Study Of Excitation Energy Transfer Research Articles

Mechanistic Study of Excitation Energy Transfer in Dye-Doped PPV Polymers

The mechanism of excitation energy transfer is studied using host−guest systems consisting of green and yellow emitting poly(phenylene vinylene) (PPV) based polymers into which red emitting dyes are dispersed. The photoluminescence from such polymer−dye systems is studied in the steady-state and time resolved. Furthermore, the electroluminescence from devices containing these polymer−dye systems as emissive layer is measured. It is shown that in such disordered polymers, characterized by dispersive exciton transport, energetic resonance between the polymer and the dye is not the only requisite for efficient energy transfer. In addition, the exciton kinetics of the combined polymer−dye system has to be taken into account. Efficient transfer of excitation energy from a disordered polymer to a dye can only occur if, at a certain energy, the polymer-to-dye exciton transfer rate is higher than the intrapolymer exciton migration rate. A consequence of the mechanism described here is that when a dye is dispersed...

Excitation Energy Transfer in the Lhca1 Subunit of LHC I-730 Peripheral Antenna of Photosystem I

Femtosecond transient absorption spectroscopy at 77 K was used to study excitation energy transfer processes in the reconstituted (r-)Lhca1 subunit of the peripheral light-harvesting antenna complex LHCI-730 of Photosystem I from higher plants. Under selective excitation of chlorophyll (Chl) b at 645 nm with 200 fs laser pulses, two energy transfer processes from Chl b to spectral forms of Chl a with lifetimes of 500 fs and 2.8 ps are induced. The subpicosecond phase reflects the energy transfer between closely located Chl b and Chl a molecules. Excitation at 670 nm induces a similar ultrafast process of energy transfer between neighboring Chl a molecules with a lifetime of 300 fs. The kinetic processes with the lifetime of 3−4 ps indicate an excitation equilibration among pools of Chl b and Chl a in the r-Lhca1, which involves the longest-wavelength spectral form of Chl a at 684 nm. This is in contrast to the Lhca4 subunit of the LHCI-730 heterodimer, which binds the longest-wavelength (red) pigments absorbing at 700−705 nm and responsible for the red fluorescence at 735 nm in LHCI. The spectral pattern of transient absorption changes in the r-Lhca1 is similar to that of the minor light-harvesting complex CP29 of PSII with which the Lhca1 shares significant similarity in protein sequences. A tentative assignment of the observed Chl spectral forms to the pigments in the Lhca1 subunit is presented based on the derivative absorption spectroscopy, transient absorption spectra, and the suggestion that the pigment−protein interactions dominate the spectral heterogeneity of the Qy transitions of Chl a and b.

Femtosecond spectral and anisotropy study of excitation energy transfer between neighbouring α-80 and β-81 chromophores of allophycocyanin trimers

Polarization pump-probe femtosecond spectroscopy was used to investigate photoinduced optical density changes in allophycocyanin (APC) trimers at 635–690 nm after excitation with 230-fs pulses at 618 nm. The initial bleaching observed at λ < 645 nm is followed by subpicosecond absorption recovery corresponding to 430 ± 40 fs recovery kinetics measured at 615 nm with 70-fs pulses. Only the red part of the APC absorption band remains strongly bleached at 3 ps after excitation. The spectral and kinetic results can be described in terms of two different models of interaction between neighbouring α-80 and β-81 chromophores of APC trimers. According to the first one, the observed subpicosecond kinetics corresponds to relaxation between the levels of excitonically coupled, spectrally identical α-80 and β-81 chromophores. Excited state absorption to doubly excited excitonic state should in this case contribute to the measured difference spectra. According to the second one, the femtosecond excitation energy transfer in APC trimers takes place between a donor chromophore absorbing predominantly at 620 nm and an acceptor chromophore absorbing at 650 nm. The high anisotropy value observed at 615 nm during the first 1.2 ps is in good agreement with the donor-acceptor model. Anisotropy values calculated in the 635–675 nm spectral region at 3 ps after excitation are in the 0.1–0.25 range corresponding to an angle of 30°–45° between donor and acceptor transition dipole orientations. The high anisotropy obtained at 658 nm during the excitation is probably due to stimulated emission of the donor chromophore.

Theoretical aspects of the effects of coupled coherent-incoherent dynamics on the primary processes of photosynthesis

The general method of the coupled coherent-incoherent mechanism including the effect of quenching has been applied to the study of excitation energy transfer in the model of the photosynthetic unit. Due to the rotational symmetry of the model used, the compact analytic form of the physical observables, e.g. time-dependent fluorescence intensities at the reaction center and antenna molecules, the rate of fluorescence quenching and the quantum yield have been derived. Based on the obtained results, the general mechanism of coupled coherent-incoherent transfer is compared to the extreme regimes of the purely coherent and purely incoherent transport. It is shown that the calculated high value of the quantum yield at infinitely long time, ≈0.93, of the incoherent transfer mechanism is not the factor evidencing the superiority of the incoherence at the energy transfer of the photosynthetic process assuming that the geometry of the photosynthetic unit is of centro-symmetric. In contrast, on the relevant pico-second time scale, this value is only ≈0.45. It is the presence of the coherent hopping coupled to incoherence that makes the energy transfer so extremely effective. Contribution of coherence increases the quantum yield on the relevant time scale up to 0.98. The time constants of the fluorescence, i.e. the rate of fluorescence quenching calculated by the general method of the coupled coherent-incoherent mechanism are in very good agreement with experiment.

Excitation energy migration and transfer in a dye pair in PMMA

Study of excitation energy transfer from acriflavine to rhodamine 6G in poly(methyl methacrylate) (PMMA) has been done at room temperature. Donor decay data are found to fit with the Förster function of energy transfer at all acceptor concentrations. However, at low acceptor concentrations the observed critical transfer distance is higher than the spectroscopically calculated value and decreases with increasing acceptor concentration and finally attains a constant value which agrees with the spectroscopic value. The results are explained on the basis of the Huber model for energy transfer (Phys. Rev. B 20 (1979) 2307, 5333).