Excitonic Dimer Research Articles

Biexponential theory of Drude dissipation via hierarchical quantum master equation

A nonperturbative quantum dissipation theory is developed based on an optimal construction of biexponential Drude bath correlation function for its influence on the system dynamics. It is an advanced hierarchical quantum master equation approach, aiming at a numerically efficient non-Markovian quantum dissipation propagator, with the support of a convenient criterion to estimate in advance its accuracy for general systems. Compared to its low level, single-exponential counterpart [R. X. Xu et al., J. Chem. Phys. 131, 214111 (2009)], the present theory remarkably improves the applicability range over all-parameter space, as tested critically with electron transfer and frequency-dispersed transient absorption of exciton dimer model systems.

Characterization of a trimeric light-harvesting complex in the diatom Phaeodactylum tricornutum built of FcpA and FcpE proteins

Fucoxanthin chlorophyll proteins (Fcps), the light-harvesting antennas of heterokont algae, are encoded by a multigene family and are highly similar with respect to their molecular masses as well as to their pigmentation, making it difficult to purify single Fcps. In this study, a hexa-histidine tag was genetically added to the C-terminus of the FcpA protein of the pennate diatom Phaeodactylum tricornutum. A transgenic strain expressing the recombinant His-tagged FcpA protein in addition to the endogenous wild type Fcps was created. This strategy allowed, for the first time, the purification of a specific, stable trimeric Fcp complex. In addition, a pool of various trimeric Fcps was also purified from the wild-type cells using sucrose density gradient ultracentrifugation and gel filtration. In both the His-tagged and the wild-type Fcps, excitation energy coupling between fucoxanthin and chlorophyll a was intact and the existence of a chlorophyll a/fucoxanthin excitonic dimer was demonstrated using circular dichroism spectroscopy. Mass spectrometric analyses of the trimeric His-tagged complex indicated that it is composed of FcpA and FcpE polypeptides. It is confirmed here that a trimer is the basic organizational unit of Fcps in P. tricornutum. From circular dichroism spectra, it is proposed that the organization of the pigments on the polypeptide backbone of Fcps is a conserved feature in the case of chlorophyll a/c containing algae.

Two-dimensional electronic spectra of an aggregating dye: simultaneous measurement of monomeric and dimeric line-shapes

We present a sequence of two-dimensional electronic spectra of a prototypical cyanine dye, whose spectral properties in aqueous solution are determined by the formation of a monomer-dimer equilibrium. Quantum-chemical methods are utilized to calculate the structure and absorption properties of the two species involved. Our spectroscopic results simultaneously characterize the spectral line-shapes of the two species in terms of underlying dynamic and static disorder, and demonstrate how the two-dimensional technique allows the exploitation of high spectral and temporal resolution in one and the same experiment. The distinctly different spectral relaxation dynamics are quantified in a two-dimensional line-shape analysis, by extracting the time dependent ratios of the diagonal and anti-diagonal peak-widths. Our findings are in line with theoretical considerations, that predict the fluctuational dynamics of an excitonic dimer state to be exchange-narrowed by excitation delocalization.

Spectroscopic Properties of Reaction Center Pigments in Photosystem II Core Complexes: Revision of the Multimer Model

