Theoretical investigation of the role of strongly coupled chlorophyll dimers in photoprotection of LHCII

Nonphotochemical quenching is the photoprotection mechanism by which the excess excitation energy absorbed by the light harvesting complex LHCII is dissipated through the protein scaffold as heat. Using the quenched structure of LHCII obtained from crystallographic experiments, the potential quenching of photoexcited excitons by aggregates of chlorophylls is theoretically investigated. In monomeric LHCII there is a hierarchy of length scales resulting in a hierarchy of energy scales that determine the interpigment direct Coulomb coupling. We propose a model whereby eight chlorophylls are coupled quantum mechanically into four dimers, with exciton transfer between these dimers and the remaining six single chlorophylls proceeding incoherently via Forster transfer. The chlorophyll dimer Chl a604-Chl b606 possesses a quasi-parallel geometry, resulting in a weakly dipole-allowed low-lying excited state. This weakly allowed state is accessible via exciton transfer to a higher, strongly allowed state followed by fast vibrational relaxation. This parallel, H-type aggregate can potentially function as an exciton trap. Calculated Forster transfer rates between single chlorophylls and chlorophyll dimers are used in a simulation of exciton transfer in monomeric LHCII to explore this possibility. It is found that Chl a604-Chl b606 has a short-lived enhanced population (on the time scale of approximately picoseconds), but not a long-time resident population. The fluorescence quantum yield of the model was calculated to be phi F = 0.38. Comparison of this result with phi F approximately 0.26 for unquenched LHCII in dilute solution and phi F approximately 0.06 for the highly quenched LHCII crystal reveals that the proposed model does not account for the quenching observed in the LHCII crystal. We therefore conclude that the formation of chlorophyll dimers is not the main cause of excitonic NPQ in LHCII.     

输运层厚度对双层有机器件复合发光的影响

采用ITO/PVK/Alq/Al双层电致发光(EL)结构,制备了三种载流子输运层厚度分别为30、 60、 120 nm,发光层厚度均为300 nm的有机薄膜EL器件,测试其EL谱及J-V特性曲线.根据有机EL器件中载流子的产生和输运过程导出了载流子复合几率及电子和空穴密度分布表示式,用以解释其发光强度随输运层厚度的变化关系,用一维无序结构载流子随机跃迁模型讨论输运层厚度对器件电流密度及启动电压的影响,探讨了载流子在薄膜中的输运过程,其理论与实验符合得很好.     

Efficient exciton transport in layers of self-assembled porphyrin derivatives

The photosynthetic apparatus of green sulfur bacteria, the chlorosome, is generally considered as a highly efficient natural light-harvesting system. The efficient exciton transport through chlorosomes toward the reaction centers originates from self-assembly of the bacteriochlorophyll molecules. The aim of the present work is to realize a long exciton diffusion length in an artificial light-harvesting system using the concept of self-assembled natural chlorosomal chromophores. The ability to transport excitons is studied for porphyrin derivatives with different tendencies to form molecular stacks by self-assembly. A porphyrin derivative denoted as ZnOP, containing methoxymethyl substituents ({meso-tetrakis[3,5-bis(methoxymethyl)phenyl]porphyrinato}zinc(II)) is found to form self-assembled stacks, in contrast to a derivative with tert-butyl substituents, ZnBuP ({meso-tetrakis[3,5-bis(tert-butyl)phenyl]porphyrinato}zinc(II)). Exciton transport and dissociation in a bilayer of these porphyrin derivatives and TiO2 are studied using the time-resolved microwave conductivity (TRMC) method. For ZnOP layers it is found that excitons undergo diffusive motion between the self-assembled stacks, with the exciton diffusion length being as long as 15 +/- 1 nm, which is comparable to that in natural chlorosomes. For ZnBuP a considerably shorter exciton diffusion length of 3 +/- 1 nm is found. Combining these exciton diffusion lengths with exciton lifetimes of 160 ps for ZnOP and 74 ps for ZnBuP yields exciton diffusion coefficients equal to 1.4 x 10(-6) m2/s and 1 x 10(-7) m2/s, respectively. The larger exciton diffusion coefficient for ZnOP originates from a strong excitonic coupling for interstack energy transfer. The findings show that energy transfer is strongly affected by the molecular organization. The efficient interstack energy transfer shows promising prospects for application of such self-assembled porphyrins in optoelectronics.     

