Excitation Energy Migration Research Articles

Enabling Förster Resonance Energy Transfer from Large Nanocrystals through Energy Migration

The stringent distance dependence of Förster resonance energy transfer (FRET) has limited the ability of an energy donor to donate excitation energy to an acceptor over a Förster critical distance (R0) of 2-6 nm. This poses a fundamental size constraint (<8 nm or ∼4R0) for experimentation requiring particle-based energy donors. Here, we describe a spatial distribution function model and theoretically validate that the particle size constraint can be mitigated through coupling FRET with a resonant energy migration process. By combining excitation energy migration and surface trapping, we demonstrate experimentally an over 600-fold enhancement over acceptor emission for large nanocrystals (30 nm or ∼15R0) with surface-anchored molecular acceptors. Our work shows that the migration-coupled approach can dramatically improve sensitivity in FRET-limited measurement, with potential applications ranging from facile photochemical synthesis to biological sensing and imaging at the single-molecule level.

Energy transfer phenomena and Judd-Ofelt analysis on Sm3+ ions in K2GdF5 crystal

The Raman, absorption, luminescence spectra and lifetimes curves of Sm3+-doped K2GdF5were measured. Based on the Judd-Ofelt analysis, the values of radiative transition probabilities, branching ratios, integrated emission cross-sections and radiative lifetimes of excited states of Sm3+ in K2GdF5 crystal were calculated. The migration of excitation energy between the Gd3+ ions and the trapping processes of Gd3+ excitation energy by Sm3+ and Tb3+ions in K2GdF5 crystal are reported. The role of the overlapping between the broad, allowed absorption bands of the RE3+ ions and the narrow absorption lines 6IJ and 6PJ of Gd3+ ions on the trapping rates of the RE3+ was discussed. The energy transfer between the Sm3+ ions was studied by the decay measurement, which has been fitted by Inokuti-Hirayama energy transfer model and revealed that electric dipole–quadrupole interaction is responsible for the energy transfer process in Sm3+ ions doped K2GdF5 crystal.

Energy Migration in Organic Solar Concentrators with a Molecularly Insulated Perylene Diimide

Maintaining high incident light absorption while minimizing luminescence reabsorption is a key challenge for organic luminescent solar concentrators (LSCs). Energy migration and trapping using light-harvesting donors and a low-energy highly emitting acceptor is one strategy to reduce the reabsorption issue. However, concentration quenching and the potential formation of quenching aggregates is a limiting factor in realizing efficient devices. We describe the synthesis of a novel molecularly insulated perylene diimide that can resist luminescence quenching at concentrations in excess of 50 mM. Photophysical measurements show the insulated perylene diimide has an excitation energy migration diffusion length of 230 ± 10 A at 60 mM in poly(methyl methacrylate). LSC devices using a mixture of the insulated perylene diimide light absorber and perylene red (LR305) as the low-energy trap emitter exhibit reduced reabsorption and a better current output than LR305 only devices. The results demonstrate that appropri...

Enhanced luminescence of 3,3′-diethyl-2,2′-thiacyanine cations adsorbed on saponite particles

Hybrid colloidal dispersions based on cationic dye 3,3′-diethyl-2,2′-thiacyanine iodide (NK88) and saponite (Sap) were prepared and their spectral properties were compared with dye solution. The effect of various NK88/Sap ratios was investigated by absorption and fluorescence spectroscopies. A detailed analysis of absorption spectra based on chemometric methods (principal component analysis and multivariate curve resolution - alternating least squares) revealed a very complex nature of these hybrid systems. Most relevant spectral components were identified as monomeric form, H- and J-aggregates. The rearrangement of initially formed dye species with time led to the increase of the amount of the J-aggregates at the expense of the H-aggregates and the monomers. The larger amounts of the H-aggregates were observed in the specimens with relatively highest dye loadings. NK88 luminescence was significantly enhanced upon the adsorption on Sap particles, but could not be assigned to a single dye form. The participation of different dye forms can explained in terms of strong couplings, such as collective exciton delocalization in dye supramolecular systems formed on Sap surface, and weak coupling between various forms via excitation energy migration and transfer. These interactions led to the emission spectra, whose profiles did not depend on the excitation wavelengths. The colloidal systems based on NK88 and Sap are perspective precursors for hybrid materials with interesting optical and photophysical properties.

