How energy funnels from the phycoerythrin antenna complex to photosystem I and photosystem II in cryptophyte Rhodomonas CS24 cells

We report an investigation of energy migration dynamics in intact cells of the photosynthetic cryptophyte Rhodomonas CS24 using analyses of steady-state and time-resolved fluorescence anisotropy measurements. By fitting a specific model to the fluorescence data, we obtain three time scales (17, 58, and 113 ps) by which the energy is transferred from phycoerythrin 545 (PE545) to the membrane-associated chlorophylls (Chls). We propose that these time scales reflect both an angular distribution of PE545 around the photosystems and the relative orientations of the donor dihydrobiliverdin (DBV) bilin and the acceptor Chl. Contrary to investigations of the isolated antenna complex, it is demonstrated that energy transfer from PE545 does not occur from a single-emitting bilin, but rather both the peripheral dihydrobiliverdin (DBV) chromophores in PE545 appear to be viable donors of excitation energy to the membrane-bound proteins. The model shows an almost equal distribution of excitation energy from PE545 to both photosystem I (PSI) and photosystem II (PSII), whose trap times correspond well to those obtained from experiments on isolated photosystems.     

Exciton delocalization and energy transport mechanisms in R-phycoerythrin

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

Excitation dynamics in the LHCII complex of higher plants: modeling based on the 2.72 Angstrom crystal structure

We have modeled steady-state spectra and energy-transfer dynamics in the peripheral plant light-harvesting complex LHCII using new structural data. The dynamics of the chlorophyll (Chl) b-->Chl a transfer and decay of selectively excited "bottleneck" Chl a and b states have been studied by femtosecond pump-probe spectroscopy. We propose an exciton model of the LHCII trimer (with specific site energies) which allows a simultaneous quantitative fit of the absorption, linear-dichroism, steady-state fluorescence spectra, and transient absorption kinetics upon excitation at different wavelengths. In the modeling we use the experimental exciton-phonon spectral density and modified Redfield theory. We have found that fast b-->a transfer is determined by a good connection of the Chls b to strongly coupled Chl a clusters, i.e., a610-a611-a612 trimer and a602-a603 and a613-a614 dimers. Long-lived components of the energy-transfer kinetics are determined by a quick population of red-shifted Chl b605 and blue-shifted Chl a604 followed by a very slow (3 ps for b605 and 12 ps for a604) flow of energy from these monomeric bottleneck sites to the Chl a clusters. The dynamics within the Chl a region is determined by fast (with time constants down to sub-100 fs) exciton relaxation within the a610-a611-a612 trimer, slower 200-300 fs relaxation within the a602-a603 and a613-a614 dimers, even slower 300-800 fs migration between these clusters, and very slow transfer from a604 to the quasi-equilibrated a sites. The final equilibrium is characterized by predominant population of the a610-a611-a612 cluster (mostly the a610 site). The location of this cluster on the outer side of the LHCII trimer probably provides a good connection with the other subunits of PSII.     

Pigment-pigment and pigment-protein interactions in recombinant water-soluble chlorophyll proteins (WSCP) from cauliflower

Plants contain water-soluble chlorophyll-binding proteins (WSCPs) that function neither as antennas nor as components of light-induced electron transfer of photosynthesis but are likely constituents of regulatory protective pathways in particular under stress conditions. This study presents results on the spectroscopic properties of recombinant WSCP from cauliflower reconstituted with chlorophyll b (Chl b) alone or with mixtures of Chl a and Chl b. Two types of experiments were performed: (a) measurements of stationary absorption spectra at 77 and 298 K and CD spectra at 298 K and (b) monitoring of laser flash-induced transient absorption changes with a resolution of 200 fs in the time domain of up to 100 ps. On the basis of a theoretical analysis outlined by Renger et al. (J. Phys. Chem. B 2007, 111, 10487) the data obtained in part (a) are interpreted within a model where tetrameric WSCP binds predominantly two Chl molecules in the form of an excitonically coupled "open sandwich" dimer with a tilt angle of about 30 degrees between the chlorin planes. The time-resolved measurements on Chl a/Chl b heterodimers are described by two exponential kinetics with time constants of 400 fs and 7 ps. These kinetics are assumed to reflect a heterogeneous population of WSCPs with Chl dimers either in excitonic coupled "open sandwich" or weakly coupled geometric arrays. The 400 fs component is assigned to excited-state relaxations from the upper to the lower excitonic level of the strongly coupled "open sandwich" dimer, while the 7-8 ps component probably indicates excitation energy transfer from 1Chl b* to Chl a in a dimer array with weak coupling due to significantly longer mutual distances between the chlorin rings.     

