Electronic Excitation Energy Transfer Research Articles

Energy Transfer in Unsymmetrical Phenylene Ethynylene Dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.

Spectroscopic and electrochemical investigation of coordination complexes of octakis(benzylthio)tetraazaporphyrincobalt(II)

Synthesis of new bichromophoric di- and pentanuclear complexes 2– 7 by datively binding (bpy) 2Ru II, (phen) 2Ru II and Cp (PPh 3)Ru II units to the periphery of [Co(OBTTAP)], 1, and their spectroscopic properties are described. IR, 1H NMR, UV–Vis, and mass spectral data were used for their characterization. Relative intensities and positions of the Soret and Q-bands absorptions in the di- and pentanuclear complexes were observed shifted vis-à-vis that in the precursor complex [Co(OBTTAP)], 1. These complexes particularly, those possessing [Co(OBTTAP)] and (bpy) 2Ru II/(phen) 2Ru II units, exhibited efficient inter-component electronic excitation energy transfer in their fluorescence excitation–emission spectra, that are suggestive of a high degree of inter-component electronic interaction in them. Also, the electrode activity of the complexes improved upon binding of the peripheral units and they exhibited multiple one-electron reversible oxidation waves in the cyclic voltammograms. These effects have been explained in terms of dπ(S)–dπ(Ru) interactions.

Influence of the Microstructure of Polymer Gels on the Electron-Excitation Energy Transfer Between Organic Molecules

The electron-excitation energy transfer (EEET) between ionic dyes in polyelectrolyte-network polymers with a different space distribution of charged segments has been investigated. It has been established that the structure of network polymers influences the efficiency of the EEET in them mainly due to the formation of a fractal distribution of interacting molecules. It is shown that the efficiency of the EEET can be controlled by changing the number and arrangement of charged segments of polyelectrolyte networks.

Energy transfer from rhodamine 6G to thionine in an anionic microemulsion

The transfer of electronic excitation energy from rhodamine 6G as an energy donor to thionine as an acceptor has been studied in anionic microemulsion by the method of fluorescence quenching and sensitization. The concentrations of both donor and acceptor used in this study are quite high, in the range of 10 −3 M. Both radiative and nonradiative processes have been taken into account, with the major contribution coming from the nonradiative path. An energy transfer efficiency up to 50% has been observed, and the rate constant of the nonradiative energy transfer from rhodamine 6G to monomeric thionine is 5 × 10 11 M −1 s −1. This indicates that the nonradiative energy transfer belongs to the long-range Forster-type of dipole–dipole interaction. The results also show why the efficiency of a photogalvanic cell based on the thionine–iron system in anionic microemulsion is enhanced by the incorporation of an auxiliary absorber.

Electronic excitation energy transfer between dye molecules in H2O and D2O solutions in the micellar phase

Processes of electronic excitation energy transfer (EEET) in H2O+D2O solutions within reversed micelles with different degrees of hydration were studied. It is ascertained that the structure of solutions forms a cluster distribution of interacting molecules having a fractal dimensionality. As a result, in regions with a locally high concentration of the dye molecules, an increase in the EEET efficiency is observed. Solubilization by reversed micelles leads to destruction of the cluster structure of the water system, with the dependence of the EEET efficiency on the degree of hydration being nonmonotonic. As the degree of hydration increases, the inhomogeneities of the structure inherent in bulk water are restored.

Injecting electronic excitation energy into an artificial antenna system through an Ru2+ complex.

The Ru2+ complex [Ru(bpy)2(bpy-ph4-Si(CH3)3)]2+ can be electrostatically bound to the negatively charged channel entrances of dye-loaded zeolite L crystals where it acts as a functional stopcock molecule. Impressive electronic triplet-singlet excitation energy transfer from the Ru2+ complex to the acceptor dye oxazine 1 (Ox1) located inside the channels can be observed when the donor molecule is selectively excited. Time-resolved luminescence experiments have been performed on the separate components and on the assembled donor-acceptor material. The luminescence lifetime of the Ru2+ complex attached to the zeolite is reduced by a factor of 30 when Ox1 acceptor molecules are present. The fluorescence decay of Ox1 incorporated in zeolite L is single exponential with a lifetime of 3 ns. The much longer lifetime in zeolite L than in solution is due to the fact, that the diethyl groups are sterically restricted when the dye is inside the host.

Energy Transfer Rates and Pathways of Single Donor Chromophores in a Multichromophoric Dendrimer Built around a Central Acceptor Core

An artificial light-harvesting dendrimer showing highly efficient electronic excitation energy transfer from four peripheral donors to one central acceptor has been investigated by single-molecule spectroscopy at low temperatures. Confocal imaging in combination with frequency selective excitation spectroscopy gives direct access to energy transfer rates of individual donors and allows the determination of energy transfer pathways within a single multichromophoric aggregate.

Dynamics of fluorescence of solid solutions of quantum dots and the organic dye DODCI: Experiment and numerical simulation

The fluorescence of solid solutions of CdSe/ZnS quantum dots and the organic dye DODCI is investigated. It is shown that nonradiative transfer of electronic excitation energy to dye molecules, which with some probability lose their acceptor properties as a result of photoisomerization or photodegradation, is responsible for a significant increase in the fluorescence intensity of a donor. The degree of polarization of the donor fluorescence attains values exceeding 0.5, which is due to the difference in the fluorescence quantum yields of donors with different orientations of the oscillator with respect to the electric vector of an excitation light wave. A numerical simulation of the experimentally observed dependences is performed.