Absorbance difference spectra associated with the light-induced formation of functional states in photosystem II core complexes from Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 (e.g., P+Pheo−,P+QA−,3P) are described quantitatively in the framework of exciton theory. In addition, effects are analyzed of site-directed mutations of D1-His198, the axial ligand of the special-pair chlorophyll PD1, and D1-Thr179, an amino-acid residue nearest to the accessory chlorophyll ChlD1, on the spectral properties of the reaction center pigments. Using pigment transition energies (site energies) determined previously from independent experiments on D1-D2-cytb559 complexes, good agreement between calculated and experimental spectra is obtained. The only difference in site energies of the reaction center pigments in D1-D2-cytb559 and photosystem II core complexes concerns ChlD1. Compared to isolated reaction centers, the site energy of ChlD1 is red-shifted by 4nm and less inhomogeneously distributed in core complexes. The site energies cause primary electron transfer at cryogenic temperatures to be initiated by an excited state that is strongly localized on ChlD1 rather than from a delocalized state as assumed in the previously described multimer model. This result is consistent with earlier experimental data on special-pair mutants and with our previous calculations on D1-D2-cytb559 complexes. The calculations show that at 5K the lowest excited state of the reaction center is lower by ∼10nm than the low-energy exciton state of the two special-pair chlorophylls PD1 and PD2 which form an excitonic dimer. The experimental temperature dependence of the wild-type difference spectra can only be understood in this model if temperature-dependent site energies are assumed for ChlD1 and PD1, reducing the above energy gap from 10 to 6 nm upon increasing the temperature from 5 to 300K. At physiological temperature, there are considerable contributions from all pigments to the equilibrated excited state P*. The contribution of ChlD1 is twice that of PD1 at ambient temperature, making it likely that the primary charge separation will be initiated by ChlD1 under these conditions. The calculations of absorbance difference spectra provide independent evidence that after primary electron transfer the hole stabilizes at PD1, and that the physiologically dangerous charge recombination triplets, which may form under light stress, equilibrate between ChlD1 and PD1.

Electronic coherence transfer in photosynthetic complexes and its signatures in optical spectroscopy

Effects of electronic coherence transfer after photoexcitation of excitonic complexes and their manifestation in optical spectroscopy are discussed. A general excitonic model Hamiltonian is considered in detail to elucidate the origin of energy relaxation in excitonic complexes. We suggest that the second-order quantum master equation for the reduced density matrix of electronic degrees of freedom provides the most suitable theoretical framework for the study of coherence transfer in photosynthetic bacteriochlorophyll complexes. Temperature dependence of the absorption band maximum of a simple excitonic dimer is interpreted in terms of coherence transfer between two excited states. The role of reorganization energy of the transitions in the magnitude of the effect is discussed. A large reorganization energy difference between the two states is found to induce significant band shift. The predictions of the theory are compared to experimental measurements of the bacterial reaction center absorption spectra of Rhodobacter sphaeroides As an example of a time-dependent spectroscopic method sensitive to coherences and possibly to their transfer, we present recent two-dimensional photon echo measurements of energy relaxation in the so-called Fenna–Matthews–Olson complex of Chlorobium tepidum, where distinct oscillatory patters predicted to be signatures of electronic coherence have been observed.

Electronic coherence transfer in photosynthetic complexes and its signatures in optical spectroscopy

Effects of electronic coherence transfer after photoexcitation of excitonic complexes and their manifestation in optical spectroscopy are discussed. A general excitonic model Hamiltonian is considered in detail to elucidate the origin of energy relaxation in excitonic complexes. We suggest that the second-order quantum master equation for the reduced density matrix of electronic degrees of freedom provides the most suitable theoretical framework for the study of coherence transfer in photosynthetic bacteriochlorophyll complexes. Temperature dependence of the absorption band maximum of a simple excitonic dimer is interpreted in terms of coherence transfer between two excited states. The role of reorganization energy of the transitions in the magnitude of the effect is discussed. A large reorganization energy difference between the two states is found to induce significant band shift. The predictions of the theory are compared to experimental measurements of the bacterial reaction center absorption spectra ofRhodobacter sphaeroidesAs an example of a time-dependent spectroscopic method sensitive to coherences and possibly to their transfer, we present recent two-dimensional photon echo measurements of energy relaxation in the so-called Fenna–Matthews–Olson complex ofChlorobium tepidum, where distinct oscillatory patters predicted to be signatures of electronic coherence have been observed.

