Seigo Yamauchi

Tohoku University, Sendai-shi, Miyagi-ken, Japan

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Publications (160)248.98 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetic control over excited states of molecules presents interest for many applications. Here we show for the first time that visible room temperature phosphorescence in multichromophoric donor–acceptor systems can be modulated by weak magnetic fields (<1 T) via magnetic field effects (MFE) on the spin dynamics in photogenerated radical pairs (RPs). The studied compounds comprise Pt porphyrin (PtP)–Rosamine B (RosB) dyads, which possess strong visible absorption bands and phosphoresce at room temperature. The observed MFE is unique in that it occurs upon direct excitation of the PtP in the dyads, whereby ultrafast quantitative formation of the local PtP triplet state precedes the occurrence of radical intermediates. A model explaining the effect is proposed, which is based on reversible electron transfer between the local triplet state and a long-lived RP. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet RP recombination channel and thereby influencing phosphorescence.
    Journal of Physical Chemistry Letters 10/2012; 3(21):3115–3119. · 6.59 Impact Factor
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    ABSTRACT: Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.
    Journal of the American Chemical Society 09/2012; 134(39):16319-26. · 10.68 Impact Factor
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    ABSTRACT: The lowest excited triplet (T(1)) ππ* states of gallium (Ga) and various rhodium (Rh) 5,10,15-trispentafluorophenyl corroles (Cors) were studied in the liquid crystal (LC) E-7 and in rigid glasses by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The triplet sublevel energies were experimentally determined by the alignment of the molecules in the LC and by magnetophotoselection in the glass. The sublevel scheme of GaCor was determined by calculating the zero field splitting (ZFS) parameters. Axial ligand effects and quantum chemical calculations were used for the sublevel assignment of RhCors. The anisotropic EPR parameters were used to determine the important higher excited states and the magnitudes of their spin-orbit coupling (SOC) contributions were evaluated. On the basis of these results and analyses, the EPR parameters and triplet lifetime were discussed for each RhCor complex.
    The Journal of Physical Chemistry A 09/2012; 116(39):9662-73. · 2.77 Impact Factor
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    ABSTRACT: The lowest excited triplet state of free-base 5,10,15-trispentafluorophenylcorrole (H3Cor) was studied in rigid glass by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. Triplet sublevels were experimentally determined by using magnetophotoselection and liquid crystal. Quantum chemical calculations of zero-field splitting parameters D and E were used for the sublevel assignment. The out-of-plane sublevel Tz was found to be the lowest; namely, D is positive for H3Cor, in contrast to previous reports. The origin of the difference is discussed in detail. Preliminary TR-EPR experiments on rhodium corrole, accompanied by the quantum chemical calculations, emphasize the important contribution of spin-orbit couplings.
    Chemical Physics Letters 01/2012; 521:64–68. · 2.15 Impact Factor
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    ABSTRACT: In alkaline aqueous solutions, 3,4-diaminobenzoate (H2(2LPDA)−) reacts with PtII to form a 1:2 (Pt:L) complex that intensely absorbs near-infrared (NIR) light at 713 nm (ε = 8.0 × 104 M−1 cm−1). The absorption disappeared at pH < 3 (in DMSO), showing pH-responsive switching of the NIR absorption. By comparing the NIR-absorbing behavior of this complex to that of a complex, [PtII(1LISQ)2]2−, containing the analogous phenylenediamine ligand [(1LISQ)2− = o-diiminobenzosemiquinonate radical], the complex can be formulated as [PtII(2LISQ)2]2−. The assignment of the entity was consistent with the redox and spectroelectrochemical behaviors and electronic spin resonance (ESR) spectroscopy. First, one-electron oxidation of [PtII(2LISQ)2]2− formed an ESR-silent complex assignable to the dimeric complex [{PtII(2LISQ)(2LIBQ)}2]2− [(2LIBQ) = o-iminobenzoquinone form] in which the two radical centers at (2LISQ)2- were antiferromagnetically coupled. Second, the one-electron reduced complex of [PtII(2LISQ)2]2− exhibited an ESR signal attributed to [PtII(2LISQ)(2LPDA)]3−; 34% of the electronic spin was located at the PtII center rather than on the (2LISQ)2- moiety. The pH-responsive switching-off of the NIR absorption was thus rationally explained by oxidation of [PtII(2LISQ)2]2− to [{PtII(2LISQ)(2LIBQ)}2]2− by the increase of the rest potential of the solution in the lower pH region.
    Inorganica Chimica Acta. 11/2011; 378(1):81–86.
