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ABSTRACT: The formation of 1:1 adducts of Pt(bpy)2(2+) (bpy = 2,2'-bipyridine) with various N bases (B) has been ascertained in water at ambient temperature by spectrophotometric titration and electrospray ionization mass spectroscopy. A pseudo-five-coordinated complex, [Pt(bpy)2(B)](2+) with a monodentating bpy, is proposed based on density functional theory calculation. The formation constants (Kc) increase with the nucleophilicity of B except for sterically hindered N-bases, indicating an associative ligand-substitution mechanism.
Inorganic Chemistry 06/2008; 47(9):3477-9. · 4.60 Impact Factor
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ABSTRACT: We report a theoretical study of molecular wires which are designed to detect a single nucleic acid base thymine, through electrical conductance change. Three model sensors with ethynylphenyl backbones having suitable detection units and capable of forming hydrogen bonds with thymine are proposed. A combination of density functional theory and nonequilibrium Green's function formalism were used for the theoretical studies. The results show that subtle changes in electronic properties of the sensors due to hydrogen bond formation with the target may give rise to measurable changes in conductance. We also report experimental conductance studies for one model sensor, which agrees qualitatively with the theoretical results.
02/2007;
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ABSTRACT: We propose a model molecular wire sensor that can detect zerovalent chromium by a strong increase in conductance when included in a metal-molecule-metal junction. An essential part of the sensor is a paracyclophane unit that binds to the metal atom. The nature and the energies of the molecular orbitals change drastically after the complex is formed, resulting in a 10- to 12-fold increase in conductance.
The Journal of Physical Chemistry B 12/2006; 110(47):23806-11. · 3.70 Impact Factor
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Tohru Nakamura,
Emiko Koyama,
Yukihiro Shimoi, Shuji Abe,
Takao Ishida,
Kiyomi Tsukagoshi,
Wataru Mizutani,
Hideo Tokuhisa,
Masatoshi Kanesato,
Ikuyo Nakai,
Hiroshi Kondoh,
Toshiaki Ohta
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ABSTRACT: Surface potential switching on gold(111) surfaces is induced by complexation/decomplexation reactions of a bipyridine (BP) derivative and palladium(II) chloride, as observed by Kelvin probe force microscopy (KFM). On the basis of the theoretical predictions, a 4-(5-phenylethynyl-2,2'-bipyridine-5'-yl-ethynyl)benzenethiol (PhBP) derivative was synthesized and used as an active monolayer to catch transition metal ions. By using the microcontact printing (CP) technique, micron-size patterned PhBP monolayers, which act as effective hosts to coordinate palladium(II) chloride, were prepared on gold(111) surfaces. The KFM signal decreases by complexation of the Pd(II) chloride in PhBP monolayers and is recovered by removal of Pd ions using an ethylenediamine solution, as confirmed by X-ray photoelectron spectroscopy. This process is reversible, indicating that the surface potential switching is realized by complexation/decomplexation of Pd(II). A CP PhBP monolayer, when it detects the target palladium ion, shows sensitivity for the picomolar level detection judged from surface potential changes in KFM measurements. The dipole moment estimated by the surface potentials is much smaller than the calculated value, indicating that mechanisms for the reduction of the surface dipole moment exist in real monolayers prepared by the CP method.
The Journal of Physical Chemistry B 06/2006; 110(18):9195-203. · 3.70 Impact Factor
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ABSTRACT: We report a theoretical study for the conformational switching of molecules on Au(111) surface in the presence of an applied electric field. Ab initio calculations for N-(2-mercaptoethyl)benzamide indicate that this molecule, when adsorbed, can exist as two stable conformers of nearly equal energies, but the dipoles point in reverse directions with respect to the surface. An electric field can be used to "switch" one conformer to the other, and during this process, there is an abrupt change in the height of the molecule from the surface. This change is expected to cause a conductance switching in the system. Further calculations for N-(2-mercaptoethyl)-4-phenylazobenzamide suggest that the experimentally observed phase switching of this molecule is associated with a conformational change accompanied by dipole reversal.
The Journal of Physical Chemistry B 04/2006; 110(9):4247-55. · 3.70 Impact Factor
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ABSTRACT: We investigated the switching behavior of small particles of an Ising-like model under constant excitation by means of Monte Carlo simulations to study photoinduced spinstate switching in nanoparticles of transition metal complexes. The threshold intensity required for that switching becomes drastically small in small particles with diameter of less than 10 pseudospins. This lower intensity results enhancement of the pseudospin fluctuation at the surface in the small particles. Our result might originate the increase of the photoinduced magnetization in nanoparticles of a Mo-Cu cyanide.
