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A random growth algorithm is used to calculate the configurations of poly(p-phenylene vinylene) chains to simulate the nonradiative excitation energy transfer. This transfer usually takes place in a small local area within a short period of time and around the excited chromophore.

Research has been conducted on the alkyl- and aryl-substituted derivatives of cis,cis-1,2,5,6-tetrasilacycloocta-3,7-dienes. The preparation of these derivatives and the isomerization of these compounds to the trans,trans-1,2,5,6-tetrasilacycloocta-3,7-dienes have been explored and described.

In our recent work [X. Yang and E. R. Bittner, J. Phys. Chem. A 118, 5196 (2014)], we showed how to construct a reduced set of nuclear motions that capture the coupling between electronic and nuclear degrees of freedom over the course of an electronic transition. We construct these modes, referred to as "Lanczos modes," by applying a search algorithm to find linear combinations of vibrational normal modes that optimize the electronic/nuclear coupling operator. Here, we analyze the irreducible representations of the dominant contributions of these modes and find that for the cases considered here, these belong to totally symmetric irreducible representations of the donor and acceptor moieties. Upon investigating the molecular geometry changes following the transition, we propose that the electronic transition process can be broken into two steps, in the agreement of Born-Oppenheimer approximation: a fast excitation transfer occurs, facilitated by the "primary Lanczos mode," followed by slow nuclear relaxation on the final electronic diabatic surface. [ABSTRACT FROM AUTHOR]

We examine the possibility of observing Bose condensation of a confined two-dimensional polariton gas in an organic quantum well. We deduce a suitable parameterization of a model polynomial Hamiltonian based upon the cavity geometry, the biexciton binding energy, and similar spectroscopic and structural data. By converting the sum-over-states to a semiclassical integration over D-dimensional phase space, we arrive at a principle of correspondence between ideal and non-ideal Bose gases that share a common critical exponent. Using our results, we can calculate the properties for a model cavity containing an anthracene thin film. [ABSTRACT FROM AUTHOR]

We use mixed classical/quantum simulations to study the time dependence of an excitation of a C=O vibration on a 3-10 helix of α-aminoisobutyric acid, a system which represents a test case for the formation of self-trapped vibrational excitation states on protein helices. Due to the inherent disorder in the system caused by the finite temperature and fluctuations in hydrogen bonding, the excitation tunnels randomly among C=O sites along the helix. Quantum forces are insufficient to establish a coherent relationship between the location of the excitation and the contraction of hydrogen bonds around this site. Our simulations indicate that the excitation frequently becomes localized on the end of the helix due to the defect in helical structure caused by unwinding. Our results generally do not support the existence of Davydov type solitons in biological helix systems under physiological conditions. [ABSTRACT FROM AUTHOR]

We report upon a theoretical study of singlet exciton migration and relaxation within a model conjugated polymer chain. Starting from poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene] polymer chains, we assume that the π-conjugation is disrupted by conformational disorder of the chain itself, giving rise to a localized Frenkel exciton basis. Electronic coupling between segments as determined by the coupling between the transition densities of the localized excitons gives rise to delocalized exciton states. Using a kinetic Monte Carlo approach to compute the exciton transfer kinetics within the manifold of either the dressed chromophore site basis or dressed eigenstate basis, we find that the decay of the polarization anisotropy of the exciton is profoundly affected by the delocalization of the exciton over multiple basis segments. Two time scales emerge from the exciton migration simulations: a short, roughly 10 ps, time scale corresponding to rapid hopping about the initial excitation site followed by a slower, 180 ps, component corresponding to long range hopping. We also find that excitations can become trapped at long times when the hopping rate to lower-energy states is longer than the radiative lifetime of the exciton. [ABSTRACT FROM AUTHOR]

For electron-phonon Hamiltonians with the couplings linear in the phonon operators, we construct a class of unitary transformations that separate the normal modes into two groups. The modes in the first group interact with the electronic degrees of freedom directly. The modes in the second group interact directly only with the modes in the first group but not with the electronic system. These transformations can be carried out independently for different types of phonon modes, e.g., high- versus low-frequency phonon bands. This construction generalizes recently introduced transformations for systems exhibiting a conical intersection topology. The separation of the normal modes into several groups allows one to develop new approximation schemes. We apply one of such schemes to study electronic relaxation at a semiconducting polymer interface. [ABSTRACT FROM AUTHOR]