Three-pulse photon echo of an excitonic dimer modeled via Redfield theory

In this article the third-order response of an excitonically coupled dimer is studied. The three-pulse photon echo signals were calculated by extracting polarization components from the total polarization in the corresponding phase-matched directions. The total nonlinear response was obtained by numeric propagation of the density matrix, with the exciton-vibrational coupling modeled via Redfield relaxation theory. The full two-dimensional three-pulse photon echo signals and the peak shift were analyzed in terms of the density-matrix dynamics of coherence dephasing and population relaxation. The location of the two-exciton state was found to be essential for proper modeling of the three-pulse photon echo. In particular, an oscillation in the three-pulse photon echo peak shift is found if the two-exciton state is displaced. The oscillations can be related to the dynamics of the one-exciton coherences.

Photophysics of an electrophosphorescent platinum (II) porphyrin in solid films

We examine electronic processes in platinum (II) octaethyl porphyrin (PtOEP) embedded in an organic solid state matrix and in the form of vacuum-evaporated neat films in conjunction with potential applications of this compound to organic photovoltaic and electrophosphorescent devices. Absorption, photoexcitation, and luminescence spectra indicate the excitonic dimers to be dominant excited states, and their dissociation underlies the charge photogeneration process. Different charge separation distance (1.5 nm and 2.6 nm) in opposite charge carrier pairs preceding dissociation can be distinguished based on the fit of the three-dimensional Onsager theory of geminate recombination to electromodulated luminescence and photoconduction measurements. The near-positive electrode concentrated triplet dimer excitons, produced by strongly 370 nm absorbed light in neat PtOEP films, are efficiently quenched by electron transfer to the metal (Al), generating the positive charge with an efficiency eta+ exceeding 0.15 at high electric fields and dominating the measured photocurrent. Their dissociation efficiency in the bulk, eta- (negatively biased illuminated electrode), is more than one order of magnitude lower than eta+. The dissociation of singlet dimer states dominates the bulk photogeneration process induced by the weakly-absorbed light at 450 nm, with comparable eta+ and eta-. The "hot excited state" underlying the temperature-increasing emission at 540 nm has been attributed to the upper excitonic component Q+ of the first absorption band Q consistent with the exciton concept applied successfully to the interpretation of all dimer-underlain spectroscopic features of PtOEP samples studied.

Spectroscopic and nonlinear characteristics of anthracene-containing silicon-organic polymers in solution

Spectroscopic characteristics, i.e. absorption, fluorescence, fluorescence excitation spectra, fluorescence decay time, fluorescence polarization degree of novel silicon-containing organic polymers including main chain anthracene groups were investigated. Three kinds of emission spectra were revealed and assigned to polaron–exciton, anthracene and anthracene dimer. The measured fluorescence polarization spectra gave evidence of directed excitation energy migration along the disordered polymeric chain. Strong quenching of anthracene fluorescence during the polaron–exciton lifetime was interpreted as a result of the interaction between two excitations that causes anthracene anion-radical formation. The third-order nonlinear susceptibility of the polymers in solution measured by the Z-scan technique at 1054 nm is 190×10 −14 cm 2 W −1.

Intramolecular dimers: a new strategy to fluorescence quenching in dual-labeled oligonucleotide probes.

Many genomics assays use profluorescent oligonucleotide probes that are covalently labeled at the 5' end with a fluorophore and at the 3' end with a quencher. It is generally accepted that quenching in such probes without a stem structure occurs through Förster resonance energy transfer (FRET or FET) and that the fluorophore and quencher should be chosen to maximize their spectral overlap. We have studied two dual-labeled probes with two different fluorophores, the same sequence and quencher, and with no stem structure: 5'Cy3.5-beta-actin-3'BHQ1 and 5'FAM-beta-actin-3'BHQ1. Analysis of their absorption spectra, relative fluorescence quantum yields, and fluorescence lifetimes shows that static quenching occurs in both of these dual-labeled probes and that it is the dominant quenching mechanism in the Cy3.5-BHQ1 probe. Absorption spectra are consistent with the formation of an excitonic dimer, an intramolecular heterodimer between the Cy3.5 fluorophore and the BHQ1 quencher.

