高等植物体内激子相干迁移与俘获

依据所建立的色素分子排列和取向的新型结构模型,利用激发能传递的广义主方程理论,提出了高等植物体内激子相干迁移与俘获的点阵理论,研究了静态荧光量子产额、定态能量传递速率和荧光强度的变化规律。指出激子相干迁移有助于活体的激发能转移与俘获,并且它有可能是活体内激子寿命的限制因素之一。     

Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies.

A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.     

Optical and conformational properties of the 7-(beta-D-ribofuranosyl) purines

The optical and conformational properties of certain 7-ribosyl purines have been studied using several experimental and theoretical methods. The CD, MCD, and absorption spectra of the 7-ribosyl purines and their cations have been determined experimentally with some solvent effects being noted. Theoretically we have used the SCF-CI and CNDO molecular orbital calculations to determine the optical properties of the bases; Simpson's bond exciton theory to determine the optical properties of the bases; Simpson's bond exciton theory to determine the optical properties of the ribose moiety; the optical activity was determined by the dipole coupling and electricmagnetic coupling equations; and the conformational energy calculations are included as a basis for excluding highly improbable conformations. On the basis of these calculations, we concluded that the oxy derivatives and the amino derivatives are predominately in the “extreme anti” conformation and “standard anti” conformation, respectively. However, the conclusions are tentative since at the present time the band assignments are not unequivocal. At least one n-π* transition has been identified in the spectra which is not notably sensitive to pH and solvent effects. Several implications of this observation are discussed.     

On the influence of pigment-protein interactions on the energy transfer processes in photosynthetic membrane structures. 2. LH2 complex of Rhodobacter sphaeroides

Low-temperature heterogeneous absorption and circular dichroism spectra of the Rb. sphaeroides LH2 complexes are calculated within the framework of the mini-exciton theory and diagonal static random disorder for the pure electronic transitions of the monomeric Bchl molecules. The coupling of Bchl molecules with the surrounding amino acid residues has been shown to change both the exciton distribution between the pigment molecules in each of the exciton states. The value of the delocalization index depends on the excitation wavelength and varies between 2-6 Bchl molecules. The optical transitions occurring at 780-790 and 820 nm have been found to be strongly mixed so that all Bchl molecules of the LH2 complex predetermine absorption in these spectral regions. On the other hand, absorption at 800 and 850 nm is mainly determined by the cycles of 9 and 18 Bchl molecules, respectively. Thus, the light energy absorbed by the B800 molecules at 800 nm is transferred to the B850 molecules by the interlevel exciton relaxation processes due to the population of the heavily mixed 820-nm exciton levels. The width of the heterogeneous absorption band for the cyclic monomeric aggregate has been shown to decrease as compared with the monomeric absorption band by square root(Ndel) time, where Ndel is the mean number of pigments over which the exciton is delocalized within the excited absorption band.     

On the influence of pigment-protein interactions on energy transfer processes in photosynthetic membrane structures. 3. The FMO complex of Chlorobium tepidum at high pressure

The low-temperature absorption spectra of the Chlorobium tepidum FMO bacteriochlorophyll-protein complex at various pressures have been calculated within the framework of mini-exciton theory. The dependences of the Qy transition energies of the monomeric pigments on pressure have been found by means of functional minimization. This functional includes the parameters of both theoretical and experimental absorption spectra at low temperatures and various pressures. The dependences obtained are compared with those derived for the exciton transition energies, which have been obtained by deconvoluting absorption spectra with seven Gaussian components at each pressure. The pressure increase has been shown to result in the increased coupling energy between both the pigment molecules themselves and pigments and amino acid residues. The pigment molecules capable of binding histidines and water molecules have been shown to have the greatest and smallest responses to increased pressure, respectively. The couplings of Bchl molecules with the surrounding amino acid residues have been shown to change both the exciton delocalization index and the exciton distribution between the pigment molecules within the protein subunit; the increased pressure does not change these parameters significantly.     

Exciton interactions in synthetic porphyrin dimers

The Soret absorption spectra of six synthetic rigid porphyrin dimers whose crystal structures have been determined are simulated using simple exciton theory. The objective is to test the validity of the point dipole and associated approximations; the electronic interaction parameters are thus calculated using data obtained from the monomer spectra, with no adjustable parameters. Satisfactory agreement between theory and experiment is obtained for one class of dimers but not for a second. This poses a challenge for semiempirical electronic structure methods as to whether improvements over the point dipole calculations can be obtained.     