The 3D [(Cu2Br2){μ-EtS(CH2)4SEt}]n material: a rare example of a coordination polymer exhibiting triplet–triplet annihilation

EtS(CH2)4SEt, L1, forms with CuI a luminescent 2D polymer [Cu4I4{μ-L1}2]n (CP1), which exhibits no triplet excitation energy migration, but with CuBr, it forms a 3D material (CP2), [(Cu2Br2){μ-L1}]n consisting of parallel (Cu2Br2S2)n layers bridged by L1's. CP2 shows T1-T1 annihilation at 298 K but not at 77 K.

Functional interpretation of the role of cyclic carotenoids in photosynthetic antennas via quantum chemical calculations

In the paper positions of the energy levels of the first three excited states of cyclic carotenoids are calculated. The pigments studied include the representatives of β-carotene homologous series containing 5, 7, 9 and 11 conjugated bonds, as well as xanthophylls: zeaxanthin, violaxanthin, lutein, canthaxanthin, astaxanthin and auroxanthin. The results are obtained using quantum chemistry computational methods. The data obtained are discussed in relation to biological functions of carotenoids and their role in the processes related to electronic excitation energy migration involving chlorophyll and oxygen molecules.

Temperature-dependent nonradiative energy transfer from Gd3+ to Ce3+ ions in co-doped LuAG:Ce,Gd garnet scintillators

Energy transfer from donor Gd3+ to acceptor Ce3+ ions was studied in low doped lutetium aluminum garnet, LuAG:Ce,Gd. High purity single crystalline films were prepared by liquid phase epitaxy. The mechanism of nonradiative energy transfer from 6PJ (Gd3+) multiplet to crystal field split 5d2 (2D) states of Ce3+ was established as long-range dipole–dipole interaction and the average critical transfer distance between Gd3+ and Ce3+ ions was found ~14Å at room temperature. It is shown that the single step energy transfer between donor–acceptor pairs is dominant while migration of excitation energy within the donor Gd3+ subsystem is only a small perturbation in the energy transfer mechanism in the studied low doped garnets.

Energy transfer based emission analysis of (Tb3+, Sm3+): Lithium zinc phosphate glasses

The present paper reports on the results pertaining to photoluminescence properties of Tb3+, Sm3+ and energy transfer from Tb3+ to Sm3+ ions in lithium zinc phosphate (LZP) glass matrix prepared by melt quenching method. Besides photoluminescence studies thermal stability for the LZP glass is also evaluated from TG-DTA measurement. Tb3+ doped glasses have exhibited a prominent green emission at 547nm assigned to 5D4→7F5 transitions on exciting at λexci=377nm. The quenching phenomenon in Tb3+ emission on varying its concentration has been discussed from cross-relaxations. Sm3+ incorporated glasses have shown strong orange emission at 603nm assigned to 4G5/2→6H7/2 transition upon exciting with λexci=404nm. The possibility of energy transfer process taking place between these two ions is understood from the significant spectral overlap of Sm3+ absorption and Tb3+ emission. Migration of excitation energy from Tb3+ ions to Sm3+ ions at λexci=375nm is evaluated from the emission spectra of (0.5mol.% Tb3++(0.5–2.0mol.%) Sm3+) co-doped glasses. The emission intensity of Sm3+ has enhanced while Tb3+ emission intensity decreased with an increase in Sm3+ concentration suggesting the occurrence of energy transfer through cross-relaxations from Tb3+ (5D4) to Sm3+ (4G5/2). The mechanism behind energy transfer process has been further explained from energy level diagram, decay profiles and confirmed by calculating energy transfer parameters (energy transfer efficiency (η) and energy transfer probability (P)) of co-doped glasses. The dipole–dipole interaction is found to be more responsible for energy transfer Tb3+ (5D4) to Sm3+ (4G5/2) ions in LZP glass matrix.

Excitation energy migration dynamics in upconversion nanomaterials.

Recent efforts and progress in unraveling the fundamental mechanism of excitation energy migration dynamics in upconversion nanomaterials are covered in this review, including short- and long-term interactions and other interactions in homogeneous and heterogeneous nanostructures. Comprehension of the role of spatial confinement in excitation energy migration processes is updated. Problems and challenges are also addressed.