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    ABSTRACT: The oxidation of a redox-active tyrosine residue Y(Z) in photosystem II (PSII) is coupled with proton transfer to a hydrogen-bonded D1-His190 residue. Because of the apparent proximity of Y(Z) to the water-oxidizing complex and its redox activity, it is believed that Y(Z) plays a significant role in water oxidation in PSII. We investigated the g-anisotropy of the tyrosine radical Y(Z)(•) to provide insight into the mechanism of Y(Z)(•) proton-coupled electron transfer in Mn-depleted PSII. The anisotropy was highly resolved by electron paramagnetic resonance spectroscopy at the W-band (94.9 GHz) using PSII single crystals. The g(X)-component along the phenolic C-O bond of Y(Z)(•) was calculated by density functional theory (DFT). It was concluded from the highly resolved g-anisotropy that Y(Z) loses a phenol proton to D1-His190 upon tyrosine oxidation, and D1-His190 redonates the same proton back to Y(Z)(•) upon reduction.
    Journal of the American Chemical Society 03/2011; 133(12):4655-60. · 10.68 Impact Factor
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    ABSTRACT: The synthesis of [14]triphyrin(2.1.1) compounds is described. In contrast with conventional subporphyrins, which consistently contain a central boron atom, free-base heteroaromatic compounds can be formed. A modified Lindsey method was used to prepare a range of different [14]triphyrins(2.1.1) in yields of up to 35% based on the reaction of diethylpyrrole (1a) and fused pyrroles of bicyclo[2.2.2]octadiene (BCOD) (2a-e) and dihydroethanonaphthalene (4a) with various aryl aldehydes. The concentration of BF(3)·OEt(2) catalyst plays the key role in determining the yield of the [14]triphyrin(2.1.1) macrocycle relative to the conventional tetrapyrrole porphyrin product. Retro-Diels-Alder reactions of 2a-e and 4a result in the formation of [14]tribenzotriphyrin (2.1.1) (3a-e) and [14]trinaphthotriphyrin(2.1.1) (5a). The effects of exocyclic ring annulation on the electronic structure are examined in detail based on optical spectroscopy, theoretical calculations, and electrochemical measurements. The availability of free-base compounds enables the formation of [Re(I)(CO)(3)(triphyrin)] (6a) and [Ru(II)(CO)(2)Cl(triphyrin)] (7a) complexes based on a modified retro-Diels-Alder reaction. X-ray structures are reported for 4a and 6a.
    Chemistry 03/2011; 17(16):4396-407. · 5.93 Impact Factor
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    ABSTRACT: The lowest excited triplet (T(1)) states of magnesium and zinc tetraphenylporphines (MgTPP and ZnTPP) were studied by time-resolved (TR) high-frequency/high-field W-band electron paramagnetic resonance (hf-EPR) spectroscopy in rigid glasses at low temperatures. Inspections of the TR-hf-EPR spectra of the spin-polarized triplets revealed that the zero field splitting (ZFS) parameters, D and E, for MgTPP and ZnTPP triplets were nearly the same. At the same time, their g-tensors were found to be different. These results are interpreted quantitatively in terms of spin-orbit couplings (SOCs) and angular momenta among the excited states, giving a magnitude of SOC in the T(1) state of ZnTPP. For the first time, both the TR-hf-EPR spectra and corresponding time profiles were acquired on the ZnTPP's triplet at room temperature in liquid paraffin solution with the populations of the electron spin states being in Boltzmann equilibrium. Because of relatively fast paramagnetic relaxation in rotating triplet at room temperature, the spectra and time profiles were free from the effects of microwave saturation that allowed for the direct measurement of the absolute intersystem crossing ratios P(x):P(y):P(z) 0.085:0.085:0.83. All of these results have demonstrated advantages and new perspectives of the W-band EPR spectroscopy.
    The Journal of Physical Chemistry B 11/2010; 114(45):14559-63. · 3.61 Impact Factor
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    ABSTRACT: The mononuclear Fe(II) phthalocyanine 2 and ball-type homobinuclear Fe(II)-Fe(II) and Cu(II)-Cu(II) phthalocyanines, 3 and 4 respectively, were synthesized from the corresponding 4,4'-[1,1'-methylenebis-(naphthalene-2,1-diyl)]bis(oxy)diphthalonitrile 1, and then ball-type heterobinuclear Fe(II)-Cu(II) phthalocyanine 5 was synthesized from 2. The novel compounds 4 and 5 have been characterized by elemental analysis, UV/vis, IR and MALDI-TOF mass spectroscopies. Electron paramagnetic resonance and magnetic circular dichroism measurements of 3, 4 and 5 were also examined. The voltammetric measurements of the complexes showed the formation of various electrochemically stable ligand- and metal-based mixed-valence species, due to the intramolecular interactions between the two MPc units, especially in ball-type binuclear iron(II) phthalocyanine. Impedance spectroscopy and d.c. conductivity measurements of 4 and 5 were performed as a function of temperature (295-523 K) and frequency (40-10(5) Hz). While room temperature impedance spectra consist of a curved line, a transformation into a full semicircle with increasing temperature was observed for both compounds.
