Energy Transfer In Molecular Systems Research Articles

Simulating Energy Transfer in Molecular Systems with Digital Quantum Computers.

Quantum computers have the potential to simulate chemical systems beyond the capability of classical computers. Recent developments in hybrid quantum-classical approaches enable the determinations of the ground or low energy states of molecular systems. Here, we extend near-term quantum simulations of chemistry to time-dependent processes by simulating energy transfer in organic semiconducting molecules. We developed a multiscale modeling workflow that combines conventional molecular dynamics and quantum chemistry simulations with hybrid variational quantum algorithm to compute the exciton dynamics in both the single excitation subspace (i.e., Frenkel Hamiltonian) and the full-Hilbert space (i.e., multiexciton) regimes. Our numerical examples demonstrate the feasibility of our approach, and simulations on IBM Q devices capture the qualitative behaviors of exciton dynamics, but with considerable errors. We present an error mitigation technique that combines experimental results from the variational and Trotter algorithms, and obtain significantly improved quantum dynamics. Our approach opens up new opportunities for modeling quantum dynamics in chemical, biological, and material systems with quantum computers.

Plasmon-activated Forster energy transfer in molecular systems

To explain the experimentally observed effect of silver nanoparticles on the fluorescence of organic dyes and the nonradiative intermolecular transfer of electronic excitation energy in multilayer nanostructures, the previously proposed theoretical model of plasmon resonance in spherical nanoparticles of metals was used. The rates of radiative and nonradiative (FRET) processes in film structures with Ag nanoparticles were calculated for fluorescein and rhodamine B molecules, as well as for two-component systems fluorescein-nile red (NR) and rhodamine B-NR. A version of the model was used that takes into account the effect of NPs on FRET between molecules, the radiative decay of donor and acceptor molecules, and the energy transfer from the dye to plasmonic nanoparticles. The calculation of the UDA rate for pairs with different energy transfer efficiency showed a greater increase in the UDA parameter for the fluorescein-nile red pair than for the rhodamine B-nile red pair. Estimation of the fluorescence enhancement factor of donor and energy acceptor molecules and the rate of energy transfer from the dye to silver NPs showed their insignificant contribution to the formation of the resulting energy transfer efficiency enhancement in the presence of plasmonic NPs. Keywords: energy transfer, silver nanoparticles, plasmon, model

Quantum dynamics of vibrational energy flow in oscillator chains driven by anharmonic interactions

A new model of vibrational energy transfer in molecular systems taking into account anharmonic (third order) interactions of localized vibrations with a chain of harmonic oscillators is developed. The role of the energy spectrum of the chain and of the magnitude of the non-linear coupling is discussed in detail by an exact numerical solution of the quantum dynamical problem based on the tensor-train (matrix product state) representation of the vibrational wave function. Results show that the type of wave packet motion is determined by the eigen-spectrum of the chain and by its excitation time. It is found that when the excitation of the chain takes place on a much shorter timescale than the energy transfer along the chain the vibrational wave packet moves in a ballistic way independently of the length of the chain. On the other hand when the excitation of the chain takes place on the timescale of the energy transfer along the chain the overall motion becomes superballistic. These findings shed new light on recent observations of ballistic energy transfer along polymethylene chains.

Ultrafast exciton dynamics in one- and two-dimensional para -sexiphenyl clusters

Laser-pulse-induced ultrafast electronic excitation energy transfer in molecular systems is studied theoretically. As a particular excitation channel, we consider the simultaneous transition of different molecules into their first excited state. Accordingly, a restricted picture of the total nonlinear response of the molecular system on laser pulse excitation is obtained, but complete insight is gained on the contribution of the first excited states. To describe the temporal evolution of this multiexciton state, a direct computation of the related density matrix is circumvented. Instead, we derive equations of motion for expectation values of interest in using the quantum master equation governing the respective density operator. The subsequently generated hierarchy of kinetic equations is truncated following a general scheme. The whole approach is applied to a chain and a disk of para-sexiphenyl molecules. Different signatures of multiexciton effects are discussed.

Calculation of coherences in Förster and modified Redfield theories of excitation energy transfer.

Förster and modified Redfield theories play one of the central roles in the description of excitation energy transfer in molecular systems. However, in the present state, these theories describe only the dynamics of populations of local electronic excitations or delocalized exciton eigenstates, respectively, i.e., the diagonal elements of the density matrix in the corresponding representation. They do not give prescription for propagating the off-diagonal elements of the density matrix (coherences). This is commonly accepted as a limitation of these theories. Here, we derive formulas for the dynamics of the coherences in the framework of Förster and modified Redfield theories and, thus, remove this limitation. For the dimer case, these formulas provide excellent correspondence with numerically exact calculations according to the hierarchical equations of motion.