Excited states of the double-stranded DNA model (A)12·(T)12 were calculated in the framework of the Frenkel exciton theory. The off-diagonal elements of the exciton matrix were calculated using the transition densities and ideal dipole approximation associated with the lowest energy ππ* excitations of the individual nucleobases as obtained from time-dependent density functional theory calculations. The values of the coupling calculated with the transition density cubes (TDC) and ideal dipole approximation (IDA) methods were found to be significantly different for the small interchromophore distances. It was shown that the IDA overestimates the coupling significantly. The effects of structural fluctuations of the DNA chain on the magnitude of dipolar coupling were also found to be very significant. The difference between the maximum and minimum values was as large as 1000 and 300 cm-1 for the IDA and TDC methods, respectively. To account for these effects, the properties of the excited states were averaged over a large number of conformations obtained from the molecular dynamics simulations. Our calculations using the TDC method indicate that the absorption of the UV light creates exciton states carrying the majority of the oscillator strength that are delocalized over at least six DNA bases. Upon relaxation, the excitation states localize over at least four contiguous bases. [ABSTRACT FROM AUTHOR]

A hierarchical electron-phonon coupling model is applied to describe the ultrafast decay of a photogenerated exciton at a donor-acceptor polymer heterojunction, via a vibronic coupling mechanism by which a charge-localized interfacial state is created. Expanding upon an earlier Communication [H. Tamura et al., J. Chem. Phys. 126, 021103 (2007)], we present a quantum dynamical analysis based on a two-state linear vibronic coupling model, which accounts for a two-band phonon bath including high-frequency C==C stretch modes and low-frequency ring torsional modes. Building upon this model, an analysis in terms of a hierarchical chain of effective modes is carried out, whose construction is detailed in the present paper. Truncation of this chain at the order n (i.e., 3n+3 modes) conserves the Hamiltonian moments (cumulants) up to the (2n+3)rd order. The effective-mode analysis highlights (i) the dominance of the high-frequency modes in the coupling to the electronic subsystem and (ii) the key role of the low-frequency modes in the intramolecular vibrational redistribution process that is essential in mediating the decay to the charge-localized state. Due to this dynamical interplay, the effective-mode hierarchy has to be carried beyond the first order in order to obtain a qualitatively correct picture of the nonadiabatic process. A reduced model of the dynamics, including a Markovian closure of the hierarchy, is presented. Dynamical calculations were carried out using the multiconfiguration time-dependent Hartree method. [ABSTRACT FROM AUTHOR]

The authors present here a simple analysis that explains the apparent strengthening of electron phonon interaction upon aggregation in conjugated polymer materials. The overall scheme is that of an intermolecular Herzberg-Teller effect whereby sidebands of a forbidden transition are activated by oppositely phased vibrations. The authors show that upon aggregation, the 0-0 emission becomes symmetry forbidden and the apparent redshift and remaining vibronic structure are due to sideband (0-1,0-2, etc.) emission. At higher temperatures, the 0-0 peak is due to thermal population in a higher lying even-parity vibronic state rather than direct emission from the odd-paritied lowest intermolecular vibronic state. [ABSTRACT FROM AUTHOR]

Polyfluorene-based polymer blends have been utilized in the development of optoelectronic devices. The constituent copolymers are chemically designed to facilitate more efficient electron/hole mobility, thereby enhancing control over exciton formation and dissociation. When appropriate pairs of these are blended together, intermolecular charged-particle localizations are induced, leading to significant intermolecular charge-transfer character and luminescence that exhibit some sensitivity to their interfacial orientation. The authors report on a time-dependent density functional theory quantum chemical investigation of the relevant excited states of the polymer blend poly[9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine]/poly(9,9-dioctylfluorene-co-benzothiadiazole. They show that the calculated excited states generally agree with experimental observations although there is a consistent underestimation of the charge-transfer states. Further, they show sensitivity to lateral shifts in interfacial stacking. Finally, solvation with a low dielectric solvent greatly stabilizes the charge-transfer states. [ABSTRACT FROM AUTHOR]

The quantum-dynamical mechanism of photoinduced subpicosecond exciton dissociation and the concomitant formation of a charge-separated state at a semiconducting polymer heterojunction is elucidated. The analysis is based upon a two-state vibronic coupling Hamiltonian including an explicit 24-mode representation of a phonon bath comprising high-frequency (C==C stretch) and low-frequency (torsional) modes. The initial relaxation behavior is characterized by coherent oscillations, along with the decay through an extended nonadiabatic coupling region. This region is located in the vicinity of a conical intersection hypersurface. A central ingredient of the analysis is a novel effective mode representation, which highlights the role of the low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree method. [ABSTRACT FROM AUTHOR]

