Processes of electronic excitation energy transfer between luminescent dye molecules in microheterogeneous systems based on polymer micellar solutions

The processes of electronic excitation energy transfer (EEET) between different-type dye molecules in polymer micellar solutions with different polyelectrolyte concentration have been analyzed. The possibility of regulating the EEET efficiency between dye molecules due to change of the structure of polymer micellar complexes has been established. The results presented are explained from the viewpoint of the fractal character of distribution of interacting particles. The experimental dependence of the fractal dimension of the distribution of dye molecules in polymer micellar solutions on the polyelectrolyte concentration has been obtained.     

Electronic excitation energy transfer between CdS quantum dots and carbon nanotubes

Stationary and transient photoluminescence of CdS quantum dots deposited on silicon substrates and carbon nanotubes is investigated. The photoluminescence spectrum of quantum dots on a silicon substrate is dominated by a band originating from electron transitions between the quantum-confinement levels in the dots. When the quantum dots are deposited on carbon nanotubes, the intensity of this band decreases significantly. Furthermore, the kinetics of the photoluminescence decay becomes faster, which brings evidence of an additional channel for the quantum-dot deexcitation. The analysis of the experimental data demonstrates that the Förster energy transfer from CdS quantum dots to carbon nanotubes is most probably responsible for this channel. The efficiency of this process exceeds 60%.     

Electronic excitation energy transfer in poly-N-epoxypropyl-3,6-dibromocarbazole

Physics of the Solid State - The mechanisms of electronic excitation energy transfer in films of poly-N-epoxypropyl-3,6-dibromocarbazole doped with pyrene, rubrene, and...     

Triplet-triplet transfer of electronic excitation energy in vapors of organic molecules

The rate constants of the triplet-triplet (T-T) transfer of energy of the electronic excitation obtained in the gas phase for two sets of donor-acceptor pairs with various spectral characteristics are compared with those predicted by the theories of nonradiative transitions. The rate constants K _TT and efficiencies β_DA of the T-T transfer are estimated from quenching of the phosphorescence of biacetyl by aromatic hydrocarbons (anthracene, 9-methylanthracene, 9,10-dimethylanthracene, 9-phenylanthracene, pyrene, and chrysene) and from quenching of the delayed fluorescence of benzophenone, 4-phenylbenzophenone, anthraquinone, carbazole, and naphthalene by biacetyl, as well as from the sensitization of phosphorescence of biacetyl by these compounds. The causes of the broad variations of β_DA in the gas phase from 0.35 (biacetyl-anthracene) to 10^?4 (biacetyl-chrysene) are discussed for the studied donor-acceptor pairs, which differ in the exothermicity of the process and in the configuration of the lowest triplet levels of the molecules participating in the transfer. The dependences of the transfer rate constants K _TT on the free energy change ΔG (the range from 0.4 to ?1.7 eV) are analyzed. It is shown that, in gas-phase systems, K _TT increase with the exothermicity of the transfer, attain a highest value, and then decrease in the range of negative ?ΔG. The observed differences in the experimental values of β_DA are discussed in terms of changes in the free energy ΔG of the process, in the energy of rearrangement of the nuclear configuration of the molecules during the transfer, and in the matrix elements of the interaction.     

Nonradiative triplet-singlet transfer of electronic excitation energy between dye molecules in the vicinity of the silver-film surface

Nonradiative triplet-singlet transfer of electronic-excitation energy between molecules of organic dyes (erythrosine and methylene blue) in a polymer film that is deposited on the surface of a silver layer is experimentally studied. It is demonstrated that the energy-transfer efficiency in such a system is lower than the efficiency in the samples without metal layers. The results of the proposed mathematical model are in qualitative agreement with the experimental data.     

Vibrational energy transfer between molecules dissolved in cryogenic liquids

Results from recent papers on vibrational energy transfer in cryogenic liquids are discussed. An overview of the significance of these results is given especially with respect to failures of the Isolated Binary Collision hypothesis for both near-resonant and non-resonant energy transfer. Preliminary results obtained for energy exchange between CO isotopes are reported and a new explanation is suggested for the solvation effects of liquid N₂/Ar mixtures on the non-resonant deactivation of N₂(v=1).     

Triplet-triplet transfer of electronic energy upon nonequilibrium vibrational excitation of triplet molecules

The specific features of the triplet-triplet (T-T) transfer of electronic excitation energy in a gas phase upon nonequilibrium vibrational excitation of the triplet molecules of a donor were studied for an anthraquinone-diacetyl donor-acceptor pair using the time-resolved slow fluoresence of anthraquinone and sensitized phosphorescence of diacetyl. It is shown that in the gas phase, which allows regular control of the number of collisions, competition between the processes of T-T transfer and intermolecular vibrational relaxation is observed for nanosecond time resolution. The T-T transfer rate for the molecular system investigated exceeded the rate of intermolecular vibrational relaxation k_V in the triplet state T₁ of the donor. The effectiveness of the T-T transfer of energy by vibrationally excited molecules turned out to be higher than the effectiveness of transfer by thermalized ones, but even the highest of them was much less than unity. An increase in the equilibrium temperature of vapors led to a decrease in the effectiveness of transfer for both vibrationally excited and thermalized triplet molecules, thus indicating the importance of the collisional complex in the intermolecular process studied. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 474–479, July–August, 2000.     

