Excitation Energy Migration Research Articles

Modelling excitation energy transfer and trapping in the filamentous cyanobacterium Anabaena variabilis PCC 7120

The phycobilisome (PBS) serves as the major light-harvesting system, funnelling excitation energy to both photosystems (PS) in cyanobacteria and red algae. The picosecond kinetics involving the excitation energy transfer has been studied within the isolated systems and intact filaments of the cyanobacterium Anabaena variabilis PCC 7120. A target model is proposed which resolves the dynamics of the different chromophore groups. The energy transfer rate of 8.5 ± 1.0/ns from the rod to the core is the rate-limiting step, both in vivo and in vitro. The PBS-PSI-PSII supercomplex reveals efficient excitation energy migration from the low-energy allophycocyanin, which is the terminal emitter, in the PBS core to the chlorophyll a in the photosystems. The terminal emitter of the phycobilisome transfers energy to both PSI and PSII with a rate of 50 ± 10/ns, equally distributing the solar energy to both photosystems. Finally, the excitation energy is trapped by charge separation in the photosystems with trapping rates estimated to be 56 ± 6/ns in PSI and 14 ± 2/ns in PSII.

Transient Depolarization Spectroscopic Study on Electronic Structure and Fluorescence Origin of Graphene Oxide.

It is well-established that the electronic states of graphene oxide (GO) consist of sp2 clusters with different sizes and the surrounding sp3 matrix according to recent reports. However, addressing the excitation energy migration/redistribution among those electronic states in GO-based complex systems from spectroscopic experiments is still a challenge. Here, we combine the time-resolved absorption and fluorescence depolarization experiments to reveal the excitation energy migration processes in electronic states in GO. We demonstrate that, in sp3 domains of GO, there are charge-transfer states between sp3-hybridized carbon atoms and the oxygen-containing functional groups, and the energy redistribution and charge migration in sp3 matrix occur on the time scale from subpicoseconds to tens of picoseconds. In contrast, the electronic states of sp2 clusters in GO are rather localized and dominantly contribute to the excitation-wavelength-dependent red fluorescence of GO.

Energy transfer investigation of Sm3+/Eu3+ CaBAl glasses

Sm3+/Eu3+ codoped CaBAl glasses were prepared by conventional melt-quenching method, and their structural and optical properties were studied. The values of volumetric density and electronic polarizability showed an increase with the addition of Sm2O3. The Fourier-transform infrared spectroscopy (FTIR) spectra indicated the existence of trigonal and tetrahedral borate groups. The N4 values revealed that Sm2O3/Eu2O3 codoping favored the conversion of structural units from BO4 to BO3. The observed absorption bands in UV/VIS spectra were attributed to Eu3+ and Sm3+ transitions, from the ground states 7F0 and 6H5/2 to the various excited states. Color coordinates at CIE diagram were studied for the codoped glasses upon exciting at 405 nm. The energy transfer from the level 4G5/2 of Sm3+ to the 5D0 level of Eu3+ was verified by means of the luminescence results. With the addition of Sm3+, emission intensity of Eu3+ has been enhanced due to migration of Sm3+ excitation energy. The mechanism for energy transfer from Sm3+ to Eu3+ was investigated by Dexter energy transfer formula and Reisfeld's theory application on photoluminescence decay curves. A decrease in the lifetime of 4G5/2 → 6H7/2 transition was observed when Sm3+ was increased, probably due to ion-ion interaction and excitation energy migration.

Energy migration captures membrane-induced oligomerization of the prion protein

Excitation energy migration via homo-Förster resonance energy transfer (homo-FRET) can serve as an intermolecular proximity ruler within complex biomolecular assemblies. Here we present a unique case to demonstrate that energy migration can be a novel and sensitive readout to capture the membrane-mediated misfolding and oligomerization of the human prion protein (PrP), which is known to undergo an aberrant conformational conversion from an α-helical form into a self-propagating aggregated β-rich state causing deadly transmissible neurodegenerative diseases. Using site-specific energy migration studies by monitoring steady-state and time-resolved fluorescence anisotropy of fluorescently-tagged PrP, we elucidate the molecular details of lipid membrane-induced oligomers. We show that the intrinsically disordered N-terminal segment is critical for lipid-induced conformational sequestration of PrP into higher-order, β-rich oligomeric species that exhibit membrane permeabilization. Our results revealed that the N-terminal regions constitute the central core of the oligomeric architecture, whereas the distal C-terminal ends participate in peripheral association with the lipid membrane. Our study will find applications in the sensitive detection and in the structural characterization of membrane-induced protein misfolding and aggregation in a variety of deadly amyloid diseases.

