阿部 修治

Substantial efforts have been made for the standardization of nanotechnology vocabulary by the ISO Technical Committee 229, which has already produced a few standards. However, in view of growing interest in the environmental, health, and safety issues of nanomaterials, the definition of nanomaterial still remains controversial in some respects, especially for regulatory purposes. This is related with dificulties in specifying requirements for a broad range of nanomaterials with a variety of applications. © The Japanese Environmental Mutagen Society.

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 (K-c) increase with the nucleophilicity of B except for sterically hindered N-bases, indicating an associative ligand-substitution mechanism.

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.

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.

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-mereaptoethyl)-4-phenylazobenzamide suggest that the experimentally observed phase switching of this molecule is associated with a conformational change accompanied by dipole reversal.

We present a theoretical study on optically controlled spin alignment in purely organic molecules. A quantum electron-spin model is applied to simulate molecular systems, in which localized spin densities on radical groups are coupled with a π-conjugated moiety through exchange interactions. We numerically solve the model up to highly excited states including all the correlation effects. It is demonstrated in the anthracene-based molecules that the lowest triplet of π moiety plays the role of ferromagnetic coupler for the localized spins. A metastable high-spin state can be generated by photoexcitation and subsequent relaxation processes. It is further clarified in the biphenyl-based molecules that such spin alignment depends sensitively on the molecular geometry and the strength of the exchange interactions. Our study offers a theoretical basis for understanding photoinduced high-spin states observed in π-conjugated molecular systems. © 2005 The American Physical Society.

We present a theoretical study on optically controlled spin alignment in purely organic molecules. A quantum electron-spin model is applied to simulate molecular systems, in which localized spin densities on radical groups are coupled with a pi-conjugated moiety through exchange interactions. We numerically solve the model up to highly excited states including all the correlation effects. It is demonstrated in the anthracene-based molecules that the lowest triplet of pi moiety plays the role of ferromagnetic coupler for the localized spins. A metastable high-spin state can be generated by photoexcitation and subsequent relaxation processes. It is further clarified in the biphenyl-based molecules that such spin alignment depends sensitively on the molecular geometry and the strength of the exchange interactions. Our study offers a theoretical basis for understanding photoinduced high-spin states observed in pi-conjugated molecular systems.

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. (c) 2005 American Institute of Physics.

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. © 2004 Elsevier B.V. All rights reserved.

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.

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.

有機磁性分子の研究分野では,近年,外から加える刺激(例えば,電子の出し入れや光励起など)を利用して分子内に複数あるスピンの整列を積極的に制御しようという研究が活発化しつつある。まず,その基本的な概念とそれを実鉦した実験を紹介する.次に,微視的なモデルを用いた我々の研究をもとに,ドーピングによる電子的制御が理論的にも可能なことを示し,ドープされた磁性分子のスピン整列機構について議論する。

We propose a method of accelerating photoinduced phase transition in conjunction with a clear-cut demonstration of the method using Monte Carlo simulations for an Ising-type model. The method involves constructing a mixture of two-component constituent units that have an opposite relative stability between their bistable states. As a simple model, an Ising-type model of two components is investigated by means of Monte Carlo simulations. The opposite relative stabilities are implemented in the energy difference between the two states of each unit. In the mixture, the phase transition proceeds under continuous excitation more rapidly than in a single-component homogeneous structure. This is due to an accelerated nucleation of the final phase in the constituent unit that is relatively close to local instability.

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.

A superstructure named staging structure has been proposed in some mixed-stack charge-transfer complexes showing neutral-ionic transitions. It consists of an alternate sequence of the ionic and neutral layers. We investigate its stability by calculations of electrostatic energies. We show the stability is determined by the difference of the intramolecular charge distribution between the ionic and neutral states. The staging structure is slightly stable than the uniform ionic or neutral state in TMB-TCNQ ((3,3',5,5')-tetramethylbenzidine(7,7,8,8)tetracyanoquinodimethane) in which the structure has been suggested accompanying the transition.

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. © 2002 American Institute of Physics.

The rate of excitation-energy transfer (EET) within the light-harvesting complex (LH) and from LH to the reaction center (RC) of photosynthetic purple bacteria is calculated, based on a formula for EET between molecular aggregates. We show that optically forbidden exciton states participate in EET processes through multipole EET interactions with the help of disorder. In the antenna systems of photosynthesis, high efficiency of energy transfer is implemented by these EET processes involving optically forbidden exciton states.

We calculated the Madelung energies of both the ground state and excited states in tetrathiafulvalene-p-chloranil by taking into account intramolecular charge distribution. The distribution is found to be significant in the neutral-ionic 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 primary process of the phase transition. A one-dimensional excited domain has the smallest energy among the possible domains with the same number of molecules when the domain is small, 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.

