Efficient Excitation Transfer Research Articles

Syntheses and Excitation Transfer Studies of Near-Orthogonal Free-Base Porphyrin–Ruthenium Phthalocyanine Dyads and Pentad

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (k(ENT)) was monitored by performing transient absorption and emission measurements using pump-probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental k(ENT) values on the order of 10(10)-10(11) s(-1), which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed.

Efficient and coherent excitation transfer across disordered molecular networks

We show that finite-size, disordered molecular networks can mediate highly efficient, coherent excitation transfer which is robust against ambient dephasing and associated with strong multisite entanglement. Such optimal, random molecular conformations may explain efficient energy transfer in the photosynthetic Fenna-Matthews-Olson complex.

Characterizing quantum-sharing of electronic excitation in molecular aggregates

A key and long standing question regarding the function of photosynthetic systems concerns the advantages that delocalized electronic excitations and their coherent dynamics could offer to robust and efficient energy transfer within and between photosynthetic light-harvesting complexes. Here we discuss how the framework of entanglement can be used to characterize the strength and spatial distribution of electronic coherences in biomolecular aggregates, why this is interesting, and how one can go about investigating possible relations between non-vanishing electronic coherences and efficient excitation transfer from donors to acceptors. As an example we discuss how certain coherences may correlate to efficient energy transfer in the Fenna-Mathews-Olson complex. Perspectives about understanding advantages of coherence-assisted energy transfer are discussed.

Efficient optical excitation transfer in layered quantum dot nanostructures networked via optical near-field interactions

We investigated the efficient optical excitation transfer in layered quantum dot structures by introducing a network of optical near-field interactions. With a density-matrix-based formalization of interdot near-field interactions, our theoretical approach allows systematic analysis of layered CdTe quantum dot systems, revealing dominant factors contributing to the efficient optical excitation transfer and demonstrating good agreement with previous experimental observations. We also show that the efficiency of optical excitation transfer could be further improved by optimizing the interaction network.

A Novel Photosynthetic Strategy for Adaptation to Low Iron Environments

Iron availability is a major limiting factor for photosynthesis and hence for life in most of the aquatic environments on earth. Cyanobacteria are important primary producers and prevail over Fe- deficiency by de-repressing the isiAB operon, which codes for the antenna protein IsiA and flavodoxin. We demonstrate that under nanomolar iron concentrations, a giant IsiA-Photosystem I supercomplex is formed, consisting of a Photosystem I trimer encircled by two complete IsiA rings with 18 and 25 copies in the inner ring and outer ring, respectively. The IsiA-Photosystem I supercomplex contains more than 850 chlorophylls and has a mass of 3.2 MDa, making it the most complex membrane protein that has been isolated to date. Ultrafast fluorescence spectroscopic results show fast and efficient excitation transfer and trapping in the supercomplex. The electron transfer throughput of Photosystem I is increased by 300%, an evolutionary adaptation that has allowed cyanobacteria to avoid oxidative stress. This adaptation of the photosynthetic apparatus confers an enormous, as-yet unrecognized, evolutionary advantage to cyanobacteria living under conditions of severe Fe stress and thereby have adapted to the modern low iron concentration in aquatic environments.1. Eldridge, M. et al. Aquatic Microbial Ecology 35, 79-91 (2004).2. Martin, J. & Fitzwater, S. Nature 331, 341-343 (1988).3. Wilhelm, S. Aquatic Microbial Ecology 9, 295-303 (1995).4. Bibby, T. S., Nield, J. & Barber, J. Nature 412, 743-5 (2001).5. Boekema, E. J. et al. Nature 412, 745-8 (2001).6. Singh, A., McIntyre, L. & Sherman, L. Plant Physiology 132, 1825 (2003).

Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport

Excitation transfer through interacting systems plays an important role in many areas of physics, chemistry, and biology. The uncontrollable interaction of the transmission network with a noisy environment is usually assumed to deteriorate its transport capacity, especially so when the system is fundamentally quantum mechanical. Here we identify key mechanisms through which noise such as dephasing, perhaps counter intuitively, may actually aid transport through a dissipative network by opening up additional pathways for excitation transfer. We show that these are processes that lead to the inhibition of destructive interference and exploitation of line broadening effects. We illustrate how these mechanisms operate on a fully connected network by developing a powerful analytical technique that identifies the invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we show how these principles can explain the remarkable efficiency and robustness of excitation energy transfer from the light-harvesting chlorosomes to the bacterial reaction center in photosynthetic complexes and present a numerical analysis of excitation transport across the Fenna–Matthew–Olson complex together with a brief analysis of its entanglement properties. Our results show that, in general, it is the careful interplay of quantum mechanical features and the unavoidable environmental noise that will lead to an optimal system performance.

