Influence of rotational dynamics on collisional depolarization and quenching of molecular luminescence in the gas phase

Theoretical expressions describing the collisional depolarization of the luminescence of polyatomic molecules in the gas phase are obtained taking into account donor molecule rotation in a dynamic excitation-quenching regime. Effects of the quenching cross-section dependence on the rotation rate are examined by numerical calculations for several models. It is shown that the decay of luminescence anisotropy intensifies substantially with the degree of rotational modulation of the energy transfer.     

Investigation of energy transfer in terbium doped Y (2)SiO(5) phosphor particles

The kinetics of luminescence of sol-gel synthesized terbium doped Y (2)SiO(5) (YSO) phosphor particles is investigated in detail with reference to Tb concentration in the 0.001%-10% range. By increasing the dopant concentration, the luminescence profile changes from a blue to a green peaked emission spectrum because of the energy transfer among centers. The inter-center energy transfer mechanism is well accounted for by the Inokuti-Hirayama (IH) kinetic model which is based on a statistical average of inter-center distance dependent decay modes of the donor luminescence. The distribution of the decay modes is implemented from the F?rster-Dexter resonance theory of energy transfer by assuming a rate constant for the energy transfer by multipolar interactions between donors and acceptors. However, the experimental results recorded in the low concentration limit show the presence of green emission contributions in the luminescence spectrum which cannot be related to the Tb concentration; for this reason an additional internal energy transfer mechanism, occurring among levels of the same center, is proposed to account for the recorded emission properties. Thus, a new and more exhaustive model which includes both the internal and external energy transfer processes is considered; the proposed model allows a better explanation of the spectroscopic features of Tb related centers in YSO crystals and discloses the critical concentration and the quantum yields of the different energy transfer mechanisms.     

能量传递过程的计算机模拟

通过对供体(D)—受体(A)间传递速率静态分布的模拟,得到了静态能量传递情况下D的发光衰减曲线,与Inokuti-Hirayama模型结果一致.模拟并讨论了D—D传递与D—A传递相关和非相关情况下D—D传递对D发光衰减的影响. 关键词：     

Kinetics of photoreactions with participation of molecular oxygen in Langmuir-Blodgett films

The influence of molecular oxygen on the luminescence of organic molecules (dyes and aromatic hydrocarbons) in Langmuir-Blodgett films is studied. Experimental luminescence kinetics curves are compared with theoretical curves calculated in terms of different mathematical models. It is shown that the best agreement with experiment is achieved using a model that takes into account the frontal diffusion of excited oxygen molecules to the interphase surface.     

Effect of energy transfer on the kinetics of the decay and anisotropy of luminescence of prodan molecules

The temporal characteristics of the decay and polarization of luminescence of prodan solutions with different concentrations, excited by a picosecond laser radiation, are studied. The kinetics of the emission decay and polarization depend on the spectral range of luminescence recording and on the luminophore concentration and reflect the relaxation processes (leading to a long-wavelength shift of the entire emission band) and, in the case of sufficiently high concentrations, nonradiative energy transfer between the prodan molecules. It was found that the evolution of luminescence anisotropy in a diluted solution depends on the recording wavelength, which allowed us to calculate the average times of the Brownian rotational diffusion for prodan in glycerol, whose maxima were determined to be about 40 ns in the spectral region near 520 nm and to decrease to 14 and 17 ns at the edges of the emission region at 450 and 560 nm, respectively.     

Triplet-triplet annihilation of eosin molecules on the anodized aluminum surface

Spectral-kinetic properties of slow-rate fluorescence caused by triplet-triplet annihilation of triplet-excited eosin molecules on the anodized aluminum surface are studied in the temperature interval 120–290 K. The decay kinetics of slow-rate luminescence of adsorbed eosin molecules is analyzed for models of random triplet excitation energy transfer and exchange-resonance energy transfer that considers the relationship between the depth of luminophor penetration into pores of anodized aluminium and the fractal dimension of its surface.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–12, October, 2004.     

Influence of steric hindrance on luminescence anisotropy of resonance dipole–dipole transfer of electronic excitation energy

An orientation factor is calculated and the effect of steric hindrance in the donor and acceptor molecules is found for the efficiency of resonance dipole–dipole transfer of electronic energy. General expressions for the acceptor luminescence anisotropy under such conditions in isotropic media (gases and liquids) are obtained taking into account orientational relaxation in ensembles of donors and acceptors. It is shown that the luminescence anisotropy of an acceptor with significant steric hindrance can increase by more than an order of magnitude compared with the case with no hindrance.     