    Dalton Transactions 09/2010; 39(35):8143-52. · 3.81 Impact Factor
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    ABSTRACT: The electronic structure of a water-soluble near-infrared (NIR)-absorbing complex was determined to be singlet diradical [PtII(LISQ)2]2– [(LISQ)2–· = o-diiminosulfobenzosemiquinonate radical] by X-ray diffraction and 1H NMR spectroscopy. Cyclic voltammetry (CV) showed redox reactions of [PtII(LISQ)2]2– to give [PtII(LISQ)(LIBQ)]– (E1 = –0.26 V vs. Fc+/Fc) and [PtII(LISQ)(LPDI)]3– (E2 = –1.46 V) [(LIBQ)– = o-diiminosulfobenzoquinonate, (LPDI)3– = o-phenylenediiminesulfonate]. The monoradical structure of the latter was confirmed by electron paramagnetic resonance (EPR) spectroscopy. Splitting of the reduction wave of [PtII(LISQ)(LIBQ)]– in CV and the weak EPR signal suggested the formation of a dimer. Because the rest potential of the solution exceeds Epa1 at pH 4.0, the decrease in the NIR absorption at pH < 4.0 can be attributed to the oxidation of [PtII(LISQ)2]2– followed by dimerization.
    Berichte der deutschen chemischen Gesellschaft 06/2010; 2010(22):3458 - 3465. · 2.94 Impact Factor
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    ABSTRACT: A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T1)3 ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g z = 2.0048, g x = g y = 2.0035 for Pt(b-iq)(CN)2 (b-iq = 3,3′bi-isoquinoline) and g z = 1.9968, g x = g y = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g e (=2.0023, g value of free electron) and that g z of ZnTPP is smaller than g e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T1(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T1(ππ*) state.
    Applied Magnetic Resonance 01/2010; 37(1):317-323. · 0.83 Impact Factor
  • ChemInform 01/2010; 30(37).
  • N. HIROTA, S. YAMAUCHI
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 25(50).
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    ABSTRACT: We reported determination of the loaded quality factor (Q) of highly overcoupled (dielectric, loop-gap, and cavity) resonators used in time-domain electron paramagnetic resonance. We introduced a microwave absorber into resonators and achieved critical-coupling. Due to the deep “Q-dip” of critical-coupling, we can easily determine the loaded Q as low as 10. The loaded Q of resonators with and without the microwave absorber was examined under various overcoupling conditions. We found that the radiation Q (Q r) can be calculated from the loaded Q of the resonator that contains the microwave absorber. We proposed a simple model that represents the loaded Q of the overcoupled resonator in terms of two parameters, Q 0 and Q r. Q 0 is the effective unloaded Q of the resonator determined for the critically coupled resonator without the microwave absorber and is independent of a degree of coupling. The model can be applied to overcoupling in which the coupling parameter (Q 0/Q r) is in the range of 1 to ca. 20.
    Applied Magnetic Resonance 01/2010; 37(1):781-794. · 0.83 Impact Factor
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    ABSTRACT: Four different types of polynuclear manganese(II) compounds with bhnq2− bridges were obtained from the reaction of Mn(CH3COO)2·4H2O with the flexible hinge-like ligand H2bhnq (H2bhnq=2,2′-bi(3-hydroxy-1,4-naphthoquinone)) by controlling the reaction solvent: [Mn3(bhnq)3(H2O)2]·10.5H2O (1), [Mn2(bhnq)2(DMSO)4]·2DMSO·CHCl3 (2), {[Mn(bhnq)(H2O)2]·2H2O}n (3), and {[Mn(bhnq)(DMF)2]}n (4). All complexes are neutral with the same Mn/bhnq 1:1 formulation ratio. Compounds 1 and 2 are discrete systems, while compounds 3 and 4 are one-dimensional ones. All compounds were characterized by X-ray diffraction and show different coordination modes for the bhnq2− ligand.
    Inorganic Chemistry Communications - INORG CHEM COMMUN. 01/2010; 13(5):636-640.
