Dynamics Of Excitation Transfer Research Articles

Tunability in 3-Site Electronic Excitation Transfer Dynamics: Insights into the Role of Perturbative Coupling.

Electronic excitation transfer dynamics in photosynthetic systems, including the Fenna-Matthews-Olson complex, are often modeled using the interaction picture of three two-level systems, also known as the 3-site system. Among the two possible configurations, uphill and downhill, a recent publication reported an intriguing correlation between population dynamics and the intersite coupling. Specifically, the uphill configuration has been shown to have a pronounced dependence on perturbations in the intersite coupling, whereas the downhill configuration displays negligible dependence. In this study, we consider a generic 3-site configuration and model site interactions through the Markovian master equation. Through this approach, we derive succinct analytical expressions for the population dynamics between the sites, shedding light on the differences in behavior between the two configurations. Using these analytical expressions, we demonstrate the range of tunability in population dynamics achievable with minimal changes in intersite coupling, and we validate these findings through simulation results. These insights into the population dynamics of a 3-site system are expected to play a crucial role in facilitating the design of efficient energy-transfer pathways in molecular systems.

Experimental and Theoretical Mutation of Exciton States on the Smallest Type-I Photosynthetic Reaction Center Complex of a Green Sulfur Bacterium Chlorobaclum tepidum.

The exciton states on the smallest type-I photosynthetic reaction center complex of a green sulfur bacterium Chlorobaculum tepidum (GsbRC) consisting of 26 bacteriochlorophylls a (BChl a) and four chlorophylls a (Chl a) located on the homodimer of two PscA reaction center polypeptides were investigated. This analysis involved the study of exciton states through a combination of theoretical modeling and the genetic removal of BChl a pigments at eight sites. (1) A theoretical model of the pigment assembly exciton state on GsbRC was constructed using Poisson TrESP (P-TrESP) and charge density coupling (CDC) methods based on structural information. The model reproduced the experimentally obtained absorption spectrum, circular dichroism spectrum, and excitation transfer dynamics, as well as explained the effects of mutation. (2) Eight BChl a molecules at different locations on the GsbRC were selectively removed by genetic exchange of the His residue, which ligates the central Mg atom of BChl a, with the Leu residue on either one or two PscAs in the RC. His locations are conserved among all type-I RC plant polypeptide, cyanobacteria, and bacteria amino acid sequences. (3) Purified mutant-GsbRCs demonstrated distinct absorption and fluorescence spectra at 77 K, which were different from each other, suggesting successful pigment removal. (4) The same mutations were applied to the constructed theoretical model to analyze the outcomes of these mutations. (5) The combination of theoretical predictions and experimental mutations based on structural information is a new tool for studying the function and evolution of photosynthetic reaction centers.

Relaxation dynamics of nonresonant excitation transfer processes assisted by coherent phonon environment

In this study, the steepest-entropy-ascent quantum thermodynamics (SEAQT) framework is used to investigate the excitation transfer (ET) dynamics of two-level nanosystems (TLSs), focusing on non-resonant processes that involve a dynamic local phonon system. In contrast to other methods based on Markovian or non-Markovian quantum master equations, SEAQT analysis always guarantees the positivity of the density operators, thus enabling the discussion of both transient and long-term dynamics of nanosystems. The findings of this study demonstrate that the relaxation time of coherent phonons, relative to the Rabi oscillation period in two TLSs, significantly affects the relaxation process of non-resonant ETs. Moreover, the degree of mutual synchronization between ET dynamics of TLSs and local (coherent) phonons can either prolong or shorten the decoherence time, presenting a way to control the coherence in nanosystems and stimulate quantum device applications.