Dynamics of excitation energy transfer in the LH1 and LH2 light-harvesting complexes of photosynthetic bacteria.

Photosynthetic light harvesting is a unique life process that occurs with amazing efficiency. Since the discovery of the structure of the bacterial peripheral light-harvesting complex (LH2), this process has been studied using a variety of advanced laser spectroscopic methods. We are now in a position to discuss the physical origins of excitation energy transfer and trapping in the LH2 and LH1 antennae of photosynthetic purple bacteria. We demonstrate that the time evolution of the state created by the light is determined by the combined action of excitonic pigment-pitment interactions, energetic disorder, and coupling to nuclear motion in a pigment-protein complex. A quantitative fit of experimental data using Redfield theory allowed us to determine the pathways and time scales of exciton and vibrational relaxation and analyze separately different contributions to the measured transient absorption dynamics. Furthermore, these dynamics were observed to be strongly dependent on the excitation wavelength. A numerical fit of this dependence turns out to be extremely critical to a variation of the structure and disorder parameters and, therefore, can be used as a test for different antenna models (disordered ring, elliptical deformations, correlated disorder, etc.). The calculated equilibration dynamics in the exciton basis allow a visualization of the exciton motion using a density matrix picture in real space.     

A model for the organization of chlorophyll-protein complex of photosystem II and analysis of its photochemical efficiency and excitation migration.

A model is proposed for the organization of chlorophyll-protein complex in photosystem II (PS II) of higher plants. The rates of exciton migration and exciton trapping have been computed using stochastic method to find out the photochemical efficiency of the dimeric PS II. Three dimeric PS II units are assumed to form a group, as transfer of the exciton to the light harvesting bed of the nearest neighbour on either side may only be effective. A relationship has been deduced between the fractions of the reaction centre traps closed and the number of jumps (J) required by the exciton for trapping. The photochemical efficiency and fluorescence quantum yield are computed using J as the parameter in an empirical equation.     

The enduring legacy of Koji Nakanishi's research on natural products and bioorganic chemistry. Part 2. Inception and establishment of the ECD exciton chirality method in 1960s to 1970s: A marvel of Nakanishi's Japanese team

The electronic circular dichroism (ECD) exciton chirality method is very useful for determining the absolute configuration (AC) of chiral compounds. In the ECD spectroscopy, the chromophore-chromophore interaction, ie, exciton coupling, is very important. For example, Harada and Nakanishi first discovered in 1969 that chiral dibenzoates exhibit exciton split bisignate Cotton effects, from the sign of which the screw sense between two long axes of benzoate chromophores, ie, the AC of dibenzoate, can be determined. This method was named the dibenzoate chirality rule and has been successfully applied to various natural products to determine their ACs. During these studies, it was also found that this CD method was expanded to encompass other aromatic and olefin chromophores like naphthalene, diene, enone, etc. Therefore, the name of the dibenzaote chirality rule was changed to the CD exciton chirality method. In 1970s, there were heated controversies about the inconsistency between X-ray Bijvoet and CD exciton chirality methods, which was a shocking and serious problem in the community of molecular chirality research. Harada and coworkers synthesized the most ideal cage compound with two anthracene chromophores to connect X-ray Bijvoet and CD exciton chitality methods and proved that these two methods are consistent with each other.     

Exciton transport in the photosynthetic unit

In this paper we present a theoretical study of the exciton transport in the photosynthetic unit and attempt to correlate the structural and organizational properties of the unit with the exciton transport efficiency. The transport mechanism is assumed to be of the Forster type. This mechanism is shown to be equivalent to the thermally assisted exciton hopping. The transport equations are solved by the Green's function technique in a more general way than has been presented previously. We thus find it possible to study the effect of the proper R^?6 coupling and random orientations of the pigment molecules. The equations are also solved for a low concentration of reaction centres when the conventional random walk picture is not valid. Various models of chlorophyll a and b mixtures are studied and by comparing their different exciton transport properties, an explanation is proposed for several observed features of the primary photochemical process.     