Excitation Energy Transfer in Intact CpcL-Phycobilisomes from Synechocystis sp. PCC 6803.

This work highlights spectroscopic studies performed on a CpcL-phycobilisome (CpcL-PBS) light-harvesting complex from cyanobacterium Synechocystis sp. PCC 6803 ΔAB strain. The CpcL-PBS antenna has the form of a single rod made up exclusively of phycocyanins (PCs), a structure that is much simpler compared to the better known and broadly studied CpcG-PBS that consists of a cylindrical core with a set of protruding PC rods. Steady-state and time-resolved fluorescence studies demonstrated that the CpcL-PBS antenna comprises two spectral forms of phycocyanobilin (PCB), one emitting at 650 nm and a second emitting at 670 nm. The latter one presumably serves as the so-called terminal energy emitter without allophycocyanin. Studies of excitation energy migration between those two PCB forms demonstrated that even small buffer alterations, commonly applied by spectroscopists to tweak buffers to be more friendly for a certain type of spectroscopy, may lead to very different experimental outcomes and, in consequence, to differences in models of excitation migration pathway in this antenna complex.

Temperature-dependent fluorescence decay and energy transfer in Nd/Cr:YAG ceramics

This work investigates the energy-transfer processes from Cr3+ to Nd3+ in yttrium aluminum garnet (YAG) ceramics by comparing the fluorescence decay of Cr3+ for Nd/Cr:YAG with that of Cr:YAG from 293 to 473 K. The experimentally determined fluorescence decay of Nd3+ excited through Cr3+ is consistent with the theoretical predictions when energy-transfer processes are taken into account by including in the rate equations the dipole-dipole interaction and energy transfer via excitation-energy migration. Furthermore, the results presented herein clarify that the efficiency of energy transfer from Cr3+ to Nd3+ increases slightly with increasing temperature.

Excitation Energy-Transfer Processes in the Sensitization Luminescence of Europium in a Highly Luminescent Complex.

The excitation energy transfer (EET) pathways in the sensitization luminescence of EuIII and the excitation energy migration between the different ligands in [Eu(fod)3dpbt] [where fod=6,6,7,7,8,8,8‐heptafluoro‐2,2‐dimethyl‐3,5‐octanedione and dpbt=2‐(N,N‐diethylanilin‐4‐yl)‐4,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,3,5‐triazine], exhibiting well‐separated fluorescence excitation and phosphorescence bands of the different ligands, were investigated by using time‐resolved luminescence spectroscopy for the first time. The data clearly revealed that upon the excitation of dpbt, the sensitization luminescence of EuIII in [Eu(fod)3dpbt] was dominated by the singlet EET pathway, whereas the triplet EET pathway involving T1(dpbt) was inefficient. The energy migration from T1(dpbt) to T1(fod) in [Eu(fod)3dpbt] was not observed. Moreover, upon the excitation of fod, a singlet EET pathway for the sensitization of EuIII luminescence, including the energy migration from S1(fod) to S1(dpbt) was revealed, in addition to the triplet EET pathway involving T1(fod). Under the excitation of dpbt at 410 nm, [Eu(fod)3dpbt] exhibited an absolute quantum yield for EuIII luminescence of 0.59 at 298 K. This work provides a solid and elegant example for the concept that singlet EET pathway could dominate the sensitization luminescence of EuIII in some complexes.

Excitation energy transfer in the far-red absorbing violaxanthin/vaucheriaxanthin chlorophyll a complex from the eustigmatophyte alga FP5.

