Acceptor Absorption Research Articles

Enhancement of Förster Energy Transfer within a Microspherical Cavity

Energy transfer from pyrene to perylene molecules co-doped within a poly(methyl methacrylate) latex microsphere was drastically accelerated relative to free space. Fluorescence spectra of the microspheres showed that the relative emission intensities of pyrene and perylene changed with the sphere diameter. Analyses of emission decay profiles clarified that Förster-type energy-transfer processes were induced and that the transfer rates increased within the microspherical cavity. This enhancement can be ascribed not only to the quantum electrodynamic effects on the pyrene emission rate, but also the cavity effect of increasing the overlapping factor between donor emission and acceptor absorption spectra.

Exciton Migration in Rigid-Rod Conjugated Polymers: An Improved Förster Model

The dynamics of interchain and intrachain excitation energy transfer taking place in a polyindenofluorene endcapped with perylene derivatives is explored by means of ultrafast spectroscopy combined with correlated quantum-chemical calculations. The experimental data indicate faster exciton migration in films with respect to solution as a result of the emergence of efficient channels involving hopping between chains in close contact. These findings are supported by theoretical simulations based on an improved Forster model. Within this model, the rates are expressed according to the Fermi golden rule on the basis of (i) electronic couplings that take account of the detailed shape of the excited-state wave functions (through the use of a multicentric monopole expansion) and (ii) spectral overlap factors computed from the simulated acceptor absorption and donor emission spectra with explicit coupling to vibrations (considered within a displaced harmonic oscillator model); inhomogeneity is taken into account by assuming a distribution of chromophores with different conjugation lengths. The calculations predict faster intermolecular energy transfer as a result of larger electronic matrix elements and suggest a two-step mechanism for intrachain energy transfer with exciton hopping along the polymer backbone as the limiting step. Injecting the calculated hopping rates into a set of master equations allows the modeling of the dynamics of exciton transport along the polyindenofluorene chains and yields ensemble-averaged energy-transfer rates in good agreement with experiment.

Sensitised green emission in an electrically active polymer doped with a fluorinated acridine

The photophysical processes leading to sensitised green emission of a blend made by a host polymer poly(vinyl-carbazole) doped with a fluorinated acridine are investigated by means of steady-state and time-resolved luminescence. The large overlap between the donor emission and the acceptor absorption produces an efficient resonant energy transfer process from the host to the dye. At low dopant concentration, the dependence of the Förster radius on the temperature is consistent with a thermally activated hopping process of the exciton within the donor prior to the resonant energy transfer to the acceptor. The activation energy for the hopping process is around 20 meV.

Optical properties of Rh6G:C337 dye mixture in the sol-gel silica matrix

The mixed dye method has received considerable attention because it is promising method to improve the efficiency of dye lasers and broaden the laser emission spectrum [1–3]. In a dye mixed solution, the excitation energy can be efficiently transferred from the donor to the acceptor. Two mechanisms involved in such an energy transfer are: (i) the radiative transfer, that is, the absorption of donor emission by an acceptor, and (ii) nonradiative transfer, that is, the excitation transfer takes place before the excited donor emits. The radiative energy transfer mechanism is found to be the dominant mechanism in dilute solutions [4, 5]. It is well known that the intermolecular energy transfer depends strongly on factors such as dye molecule concentrations, spectral overlap between the acceptor absorption spectra and the donor fluorescence spectra [6, 7]. Most of the studies conducted so far report investigation of energy transfer between dyes in liquid media. To our knowledge, no work has been reported on the energy transfer of Rh6G:C337 dye mixture in the sol-gel silica matrix. In this paper, the optical properties of Rh6G: C337 dye doped sol-gel silica matrices are investigated. The dye doped sol-gel silica samples were prepared by hydrolysis and polycondensation of tetraethosysilane (TEOS), with formamide as a

Temperature study of the photoluminescence of a single InAs/GaAs quantum dot

The temperature evolution of the micro-photoluminescence (PL) spectra of a single InAs/GaAs quantum dot is studied from 5 to 65 K for different excitation energies. For excitation energies within the shallow acceptor absorption region below the GaAs bandgap a strong increase of the PL intensity (more than 10 times) is observed at 45 K with respect to 5 K. Besides, with increasing the temperature the PL spectrum redistributes in favour of a more neutral exciton configuration, contrary to the tendency observed when exciting outside this region. The results are explained in terms of separate optical generation of excess electrons and thermal generation of excess holes via the shallow acceptor levels in the GaAs barriers, resulting in an efficient carrier diffusion in the wetting layer and capture into the quantum dot. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical membrane for o-nitroaniline based on fluorescence energy transfer between a small molecule and a conjugated polymer

