Leszek Kułak

Nonradiative excitation energy transport in one-component disordered systems.

AbstractHigh-accuracy Monte Carlo simulations of the time-dependent excitation probabilityGs(t) and steady-state emission anisotropyrM/r0M for one-component three-dimensional systems were performed. It was found that the values ofrM/r0M obtained for the averaged orientation factor

Excitation energy migration in uniaxially oriented polymer films

\overline {\kappa ^2 }

Monte-Carlo Simulation of Energy Migration in Uniaxially Oriented Polymer Films

only slightly overrate those obtained for the real values of the orientation factor κik2. This result is essentially different from that previously reported. Simulation results were compared with the probability coursesGs(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 fromGs(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 7.5·10−3M 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 forλex 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 new method for extending the utilizable range of Förster resonance energy transfer (FRET) is proposed and tested by the Monte Carlo technique. The obtained results indicate that the efficiency of FRET can be significantly enhanced at a given distance if the energy transfer takes place toward multiple acceptors that are closely located on a macromolecule instead of a single acceptor molecule as it is currently used in FRET analysis. On the other hand, reasonable FRET efficiency can be obtained at significantly longer distances than in the case of a single acceptor. Randomly distributed and parallel orientated acceptor transition moments with respect to the transition moment of the donor molecule have been analyzed as two extreme cases. As expected, a parallel orientation of donor and acceptor transition moments results in a more efficient excitation energy transfer. This finding could be used to directly reveal the assembly/deassembly of large protein complexes in a cell by fluorescence microscopy.

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

Abstract Multistep nonradiative excitation energy transport is for the first time studied in uniaxially stretched polymer films. Concentration depolarization studies performed for 3,3 ′ -diethylthiacarbocyanine iodide (DTCI) for stretched (system I) and unstretched (system II) polyvinyl alcohol films (PVA) yield extremely different results. As expected, disordered system II exhibits strong concentration depolarization due to energy migration between identical fluorophores. However, for stretched films, concentration depolarization effect is much weaker and it occurs at higher dye concentrations compared to system II. This fact is mostly due to strongly modified angular distribution of dipole moments of molecules taking part in the excitation energy migration.

Donor fluorescence decay in the presence of forward and reverse excitation energy transport in two-component disordered systems

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.

The influence of water structure on the rotational depolarization of fluorescence

Nonradiative electronic energy migration between identical fluorophores is studied numerically in uniaxially oriented polymer films. The reorientation effect of dipole moments induced by film stretching leads to extremely different concentration- and time-courses of emission anisotropy compared to those in disordered system. In particular, the effect of a much weaker concentration depolarization of fluorescence is due to the fact that not only primarily excited molecules contribute to emission anisotropy in oriented films.

Electronic Excitation Energy Migration in Partly Ordered Polymeric Films

A Monte Carlo simulation of the concentration dependence of the fluorescence quantum yield ŋM and emission anisotropyrM 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 distancesR0MM andR0MT and for several values ofR0TM andR0TT, 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 ofrM as well as those of ŋM. For emission anisotropyrM an effect of repolarization was observed which decreases rapidly with increasingR0TM andR0TT. 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.