Quantum master equation approach to the second hyperpolarizability of nanostar dendritic systems

We investigate the dynamic second hyperpolarizability (gamma) of nanostar dendritic systems using the quantum master equation approach. In the nanostar dendritic systems composed of three-state monomers, the multistep exciton states are obtained by the dipole-dipole interactions, and the directional energy transport, i.e., exciton migration, from the periphery to the core is predicted to occur by the relaxation between exciton states originating in the exciton-phonon coupling. The effects of the intermolcecular interaction and the exciton migration, i.e., exciton relaxation, on the gamma in the third-harmonic generation (THG) are examined in the three-photon off- and on- resonance regions using the two-exciton model. Furthermore, the method for analysis of spatial contributions of excitons to gamma is presented by partitioning the total gamma into the one- and two-exciton contributions. It turns out that the exciton relaxation between exciton states causes significant broadening of the spectra of gamma and their mutual overlap as well as the relative increase of two-exciton contributions in the nanostar dendritic system.     

Percolative versus homogenous energy transport kinetics: time-resolved donor and acceptor fluoresence of isotopic mixed naphthalene crystals

Tune-resolved spectral studies of the singlet exciton transport threshold in doped naphthalene-perdeuteronaphthalene mixed crystals are presented. While a homogeneous kinetic model adequately explains many of the results, some strong deviations from its predictions are found. These are attributed to clusterization effects.     

One-dimensional exciton diffusion in perylene bisimide aggregates

The dynamics and mobility of excitons in J-aggregates of perylene bisimides are investigated by transient absorption spectroscopy with a time resolution of 50 fs. The transient spectra are compatible with an exciton delocalization length of two monomers and indicate that vibrational and configurational relaxation processes are not relevant for the spectroscopic properties of the aggregates. Increasing the pump pulse energy and in that way the initial exciton density results in an accelerated signal decay and pronounced exciton-exciton annihilation dynamics. Modeling the data by assuming a diffusive exciton motion reveals that the excitons cannot migrate freely in all three directions of space but their mobility is restricted to one dimension. The observed anisotropy supports this picture and points against direct Fo?rster-transfer-mediated annihilation between the excitons. A diffusion constant of 1.29 nm(2)/ps is deduced from the fitting procedure that corresponds to a maximal exciton diffusion length of 96 nm for the measured exciton lifetime of 3.6 ns. The findings indicate that J-aggregates of perylene bisimides are promising building blocks to facilitate directed energy transport in optoelectronic organic devices or artificial light-harvesting systems.     

Disorder-induced exciton localization and violation of optical selection rules in supramolecular nanotubes

Using numerical simulations, we study the effect of disorder on the optical properties of cylindrical aggregates of molecules with strong excitation transfer interactions. The exciton states and the energy transport properties of such molecular nanotubes attract considerable interest for application in artificial light-harvesting systems and energy transport wires. In the absence of disorder, such nanotubes exhibit two optical absorption peaks, resulting from three super-radiant exciton states, one polarized along the axis of the cylinder, the other two (degenerate) polarized perpendicular to this axis. These selection rules, imposed by the cylindrical symmetry, break down in the presence of disorder in the molecular transition energies, due to the fact that the exciton states localize and no longer wrap completely around the tube. We show that the important parameter is the ratio of the exciton localization length and the tube's circumference. When this ratio decreases, the distribution of polarization angles of the exciton states changes from a two-peak structure (at zero and ninety degrees) to a single peak determined by the orientation of individual molecules within the tube. This is also reflected in a qualitative change of the absorption spectrum. The latter agrees with recent experimental findings.     

Exciton migration by ultrafast Förster transfer in highly doped matrixes

The energy transfer between dye molecules and the mobility of the corresponding excitons are investigated in polymethyl methacrylate films highly doped with perylene bisimide dyes. The dynamics is measured by group delay corrected, femtosecond broad-band spectroscopy revealing the transfer route via absorption changes that are specific for the participating species. In films doped with 0.14 M perylene orange an ultrafast homotransfer between the dye molecules is found by analyzing the loss of the excitation-induced anisotropy. The process exhibits a stretched exponential time dependence which is characteristic for F?rster energy transfer between immobilized molecules. The transfer time is 1.5 ps for an average transfer distance of 2.3 nm and results in a high mobility of the optically generated excitons. In addition, we find that the excitons move to perylene orange dimers, which have formed in low concentration during the sample preparation. The observed energy transfer time is slightly shorter than expected for a direct F?rster transfer and indicates that exciton migration by multistep transfer between the monomers speeds up the transport to the dimers. In samples doped with perylene orange and perylene red heterotransfer to perylene red takes place with transfer times down to 600 fs. The mechanism is F?rster transfer as demonstrated by the agreement with calculations assuming electric dipole interaction between immobilized and statistically distributed donor and acceptor units. The model predicts the correct time dependence and concentration scaling for highly doped as well as diluted samples. The results show that ultrafast exciton migration between dye molecules in highly doped matrixes is an attractive and efficient mechanism to transport and collect energy in molecular systems and organic electronic devices. Further optimization should lead to a loss-free transport over distances typical for the thickness of active layers in these systems.     