Optical activity of Sn-variants of oxygen deficient centers in fluorine-modified silica

Photoluminescence in fluorine-modified Sn-doped silica has been analyzed by means of synchrotron radiation in the UV and vacuum-UV, from 120 to 330 nm, looking at the optical activity of oxygen-deficient-centers ODC(II) in Sn-substituted cationic sites. The comparison between F-modified Sn-doped samples and previous data on F-free Sn-doped material evidences differences in the intensity of the 3.2 eV emission band excited at 3.7 eV, and in the thermal dependence of the intensity of this emission excited via intersystem crossing. The role of fluorine in modifying the optical activity of ODC(II) and in the SnO 2 clustering is discussed, showing that an efficient excitation transfer may be activated from SnO 2 to the Sn-variant of ODC(II).

Excitation mechanism of europium ions embedded into TiO 2 nanocrystalline matrix

In this work, the excitation mechanism of Eu 3+ ions embedded into nanocrystalline TiO 2 film obtained by Low Pressure Hot Target Reactive Sputtering has been investigated by means of photoluminescence, photoluminescence excitation, transmission and surface photovoltage spectroscopy. The strong emission related to europium ions in the visible range has been observed at a non resonant excitation wavelength suggesting an efficient excitation transfer from the matrix to the ions. Based on obtained results, indirect excitation mechanism of europium ions has been confirmed and discussed. Moreover, surface activity strongly depending on environment conditions, has been also observed.

Influence of Annealing on Europium Photoexcitation Doped into Nanocrystalline Titania Film Prepared by Magnetron Sputtering

Nanocrystalline films doped by Eu were obtained by low-pressure hot target reactive sputtering at different annealing temperatures and then characterized both structurally and optically. Strong emission assigned to europium ions was observed in the visible range. No emission related to the matrix was detected, indicating an efficient excitation transfer from the matrix to the ions under nonresonant excitation. Europium emission strongly depends on the annealing temperature and reaches a maximum intensity for the sample annealed at before dropping sharply at higher temperatures. Emission, absorption, and photoluminescence excitation measurements indicate that this effect results from the influence of defect formation at higher temperatures.

Excitation Transfer in Functionalized Carbon Nanotubes

Energy flow: Efficient excitation transfer from porphyrins to nanotubes occurs when the porphyrin molecules, which act as antenna, are flattened (π-stacked, see figure) on the nanotube wall. The photoluminescence of the nanotubes is enhanced when the excitation energy is tuned in resonance with the absorption band of the porphyrins.

Structure of the light-harvesting bacteriochlorophyll c assembly in chlorosomes from Chlorobium limicola determined by solid-state NMR

We have determined the atomic structure of the bacteriochlorophyll c (BChl c) assembly in a huge light-harvesting organelle, the chlorosome of green photosynthetic bacteria, by solid-state NMR. Previous electron microscopic and spectroscopic studies indicated that chlorosomes have a cylindrical architecture with a diameter of approximately 10 nm consisting of layered BChl molecules. Assembly structures in huge noncrystalline chlorosomes have been proposed based mainly on structure-dependent chemical shifts and a few distances acquired by solid-state NMR, but those studies did not provide a definite structure. Our approach is based on (13)C dipolar spin-diffusion solid-state NMR of uniformly (13)C-labeled chlorosomes under magic-angle spinning. Approximately 90 intermolecular C C distances were obtained by simultaneous assignment of distance correlations and structure optimization preceded by polarization-transfer matrix analysis. It was determined from the approximately 90 intermolecular distances that BChl c molecules form piggyback-dimer-based parallel layers. This finding rules out the well known monomer-based structures. A molecular model of the cylinder in the chlorosome was built by using this structure. It provided insights into the mechanisms of efficient light harvesting and excitation transfer to the reaction centers. This work constitutes an important advance in the structure determination of huge intact systems that cannot be crystallized.