Blow-up for Semidiscretisations of a Semilinear Schrodinger Equation with Dirichlet Condition

Theoretical study of the phenomenon of blow-up solutions for semilinear Schrödinger equations has been the subject of investigations of many authors. It is said that the maximal time interval of existence of the solution blows up in a finite time when this time is finite, and the solution develops a singularity in a finite time. In fact, semilinear Schrhödinger equation models a lot of physical phenomenon such as nonlinear optics, energy transfer in molecular systems, quantum mechanics, seismology, plasma physics. In the past, certain authors have used numerical methods to study the phenomenon of blow-up for semilinear Schrödinger equations. They have considered the same problem and one proves that the energy of the system is conserved, and the method used to show blow-up solutions are based on the energy's method. This paper proposes a method based on a modification of the method of Kaplan using eigenvalues and eigenfunctions to show that the semidiscrete solution blows up in a finite time under some assumptions. The semidiscrete blow-up time is also estimate. Similar results are obtain replacing the reaction term by another form to generalise the result. Finally, this paper propose two schemes for some numerical experiments and a graphics is given to illustrate the analysis.

ChemInform Abstract: Design and Properties of Porphyrin‐Based Singlet Oxygen Generator

AbstractReview: 56 refs.

Design and Properties of Porphyrin‐based Singlet Oxygen Generator

AbstractIn this review, we address recent studies of porphyrin‐based molecular systems for efficient singlet oxygen (1O2) generation. Porphyrins have a highly conjugated, delocalized 18‐π electron system (for the shortest cyclic path) that is responsible for the strong absorption and emission characteristics in the visible region. Singlet oxygen is useful for applications in photodynamic cancer therapy (PDT), photooxidation of toxic molecules, and photoproduction of important intermediates for various chemicals owing to its high oxidation ability. Porphyrin and its analogues have been investigated as photosensitizers for 1O2 generation. The production of 1O2 is successfully regulated by photophysical parameters, such as intersystem crossing, triplet state (T1) lifetime, and photosensitizer to 3O2 energy transfer in molecular systems. Introduction of substituents with heavy atoms (halogen, metal) and radicals onto the photosensitizer (PS) exerts a strongly positive impact on intersystem crossing of the singlet excited state of the PS, promoting generation of the triplet excited state. Environmental conditions, such as solvent and pH, may also influence 1O2 generation. As a means to limit cellular photodamage, two‐photon absorption and DNA switches for 1O2 generation have been proposed and developed. Achieving control of singlet oxygen generation is pertinent to many arenas at the cutting edge of modern science, ranging from the chemical industry to biomedical applications.

A Novel 2,6-Bis(8’-quinolinyl)pyridine Rhenium Compound

Light-induced electron and energy transfer in molecular systems is being extensively studied in view of artificial photosynthesis [1-3], molecular electronics [4] and biomedical applications. [5] An ideal photosensitizer should absorb light across a significant portion of the visible spectrum and possess an appropriately aligned excited state energy with sufficiently long lifetime to promote the desired electron transfer process. In this context, complexes of Re(I) containing polypyridine or diimine donors, usually with three carbon monoxide co-ligands, continue to be of great interest. These complexes have been well-investigated because of their intense visible emission properties, capability as building blocks for supramolecular systems, and catalytic activities. [6-8] However, photoinduced CO ligand dissociation, lack of significant visible light absorption, and the inherent potential chirality of polypyridine donors hinders the overall effectiveness of these systems. Previously, we prepared a homologous series of meridionallycoordinated tridentate terpyridine Re(I) complexes containing two carbon monoxide co-ligands. [9,10] The complexes all absorb light throughout a significant portion of the visible spectrum, and several of them absorb light throughout the entire visible A Novel 2,6-Bis(8’-quinolinyl)pyridine Rhenium Compound

Coherent versus incoherent excitation energy transfer in molecular systems

We investigate the Markovian limit of a polaronic quantum master equation for coherent resonance energy transfer proposed recently by Jang et al. [J. Chem. Phys. 129, 101104 (2008)]. An expression for the rate of excitation energy transfer (EET) is derived and shown to exhibit both coherent and incoherent contributions. We then apply this theory to calculated EET rates for model dimer systems, and demonstrate that the small-polaron approach predicts a variety of dynamical behaviors. Notably, the results indicate that the EET dynamical behaviors can be understood by the interplay between noise-assisted EET and dynamical localization, while both are well captured by the polaron theory. Finally, we investigate bath correlation effects on the rate of EET and show that bath correlations (or anti-correlations) can either enhance or suppress EET rate depending on the strength of individual system-bath couplings. In summary, we introduce the small-polaron approach as an intuitive physical framework to consolidate our understanding of EET dynamics in the condensed phase.

Energy transfer in molecular systems at the surface of metal solids and nanoparticles

Nonradiative transfer of electronic excitation energy between molecules at a flat and curved conductor surface was studied. It was shown that the effective mechanism of energy transfer in a such a system can involve surface plasmons. The type of the distance dependence and the characteristics of anisotropy of the energy transfer rate in a donor-acceptor pair of adsorbates were established. Comparative estimates of the efficiency of direct dipole-dipole energy transfer and the plasmon pathway of transfer were made. It was found that the contribution of the plasmon mechanism to the total rate of energy transfer dominates (exceeds by one to two orders of magnitude the rate of transfer in the absence of conducting bodies from the system) when the molecules occur close to the metal surface.

