Excitation Energy Transfer Rate Research Articles

How does the β-ionone effect the spectra of chlorophyll and the excitation energy transfer in solution

This paper presents the excitation spectra and fluorescence spectra of mixed solution containing chlorophyll a(Chla), chlorophyll b(Chlb) and β-ionone of different concentration, investigates the spectral changes of Chla and Chlb caused by β-ionone through optimizing the frontier orbital of Chl-β-ionone compound, and calculates the energy transfer rate between Chla and Chlb by means of the fluorescence Gauss decomposition. The results show that the β-ionone interacts with Chl mainly via the connection between the magnesium atom in Chl molecule and the carbonyl in β-ionone molecule. The formation of Chl-β-ionone compound changes the charge distribution of Chl so greatly that it leads to the red-shift of the excitation spectra and blue-shift of the fluorescence spectra of Chls. Also in the compound the electronic energy located on carbon double bond of β-ionone can be transferred to Chl molecule by the Mg…O bond, which enhances the fluorescence intensity greatly. The calculation of the excitation energy transfer (EET) rate indicates that a small amount of β-ionone can promote the EET between Chla and Chlb.

Excitation Energy Transfer within Covalent Tetrahedral Perylenediimide Tetramers and Their Intermolecular Aggregates

Perylenediimides (PDIs) offer a number of attractive characteristics as alternatives to fullerenes in organic photovoltaics (OPVs), including favorable orbital energetics, high extinction coefficients in the visible spectral region, photostability, and the capacity to self-assemble into ordered nanostructures. However, energy transfer followed by charge separation in PDI assemblies must kinetically out-compete excimer formation that limits OPV performance. We report on the excitation energy transfer (EET) rate in a covalently linked PDI tetramer in which the PDI chromophores are arranged in a tetrahedral geometry about a tetraphenyladamantane core. Transient absorption spectroscopy of the tetramer in CH2Cl2 reveals a laser intensity-dependent fast absorption decay component indicative of singlet–singlet annihilation resulting from intramolecular EET. Femtosecond fluorescence anisotropy measurements show that the EET time constant τ = 6 ps, which is similar to that predicted for a through-space Förster EET mechanism. Concentration-dependent steady-state spectroscopic studies reveal the formation of intermolecular aggregates of the tetramers in toluene. The aggregates are formed by cofacial π-stacking interactions between PDIs of neighboring tetramers. Transient absorption spectra of the aggregated tetramers in toluene solution demonstrate long-lived excited-state decay dynamics (τ ∼ 30 ns) in agreement with previous observations of PDI excimers.

Modeling of Various Optical Spectra in the Presence of Slow Excitation Energy Transfer in Dimers and Trimers with Weak Interpigment Coupling: FMO as an Example

We present an improved simulation methodology to describe nonphotochemical hole-burned (NPHB) spectra. The model, which includes both frequency-dependent excitation energy transfer (EET) rate distributions and burning following EET, provides reasonable fits of various optical spectra including resonant and nonresonant holes in the case of FMO complex. A qualitative description of the NPHB process in light of a very complex protein energy landscape is briefly discussed. As an example, we show that both resonant and nonresonant HB spectra obtained for the 825 nm band of the trimeric FMO of C. tepidum are consistent with the presence of a relatively slow EET between the lowest energy states of the monomers of the trimer (mostly localized on BChl a 3), with a weak (∼1 cm(-1)) coupling between these states revealed via calculated emission spectra. We argue that the nature of the so-called 825 nm absorption band of the FMO trimer, contrary to the presently accepted consensus, cannot be explained by a single transition.

Improved Variational Master Equation Theory for the Excitation Energy Transfer

In photosynthetic light harvesting systems, the strength of the excitation energy transfer (EET) interaction between pigments and the strength of the exciton–phonon coupling are of a comparable magnitude. Established theories are able to reproduce the EET processes for limiting cases, but the intermediate case has proven to be more difficult. We present here an improvement of the quantum master equation theory based on the variational principle to adequately describe the EET under intermediate conditions. The modified variational parameter is determined by a free-energy minimization based on the second Bogoliubov inequality. We show that the perturbation term given by our modified theory leads to a reorganization energy dependence of the EET rate that is closer to that determined by the hierarchical equation of motion.