This work highlights spectroscopic investigations on a new representative of photosynthetic antenna complexes in the LHC family, a putative violaxanthin/vaucheriaxanthin chlorophyll a (VCP) antenna complex from a freshwater Eustigmatophyte alga FP5. A representative VCP-like complex, named as VCP-B3 was studied with both static and time-resolved spectroscopies with the aim of obtaining a deeper understanding of excitation energy migration within the pigment array of the complex. Compared to other VCP representatives, the absorption spectrum of the VCP-B3 is strongly altered in the range of the chlorophyll a Qy band, and is substantially red-shifted with the longest wavelength absorption band at 707nm at 77K. VCP-B3 shows a moderate xanthophyll-to-chlorophyll a efficiency of excitation energy transfer in the 50-60% range, 20-30% lower from comparable VCP complexes from other organisms. Transient absorption studies accompanied by detailed data fitting and simulations support the idea that the xanthophylls that occupy the central part of the complex, complementary to luteins in the LHCII, are violaxanthins. Target analysis suggests that the primary route of xanthophyll-to-chlorophyll a energy transfer occurs via the xanthophyll S1 state.

Mapping the excitation energy migration pathways in phycobilisomes from the cyanobacterium Acaryochloris marina

In this study, we use ultrafast time-resolved absorption and fluorescence spectroscopies to examine A. marina phycobilisomes isolated from cells grown under light of different intensities and spectral regimes. Investigations were performed at room temperature and at 77 K. The study demonstrates that if complexes are stabilized by high phosphate (900 mM) buffer, there are no differences between them in temporal and spectral properties of fluorescence. However, when the complexes are allowed to disassemble into trimers in low phosphate (50 mM) buffer, differences are clearly observed. The fluorescence properties of intact or disassembled phycobilisomes from cells grown in low intensity white light are unresponsive to variation in phosphate concentration. This antenna complex was further studied in detail with application of femtosecond time-resolved absorption at room temperature. Combined spectroscopic and kinetic analysis of time-resolved fluorescence and absorption data of this antenna allowed us to identify spectrally different forms of phycocyanobilins and to propose a simplified model of how they could be distributed within the phycobilisome structure.

Quantitative evaluation of energy migration between identical chromophores enabled by breaking symmetry

Energy migration between the identical chromophores is a necessary process in both natural and artificial photosynthesis. The distance and orientation dependence of energy migration have not been experimentally investigated in detail. Here we propose a method to investigate energy migration. Two fluorophores are introduced into one strand of a DNA duplex with a quencher placed opposite one of fluorophores. This design enables asymmetrization of identical fluorophores and allows one fluorophore to behave as an acceptor. The emission intensities and lifetimes decrease depending on the efficiency of energy migration. Distance and orientation dependence are successfully quantified, and the excitation energy migration efficiencies measured are in excellent agreement with those calculated based on Förster theory. We also demonstrate that multi-step energy migration among four fluorophores can be estimated from the theory. These results may provide a basis for design and preparation of efficient light-harvesting photonic devices and chemical probes.

Nonradiative energy transfer and ion-ion interactions in terbium-praseodymium coactivated borate glass

A study of energy transfer between Tb3+ and Pr3+ in borate glass is being presented. From our measurement of fluorescence emission and decay times of donor (Tb3+), in the presence of different concentrations of acceptor (Pr3+), the energy transfer probability (Pda) and transfer efficiency (ɳT) have been calculated. Analyzing the data it is observed that the non-radiative energy transfer in our borate glass system occurs by dipole-dipole interactions, contrary to the observations made by Nakazawa and Shionoya (dipole-quadrupole interaction) in phosphate glass. A linear dependence of Pda versus C at low acceptor concentration suggests the migration of excitation energy among donor ions before the energy is finally transferred to the acceptor ion. At high acceptor concentrations a plot of Pdavs (C0 + C)2; where C0 is donor concentration and C is the acceptor concentration, presents a linear dependence suggesting thereby that one donor interacts with two neighbouring acceptors consistent with Fong-Diestler mechanisms of dipole-dipole energy transfer interactions.

