Study Of Energy Transfer Processes Research Articles

NaGdF4:Yb, Er, Tm Upconversion Nanoparticles for Bioimaging in Shortwave-Infrared Range: Study of Energy Transfer Processes and Composition Optimization

Upconversion nanoparticles are promising for many applications. For triple-doped nanoparticles (NPs), the luminescence intensity shows a non-linear dependence on the rare-earth ion concentration, making it difficult to obtain bright phosphors with high energy output. We investigated the energy transfer processes in β-NaGdF4:Yb-Er-Tm NPs and considered strategies for increasing the thulium luminescence intensity, in particular, the use of core–shell structures. The luminescence spectra were analyzed in the short-wavelength infrared (SWIR) and visible (VIS) regions. The Er3+ and Tm3+ luminescence lifetimes in the VIS region were measured to study the energy transfer processes between the active ions. The quenching of the Tm3+ luminescence in the SWIR region was observed. However, both Er3+ and Tm3+ luminescence bands were observed in the VIS range. We attribute these effects to energy transfer between Tm3+ 3F4 → 3H6 and Er3+ 4I13/2 → 4I9/2, which occurs due to overlap of Er3+ and Tm3+ luminescence bands, and also to competition between Er3+ and Tm3+ for energy transfer from Yb3+. For core–shell NPs, when Tm3+ and Er3+ are separated into adjacent layers, quenching cannot be avoided, likely due to the mutual diffusion of ions during shell synthesis. The most optimal strategy to obtain luminescence in the SWIR range is to use an inert intermediate shell between the layers containing Tm3+ and Er3+.

Study of energy transfer processes between rare earth ions and photosensitizer molecules for photodynamic therapy with IR-excitation

Today, photodynamic therapy is one of the most promising minimally invasive methods of treatment of various diseases, including cancer. The main limitation of this method is the insufficient penetration into the tissue of laser radiation used to activate photosensitizer molecules, which makes it difficult to carry out therapy in the treatment of large or deep-seated tumors. In this regard, there is a great interest in the development of new strategies for photodynamic therapy using infrared radiation for excitation, the wavelengths of which fall into the “transparency window” of biological tissues. In this work, it was proposed to use upconversion NaGdF4 :Yb:Er nanoparticles (UCNP), which absorb infrared excitation and serve as a donor that transfers energy to the photosensitizer. Photosens and phthalosens were chosen as the most promising photosensitizers for the study. The aim of this work was to study the energy transfer processes between upconversion nanoparticles doped with rare-earth ions and photosensitizer molecules. in order to excite photosensitizers with IR radiation and carry out photodynamic therapy of deep-seated neoplasms. Using spectroscopic and time-resolved methods, it has been demonstrated that there is an efficient energy transfer between upconversion particles and photosensitizers phthalosens and photosens. The calculated efficiency of energy transfer by the Foerster mechanism was 41% for the UCNP + photosens system and 69% for the UCNP + phthalosens system. It has been experimentally and theoretically proved that there is a binding of photosensitizer molecules with UCNP by means of surfactants, leading to a reduction in the distance between them, due to which effective nonradiative energy transfer is realized. The generation of singlet oxygen by the phthalosens photosensitizer upon excitation by means of energy transfer from UCNP, excited at 980 nm wavelength of, has been demonstrated.

Energy transfer in luminescence composite materials based on mesogenic europium (III) complexes and some semiconducting polymers according to quantum-chemical simulation

A TDDFT study of energy transfer processes in luminescence composite materials based on mesogenic europium (III) complexes with substituted β-diketones and Lewis bases and some semiconducting polymers is presented. The calculated energies of the lowest singlet and triplet excited states of ligands and polymers were used to construct the energy level diagrams and indentify the main channels of intramolecular energy transfer. A system of a complex and a polymer, which is most preferable for practical application in optoelectronics, was selected. The simulated data agree well with the experiment.