Gene regulatory networks: a coarse-grained, equation-free approach to multiscale computation

We present computer-assisted methods for analyzing stochastic models of gene regulatory networks. The main idea that underlies this equation-free analysis is the design and execution of appropriately initialized short bursts of stochastic simulations; the results of these are processed to estimate coarse-grained quantities of interest, such as mesoscopic transport coefficients. In particular, using a simple model of a genetic toggle switch, we illustrate the computation of an effective free energy Phi and of a state-dependent effective diffusion coefficient D that characterize an unavailable effective Fokker-Planck equation. Additionally we illustrate the linking of equation-free techniques with continuation methods for performing a form of stochastic "bifurcation analysis"; estimation of mean switching times in the case of a bistable switch is also implemented in this equation-free context. The accuracy of our methods is tested by direct comparison with long-time stochastic simulations. This type of equation-free analysis appears to be a promising approach to computing features of the long-time, coarse-grained behavior of certain classes of complex stochastic models of gene regulatory networks, circumventing the need for long Monte Carlo simulations.     

Exciton migration in rigid-rod conjugated polymers: an improved Förster model

The dynamics of interchain and intrachain excitation energy transfer taking place in a polyindenofluorene endcapped with perylene derivatives is explored by means of ultrafast spectroscopy combined with correlated quantum-chemical calculations. The experimental data indicate faster exciton migration in films with respect to solution as a result of the emergence of efficient channels involving hopping between chains in close contact. These findings are supported by theoretical simulations based on an improved Forster model. Within this model, the rates are expressed according to the Fermi golden rule on the basis of (i) electronic couplings that take account of the detailed shape of the excited-state wave functions (through the use of a multicentric monopole expansion) and (ii) spectral overlap factors computed from the simulated acceptor absorption and donor emission spectra with explicit coupling to vibrations (considered within a displaced harmonic oscillator model); inhomogeneity is taken into account by assuming a distribution of chromophores with different conjugation lengths. The calculations predict faster intermolecular energy transfer as a result of larger electronic matrix elements and suggest a two-step mechanism for intrachain energy transfer with exciton hopping along the polymer backbone as the limiting step. Injecting the calculated hopping rates into a set of master equations allows the modeling of the dynamics of exciton transport along the polyindenofluorene chains and yields ensemble-averaged energy-transfer rates in good agreement with experiment.     

Exciton delocalization and energy transport mechanisms in R-phycoerythrin

Energy transport mechanisms in R-Phycoerythrin (RPE), a light harvesting protein located at the top of the phycobilisome antenna in red algae, are investigated using nonlinear optical spectroscopies and theoretical models. The RPE hexamer possesses a total of 30 bilin pigments, which can be subdivided into three classes based on their molecular structures and electronic resonance frequencies. Of particular interest to this study is the influence of exciton delocalization on the real-space paths traversed by photoexcitations as they concentrate on the lowest energy pigment sites. Transient grating measurements show that significant nuclear relaxation occurs at delay times less than 100 fs, whereas energy transport spans a wide range of time scales depending on the proximity of the initial and final states involved in the process. The fastest energy transport dynamics within the RPE complex are close to 1 ps; however, evidence for sub-100 fs exciton self-trapping is also obtained. In addition, photon echo experiments reveal vibronic interactions with overdamped and underdamped nuclear modes. To establish signatures of exciton delocalization, energy transport is simulated using both modified Redfield and Fo?rster theories, which respectively employ delocalized and localized basis states. We conclude that exciton delocalization occurs between six pairs of phycoerythrobilin pigments (i.e., dimers) within the protein hexamer. It is interesting that these dimers are bound in locations analogous to the well-studied phycocyanobilin dimers of cyanobacterial allophycocyanin and c-phycocyanin in which wave function delocalization is also known to take hold. Strong conclusions regarding the electronic structures of the remaining pigments cannot be drawn based on the present experiments and simulations due to overlapping resonances and broad spectroscopic line widths, which prevent the resolution of dynamics at particular pigment sites.     