How Does the β-ionone Effect the Spectra of Chlorophyll and the Excitation Energy Transfer in Solution

This paper presents the excitation spectra and fluorescence spectra of mixed solution containing chlorophyll a(Chla), chlorophyll b(Chlb) and β ionone of different concentration, investigates the spectral changes of Chla and Chlb caused by β ionone through optimizing the frontier orbital of Chl β ionone com pound, and calculates the energy transfer rate between Chla and Chlb by means of the fluorescence Gauss decomposition. The results show that the β ionone interacts with Chl mainly via the connection between the magnesium atom in Chl molecule and the carbonyl in β ionone molecule. The formation of Chl β ionone compound changes the charge distribution of Chl so greatly that it leads to the red shift of the excitation spec tra and blue shift of the fluorescence spectra of Chls. Also in the compound the electronic energy located on carbon double bond of β ionone can be transferred to Chl molecule by the Mg...O bond, which enhances the fluorescence intensity greatly. The calculation of the excitation energy transfer (EET) rate indicates that a small amount of β ionone can promote the EET between Chla and Chlb. DOI: 10.1134/S0030400X14100154 CONDENSED MATTER SPECTROSCOPY

Network characteristics of individual pigments in cyanobacterial photosystem II core complexes

Part of the excitation energy transfer (EET) characteristics of the photosystem II (PSII) comes from the interconnection between pigments. To understand the correlation between the EET and the pigments’ interaction structure, we construct a network from the EET rates which are related to both the distance between the pigments (chlorophylls and pheophytins) and their spatial orientations. Especially, we investigate how well the PS II core complex’s EET functionality can be explained by using only the network topology in Thermosynechococcus vulcanus 1.9 °A. Starting from the Forster theory, we construct a network of EET pathways. For an analysis of the network structure, we calculate common network-structural measures like betweenness centrality, eigenvector centrality and weighted clustering. These measures can reflect the role of individual pigments in the EET network. In our work, we found that some well-known properties were reproduced by the network analysis of the simplified network, which means that the topology of the network encodes functionally relevant information. For example, from the network structural analysis, we can infer that most of the chlorophyll molecules (clorophylls) in the pigment-protein complex CP47 have heightened probability to transfer energy compared with other chlorophylls. We also see that the active branch chlorophylls in the reaction center are characterized by a high eigenvector centrality, a high betweenness centrality and a low weighted clustering coefficient. This is indicative of functionally important vertices.

Theory and calculation for the electronic coupling in excitation energy transfer

Excitation energy transfer (EET) is a process where the electronically excitation is transferred from a donor to an acceptor. EET is widely seen in both natural and in artificial systems, such as light-harvesting in photosynthesis, the fluorescence resonance energy transfer technique, and the design of light-emitting molecular devices. In this work, we outline the theories describing both singlet and triplet EET (SEET and TEET) rates, with a focus on the physical nature and computational methods for the electronic coupling factor, an important parameter in predicting EET rates. The SEET coupling is dominated by the Coulomb coupling, and the remaining short-range coupling is very similar to the TEET coupling. The magnitude of the Coulomb coupling in SEET can vary much, but the contribution of short-range coupling has been found to be similar across different excited states in naphthalene. The exchange coupling has been believed to be the major physical contribution to the short-range coupling, but it has been pointed out that other contribution, such as the orbital overlap effect is similar or even larger in strength. The computational aspects and the subsequent physical implication for both SEET and TEET coupling values are summarized in this work. © 2013 The Authors. International Journal of Quantum Chemistry Published by Wiley Periodicals, Inc.

