Exciton dissociation in conjugated polymers

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.     

Exciton mobility and trapping in a MALDI matrix

Energy transfer (ET) from excited matrix to fluorescent traps is used to probe the mobility of excitations in the matrix-assisted laser desorption/ionization (MALDI) matrix material 2,5-dihydroxybenzoic acid. The dependence of host and guest fluorescence on excitation density (laser intensity) and trap concentration gives clear evidence for long-range energy transport in this matrix. This conclusion is further supported by time-resolved emission data showing a 2 ns delay between matrix and trap emission. Rate equation and random walker models give good agreement with the data, allowing determination of hopping, collision, and trapping parameters. Long-range energy transfer contributes to the pooling reactions which can lead to primary ions in MALDI. The results validate the pooling aspect of the prior quantitative MALDI ionization model (J. Mass Spectrom. 2002, 37, 867-877). It is shown that exciton trapping can decrease MALDI ion yield, even at low trap concentration.     

Controlling excitonic wavepacket motion in the PS1 core-antenna system

Sub-picosecond laser pulse driven localization of electronic excitation energy is suggested for a biological chromophore complex. Based on an exciton model of the photosynthetic core antenna PS1 of Synechococcus elongatus the shape of the respective laser pulse is calculated using optimal control theory combined with a density matrix theory accounting for energy relaxation and dephasing caused by the protein environment. As a target observable we choose the population oscillation after photo-excitation between the two Chlorophylls forming the special pair. The temperature dependence of the related control yield is studied as well as its dependence on the pulses duration.     

Excitonic energy transfer in light-harvesting complexes in purple bacteria

Two distinct approaches, the Frenkel-Dirac time-dependent variation and the Haken-Strobl model, are adopted to study energy transfer dynamics in single-ring and double-ring light-harvesting (LH) systems in purple bacteria. It is found that the inclusion of long-range dipolar interactions in the two methods results in significant increase in intra- or inter-ring exciton transfer efficiency. The dependence of exciton transfer efficiency on trapping positions on single rings of LH2 (B850) and LH1 is similar to that in toy models with nearest-neighbor coupling only. However, owing to the symmetry breaking caused by the dimerization of BChls and dipolar couplings, such dependence has been largely suppressed. In the studies of coupled-ring systems, both methods reveal an interesting role of dipolar interactions in increasing energy transfer efficiency by introducing multiple intra/inter-ring transfer paths. Importantly, the time scale (4 ps) of inter-ring exciton transfer obtained from polaron dynamics is in good agreement with previous studies. In a double-ring LH2 system, non-nearest neighbor interactions can induce symmetry breaking, which leads to global and local minima of the average trapping time in the presence of a non-zero dephasing rate, suggesting that environment dephasing helps preserve quantum coherent energy transfer when the perfect circular symmetry in the hypothetic system is broken. This study reveals that dipolar coupling between chromophores may play an important role in the high energy transfer efficiency in the LH systems of purple bacteria and many other natural photosynthetic systems.     

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.     

Ultrafast exciton transfers in DNA and its nonlinear optical spectroscopy

We have calculated the nonlinear response function of a DNA duplex helix including the contributions from the exciton population and coherence transfers by developing an appropriate exciton theory as well as by utilizing a projector operator technique. As a representative example of DNA double helices, the B-form (dA)10-(dT)10 is considered in detail. The Green functions of the exciton population and coherence transfer processes were obtained by developing the DNA exciton Hamiltonian. This enables us to study the dynamic properties of the solvent relaxation and exciton transfers. The spectral density describing the DNA base-solvent interactions was obtained by adjusting the solvent reorganization energy to reproduce the absorption and steady-state fluorescence spectra. The time-dependent fluorescence shift of the model DNA system is found to be ultrafast and it is largely determined by the exciton population transfer processes. It is further shown that the nonlinear optical spectroscopic techniques such as photon echo peak shift and two-dimensional photon echo can provide important information on the exciton dynamics of the DNA double helix. We have found that the exciton-exciton coherence transfer plays critical roles in the peculiar energy transfer and ultrafast memory loss of the initially created excitonic state in the DNA duplex helix.     

