Aggregation of Chlorophyll a' in Aqueous Methanol

An aggregate of chlorophyll a' (Chi a' , C13²-epimer of ChI a ) formed in a methanovwater (40160, vol/vol) mixed solvent was examined by visible absorption, circular dichro-ism (CD), fluorescence and resonance Raman spectrosco-pies in relation to its possible involvement in the core of photosystem I reaction center. The Chl a' aggregate exhibited a sharp, double-peaked absorption spectrum (690 and 715 nm) accompanied by an intense, conservative CD signal. The fluorescence excitation polarization spectrum showed that the doublet results from the exciton splitting in a single aggregate species. Time-dependent changes in the spectroscopic properties clearly point to a simple transformation process from one molecular species to another, though a minor component appears to coexist. This conclusion was supported also by the principal multicom-ponent spectral estimation analysis of the transients of absorption spectra. The species formed at the initial stage is most probably a T-shaped dimer or oligomer, which is then gradually converted into the final major product, presumably a stacked dimer. In both of these states, the Chl molecules are linked together via direct coordination of the C13'keto carbonyl oxygen onto the Mg atom of neighboring molecules, as suggested by almost identical resonance Raman spectra in ordinary and deuterated methanovwater mixed solvents. The stacked dimers probably further associate to form a colloidal state in this solvent system. Based on these results, a model for the Chl a' aggregation is proposed.     

Absorption,fluorescence and stimulated fluorescence by polaritons in thin anthracene crystals. A re-interpretation of spectroscopic properties of anthracene

The transmission, absorption (excitation spectra) and fluorescence spectra of thin (? 47 nm) free mounted anthracene flakes have been measured. True absorption in b polarization in the region of the lowest exciton state occurs as a result of scattering by phonons. It has a minimum near the transverse exciton frequency and a maximum near the longitudinal exciton frequency, in agreement with expected polariton behaviour. Thickness dependent polariton states have finite absorption and fluorescence transition probabilities (due to crystal inhomogeneities) and are observed below the transverse exciton frequency. These polariton states represent the energy reservoir for excitation energy in the bulk of the crystal. A surface-induced exciton state is the origin of the sharp line fluorescence from pure crystals and accounts for the high efficiency of stimulated fluorescence at low temperatures. Stimulated fluorescence can also be observed from the polariton modes when excitation occurs in these modes. Resonance interactions between polariton modes and impurity levels represent an important pathway for fluorescence quenching in crystalline anthracene.     

PHYCOERYTHROCYANIN: ITS SPECTROSCOPIC BEHAVIOR AND PROPERTIES

Abstract— Phycoerythrocyanin is a biliprotein found in very few blue-green algae. Its properties have been examined under three conditions: in whole cells, in light-harvesting organelles (the phycobilisomes). and as an isolated protein. Absorption and fluorescence bands characteristic of the isolated protein are essentially the same as those in intact cells of the blue-green alga Anabaena variabilis. The same spectroscopic hallmarks are observed in purified phycobilisomes. Dissociation of these physobilisomes at low-phosphate concentrations resulted in increased phycoerythrocyanin fluorescence. This time-dependent increase in fluorescence demonstrates the function of this biliprotein in excitation-energy transfer to the other biliproteins when the organelles are intact. The relative stabilities of the various heteroprotein bonds within the phycobilisomes are shown to possess differing phosphate ion dependencies. Studies on the isolated protein from Mastigocladus laminosus include fluorescence measurements at both 23 and-196°C, as is generally observed with biliproteins, although phycoerythrocyanin has complex visible absorption and excitation spectra, only a single emission band is observed.     

Density Functional Study of Tetraphenylporphyrin Long-Range Exciton Coupling

The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intensity is consistent with TPP separations of over 40 Å. It is found that a hybrid functional with fully long-range corrected range-separated exchange performs best for full TDDFT calculations of the dimer. The range-separation parameter is optimally tuned for TPP, resulting in a good quality TPP absorption spectrum and small DFT delocalization error (measured by the curvature of the energy calculated as a function of fractional electron numbers). Calculated TDDFT data for the absorption spectra of TPP are also used as input for a ‘matrix method’ (MM) model of the exciton CD. For long-range exciton CD, comparison of MM spectra with full TDDFT CD spectra for the dimer shows that the matrix method is capable of producing very accurate results. A MM spectrum obtained from TPP absorption data calculated with the nonhybrid Becke88–Perdew86 (BP) functional is shown to match the experimental brevetoxin spectrum ‘best’, but for the wrong reasons.     