Pyrene-Naftylamide Bifluorophore for Spectra-Converting Media

New bifluorophore containing pyrene as an excitation energy donor and naphtylamide as an acceptor is synthesized and its optical properties are studied. It is founded that excitation spectrum of the bifluorophore is almost the sum of its constituents—pyrene and naftylamide. At the same time, in the luminescence spectrum there only the peak of acceptor luminescence is observed, which indicates the effective radiationless energy transfer from the donor to the acceptor. This is also proved by time resolved measurements of bifluorophore decay. The decay rate was calculated from decay curve and appear to be 0.25 нс^?1. The same value obtained from the Förster’s theory is almost by an order as high. It indicates that Förster’s does not applicable in this particular case. Instead, we have to use the electron excitation density functions approach.     

Study of energy transfer in a solution of coumarin 460 and rhodamine 6G by time-resolved laser-induced fluorescence spectroscopy

A multi-photon fast analog technique has been used to study nonradiative energy transfer from coumarin 460 to rhodamine 6G molecules. At low acceptor concentrations (<10^-2 mol/1) the fluorescence decay from coumarin deviates markedly from monoexponential behavior. The complex decay of the donor fluorescence reflects the time-dependent population of donor-acceptor pairs. Various energy transfer kinetics were investigated. The donor decay was found to be consistent with Förster kinetics and the critical energy transfer radius was found to be (5.46±0.10) nm.     

Calculation of kinetics of cooperative luminescence quenching in the nearest-neighbor approximation

The luminescence decay kinetics of donor centers that is caused by the down-conversion process, in which the donor excitation energy is transferred to a pair of acceptors, has been calculated in the nearestneighbor approximation for media of different dimensions. The interaction of donor and acceptor centers has been assumed to be multipolar. Two possible dependences of the probability of down-conversion on the intercenter distance have been examined. In both cases, it has been confirmed that the time dependence of the luminescence kinetics is stretched exponential.     

Electronic-Excitation Energy Transfer between Dye Molecules Adsorbed in One-Dimensional Photonic Crystals

Electronic-excitation energy transfer between molecules of Coumarin 7 (donor) and Rhodamine B (acceptor) have been experimentally investigated in one-dimensional photonic crystals based on oxidized mesoporous silicon and in a similar matrix, which does not possess the properties of a photonic crystal. The efficiencies of excitation transfer between donor and acceptor molecules have been determined based on the donor-fluorescence quenching and sensitized acceptor luminescence. It is established that the efficiency of electronic-excitation energy transfer in a photonic crystal increases in comparison with that for porous silicon.     

稀土离子发光体系中能量传递和迁移模型的研究

基于以前综合V-F模型(考虑施主受主(D-A)能量传递)和B模型(考虑施主施主(D-D)能量迁移)而提出的V-F-B综合模型,对La_1-xEr_xF₃体系中Er³⁺离子四种低掺杂浓度下(x=0.001,0.003,0.01,0.02)295K时⁴S_3/2→⁴I_15/2发光的时间演化曲线成功地重新进行了数值拟合.结果表明该体系内D-A间交叉弛豫能量传递是偶极-偶极作用,作用常数C_DA为4.75×10^-41cm⁶/s,与Okamoto等人原用YT扩散模型得到的结果一致.而拟合得到的四个D-D平均跳跃时间τ₀粗略地与x_D^α次方(x_D为D离子浓度)成正比(α≈-1.237),并不遵从τ₀很小时与x_D的-2次方成正比的理论关系.同时,通过把D-D迁移和D-A传递同时同样地纳入V-F模型还粗略估计了D-D平均跳跃时间τ₀′的值,发现它与V-F-B给出的τ₀拟合值比较一致,说明V-F-B模型在一定近似程度上是内部自恰的、合理的. 关键词： 能量传递 能量迁移 跳跃时间 交叉弛豫     

激子旋转弛豫对低掺杂卟啉侧链聚合物荧光衰变过程的影响

利用激子旋转扩散理论研究了一类低掺杂卟啉侧链聚合物中卟啉侧链基团的旋转对其发光动力学过程的影响.研究表明,卟啉侧链基团的旋转行为是导致激发态无辐射能量弛豫的重要途径.基团旋转越容易,能量弛豫速度越快,这可导致一个快速的荧光衰变动力学过程.在卟啉低掺杂浓度和聚合物分子链间距离较大的情况下,卟啉侧链基团的旋转成为影响荧光寿命和发光效率的主要因素.对实验测得的两种样品的荧光弛豫过程进行了拟合,理论结果与实验结果符合较好. 关键词： 激子旋转弛豫 瞬态荧光 卟啉侧链聚合物     

5,10-二(对-氨基)四苯基卟啉和吖啶黄之间激发能量的传递

利用Forster能量传递机制,研究了氯仿溶液中吖啶黄向5、10-二(对-氨基)四苯基卟啉(简称TBP)的激发能量传递和TBP.浓度变化时对能量传递速度常数的影响.实验结果表明,吖啶黄和TBP之间具备能量共振传递的条件,并且计算了能量共振传递的临界距离为0.42nm、传递效率为0.72和传递速度常数为1.42×107s-1.这些数据对复合膜光电转换器件的设计将具有重要意义.     

