Energy Transfer Steps Research Articles

Electronic resonance with anticorrelated pigment vibrations drives photosynthetic energy transfer outside the adiabatic framework

The delocalized, anticorrelated component of pigment vibrations can drive nonadiabatic electronic energy transfer in photosynthetic light-harvesting antennas. In femtosecond experiments, this energy transfer mechanism leads to excitation of delocalized, anticorrelated vibrational wavepackets on the ground electronic state that exhibit not only 2D spectroscopic signatures attributed to electronic coherence and oscillatory quantum energy transport but also a cross-peak asymmetry not previously explained by theory. A number of antennas have electronic energy gaps matching a pigment vibrational frequency with a small vibrational coordinate change on electronic excitation. Such photosynthetic energy transfer steps resemble molecular internal conversion through a nested intermolecular funnel.

Near-infrared quantum cutting in Pr3+/Yb3+ co-doped transparent tellurate glass via two step energy transfer

A series of Pr3+/Yb3+ co-doped transparent tellurate glass samples were prepared by high-temperature melting method. The optical properties and energy transfer process from Pr3+ to Yb3+ were investigated. On excitation of Pr3+ ions with a blue visible photon at 449nm, a near infrared emission at 1001nm were observed in the Pr3+/Yb3+ co-doped tellurate glass, which is attributed to 2F2/5→2F2/7 transition of Yb3+ ions. The feeding of Yb3+ ions is shown to be two step energy transfer from Pr3+ through 1G4 multiplet by excitation and emission spectra. The maximum total quantum efficiency reaches 136.7% when the co-doped Yb3+ mole concentration is 5%. The efficient down-conversion near infrared emission of Pr3+/Yb3+ co-doped tellurate glass has potential application in enhancing the conversion efficiency of crystalline silicon based solar cells.

Intense upconversion luminescence and origin study in Tm3+/Yb3+ codoped calcium scandate

Doping concentration optimized CaSc2O4:0.004Tm3+/0.1Yb3+ shows stronger upconversion luminescence (UCL) than doping concentration also optimized typical oxide upconverting phosphor Y2O3:0.004Tm3+/0.1Yb3+ upon 980 nm laser diode pump. The two-step up converted 3H4 → 3H6 near infrared emission peaked around 800 nm and the three-step up converted 1G4 → 3H6 blue emission around 480 nm of Tm3+ is enhanced by a factor of 3.5 and 2.2, respectively. On analyzing the 2F5/2 → 2F7/2 emission intensities and decay curves of Yb3+ in the two hosts, we reveal that Yb3+ in CaSc2O4 exhibits both a larger absorption cross section at 980 nm and Yb3+ → Tm3+ first step energy transfer coefficient (9.29 × 10−17 cm3 s−1) than that (2.87 × 10−17 cm3 s−1) in Y2O3, indicating that CaSc2O4 is an excellent host for achieving very intense UCL in Tm3+/Yb3+ codoped oxide upconverting materials.

Primary steps of electron and energy transfer in photosystem I: Effect of excitation pulse wavelength

Time-resolved differential spectra of photosystem I complex were obtained by the "pump-probe" technique with 25-fs pulses with maxima at 670, 700, and 720 nm. The ratio between the number of excited chlorophyll molecules of the antenna and of the reaction center was shown to depend on spectral characteristics of the pump pulses. In all cases, an ultrafast (<150 fs) formation of the primary radical pair P700(+)A(0)() was recorded. However, on excitation by pulses with maxima at 670 or 700 nm, detection of the charge separation was masked by the much more intensive bleaching at the chlorophyll Q(y) band due to excitation of the bulk antenna chlorophylls. We show that triggering the charge separation by 25-fs pulses centered at 720 nm allows to detect more clearly kinetics of formation of the primary and secondary ion-radical pairs. The findings help to explain possible reasons for discrepancies of kinetics of primary steps of electron transfer detected in different laboratories.