A new o-nitroaniline-sensitive optical membrane has been proposed based on the fluorescence energy transfer between a small molecule 1,2-di(1-naphthyl) acetylene (DNAL, donor) and a conjugated polymer poly( p-phenylene ethynylene) (PPP, acceptor). There is large spectral overlap between the donor emission and the acceptor absorption. When DNAL and PPP were immobilized in plasticized poly(vinyl chloride) (PVC), energy transfer occurred from small molecule DNAL to conjugated polymer PPP. The fluorescence of the optical membrane based on DNAL was strongly quenched by o-nitroaniline, but that of PPP only weakly. When the optical membrane containing the DNAL/PPP contacted a solution of o-nitroaniline, the efficiency of fluorescence quenching of PPP was increased greatly. Under the optimum condition, the measuring range of o-nitroaniline was from 4.0×10 −4 to 4.0×10 −7 mol l −1. The present membrane exhibited good reproducibility, reversibility and selectivity in the presence of some common ions and organic compounds. The proposed optical membrane was applied to the direct analysis of spiked o-nitroaniline in Xiang River water and the results obtained were satisfactory.

Analysis of resonance energy transfer in model membranes: role of orientational effects

The model of resonance energy transfer (RET) in membrane systems containing donors randomly distributed over two parallel planes separated by fixed distance and acceptors confined to a single plane is presented. Factors determining energy transfer rate are considered with special attention being given to the contribution from orientational heterogeneity of the donor emission and acceptor absorption transition dipoles. Analysis of simulated data suggests that RET in membranes, as compared to intramolecular energy transfer, is substantially less sensitive to the degree of reorientational freedom of chromophores due to averaging over multiple donor-acceptor pairs. The uncertainties in the distance estimation resulting from the unknown mutual orientation of the donor and acceptor are analyzed.

Designing light-harvesting antenna systems based on superradiant molecular aggregates

The efficiency of synthetic light-harvesting antenna systems for energy transfer and trapping that incorporate complex assemblies of chromophores can be significantly dictated by mixed states that arise through electronic couplings among aggregates of donor or acceptor chromophores. Such electronic interactions – that perturb donor emission spectra, radiative decay rate, and acceptor absorption spectra – cannot be accounted for by Förster theory. In the present report, use of collective superradiant donor and/or acceptor transitions to mediate rapid `supertransfer' of excitation energy is considered. Design principles that can be used to incorporate complex molecular assemblies into light-harvesting devices are elucidated by calculations of energy supertransfer dynamics in model assemblies. It is concluded that design based only on donor emission and acceptor absorption spectra can be misguiding because of the importance of the precise arrangement of the molecular constituents that collectively give rise to those electronic transitions.

Pr3+- and Pr3+/ Er3+-Doped Selenide Glasses for Potential 1.6 mm Optical Amplifier Materials

1.6 µm emission originated from Pr3+: (3F3, 3F4) → 3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses was investigated under an optical pump of a conventional 1480 nm laser diode. The measured peak wavelength and full-width at half-maximum of the fluorescent emission are ~1650 nm and ~120 nm, respectively. A moderate lifetime of the thermally coupled upper manifolds of ~212 ± 10 µs together with a high stimulated emission cross-section of ~(3 ± 1)×10−−20 cm2 promises to be useful for 1.6 µm band fiber-optic amplifiers that can be pumped with an existing high-power 1480 nm laser diode. Codoping Er3+ enhances the emission intensity by way of a nonradiative Er3+: 4I13/2 → Pr3+: (3F3, 3F4) energy transfer. The Dexter model based on the spectral overlap between donor emission and acceptor absorption describes well the energy transfer from Er3+ to Pr3+ in these glasses. Also discussed in this paper are major transmission loss mechanisms of a selenide glass optical fiber.