The majot red form in Spirulina trimers (C709) is the low energy band of an excitonic dimer

The strongly red shifted chlorophyll antenna forms associated with photosystem I are intriguing both from the standpoint of their biological function and the physicochemical interactions which give rise to them. Due to their uncommonly strong electron/phonon coupling and charge transfer characteristics it is often suggested that they are brought about by strong Coulombic interactions with other chlorophylls leading to the formation of excitonic dimers. The accompanying excitonic splitting would then give rise to a low energy, red shifted, absorption band. In the present study direct evidence for this suggestion is presented. Selective destruction of the strongly dichroic red shifted chlorophyll state (CD) and absorption maxima at 709nm and extinguishing near 725nm) in photosystem I trimers from Spirulina by either high intensity illumination (photobleaching) or incubation with low concentrations of Triton X-100 is accompanied by changes in the circular dichroism spectrum of the same amplitude and of opposite sign at 677nm. The difference spectra band shapes are strongly suggestive of a dimeric chlorophyll structure with excitonic bands peaking at these two wavelengths. In the assumption that the monomer absorption bands are of similar energy the interaction energy is in excess of 300cm-1. The observation that the red CD tail extinguishes near 725nm at room temperature while the absorption spectrum does so at 750nm is discussed in termsof the low temperature CD spectrum.

Excitonic-dimer model with oscillatory Fano damping

The dynamical properties of an excitonic dimer coupled to a harmonic oscillator are analysed as a simple model for the problem of self-localization of excitons in crystalline systems. If the oscillator is treated classically and a damping term is included in a phenomenological way, the system relaxes to its lowest-energy state, which is either a symmetrical or a site-trapped, symmetry-broken state depending on the values of the system parameters. By quantum mechanical analysis, employing the accurate eigensolutions of the system obtained in previous work, it is possible to establish a semi-quantitative argument suggesting that this symmetry-breaking behaviour is an artifact of the semiclassical approximation. This argument is based, on the one hand, on group theoretical requirements and, on the other, on the exploitation of a Fano-type model for the coupling of the local oscillator to a bath. An illustration is made for the down-cascading of the excitonic energy to the final state.

Dynamics of the dimer-oscillator model with Fano damping

The dynamical properties of an excitonic dimer coupled to a harmonic oscillator are analyzed as a simple model for the problem of self-localization of excitons in crystalline systems. If the oscillator is treated classically and a damping term is included in a phenomenological way, the system relaxes to its lowest-energy state, which is either a symmetrical or a site-trapped, symmetry-broken state depending on the values of the system parameters. When the system is treated quantum mechanically, and the accurate eigenstates of the system obtained in previous work are used, it is possible to establish a semiquantitative argument suggesting that this symmetry-breaking behavior is an artifact of the semiclassical approximation.

Delay of excitonic self-trapping (bottleneck states)

The conventional barrier model of exciton self-trapping can be shown to display divergencies, whence a new explanation of the retarded luminescence phenomena (RLP) is necessary. In a previous work a novel (“exotic”) sequence of exciton-phonon states has been found in the excitonic dimer case which provides the desired new theoretical concept for the RLP. Presently, this work is extended to the trimeric, tetrameric and to the E-e Jahn-Teller situations. It is shown that in each of these extensions the exotic sequences again make their appearance. These states are of non-self-trapping nature, and by way of their small extension in the Q-space they may acquire a “bottleneck” role in the process of energy dissipation.

Circular dichroism of large molecules

AbstractThe circular dichroism of molecules, which are large compared to the wavelength of light, is considered. Explicit expressions are obtained for the circular dichroism and absorption of an exciton dimer and of a free particle on a helix. The dimensions are described for which the dipole approximation for the optical properties fails.