OPERATIONAL IRRIGATION MANAGEMENT: MODERN CHALLENGES, REALITIES AND VISIONS

Introduction. The current challenges in water and agriculture management in Ukraine wield major influence on the development of reclamation science and practice. These challenges and the realities of irrigated farming require a revision of traditional decision-making methods and criteria to ensure resource-efficient irrigation management. Analytical and experimental studies were conducted to evaluate existing irrigation practices, develop a vision for its development over the next 20-30 years, and evaluate the prospects for the use of certain innovative products that can be implemented for irrigation management under existing economic conditions and in the future. The purpose of the research was to improve the methods of operational irrigation management and support the adoption of appropriate strategic decisions to achieve resource efficiency in irrigated agriculture. The following tasks were solved: to investigate the temporal and spatial variability of the natural and economic conditions of irrigation use; to determine the basic directions of models and algorithms improvement for operational irrigation planning taking into account the spatial and temporal variability of natural and economic conditions of real production; to evaluate perspective directions of development of irrigation planning methods to ensure resource efficiency of management in the current agricultural practice. Methods and methodologies. The research was conducted during 2012-2019 at the farms of Kherson and Zaporizhzhya regions. Testing and pilot implementation of the operational irrigation planning information system “GIS Polyv” has been carried out. The studies were carried out on 306 fields, the total area of which was 9266.09 ha, the main crops were soybean, sunflower, winter wheat, alfalfa and winter rape. Research methods included on-site observations, modelling, remote sensing, and method of system analysis. Results and discussion. The role of on-site and space agro-monitoring for the correction of bioclimatic coefficients of crop water consumption taking into account the space-time variability of the actual biomass has been substantiated and demonstrated. For the adaptation of irrigation management to the conditions of air drought, it is proposed to use an additional criterion for making decisions on crop cooling, which is determined by the maximally permissible temperature duration at the vegetation surface above the physiologically acceptable level. It is established that under conditions of air drought, in addition to slowing the growth of biomass, physiological processes occur in the leaves and reproductive organs of plants, due to the increase in the temperature of the vegetation surface. According to studies of energy transfer processes in crops during periods of atmospheric drought, an increase in the use of a share of thermal energy for turbulent exchange has been found compared to the volumes of energy that is evaporated. The vision of the future development of methods of operational irrigation planning based on modern agricultural information platforms has been presented. It will allow to choose a method of operational irrigation management, based on the capabilities of each farm economy and to provide "on-line" consulting for water user organizations or farm personnel.

Ab initio study of energy transfer pathways in dinuclear lanthanide complex of europium(III) and terbium(III) ions.

An ab initio XMCQDPT2/CASSCF study of energy transfer processes in the dinuclear lanthanide complex [(Acac)3Eu(μ-Bpym)Tb(Acac)3] (Acac is acetylacetonate, and Bpym is 2,2'-bipyrimidine) and a corresponding computational procedure are presented. Because ligands in lanthanide complexes weakly interact with each other, the large dinuclear complex bearing seven organic ligands is divided into fragments that reproduce the electrostatic effects of the ions on the electronic and geometrical structure of the ligands. The multireference XMCQDPT2/CASSCF approach is directly applied to these relatively small fragments with reasonable computational cost. The calculated energies of the singlet and triplet excited states agree well with the experiment. Based on the calculated energies, the energy level diagrams of the complex are constructed and intramolecular energy transfer channels are determined.

Time-resolved methods in biophysics. 4. Broadband pump—probe spectroscopy system with sub-20 fs temporal resolution for the study of energy transfer processes in photosynthesis

In this paper we discuss how to push the temporal resolution limits of transient absorption spectroscopy in order to detect very fast processes (energy relaxation, energy or charge transfer, vibrational coherence) taking place in molecules of biological relevance. After reviewing the main principles of femtosecond pump-probe spectroscopy, we describe an experimental setup based on two synchronized non-collinear optical parametric amplifiers (NOPAs). Each NOPA can be independently configured to generate ultra-broadband sub-10 fs visible pulses, tunable 10-15 fs visible pulses, tunable 15-40 fs near-infrared pulses (900-1500 nm). This system enables to perform pump-probe experiments over nearly two octaves of spectrum with sub-20 fs temporal resolution. We then present an application example highlighting the capability of this instrument to track excited state dynamics in biomolecules on the sub-100 fs timescale: the study of carotenoid-bacteriochlorophyll energy transfer processes in peripheral light-harvesting complexes (LH2) from purple bacteria. We show that, by comparing excited-state dynamics of the carotenoids in organic solvents and inside the LH2 complexes, it is possible to visualize in the time domain the primary events in photosynthesis.