Energy migration processes in phosphate nanocrystals: Size and dimensionality dependence

Peculiarities of electronic excitation energy migration in phosphate nanocrystals with three-dimensional (EuPO4) and one-dimensional (EuP3O9) arrangement of regular ions under variation of concentration of both energy traps (Nd3+ ions) and scattering centres (La3+ ions) are discussed in the paper. Processes of energy migration in both EuPO4 and EuP3O9 nanocrystals are phonon-assisted ones due to absence of resonance between energy levels of adjacent Eu3+ ions. The distance of energy migration for both EuPO4 and EuP3O9 nanocrystals was found to be equal to tens of nanometres, however, for EuPO4 nanocrystals energy migration leads to stronger quenching of Eu3+ luminescence. For 10 nm EuPO4 nanocrystals sufficient hampering of energy migration is observed due to the depletion of the low-energy phonon spectrum as a result of the phonon confinement effect.

Precisely Tailoring Upconversion Dynamics via Energy Migration in Core-Shell Nanostructures.

Upconversion emission dynamics have long been believed to be determined by the activator and its interaction with neighboring sensitizers. Herein this assumption is, however, shown to be invalid for nanostructures. We demonstrate that excitation energy migration greatly affects upconversion emission dynamics. “Dopant ions’ spatial separation” nanostructures are designed as model systems and the intimate link between the random nature of energy migration and upconversion emission time behavior is unraveled by theoretical modelling and confirmed spectroscopically. Based on this new fundamental insight, we have successfully realized fine control of upconversion emission time behavior (either rise or decay process) by tuning the energy migration paths in various specifically designed nanostructures. This result is significant for applications of this type of materials in super resolution spectroscopy, high‐density data storage, anti‐counterfeiting, and biological imaging.

Precisely Tailoring Upconversion Dynamics via Energy Migration in Core–Shell Nanostructures

AbstractUpconversion emission dynamics have long been believed to be determined by the activator and its interaction with neighboring sensitizers. Herein this assumption is, however, shown to be invalid for nanostructures. We demonstrate that excitation energy migration greatly affects upconversion emission dynamics. “Dopant ions’ spatial separation” nanostructures are designed as model systems and the intimate link between the random nature of energy migration and upconversion emission time behavior is unraveled by theoretical modelling and confirmed spectroscopically. Based on this new fundamental insight, we have successfully realized fine control of upconversion emission time behavior (either rise or decay process) by tuning the energy migration paths in various specifically designed nanostructures. This result is significant for applications of this type of materials in super resolution spectroscopy, high‐density data storage, anti‐counterfeiting, and biological imaging.

808 nm excited energy migration upconversion nanoparticles driven by a Nd3+-Trinity system with color-tunability and superior luminescence properties.

We have developed energy migration upconversion (EMU) nanoparticles (UCNPs) with optimal Nd3+-sensitization under excitation of an 808 nm laser to avoid over-heating effects caused by a 980 nm laser while maximizing the excitation efficiency. To realize efficient 808 nm sensitization, a "Nd3+-Trinity system" was implemented in the energy migration upconversion (EMU) cores (NaGdF4:Yb,Tm@NaGdF4:Yb,X, X = Eu/Tb), resulting in a core-multishell structure of EMU cores (accumulation layer@activation layer)@transition layer@harvest layer@activation layer. The spatially separated dopants and optimized Yb3+/Nd3+ content effectively prevented severe quenching events in the UCNPs and their Nd3+-sensitized EMU-based photoluminescence mechanism was studied under 808 nm excitation. These Nd3+-Trinity EMU system UCNPs presented enhanced upconversion luminescence and prolonged lifetime compared to the 980 nm excited UCNPs of the EMU system. It is proposed that 975 nm and 1056 nm NIR photons induced from the Nd3+ → Yb3+ energy transfer facilitate the Tm3+ accumulation process due to the matched energy gaps, which contributes to the extended lifetimes. More importantly, the synthesized UCNPs had a small average size of sub-15 nm and they not only exhibited color-tunability via Eu3+/Tb3+ activators, but also released a larger portion of Tm3+ red emission at 647 nm and had better penetration ability in water under 808 nm excitation, which are favorable for bioimaging applications.

Effect of Differential Self-Assembly on Mechanochromic Luminescence of Fluorene-Benzothiadiazole-Based Fluorophores.