Characterization of light-induced, volumetric steam generation in nanofluids

Solar thermal energy has shown remarkable growth in recent years [1]. At the same time, research has shown that nanofluids – suspensions of nanoparticles in conventional fluids – can potentially enhance the efficiency of these solar thermal systems [2–7]. The fundamental advantage of this type of collector is that it can be designed to minimize the number of energy transfer steps, thereby reducing losses in converting sunlight (via thermal energy) to electricity. To investigate the inherent advantages of an optically tailored nanofluid system, we have experimentally tested the feasibility of volumetrically absorbing, direct steam collection mediums. In this study, we use a transparent testing apparatus where concentrated laser light at 532nm (a wavelength very near the solar spectrum peak) is incident on optically absorbing mediums (samples). The samples compared in this study are black dyes, black painted surfaces, and nanofluids – with de-ionized water as a base fluid for each. Each of these converts laser light energy to heat in a localized region. The heated region is monitored with a digital camera and an infrared camera, simultaneously. Resulting temperature profiles and bubble dynamics are compared for the different samples. For pure water with a black backing, high temperatures (>300°C) are observed with a laser input of ∼287W/cm2. For the same laser input, nanofluids show lower peak temperatures as compared to water with a black backing, but up to 50% more vapor generation. Vapor bubbles are seen to be larger in pure water as compared to nanofluids or black dyes. These results indicate volumetric absorbers (notably nanofluids) provide a more effective coupling between light energy and liquid-to-vapor phase change than surface absorbers. That is, our results indicate that a well-designed volumetric absorbing, direct steam nanofluid collector can potentially enhance the efficiency of the light-to-steam conversion in a solar thermal system.

Photon management in La2BaZnO5:Tm3+, Yb3+ and La2BaZnO5:Pr3+, Yb3+ by two step cross-relaxation and energy transfer

A luminescence study from UV to NIR of La2BaZnO5:Tm3+, Yb3+ and La2BaZnO5:Pr3+, Yb3+ is reported. Emission spectra of La2BaZnO5:Tm3+, Yb3+ reveal an unexpected high intensity of the Tm3+3F4→3H6 emission at 5000cm−1 upon excitation in the Tm3+1G4 level. This unusual efficiency is explained by two successive cross-relaxation steps between Tm3+ ions, resulting in potential emission of three NIR photons for each absorbed visible photon. Emission and decay spectra of La2BaZnO5:Pr3+, Yb3+ give evidence for a Pr3+ (3P0→1G4) to Yb3+ (2F7/2→2F5/2) energy transfer. The Pr3+ (1G4→3H4) to Yb3+ (2F7/2→2F5/2) energy transfer could not be derived from our data.

Sensitized Upconversion Emissions of Tb3+ by Tm3+ in YF3 and NaYF4 Nanocrystals

Yb3+-Tm3+-Tb3+-codoped YF3 and NaYF4 nanocrystals (NCs) were synthesized using a simple hydrothermal method. Under 980 nm excitation, violet and ultraviolet upconversion (UC) emissions of 5D3 --> 7FJ (J = 6, 5, 4) and 5D4 --> 7FJ (J = 6, 5, 4, 3) of Tb3+ ions were observed with the fluoride NCs. In the Yb-Tm-Tb codoped NCs, energy transfer (ET) processes from Tm3+ to Tb3+ were proposed to be the main mechanisms for the UC emissions of Tb3+ ions. They are more efficient than the phonon assisted cooperative sensitization of the Yb3+ couple proposed previously for similar material system. The analysis of power dependence indicated that populating the 5D4 level of the Tb3+ ions was a four photon UC process, which demonstrated the existence of the two step ET process of Yb3+ --> Tm3+ --> Tb3+. It was also found that UC luminescence properties of Tb3+ ions were sensitive to crystal structures.