Study on energy transfer between carotenoid and chlorophyll a in cytochrome b 6 f complex from Bryopsis corticulans

The excitation energy transfer between carotenoid and chlorophyll (Chl) in the cytochrome b ( 6 ) f complex from Bryopsis corticulans (B. corticulans), in which the carotenoid is 9-cis-alpha-carotene, was investigated by means of fluorescence excitation and sub-microsecond time-resolved absorption spectroscopies. The presence of efficient singlet excitation transfer from alpha-carotene to Chl a was found with an overall efficiency as high as approximately approximately 24%, meanwhile the Chl a-to-alpha-carotene triplet excitation transfer was also evidenced. Circular dichroism spectroscopy showed that alpha-carotene molecule existed in an asymmetric environment and Chl a molecule had a certain orientation in this complex.

Excitation process and photoluminescence properties of Tb 3+ and Eu 3+ ions in SnO 2 and in SnO 2: Porous silicon hosts

Tin oxide (SnO 2)-layers-doped terbium and europium ions are elaborated by the sol–gel method on silicon substrates. After annealing at 500 °C, the transmission electron microscopy revealed a crystallization of tin oxide. The emission properties of rare-earth in SnO 2 are studied systematically against temperature annealing and Tb 3+ concentration. The PL spectrum is optimal after annealing at 900 °C and the corresponding photoluminescence (PL) decay is nearly exponential, showing that the sample is homogenous and the PL process can be described by two levels system. The concentration effect shows a quenching of the PL intensity for Tb 3+ concentration above 4%. From the investigation of the decay rate from the 7F 5 state within terbium concentration, we show that self-quenching is insured by dipole – dipole interaction. The evolutions of both PL intensity and PL lifetime versus temperature are studied. The PL intensity and PL lifetime are enhanced by deposing SnO 2:Tb 3+ and SnO 2:Eu 3+ in porous silicon. We show that an efficient excitation transfer from Si nanocrystallites to RE ions can occur.

Functional Consequences of the Organization of the Photosynthetic Apparatus in Rhodobacter sphaeroides: I. QUINONE DOMAINS AND EXCITATION TRANSFER IN CHROMATOPHORES AND REACTION CENTER·ANTENNA COMPLEXES

The purpose of this study was to gain information on the functional consequences of the supramolecular organization of the photosynthetic apparatus in the bacterium Rhodobacter sphaeroides. Isolated complexes of the reaction center (RC) with its core antenna ring (light-harvesting complex 1 (LH1)) were studied in their dimeric (native) form or as monomers with respect to excitation transfer and distribution of the quinone pool. Similar issues were examined in chromatophore membranes. The relationship between the fluorescence yield and the amount of closed centers is indicative of a very efficient excitation transfer between the two monomers in isolated dimeric complexes. A similar dependence was observed in chromatophores, suggesting that excitation transfer in vivo from a closed RC.LH1 unit is also essentially directed to its partner in the dimer. The isolated complexes were found to retain 25-30% of the endogenous quinone acceptor pool, and the distribution of this pool among the complexes suggests a cooperative character for the association of quinones with the protein complexes. In chromatophores, the decrease in the amount of photoreducible quinones when inhibiting a fraction of the centers implies a confinement of the quinone pool over small domains, including one to six reaction centers. We suggest that the crowding of membrane proteins may not be the sole reason for quinone confinement and that a quinone-rich region is formed around the RC.LH1 complexes.

Efficient excitation transfer from polymer to nanocrystals

We quantify experimentally the efficiency of excitation transfer from a semiconducting polymer matrix to quantum dot nanocrystals. We study 5±0.5 nm PbS nanocrystals embedded in MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)]) polymer. We determine the excitation transfer efficiency from normalized photoluminescence excitation measurements. When the composites are made using as-synthesized PbS nanocrystals capped by oleate ligands, the excitation transfer efficiency is about 20%. Replacing these ligands with shorter chains results in a factor-of-3 enhancement in the excitation transfer efficiency. Our findings provide guidance to the realization of efficient electroluminescent devices.

Morphology and optical spectra of metal iodate microcrystals in porous matrices

Microcrystals of iodates of mercury, lead, thallium, and bismuth were grown in pores of glass and polymer matrices from solutions or through sublimation in vacuum. Images of these microcrystals embedded in matrices were obtained with an electron microscope. Exciton absorption and luminescence spectra of the microcrystals revealed a strong difference in the temperature regions of stability of the various structural modfications between bulk crystals and microcrystals of some iodates. The absorption and luminescence spectra are broadened inhomogeneously because of considerable size dispersion of the microcrystals and exhibit quantum confinement effects. The exciton emission spectra show that exciton interaction in microcrystals becomes significant at very low optical pumping levels because of efficient excitation transfer from the matrix.

