Bogolyubov Institute for Theoretical Physics
of the National Academy of Sciences of Ukraine

Department of Quantum Theory of Molecules and Crystals
  • A microscopic model of quantum transitions in the generalized few-level system has been proposed. Several mechanisms of variation of the positions of energy levels have been taken into account for a coarse-grained description of flexible macromolecular structures with the hierarchy of characteristic times. Particularly, the additions from an initial chaotization of energy levels, their adiabatic stochastization, as well as their nonadiabatic relaxation have been determined. For the general case of chaotic distribution of fluctuating energy levels, weakly coupled with the environment, the probabilities of between-level transitions have been found in the analytical form. This expression comprises an explicit dependence on dimensionality of the energy levels, their stochastic broadening and their relaxation coupling parameters for environment-induced transitions, thus leading to formation of the physical conditions for effective functioning of biomacromolecules
    E.G. Petrov, V.I. Teslenko, O.L. Kapitanchuk, V.O. Leonov
  • A formation of stationary current and electroluminescence in a molecular diode in the case of asymmetric coupling of the molecule to the attached electrodes has been studied in the framework of kinetic theory of electron transport. The conditions have been found when a photocurrent as well as an electroluminescence achieves their maximum values. These conditions are realized when energy of the photoexcited molecule in the diode becomes, under the interelectrode bias voltage as well as the electric field of the gate electrode, lower the energy of the single-charged molecule. It is shown that both mechanisms of interelectrode electron transport associated with the sequential electron hoppings and the inelastic tunneling, contribute to the excitation of the molecule during a resonance regime.
    E.G. Petrov, Ye.V. Shevchenko, V.O. Leonov
  • A kinetic model is developed to describe the of a few-phase ligand-receptor structure in the condensed medium. The rate constant for transitions between the energy levels of system has been found with regard for the dynamic, fluctuation and entropic system's parameters, and a ligand concentration and environment temperature as well. In the case of irreversible dynamics of level populations, the concentration dependences of their peak amplitudes are shown to acquire anomalous character. In particular, the log odds of the output state with respect to the cooperativity of the transient process is fractional and is limited from below by the quantity 2/е. Moreover, the approach of the log odds to this value is critical and leads to the break in the stability of the irreversible system. For low temperature, this effect occurs within a rather narrow range of rates of kinetic stages. The temperature increase diminishes the effect and thus increases the degree of control of the transient process.
    V.I. Teslenko, O.L. Kapitanchuk
  • Laser-pulse induced transient currents through a molecular junction are studied in the framework of a density matrix theory. By focusing on the sequential transport regime the effects of collective plasmon excitations of the leads are investigated. A remarkable current enhancement of transient as well as time-averaged dc currents is found due to simultaneous plasmon excitations in the leads. The effects of intra-molecular vibrational energy redistribution and its important role on both transient and the averaged dc-currents are also considered. A vibrational progression becomes visible if the dc caused by a huge sequence of laser pulses is drawn versus the applied voltage. This can be considered as a dynamically determined IV-characteristic.
    Y. Zelinskyy
  • The orderings of classical one-dimensional structures of charges have been investigated in equilibrium states of Coulomb clusters with a confining potential originated from the uniform cylindrical background. Regions of parameters compatible with ideal structures or quite ideal ones on a finite length scale have been defined. Specificity of the physical mechanism that results in the high ordering of structures is discussed. Characteristics of ideal structures have been obtained and the algorithm for estimations of orderings in one-dimensional structures in a general case has been given.
    S.Ya. Goroshchenko
  • A theory of triplet excitation transfer in a donor-oxygen-acceptor systems is developed for the conditions when the transfer is mediated by an oxygen molecule. It is shown that the oxygen molecule can be involved in both real and virtual intramolecular singlet-triplet conversions in the course of the process under consideration. Expressions describing a superexchange donor-acceptor coupling owing to a participation of the bridging twofold degenerate oxygen's virtual singlet state are derived and the transfer kinetics including the sequential (hopping) and coherent (distant) routes are analyzed. Applicability of the theoretical description to the pigment-protein cytochrome complex, by considering the triplet excitation transfer from the chlorophyll molecule to distant carotene, is explained using the above theoretical model.
    E.G. Petrov
  • The mechanisms of current formation and electroluminescence in "electrode-photochromic molecule-electrode" system are derived under the assumption that electric field is applied to both the electrodes and the molecule. It is shown that in the charge-neutral molecule, a contribution in photocurrent and electroluminescence comes from sequential and tunnel electron transfer pathways. The conditions for resonance electron transmission through the molecule are found to attain the most effective electroluminescence and current through a molecule.
    E.G. Petrov, V.O. Leonov, Ye.V. Shevchenko, O.L. Kapitanchuk
  • Density functional theory calculations were applied to the study of changes in structural and electronic properties of diarylethen-based molecule in the result of conformational switching between open and closed state, energy barriers of corresponding transitions were calculated. Effect of molecular environment on the barrier value of transition state was found to be significant. Experimentally observed field-induced switching of a molecule in the monolayer was substantiated.
    O.L. Kapitanchuk
  • The transient response of the molecular junctions attached to a left and a right lead has been studied at the conditions when a single voltage pulse or a sequence of pulses is applied. The transient currents as well as a quantity detectable in the experiment the averaged dc-current, resulting from a single voltage pulse or a sequence of pulses is calculated. The important role of intra molecular vibrational relaxation on transient and dc-currents are highlighted. Based on above findings the approach to determine strength of intramolecular relaxation is suggested.