Energy transfer processes and heat generation in Yb3+-doped phosphate glasses

In this work we present a study of energy transfer processes in Yb3+-doped phosphate glasses with different OH− contents and Yb3+ concentrations. Fluorescence and thermal lens (TL) techniques were used in order to analyze the Yb3+–OH− and Yb3+-other impurity interactions. The role of OH− groups is also presented and discussed. The results show that in the low concentration region up to ∼3×1020Ybions∕cm3, the interaction between Yb3+ and OH radicals is the dominant process reducing the lifetime, increasing the TL dioptric power, and then endangering the performance of the system. For the high Yb3+ concentration limit, the Yb3+-other impurity interaction, which is favored by energy migration, is comparable to the Yb3+–OH− energy transfer, even for high OH− concentration. The nonradiative decay rate due to hydroxyl groups follows Förster-Dexter theory [Ann. Phys. 2, 55 (1948); J. Chem. Phys. 21, 836 (1953)] except at low Yb3+ concentration, being more active at low OH− levels, where quenching rate is probably due to several impurities.

Efficient Light Harvesting in Dye-Endcapped Conjugated Polymers Probed by Single Molecule Spectroscopy

The development of sophisticated microscopic models of energy transfer in linear multichromophoric systems such as conjugated polymers is rarely matched by suitable experimental studies on the microscopic level. To assess the roles of structural, temporal, and energetic disorder in energy transfer, single molecule spectroscopic investigations of the elementary processes leading to energetic relaxation in conjugated polymers are desirable. We present a detailed study of energy transfer processes occurring in dye-endcapped conjugated polymer molecules on the single molecule level. These processes are mostly masked in ensemble investigations. Highly efficient intramolecular energy transfer along a single polyindenofluorene chain to a perylene endcap occurs in many instances and is resolved in real time. We further consider the spectral emission characteristics of the single molecule, the polarization anisotropy which reveals the chain conformation, the fluorescence intermittency, and the temperature dependence and conclude that the efficiency of energy transfer in the ensemble is controlled by the statistics of the individual molecules. The weak thermal activation of energy transfer indicates the involvement of vibrational modes in interchromophoric coupling. Whereas backbone-endcap coupling is strong, the rate-limiting step for intramolecular energy transfer is the migration along the backbone. The results are particularly relevant to understanding undesired exciton trapping on fluorenone defects in polyfluorenes.

Study of energy transfer processes in CH as prerequisite for quantitative minor species concentration measurements

Energy transfer effects on CH A-X (0,0) laser-induced fluorescence spectra have been studied for the first time in rich methane and acetylene atmospheric-pressure flames using a picosecond laser system in conjunction with a streak camera. Results allowed determination of typical fluorescence lifetimes of levels in the A state populated by rotational energy transfer (RET), and of state-dependent quenching. These values are used to extend the LASKIN simulation program that has originally been developed to study energy transfer processes in OH, for modelling of spectra of the CH A-X (0,0) transition. Results show good agreement between experiment and simulation both in the spectrally as well as in the temporally-resolved regime.

The Chemical Desymmetrisation of Two- and Three-Dimensional Polyphenylenes as a Key Step to Functional Nanoparticles

Desymmetrisation of macromolecules is an important process in the preparation of functional nanoparticles for a variety of potential applications. Efficient routes for the directed stepwise synthesis of desymmetrised functional two- and three-dimensional polyphenylenes are here presented. Building blocks substituted with different functional groups have been prepared and combined to make functionalised hexaphenylbenzenes. Cyclodehydrogenation of these gives unique hexa-peri-hexabenzocoronenes with different types of substituents in various substitution patterns. These show remarkable phase-forming properties, which can be controlled through their substitution patterns, making them attractive candidates for use in electronic aplications. Selective silylation of a tetrahedrally symmetric core molecule followed by stepwise addition of substituted dendrons leads to dendrimers selectively functionalised on the periphery by different functional groups in well-defined spatial relationships to each other. By this method dendrimers have been made with two diffferent chromophores permitting study of energy transfer processes, and dendrimers with functional groups permitting their attachment to electrodes or biological systems with posssible applications as biosensors.

STUDY OF ENERGY TRANSFER IN ORGANIC DYE PAIRS USING THERMAL LENS TECHNIQUE

It is demonstrated that the dual beam thermal lens technique can be very effectively used for the study of energy transfer processes in organic dye mixtures. The energy transfer rate and critical transfer radius in Rhodamine 6G — Rhodamine B are calculated using this technique. The observed results support Forster type resonance transfer due to long-range dipole–dipole interaction as reported by earlier workers using other techniques.