Spatio-temporal description of the origin of optical-near-field interactions

Optical-near-field interactions occur via virtual photons that are non-resonant with the nanomaterials. Since the virtual photon exists only on a timescale shorter than the timescale of the interaction between the propagating light and the nanomaterial, the optical-near-field interactions exhibit non-Markovian temporal properties. We propose the spatio-temporal description of excitation transfer dynamics via virtual photons, and clearly show that the spatial properties of optical-near-field interactions at a short distance originate from the non-Markovian temporal properties on short timescale. Thus, the origin of optical-near-field interactions can be understood using the spatio-temporal description of the non-Markovian dynamics.

Energy transfer times in fluorographene-based biomimetic light harvesting antennae

It has been demonstrated earlier that graphene-like defects on fluorographene can act as molecules in biomimetic molecular light-harvesting antennae. In competition with radiative and non-radiative losses, transfer time of excitations in an antenna measures its performance. We report on the optimal conditions for excitation energy transfer in artificial antennae built from selected types of fluorographene defects. The excitation transfer dynamics is calculated based on the Frenkel exciton model using hierarchical equations of motion for different values of temperature, system-environment reorganization energy and bath correlation time to study a possible range of parameters pertaining to the fluorographene material. We also study possible energy funnelling in the third dimension for two parallel fluorographene sheets with defects. We conclude that the strength of system-environment interaction is more important for the efficient energy funnelling in our proposed material than a precise control over the structure of artificial antennae.

Motion-Enhanced Quantum Entanglement in the Dynamics of Excitation Transfer**Supported by the National Natural Science Foundation of China under Grant Nos 11374085, 61073048 and 11274010, the Specialized Research Fund for the Doctoral Program of Higher Education under Grant No 20113401110002, the 211 Project of Anhui University, the Anhui Provincial Natural Science Foundation under Grant No 1408085MA20, the Personnel Department of Anhui

We investigate the dynamics of entanglement in the excitation transfer through a model consisting of three interacting molecules coupled to environments. It is shown that the entanglement can be further enhanced if the distance between the molecules is oscillating. Our results demonstrate that the motional effect plays a constructive role on quantum entanglement in the dynamics of excitation transfer. This mechanism might provide a useful guideline for designing artificial systems to battle against decoherence.

Environmental Coupling and Population Dynamics in the PE545 Light-Harvesting Complex

Abstract Long-lived quantum coherences have been shown experimentally in the Fenna–Matthews–Olson (FMO) complex of green sulfur bacteria as well as in the phycoerythrin 545 (PE545) photosynthetic antenna system of marine algae. A combination of classical molecular dynamics simulations, quantum chemistry and quantum dynamical calculations is employed to determine the excitation transfer dynamics in PE545. One key property of the light-harvesting system concerning the excitation transfer and dephasing phenomena is the spectral density. This quantity is determined from time series of the vertical excitation energies of the aggregate. In the present study we focus on the quantum dynamical simulations using the earlier QM/MM calculations as input. Employing an ensemble-averaged classical path-based wave packet dynamics, the excitation transfer dynamics between the different bilins in the PE545 complex is determined and analyzed. Furthermore, the nature of the environmental fluctuations determining the transfer dynamics is discussed.

Environmental coupling and population dynamics in the PE545 light-harvesting complex

Long-lived quantum coherences have been shown experimentally in the Fenna–Matthews–Olson (FMO) complex of green sulfur bacteria as well as in the phycoerythrin 545 (PE545) photosynthetic antenna system of marine algae. A combination of classical molecular dynamics simulations, quantum chemistry and quantum dynamical calculations is employed to determine the excitation transfer dynamics in PE545. One key property of the light-harvesting system concerning the excitation transfer and dephasing phenomena is the spectral density. This quantity is determined from time series of the vertical excitation energies of the aggregate. In the present study we focus on the quantum dynamical simulations using the earlier QM/MM calculations as input. Employing an ensemble-averaged classical path-based wave packet dynamics, the excitation transfer dynamics between the different bilins in the PE545 complex is determined and analyzed. Furthermore, the nature of the environmental fluctuations determining the transfer dynamics is discussed.