Anti-correlated spectral motion in bisphthalocyanines: evidence for vibrational modulation of electronic mixing

We exploit a coherently excited nuclear wave packet to study nuclear motion modulation of electronic structure in a metal bridged phthalocyanine dimer, lutetium bisphthalocyanine, which displays two visible absorption bands. We find that the nuclear coordinate influences the energies of the underlying exciton and charge resonance states as well as their interaction; the interplay of the various couplings creates unusual anti-correlated spectral motion in the two bands. Excited state relaxation dynamics are the same regardless of which transition is pumped, with decay time constants of 1.5 and 11 ps. The dynamics are analyzed using a three-state kinetic model after relaxation from one or two additional states faster than the experimental time resolution of 50-100 fs.     

Photophysical properties of porphyrin tapes

The novel fused Zn(II)porphyrin arrays (Tn, porphyrin tapes) in which the porphyrin macrocycles are triply linked at meso-meso, beta-beta, beta-beta positions have been investigated by steady-state and time-resolved spectroscopic measurements along with theoretical MO calculations. The absorption spectra of the porphyrin tapes show a systematic downshift to the IR region as the number of porphyrin pigments increases in the arrays. The fused porphyrin arrays exhibit a rapid formation of the lowest excited states (for T2, approximately 500 fs) via fast internal conversion processes upon photoexcitation at 400 nm (Soret bands), which is much faster than the internal conversion process of approximately 1.2 ps observed for a monomeric Zn(II)porphyrin. The relaxation dynamics of the lowest excited states of the porphyrin tapes were accelerated from approximately 4.5 ps for the T2 dimer to approximately 0.3 ps for the T6 hexamer as the number of porphyrin units increases, being explained well by the energy gap law. The overall photophysical properties of the porphyrin tapes were observed to be in a sharp contrast to those of the orthogonal porphyrin arrays. The PPP-SCI calculated charge-transfer probability indicates that the lowest excited state of the porphyrin tapes (Tn) resembles a Wannier-type exciton closely, whereas the lowest excited state of the directly linked porphyrin arrays can be considered as a Frenkel-type exciton. Conclusively, these unique photophysical properties of the porphyrin tapes have aroused much interest in the fundamental photophysics of large flat organic molecules as well as in the possible applications as electric wires, IR sensors, and nonlinear optical materials.     

Ultrafast photo-induced charge transfer unveiled by two-dimensional electronic spectroscopy

The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology, and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements, and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.     

Polymer dynamics in torsion space

The large scale motion of proteins, or covalently bonded polymers in general, is governed by the dynamics of the torsion angles, with bond lengths and bond angles kept approximately constant. In the present work, the Lagrangian equations of torsion motion are derived for a general macromolecule. The dynamics is implemented numerically for a test protein, using the velocity Verlet method as the integrator. The results indicate time steps of up to about 30 fs can be used for short time (up to at least 20 ps) simulations, before the dynamics and energy start to differ significantly from results obtained with smaller time steps. For longer time simulations, up to 1000 ps, a time step of 10 fs is relatively safe. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

The dependence of singlet exciton relaxation on excitation density and temperature in polycrystalline tetracene thin films: kinetic evidence for a dark intermediate state and implications for singlet fission

The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A. M. Muller, D. Gosztola, and C. J. Bardeen, J. Chem. Phys. 133, 144506 (2010)]. A study of the intensity dependence of the singlet state decay is conducted to understand the origins of the discrepancy between the broadband transient absorption and fluorescence experiments seen previously. High-sensitivity single channel transient absorption experiments allow us to compare the transient absorption dynamics to the fluorescence dynamics measured at identical laser fluences. At high excitation densities, an exciton-exciton annihilation rate constant of ~1 × 10(-8) cm(3) s(-1) leads to rapid singlet decays, but at excitation densities of 2 × 10(17) cm(-3) or less the kinetics of the transient absorption match those of the fluorescence. At these lower excitation densities, both measurements confirm that the initially excited singlet state relaxes with a decay time of 80 ± 3 ps, not 9.2 ps as claimed in the earlier paper. In order to investigate the origin of the singlet decay, the wavelength-resolved fluorescence dynamics were measured at 298 K, 77 K, and 4 K. A high-energy J-type emitting species undergo a rapid (~100 ps) decay at all temperatures, while at 77 K and 4 K additional species with H-type and J-type emission lineshapes have much longer lifetimes. A global analysis of the wavelength-dependent decays shows that the initial ~100 ps decay occurs to a dark state and not via energy transfer to lower energy bright states. Varying the excitation wavelength from 400 nm to 510 nm had no effect on the fast decay, suggesting that there is no energy threshold for the initial singlet relaxation. The presence of different emitting species at different temperatures means that earlier interpretations of the fluorescence behavior in terms of one singlet state that is short-lived due to singlet fission at high temperatures but long-lived at lower temperatures are probably too simplistic. The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.     