Optimal fold symmetry of LH2 rings on a photosynthetic membrane

An intriguing observation of photosynthetic light-harvesting systems is the N-fold symmetry of light-harvesting complex 2 (LH2) of purple bacteria. We calculate the optimal rotational configuration of N-fold rings on a hexagonal lattice and establish two related mechanisms for the promotion of maximum excitation energy transfer (EET). (i) For certain fold numbers, there exist optimal basis cells with rotational symmetry, extendable to the entire lattice for the global optimization of the EET network. (ii) The type of basis cell can reduce or remove the frustration of EET rates across the photosynthetic network. We find that the existence of a basis cell and its type are directly related to the number of matching points S between the fold symmetry and the hexagonal lattice. The two complementary mechanisms provide selection criteria for the fold number and identify groups of consecutive numbers. Remarkably, one such group consists of the naturally occurring 8-, 9-, and 10-fold rings. By considering the inter-ring distance and EET rate, we demonstrate that this group can achieve minimal rotational sensitivity in addition to an optimal packing density, achieving robust and efficient EET. This corroborates our findings i and ii and, through their direct relation to S, suggests the design principle of matching the internal symmetry with the lattice order.

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.

ChemInform Abstract: Excitation Energy Transfer in Multiporphyrin Arrays with Cyclic Architectures: Towards Artificial Light‐Harvesting Antenna Complexes

AbstractReview: 60 refs.

Excitation energy migration processes in various multi-porphyrin assemblies

The electronic interactions and excitation energy transfer (EET) processes of a variety of multi-porphyrin arrays with linear, cyclic and box architectures have been explored. Directly meso-meso linked linear arrays (Z(N)) exhibit strong excitonic coupling with an exciton coherence length of approximately 6 porphyrin units, while fused linear arrays (T(N)) exhibit extensive π-conjugation over the whole array. The excitonic coherence length in directly linked cyclic porphyrin rings (CZ(N)) was determined to be approximately 2.7 porphyrin units by simultaneous analysis of fluorescence intensities and lifetimes at the single-molecule level. By performing transient absorption (TA) and TA anisotropy decay measurements, the EET rates in m-phenylene linked cyclic porphyrin wheels C12ZA and C24ZB were determined to be 4 and 36 ps(-1), respectively. With increasing the size of C(N)ZA, the EET efficiencies decrease owing to the structural distortions that produce considerable non-radiative decay pathways. Finally, the EET rates of self-assembled porphyrin boxes consisting of directly linked diporphyrins, B1A, B2A and B3A, are 48, 98 and 361 ps(-1), respectively. The EET rates of porphyrin boxes consisting of alkynylene-bridged diporphyrins, B2B and B4B, depend on the conformation of building blocks (planar or orthogonal) rather than the length of alkynylene linkers.

Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems

Two-dimensional photon-echo experiments indicate that excitation energy transfer between chromophores near the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides occurs coherently with decoherence times of hundreds of femtoseconds, comparable to the energy transfer time scale in these systems. The original explanation of this observation suggested that correlated fluctuations in chromophore excitation energies, driven by large scale protein motions could result in long lived coherent energy transfer dynamics. However, no significant site energy correlation has been found in recent molecular dynamics simulations of several model light harvesting systems. Instead, there is evidence of correlated fluctuations in site energy-electronic coupling and electronic coupling-electronic coupling. The roles of these different types of correlations in excitation energy transfer dynamics are not yet thoroughly understood, though the effects of site energy correlations have been well studied. In this paper, we introduce several general models that can realistically describe the effects of various types of correlated fluctuations in chromophore properties and systematically study the behavior of these models using general methods for treating dissipative quantum dynamics in complex multi-chromophore systems. The effects of correlation between site energy and inter-site electronic couplings are explored in a two state model of excitation energy transfer between the accessory bacteriochlorophyll and bacteriopheophytin in a reaction center system and we find that these types of correlated fluctuations can enhance or suppress coherence and transfer rate simultaneously. In contrast, models for correlated fluctuations in chromophore excitation energies show enhanced coherent dynamics but necessarily show decrease in excitation energy transfer rate accompanying such coherence enhancement. Finally, for a three state model of the Fenna-Matthews-Olsen light harvesting complex, we explore the influence of including correlations in inter-chromophore couplings between different chromophore dimers that share a common chromophore. We find that the relative sign of the different correlations can have profound influence on decoherence time and energy transfer rate and can provide sensitive control of relaxation in these complex quantum dynamical open systems.