We study theoretically the influence of both diagonal and off-diagonal disorder on the absorption spectra of the light-harvesting antenna complex LH2 consisting of two circular aggregates, B850 and B800, of bacteriochlorophyll pigments in photosynthetic purple bacteria. Off-diagonal disorder, i.e., randomness in excitonic couplings between molecules, is introduced by a model of disorder in the position of each pigment molecule embedded in proteins. We demonstrate that a large contribution of positional disorder provides a natural explanation for the experimental fact that the excitonic B850 absorption peak is broader than that of monomeric B800 in spite of motional narrowing. (C) 2001 Elsevier Science B.V. All rights reserved.

Two-photon absorption (TPA) spectra of molecular aggregates are calculated for asymmetric molecules with a difference static dipole between the ground and excited states. If the molecules are aligned symmetrically such that their difference static dipoles cancel out as a whole, some of dipole-forbidden exciton states are two-photon allowed. This effect is important not only for probing forbidden excitons as well as for resonance enhancement of optical nonlinearity. Examples of dimers, and linear and circular aggregates are discussed. As an interesting application, the TPA spectrum is calculated for circular light-harvesting antenna complexes in photosynthetic bacteria. (C) 2001 Elsevier Science B.V. All rights reserved.

光合成は地球上全生命のエネルギー源であり,その研究の歴史は200年以上に及ぶ.しかし微視的構造に基づいた理解が進んだのは,この15年間にすぎない.特に光捕集アンテナ複合体の立体構造が明らかにされたのは5年前である.ここでは,アンテナ系で見られる高速な励起エネルギー伝達が,そこでの励起子準位によって実現されていることを述べる.生体機能において幾分子にもわたって空間的に広がった量子状態が重要な役割を演じる初めての例である.

The rate of excitation-energy transfer (EET) within the light-harvesting complex (LH) and from LH to the reaction center (RC) of photosynthetic purple bacteria is calculated, based on a formula for EET between molecular aggregates. We show that optically forbidden exciton states participate in EET processes through multipole EET interactions with the help of disorder. In the antenna systems of photosynthesis, high efficiency of energy transfer is implemented by these EET processes involving optically forbidden exciton states.

A novel microscopic mechanism of bidirectional structural changes Is proposed for the photoinduced magnetic phase transition in Co-Fe Prussian blue analogs 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.

生体細胞中にはさまざまな生理機能を実現するナノスケールの装置が存在する.光合成の初期過程を担う光捕集アンテナ系と呼ばれる模タンパク角素複合体もそのようなナノ装置の例である.ここでは,ある種の光合成細菌で見いだされた環状のアンテナ系を例にとり,その色素分子集合体の励起子状態の特徴と,集合体間の励起エネルギー移動の機構について解説する.

The rate of excitation-energy transfer (EET) within the light-harvesting complex LH2 of photosynthetic purple bacteria is calculated, based on a new formula for EET between molecular aggregates with a small mutual distance, by taking into account both the exciton-phonon and exciton-disorder interactions. It is elucidated that EET from B800 to higher and optically forbidden exciton states in B850 is the dominant mechanism of the rapid EET. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

The mechanism of the heat-induced charge transfer observed in Prussian-blue compounds K1-2xCo1+xFe(CN)(6) is studied theoretically in connection with photoinduced magnetism. In the case of x = 0, our first-principles band calculation confirms that the band-gap excitation corresponds to charge transfer from Fe-d epsilon to Co-d gamma orbitals. By increasing the lattice constant (specifically, by elongating the Co-N bond in the crystal), the band gap decreases systematically due to the variation of the crystal field at each Co site. In the nonstoichiometric case (x not equal 0), we demonstrate on the basis of ab initio cluster calculations that a small increase in the Co-N distance is sufficient to cause charge transfer between Fe and Co atoms located near Fe vacancies accompanied by water molecules. This mechanism by electron-lattice coupling combined with disorder explains most of experimental findings on heat-induced changes and is suggested to be responsible also for the photoinduced magnetic transition. [S0163-1829(99)03141-0].

Excitation-energy transfer (EET) has been observed to be very rapid from B800 to B850, between two circular aggregates of bacteriochlorophyll molecules (BChls), in the Light-harvesting antenna complex LH2 of photosynthetic purple bacteria. This rapid EET cannot be understood within the framework of Forster' s formula,. since the luminescence spectrum of B800 overlaps little with the absorption spectrum of B850. The present work shows that it can be rationalized on the basis of a recently proposed formula for EET between molecular aggregates. The formula differs from Forster"s one when excited stares are excitonic at least in one of the donor and the acceptor aggregates with a mutual distance not much larger than their physical sizes, as in the present case. Excited states of B850 are regarded as excitonic, while those of B800 as monomeric. The exciton-phonon coupling was taken into account over all orders for B800 and within a self-consistent second-order perturbation for B850. Calculated rates of the EET are in the range of 10(11)-10(12) S-1, increasing weakly with temperature, in good agreement with experiments. We demonstrate that this rapid EET occurs to optically forbidden exciton slates of B850, without total transition dipole, due to strong interaction of st transition dipole on a BChl in B800 with those on nearby BChls in B850.