A Theory of Electronic Energy Transfer in Complex Molecular Systems

The Förster theory for energy transfer is critically reviewed in the context of the present-day theory of nonradiative transitions, and the fundamental tenets of the Förster theory are shown to be erroneous. A new theory of electronic energy transfer is constructed taking into account the electronic transition theory. The intermolecular interaction between donor and acceptor molecules is assumed to perturb electron states of isolated molecules before the donor-molecule excitation. A distinguishing characteristic of the intermolecular interaction is spatial delocalization of the wave functions of electron states of the interacting molecules. It is this fact that has made it possible to realize ordinary photophysical processes between electron states of various molecules in a bimolecular system. In the experiments under study, the result of the intermolecular nonradiative photoprocess is given as evidence of electronic energy transfer from the donor molecule to the acceptor molecule.     

Energy transfer between Yb3+ and Er3+ ions in an Er,Yb:YAl3(BO3)4 crystal

The electron-excitation energy transfer between Er³⁺ and Yb³⁺ ions in Er,Yb:YAl₃(BO₃)₄ crystals has been investigated. Crystals with different concentrations of active ions have been grown, and their luminescence decay kinetics in the spectral range near 1 μm have been experimentally measured. The energy-transfer microparameters have been calculated. It is shown that, to correctly describe the energy transfer in Er,Yb:YAl₃(BO₃)₄ crystals, one must take into account the interactions of higher orders than dipole-dipole interactions.     

Effects of Internal Rotations on the Time-Resolved Fluorescence in a Bichromophoric Protein System Under the Energy Transfer Interaction

Effects of internal rotations of chromophores under the energy transfer interaction in proteins on the time-resolved fluorescence were examined by numerical calculations. Expressions used for the calculations are based on the approximations that the energy transfer takes place according to Foöurster's mechanism and the rotational motions of the energy donor and acceptor along the surfaces of cones are described by a set of rotational diffusion equations. The intensity decay of the donor depended a little on the rotational diffusion coefficient of the donor in some cases, while that of the acceptor did very little. Anisotropy of the donor decayed faster as the diffusion coefficient of the donor increased. Anisotropy decay of the acceptor markedly depended not only on the mutual configuration of the pair in the protein, but also on the diffusion coefficient of the donor. The dependence of the time-resolved fluorescence on the diffusion coefficient of the acceptor was not as great as that of the donor.     

Förster resonance energy transfer in hybrid associates of colloidal Ag<Subscript>2</Subscript>S quantum dots with thionine molecules

Nonradiative resonance energy transfer in hydrophilic hybrid associates of thionine molecules (TH⁺) with colloidal Ag₂S quantum dots (QDs) with average diameter of 3.5 nm was studied. Photoluminescence spectra and its decay shown that for these systems the supplemental photosensitization of recombination luminescence of Ag₂S QDs (1200 nm) from the region of TH⁺ fluorescence (618 nm) is possible. It was found that the average lifetime of TH⁺ molecules luminescence is shortened during their association with Ag₂S QDs. Approximation of luminescence decay by stretched exponent with value of parameter β =?0.5 indicates on the inductive-resonance dipole-dipole (Förster) mechanism of nonradiative energy transfer (FRET). The efficiency of FRET was 0.29–0.41.     

Highly Selective Recognition on Enatioselective of R(+) and S(−)-1,1-Bi(2-Naphthol) Using of Eu(dap)<Subscript>3</Subscript> Complex as Photo Probe

The chiral recognition phenomenon was observed in enantioselective and excited-state energy transfer processes. Based on bimolecular luminescence quenching kinetics for a system containing chiral molecules, the quenching efficiency was evaluated by Stern-Volmer equation for a system containing a chiral R(+) and S(−) resolved quencher species. The utility of this methodology is confirmed by examining the enantio-selective excited-state quenching between Eu(dpa)₃ complex (where dpa = pyridine-2,6-dicarboxylate) acting as the energy donor to R(+)-1,1-Bi(2-Naphthol) and S(−)-1,1-Bi(2-naphthol) as the energy acceptor was studied in solution. The results of this study confirm the utility of luminescence measurements as a probe of chiral discriminatory behavior.     

Femtosecond luminescence decay due to exciton energy transfer in single-walled carbon nanotube bundles

We investigated luminescence decay kinetics in single-walled carbon nanotubes in large bundles excited by femtosecond pulses. The time constant of luminescence decay becomes longer with decrease in photon energy: 40 fs at 1.2 eV and 380 fs at 0.6 eV. This behavior is explained by exciton energy transfer from an excited to neighboring tubes.