Excitation energy transfer from polystyrene to dye in 40-nm diameter microspheres

When dye molecules are placed into a solid, like a polymer matrix, the opportunity exists for resonant excitation energy transfer from the solid to the dye. We observe a 51% excitation transfer efficiency from polystyrene to the fluorescent dye BODIPY ® 564/591 in 40-nm polystyrene microspheres manufactured for use as fluorescent bioprobes. The Förster radius for this transfer is 19 Å. Despite this modest value for R 0, the high concentration of dye in these spheres permits efficient excitation transfer. Absorption, fluorescence, and fluorescence excitation spectra for the polystyrene spheres with and without dye are reported. Our work highlights the idea that the solid need not just be a structural entity in the creation of a fluorescent material. It can play the role of an element in an excitation transfer network. The solid's absorption of light can sensitize fluorescence from the dye molecules through excitation transfer, increasing the overall emission of the dye. Application to the area of biological fluorescence probe development is discussed.

Energy transfer and optical gain studies of FDS: Rh B dye mixture investigated under cw laser excitation

Radiative and non-radiative (Forster type) energy transfer processes in a dye mixture of FDS and Rh B in methanol under cw Ar ion laser excitation were investigated for fixed donor concentration and varying acceptor concentration. It is found that most of the pump power absorbed by FDS is transferred to Rh B as a useful pump power. Transfer probability (PDA), transfer efficiency both radiative (ηR) and non-radiative (ηNR) and optical gain (G) of the system were studied for various pump powers. The gain characteristics of Rh B are found to alter due to the change in the effective fluorescence lifetime caused by energy transfer reaction. Theoretical calculations were also done to find the total transfer efficiency (ηT) at various acceptor concentrations to identify the appropriate energy transfer mechanism responsible for gain enhancement in Rh B. Both radiative and non-radiative transfer processes are taken into consideration in all the calculations. Various energy transfer parameters viz. radiative rate constant (KR), non-radiative rate constant (KNR), critical concentration (C0), critical radius (R0) and half quenching concentration ([A]1/2) are calculated by using the Stern–Volmer plots and concentration dependence of radiative and non-radiative transfer efficiencies. Concentration and pump power dependence of the peak gain and lasing wavelengths of the energy transfer dye lasers (ETDL) have also been studied. The experimental results show that the dominant mechanism responsible for the efficient excitation transfer in this mixture is of radiative nature, whereas the long range dipole–dipole (d–d) interaction (Forster type) is comparatively smaller.

Primary processes in photosystem I reaction center at cryogenic temperatures

Structural model at a molecular resolution has been reported for the photosystem I reaction center, where there are 6 chlorophyll molecules (Chl) in the electron transfer system. It has been, however, difficult to detect the elementary processes in the electron-transfer system. This is partly due to the large number (about 80 Chls) of antenna chlorophylls that surround the electron transfer system. We have applied a femtosecond transient absorption method to photosystem I reaction center with a reduced number of chlorophylls (about 12) per its primary donor chlorophyll (P700). The temperature of the sample was cooled to as low as 9 K, where uphill excitation transfers were suppressed. We thus aimed at revealing the primary processes in the electron transfer system when thermal activation does not work. Under the P700-preoxidized conditions, about 90 % of the initial photobleaching signal of the Chls upon a nonselective excitation at 630 nm decayed within 120 ps at 9 K. This ratio was found to be comparable to that at 290 K. This suggests that, for the most Chls, there is at least one energetically downhill and efficient excitation-transfer pathway to P700+. Under the P700-neutral conditions, the transient absorption in the 720 ? 780 nm region upon a preferential excitation of P700 at 9 K was measured. This has enabled us to understand the photochemistry from the excited state of P700, without interference from excited states of the other chlorophylls.

Photoinduced Charge Transfer in MBB-PPV/C60 Composite Film

The photoconductive properties of MBB-PPV/C60 composite films fabricated by the physical jet deposition (PJD) technique have been studied on the basis of the photoinduced charge transfer process. Under the photoexcitation, the transverse photoconductivity of the composite films is greatly enhanced by 4 orders of magnitude due to the efficient excitation transfer process from MBB-PPV (as donor) to C60 (as acceptor) occurring at the interface, the separation of charges and the existence of in-layer free carriers. The results indicate that the conjugated polymer/fullerene composite films optimized by an appropriate configuration have potential applications in photoelectric devices and photonic fields.