Energy transfer via guest excitons in isotopic mixed naphthalene crystals

The temperature dependence of the fluorescence spectra of aggregates in naphthalene-perdeuteronaphthalene mixed crystals has been investigated between 1.4 and 70 K and for concentrations up to 50% naphthalene. It is shown that the most abundant traps — the monomer guest molecules — transfer energy like a guest exciton band 48 cm^?1 below the host exciton band. With increasing temperature, the excitation energy is redistributed between the different aggregate traps by thermal activation into the monomer states. The energy transfer constant within the monomer exciton band is measured as a function of concentration. It is suggested that dipole-dipole interaction between the monomer guests is responsible for the energy transfer via guest excitons.     

Influence of donor:acceptor ratio on charge transfer dynamics in non-fullerene organic bulk heterojunctions

The donor:acceptor(D:A) blend ratio plays a very important role in affecting the progress of charge transfer and energy transfer in bulk heterojunction(BHJ) orga nic solar cells(OSCs).The proper D:A blend ratio can provide maximized D/A interfacial area for exciton dissociation and appro p riate domain size of the exciton diffusion length,which is beneficial to obtain high-performance OSCs.Here,we comprehensively investigated the relationship between various D:A blend ratios and the charge transfer and energy transfer mechanisms in OSCs based on PBDB-T and non-fullerene acceptor IT-M.Based on various D:A blend ratios,it was found that the ratio of components is a key factor to suppress the formation of triplet states and recombination energy losses.Rational D:A blend ratios can provide appropriate donor/accepter surface for charge transfer which has been powerfully verified by various detailed experimental results from the time-resolved fluorescence measurement and transient absorption(TA) spectroscopy.Optimized coherence length and crystallinity are verified by grazing incident wide-angle X-ray scattering(GIWAXS) measurements.The results are bene ficial to comprehend the effects of various D:A blend ratios on charge transfer and energy transfer dynamics and provides constructive suggestions for rationally designing new materials and feedback for photovoltaic performance optimization in non-fullerene OSCs.     

Long distance energy transfer in a polymer matrix doped with a perylene dye

Exciton migration over long distances is a key issue for various applications in organic electronics. We investigate a disordered material system which has the potential for long exciton diffusion lengths in combination with a high versatility. The perylene bisimide dye Perylene Red is incorporated in a polymer matrix with a high concentration. The dye molecules represent active sites with a narrow energy distribution for the electronically excited states. Excitons can be efficiently exchanged between them by F?rster resonance energy transfer (FRET). The narrow energy distribution reduces drastically the trapping probability of the excitons compared to polymers and allows for long transfer distances. To characterize the mobility of the excitons and their diffusion length the dye Oxazine 1 is added as an acceptor in low concentration and the transfer probability to the acceptor is determined by measuring the reduction of Perylene Red fluorescence. The quenched quantum yield is measured for dye concentrations varying from 0.05?M to 0.15 M for Perylene Red and from 0.3 mM to 3 mM for Oxazine 1. The experimental results are compared to a model which assumes that excitons can diffuse through the material by FRET between Perylene Red sites and are trapped at an acceptor with a final hetero FRET step. We find a quite good match between theory and experiment though the observed diffusion constant is about two times smaller than the calculated one. The exciton diffusion length extracted from the data is 30 nm for a Perylene Red concentration of 0.1 M and demonstrates that long distance energy transfer is possible in this disordered material system.     

Energy transfer for systems possessing long-range rates of capture

The authors' recent theory of exciton trapping and sensitized luminescence is extended to cover long-range capture processes. Expressions are given for the energy transfer rate k_s, and the quantum yield ϕ_G, which show the interplay of exciton migration with the strength and range of the capture process.     