Demonstration of Controlling the Spatiotemporal Dynamics of Optical Near-Field Excitation Transfer in Y-Junction Structure Consisting of Randomly Distributed Quantum Dots

Solution searching devices that operate on the basis of controlling the spatiotemporal dynamics of excitation transfer via dressed photon interactions between quantum dots have been proposed. Long-range excitation transfer based on dressed photon interactions between randomly distributed quantum dots is considered to be effective in realizing such devices. Here, we successfully controlled the spatiotemporal dynamics of excitation transfer using a Y-junction structure consisting of randomly dispersed CdSe/ZnS core-shell quantum dots. This Y-junction structure has two “output ends” and one “tap end.” By exciting one output end with control light, we observed increased excitation transfer to the other output end via a state-filling effect. Conversely, we observed reduced excitation transfer to the output ends by irradiating the tap end with control light, due to excitation of defect levels in the tap end. These results show the possibility of controlling the optical excitation transfer dynamics between multiple quantum dots.

Excitation transfer pathways in excitonic aggregates revealed by the stochastic Schrödinger equation

We derive the stochastic Schrödinger equation for the system wave vector and use it to describe the excitation energy transfer dynamics in molecular aggregates. We suggest a quantum-measurement based method of estimating the excitation transfer time. Adequacy of the proposed approach is demonstrated by performing calculations on a model system. The theory is then applied to study the excitation transfer dynamics in a photosynthetic pigment-protein Fenna-Matthews-Olson (FMO) aggregate using both the Debye spectral density and the spectral density obtained from earlier molecular dynamics simulations containing strong vibrational high-frequency modes. The obtained results show that the excitation transfer times in the FMO system are affected by the presence of the vibrational modes; however, the transfer pathways remain the same.

Constrained dynamics of excitation transfer due to a structured nano system

Optical near-field phenomena observed in nanoscale systems were recently proposed to be applied to nanofabrication and nanophotonic devices. One of the elementary near-field processes is excitation transfer that originates from near-field photons localized in nanoscale space, which are different from propagating far-field photons. In recent experiments, excitation transfer has been controlled by semiconductor nanostructures under external fields. In this paper, we present a full-quantum-mechanical theory of near-field excitation transfer, and clarify the mechanism of effective interaction between matter and near-field photons.

Time-resolved fluorescence and fluorescence anisotropy of calix[4]arene: Elucidation of the excitation energies of various conformers

Fluorescence, fluorescence excitation, time-resolved fluorescence spectroscopic, time-dependent fluorescence anisotropy, and time-dependent (TD) DFT studies were carried out to elucidate the excitation energies, fluorescence decay kinetics, and excitation transfer dynamics of calix[4]arene. We found that TD DFT calculations help to analyze the absorption spectra by elucidating the variation of the oscillator strengths with the excitation energies of various conformers involved and to clarify their relative importance. The TD DFT calculations imply that a cone-shaped conformer plays the most important role in the absorption spectrum. Fluorescence in a methanol solvent has a lifetime of 3.11±0.1ns and the decay kinetics is not found to have particular propensity with respect to solvent polarity. When the molecules in a glass matrix at 77K were excited by short laser pulses, fluorescence anisotropy was observed. From the time-dependent fluorescence anisotropy, we derived the energy transfer rate constant and fluorescence anisotropy decay time, respectively: ket=1.7×109s-1,τh=0.59ns.