Infrared excitation induced upconversion fluorescence properties and photoelectric effect of NaYbF4:Tm3+@TiO2core–shell nanoparticles

NaYbF4:Tm3+@TiO2 core–shell nanoparticles were successfully synthesized by the hydrothermal method, followed by hydrolysis of tetrabutyl titanate (TBOT). The ultraviolet and blue UC luminescence are decreased significantly for NaYbF4:Tm3+@TiO2 with a 980 nm semiconductor laser, which originated from the 1D2 → 3H6 and 1D2 → 3F4 transitions of Tm3+, respectively. The inner photoelectric effect of NaYbF4@TiO2 core–shell structure is also found by exciting the core–shell structure under 980 nm excitation, which is attributed to excitation energy migration from the NaYbF4:Tm3+ core to the TiO2 shell. This study suggests a promising system to utilize the near-infrared energy of sunlight for photoelectrical applications based on TiO2, which will be the efficient infrared photoelectric materials in future.

In Depth Analysis of the Quenching of Three Fluorene–Phenylene-Based Cationic Conjugated Polyelectrolytes by DNA and DNA Bases

The interaction of three cationic poly {9,9-bis[N,N-(trimethylammonium)hexyl]fluorene-co-1,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR36(I),12(Br),100(Br)) with both double and single stranded, short and long, DNA and DNA bases have been studied by steady state and time-resolved fluorescence techniques. Fluorescence of PFP-NR3 polymers is quenched with high efficiency by DNA (both double and single stranded) and DNA bases. The resulting quenching plots are sigmoidal and are not accurately described by using a Stern-Volmer quenching mechanism. Here, the quenching mechanism is well modeled in terms of an equilibrium in which a PFP-NR3/DNA aggregate complex is formed which brings polymer chains into close enough proximity to allow interchain excitation energy migration and quenching at aggregate or DNA base traps. Such an analysis gives equilibrium constants of 8.4 × 10(6) (±1.2 × 10(6)) M(-1) for short-dsDNA and 8.6 × 10(6) (±1.7 × 10(6)) M(-1) for short-ssDNA with PFP-NR36(I).

Enhancing multiphoton upconversion through energy clustering at sublattice level

The applications of lanthanide-doped upconversion nanocrystals in biological imaging, photonics, photovoltaics and therapeutics have fuelled a growing demand for rational control over the emission profiles of the nanocrystals. A common strategy for tuning upconversion luminescence is to control the doping concentration of lanthanide ions. However, the phenomenon of concentration quenching of the excited state at high doping levels poses a significant constraint. Thus, the lanthanide ions have to be stringently kept at relatively low concentrations to minimize luminescence quenching. Here we describe a new class of upconversion nanocrystals adopting an orthorhombic crystallographic structure in which the lanthanide ions are distributed in arrays of tetrad clusters. Importantly, this unique arrangement enables the preservation of excitation energy within the sublattice domain and effectively minimizes the migration of excitation energy to defects, even in stoichiometric compounds with a high Yb(3+) content (calculated as 98 mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er(3+) with an intensity that is more than eight times higher than previously reported. Our results highlight that the approach to enhancing upconversion through energy clustering at the sublattice level may provide new opportunities for light-triggered biological reactions and photodynamic therapy.

The energy flux theory 35 years later: formulations and applications

Several models have been proposed for the energetic behavior of the photosynthetic apparatus and a variety of experimental techniques are nowadays available to determine parameters that can quantify this behavior. The Energy Flux Theory (EFT) developed by Strasser 35years ago provides a straightforward way to formulate any possible energetic communication between any complex arrangement of interconnected pigment systems and any energy transduction by these systems. We here revisit the EFT, starting from the basic general definitions and equations and presenting applications in formulating the energy distribution in photosystem (PS) II units with variable connectivity, as originally derived, where certain simplifications were adopted. We then proceed to the derivation of equations for a PSII model of higher complexity, which corresponds, from the formalistic point of view, to the later formulated and now broadly accepted exciton-radical-pair model. We also compare the formulations derived with the EFT with those obtained, by different approaches, in the classic papers on energetic connectivity. Moreover, we apply the EFT for the evaluation of the excitation energy distribution between PSII and PSI and the distinction between state transitions and PSII to PSI excitation energy migration. Our analysis demonstrates that the EFT is a powerful approach for the formulation of any possible model, at any complexity level, even of models that may be proposed in the future, with the advantage that any possible energetic communication or energy transduction can be easily formulated mathematically by trivial algebraic equations. Moreover, the biophysical parameters introduced by the EFT and applicable for any possible model can be linked with obtainable experimental signals, provided that the theoretical resolution of the model does not go beyond the experimental resolution.