Supramolecular self-assembly is an excellent tool for controlling the optical and electronic properties of chromophore-based molecular systems. Herein, we demonstrate how differential self-assembly affects mechanoresponsive luminescence of fluorene-benzothiadiazole-based fluorophores. We have synthesized two donor–acceptor–donor-type conjugated oligomers consisting of fluorene as the donor and benzothiadiazole as the acceptor. For facile self-assembly, both molecules are end-functionalized with hydrogen-bonding amide groups. Differential self-assembly was induced by attaching alkyl chains of different lengths onto the fluorene moiety: hexyl (FB-C6) and dodecyl (FB-C12). The molecules self-assemble to form well-defined nanostructures in nonpolar solvents and solvent mixtures. Although their optical properties in solution are not affected by the alkyl chain length, significant effects were observed in the self-assembled state, particularly in the excitation energy migration properties. As a result, remarkable differences were observed in the mechanochromic luminescence properties of the molecules. A precise structure–property correlation is made using UV–visible absorption and fluorescence spectroscopy, time-correlated single-photon counting analysis, scanning electron microscopy, and X-ray diffraction spectroscopy.

Modulation of the Photophysical Properties of 1‐Azathioxanthones by Eu3+, Gd3+, Tb3+, and Yb3+ Ions in Methanol

The design of bright fluorescent probes based on lanthanide centered emission relies on efficient energy transfer from antenna chromophores to lanthanide centers. Here, we set out to explore the excited state energy migration pathways using collisional quenching of three 1‐azathioxanthones by four lanthanide(III) ions. The results show that each combination of lanthanide(III) ion and chromophore must be considered as a unique case, as the rate constants that determine the outcome of the excited state energy migration is modulated differently in each system. For the three dye systems investigated here, the effect of the lanthanide(III) ions is that two dye systems show a strong increase in inter‐system crossing leading to increased fluorescence yields and room temperature phosphorescence, while the fluorescence is quenched in the third dye system. Contrasting our results with literature data leads us to conclude that freely diffusing dyes and ions do not shed light on which properties will be found for linked systems.

Higher Energy Gap Control of Fluorescence in Conjugated Polymers: Turn-On Amplifying Chemosensor for Hydrogen Sulfide

The significance of conjugated polymers for sensing applications stems from the intrinsic amplification of analyte binding events which is related to the efficient excitation energy migration in the polymers. Within the framework of conventional design of amplifying sensory polymers, interaction with an analyte creates a lower energy gap site within the conjugated backbone which acts as a trap for randomly migrating excitons typically producing a fluorescence quenching effect (turn-off sensors). Herein, we describe an alternative “higher energy gap” paradigm, a general design concept surprisingly overlooked so far. Within this paradigm, reaction with an analyte generates a higher energy gap site in the conjugated backbone. This higher energy gap site diminishes exciton migration length and causes pronounced enhancement of the polymer fluorescence (turn-on response). This effect benefits from the signal amplification and therefore can be used universally to design a wide range of fluorescent sensors. To de...

Processes of energy migration in mixed europium–lanthanum magnesium borate nanocrystals

ABSTRACTProcesses of excitation energy migration in europium magnesium borate nanocrystals at different concentrations of neodymium and lanthanum ions were investigated by steady-state and time-resolved luminescence spectroscopy. Energy migration in neodymium doped mixed europium–lanthanum magnesium borate nanocrystals is well fitted by ∼t0.33 dependence indicating one-dimensional transport to neodymium energy acceptors through europium chains. Variation of neodymium and lanthanum concentrations have shown that at room temperature excitation can migrate over the distances up to 100 nm that is comparable with the size of nanocrystal.

Photophysical and conformation properties of luminescent-labeled star poly-2-isopropyl-2-oxazolines based on octa-tert-butylcalix[8]arene in solution

The synthesis of luminescent-labeled copolymers of octa-tert-butylcalix[8]arene with 8 poly-2-isopropyl-2-oxazoline arms is carried out. Each macromolecule involves one luminescent label of the anthracene structure located at the end of one of the arms. The luminescent properties of the copolymers in water and methanol are studied. It is shown that, in aqueous solutions, the formation of intermolecular aggregates with a hydrophobic core and a hydrophilic shell results in a new photophysical process of migration of electron excitation energy accompanied by a decrease in the solution luminescence polarization. The interaction of oxazoline arms with poly(methacrylic acid) via the mechanism of interpolymer complex formation implies a weakening of the efficiency of energy migration.