Pure optical dephasing dynamics in semiconducting single-walled carbon nanotubes

We report a detailed study of ultrafast exciton dephasing processes in semiconducting single-walled carbon nanotubes employing a sample highly enriched in a single tube species, the (6,5) tube. Systematic measurements of femtosecond pump-probe, two-pulse photon echo, and three-pulse photon echo peak shift over a broad range of excitation intensities and lattice temperature (from 4.4 to 292 K) enable us to quantify the timescales of pure optical dephasing (T(2)(*)), along with exciton-exciton and exciton-phonon scattering, environmental effects as well as spectral diffusion. While the exciton dephasing time (T(2)) increases from 205 fs at room temperature to 320 fs at 70 K, we found that further decrease of the lattice temperature leads to a shortening of the T(2) times. This complex temperature dependence was found to arise from an enhanced relaxation of exciton population at lattice temperatures below 80 K. By quantitatively accounting the contribution from the population relaxation, the corresponding pure optical dephasing times increase monotonically from 225 fs at room temperature to 508 fs at 4.4 K. We further found that below 180 K, the pure dephasing rate (1/T(2)(*)) scales linearly with temperature with a slope of 6.7 ± 0.6 μeV/K, which suggests dephasing arising from one-phonon scattering (i.e., acoustic phonons). In view of the large dynamic disorder of the surrounding environment, the origin of the long room temperature pure dephasing time is proposed to result from reduced strength of exciton-phonon coupling by motional narrowing over nuclear fluctuations. This consideration further suggests the occurrence of remarkable initial exciton delocalization and makes nanotubes ideal to study many-body effects in spatially confined systems.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

Picosecond relaxation dynamics in polydiacetylene-pTS

Time-resolved absorption spectroscopy as well as transient grating measurements were performed on polydiacetylene-pTS. Singlet exciton lifetimes of 0.8 ± 0.2 ps at 300 K and 1.1 ± 0.2 ps at 5 K were measured. We propose that the observed non-radiative relaxation proceeds through a conformationally relaxed singlet state which lives for roughly 1.0 and 3.4 ps at 300 and 5 K, respectively. Data pertinent to the formation of triplet excitons are also presented and discussed.     

Femtosecond dynamics of excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

The dynamics of excited-state intramolecular proton transfer (ESIPT) and of relaxation processes in o-tosylaminobenzoic acid (TAC) and o-acetylaminobenzoic acid (AAC) have been studied by femtosecond absorption spectroscopy with a time resolution of 30 fs. The ESIPT characteristic time in the TAC dimer and monomer and in AAC monomer is 50 fs. The excited product of photoinduced proton transfer in the monomer undergoes effective radiationless deactivation with a characteristic time of 30 ps, one of the channels of which is internal rotation followed by intersystem crossing and internal conversion. The product of ESIPT in the TAC dimer deactivates preferentially into the ground state via radiative transition with a time of 291 ps. ESIPT in the AAC dimer is thermodynamically unfavorable and occurs with a low yield.     

Communication: Time-domain measurement of high-pressure N2 and O2 self-broadened linewidths using hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering

The direct measurement of self-broadened linewidths using the time decay of pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps RCARS) signals is demonstrated in gas-phase N(2) and O(2) from 1-20 atm. Using fs pump and Stokes pulses and a spectrally narrowed ps probe pulse, collisional dephasing rates with time constants as short as 2.5 ps are captured with high accuracy for individual rotational transitions. S-branch linewidths of N(2) and O(2) from ~0.06 to 2.2 cm(-1) and the line separation of O(2) triplet states are obtained from the measured dephasing rates and compared with high-resolution, frequency-domain measurements and S-branch approximations using the modified exponential gap model. The accuracy of the current measurements suggests that the fs/ps RCARS approach is well suited for tracking the collisional dynamics of gas-phase mixtures over a wide range of pressures.     