Journal of Physics Conference Series 09/2005; 21(1):56.
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ABSTRACT: Theoretical investigation with Monte Carlo simulations predicts that thermal spin-switching hysteresis of transition-metal complexes appears even in nanoparticles, but the hysteresis width does not depend only on the interaction strength between molecules but also strongly on the shape and size of the particles.
Chemical Communications 09/2005; · 6.17 Impact Factor
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ABSTRACT: The mechanism of charge-induced high spin is studied in pi-conjugated molecules by means of a model-Hamiltonian approach. Intersite Coulomb interactions are taken into account in a pi-conjugated moiety, which is coupled with two localized spins through exchange interactions. We clarify spin alignment in neutral and oxidized states by exact numerical calculations including all the correlation effects. In thianthrene-based molecules, one-electron oxidation induces strong ferromagnetic correlation between the localized spins irrespective of the spin alignment in the neutral state. The localized spins are coupled to the delocalized hole spin ferromagnetically, leading to a high-spin state in the oxidized molecule. Our calculations on structural dependence and effective exchange interaction are consistent with the recent experiment of thianthrene bis(nitronyl nitroxide). By comparing the thianthrene-based molecule with the anthracene-based one, we clarify the role of superexchange interactions via the sulfur atoms.
The Journal of Chemical Physics 07/2005; 122(24):244324. · 3.33 Impact Factor
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ABSTRACT: The performance of a molecular sensor for detecting nucleic acid base was studied by molecular orbital calculations of model compounds. The model compounds consist of a detecting unit (pyridine derivative), a conducting unit (benzene) and a connection between them. The changes of the HOMO, LUMO energy levels and charge distributions of the conducting unit by the hydrogen bonds formation between the detecting unit and thymine were studied. The calculations show that the choice of the connection and its position are significantly important to efficiently transmit the electronic effects to the conducting unit. The comparison with aminopyridine, where the pyridine ring plays both detecting and conducting units, shows that the electronic effects are transmitted quite efficiently, if the connection is properly selected. The size of the electronic effects by the formation of the hydrogen bonds were also compared with those induced by the change of the torsional angle between the pyridine and benzene rings and by substitution of the benzene ring.
Biosensors and Bioelectronics 02/2005; 20(7):1452-7. · 5.60 Impact Factor
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ABSTRACT: Control of intramolecular spin alignment is studied theoretically in a model of polyene-based molecular magnets in which delocalized $\pi$ electrons are coupled with localized radical spins. In a previous paper [Phys. Rev. Lett. 90 (2003) 207203], we have demonstrated that charge doping is an effective way to realize controllable high-spin in the $\pi$-conjugated molecular magnets. In this paper, we clarify the dependence of spin-alignment on the exchange interaction between the localized spin and $\pi$ electron and the electron-electron interactions. The antiferromagnetic exchange interaction plays a role different from the ferromagnetic counterpart in doped molecules. To understand complex interplay of charge and spin degrees of freedom in the doped systems, we carry out a systematic study on the phase diagram of spin alignment in the parameter space. The mechanism of the spin alignment is discussed based on the spin densities of $\pi$ electrons. The calculated results are consistent with experiments, providing a theoretical basis for the control of spin alignment. Comment: Journal of the Physical Society of Japan, in press
09/2004;
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ABSTRACT: The stability of staging states has been investigated for mixed-stack charge-transfer complexes, TMB-TCNQ [(3,3′,5,5′)-tetramethylbenzidine-(7,7,8,8)tetracyanoquinodimethane], DMTTF-CA (dimethyltetrathiafulvalene-p-chloranil), and TTF-CA (tetrathiafulvalene-p-chloranil) crystals, by the calculation of their electrostatic energies considering intramolecular charge distributions. The staging state, a superstructure consisting of an alternative sequence of the ionic and neutral domains, is supposed to be advantageous for reducing the repulsive interaction between neighboring chains in the ionic phase. However, our calculations indicate that the electrostatic interchain interaction is attractive, implying that the staging state is disadvantageous. Nevertheless, a staging state can be marginally stabilized around the neutral–ionic transition in TMB-TCNQ although the energy difference between the staging state and the uniform ionic state is quite small.
Phys. Rev. B. 08/2004; 70(8).
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ABSTRACT: Intramolecular spin alignment in pi-conjugated molecules is studied theoretically in a model of a Peierls-Hubbard chain coupled with two localized spins. By means of the exact diagonalization technique, we demonstrate that a spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic doping, depending on the chain length, the positions of the localized spins, and the sign of the electron-spin coupling. The calculated results provide a theoretical basis for understanding the mechanism of spin alignment recently observed in a diradical donor molecule.