Photophysical processes in aromatic polyimides. Studies with model compounds

Emission mechanism in an aromatic polyimide, PI(BPDA/PDA), derived from biphenyltetracarboxylic dianhydride and p-phenylene diamine were studied with ultraviolet visible absorption and fluorescence spectra of a series of the model compounds. The excitation spectrum of the intermolecular charge-transfer (CT) fluorescence peaking around 550 nm of PI(BPDA/PDA) thin film was completely consistent with the absorption spectrum, indicating that the intermolecular CT fluorescence emission of PI(BPDA/PDA) film is not caused by direct excitation of the CT absorption band, but by light absorption due to structural units in the polymer backbone. The UV-vis. absorption spectra of the model compounds corresponding to the structural units in PI(BPDA/PDA) showed that the longest wavelength absorption band is due to the biphenylbisimide moiety. The band was assigned as π, π* transition with the polarization spectrum of the model compound. The fluorescence spectra of the model compounds changed sensitively depending on the conformation around N-phenyl bond. The lifetime measurement for the model compounds suggested that intramolecular CT process occurs very rapidly. © 1993 John Wiley & Sons, Inc.     

Photophysical properties of long rodlike meso-meso-linked zinc(II) porphyrins investigated by time-resolved laser spectroscopic methods

The molecular design of directly meso-meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso-meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413-414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[pi/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm(-1) for the exciton coupling energy in the S(2) state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S(2) --> S(1) internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (approximately 1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.     

The electronic absorption spectra of pyridine azides, solvent-solute interaction

The electronic absorption spectra of: 2-, 3-, and 4-azidopyridines have been investigated in a wide variety of polar and non-polar solvents. According to Onsager model, the studied spectra indicate that the orientation polarization of solvent dipoles affects the electronic spectrum much stronger than the induction polarization of solvent dipoles. The effect of solvent dipole moment predominates that of solvent refractive index in determining the values of band maxima of an electronic spectrum. The spectra of azidopyridines differ basically from these of pyridine or mono-substituted pyridine. Results at hand indicate that the azide group perturbs the pyridine ring in the case of 3-azidopyridine much more than it does in the case of 2-azidopyridine. This result agrees with the predictions of the resonance theory. Although the equilibrium <==> azide tetrazole is well known, yet the observed spectra prove that such an equilibrium does not exist at the studied conditions. The spectra of the studied azidopyridines are characterized by the existence of overlapping transitions. Gaussian analysis is used to obtain nice, resolved spectra. All the observed bands correspond to pi-->pi* transitions, n-->pi* may be overlapped with the stronger pi-->pi* ones.     

Temperature-dependent relaxation of excitons in tubular molecular aggregates: Fluorescence decay and stokes shift

We report temperature-dependent steady-state and time-resolved fluorescence studies to probe the exciton dynamics in double-wall tubular J-aggregates formed by self-assembly of the dye 3,3'-bis(3-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine. We focus on the lowest energy fluorescence band, originating from the inner cylindrical wall. At low temperatures, the experiments reveal a nonexponential decay of the fluorescence, with a typical time scale that depends on the emission wavelength. At these temperatures we also find a dynamic Stokes shift of the fluorescence spectrum and its nonmonotonic dependence on temperature under steady-state conditions. All these data indicate that below about 20 K the excitons in the lowest fluorescence band do not reach thermal equilibrium before emission occurs, while above about 60 K thermalization on this time scale is complete. By comparing the two lowest fluorescence bands, we also find indications for fast energy transfer from the outer to the inner wall. We show that the Frenkel exciton model with diagonal disorder, which previously has been proposed to explain the absorption and linear dichroism spectra of these aggregates, yields a quantitative explanation to the observed dynamics. To this end, we extend the model to account for weak phonon-induced scattering of the localized exciton states; the spectral dynamics are then described by solving a Pauli master equation for the exciton populations.     

A linear dichroism study of colloidal silver in stretched polymer films

The visible absorption spectrum of silver colloids incorporated in poly vinyl alcohol films has been resolved into five Gaussian bands of different polarization. The energy positions and relative intensities of the bands can be explained using an exciton model in which dipole-dipole coupling occurs between silver particles in a linear chain arrangement. An important conclusion that stems from the analysis is that surface-enhanced Raman scattering occurs only when the exciting light is polarized parallel to the main axis of the linear chain colloidal aggregate.     

Theoretical study on the absorption spectra of pseudoisocyanine bromide (PIC-Br) molecular J-aggregates

The linear dichroic absorption spectrum of pseudoisocyanine bromide (PIC-Br) molecular J-aggregates is theoretically analyzed by using the dynamical coherent potential approximation and numerical calculations. The absorption spectra with the electric field polarization parallel and perpendicular to the one-dimensional axis of the oriented J-aggregates are well explained in terms of Davydov components. Above the one-phonon continuum in the dispersion curve of the exciton–phonon system, splitting off of an exciton polaron band is observed. This exciton polaron state is assigned to the largest peak in the case of the latter polarization. The absorption intensity differences between the lowest peak and the 2nd peak observed in two polarization conditions are simply explained by the character of relevant states.     