Photoluminescence Dynamics of CdSe QD/Polymer Langmuir–Blodgett Thin Films: Morphology Effects

Thin films of colloidal semiconductor CdSe quantum-dots (QDs) and a styrene/maleic anhydride copolymer were prepared by the Langmuir–Blodgett technique and their photoluminescence was characterized by confocal fluorescence lifetime microscopy. In the films, the photoluminescence dynamics are strongly affected by excitation energy migration between close-packed quantum-dots and energy trapping by surface-defective QDs or small clusters of aggregated QDs. Polymer/QD films with more aggregated QD regions exhibit lower PL intensities and faster decays, which we attribute primarily to the increased ability of excitations to find trap sites among more close-packed QD regions. This was confirmed by comparing films from bilayer or cospreading deposition, and varying the QD-to-polymer composition or the surface pressure at deposition. The more regular films, such as those obtained by bilayer deposition at high-pressure, display more surface emission intensity and decays with less accentuated curvature. Decay analysis was performed with a model that accounts for excitation energy migration and trapping in the films. In the framework of the model, the photoluminescence dynamics are related to film morphology through the density of energy traps. A lower amount of QD clustering in the films reflects on a lower density of energy traps and, thus, on more emissive QD films, as inferred here for bilayer films relative to cospreading films.

The real role of active-shell in enhancing the luminescence of lanthanides doped nanomaterials

Although it is widely recognized that doping sensitizers in the shell can improve significantly the luminescence of lanthanides doped nanocrystals, lack of an unambiguous picture of relevant luminescence enhancement mechanism seriously hinders the optimization of this approach. In this work, the complete processes of excitation energy migration, from photon absorption to emission, was dissected to unravel the role of sensitizers doped in shell in every individual stage. We revealed that the essence of doping sensitizers in the shell is just to increase the absorption efficiency whereas the quantum yield is lessened simultaneously. The optimal sensitizer doping concentration is also fixed to achieve the best luminescence performance.

Excitation energy migration in yellow fluorescent protein (citrine) layers adsorbed on modified gold surfaces

The nature of functional proteins adsorbed on solid surfaces is interesting from the perspective of developing of bioelectronics and biomaterials. Here we present evidence that citrine (one of yellow fluorescent protein variants) adsorbed on modified gold surfaces would not undergo denaturation and energy transfer among the adsorbed citrine molecules would occur. Gold substrates were chemically modified with 3-mercaptopropionic acid and tert-butyl mercaptan for the preparation of hydrophilic and hydrophobic surfaces, respectively. A pure solution of citrine was dropped and dried on the modified gold substrates and their surface morphology was studied with scanning tunnelling microscopy (STM). The obtained STM images showed multilayers of citrine adsorbed on the modified surfaces. On hydrophobic surfaces, citrine was adsorbed more randomly, formed various non-uniform aggregates, while on hydrophilic surfaces, citrine appeared more aligned and isolated uniform protein clusters were observed. Fluorescence lifetime and anisotropy decay of these dried citrine layers were also measured using the time correlated single photon counting method. Fluorescence anisotropy of citrine on the hydrophobic surface decayed faster than citrine on the hydrophilic surface. From these results we concluded that fluorescence energy migration occurred faster among citrine molecules which were randomly adsorbed on the hydrophobic surface to compare with the hydrophilic surface.

Excitation energy migration and trapping on the surface of fluorescent poly(acrylic acid)-grafted polymer particles

The surface of poly(methyl methacrylate) particles with different amounts of a grafted layer of poly(acrylic acid) was labeled with varying degrees of an amino derivative of fluorescein isothiocyanate. The resulting fluorescent polymer particles were analyzed by absorption spectroscopy and by steady-state and time-resolved fluorescence spectroscopy including measurements of the fluorescence anisotropy. The combined results indicate that the overall decrease in fluorescence intensity with increasing surface concentrations of the fluorophore can be traced back to the formation of non-fluorescent aggregates. A mechanism is proposed, in which the excitation energy migrates between identical fluorophores until it is transferred to non-fluorescent aggregates acting as an energy trap. Increases in the surface fluorophore concentration increase both the probability for energy transfer between identical fluorophores and the probability for energy transfer to non-fluorescent aggregates. Furthermore, we suggest that this mechanism also applies to fluorescent protein conjugates and rationalizes the nonlinear dependence of the fluorescence emission on the labeling density.