Defect induced changes on the excitation transfer dynamics in ZnS/Mn nanowires

Transients of Mn internal 3d5 luminescence in ZnS/Mn nanowires are strongly non-exponential. This non-exponential decay arises from an excitation transfer from the Mn ions to so-called killer centers, i.e., non-radiative defects in the nanostructures and is strongly related to the interplay of the characteristic length scales of the sample such as the spatial extensions, the distance between killer centers, and the distance between Mn ions. The transients of the Mn-related luminescence can be quantitatively described on the basis of a modified Förster model accounting for reduced dimensionality. Here, we confirm this modified Förster model by varying the number of killer centers systematically. Additional defects were introduced into the ZnS/Mn nanowire samples by irradiation with neon ions and by varying the Mn implantation or the annealing temperature. The temporal behavior of the internal Mn2+ (3d5) luminescence is recorded on a time scale covering almost four orders of magnitude. A correlation between defect concentration and decay behavior of the internal Mn2+ (3d5) luminescence is established and the energy transfer processes in the system of localized Mn ions and the killer centers within ZnS/Mn nanostructures is confirmed. If the excitation transfer between Mn ions and killer centers as well as migration effects between Mn ions are accounted for, and the correct effective dimensionality of the system is used in the model, one is able to describe the decay curves of ZnS/Mn nanostructures in the entire time window.

Energy transfer dynamics in an RC–LH1–PufX tubular photosynthetic membrane

Light absorption and the subsequent transfer of excitation energy are the first two steps in the photosynthetic process, carried out by protein-bound pigments, mainly bacteriochlorophylls (BChls), in photosynthetic bacteria. BChls are anchored in light-harvesting (LH) complexes, such as light-harvesting complex I (LH1), which directly associates with the reaction center (RC), forming the RC–LH1 core complex. In Rhodobacter sphaeroides, RC–LH1 core complexes contain an additional protein, PufX, and assemble into dimeric RC–LH1–PufX core complexes. In the absence of LH complex II (LH2), the former complexes can aggregate into a helically ordered tubular photosynthetic membrane. We have examined the excitation transfer dynamics in a single RC–LH1–PufX core complex dimer using the hierarchical equations of motion for dissipative quantum dynamics that accurately, yet in a computationally costly manner, treat the coupling between BChls and their protein environment. A widely employed description, the generalized Förster (GF) theory, was also used to calculate the transfer rates of the same excitonic system in order to verify the accuracy of this computationally cheap method. Additionally, in light of the structural uncertainties in the Rba. sphaeroides RC–LH1–PufX core complex, geometrical alterations were introduced into the BChl organization. It is shown that the energy transfer dynamics are not affected by the considered changes in the BChl organization and that the GF theory provides accurate transfer rates. An all-atom model for a tubular photosynthetic membrane is then constructed on the basis of electron microscopy data, and the overall energy transfer properties of this membrane are computed.

Fluorescent resonant excitation energy transfer in linear polyenes

We have studied the dynamics of excitation transfer between two conjugated polyene molecules whose intermolecular separation is comparable to the molecular dimensions. We have employed a correlated electron model that includes both the charge-charge, charge-bond, and bond-bond intermolecular electron repulsion integrals. We have shown that the excitation transfer rate varies as inverse square of donor-acceptor separation R(-2) rather than as R(-6), suggested by the Forster type of dipolar approximation. Our time-evolution study also shows that the orientational dependence on excitation transfer at a fixed short donor-acceptor separation cannot be explained by Forster type of dipolar approximation beyond a certain orientational angle of rotation of an acceptor polyene with respect to the donor polyene. The actual excitation transfer rate beyond a certain orientational angle is faster than the Forster type of dipolar approximation rate. We have also studied the excitation transfer process in a pair of push-pull polyenes for different push-pull strengths. We have seen that, depending on the push-pull strength, excitation transfer could occur to other dipole coupled states. Our study also allows for the excitation energy transfer to optically dark states which are excluded by Forster theory since the one-photon transition intensity to these states (from the ground state) is zero.

Shared-mode assisted resonant energy transfer in the weak coupling regime

Recent work has suggested that correlations in the environments of chromophores can lead to a change in the dynamics of excitation transfer in both the coherent and incoherent limits. An example of this effect that is relevant to many single molecule experiments occurs in the standard Forster model for resonant energy transfer (RET). The standard formula for the FRET rate breaks down when the electronic excitations on weakly interacting donor and acceptor couple to the same vibrational modes. The transfer rate can then no longer be factored into donor emission and acceptor absorption lineshapes, but must be recast in terms of a renormalized phonon reorganization energy accounting for the magnitude and sign of the excitation-vibration couplings. In this paper, we derive theoretically how the FRET rate depends on the shared mode structure and coupling, examine the simplified case of Gaussian lineshapes and then provide a quantitative calculation for a system of current interest.