A theoretical study on the exciton circular dichroism of propeller‐like metal complexes of bipyridine and tripodal tris(2‐pyridylmethyl)amine derivatives

Time‐dependent density functional theory (TD‐DFT) has been employed to simulate the circular dichroism (CD) spectra of bipyridyl ruthenium(II) complexes as well as zinc(II) and copper(II) complexes containing tris(2‐pyridylmethyl)amine (TPA) derivatives. A qualitative model is used to account for the mechanism by which the bis‐ and tris‐bipyridine complexes (or analogous systems) exhibit exciton CD. The model is further used to predict the sign of the exciton CD bands. The predictions are in agreement with experiment and DFT calculations. A comprehensive analysis is presented of the subtle differences in the CD spectra of this series of related complexes. Chirality, 2011. © 2010 Wiley‐Liss, Inc.     

Electronic and vibrational exciton coupling in oxidized trianglimines

Readily available chiral trianglimine and their (poly)oxygenated congeners represent a unique class of macrocyclic rigid compounds optimal for testing electronic and vibrational circular dichroism exciton chirality methods. Electronic and vibrational circular dichroism spectra of such trianglimines are strongly affected by polar substituents in macrocycle skeletons. Double substitution by OH groups in each aromatic fragment of the macrocycle causes sign reversal of the exciton couplet in the region of the strongest UV absorption. On the other hand, electronic circular dichroism spectrum of the macrocycle having 2 methoxy groups shows 2 exciton couplets—the long‐wavelength positive and the second of the negative sign, observed at the shorter wavelengths. VCD spectra of macrocyclic imines show vibrational exciton couplets in the region of strong C=N stretches. The signs of these couplets are positive and the opposite of the diamine chirality. For trianglimine macrocycles the interpretation of VCD spectra in terms of excitons is much more convincing than for electronic circular dichroism spectra. By contrast, trans‐1,2‐diaminocyclohexane–based vicinal diimines, being a one‐third of the respective macrocycle, do not exhibit any vibrational exciton effect. Experimental data were confronted with DFT calculations. We observed good‐to‐excellent agreement between experimental and computed data.     

Ultraviolet circular dichroism of polyproline and oriented collagen

The ultraviolet absorption, linear dichroism, circular dichroism, and oriented circular dichroism of collagen are reported and the spectra are resolved into a self-consistent set of bands in accord with exciton theory. The parallel band at 200 nm has 40% of the π → π* intensity; the perpendicular band is placed at 189 nm yielding a splitting of 2700 cm^?1. The circular dichroism is resolved into two Gaussians at λ_∥ and λ_τ (rotational strengths +14 × 10^?40 and ?32 × 10^?40 esu². cm²) plus a large non-Gaussian (“helix”) band with ampplitude ?25,000° at 201 nm. These data appear to be in reasonably good accord with recent calculations. Measurements of the absorption, linear dichroism and circular dichroism of polyproline I and II are also reported and are resolved into their component bands. Polyproline I is in good accord with exciton theory, whereas polyproline II remains unsatisfactory.     

Spectral dependence of energy transfer in wild-type peripheral light-harvesting complexes of photosynthetic bacteria

The precise position of the upper exciton component and relevant vibronic transitions of the B850 ring in peripheral light-harvesting complexes from purple photosynthetic bacteria are important values for determining the exciton bandwidth and electronic structure of the B850 ring. To determine the presence of these components in wild-type LH2 complexes the pump-probe femtosecond transient spectra obtained with excitation into the 730-840 nm spectral range are analyzed. We show that at excitation wavelengths less than 780 nm B850 absorption bands are present and that, in accordance with exciton theory, these bands peak further in the blue when the lowest optically allowed transition is more red-shifted.     

Interactions between purine derivatives: Electronic spectral studies. I. Electronic transitions in caffeine monomer and exciton coupling in caffeine dimer

The concentration dependence of the ultraviolet absorption spectrum of aqueous solutions of caffeine has been studied. Individual species spectra have been derived for the monomer, dimer, and tetramer of caffeine. The emission spectrum of caffeine in aqueous solution and the dichroic spectra in oriented poly(vinyl alcohol) and polyethylene films have been measured. The long-wavelength tail of the absorption spectrum of caffeine in non-polar environment has been found to incorporate at least one carbonyl(π*, n) transition. Dichroic spectral data and molecular orbital calculations have been used to assign transition moment directions to the (π*,π) transitions. The lowest energy (π*,π) transition, responsible for the near-ultraviolet absorption peak in aqueous solution of caffeine, has been used for the study of degenerate exciton interactions in the dimeric species of caffeine. Assuming that the caffeine molecules in the dimer are stacked in parallel planes, theoretical calculations of the ground-state interactions and of the degenerate exciton interactions have been combined with experimental data and a unique model for the dimer of caffeine has been derived. The transfer rate of energy between the molecules in the dimer is of the order of 10¹³_S^?1.     

