Influence of donor-donor transport on excitation energy transfer

Energy migration and transfer from acriflavine to rhodamine B and malachite green in poly (methylmethacrylate) have been investigated using the decay function analysis. It is found that the influence of energy migration in energy transfer can be described quite convincingly by making use of the theories of Loring, Andersen and Fayer (LAF) and Huber. At high acceptor concentration direct donor-acceptor transfer occurs through Förster mechanism.     

Nonradiative excitation energy transport in one-component disordered systems

High-accuracy Monte Carlo simulations of the time-dependent excitation probabilityG ^s (t) and steady-state emission anisotropyr _M /r _0M for one-component three-dimensional systems were performed. It was found that the values ofr _M /r _0M obtained for the averaged orientation factor only slightly overrate those obtained for the real values of the orientation factor _ik ² . This result is essentially different from that previously reported. Simulation results were compared with the probability coursesG ^s (t) andR(t) obtained within the frameworks of diagrammatic and two-particle Huber models, respectively. The results turned out to be in good agreement withR(t) but deviated visibly fromG ^s (t) at long times and/or high concentrations. Emission anisotropy measurements on glycerolic solutions of Na-fluorescein and rhodamine 6G were carried out at different excitation wavelengths. Very good agreement between the experimental data and the theory was found, with _ex0-0 for concentrations not exceeding 3.5·10^–2 and 7.5·10^–3 M in the case of Na-fluorescein and rhodamine 6G, respectively. Up to these concentrations, the solutions investigated can be treated as one-component systems. The discrepancies observed at higher concentrations are caused by the presence of dimers. It was found that for _ex <_0-0 (Stokes excitation) the experimental emission anisotropies are lower than predicted by the theory. However, upon anti-Stokes excitation (_ex>_0-0), they lie higher than the respective theoretical values. Such a dispersive character of the energy migration can be explained qualitatively by the presence of fluorescent centers with 0-0 transitions differing from the mean at _0-0.     

Direct and reverse energy transport in systems of monomers and imperfect traps: Monte Carlo simulations

A Monte Carlo simulation of the concentration dependence of the fluorescence quantum yield _M and emission anisotropyr _M of a system containing dye molecules in the form of monomers M and clusters T (statistical pairs and trimers) playing the role of the imperfect traps for nonradiative excitation energy transfer (NET) has been carried out. The simulation has been made for determined values of Förster critical distancesR ₀ ^MM andR ₀ ^MT and for several values ofR ₀ ^TM andR ₀ ^TT , assuming that the energy may be transferred from M^* to T as well as from T^* to M (reverse nonradiative energy transfer, RNET). It was shown that the RNET process in the range of high concentrations may strongly change the values ofr _M as well as those of _M. For emission anisotropyr _M an effect of repolarization was observed which decreases rapidly with increasingR ₀ ^TM andR ₀ ^TT . A very good agreement between the simulation results of _M and the theoretical model with no adjustable parameters was found. In the model, the RNET process and influence of correlation between active molecules on energy migration among monomers were taken into account.     

Phonon-assisted excitation energy transfer in photosynthetic systems

The phonon-assisted process of energy transfer aiming at exploring the newly emerging frontier between biology and physics is an issue of central interest.This article shows the important role of the intramolecular vibrational modes for excitation energy transfer in the photosynthetic systems.Based on a dimer system consisting of a donor and an acceptor modeled by two two-level systems,in which one of them is coupled to a high-energy vibrational mode,we derive an effective Hamiltonian describing the vibration-assisted coherent energy transfer process in the polaron frame.The effective Hamiltonian reveals in the case that the vibrational mode dynamically matches the energy detuning between the donor and the acceptor,the original detuned energy transfer becomes resonant energy transfer.In addition,the population dynamics and coherence dynamics of the dimer system with and without vibration-assistance are investigated numerically.It is found that,the energy transfer efficiency and the transfer time depend heavily on the interaction strength of the donor and the high-energy vibrational mode,as well as the vibrational frequency.The numerical results also indicate that the initial state and dissipation rate of the vibrational mode have little influence on the dynamics of the dimer system.Results obtained in this article are not only helpful to understand the natural photosynthesis,but also offer an optimal design principle for artificial photosynthesis.     