Dimensionality-dependent energy transfer in polymer-intercalatedSnS2nanocomposites

We have investigated the influence of dimensionality on the excitation-transfer dynamics in a conjugated polymer blend. Using time-resolved photoluminescence spectroscopy, we have measured the transfer transients for both a three-dimensional blend film and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of an inorganic SnS2 matrix. We compare the experimental data with a simple, dimensionality- dependent model based on electronic coupling between electronic transition moments taken to be point dipoles. Within this approximation, the energy-transfer dynamics is found to adopt a three-dimensional character in the solid film and a two-dimensional nature in the monolayers present in the SnS2-polymer nanocomposite.

Dynamics of Excitation Transfer Inside InAs/GaAs Quantum Dot System

We present time-resolved photoluminescence investigations of InAs/GaAs structures containing quantum dots with the ground state at 1.43 eV. State fllling efiect and a Pauli blocking efiect were clearly observed. These efiects signiflcantly in∞uenced dynamics of excitation transfer from upper to lower state inside a dot leading to non-exponential dynamics. Numerical model based on nonlinear rate equations was proposed. The model described well the experimental data providing values of: lifetime of the ground state 0:53 § 0:03 ns, lifetime of excited state (when the ground state is full) 1:1 § 0:2 ns, and internal relaxation time (when the ground state is empty) 0:07 § 0:01 ns.

β-Carotene to Chlorophyll Singlet Energy Transfer in the Photosystem I Core of Synechococcus elongatus Proceeds via the β-Carotene S2 and S1 States

In this work, we describe the ultrafast excitation transfer dynamics of β-carotene in trimeric photosystem I preparations from Synechococcus elongatus. β-Carotene was excited with a femtosecond laser, and the changes in absorption were followed in the spectral region 490−740 nm. The lifetime of the β-carotene S2 (1Bu+) states is ∼60 fs, significantly shorter than that in nonpolar solution (150 fs), demonstrating efficient energy transfer to the chlorophylls from these states. About 70% of the remaining population of β-carotene S1 (2Ag-) states is not in contact with the chlorophylls (lifetime 10 ps), whereas ∼30% transfers its energy efficiently to chlorophylls (lifetime 3 ps). We ascribe the S1 transfer to a fraction of the β-carotenes (possibly the ones adopting a cis configuration) that exhibit favorable π−π stacking with nearby chlorophylls. We estimate that ∼70% of initial β-carotene S2 excitations is transferred to the chlorophylls, namely, ∼60% from S2 on a 60 fs time scale and ∼10% from S1 on a 3 ...

Excitation Transfer in Aggregated and Linearly Confined Poly(p-phenylene vinylene) Chains

We simulate the nonradiative excitation energy transfer in an ensemble of poly(p-phenylene vinylene) (PPV) chains the configurations of which were calculated using a random growth algorithm. The polymers are viewed as a series of phenyl−vinyl oligomers of various lengths, separated from one another through conformational defects. Initially excited chromophores, corresponding to short segments of PPV oligomers in the polymer, transfer excitations nonradiatively to longer oligomers of lower electronic energy with transfer rates as described by Förster theory. Transfer occurs in a short period of time (<50 ps) and within a small local area (∼52 Å) around the initially excited chromophore. We find in our simulations that the majority of the excitation gets trapped in local minima corresponding to medium length oligomers (with six to eight phenyl rings) and does not eventually find the global energy minimum. Excitation transfer dynamics are presented for PPV chains inserted into mesoporous silica matrixes, which are believed to direct the excitation energy from the parts of the chains outside the pores to those inside the pores. We show that this funneling is most efficient if the chains outside the pores are close enough to the target chromophores inside the pores, within a distance defined by an average transfer correlation length.