Flow of Excitation Energy in the Cryptophyte Light-Harvesting Antenna Phycocyanin 645

We report a detailed description of the energy migration dynamics in the phycocyanin 645 (PC645) antenna complex from the photosynthetic alga Chroomonas CCMP270. Many of the cryptophyceae are known to populate greater depths than most other algal families, having developed a 99.5% efficient light-harvesting system. In this study, we used femtosecond time-resolved spectroscopy and global analysis to characterize the excited-state dynamics of PC645. Several different pump colors were selected to excite different fractions of the four phycobiliprotein pairs present in the complex. Measurements were also performed at cryogenic temperature to enhance spectral resolution and selectively promote downhill energy transfers. Upon excitation of the highest-energy bilins (dihydrobiliverdins), energy is transferred from the core of the complex to the periphery within 0.82 ps. Four bilins (mesobiliverdin (MBV) A/B and phycocyanobilins (PCB) 158C/D), which are responsible for the central band of the absorption spectrum, show concerted spectral dynamics. These chromophores show a biphasic decay with lifetimes of 0.6 ps (MBV) and 5–7 ps (PCB 158) to the lowest bilin pair (PCB 82C/D) absorbing around 650–657 nm. Within this lifetime of several picoseconds, the excitations reach the PCB 82 bilins on the two poles at the smaller sides of PC645. A slow 44–46 ps energy transfer step to the lowest-energy PCB 82 bilin concludes the dynamics.

Long-Lived, Charge-Shift States in Heterometallic, Porphyrin-Based Dendrimers Formed via Click Chemistry

A series of multiporphyrin clusters has been synthesized and characterized in which there exists a logical gradient for either energy or electron transfer between the porphyrins. A central free-base porphyrin (FbP), for example, is equipped with peripheral zinc(II) porphyrins (ZnP) which act as ancillary light harvesters and transfer excitation energy to the FbP under visible light illumination. Additional energy-transfer steps occur at the triplet level, and the series is expanded by including magnesium(II) porphyrins and/or tin(IV) porphyrins as chromophores. Light-induced electron transfer is made possible by incorporating a gold(III) porphyrin (AuP(+)) into the array. Although interesting by themselves, these clusters serve as control compounds by which to understand the photophysical processes occurring within a three-stage dendrimer comprising an AuP(+) core, a second layer formed from four FbP units, and an outer layer containing 12 ZnP residues. Here, illumination into a peripheral ZnP leads to highly efficient electronic energy transfer to FbP, followed by charge transfer to the central AuP(+). Charge recombination within the resultant charge-shift state is intercepted by secondary hole transfer to the ZnP, which occurs with a quantum yield of around 20%. The final charge-shift state survives for some microseconds in fluid solution at room temperature.

Ultrafast Photosensitization of Phthalocyanines through Their Axial Ligands

We have studied the energy transfer properties of a novel silicon phthalocyanine that coordinates two anthracene-9-carboxylate groups in the form of trans axial ligands. Our objectives were to generate a system with auxiliary chromophores that enhance the light absorption properties of the macrocycle in a specific region in the UV and to evaluate the efficiency and time scales for energy transfer. The ligand coordination through a carboxylate group directly attached to the anthracenic system allows for close proximity of the donor and acceptor chromophores. The energy transfer process was observed to be nearly 100% efficient and to occur on a time scale of 370 fs. From the energy relations of the donor and acceptor states and the observed dynamics, the initial energy transfer step is likely to involve upper electronic states of the phthalocyanine rather than the states of the lowest-energy vibroelectronic Q bands.

Self-Assembled Quantum Dot-Sensitized Multivalent DNA Photonic Wires

Combining the inherent scaffolding provided by DNA structure with spatial control over fluorophore positioning allows the creation of DNA-based photonic wires with the capacity to transfer excitation energy over distances greater than 150 Å. We demonstrate hybrid multifluorophore DNA-photonic wires that both self-assemble around semiconductor quantum dots (QDs) and exploit their unique photophysical properties. In this architecture, the QDs function as both central nanoscaffolds and ultraviolet energy harvesting donors that drive Förster resonance energy transfer (FRET) cascades through the DNA wires with emissions that approach the near-infrared. To assemble the wires, DNA fragments labeled with a series of increasingly red-shifted acceptor-dyes were hybridized in a predetermined linear arrangement to a complementary DNA template that was chemoselectively modified with a hexahistidine-appended peptide. The peptide portion facilitated metal-affinity coordination of multiple hybridized DNA-dye structures to a central QD completing the final nanocrystal-DNA photonic wire structure. We assembled several such hybrid structures where labeled-acceptor dyes were excited by the QDs and arranged to interact with each other via consecutive FRET processes. The inherently facile reconfiguration properties of this design allowed testing of alternate formats including the addition of an intercalating dye located in the template DNA or placement of multiple identical dye acceptors that engaged in homoFRET. Lastly, a photonic structure linking the central QD with multiple copies of DNA hybridized with 4-sequentially arranged acceptor dyes and demonstrating 4-consecutive energy transfer steps was examined. Step-by-step monitoring of energy transfer with both steady-state and time-resolved spectroscopy allowed efficiencies to be tracked through the structures and suggested that acceptor dye quantum yields are the predominant limiting factor. Integrating such DNA-based photonic structures with QDs can help create a new generation of biophotonic wire assemblies with widespread potential in nanotechnology.