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    ABSTRACT: LOV domains belong to the PAS domain superfamily, which are found in a variety of sensor proteins in organism ranging from archaea to eukaryotes, and they noncovalently bind a single flavin mononucleotide as a chromophore. We report the Raman spectra of the dark state of LOV domain in FKF1 from Arabidopsis thaliana. Spectra have been also measured for the signaling state, where a cysteinyl-flavin adduct is formed upon light irradiation. Most of the observed Raman bands are assigned on the basis of normal mode calculations using a density functional theory. We also discuss implication for the analysis of the infrared spectra of LOV domains. The comprehensive assignment provides a satisfactory framework for future investigations of the photocycle mechanism in LOV domains by vibrational spectroscopy.
    The Journal of Physical Chemistry B 04/2009; 113(9):2913-21. · 3.61 Impact Factor
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    ABSTRACT: Ordered microporous carbon that was synthesized in the nanochannels of zeolite Y is characterized by an extremely large surface area, surprisingly uniform micropores and a long-range periodicity originating from the parent zeolite. However, the molecular structure of this zeolite templated carbon (ZTC) has been completely unknown. In this study, an attempt was made to construct a possible molecular model for ZTC. The proposed model is made up of buckybowl-like nanographenes assembled into a three-dimensionally regular network, which reflects all the experimental results obtained from Raman spectroscopy, electron energy-loss spectroscopy, and previously obtained analyses with several other means. Starting from this idealized model, possible forms of defects that would be included in the actual ZTC were also investigated. Moreover, the amount and the type of oxygen functional groups were analyzed and, as per the results, some functional groups were bound to the edge sites of each buckybowl unit in the molecular model. The elemental composition, pore curvature, pore size and pore volume and surface area estimated from such oxygen-containing model agree well with the corresponding experimentally obtained results. The present model can be considered as a reasonable starting point for future refinements of the structure of this quite novel carbon material.
    Carbon. 01/2009;
  • Bulletin of The Chemical Society of Japan - BULL CHEM SOC JPN. 01/2009; 82(1):58-64.
  • Chemistry Letters - CHEM LETT. 01/2009; 38(1):68-69.
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    ABSTRACT: Neuronal PAS protein 2 (NPAS2), a heme-binding transcriptional regulatory factor, is involved in circadian rhythms. Period homologue (Per) is another important transcriptional regulatory factor that binds to cryptochrome (Cry). The resultant Per/Cry heterodimer interacts with the NPAS2/BMAL1 heterodimer to inhibit the transcription of Per and Cry. Previous cell biology experiments indicate that mouse Per2 (mPer2) is also a heme-binding protein, and heme shuttling between mPer2 and NPAS2 may regulate transcription. In the present study, we show that the isolated PAS-A domain of mPer2 (PAS-A-mPer2) binds the Fe(III) protoporphyrin IX complex (hemin) with a heme:protein stoichiometry of 1:1. Optical absorption and EPR spectroscopic findings suggest that the Fe(III)-bound PAS-A-mPer2 is a six-coordinated low-spin complex with Cys and an unknown axial ligand. A Hg (2+) binding study supports the theory that Cys is one of the axial ligands for Fe(III)-bound PAS-A-mPer2. The dissociation rate constant of the Fe(III) complex from PAS-A-mPer2 (6.3 x 10 (-4) s (-1)) was comparable to that of the heme-regulated inhibitor (HRI), a heme-sensor enzyme (1.5 x 10 (-3) s (-1)), but markedly higher than that of metmyoglobin (8.4 x 10 (-7) s (-1)). As confirmed by a Soret absorption spectral shift, heme transferred from the holo basic helix-loop-helix PAS-A of NPAS2 to apoPAS-A-mPer2. The Soret CD spectrum of the C215A mutant PAS-A-mPer2 protein was markedly different from that of the wild-type protein. On the basis of the data, we propose that PAS-A-mPer2 is a heme-sensor protein in which Cys215 is the heme axial ligand.
    Biochemistry 07/2008; 47(23):6157-68. · 3.38 Impact Factor

Publication Stats

691 Citations
248.98 Total Impact Points

Institutions

  • 1991–2011
    • Tohoku University
      • • Institute of Multidisciplinary Research for Advanced Materials (IMRAM)
      • • Department of Chemistry
      Sendai-shi, Miyagi-ken, Japan
  • 2009
    • Saga University
      • Department of Chemistry and Applied Chemistry
      Saga-shi, Saga-ken, Japan
  • 2008
    • Russian Academy of Sciences
      • N. N. Semenov Institute of Chemical Physics
      Moscow, Moscow, Russia
  • 1982–2007
    • Kyoto University
      • Division of Chemistry
      Kyoto, Kyoto-fu, Japan
  • 2002
    • University of California, Riverside
      • Department of Chemistry
      Riverside, CA, United States