CdSe−ZnS Quantum Dots as Resonance Energy Transfer Donors in a Model Protein−Protein Binding Assay

Specific binding of biotinilated bovine serum albumin (bBSA) and tetramethylrhodamine-labeled streptavidin (SAv−TMR) was observed by conjugating bBSA to CdSe−ZnS core−shell quantum dots (QDs) and observing enhanced TMR fluorescence caused by fluorescence resonance energy transfer (FRET) from the QD donors to the TMR acceptors. Because of the broad absorption spectrum of the QDs, efficient donor excitation could occur at a wavelength that was well resolved from the absorption spectrum of the acceptor, thereby minimizing direct acceptor excitation. Appreciable overlap of the donor emission and acceptor absorption spectra was achieved by size-tuning the QD emission spectrum into resonance with the acceptor absorption spectrum, and cross-talk between the donor and acceptor emission was minimized because of the narrow, symmetrically shaped QD emission spectrum. Evidence for an additional, nonspecific QD−TMR energy transfer mechanism that caused quenching of the QD emission without a corresponding TMR fluorescence enhancement was also observed.

Effect of Pressure on the Emission Efficiencies of a Series of Phenylacetylene Dendrimers

The effect of pressure to 60 kbar has been measured on the emission intensity of five dendritic macromolecules. The emission efficiency decreases with increasing pressure in all cases, but to a greater extent for the larger dendrimers. An explanation is given in terms of changing efficiency of Forster energy transfer. The efficiency is expressed in terms of an effective “Forster distance” R0, a parameter that depends on the overlap between the donor emission and acceptor absorption. From the peak shifts, it appears that R0 decreases significantly with pressure, and an argument is given to explain why this decrease is more important for the larger dendrimers.

Enhanced intersystem crossing in donor/acceptor systems based on zinc/iron or free-base/iron porphyrins.

The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(III) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 A donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S1-->T1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Forster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for electron transfer.

Simultaneous photo-excitation of Li acceptor and shallow donors in ZnSe

We observed the donor-assisted infrared absorption of Li acceptor in bulk ZnSe. Two kinds of donors, Li int and/or Ga Zn, make transition from 1S to 2S state during photo-excitation of Li acceptor. The transition energies are estimated at 6.3 and 20.2 meV for Li int and Ga Zn, respectively. Fano resonances of the valence band state with the simultaneous excited states of a Li acceptor and donors were also observed.

Electronic energy transfer in a microcavity

We study the microcavity effect on energy transfer via the electromagnetic field. The modification of the optical properties of the microcavity due to the presence of absorbing acceptors is taken into account. We analyze different cases that may be realized depending on the characteristics of the acceptor absorption band: the case of strong and broad absorption, when the cavity mode is practically destroyed; the regime of weak absorption, when the cavity mode is still well defined and just acquires some additional broadening; and the strong-coupling regime, when the acceptor absorption has the shape of a strong and narrow peak and two polariton branches appear, formed due to coherent mixing of the acceptor excitations and the cavity mode. In our calculations we use a realistic model for the cavity mirrors (dielectric or metallic), which is important since the mirrors introduce an additional decay channel for the donor excitation besides the transfer to the acceptors. The distribution of the energy between different decay channels is analyzed in detail.

Tunable Energy Transfer from Dicyanoaurate(I) and Dicyanoargentate(I) Donor Ions to Terbium(III) Acceptor Ions in Pure Crystals

A temperature-dependent photoluminescence study is reported for single crystals of M[Au(CN)2]3 and M[Ag(CN)2]3 (M = Tb; Gd; Y). The results indicate that in both Tb[Au(CN)2]3 and Tb[Ag(CN)2]3 exclusive excitation of the donor leads to sensitized luminescence for the acceptor, characteristic of the 5D4 → 7FJ (J = 0−6) transition of Tb(III). However, the sensitized luminescence is much stronger in Tb[Ag(CN)2]3 than in Tb[Au(CN)2]3 due to a larger spectral overlap between the [Ag(CN)2-] emission and the Tb(III) absorption. Upon increasing the temperature, energy transfer is enhanced in Tb[Ag(CN)2]3 but inhibited in Tb[Au(CN)2]3. In Tb[Ag(CN)2]3, a large spectral overlap exists between the [Ag(CN)2-] donor emission and the Tb(III) acceptor absorption at all temperatures. The Tb(III) sensitized emission is strong at all temperatures and is enhanced upon a temperature increase while the [Ag(CN)2-] emission is quenched. An activation energy of 53.7 cm-1 (±2.5 cm-1) has been calculated for the energy transfer pro...