PS II model based analysis of transient fluorescence yield measured on whole leaves of Arabidopsis thaliana after excitation with light flashes of different energies

Our recently presented PS II model (Belyaeva et al., 2008) was improved in order to permit a consistent simulation of Single Flash Induced Transient Fluorescence Yield (SFITFY) traces that were earlier measured by Steffen et al. (2005) on whole leaves of Arabidopsis (A.) thaliana at four different energies of the actinic flash. As the essential modification, the shape of the actinic flash was explicitly taken into account assuming that an exponentially decaying rate simulates the time dependent excitation of PS II by the 10ns actinic flash. The maximum amplitude of this excitation exceeds that of the measuring light by 9 orders of magnitude. A very good fit of the SFITFY data was achieved in the time domain from 100ns to 10s for all actinic flash energies (the maximum energy of 7.5×1016 photons/(cm2flash) is set to 100%, the relative energies of weaker actinic flashes were of ∼8%, 4%, ∼1%). Our model allows the calculation and visualization of the transient PS II redox state populations ranging from the dark adapted state, via excitation energy and electron transfer steps induced by pulse excitation, followed by final relaxation into the stationary state eventually attained under the measuring light. It turned out that the rate constants of electron transfer steps are invariant to intensity of the actinic laser flash. In marked contrast, an increase of the actinic flash energy by more than two orders of magnitude from 5.4×1014 photons/(cm2flash) to 7.5×1016 photons/(cm2flash), leads to an increase of the extent of fluorescence quenching due to carotenoid triplet (3Car) formation by a factor of 14 and of the recombination reaction between reduced primary pheophytin (Phe−) and P680+ by a factor of 3 while the heat dissipation in the antenna complex remains virtually constant.The modified PS II model offers new opportunities to compare electron transfer and dissipative parameters for different species (e.g. for the green algae and the higher plant) under varying illumination conditions.

Enhanced 1G 4 emission in NaLaF 4: Pr 3+, Yb 3+ and charge transfer in NaLaF 4: Ce 3+, Yb 3+ studied by fourier transform luminescence spectroscopy

A high resolution luminescence study of NaLaF 4: 1%Pr 3+, 5%Yb 3+ and NaLaF 4: 1%Ce 3+, 5%Yb 3+ in the UV to NIR spectral range using a InGaAs detector and a fourier transform interferometer is reported. Although the Pr 3+( 3P 0 → 1G 4), Yb 3+( 2F 7/2 → 2F 5/2) energy transfer step takes place, significant Pr 3+ 1G 4 emission around 993, 1330 and 1850 nm is observed. No experimental proof for the second energy transfer step in the down-conversion process between Pr 3+ and Yb 3+ can be given. In the case of NaLaF 4: Ce 3+, Yb 3+ it is concluded that the observed Yb 3+ emission upon Ce 3+ 5d excitation is the result of a charge transfer process instead of down-conversion.