We propose a lattice fermion model suitable for studying the ultrafast photoexcitation dynamics of ordered chains of deoxyribonucleic acid (DNA) polymers. The model includes both parallel (intrachain) and perpendicular (cross-chain) terms as well as diagonal cross-chain terms coupling neighboring bases. The general form of our Hamiltonian is borrowed from lattice fermion models of quantum chromodynamics. The band structure for this model can be determined analytically, and we use this as a basis for computing the singly excited states of the poly(dA)poly(dT) DNA duplex using configuration interaction singles. Parameters for the model are taken from various literature sources and our own ab initio calculations. Results indicate that the excited states consist of a low energy band of dark charge-separated states followed by separate bands of delocalized excitonic states which have weak mixing between the thymidine and adenosine sides of the DNA chain. We then propose a lattice exciton model based upon the transition dipole-dipole couplings between bases and compare the analytical results for the survival probability of an initially localized exciton to exact numerical results. The results herein underscore the competing role of excitonic and charge-transfer dynamics in these systems. [ABSTRACT FROM AUTHOR]

The hydrodynamic equations of motion associated with the de Broglie-Bohm formulation of quantum mechanics are solved using a meshless method based upon a moving least-squares approach. An arbitrary Lagrangian-Eulerian frame of reference and a regridding algorithm which adds and deletes computational points are used to maintain a uniform and nearly constant interparticle spacing. The methodology also uses averaged fields to maintain unitary time evolution. The numerical instabilities associated with the formation of nodes in the reflected portion of the wave packet are avoided by adding artificial viscosity to the equations of motion. A new and more robust artificial viscosity algorithm is presented which gives accurate scattering results and is capable of capturing quantum resonances. The methodology is applied to a one-dimensional model chemical reaction that is known to exhibit a quantum resonance. The correlation function approach is used to compute the reactive scattering matrix, reaction probability, and time delay as a function of energy. Excellent agreement is obtained between the scattering results based upon the quantum hydrodynamic approach and those based upon standard quantum mechanics. This is the first clear demonstration of the ability of moving grid approaches to accurately and robustly reproduce resonance structures in a scattering system. [ABSTRACT FROM AUTHOR]

We present a methodology for computing photocurrent production in molecular semiconducting molecules. Our model combines a single-configuration interaction picture with the nonequilibrium Green’s function approach to compute the current response of a molecular semiconducting wire following excitation. We give detailed analysis of the essential excitonic, charge transfer, and dipole states for poly-(phenylenevinylene) chains of length 32 and 48 repeat units under an electric field bias and use this to develop a reduced dimensional tunneling model which accounts for chain-length and field-dependent behavior. In this paper, we consider the decay of an excited electron/hole state on a molecular wire under bias attached to semiconducting leads at either end. We find that the current produced by the decay of an excitation depends not only upon the lifetime of the state, as given by the imaginary part of its complex eigenvalue, but also upon the net charge on terminal ends of the molecule. We also find that weakly bound electron/hole charge-transfer pairs can decay into the continuum via field induced tunneling and produce a net current whereas excitonic states decay via tunneling but give no net current contribution. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper we develop an approximate methodológy for estimating the multidimensional quantum density associated with a statistical bundle of de Broglie–Bohm trajectories. The quantum density is constructed as a discrete sum of nonequivalent Gaussian components. We incorporate the ideas of Bayesian statistical analysis and an expectation-maximization procedure to compute an approximate quantum force that drives the statistical ensemble quantum trajectories. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spin-dependent electron-hole (e - h) recombination in poly(p-phenylenevinylene) chains is modeled by the dissipative dynamics of the multilevel electronic system coupled to the phonon bath. The underlying Hamiltonian incorporates the Coulomb and exchange interactions of spin-singlet and spin-triplet monoexcitations in Wannier-orbital basis and their coupling to the prominent Franck-Condon active modes. In agreement with experiment, we obtain that the ratio of singlet versus triplet exciton formation rates is strongly conjugation-length dependent and increasing on going from the model dimer to the extended chain. The result is rationalized in terms of a cascade interconversion mechanism across the electronic levels. In parallel to the direct formation of spin-dependent excitons, e - h capture is found to generate long-lived charge-transfer states, whose further phonon-mediated relaxation to the bottom of the density of states is hindered by the near e - h symmetry of conjugated hydrocarbons. Being nearly spin independent, such states most likely form an intersystem crossing pre-equilibrium, from which the singlet e - h binding channel is about ten times faster than the triplet one. [ABSTRACT FROM AUTHOR]