The linear dichroic absorption spectrum of pseudoisocyanine bromide (PIC-Br) molecular J-aggregates is theoretically analyzed by using the dynamical coherent potential approximation and numerical calculations. The absorption spectra with the electric field polarization parallel and perpendicular to the one-dimensional axis of the oriented J-aggregates are well explained in terms of Davydov components. Above the one-phonon continuum in the dispersion curve of the exciton-phonon system, splitting off of an exciton polaron band is observed. This exciton polaron state is assigned to the largest peak in the case of the latter polarization. The absorption intensity differences between the lowest peak and the 2nd peak observed in two polarization conditions are simply explained by the character of relevant states. (C) 1998 Elsevier Science B.V. All rights reserved.

Competition between polaron and bipolaron in conjugated 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.

We calculate rates of excitation-energy transfer (EET) from B800 to B850 within the light-harvesting antenna complex LH2 of photosynthetic purple bacteria. The calculation is based on anew formula which incorporates EET interactions between transition dipoles distributed on individual chromophores in the acceptor molecular aggregate and those in the donor. The calculated rate is in good agreement with experiments, which could not be explained by directly applying Forster's theory.

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.

Interchain hopping of solitons and polarons in polyacetylene is studied by numerical simulations of their motion under the electric field. The used model is the Su-Schrieffer-Heeger model supplemented with the intrachain electron-electron interaction and dopant potential. We find that charged solitons can hop to the opposite chain by forming a pair (bipolaron). 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 effective than that of a soliton pair.

Third-order nonlinear optical spectra chi((3)) Of conjugated polymers are theoretically studied within the treatment of double-excitation configuration interaction for the Pariser-Parr-Pople model. We consider a realistic situation of dephasing rates being much larger than depopulation rates. It is shown that on the low-energy side of the one-photon resonance, the absolute value of chi((3))(-omega;omega,-omega,omega) is approximately proportional to the linear absorption spectrum alpha(omega). Additional peaks show up in the chi((3)) spectrum at frequencies corresponding to energy differences among excited states. Their intensities can become comparable to and even stronger than ordinary two-photon absorption peaks in the same energy region. The same is true for the electromodulation spectrum chi((3))(-omega;omega,O,O). In this situation the product of chi((3))(-omega;omega,O,O) and alpha(omega) is shown to be approximately proportional to chi((3))(-omega;omega-omega,omega).

We have theoretically studied chi((3)) spectra of degenerated four wave mixing and electroabsorption for conjugated polymers within a treatment of double-excitation configuration interaction for the Pariser-Parr-Pople model. Comparison between the generally accepted Orr-Ward formula and the density matrix approach in deriving chi((3)) reveals in the latter case some new features of the nonlinear spectra, for example, additional resonances, and demonstrates an importance of the density matrix approach for analyzing correctly the regions of the two-photon and saturated absorptions in conjugated polymers.

We have calculated the two-photon absorption (TPA) spectrum in the Pariser-Parr-Pople model of conjugated polymers. Configuration interactions among all the single and double excitations are taken into account for relatively long chains. It is shown that the maximum of TPA decreases with increasing interaction strength or with increasing bond alternation. For moderate interaction strength and bond alternation, the overall TPA spectrum is characterized by two energy regions: the low-energy part due to mainly one-electron excitations (excitonic states), and the higher-energy part with contributions mainly from two-electron excited states (biexcitonic states). The high-energy TPA is about one order of magnitude stronger than the low-energy part. For stronger interaction or smaller bond alternation, the single and double excitations are fairly mixed, and the distinction between the excitonic and the biexcitonic regions is lost.

Triplet exciton polarons in nondegenerate conjugated polymers are studied theoretically. Both electron-electron and electron-lattice interactions are taken into account in the Pariser-Parr-Pople model. The electronic and lattice structures of exciton polarons and their adiabatic energy surfaces are calculated by means of the unrestricted Hartree-Fock and the single-configuration-interaction methods. The calculated absorption spectrum of a triplet exciton polaron explains well the photoinduced absorption (PA) spectrum observed in polydiaectylene. An excitonic effect turns out to be crucial in explaining the narrow-peak profile of the observed PA spectrum. The present theory may well apply to the similar PA observed in poly(p-phenylene vinylene).