Excitation energy transfer in multiporphyrin arrays with cyclic architectures: towards artificial light-harvesting antenna complexes

Since highly symmetric cyclic architecture of light-harvesting antenna complex LH2 in purple bacteria was revealed in 1995, there has been a renaissance in developing cyclic porphyrin arrays to duplicate natural systems in terms of high efficiency, in particular, in transferring excitation energy. This tutorial review highlights the mechanisms and rates of excitation energy transfer (EET) in a variety of synthetic cyclic porphyrin arrays on the basis of time-resolved spectroscopic measurements performed at both ensemble and single-molecule levels. Subtle change in structural parameters such as connectivity, distance, and orientation between neighboring porphyrin moieties exquisitely modulates not only the nature of interchromophoric interactions but also the rates and efficiencies of EET. The relationship between the structure and EET dynamics described here should assist a rational design of novel cyclic porphyrin arrays, more contiguous to real applications in artificial photosynthesis.

Effects of the Distributions of Energy or Charge Transfer Rates on Spectral Hole Burning in Pigment–Protein Complexes at Low Temperatures

Effects of the distributions of excitation energy transfer (EET) rates (homogeneous line widths) on the nonphotochemical (resonant) spectral hole burning (SHB) processes in photosynthetic chlorophyll-protein complexes (reaction center [RC] and CP43 antenna of Photosystem II from spinach) are considered. It is demonstrated that inclusion of such a distribution results in somewhat more dispersive hole burning kinetics. More importantly, however, inclusion of the EET rate distributions strongly affects the dependence of the hole width on the fractional hole depth. Different types of line width distributions have been explored, including those resulting from Förster type EET between weakly interacting pigments as well as Gaussian ones, which may be a reasonable approximation for those resulting, for instance, from so-called extended Förster models. For Gaussian line width distributions, it is possible to determine the parameters of both line width and tunneling parameter distributions from SHB data without a priori knowledge of any of them. Concerning more realistic asymmetric distributions, we demonstrate, using the simple example of CP43 antenna, that one can use SHB modeling to estimate electrostatic couplings between pigments and support or exclude assignment of certain pigment(s) to a particular state.

Site, Rate, and Mechanism of Photoprotective Quenching in Cyanobacteria

In cyanobacteria, activation of the Orange Carotenoid Protein (OCP) by intense blue-green light triggers photoprotective thermal dissipation of excess absorbed energy leading to a decrease (quenching) of fluorescence of the light harvesting phycobilisomes and, concomitantly, of the energy arriving to the reaction centers. Using spectrally resolved picosecond fluorescence, we have studied cells of wild-type Synechocystis sp. PCC 6803 and of mutants without and with extra OCP (ΔOCP and OverOCP) both in the unquenched and quenched state. With the use of target analysis, we managed to spectrally resolve seven different pigment pools in the phycobilisomes and photosystems I and II, and to determine the rates of excitation energy transfer between them. In addition, the fraction of quenched phycobilisomes and the rates of charge separation and quenching were resolved. Under our illumination conditions, ∼72% of the phycobilisomes in OverOCP appeared to be substantially quenched. For wild-type cells, this number was only ∼29%. It is revealed that upon OCP activation, a bilin chromophore in the core of the phycobilisome, here called APC(Q)(660), with fluorescence maximum at 660 nm becomes an effective quencher that prevents more than 80% of the excitations in the phycobilisome to reach Photosystems I and II. The quenching rate of its excited state is extremely fast, that is, at least (∼240 ± 60 fs)(-1). It is concluded that the quenching is most likely caused by charge transfer between APC(Q)(660) and the OCP carotenoid hECN in its activated form.