Temperature dependence of solvation dynamics and anisotropy decay in a protein: ANS in bovine serum albumin

Temperature dependence of solvation dynamics and fluorescence anisotropy decay of 8-anilino-1-naphthalenesulfonate (ANS) bound to a protein, bovine serum albumin (BSA), are studied. Solvation dynamics of ANS bound to BSA displays a component (300 ps) which is independent of temperature in the range of 278-318 K and a long component which decreases from 5800 ps at 278 K to 3600 ps at 318 K. The temperature independent part is ascribed to a dynamic exchange of bound to free water with a low barrier. The temperature variation of the long component of solvation dynamics corresponds to an activation energy of 2.1 kcal mol(-1). The activation energy is ascribed to local segmental motion of the protein along with the associated water molecules and polar residues. The time scale of solvation dynamics is found to be very different from the time scale of anisotropy decay. The anisotropy decays are analyzed in terms of the wobbling motion of the probe (ANS) and the overall tumbling of the protein.     

Dynamical changes of hemoglobin and its surrounding water during thermal denaturation as studied by quasielastic neutron scattering and temperature modulated differential scanning calorimetry

We have investigated the dynamical behavior of both the protein hemoglobin and its surrounding water during the denaturation process using modulated temperature differential scanning calorimetry, quasielastic neutron scattering, and frequency dependent conductivity measurements. To distinguish between the scattering from the protein and its surrounding water, neutron scattering measurements were performed on both a fully hydrogenated sample as well as a sample where the water and the exchangeable hydrogen atoms on the protein surface were deuterated. The experimental data show that the unfolding and aggregation processes are substantially overlapping in temperature. The unfolding process occurs in the approximate temperature range of 315-345 K, whereas the aggregation process starts around 330-335 K and is completed at 360 K. Furthermore, the results suggest that the secondary structure of the protein unfolds at about 325 K, and that this leads to an increased number of water molecule hydrogen bonded to the protein. Thus, the unfolding of the secondary structure reduces the number of mobile (on the experimental time scale of about 50-100 ps) water molecules. In contrast, the aggregation of protein molecules seems to have a minor effect on the dynamics of its surrounding water. In the case of the protein dynamics there are competing effects from unfolding and aggregation. The unfolding process increases the flexibility of the protein, whereas the initial aggregation reduces its dynamics. The conductivity seems to be negatively affected by both reduced water mobility and an aggregation of protein molecules.     

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.     

Observation of a fluorescent dimer of a sulfonated phthalocyanine

With very few exceptions, phthalocyanine dimers are found to be nonfluorescent. We report here the observation of a fluorescent dimer of a tetrasulfonated copper phthalocyanine in ethanol and water. Fluorescence excitation and emission spectra at room temperature and at 77 K are presented. These are consistent with the conventional model of exciton coupling in a cofacial dimer.     

Subunit separation (alpha,alpha',beta) of cryptomonad biliproteins

Abstract— Phycocyanin 645, phycocyanin 612 and phycoerythrin 545 are biliproteins isolated from the cryptomonad algae, Chroornonas species, Hemiselmis virescens and Rhodomonas lens, respectively. The protein has α and β subunits. which are separated on an ion exchange column by using a urea gradient and omitting 2-mercaptoethanol from the solvent. This separation establishes that the α and β subunits are not joined by disulfide bonds. In addition it has recently been shown that mercaptoethanol can produce spurious results in the calculation of the chromophore contents of these biliproteins (Guard-Friar and MacColl, 1984). The mercaptoethanol-free experiments allow analysis of the chromophore content in a rapid and artifact-free manner. When the ion exchange chromatography of phycocyanin 645 is manipulated by changing the type of urea gradient, two distinct a subunit fractions are obtained. These two fractions have identical visible absorption spectra but different amino acid compositions. At least two different gene products are, thus. responsible for the a subunits. The sole chromophore on the two a subunits of phycocyanin 645 is the unique 697-nm bilin as seen in acidic urea. Its reactivity with mercaptoethanol is determined. The α subunits of phycoerythrin 545 have two different bilins: cryptoviolin and phycoerythrobilin. Phycocyanobilin is the chromophore on the α subunit of phycocyanin 612.