An unique method of determining the excitation energy migration coefficient in organic liquid scintillators

An experimental method is described to determine the excitation energy migration coefficient Λ in an organic liquid scintillator by measuring the energy transfer rate parameterk ₃ and the quenching rate parameterk _7b as a function of temperature in the range of 20°C–70°C. In this experiment we have used toluene as donor, ethyl-1-n-butyl-2-methyl-5-hydroxyindole-3-carboxylate (EBMHC) as acceptor and bromobenzene as quencher. The values of Λ are found to be in good agreement with literature values. Hence this method can be used to determine the migration coefficient in organic liquid scintillators.     

A comparative study of the energy migration and energy transfer in highly doped Nd:YAG single crystal

The donor-donor (D-D) energy migration interaction parameter C_DD in high-concentration Nd³⁺-doped YAG laser crystal is estimated, for the first time, by using the Yokota-Tanimoto (Y-T) model and the spectral overlap model (SOM) of Kushida. Firstly, the experimental luminescence decay curves of ⁴F_3/2 state of Nd³⁺ ions in YAG laser crystal at room temperature for 2.0 and 3.0 at% Nd³⁺ concentrations reported by Mao are fitted successfully by using the Y-T model and the parameter C_DD is obtained to be 1.50×10⁻³⁹ cm⁶/s. Secondly, the parameter C_DD is also directly calculated by using the SOM of Kushida: C_DD is calculated to be 2.73×10⁻³⁹ cm⁶/s. By comparing the energy migration interaction parameter C_DD and the donor-acceptor (D-A) energy transfer interaction parameter C_DA (1.794×10⁻⁴⁰ cm⁶/s), it is concluded that energy migration rate between Nd³⁺ ions in YAG laser crystal was about 11 times larger than energy transfer rate, and that energy migration would play a very important role in high-concentration Nd³⁺ -doped YAG laser crystal.     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Environmental effects on electronic excitation energy transfer in organic liquid scintillators

The efficiency of electronic excitation energy transfer from toluene to a dissolved fluorescent indole, is determined experimentally in systems comprising neat toluene, 1:9 mixture of toluene-cyclohexane and 1:9 mixture of toluene-paraffin as a function of temperature in the range 293–353°K (viscosity range 0.6–39 cP). The results are discussed in terms of the influence of temperature, dilution and viscosity of the medium on the energy transfer efficiency in binary organic scintillator systems.     

Light absorption and excitation energy transfer calculations in primitive photosynthetic bacteria

In photosynthetic organisms, light energy is converted into chemical energy through the light absorption and excitation energy transfer (EET) processes. These processes start in light-harvesting complexes, which contain special photosynthetic pigments. The exploration of unique mechanisms in light-harvesting complexes is directly related to studies, such as artificial photosynthesis or biosignatures in astrobiology. We examined, through ab initio calculations, the light absorption and EET processes using cluster models of light-harvesting complexes in purple bacteria (LH2). We evaluated absorption spectra and energy transfer rates using the LH2 monomer and dimer models to reproduce experimental results. After the calibration tests, a LH2 aggregation model, composed of 7 or 19 LH2s aligned in triangle lattice, was examined. We found that the light absorption is red shifted and the energy transfer becomes faster as the system size increases. We also found that EET is accelerated by exchanging the central pigments to lower energy excited pigments. As an astrobiological application, we calculated light absorptions efficiencies of the LH2 in different photoenvironments.     