PHOTOVOLTAIC EFFECT OF CHLOROPHYLL a–QUINONE SYSTEMS IN MULTILAYER ARRAYS

Abstract— Photovoltaïc cells made of an array of chlorophyll a (Chi a ) monolayers between an aluminum and a silver electrode have been analyzed. These cells are characterized by charge carrier production due to the dissociation of singlet excitons. The exciton diffusion length λ∼ 300 ± 100 Å. The optimum thickness of these cells consists of an array of 44 monolayers for which a power conversion efficiency η= 0.038% and a quantum yield ø= 0.49% has been measured at 678 nm using an incident light intensity of 0.3 W/m². In these cells, about 60% of the collected charges are generated by the exciton dissociation in the bulk of the semiconductor and 40% by the exciton dissociation at the aluminum electrode. The behaviour of Chl a photovoltaïc cells doped with two quinones has also been analyzed. The two quinones are vitamin K¹ and N,N-distearoyl-1,4-diaminoanthraquinone (SAQ). The photovoltaic properties of Chi a remain practically unchanged when quinone is not in the same monolayer as Chi a. But when Chi a and quinone are in the same monolayer, up to a molecular ratio of 1:0.3, there is a drastic decrease in the efficiencies and quantum yields of the cells. The photovoltaïc behaviour varies in parallel with the extinction of Chi a fluorescence by the quinones in contrast with what is reported for strong electron acceptors adsorbed on photoconductors. An electron transfer from the singlet state of Chi a to the quinones is proposed on the basis of the variation in the quantum yield for current production with the energy of the incident photon.     

Influence of diffusion on nonradiative energy transfer between FMN molecules in aqueous solutions

Concentration dependence of photoluminescence quantum yield of FMN aqueous solutions (66mM potassium phosphate buffer, pH 7.0) is investigated over the concentration range from 6.31x10(-5) M to 1.8x10(-2) M at temperatures 298.2 and 323.9K. Experimental data are compared with those obtained theoretically based on two different models of excitation energy transfer and migration in the system of FMN monomers and dimers. The first model does not take the material diffusion into account [Acta Phys. Acad. Sci. Hung. 30 (1971) 145] and the second model is based on the second-order transfer rates which are diffusion dependent [Chem. Phys. Lett. 41 (1976) 139; J. Lumin. 27 (1982) 441]. The comparison shows that the process of material diffusion cannot be neglected in the solutions studied as the relative contribution of the diffusion accelerated nonradiative energy transfer to the total drop of the quantum yield can be even higher then 70%. It is also shown, that in order to obtain a good agreement of the experimental and theoretical data it is necessary to introduce into the theory an additional channel of deactivation for the excitation energy. It is proposed that this additional channel can be partial degradation of excitation energy during its migration between the monomers.     

Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.     

Exciton binding energy in semiconducting single-walled carbon nanotubes

The exciton binding energy serves as a critical criterion for identification of the nature of elementary excitations (neutral excitons versus a pair of charged carriers) in semiconductor materials. An exciton binding energy of 0.41 eV is determined experimentally for a selected nanotube type, the (8,3) tube, confirming the excitonic nature of the elementary excitations. This determination is made from the energy difference between an electron-hole continuum and its precursor exciton. The electron-hole continuum results from dissociation of excitons following extremely rapid exciton-exciton annihilation and possibly also ultrafast relaxation from the second to the first exciton states and is characterized by distinct spectroscopic and dynamic signatures.     

Reducing the Exciton Binding Energy of Donor–Acceptor‐Based Conjugated Polymers to Promote Charge‐Induced Reactions

Exciton binding energy has been regarded as a crucial parameter for mediating charge separation in polymeric photocatalysts. Minimizing the exciton binding energy of the polymers can increase the yield of charge‐carrier generation and thus improve the photocatalytic activities, but the realization of this approach remains a great challenge. Herein, a series of linear donor–acceptor conjugated polymers has been developed to minimize the exciton binding energy by modulating the charge‐transfer pathway. The results reveal that the reduced energy loss of the charge‐transfer state can facilitate the electron transfer from donor to acceptor, and thus, more electrons are ready for subsequent reduction reactions. The optimized polymer, FSO‐FS, exhibits a remarkable photochemical performance under visible light irradiation.     

Heterogeneous exciton dynamics revealed by two-dimensional optical spectroscopy

We show that optical two-dimensional (2D) spectroscopy can recover ultrafast heterogeneous dynamics of closely spaced delocalized exciton states from a molecular exciton manifold characterized by a single absorption band. The complete experimental third-order nonlinear optical response from room-temperature J-aggregates in liquid phase is reproduced for the first time with self-consistent Frenkel exciton theory combined with modified Redfield theory. We show that exciton relaxation between the exciton states and nuclear-motion-induced exchange-narrowed energy fluctuations of individual delocalized exciton states can be distinguished because these two processes lead to a distinctively different evolution of the absolute 2D spectrum. Our technique also allows recovery of the variation of the exciton relaxation rates as well as the degree of exciton delocalization across the absorption band.     

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.     

Nonmonotonic energy harvesting efficiency in biased exciton chains

We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well.     

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.