Spectroscopic elucidation of uncoupled transition energies in the major photosynthetic light-harvesting complex, LHCII

Electrostatic couplings between chromophores in photosynthetic pigment-protein complexes, and interactions of pigments with the surrounding protein environment, produce a complicated energy landscape of delocalized excited states. The resultant electronic structure absorbs light and gives rise to energy transfer steps that direct the excitation toward a site of charge separation with near unity quantum efficiency. Knowledge of the transition energies of the uncoupled chromophores is required to describe how the wave functions of the individual pigments combine to form this manifold of delocalized excited states that effectively harvests light energy. In an investigation of the major light-harvesting complex of photosystem II (LHCII), we develop a method based on polarized 2D electronic spectroscopy to experimentally access the energies of the S(0)-S(1) transitions in the chromophore site basis. Rotating the linear polarization of the incident laser pulses reveals previously hidden off-diagonal features. We exploit the polarization dependence of energy transfer peaks to find the angles between the excited state transition dipole moments. We show that these angles provide a spectroscopic method to directly inform on the relationship between the delocalized excitons and the individual chlorophylls through the site energies of the uncoupled chromophores.

A Pre-organised Truxene Platform for Phosphorescent [Ru(bpy)2] and [Os(bpy)2] Metal Centres: A Clear-Cut Switch from Förster- to Dexter-Type Energy-Transfer Mechanism

We report on the synthesis, optical properties and energy-transfer features of a series of transition-metal-containing complexes and dyads, based on a pre-organised truxene scaffold. In this series, the [Ru(bpy)(3)](2+) and [Os(bpy)(3)](2+) photoactive terminals are coupled to the bridging aromatic truxene core through rigid ethynyl linkers. The photoinduced energy-transfer processes taking place from the Ru- to the Os-based levels, and from the truxene bridging ligand to the terminal-metal chromophores are studied and the pathways and mechanisms are discussed. The photoinduced energy-transfer process observed in the dyad proceeds rapidly through: i) an efficient (1)L-->(1)Os direct energy transfer followed by intersystem crossing to (3)Os, and ii) a fast (1)L-->(1)Ru energy-transfer step and subsequent intersystem crossing to (3)Ru followed by a (3)Ru-->(3)Os energy-transfer process. The first (1)L-->(1)Os and (1)L-->(1)Ru steps are controlled by a dipole-dipole interaction (Förster mechanism), whereas the subsequent (3)Ru-->(3)Os step proceeds by means of a long-range (approximately 24 A) through-bond mediated Dexter mechanism, facilitated by the conjugation along the bpy-truxene-bpy molecular axis.

Energy transfer processes in the isolated core antenna complexes CP43 and CP47 of photosystem II

The energy equilibration and transfer processes in the isolated core antenna complexes CP43 and CP47 of photosystem II have been studied by steady-state and ultrafast (femto- to nanosecond) time-resolved spectroscopy at room temperature. The annihilation-free femtosecond absorption data can be described by surprisingly simple sequential kinetic models, in which the excitation energy transfer between blue and red states in both antenna complexes is dominated by sub-picosecond processes and is completed in less than 2 ps. The slowest energy transfer steps with lifetimes in the range of 1–2 ps are assigned to transfer steps between the chlorophyll layers located on the stromal and lumenal sides. We conclude that these ultrafast intra-antenna energy transfer steps do not represent a bottleneck in the rate of the primary processes in intact photosystem II. Since the experimental energy equilibration rates are up to a factor of 3–5 higher than concluded previously, our results challenge the conclusions drawn from theoretical modeling.

Efficient visible to infrared quantum cutting through downconversion with the Er3+–Yb3+ couple in Cs3Y2Br9

Downconversion of one visible photon to two near-infrared photons may increase the efficiency of c-Si solar cells by 30%. The lanthanide ion couple Er3+–Yb3+ is well known for efficient upconversion but for the reverse process, downconversion, fast multiphonon relaxation from the F47/2 level has been shown to compete with downconversion. Here we report efficient downconversion for the Er–Yb couple in Cs3Y2Br9. The low phonon energy in this bromide host suppresses multiphonon relaxation and efficient two step energy transfer from the F47/2 level of Er3+ is observed and results in strong 1000 nm emission from Yb3+. Based on emission spectra and luminescence life time measurements an intrinsic downconversion efficiency close to 200% is determined.