A study of energy transfer processes in zinc–porphyrin films using Monte Carlo simulation of fluorescence decay

Energy transfer in thin films of zinc tetra(-octylphenyl)-porphyrin (ZnTOPP) doped with the copper (CuTOPP) or free-base (H 2TOPP) analogues were investigated by time-resolved fluorescence and Monte Carlo simulations, applied to a single domain model of parallel porphyrin stacks. Rate constants for intra- and inter-stack energy transfer are (0.8–1.1)×10 12 and (71–91)×10 9 s −1 , respectively. The fluorescence lifetimes for ZnTOPP and H 2TOPP films are (1.80–1.88)×10 −9 and (6.8–7.3)×10 −9 s. The rate constant for H 2TOPP to ZnTOPP back transfer is (8.8–9.4)×10 6 s −1 . The results agree with those of a previous analytical analysis [1].

Selective doping of multilayer organic light emitting devices

Doping of organic light emitting diodes (OLEDs) has well-established benefits such as tuning emission wavelengths, as well as enhancing device lifetime and quantum efficiency. The use of low percentage doping within the emissive layer is an established tool for the study of energy transfer processes in OLEDs, allowing one to trace electron and hole movement and exciton formation. Delta doping, in which a thin layer of the dopant material alone is incorporated in the device, has the added advantage of a thinner sensing layer and an electroluminescence spectrum distinct from that of the host material. In an ongoing effort to further our understanding of electroluminescent emission processes, we have fabricated multilayer OLEDs which incorporate a narrow (<1 nm) delta-doped DCM region within the emissive layer. The devices studied were deposited on indium tin oxide (ITO) on glass substrates using thermal evaporation, with a structure of ITO, TPD (40 nm)/selectively doped Alq3 (40 nm)/Al (100 nm). The relationship between the doping profile and the emission characteristics will be discussed.

Energy transfer processes in organized media. II. Generalization of the crystal model for dipole–dipole interactions in cubic sites

The study of energy transfer processes between optical centers in crystalline cubic materials is reexamined within the conceptual framework of the crystal model. The initial assumption of modest transfer rates is relaxed, thereby correcting the master equation of the crystal model and making it possible to explain the nonexponential behavior of the radiation decay curves from excited states. This study shows that such experimental trends are the result of a purely statistical effect related to the fluctuation in the population of optically active ions in a crystal lattice. The corrected crystal model does not need to invoke migration of excitation processes to explain the fast exponential decay observed in crystals with a high concentration of optical centers.

Energy transfer processes in organized media. I. A crystal model for cubic sites

A general study of energy transfer processes in organized condensed matter is presented. Considering the nature of these media, i.e., the periodic properties of the lattice, a theoretical microscopic model is worked out for the general case. It takes full advantage of the translational periodicities in the crystalline phase, using the real discrete distribution of ions in the lattice. A general master equation is obtained and, for the sake of completeness, a particular case is analyzed for the dipole–dipole nonradiative mechanism for energy transfer processes in the elpasolite lattices where the Ln3+ impurities are in cubic sites.

CHLOROPHYLLS IN POLYMERS. I. STATE OF CHLOROPHYLL a IN UNSTRETCHED POLYMER SYSTEMS

Abstract— Model systems for the study of energy transfer processes are useful for the elucidation of the various factors governing the mechanism of energy transfer in photosynthetic systems. Here we describe the characterization of two systems, consisting of chlorophyll a incorporated in anhydrous nitrocellulose and polyvinylalcohol films. First, optical spectroscopy and time‐resolved fluorescence techniques are used to characterize the state of the chlorophyll molecules in the films. We find that in nitrocellulose films the state of chlorophyll a depends strongly on the ratio of nitrocellulose to dimeth‐ylsulfoxide in the solutions from which the films are cast. The state of chlorophyll a in polyvinylalcohol films does not depend on the amount of polymer originally dissolved in dimethylsulfoxide. In these films the pigment is monomeric at low concentrations of chlorophyll a, but aggregates are formed at much lower concentrations than in nitrocellulose. The latter fact is explained by the existence of pockets in polyvinylalcohol, leading to high local concentrations.To further test the suitability of the nitrocellulose polymer films as model systems for energy transfer processes, time‐resolved fluorescence anisotropy profiles are measured in dependence of the concentration of pigments in the matrix. Fits of the observed decay profiles to the predicted decay show good correspondence, as long as no traps are present. Furthermore, the fitted decay times yield the correct value of the Forster radius R0 as compared to the value obtained spectroscopically. We thus conclude that the chlorophyll a‐nitrocellulose system can be very appropriate for the study of energy transfer processes between photosynthetic pigment, since the pigments are uniformally distributed in the matrix.