A non-perturbation method for the optical properties of helical polymers

The optical absorption and circular dichroism (CD) of poly-L -alanine in an α-helical conformation are computed in terms of monomer properties by use of Bogoliubov exciton theory (a non-perturbation method). It was shown earlier that a proper formulation of the theory of CD for large molecular systems permits the use of periodic boundary conditions, a feature which greatly simplifies calculations as well as conceptual understanding. If is found that the shape of the observed CD spectrum can be explained in terms of the two rotational strength components and an electric dipole strength component referred to as the helix term. Comparisons of the results of perturbation theory and Bogoliubov exciton theory show that perturbation theory is valid for absorption but is inadequate for CD if the coupling among monomers is sufficiently large lo predict the observed hypochromism.     

Excitons in Metal‐Halide Perovskites

The unprecedented increase of the power conversion efficiency of metal‐halide perovskite solar cells has significantly outpaced the understanding of their fundamental properties. One of the biggest puzzles of perovskites has been the exciton binding energy, which has proved to be difficult to determine experimentally. Many contradictory reports can be found in the literature with values of the exciton binding energy from a few meV to a few tens of meV. In this review the results of the last few years of intense investigation of the exciton physic in perovskite materials are summarized. In particular a critical overview of the different experimental approaches used to determine exciton binding energy is provided. The problem of exciton binding energy in the context of the polar nature of perovskite crystals and related polaron effects which have been neglected to date in most of work is discussed. It is shown that polaron effects can reconcile at least some of the experimental observations and controversy present in the literature. Finally, the current status of the exciton fine structure in perovskite materials is summarized. The peculiar carrier–phonon coupling can help to understand the intriguing efficiency of light emission from metal‐halide perovskites.     

Testing the vibrational exciton and the local mode models on the instructive cases of dicarvone,dipinocarvone, and dimenthol vibrational circular dichroism spectra

The vibrational circular dichroism (VCD) spectra of dicarvone ( 1 ), dipinocarvone ( 2 ), and dimenthol ( 3 ) have been recorded in the range 900–3200 cm⁻¹, encompassing the mid-infrared (mid-IR), the CO stretching, and the CH-stretching regions. For compound 3 also, the fundamental and the first overtone OH stretching regions have been investigated by IR/NIR absorption and VCD. Density functional theory (DFT) calculations allow one to interpret the IR and VCD spectra and to confirm the configuration/conformational studies previously conducted by X-ray diffraction. The most intense VCD signals are associated with the vibrational normal modes involving symmetry-related groups close to the CC bond connecting covalently the two molecular units. The vibrational exciton (VCDEC) model is fruitfully tested on the VCD data of compounds 1 and 2 for the spectroscopic regions at ~1700 cm⁻¹, and the local mode model is tested on compound 3 at ~3500 and ~6500 cm⁻¹. For compounds 1 and 2 also, ECD spectra are reported, and the exciton mechanism is tested also there, and connections to the VCDEC model are examined.     

Energy transfer and fluorescence mechanisms in photosynthetic membranes

Energy transfer in photosynthetic membranes involves the migration of excitons from light‐harvesting antenna chlorophyll‐protein complexes to the reaction center complexes. Recent efforts have focused on determining the time of arrival of excitons (trapping times) at the reaction centers following excitation with a single picosecond laser pulse. Three different approaches have been utilized: (1) determination of appearance of separated charges within the reaction centers by differential absorbtion spectroscopy, (2) determination of appearance of separated charges by fast photoemf measurements, and (3) kinetics of decay of fluorescence. The first two methods provide more direct information on exciton trapping by reaction centers than fluorescence methods, but are experimentally difficult to realize. Therefore, much activity has centered around the accurate measurement and analysis of fluorescence‐decay profiles by single‐photon counting methods. In green plants, about three different components with lifetimes of about 100 psec, 200 to 500 psec, and >1 nsec, have been reported. The first two components are believed to be related to trapping rates by reaction centers, while the third component is attributed to a charge recombination (Klimov) mechanism. Results from photoemf and exciton‐exciton annihilation experiments are consistent with the interpretation that the first decay component reflects exciton‐trapping rates. A critical analysis and discussion of these fast energy‐transfer phenomena in photosynthetic membranes of green plants are offered in this review.