Spectral characteristics of luminescence of solutions of complex molecules under conditions of electronic excitation transfer to a saturable acceptor in the case of inhomogeneous broadening of electronic energy levels

Spectral characteristics of solutions of complex molecules under conditions of inhomogeneous broadening of energy levels are considered in the case when the nonlinear dependence of the population of molecular states on the excitation intensity is determined not by saturation of molecular levels but by exchange of the electronic excitation energy with the environment. Calculations have shown that the dependence of the position of the fluorescence spectrum on the excitation frequency is nonmonotonic in solutions of the type and varies substantially with the excitation intensity. Belarusian State University, 4, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 164–168, March–April, 1997.     

Internal mode mechanism for collective energy transport in extended systems

We study directed energy transport in homogeneous nonlinear extended systems in the presence of homogeneous ac forces and dissipation. We show that the mechanism responsible for unidirectional motion of topological excitations is the coupling of their internal and translation degrees of freedom. Our results lead to a selection rule for the existence of such motion based on resonances that explain earlier symmetry analysis of this phenomenon. The direction of motion is found to depend both on the initial and the relative phases of the two harmonic drivings, even in the presence of noise.     

Kinetics of fluorescence polarization for solid bichromophore solutions in intense pulse excitation

The kinetics of fluorescence polarization in intense pulse excitation of solid disordered solutions of bichromophores that consist of complex molecules of two types between which there can be inductiveresonance transfer of electron-excitation energy is theoretically investigated. Variants of fluorescence excitation by single pulses and pulse trains are considered. The lifetime of the fluorescence of a given solution increases with the intensity of the exciting pulses. The possibility of controlling the duration of fluorescence attenuation for donor molecules incorporated into the bichromophores by the action of luminescence radiation at the frequency of acceptor-molecule absorption on the solution is demonstrated. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii Vol. 65, No. 4, pp. 546–550, July–August, 1998.     

The energy transfer mechanism in Ce,Tb co-doped LaF3 nanoparticles

LaF₃ pure host, LaF₃:Ce, LaF₃:Tb as well as LaF₃:Ce,Tb phosphors were synthesized by the hydrothermal method. X-ray diffraction measurements were in good agreement with the standard data of LaF₃ from JCPDS card No. 32-0483 and indicated that the material was nanocrystalline with an average particle size of about 36 nm. Photoluminescence spectra of co-doped samples revealed that the Ce³⁺ emission was quenched while Tb³⁺ emission was enhanced, implying that energy was transferred from Ce³⁺ (the donor) to Tb³⁺ (the acceptor) in this system. The luminescence intensities and lifetimes of the donor for different concentrations of the acceptor were fitted to theoretical models in order to investigate the energy transfer mechanism. The quadrupole–quadrupole and exchange interaction mechanisms gave the best fit between the experimental data and the theoretical curves. The effective average Bohr radius from the fit to the exchange model is 0.095 nm. Since this is close to the ionic radii of the Ce³⁺ and Tb³⁺ ions, it suggests that the exchange interaction mechanism contributes to the energy transfer.     

Photoinduced energy transfer in blend films of hole and electron transport materials

The photoluminescence properties of the blend films consisting of the hole transport and electron transport materials, PVK and Alq₃, are studied by steady-state and time-resolved photoluminescence (PL) spectroscopy. Both the relative intensity and the photoluminescence lifetime are intensively dependent of the weight ratios of PVK and Alq₃. The detailed analysis of experiment data provides clear evidence for a Förster energy transfer from excited PVK, as donor, to Alq₃, as acceptor, based on nonradiative resonant transfer mechanism, and allows the determination of Förster radius and the concentration dependent energy transfer efficiency.     