The neutral excitations in poly(p-phenylenevinylene) are studied in conjunction with the vibronic structure of the lowest optical transitions. Combining the configuration interaction of Wannier-localized electron–hole pairs with an empirical description of electron–phonon coupling, we obtain the potential energy surfaces of monoexcited states and the Condon electron–vibrational spectra in absorption and emission. The S[sub 1]→S[sub 0] luminescence band shape is found compatible with self-localization of S[sub 1] within about 10 monomers, driven exclusively by electron–phonon coupling. The singlet and triplet polaron–excitons are exchange–split by about 1 eV and differ substantially in terms of average electron–hole separation. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Structures of poly-(p-phenylene vinylene) (PPV) were generated using a random growth algorithm. The algorithm assumes that the configuration of a part of the polymer (a few monomers length) can be sampled from the configuration of a PPV trimer. The probabilities of the configurations of the trimer are taken as the Boltzmann weight of the energies. We constructed several types of polymers with different numbers of cis-defects which were added to the polymer either uniformly or randomly distributed within the entire polymer. Polymer characteristics, such as conjugation length, end-to-end distance, and radius of gyration, were also calculated. The trends of these characteristics were found to be inversely proportional with the number of cis-defects in the polymer. Although average conjugation lengths are generally independent of the distribution of cis-defects, the morphology of the polymer is dependent on cis-defect distribution. This suggests that conformational disorder rather than cis-defect density is the determining factor in exciton localization and diffusion in these systems. Finally, we derive a simple model similar to the Ising model that relates the energy needed to break conjugation to the average conjugation length. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Evaluates the use of Condon approach to calculate the excitation and emission band shapes for the electronic transitions. Increase of vibronic progression with oligomeric; Conjugation of oligomers; Obtainment of spectral profiles as a superposition of Lorentzian line shapes.

A variational quantum hydrodynamic approach is used for examining the structure and dynamics of [(Ne).sub.n] clusters at both T = 0K and for temperatures spanning the solid-to-liquid transition in bulk Ne. The results have predicted the negative heat capacity for very small clusters that are attributed to the nonadditive nature of the total free-energy for very small systems.

A theoretical investigation on various semiconducting materials that exhibit photovoltaic and photoluminescent properties is presented. Focus is laid on the relaxation dynamics that occur upon photoexciton of a couple of type II donor-acceptor heterojunction systems.

A theoretical treatment is provided, wherein the spontaneous emission rates for the singlet manifold is included and the competition between radiative processes is studied. It is also shown that the formation of the emissive singlet exciton is accelerated in a regular acceptor - donor copolymer because of the removal of the charge transfer states.

Describes a method to do molecular dynamics in an electronically excited state surface in physical chemistry. Observation of the method within the random phase approximation (RPA); Use of a dynamical variational treatment of the RPA frequency; Derivation of coupled equations of motion for the RPA amplitudes.