Quantum Yield of Charge Separation in Photosystem II: Functional Effect of Changes in the Antenna Size upon Light Acclimation

We have studied thylakoid membranes of Arabidopsis thaliana acclimated to different light conditions and have related protein composition to excitation energy transfer and trapping kinetics in Photosystem II (PSII). In high light: the plants have reduced amounts of the antenna complexes LHCII and CP24, the overall trapping time of PSII is only ∼180 ps, and the quantum efficiency reaches a value of 91%. In low light: LHCII is upregulated, the PSII lifetime becomes ∼310 ps, and the efficiency decreases to 84%. This difference is largely caused by slower excitation energy migration to the reaction centers in low-light plants due to the LHCII trimers that are not part of the C2S2M2 supercomplex. This pool of "extra" LHCII normally transfers energy to both photosystems, whereas it transfers only to PSII upon far-red light treatment (state 1). It is shown that in high light the reduction of LHCII mainly concerns the LHCII-M trimers, while the pool of "extra" LHCII remains intact and state transitions continue to occur. The obtained values for the efficiency of PSII are compared with the values of Fv/Fm, a parameter that is widely used to indicate the PSII quantum efficiency, and the observed differences are discussed.

Temporal Switching of Homo-FRET Pathways in Single-Chromophore Dimer Models of π-Conjugated Polymers

A set of π-conjugated oligomer dimers templated in molecular scaffolds is presented as a model system for studying the interactions between chromophores in conjugated polymers (CPs). Single-molecule spectroscopy was used to reveal energy transfer dynamics between two oligomers in either a parallel or oblique-angle geometry. In particular, the conformation of single molecules embedded in a host matrix was investigated via polarized excitation and emission fluorescence microscopy in combination with fluorescence correlation spectroscopy. While the intramolecular interchromophore conformation was found to have no impact on the fluorescence quantum yield, lifetime, or photon statistics (antibunching), the long-term nonequilibrium dynamics of energy transfer within these bichromophoric systems was accessible by studying the linear dichroism in emission at the single-molecule level, which revealed reversible switching of the emission between the two oligomers. In bulk polymer films, interchromophore coupling promotes the migration of excitation energy to quenching sites. Realizing the presence and dynamics of such interactions is crucial for understanding limitations on the quantum efficiency of larger CP materials.

Luminescence of Ln3+ lanthanide complexes in polymer matrices

The photoluminescence properties of Tb3+ and Eu3+ complexes with polymer ligands containing various kinds of complexing groups—namely, carboxyl (5–20 mol %), pyridylquinoline (5 mol %), or pyridylnaphthyl (5 and 10 mol %) groups—in solution and block are considered. The chemical structure of a neutral comonomer (methyl methacrylate; styrene; isopropyl-, phenyl-, or benzylmethacrylamide; and N-vinylamides) in polymer ligands is varied. The intensity of photoluminescence is dependent not only on the chemical nature of a complexing group but also the chemical nature of a neutral comonomer and a spacer. Variation in the nature of a comonomer and a ligand makes it possible to prepare complexes in which a high luminescence of a low-molecular-mass complex is preserved and advantages inherent in a polymer complex are acquired. The effect of the nature of the polymer matrix (photoactive and photoinert) on the efficiency of electronic-excitation energy transfer is ascertained. The data on the photoluminescence of metal-polymer complexes that are based on polymer ligands containing vinylcarbozole units and that possess hole conductivity make it possible to regard them as materials for electroluminescence. The intensity of photoluminescence of these complexes is related to the competition of oppositely directed photophysical processes in a macromolecule: formation of excimers and migration of electronic excitation energy. An analysis of the published data and of the results of the authors shows that detailed studies of these polymer systems in solution and in matrices are needed to gain insight into the relationship between photo- and electroluminescence properties of metal-polymer complexes, because the matrix plays different roles in photoluminescence and electroluminescence (inner filter or conduction); as a consequence, the emission spectra may differ appreciably. It is shown that the efficiency of electroluminescence may be improved if the transfer of energy from the lanthanide ligand in a complex to the conducting matrix is decreased.