General model of nonradiative excitation energy migration on a spherical nanoparticle with attached chromophores.

Theory of multistep excitation energy migration within the set of chemically identical chromophores distributed on the surface of a spherical nanoparticle is presented. The Green function solution to the master equation is expanded as a diagrammatic series. Topological reduction of the series leads to the expression for emission anisotropy decay. The solution obtained behaves very well over the whole time range and it remains accurate even for a high number of the attached chromophores. Emission anisotropy decay depends strongly not only on the number of fluorophores linked to the spherical nanoparticle but also on the ratio of critical radius to spherical nanoparticle radius, which may be crucial for optimal design of antenna-like fluorescent nanostructures. The results for mean squared excitation displacement are provided as well. Excellent quantitative agreement between the theoretical model and Monte-Carlo simulation results was found. The current model shows clear advantage over previously elaborated approach based on the Padé approximant.

Förster resonance energy transfer and trapping in selected systems: analysis by Monte-Carlo simulation.

Monte-Carlo simulation method is described and applied as an efficient tool to analyze experimental data in the presence of energy transfer in selected systems, where the use of analytical approaches is limited or even impossible. Several numerical and physical problems accompanying Monte-Carlo simulation are addressed. It is shown that the Monte-Carlo simulation enables to obtain orientation factor in partly ordered systems and other important energy transfer parameters unavailable directly from experiments. It is shown how Monte-Carlo simulation can predict some important features of energy transport like its directional character in ordered media.

Electronic excitation energy migration in partly ordered polymeric films.

Multistep intermolecular energy migration between elongated fluorophores (carbocyanines) in uniaxially oriented polymer films is studied based on fluorescence depolarization and Monte-Carlo simulations. Contrary to disordered systems it is found experimentally that the concentration depolarization of fluorescence in uniaxially oriented films is extremely weak despite effective energy migration. Based on the concentration depolarization experiment in the ordered matrix it is possible to estimate the angle between absorption and fluorescence transition moments of carbocyanines. The values of that angle are very close to those obtained from other methods.

Excitation energy transport and trapping in concentrated solid solutions of flavomononucleotide.

Excitation energy transport and trapping is studied for monomer-fluorescent dimer system of flavomononucleotide (FMN) in polyvinyl alcohol films (PVA). It is shown that the theory neglecting reverse energy transfer (RET) from dimers to monomers does not allow for the explanation of concentration quenching and concentration depolarization results presented herein. Much better agreement has been obtained using generalized energy transport theory in which fluorescent dimers are treated as imperfect traps for excitation energy. Such parameters like the dimer quantum yield and its emission anisotropy are estimated.

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[Formula: see text] only slightly overrate those obtained for the real values of the orientation factor κ 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 λex≈λ0-0 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 forλ ex <λ0-0 (Stokes excitation) the experimental emission anisotropies are lower than predicted by the theory. However, upon anti-Stokes excitation (λex>λ0-0), 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 λ0-0.

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 ŋM 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 ŋM. 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 ŋM 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.

Investigations of the excitation energy transport mechanism in donor-acceptor systems.

Measurements of fluorescence quantum yield ηD/ηoD of Na-fluorescein (donor; D) versus concentration of rhodamine B (acceptor; A) in viscous solutions have been carried out. The donor concentration in these solutions was as follows:C D=2·10(-2) M (system I), 1.5·10(-2) M (II), 10(-2) M (III), 3·10(-3) M (IV), and 5·10(-5) M (V). The experimental results have been compared with current theories of nonradiative electronic energy transfer (NEET). In the case of very strong migration (systems I, II, and III), a significant influence of correlations (between configurations of D and A molecules in the surroundings of successively excited donors) on quantum yield ηD/ηoD has been determined. Experimental values have been found to be clearly higher in comparison with those predicted theoretically. The influence of possible factors on the decrease in the effectiveness of excitation energy transport to traps-acceptors in systems of very strong migration has been discussed.