Organic Photovoltaics: Elucidating the Ultra‐Fast Exciton Dissociation Mechanism in Disordered Materials

Organic photovoltaics (OPVs) offer the opportunity for cheap, lightweight and mass‐producible devices. However, an incomplete understanding of the charge generation process, in particular the timescale of dynamics and role of exciton diffusion, has slowed further progress in the field. We report a new Kinetic Monte Carlo model for the exciton dissociation mechanism in OPVs that addresses the origin of ultra‐fast (<1 ps) dissociation by incorporating exciton delocalization. The model reproduces experimental results, such as the diminished rapid dissociation with increasing domain size, and also lends insight into the interplay between mixed domains, domain geometry, and exciton delocalization. Additionally, the model addresses the recent dispute on the origin of ultra‐fast exciton dissociation by comparing the effects of exciton delocalization and impure domains on the photo‐dynamics.This model provides insight into exciton dynamics that can advance our understanding of OPV structure–function relationships.     

Electronic energy transfer in mixed organic solids: Anderson localization or delocalization?

A simple Anderson transition model, ignoring guest clusterization, excitation lifetime, sensor concentration, exciton-phonon coupling and thermalization, appears to be incompatible with the critical concentrations observed for triplet exciton transport in several ternary crystal systems. Dynamic percolation, involving hopping or tunneling through long-range clusters, remains our suggested model.     

Dephasing and Dissipation in a Source–Drain Model of Light‐Harvesting Systems

The energy transport process in natural‐light‐harvesting systems is investigated by solving the time‐dependent Schrödinger equation for a source–network–drain model incorporating the effects of dephasing and dissipation, owing to coupling with the environment. In this model, the network consists of electronically coupled chromophores, which can host energy excitations (excitons) and are connected to source channels, from which the excitons are generated, thereby simulating exciton creation from sunlight. After passing through the network, excitons are captured by the reaction centers and converted into chemical energy. In addition, excitons can reradiate in green plants as photoluminescent light or be destroyed by nonphotochemical quenching (NPQ). These annihilation processes are described in the model by outgoing channels, which allow the excitons to spread to infinity. Besides the photoluminescent reflection, the NPQ processes are the main outgoing channels accompanied by energy dissipation and dephasing. From the simulation of wave‐packet dynamics in a one‐dimensional chain, it is found that, without dephasing, the motion remains superdiffusive or ballistic, despite the strong energy dissipation. At an increased dephasing rate, the wave‐packet motion is found to switch from superdiffusive to diffusive in nature. When a steady energy flow is injected into a site of a linear chain, exciton dissipation along the chain, owing to photoluminescence and NPQ processes, is examined by using a model with coherent and incoherent outgoing channels. It is found that channel coherence leads to suppression of dissipation and multiexciton super‐radiance. With this method, the effects of NPQ and dephasing on energy transfer in the Fenna–Matthews–Olson complex are investigated. The NPQ process and the photochemical reflection are found to significantly reduce the energy‐transfer efficiency in the complex, whereas the dephasing process slightly enhances the efficiency. The calculated absorption spectrum reproduces the main features of the measured counterpart. As a comparison, the exciton dynamics are also studied in a linear chain of pigments and in a multiple‐ring system of light‐harvesting complexes II (LH2) from purple bacteria by using the Davydov D₁ ansatz. It is found that the exciton transport shows superdiffusion characteristics in both the chain and the LH2 rings.     

Negative polaron and triplet exciton diffusion in organometallic "molecular wires"

The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL(2)-C≡C-Ph-C≡C-](n) (where L = PBu(3) and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of ~3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of ~27 ps for triplets and <10 ps for electrons.     