Electronic excitation dynamics and energy transfer in lithium-gadolinium borates doped by rare earths

This paper reports on luminescence studies of lithium borate Li₆Gd(BO₃)₃ doped with Eu³⁺ and Ce³⁺ and Li₆Eu(BO₃)₃ crystals upon selective excitation by synchronous radiation in the pump energy region 3.7–27 eV at temperatures of 10 and 290 K. The effective energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ is found to operate by the resonant mechanism, as well as through electron-hole recombination. A study is made of the fast decay kinetics of the Ce³⁺-center activator luminescence under intracenter photoexcitation and excitation in the interband transition region. The mechanisms underlying luminescence excitation and radiative relaxation of electron states of rare-earth ions are analyzed and energy transfer processes active in these crystals are discussed.     

The diffusion and non-radiative transfer of excitation energy in molecular crystals containing foreign molecules

A study is made of the non-radiative transfer and diffusion of excitation energy in molecular crystals containing foreign molecules and its influence on the luminescent properties of these crystals.

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Vibration-assisted coherent excitation energy transfer in a detuned dimer

The important role of high-energy intramolecular vibrational modes for excitation energy transfer in the detuned photosynthetic systems is studied. Based on a basic dimer model which consists of two two-level systems(pigments)coupled to high-energy vibrational modes, we find that the high-energy intramolecular vibrational modes can enhance the energy transfer with new coherent transfer channels being opened when the phonon energy matches the detuning between the two pigments. As a result, the energy can be effectively transferred into the acceptor. The effective Hamiltonian is obtained to reveal the strong coherent energy exchange among the donor, the acceptor, and the high-energy intramolecular.A semi-classical explanation of the phonon-assisted mechanism is also shown.     

Conformational transitions in single polymer molecules modeled with a complete energy landscape: continuous two-state model

An extension of the two-state Freely Jointed Chain model is presented in which the discrete energies of the two conformers are replaced by continuous functions of the conformer length. The statistical mechanics is initially developed in the Gibbs ensemble and leads to a conformational multi-state model. This is used to fit the equilibrium force-extension curve for Dextran. The continuous model also allows the use of Transfer Matrix methods to calculate all statistical properties in the Helmholtz ensemble, including thermal fluctuations. The latter are obtained with near perfect agreement to experiment.     

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.     

Electronic energy transfer in benzophenone adlayer

The extent to which energy transfer occurs in electronically excited organic adlayer films on dielectric surfaces is investigated. Migration and subsequent trapping of the energy in the film are observed by pumping the singlet state of an organic adlayer of benzophenone and by monitoring the phosphorescence and fluorescence lifetimes. To observe the effects of adsorption, benzophenone was chosen as the adlayer because the energies of its well characterizedn, carbonyl states are remarkably sensitive to solvent interactions. Upon excitation with a nitrogen laser, the perturbation on the electronic states of benzophenone by the substrate caused the emergence of the normally absent fluorescence from the adlayer traps at the interface between the surface of the dielectric substrate and the adlayer. Energy transfer to this interface was observed as a function of film thickness. On the surface of a single crystal of an organic crystal, naphthalene, energy transfer from the adlayer to the substrate was observed, whereas such transfer was not energetically possible with the other dielectric surfaces.     

Strongly localized states in a single carbon nanotube with polymer molecules

A single-walled carbon nanotube (SWCNT) with conjugated polymer molecules is analyzed via optical spectroscopy. The presence of strongly localized excitonic states in the SWCNT is confirmed using time-integrated photoluminescence (PL). The PL spectrum exhibits extremely narrow width (~0.8 meV) which is attributed to the strong confinement of the states by polymer molecules. In addition, I observed that the excited states are gradually filled as a function of the excitation power, which supports the localized excitonic behavior. Only the ground excitonic state is observed at low excitation powers, but three additional PL peaks appear as the excitation power is increased. Especially, the power-dependent PL spectrum shows a blueshift and increased width, which can be elucidated in terms of quantum confined stark effect and the screening of induced electric fields. Overall, I demonstrate that the presence of polymer molecules induces several localized states in a single SWCNT.     

共轭聚合物单分子构象和能量转移特性研究

利用基于宽场显微光学系统的单分子散焦成像技术测量了不同构象poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT)共轭聚合物单分子的光物理与动力学特性.通过分析共轭聚合物单分子的荧光轨迹和对应的发射偶极取向变化识别共轭聚合物单分子发光单元,发现延伸构象下的单分子呈现多发色团发光特性,而折叠构象下的单分子保持高效链间能量转移,呈现单个发色团发光特性.共轭聚合物单分子构象对能量转移效率的影响可用于研究基于共轭聚合物的光电器件和分子器件.