaluated by simultaneous band deconvolution analysis, using both electronic absorption and magnetic circular dichroism spectra. The preparation of these TAP compounds has made it possible to adjust the Q-band position in a stepwise manner between ca. 600 and 750 nm. The first reduction and oxidation potentials of the TAP ring shift negatively with increasing number and size of the fused aromatics. The extent of the shift is found to be very small for the LUMOs but significant for the HOMOs. These spectroscopic and electrochemical properties are almost perfectly reproduced by molecular orbital calculations within the framework of the Pariser-Parr-Pople approximation. In particular, a small variation of the LUMO level and large destabilization of the HOMO level on ring expansion are rationalized from the extent of stretch of molecular orbitals: i.e., since the LUMOs are localized in the central TAP moiety irrespective of the molecular size, while the HOMOs have appreciable coefficients even over the fused aromatics, the HOMO level destabilizes while the LUMO level remains constant with increasing molecular size. In one CoTAP derivative, Co(III/II) and the first ligand oxidation couples occur experimentally at the same potential.

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PubMed2002

Synthesis and characterization of phthalocyanines with direct Si-Si linkages.

Several cofacial phthalocyanines (Pcs) with an Si-Si linkage were obtained by one-step condensation of 1H-isoindole-1,3(2H)-diimine with hexachlorodisilane as template in quinoline. They were characterized by gel-permeation chromatography, IR, NMR spectroscopy, and mass spectrometry, and cyclic voltammetry. The results strongly suggest that we indeed obtained Pc dimers directly linked by an Si-Si bond using this novel concept of utilizing a compound/salt with an element-element bond as a template. The cofacial dimer structures are reasonably supported by X-ray absorption near-edge structure (XANES), electronic absorption and magnetic circular dichroism (MCD) spectra, and molecular orbital (MO) calculations. Interestingly, they show an electronic absorption spectrum very similar to that of silicon tetrabenztriazacorrole (SiTBC).

Journal Article↗
PubMed2002

Spectroscopy, electrochemistry, and molecular orbital calculations of metal-free tetraazaporphyrin, -chlorin, -bacteriochlorin, and -isobacteriochlorin.

Metal-free tetraazachlorin (TAC), -bacteriochlorin (TAB), and -isobacteriochlorin (TAiB) were characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence, and time-resolved ESR (TR-ESR) spectroscopy, and by cyclic voltammetry. The results are compared with those of metal-free tetraazaporphyrin (TAP). The potential difference DeltaE between the first oxidation and reduction couples decreases in the order TAP>TAiB>TAC>TAB. The splitting of both the Q and Soret bands decreases in the order TAB>TAC>TAP>TAiB. Corresponding to the split absorption bands, MCD spectra show a minus-to-plus pattern with increasing energy in both the Q and Soret regions, which suggests that the energy difference between the HOMO and second HOMO is larger than that between the LUMO and second LUMO. These spectroscopic properties and redox potentials were reproduced by molecular orbital calculations using the ZINDO/S Hamiltonian. The fluorescence quantum yields of the reduced species are much smaller than that of TAP. The zero-field splitting (ZFS) parameters D and E of the excited triplet states (T1) of these species decrease and increase, respectively, on going from TAP to TAC and further to TAB. The D and E values of TAiB are larger than those of the other species. The results are supported by the absence of interaction between the spin over reduced pyrrole moieties of the HOMO and over the LUMO, and by calculations of ZFS under a half-point-charge approximation.

Chemical Phenomena↗