Influence of the energy transfer in a single donor–acceptor pair on the photon distribution functions in its fluorescence

Fluorescence of a single donor–acceptor pair is considered. A donor molecule is described by a three-level energy scheme. It has a ‘dark’ state, which makes donor fluorescence blinking. An acceptor molecule is described by two-level scheme. Its fluorescence is of continuous type. Strong dependence of the distributions w N D ( T ) and w N A ( T ) for photons emitted by the donor and the acceptor on the electronic excitation energy transfer rate F between the two molecules is demonstrated. For instance, at F ≅ 1 / T D , where 1 / T D is the donor fluorescence rate, the fluorescence of the two-level acceptor starts blinking. The photon number distributions at F ≪ 1 / T D u F ≫ 1 / T D are considered as well.

A Mixed Quantum–Classical Description of Excitation Energy Transfer in Supramolecular Complexes: Förster Theory and beyond

Electronic excitation energy transfer (EET) is described theoretically for the chromophore complex P(4) formed by a butanediamine dendrimer to which four pheophorbide-a molecules are covalently linked. To achieve a description with atomic resolution, and to account for the effect of an ethanol solvent, a mixed quantum-classical methodology is utilized. Room-temperature molecular dynamics simulations are used to describe the nuclear dynamics, and EET is accounted for in utilizing a mixed quantum-classical formulation of the transition rates. Therefore, the full quantum expression of the EET rates is given and the change to a mixed quantum-classical version is briefly explained. The description results in the calculation of transition rates which coincide rather satisfactory with available experimental data on P(4). It is also shown that different assumptions of classical Förster theory are not valid for P(4). The temporal behavior of EET deduced from the rate equations is confronted with that following from the solution of the time-dependent Schrödinger equation entering the mixed quantum-classical description of EET. From this we can conclude that EET in flexible chromophore complexes such as P(4) can be rather satisfactory estimated by single transition rates. A correct description, however, is only achievable by using a sufficiently large set of rates that correspond to the various possible equilibrium configurations of the complex.

Ultrafast carotenoid band shifts correlated with Chlz excited states in the photosystem II reaction center: are the carotenoids involved in energy transfer?

We show a correlation between the electronic excitation of the peripheral chlorophylls (Chls(Z)) of the photosystem II reaction center and a shift of the S(2) absorption bands of β-carotene, and suggest that the carotenoids may enhance the excitation energy transfer rate from these chlorophylls to the central cofactors.

Analysis of excitation energy transfer in quantum dot clusters in the presence of nonluminescent dots

AbstractWe studied the excitation energy transfer (EET) process in CdSe/CdZnS quantum dot (QD) clusters by measuring time‐ and spectrally‐resolved fluorescence intensities. Radiative and non‐radiative exciton recombinations that occurs after EET were evaluated: the latter occurred in a large class of QD ensembles because of the presence of nonluminescent (dark) QDs. Numerical simulation confirmed the effect of the dark QDs on time‐dependent fluorescence intensities, and the results were compared with that of the experiment. Nonradiative recombination occurring after EET was found to be dominant in the low quantum yield of QDs. The EET rate as a function of QD energy was successfully calculated on the basis of these findings. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanism of excitation energy transfer between ring‐shaped aggregates of pigments

AbstractWe theoretically study the mechanism of excitation energy transfer (EET) between two ring‐shaped aggregates of pigments. Dynamics of the pigment system and the light field are described by a Markovian quantum master equation and the Maxwell's equations, respectively. Self‐consistently solving these equations, we investigate the interplay between light and aggregates of pigments. From our calculation, it is revealed that the isotropic property gives two coupling constants, which prevents the deterioration of the EET rate between anisotropic dipoles. The distance dependence of the coupling strengths is also calculated for all excited states of the aggregate, which shows that each excited state has a different distance dependence of the coupling strength.