纳米体系中发光能隙展宽的研究

采用两种简单的一维模型,通过直接严格求解该模型下薛定鄂方程给出晶场中的限域情况下电子能级结构以及纳米体系中电子的限域能,从而研究体系的发光蓝移现象.基于限域能的计算,讨论了纳米体系的限域能和发光蓝移随尺寸以及势场的变化关系.结果表明：导带底的电子能级随体系的尺寸增大而降低;而价带顶则随粒径的增大而升高,因此,限域能随纳米尺寸的减小迅速增大.同时将计算结果与通常文献中所采用的有效质量计算的结果进行比较,可以看出二者有明显差别,且尺寸越小,其差异越大.因此,限域能随尺寸的变化并不严格满足通过有效质量方法计算出 关键词： 蓝移 能隙 量子限域效应 量子尺寸     

Concentration self-quenching in solid solutions of complex organic molecules under intense excitation

A theoretical study was carried out to investigate concentration self-quenching which is nonlinear in intensity in a molecular system caused by nonradiative electronic excitation energy transfer to relatively long-lived acceptors that are formed in a solution and whose function is performed by molecules that have acquired the triplet state. It is shown that at ratios of constants of radiative and nonradiative transitions typical of complex molecules at concentrations of ∼10⁻³–10⁻² mole/liter substantial (2- to 10-fold) fluorescence takes place at intensities of excitation of ∼10⁴–10³ of the intensity of saturation for a singlet subsystem. Polarization characteristics of the system are analyzed. Belarusian State University, 4, F. Skorina Ave., 220050, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 729–733, November–December, 1997.     

Processes of the excitation energy migration and transfer in Ce-doped alkali gadolinium phosphates studied with time-resolved photoluminescence spectroscopy technique

Spectral-kinetic characteristics of Gd³⁺ and Ce³⁺ luminescence from a series of Ce³⁺-doped alkali gadolinium phosphates of MGdP₄O₁₂ type (M=Li, Na, Cs) have been studied within 4.2-300 K temperature range using time-resolved luminescence spectroscopy techniques. The processes of energy migration along the Gd³⁺ sub-lattice and energy transfer between the Gd³⁺ and Ce³⁺ ions have been investigated. Peculiarities of these processes have been compared for MGdP₄O₁₂ phosphate hosts with different alkali metal ions. A contribution of different levels from the ⁶P_j multiplet of the lowest Gd³⁺ excited state into the energy migration and transfer processes has been clarified. The phonon-assisted occupation of high-energy ⁶P_{5/2, 3/2} levels by Gd³⁺ in the excited ⁶P_j state has been revealed as a shift of Gd³⁺⁶P_j→⁸S_7/2 emission into the short-wavelength spectral range upon the temperature increase. The relaxation of excited Gd³⁺ via phonon-assisted population of Gd³⁺⁶P_5/2 level (next higher one to the lowest excited ⁶P_7/2) is supposed to be responsible for the rise in probability of energy migration within the Gd³⁺ sub-lattice initiating the Gd³⁺→Ce³⁺ energy transfer at T<150 K, whereas further intensification of Gd³⁺→Ce³⁺ energy transfer at T>150 K is explained by the increase in probability of Gd³⁺ relaxation into the highest ⁶P_3/2 level of the ⁶P_j multiplet. An efficient reversed Ce³⁺→Gd³⁺ energy transfer has been revealed for the studied phosphates at 4.2 K.     

Inverse bottleneck in Eu-Mn energy transfer

The energy transfer (ET) between Eu²⁺ and Mn²⁺ in Ca₅(PO₄)₃Cl has been investigated. At low intensities under 405 nm excitation, time-resolved experiments provide microscopic parameters for the energy transfer between adjacent Eu²⁺ and Mn²⁺ ions. At high intensities, we observe a non-linear component in the energy transfer process due to ground state depletion of the Mn²⁺ ions, leading not to a reduction, but to an increase in the energy transfer rate between Eu²⁺ and Mn²⁺ ions and also energy transfer between excited Mn²⁺ ions. This results in a sub-linear response of both Eu²⁺ and Mn²⁺ luminescence as a function of intensity. Our observations are quantitatively described by a model using energy transfer to Mn²⁺ ions in both the ground and the excited state.