Dependence of the collisional relaxation of highly vibrationally excited polyatomic molecules on the population distribution function

The influence of the population distribution function on the collisional relaxation of highly vibrationally excited polyatomic molecules is analyzed in a non-reactive system. Unimodal and bimodal energy distributions are considered. Calculations made with unimodal energy distributions showed that the energy decay is almost independent of its initial shape and also of the collisional transition probability models, provided they have the same dependence on energy. When the initial distribution is bimodal, the rate of energy decay, for constant average energy transferred, depends on the fraction of molecules excited, but if the decay is exponential, the energy loss profile is almost independent of the fraction q of molecules excited. These results are discussed in relation to the use of IR multiphoton absorption as an experimental technique for the study of energy transfer processes.

Energy-transfer processes in lanthanide dinuclear complexes with p-tert-butylcalix[8]arene: an example of dipole-dipolar mechanism

The study of energy-transfer processes between Tb(III) and various Ln(III) ions (Ln = Ho, Nd, Eu) in dinuclear complexes of p-tert-butylcalix[8]arene, [(Tb[sub x]Ln[sub 2[minus]x])(LH[sub 2])(DMF)[sub 5]][center dot]4DMF, is reported. Easily available crystals allow an extended work on these transfers. The compounds contain isolated molecules, so that a simple model accounts for the observed luminescence decays. Only two types of interacting Tb-Ln pairs need to be taken into account, one at 3.7 A and the other at 11 A. A rate equation is developed to quantify the energy transfer, and the analyzed data give an interaction strength sequence proportional to the overlap integral, F[sub Tb-Ho] > F[sub Tb-Nd] > F[sub Tb-Eu]. Moreover, the good fits obtained point unambiguously to a dipole-dipolar character of the energy-transfer processes. The low-lying ligand-to-metal charge-transfer state (LMCT) increases the absorption probabilities of Eu(III) f-f transitions considerably, and the resulting quasi-allowed [sup 5]D[sub o] [yields] [sup 7]F[sub o] transition plays an important role in the energy-transfer process. This feature permits us to assign a dipolar character to the interaction responsible for the energy transfer. The low-lying LMCT state also induces a strong intramolecular Eu-Eu interaction. 20 refs., 3 figs., 4 tabs.

Photoluminescence Study of Energy Transfer Processes in Erbium Doped AlxGal−xAs Grown by MBE

ABSTRACTSharp photoluminescence from the intra-4f shell of Er3+ is observed from erbium doped AlxGal-xAs (0 ≤x ≤ 1) grown by molecular beam epitaxy. The intensity of the luminescence from the erbium is strongly dependent on the aluminum composition with a maximum at x ≈ 0.6. We will present a model that explains the variation in intensity based on the energy transfer coupling efficiency between the host semiconductor and the optically active erbium ions. The coupling efficiency is dominated by the alignment or misalignment of the erbium energy levels with the energy bands of the host semiconductor and by the excess carrier lifetime in the host. The data and model, which are presented here for the first time, are consistent with our previous work on the effects of co-doping with Be or Si and with other workers' measurements of thermal quenching in rare earth doped semiconductors.

Collisional relaxation of highly excited vibrational levels of the I2 X 1Σ+g state using an I2 optically pumped laser

A new technique for the study of energy transfer processes using a cw optically pumped laser (OPL) is presented. Population removal rates from individual vibration–rotation levels of a nonfluorescing state may be measured. The rate measured is that out of the lower level of the lasing transition. A limiting case of a simple kinetic model, involving seemingly contradictory requirements of high OPL circulating power and low pump power, is described and experimentally realized. Rate coefficients are extracted by observing the change in spontaneous emission (side fluorescence) intensity from the OPL gain medium with and without OPL oscillation. Removal rates from several highly excited vibrational levels of I2 X 1Σ+g are measured using an Ar+ laser pumped I2 BO+u–X 1Σ+g(v′=43, v″=42, 64, and 76–83) laser.