In this paper, we explore in detail the way in which quantum decoherence is treated in different mixed quantum-classical molecular dynamics algorithms. The quantum decoherence time proves to be a key ingredient in the production of accurate nonadiabatic dynamics from computer simulations. Based on a short time expansion to a semiclassical golden rule expression due to Neria and Nitzan [J. Chem. Phys. 99, 1109 (1993)], we develop a new computationally efficient method for estimating the decay of quantum coherence in condensed phase molecular simulations. Using the hydrated electron as an example, application of this method finds that quantum decoherence times are on the order of a few femtoseconds for condensed phase chemical systems and that they play a direct role in determining nonadiabatic transition rates. The decay of quantum coherence for the solvated electron is found to take ≊50% longer in D2O than in H2O, providing a rationalization for a long standing puzzle concerning the lack of experimentally observed isotope effect on the nonadiabatic transition rate: Although the nonadiabatic coupling is smaller in D2O due to smaller nuclear velocities, the smaller coupling in D2O adds coherently for a longer time than in H2O, leading to nearly identical nonadiabatic transition rates. The implications of this isotope dependence of the nonadiabatic transition rate on changes in the quantum decoherence time for electron transfer and other important chemical reactions are discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We address the issue of quantum decoherence in mixed quantum-classical simulations. We demonstrate that restricting the classical paths to a single path among all the quantum paths affects a coarse graining of the quantum paths. Such coarse graining causes the quantum paths to lose coherence as the various possible classical paths associated with each quantum state diverge. This defines a reduction mapping of the quantum density matrix, and we derive a quantum master equation suitable for mixed quantum-classical systems. The equation includes two terms: first, the ordinary quantum Liouvillian which is parametrized by a single classical path, and second, a quantum decoherence term that includes both a coherence time and length scale which are determined by the dynamics of the classical paths. Model calculations for electronic coherence loss in nonadiabatic mixed quantum-classical dynamics are presented as examples. For a model charge transfer chemical reaction with nonadiabatic transitions, application of the present formulation reveals that nonadiabaticity is diminished as the decoherence timescale becomes shorter and adiabatic dynamics are recovered in the limit of rapid decoherence. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The role of shape resonances and many-body effects on universal quantum sticking of ultracold atoms onto solid surfaces is examined analytically and computationally using an exactly solvable representation of the Dyson equation. We derive the self-energy renormalization of the transition amplitude between an ultracold scattering atom and the bound states on the surface in order to elucidate the role of virtual phonon exchanges in the limiting behavior of the sticking probability. We demonstrate that, to first order in the interactions for finite ranged atom–surface potentials, virtual phonons can only rescale the strength of the atom–surface coupling and do not rescale the range of the coupling. Thus, universal sticking behavior at ultralow energies is to be expected for all finite ranged potentials. We demonstrate that the onset of the universal sticking behavior depends greatly on the position of the shape resonance of the renormalized potential and for sufficiently low energy shape resonances, deviations from the universal s(E)∝[Square_Root]E can occur near these energies. We believe that this accounts for many of the low energy sticking trends observed in the scattering of submillikelvin H atoms from superfluid 4He films. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

He scattering from Xe monolayers adsorbed onto Ag substrates is studied using a quantum stochastic method. Both the He atom and the surface and bulk modes are treated quantum mechanically using a mean-field partitioning of the quantum Liouville equation. The resulting equations of motion permit the evaluation of the reduced density matrix of the He atom subsystem by averaging over a series of ‘‘quantum trajectories.’’ The final reduced density matrix of the atomic subsystem, when projected onto the asymptotic states, provides an accurate estimate of energy transfer processes between the atom and the surface. Our calculations indicate that inelastic energy transfer is enhanced near adsorption resonances due primarily to the increased lifetime of the atom near the surfaces as evidenced by marked deviations from the Debye–Waller thermal attenuation rates. [ABSTRACT FROM AUTHOR]

The zero energy scattering limit of inelastic molecule–surface scattering is studied within the context of a multiphonon expansion of the molecule–bath wave function. By assuming that at low scattering energies the expansion may be truncated at first order in the phonon operators, we derived a closed form solution to the Lippmann Schwinger equation for the scattering wave function which includes a nonlocal and energy dependent self-energy term which correctly incorporates virtual phonon transitions in the elastic channel. The closure relation results from the use of a discrete spectral (L2) form of the inelastic channel Greens functions. We compute the zero energy limit of these wave functions and discuss the trapping and reflection of cold atoms from ultracold surfaces. Our results indicate that for realistic atom surface interactions the low energy limit of the sticking coefficient, s, can deviate markedly from the expected s∝E1/2 behavior and is shown to approach a constant nonzero limiting value. This trend is consistent with recent experimental work involving the sticking of spin polarized hydrogen atoms on liquid He films. [ABSTRACT FROM AUTHOR]

We present a general, fully quantum mechanical theory for molecule surface scattering at finite temperature within the time dependent Hartree (TDH) factorization. We show the formal manipulations which reduce the total molecule–surface–bath Schrödinger equation into a form which is computationally convenient to use. Under the TDH factorization, the molecular portion of the wavefunction evolves according to a mean-field Hamiltonian which is dependent upon both time and temperature. The temporal and thermal dependence is due to stochastic and dissipative terms that appear in the Heisenberg equations of motion for the phonon operators upon averaging over the bath states. The resulting equations of motion are solved in one dimension self consistently using quantum wavepackets and the discrete variable representation. We compute energy transfer to the phonons as a function of surface temperature and initial energy and compare our results to results obtained using other mean-field models, namely an averaged mean-field model and a fully quantum model based upon a dissipative form of the quantum Liouville equation. It appears that the model presented here provides a better estimation of energy transfer between the molecule and the surface. [ABSTRACT FROM AUTHOR]