Physical Review Letters 06/2003; 90(20):207203. · 7.37 Impact Factor
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ABSTRACT: By means of Monte Carlo simulations on a kinetic model, we demonstrate that the efficiency of a photoinduced phase change can in general be enhanced drastically by using a superstructure of an appropriate combination of two components. This is due to the accelerated nucleation of converted domains in the structural blocks relatively close to local instability. The present mechanism provides a general guideline on the design of photocontrollable materials with potential applications for memory and storage devices. Comment: 4 pages, 3figures, accepted for publication in Appl. Phys. Lett
02/2002;
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ABSTRACT: By means of Monte Carlo simulations for a kinetic Ising model on a two-dimensional square lattice, we demonstrate that a photoinduced phase transition is accomplished more rapidly even for weaker light in a stripe-structure consisting of two kinds of constituent units than in a homogeneous structure. This is due to accelerated nucleation of a final phase in the constituent unit that is relatively close to instability locally.
Phase Transitions: A Multinational Journal. 01/2002; 75(7-8):753-758.
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ABSTRACT: We calculated the Madelung energies of both the ground state and excited states in tetrathiafulvalene - p-chloranil (TTF-CA) by taking into account intramolecular charge distribution. The distribution is found to be significant in the neutral-ionic (NI) transition. In the ionic phase, the Madelung energy depends more strongly on the intermolecular distance perpendicular to the $\pi$-stacking chains than on that along the chains. This indicates that simple single-chain models neglecting interchain electrostatic coupling are not adequate. The gain of the Madelung energy due to dimerization is concluded to be small compared with the other structural changes. We also calculated the formation energy of excited state domains, which appear in the initial process of the phase transition. A one-dimensional excited domain has the smallest energy among the possible domains with the same number of molecules, and the energy per molecule is considerably reduced in increasing the domain size. It is consistent with the experimental suggestion that a large number of excitations were generated by only one photon. Comment: 6 pages, 6 figures, Phys. Rev. B, in press
07/2001;
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ABSTRACT: A novel microscopic mechanism of bi-directional structural changes is proposed for the photo-induced magnetic phase transition in Co-Fe Prussian blue analogues on the basis of ab initio quantum chemical cluster calculations. It is shown that the local potential energies of various spin states of Co are sensitive to the number of nearest neighbor Fe vacancies. As a result, the forward and backward structural changes are most readily initiated by excitation of different local regions by different photons. This mechanism suggests an effective strategy to realize photoinduced reversible phase transitions in a general system consisting of two local components. Comment: 4 pages, LaTex, 3 figures, to appear in Phys. Rev. Lett
06/2000;
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ABSTRACT: Competition between polaron and bipolaron inconjugated polymers with nondegenerate ground state is systematically studied in the extended Hubbard-Peierls model with the symmetry-breaking Brazovskii-Kirova term, using the density matrix renormalization group method combined with lattice optimization in the adiabatic approximation. We demonstrate that the relative stability of a bipolaron over two separated polarons sensitively depends on both on-site Hubbard $U$ and nearest-neighbor repulsion $V$. When $U$ is much larger than $V$, the bipolaron state is more stabilized compared with mean field calculations. Comment: 4 pages, RevTeX, 3 eps figures. To appear in Journal of the Physical Society of Japan
03/1998;
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ABSTRACT: Interchain hopping of solitons and polarons in polyacetylene is studied by numerical simulation of their motion under an electric field. Use is made of the Su-Schrieffer-Heeger model supplemented with intrachain electron-electron interactions and dopant potentials. We find that charged solitons can hop to the opposite chain by forming bound pairs (bipolarons). For the case of polarons also, hopping in a pair is more favorable than single polaron hopping. Interchain hopping of a polaron pair is more efficient than that of a soliton pair. Comment: 10 pages, 7 Postscript figures, RevTeX, to appear in J. Phys. Soc. Jpn.(1997) Apr
02/1997;
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ABSTRACT: Exciton effects on conjugated polymers are investigated in the soliton lattice system. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions treated by the single-excitation configuration-interaction method. The soliton band is present in the Peierls gap of the doped system. There appears a new kind of the exciton where an electron-hole pair is excited between the soliton band and the continuum states. We find that the oscillator strengths accumulate rapidly at this exciton as the soliton concentration increases. The contribution from the lowest exciton is more than 90% at the 10% doping. The third-harmonic generation (THG) at off-resonance frequencies is calculated as functions of the soliton concentration and the chain length of the polymer. The optical nonlinearity by the THG at the 10% doping increases by the factor about 10^2 from that of the neutral system.
11/1995;
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Molecular Crystals and Liquid Crystals. 10/1995; 267(1):329-334.