Transport of excitation energy in a three-dimensional doped molecular crystal. IV. Fourth-order propagation,exciton clothing,and exciton diffusion

The problem of excitons in interaction with phonons in a molecular crystal has been reinvestigated as a continuation of our earlier work. The exciton-phonon interaction has been taken to be linear in lattice displacements. The external medium, the phonon assembly, has been considered to be in thermal equilibrium. Following Simons, we have incorporated the effects of the medium on the exciton dynamics into a time-dependent effective potential that contains the equilibrium average exciton-phonon interaction as well as terms arising from the fluctuations in the medium's coordinates about their equilibrium values. A correlation function that represents the probability of exciton transfer has been given in the interaction picture. The time evolution of this correlation function has been determined by following Kubo's technique of cumulant expansion. The zeroth-, second-, and fourth-order contributions to the correlation function have been calculated in this way. The second- and fourth-order contributions have been diagrammatically represented. The second-order contribution has been explicitly calculated in different physical limits, namely, the slow exciton and the slow phonon limits at high and low temperatures and for very large and very small time. A few simple formulas for the transfer probability of a bare exciton in a molecular crystal of cubic symmetry have been derived from the Debye approximation for the dispersion of phonons. It has been specifically shown that the sum over phonon modes in the large time dynamics leads to a fully destructive interference in second order at a very low temperature and gives rise to a diffusive transport at a high enough temperature. A natural way of clothing the excitons has been considered and the clothed exciton has been represented diagrammatically. The dressing requires the correlation function to be redefined in terms of the clothed states and the clothed operators. The clothed exciton correlation function that represents the probability of transfer of excitons fully clothed by the phonons in thermal equilibrium turns out to be identical with the bare exciton correlation function. This attaches a novel interpretation to the correlation function which was originally defined by Simons. Transfer probabilities for a clothed exciton in a cubic crystal has been explicitly worked out for different physical limits under the Debye model of phonon dispersion. From these results a few expressions for the macroscopic diffusion coefficient of the clothed exciton have been obtained. A few critical comments have been incorporated. © 1996 John Wiley & Sons, Inc.     

The importance of p-n junction interfaces for efficient small molecule-based organic solar cells

The efficiency of small-molecule solar cells critically depends on the match of the junction of the donor and acceptor semiconductors used in these devices to create charged carriers and on the mobility of individual components to transport holes and electrons. In the present study, a 2% efficient bilayer organic solar cell consisting of a p-type semiconductor, pentacene, and an n-type semiconductor, N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(7)), is fabricated. The morphology of PTCDI-C(7) interestingly follows pentacene due to the matched surface energy of these two active layers and the easily deposited PTCDI-C(7) monomers on an inclined plane of the pentacene grains. This condition results in the low trap states in the PTCDI-C(7) film and at the pentacene/PTCDI-C(7) interface for the enhancement of exciton dissociation and carrier transport compared with the photoactive layer comprised of pentacene and N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(13)). The detailed exciton and carrier transport mechanisms are investigated using time-resolved photoluminescence and X-ray diffraction spectroscopy.     

Gas diffusion in polycrystalline silicalite membranes investigated by 1H pulse field-gradient NMR

1H pulse field-gradient (PFG) spin-echo NMR was performed to measure the diffusivity of methane in a polycrystalline MFI-type silicalite membrane. Measured diffusivities decreased with an increase in the diffusion distance and converged to the constant value. This result suggests the presence of a transport barrier in the membrane. The long-time diffusivity in the membrane was 3.7 x 10(-9) m2/s, which was a factor of 3 smaller than reported values in a single crystal. The distance between the transport barriers was estimated to be much larger than 6 mum from the relationship of diffusivity with displacement. It should be noted that the estimated distances were larger than the smallest dimension of the crystals appearing in the membrane surface. Gas permeation and pervaporation tests were carried out on the same sample for which NMR measurements were taken. The estimated methane flux using measured long-time diffusivity by the permeation theory overestimated the experimental value, although it is closer to the experimental value than the value estimated using the short-time diffusivity. These results mean that the methane diffusivity in a silicalite membrane is much smaller than that in a single crystal.     

Energy-transfer and charge-separation pathways in the reaction center of photosystem II revealed by coherent two-dimensional optical spectroscopy

The excited state dynamics and relaxation of electrons and holes in the photosynthetic reaction center of photosystem II are simulated using a two-band tight-binding model. The dissipative exciton and charge carrier motions are calculated using a transport theory, which includes a strong coupling to a harmonic bath with experimentally determined spectral density, and reduces to the Redfield, the F?rster, and the Marcus expressions in the proper parameter regimes. The simulated third order two-dimensional signals, generated in the directions -k(1)+k(2)+k(3), k(1)-k(2)+k(3), and k(1)+k(2)-k(3), clearly reveal the exciton migration and the charge-separation processes.