Synthesis and photophysical properties of kinetically stable complexes containing a lanthanide ion and a transition metal antenna group

A series of bimetallic complexes has been prepared in which an octadentate DOTA-monoamide pocket containing a bound lanthanide ion is linked covalently to a Re(I) or Ru(II) bipyridyl unit via an alkyl spacer group. The transition metal chromophores incorporated in this way act as effective sensitisers for lanthanide-centred luminescence. The rate and efficiency of energy transfer are dependent upon the nature of the spacer group, and upon the nature of the lanthanide acceptor. For the RuNd diad, there is a long rise-time associated with the energy-transfer step, such that energy transfer is rate determining in H(2)O, but not in D(2)O. The results also lead us to suggest that energy transfer may precede formation of the (3)M((Ru/Re))L((bpy))CT state and may be a competitive deactivation pathway for the precursor state ((1)M((Ru/Re))L((bpy))CT).

Analysis of the Contribution of Charge Transport in Iodine-125-Induced DNA Damage

Auger electron emitters like (125)I are the radionuclides of choice for gene-targeted radiotherapy. The highly localized damage they induce in DNA is produced by three mechanisms: direct damage by the emitted Auger electrons, indirect damage by diffusible free radicals produced by Auger electrons traveling in water, and charge neutralization of the residual, highly positively charged tellurium daughter atom by stripping electrons from covalent bonds of neighboring residues. The purpose of our work was to determine whether these mechanisms proceed through an intermediate energy transfer step along DNA. It was proposed that this intermediate step proceeds through the charge transport mechanism in DNA. Conventional charge transport has been described as either a hopping mechanism initiated by charge injection into DNA and propagated by charge migration along the DNA or a tunneling mechanism in which charge moves directly from a donor to an acceptor within DNA. Well-known barriers for the hopping mechanism were used to probe the role of charge transport in (125)I-induced DNA damage. We studied their effect on the distribution of DNA breaks produced by the decay of (125)I in samples frozen at -80 degrees C. We found that these barriers had no measurable effect on the distribution of (125)I-induced breaks.

Singlet−Singlet Energy Transfer in Self-Assembled Systems of the Cationic Poly{9,9-bis[6-N,N,N-trimethylammonium)hexyl]fluorene-co-1,4-phenylene} with Oppositely Charged Porphyrins

Electronic energy transfer has been studied between the cationic conjugated polyelectrolyte, poly{9,9-bis[6-N,N,N-trimethylammonium)hexyl]fluorene-co-1,4-phenylene} dibromide (HTMA-PFP), and three, oppositely charged meso-tetrakis-phenylporphyrinsulfonates in buffered (pH = 9.2), 4% (v/v) dimethyl sulfoxide-water (DMSO-water) solutions using steady-state and time-resolved fluorescence. Energy transfer was indicated by the decrease in intensity of the fluorescence band of the HTMA-PFP donor, by a corresponding increase in fluorescence of the porphyrin acceptors, by a band in the excitation spectrum of the porphyrin corresponding to the polymer absorption, and by the fact that the decay of the polymer emission observed at 423 nm was accompanied by the grow-in of porphyrin emission at 650 nm in time-resolved measurements. It is suggested that the energy transfer may involve upper excited states of the acceptor. The Förster equation and the experimental spectral overlap between donor fluorescence and acceptor absorption were used to calculate Förster radii for the three systems. Both steady-state and dynamic Stern-Volmer plots were nonlinear at high acceptor concentrations, and quenching rate constants calculated from the slopes of the initial linear region and the HTMA-PFP fluorescence lifetime were orders of magnitude greater than expected for a diffusion-controlled process, strongly supporting the idea that energy transfer occurs in self-assembled species formed by association (through ion pairing) of the polymer and porphyrins. There are indications that these aggregates involve more than one polymer chain. Picosecond time-resolved measurements on the HTMA-PFP fluorescence decay showed a short-lived component, attributed to the energy-transfer step, and two longer lived decays, which may be associated with exciton migration along the chain and the fluorescence decay of the polymer backbone, respectively. From considerations of the probable distance between donor and acceptor it is suggested that the Forster mechanism, assuming point dipoles, is inadequate for this system and that more detailed calculations, considering the actual sizes of the donor and acceptor, are necessary.