Porphyrin boxes constructed by homochiral self-sorting assembly: optical separation, exciton coupling, and efficient excitation energy migration

meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the F?rster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex.     

Excitonic interactions in covalently-linked porphyrin dimers

We have studied the absorption spectra, emission spectra, and fluorescence excitation polarization spectra of a series of free base and diprotonated etioporphyrin-I dimers covalently linked through (CH₂)_n bridges, n = 0–8. The absorption spectra of the n = 0 and n = 1 dimer show red shifts, which are largest (≈15 mm) for the Soret band of the n = 0 dimer. The Soret bands of the diprotonated dimers n = 0–3 show splitting (≈500–1000 cm^?1) which can be interpreted by an exciton model assuming a reasonable geometry. The fluorescence spectra and quantum yields are similar to that of the monomer, except for the same red shift seen in absorption; however, the n = 0 diprotonated dimer shows an anomalo vibronic structure. The fluorescence excitation polarization spectra for the n = 0 and the n = 1 dimers differ substantially from the monomer; dimers n ? 3 have fluorescence excitation polarization spectra that suggest that some of the excitation stays localized in one moiety while the r hops to the dimer partner.     

THE BEHAVIOR OF DYE-MUCOPOLYSACCHARIDE COMPLEXES—I. SPECTROSCOPIC STUDIES OF THE SYSTEM CHONDROITIN SULFATE A-ACRIDINE ORANGE

Abstract —Absorption, fluorescence, and circular dichroism (CD) spectra have been obtained for a mucopolysaccharide—cationic dye complex. The dye acridine orange exhibits a blue shift in its absorption maximum and a red shift in its fluorescence maximum when mixed with chondroitin sulfate A in aqueous solution. The spectral shifts of the dye appear to be reversed by addition of salt, divalent cations being more effective than monovalent cations. The complex exhibits induced optical activity in the visible absorption spectrum of acridine orange. The biphasic nature of the CD curve is compatible with interpretations involving chromophore coupling.     

Induced chirality upon crocetin binding to human serum albumin: origin and nature

Binding to human serum albumin (HSA) of the natural, achiral carotenoid crocetin, having hypocholesterolemic and antitumour effects, was investigated in detail by circular dichroism (CD) and absorption spectroscopy. It has been shown that in the visible absorption region the crocetin–HSA complex exhibits a well-defined induced circular dichroic spectrum with two major bands of opposite sign, proving excitonic interaction between carotenoids bound in a left-handed chiral arrangement on the albumin molecule. In the course of CD titration experiments, palmitic acid gradually decreased the exciton band intensities indicating that crocetin and palmitic acid have common binding sites on HSA. To investigate potential sources of the intermolecular excitonic interaction, molecular modeling studies were performed fitting crocetin molecules to the long-chain fatty acid binding sites of HSA, determined recently by X-ray crystallographic measurements. The results suggest that binding of crocetin to domain III of the albumin might be responsible for the observed intermolecular exciton coupling. Crocetin binding was accompanied by a significant red shift in the visible absorption spectrum which has showed no excitonic contribution but rather indicates the higher polarizability of the protein environment.     

Spectroscopy and dynamics of jet-cooled 1,1′ binaphthyl

Fluorescence excitation and dispersed fluorescence spectra of jet-cooled 1,1′-binaphthyl are reported and analysed. The spectra indicate that in the ground and excited state the naphthalene rings are perpendicular to one another. The spectra can be further interpreted in terms of an exciton model with an exciton splitting of 21 cm^?1 in the origin. From the structureless emission spectrum and lifetime it is concluded that, in the isolated molecule, efficient vibrational relaxation occurs through conversion of vibrational into librationaI energy.     

Polarized absorption and anomalous temperature dependence of fluorescence depolarization in cylindrical J-aggregates

We study chiral J-aggregates of the amphiphilic dye 1A that are spontaneously and asymmetrically generated from achiral dye monomers. These aggregates occur in two types. One type possesses a threefold split J-absorption band and forms micrometer-sized superhelices. The other type has a twofold split J-absorption band and forms smaller nanoparticles. We show that the analysis of optical experiments with polarized light in terms of an exciton model yields strong indications that the smaller aggregates have a cylindrical structure as well; the lower exciton band is polarized along the cylinder axis, while the higher band is polarized perpendicular to it. Our analysis allows for an estimate of the number of molecules around the circumference of the cylinder. Fluorescence–polarization excitation spectra at room temperature confirm the cylinder model. At low temperature, these spectra reveal a surprising loss of fluorescence polarization upon excitation in the higher exciton band. Possible explanations for this observation are discussed.     