Beyond the Förster theory of excitation energy transfer: importance of higher-order processes in supramolecular antenna systems

Electronic excitation energy transfer in molecular systems is connected with the de-excitation of one molecule and the excitation of the other. Mostly, it can be understood in terms of Förster (or fluorescence) resonance energy transfer. An increasing interest in the optimization of artificial light harvesting systems, however, requires a more detailed study going beyond the standard Förster scheme. There are two main routes to do this. Considering a Coulombic less-strongly coupled system first, coherences among different chromophores may be considered in the framework of perturbation theory with higher-order mechanisms correcting the standard second-order description. Secondly, if inter-chromophore coherences are dominant and delocalized Frenkel-exciton states are formed, it becomes of some importance to study their decay due to the coupling to vibrational degrees of freedom. While we will also comment on this latter mechanism, the first description based on localized excitations will be the main focus. A general higher-order theory resulting in respective transition rates and rate equations is utilized. Its capability is demonstrated when presenting a systematic description of short-range and long-range corrections to the basic Förster mechanism. Accordingly, a unique description of bridge-mediated and photon-mediated long-range electronic excitation energy transfer is offered. Moreover, short-range excitation energy transfer appearing as a two electron exchange is also discussed. And, the exciton-exciton annihilation process present at higher optical excitation intensities is described as a direct higher-order transition. The related higher-order vibrational correlation functions are presented and estimated for the reference case where the coupling to vibrational degrees of freedom either of intra-molecular or inter-molecular type is reduced to a simple electronic state dephasing process.

Long-range resonance energy transfer in molecular systems.

The current state of understanding of molecular resonance energy transfer (RET) and recent developments in the field are reviewed. The development of more general theoretical approaches has uncovered some new principles underlying RET processes. This review brings many of these important new concepts together into a generalization of Förster's original theory. The conclusions of studies investigating the various approximations in Förster theory are summarized. Areas of present and future activity are discussed. The review covers Förster theory for donor-acceptor pairs and electronic coupling for singlet-singlet, triplet-triplet, and superexchange-mediated energy transfer. This includes the transition density picture of Coulombic coupling as well as electronic coupling between molecular aggregates (excitons). Spectral overlaps and ensemble energy transfer rates in disordered aggregates, the role of dielectric properties of the medium, weak versus strong coupling, and new models for energy transfer in complex molecular assemblies are also described.

On the problem of directed signal transmission and energy transfer in molecular systems

A new physical interpretation of the mechanism of directed energy transfer and signal transmission in molecular systems along one-dimensional chains capable of successive isomerization is proposed. The photoacceptor—transmitting chain—reaction center system is considered as a model. Conditions under which the directed energy transfer occurs from the photoacceptor to the reaction center along the chain are analyzed. The time of signal transmission depends on the chain length, and the transmission coefficient is mainly determined by the characteristics of the initial and final elements in the whole system.

Potential Parameters and Energy Dependence of Rotational Energy Transfer in Molecular Systems

The cross-sections for rotational transitions in a diatomic molecule due to collision with an atom were computed. An empirical relation is obtained that gives the dependence of cross-sections on the energy of the colliding system and the parameters of the interaction potential energy surface. The results are interpreted to investigate the mechanism of the rotational energy transfer process. The cross-sections are found to depend linearly on the torque, range of the potential, and the collision time. Depending on the variation in the location of classical turning point with change in energy, the cross-sections may increase or decrease with energy or cross-section versus energy curve may exhibit maxima or minima. PACS numbers: 35.20.-i

Classical Aspects of Energy Transfer in Molecular Systems

The decay time of the luminescence of a molecule S in front of a metal mirror depends markedly on its distance from the mirror. This phenomenon is quantitatively explained by considering the radiation field of this dipole, given by Hertz classical equation. This field arrives at the molecule, after being reflected at the mirror, with a retardation of the order of 10−15 sec. The decay time of the luminescence depends on the phase shift produced by this retardation, and thus on the ratio of the distance of the oscillator from the mirror, and the wavelength of the emitted light. By measuring the distance dependence of the decay time of the luminescence this retardation effect can be studied. In quantum-mechanical terms the phenomenon can be described as being due to a stimulation or inhibition of the emission of the light quantum. In contrast to the known cases of stimulated emission, the stimulating field is the radiation field of the emitter quantum itself. The energy transfer from an excited molecule S to an acceptor A can be treated in a similar manner by considering the phenomenon as a retardation effect. In classical terms the field of S induces A to oscillate, and the induced field of A arriving at S slows down this oscillator. Simple equations are given for the energy transfer from an excited dipole or quadrupole, and for a row of many dipoles, oscillating in phase, to a weakly absorbing acceptor layer. The latter case is considered as a model for a J-aggregating dye and by comparison with experimental data conclusions concerning the size of a J aggregate are drawn.