Twentyfold blue upconversion emission enhancement through thermal effects in Pr3+/Yb3+-codoped fluoroindate glasses excited at 1.064 μm

Infrared-to-visible upconversion emission enhancement through thermal effects in Yb3+-sensitized Pr3+-doped fluoroindate glasses excited at 1.064 μm is investigated. A twentyfold increase in the 485 nm blue emission intensity as the sample temperature was varied from 20 to 260 °C was observed. The visible upconversion fluorescence enhancement is ascribed to the temperature dependent multiphonon-assisted anti-Stokes excitation of the ytterbium sensitizer and excited-state absorption of the praseodymium acceptor. A model based upon conventional rate equations considering a temperature dependent effective absorption cross section for the 2F7/2→2F5/2 transition of the Yb3+ and 1G4→3P0 excited-state absorption of the Pr3+, agrees very well with the experimental results.

Synthesis and Steady-State Photophysical Properties of Dye-Labeled Dendrimers Having Novel Oligothiophene Cores: A Comparative Study

Novel chromophore-labeled dendrimers with penta- and heptathiophene cores and coumarin-2 chromophores at their periphery have been shown to be very efficient light-harvesting systems. Excitation of the peripheral coumarin-2 chromophores results in energy transfer to the oligothiophene cores as a result of the large overlap between the donor emission spectrum and the acceptor absorption spectrum, as well as the large transition dipole moments of the oligothiophenes. Although these core dyes have low fluorescence quantum yields, their emission intensity is significantly enhanced by the ability of the large light-harvesting dendron to funnel absorbed energy to the core. Because of the large Stokes shift of the oligothiophenes, the emission spectrum of the dendrimers was red-shifted by 200 nm from the excitation wavelength. Additionally, it was found that oligothiophene orientationend functionalization vs central functionalizationdid not have a significant effect on energy-transfer efficiency.

Excitation energy transfer between acriflavine and rhodamine 6G as a pH sensor

Excitation energy transfer between acriflavine (donor) to rhodamine 6G (acceptor) molecules in water with varying pH has been studied by using steady state measurements. From absorption and fluorescence spectra of donor and acceptor, overlap integrals for donor fluorescence and donor absorption ( Ω DD) as well as donor fluorescence and acceptor absorption ( Ω DA) have been calculated. The corresponding critical transfer distances for dipole–dipole Forster mechanism of excitation energy transfer between donor–donor ( R 0D) and donor–acceptor ( R 0A), reduced concentration and efficiency of energy transfer have also been calculated. It has been found that the Ω DA change with pH whereas Ω DD remain unchanged, resulting in change in energy transfer from donor to acceptor with pH, but no change in energy migration. The efficiency of excitation energy transfer is maximum for a highly basic solution and it decreases with a decrease in pH. The energy transfer efficiency, reduced concentration, and overlap integrals show a linear dependence on pH. It has been proposed that the system may be used as a wide range (1.4 to 12) pH sensor with an accuracy of 0.01.

Porphyrin Core Star Polymers: Synthesis, Modification, and Implication for Site Isolation

Porphyrin Core Star Polymers: Synthesis, Modification, and Implication for Site Isolation

Localization and function of ferredoxin:NADP(+) reductase bound to the phycobilisomes of Synechocystis.

Each phycobilisome complex of the cyanobacterium Synechocystis PCC 6803 binds approximately 2.4 copies of ferredoxin:NADP(+) reductase (FNR). A mutant of this strain that carries an N-terminally truncated version of the petH gene, lacking the 9 kDa domain of FNR that is homologous to the phycocyanin-associated linker polypeptide CpcD, assembles phycobilisome complexes that do not contain FNR. Phycobilisome complexes, consisting of the allophycocyanin core and only the core-proximal phycocyanin hexamers from mutant R20, do contain a full complement of FNR. Therefore, the binding site of FNR in the phycobilisomes is not the core-distal binding site that is occupied by CpcD, but in the core-proximal phycocyanin hexamer. Phycobilisome complexes of a mutant expressing a fusion protein of the N-terminal domain of FNR and green fluorescent protein (GFP) contain this fusion protein in tightly bound form. Calculations of the fluorescence resonance energy transfer (FRET) characteristics between GFP and acceptors in the phycobilisome complex indicate that their donor-acceptor distance is between 3 and 7 nm. Fluorescence spectroscopy at 77K and measurements in intact cells of accumulated levels of P700(+) indicate that the presence of FNR in the phycobilisome complexes does not influence the distribution of excitation energy of phycobilisome-absorbed light between photosystem II and photosystem I, and also does not affect the occurrence of 'light-state transitions'.