Heme-Peptide Models for Hemoproteins. 2. N-Acetylmicroperoxidase-8: Study of the pi-pi Dimers Formed at High Ionic Strength Using a Modified Version of Molecular Exciton Theory

AcMP8 is the Cys-14-acetylated water-soluble heme-octapeptide fragment obtained proteolytically from cytochrome c. Two successive dimerization equilibria are observed with increasing ionic strength in aqueous solution at neutral pH (part 1, preceding article). The electronic spectra of the two pi-pi dimers were extracted from the absorption envelopes at 2.01 and 4.02 M ionic strength and resolved by Gaussian analysis. The principal transitions were assigned using a tailored version of molecular exciton theory based on coupling of the main x- and y-polarized transition dipole moments of the interacting heme groups. The spectra of both pi-pi dimers indicate that the y-polarized exciton states are blue-shifted relative to the excited states of the monomer, while the x-polarized exciton states exhibit a red shift. These shifts were correctly predicted by a simple dipole-dipole coupling model. From an analysis of the resultant transition dipole moments to the exciton states with B(x)()(0,0) and B(y)()(0,0) character and the magnitudes of their red and blue exciton shifts, respectively, we have determined the dipole-dipole interaction geometries for both dimers. The principal difference between the interaction geometry in the first dimer and that in the second is a stronger interaction for the y-polarized transition dipoles and somewhat weakened interaction for the x-polarized transition dipoles. From an analysis of available crystallographic data for porphyrin and metalloporphyrin pi-pi dimers (Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1) and the results of our exciton model, we conclude that the origin of the coordinate system for the Soret transition dipole moments of AcMP8 is not metal-centered. Furthermore, since the true directions of the x- and y-axes of the low-symmetry heme chromophore in AcMP8 are unknown, we have not been able to determine the structures of the pi-pi dimers from a knowledge of their transition dipole-dipole interaction geometries. This study therefore highlights one of the shortfalls of molecular exciton theory.     

The role of excitons' quasiequilibrium in the temperature dependence of the poly(9,9-dioctylfluorene) beta phase photoluminescence

We investigated the temperature dependence of the poly(9,9-dioctylfluorene) beta phase photoluminescence (PL) spectra in spin coated thin films from tetrahydrofuran solutions. As the temperature increases from 18 to 300 K a continuous blueshift of the 0-0 PL peak of about 25 meV and an increase of the peak full width at half maximum (FWHM) of about 49 meV are observed. We show that the PL spectra temperature dependence is not due to a temperature dependent average conjugation length, as often assumed, but instead it can be quantitatively explained in the frame of a thermal quasiequilibrium model for excitons in an inhomogeneously broadened excited states distribution. We demonstrate that the emission blueshift and broadening are mainly due to the increase of the excitons' temperature with the sample one. This effect is partially compensated by an increasing efficiency of the exciton energy migration. The interplay between these two processes quantitatively explains the observed temperature dependence of the PL peak energy and of its FWHM. On the contrary we show that the PL spectra are almost independent of the absorption blueshift with temperature.     

Effects of (multi)branching of dipolar chromophores on photophysical properties and two-photon absorption

To investigate the effect of branching on linear and nonlinear optical properties, a specific series of chromophores, epitome of (multi)branched dipoles, has been thoroughly explored by a combined theoretical and experimental approach. Excited-state structure calculations based on quantum-chemical techniques (time-dependent density functional theory) as well as a Frenkel exciton model nicely complement experimental photoluminescence and one- and two-photon absorption findings and contribute to their interpretation. This allowed us to get a deep insight into the nature of fundamental excited-state dynamics and the nonlinear optical (NLO) response involved. Both experiment and theory reveal that a multidimensional intramolecular charge transfer takes place from the donating moiety to the periphery of the branched molecules upon excitation, while fluorescence stems from an excited state localized on one of the dipolar branches. Branching is also observed to lead to cooperative enhancement of two-photon absorption (TPA) while maintaining high fluorescence quantum yield, thanks to localization of the emitting state. The comparison between results obtained in the Frenkel exciton scheme and ab initio results suggests the coherent coupling between branches as one of the possible mechanisms for the observed enhancement. New strategies for the rational design of NLO molecular assemblies are thus inferred on the basis of the acquired insights.     

Circular dichroism and absorption spectroscopy of merocyanine dimer aggregates: molecular properties and exciton transfer dynamics from time-dependent quantum calculations

A wave packet approach to the calculation and interpretation of circular dichroism (CD) spectra is applied to the spectroscopy of aggregates of a merocyanine dye. A combined analysis of absorption and CD spectra allows for the extraction of geometric information and excited state electronic coupling. It is shown that in the case of dimer aggregates of a chiral merocyanine dye, it is possible to infer the dynamics of an exciton transfer directly from the CD spectrum. This relation is established via the Fourier relation to a time-dependent correlation function reflecting the quantum dynamics in the dye aggregate.