    Y. Zelinskyy
  • The method for a control of relaxation transitions in a quantum system by the stochastic fields, which are created in a macroscopic environment owing to the thermodynamic fluctuations of position of the energy levels, is proposed. By means of averaging the stochastic Liouville equation for the density matrix of the whole system, the master kinetic equation for probabilities of populations of the quantum system's states, are found. It is shown that in nanoscopic system, the transition rates allow themselves to provide a unified description of kinetic processes as in the case of temperature independent transitions, and in the Arrhenius's regime realized for temperature activated transitions as well.
    V.I. Teslenko, E.G. Petrov
  • In the model of classic Coulomb cluster with confining potential supported by a uniform cylindrical background, the formation of spiral layered structures disposed perpendicularly to a cylindrical axis, is predicted. It is shown that self-organization of such a structure and distribution of the charges within the structure is controlled by Coulomb barriers between the layers, the precise form of which is determined by geometric and charge parameters of a cluster.
    S.Ya. Goroshchenko
  • Mechanism of formation of electroluminescence is studied for a linear molecule embedded between the metallic electrodes. It is shown that an electroluminescence can appear if the photochromic group of the molecule is separated from the electrodes by covalent bonds. Electroluminescence is formed in the conditions when a difference of chemical potentials of the electrodes exceeds energy of excitation in the molecule and, additionally, an electron transfer from the donating electrode to the molecule occurs in a resonant regime. In this case, a molecular excitation is formed owing to electron hoppings between the electrodes and the molecule as well as due to an inelastic interelectrode electron tunneling accompanied by a virtual participation of the charged states of the molecule.
    V.O. Leonov, Ye.V. Shevchenko, E.G. Petrov
  • Role of intramolecular exchange interaction in formation of the transient and steady-state currents in a molecular diode is clarified. It is shown that at weak exchange interaction between the unpaired electrons of an optically excited molecule and, thus, at negligible energy splitting between the excited singlet and triplet molecular states , the steady-state photocurrent is close in magnitude to the peak photocurrent, whereas the same current is suppressed when the exchange interaction is strong. The reason is that a molecule in the triplet state exhibits itself as a trap blocking an electron transfer between electrodes.
    V.O. Leonov, E.G. Petrov
  • Conductance properties of a non-conjugated organic molecule that contains the localized bonds, have been studied with taken into consideration a distribution of an electron density within the molecule. It is shown that conductance properties of such molecule are determined by the electrons situated at the completely occupied molecular orbitals. As to an electron transmission along the molecule, it results from a superexchange interaction between the localized orbitals whereas a nonlinear current is associated with an interelectrode electron tunneling controlled by the noted localized orbitals. Theoretical results obtained are in a good agreement with experimental data concerning the trimethylsil-acetilene molecule with two triple and one double bonds.
    O.L. Kapitanchuk, E.G. Petrov
  • A microscopic model of the non-stationary system contacting to the condensed medium is proposed. By the example of a biomolecular receptor, the role of medium fluctuations of dichotomous and trichotomous types in achieving a control of the dependent on temperature kinetic and cooperative receptor's properties is discovered. It is found out that comparing to dichotomous fluctuations the presence of trichotomous fluctuations does not influence the possibility of emerging the fact of fractal cooperativity, which is general for receptors, but due to the effect of stochastic narrowing of the width of energy levels can rather make the process of their temperature-independent desensitization several times faster.
    V.I. Teslenko
  • A density matrix based theoretical description of sequential electron transmission through a single molecule sandwiched between two nanoelectrodes and placed in a nanoresonator is presented. Electroluminescence of a molecular junction has been calculated and an enhancement of the related spectra by more than three orders of magnitude could be demonstrated by changing the resonance condition between the plasmon hybrid level of the nanoresonartor and the molecular transition. A selective enhancement of different parts of the vibrational progression in the emission spectrum becomes possible by varying the distance between the nanoresonators.
    Ya.R. Zelinsky
  • The evolution of averaged occupancies of molecular quasi-isoenergetic state in molecular systems where alternation of molecular energy levels is caused by discrete dichotomous and trichotomous stochastic fields, is studied. The dependence of the kinetics of transition processes on low and high frequency parameters of molecular levels fluctuation is characterized in detail. A novel physical mechanism for temperature-independent transitions in flexible molecular systems is proposed. This mechanism becomes effective when the conformation transitions between quasi-isoenergetic molecular states take place. At room temperatures, stochastic broadening of molecular energy levels predominates the energy of low-frequency vibrations accompanying transition. This leads to cancellation of the temperature dependence in the stochastically averaged rate constants. The proposed mechanism explains the temperature-independent onset of P2X3 receptor desensitization in neuronal membranes, as well as degradation of PER2 protein in embryonic fibroblasts.
    E.G. Petrov, V.I. Teslenko
  • The concentrations of the injected by electric field charges across the 2D layers in fullerite-based FET are found. The relation between total amount of charge carriers and realistic electric field magnitude in FET structure is determined analytically. Maximal possible field-effect doping rate of fullerene molecules is estimated in break-down limit of gate dielectrics.
    O.L. Kapitanchuk