Fluorescence of CdSe/ZnS quantum dots in solid solutions in the presence of organic molecules DODCI

Fluorescence characteristics of solid solutions of CdSe/ZnS quantum dots (donor) and organic dye 3,3′-Diethyloxadicarbocyanine iodide (acceptor) in polymethylmetacrylate films have been investigated. The fluorescence intensity increase caused by the exciting radiation at the frequency of the QD absorption has been found for the films containing both QDs and DODCI. The abrupt rise of the fluorescence polarization degree has been observed at the high-frequency edge of the radiation spectrum of these films. Possible processes (nonradiative electronic excitation energy transfer, acceptor photodegradation) accounting for the experimental dependences are also discussed.

Electron-excitation energy transfer in a heterocyclic bichromophore cooled in a supersonic jet

Electron-excitation energy transfer in a heterocyclic bichromophore cooled in a supersonic jet

Evidence for Highly Selective Supramolecular Formation between Perylene/γ-CD and Pyrene/γ-CD Complexes in Water

We have studied the simultaneous complexation of pyrene and perylene with γ-cyclodextrin in water by means of electronic excitation energy transfer. We found evidence that although in the solution there are present three different pyrene/γ-cyclodextrin complexes with stoichiometries of 1:1, 1:2, and 2:2 the (perylene)1/(γ-cyclodextrin)2 complex that is formed associates exclusively with the excimer-emitting (pyrene)2/(γ-cyclodextrin)2 adduct and not at all with the others. Moreover, this effect is not observed when pyrene is replaced by some of its homologues, which also form 2:2 excimer-emitting complexes with γ-cyclodextrin.

Energy transfer and fluorescence quenching in complexes of polymethine dyes with human serum albumin.

Electronic excitation energy transfer (EET) between molecules of polymethine dyes bound to human serum albumin (HSA) has been established and studied by absorption and fluorescence spectroscopy as well as by fluorescence decay measurements. In this system, excitation of the donor dye molecule leads to fluorescence of the acceptor dye molecule, both bound to HSA, with donor fluorescence quenching by the acceptor. The short distance between the donor and the acceptor (25-28 A) revealed from the Forster model of EET as well as some spectroscopic data show that both molecules are probably located in the same binding domain of HSA. The role of HSA is to bring donor and acceptor molecules together to a distance adequate to achieve EET as well as to increase the donor and acceptor fluorescence quantum yields. Efficient quenching of the intrinsic HSA fluorescence by some polymethine dyes (oxonols) is observed. The experimental results fit well a model for the formation of a weakly fluorescent dye-HSA complex; the quencher in this complex should be located in the immediate vicinity of the HSA fluorophore group (Trp(214)).

Electronic energy transfer between dye molecules in structured solutions of H2O and D2O

The structure of H2O+D2O solutions was studied by correlation spectroscopy of scattered light. The correlation function and size of scatterers were both found to depend nonmonotonically on the D2O concentration in the H2O+D2O mixture. Processes of transfer of electronic excitation energy between dye molecules of different types in H2O+D2O solutions were studied. The efficiency of these processes was found to depend extremally on the concentration of the components of the solution. A fractal distribution of the interacting dye molecules is ascertained from the experimental data. The dependence of the fractal dimensionality of the dye solutions on the D2O concentration in the D2O+H2O mixture is determined.

Triplet-triplet transfer of electronic excitation energy in vapors of organic molecules

The rate constants of the triplet-triplet (T-T) transfer of energy of the electronic excitation obtained in the gas phase for two sets of donor-acceptor pairs with various spectral characteristics are compared with those predicted by the theories of nonradiative transitions. The rate constants K TT and efficiencies βDA of the T-T transfer are estimated from quenching of the phosphorescence of biacetyl by aromatic hydrocarbons (anthracene, 9-methylanthracene, 9,10-dimethylanthracene, 9-phenylanthracene, pyrene, and chrysene) and from quenching of the delayed fluorescence of benzophenone, 4-phenylbenzophenone, anthraquinone, carbazole, and naphthalene by biacetyl, as well as from the sensitization of phosphorescence of biacetyl by these compounds. The causes of the broad variations of βDA in the gas phase from 0.35 (biacetyl-anthracene) to 10−4 (biacetyl-chrysene) are discussed for the studied donor-acceptor pairs, which differ in the exothermicity of the process and in the configuration of the lowest triplet levels of the molecules participating in the transfer. The dependences of the transfer rate constants K TT on the free energy change ΔG (the range from 0.4 to −1.7 eV) are analyzed. It is shown that, in gas-phase systems, K TT increase with the exothermicity of the transfer, attain a highest value, and then decrease in the range of negative −ΔG. The observed differences in the experimental values of βDA are discussed in terms of changes in the free energy ΔG of the process, in the energy of rearrangement of the nuclear configuration of the molecules during the transfer, and in the matrix elements of the interaction.

Migration transfer of electronic excitation energy in activated solids

An analytic expression for the kinetics of quenching of donor luminescence in the presence of acceptors with allowance for migration of energy over the donor subsystem is derived. An extended criterion for the applicability of the hopping migration model is obtained.

T → S Energy Transfer in Some Molecular Complexes

Calculations on the bimolecular complexes of acetophenone or benzophenone with anthracene and its substituted derivatives were carried out using a standard quantum-chemical approach to molecular systems. The deactivation pathways for lower triplet excited states of acetophenone and benzophenone were established. The probability of energy transfer from the energy donor to acceptor in the complexes was considered. The analysis of calculation results showed that the T S transfer of electronic excitation energy in these com- plexes is feasible only in the case of a small distance between the molecules, and the efficiency of this transfer is higher in the acetophenone rather than benzophenone complexes.

Triplet‐Triplet Electron Excitation Energy Transfer in the Gas Phase for the Carbazol–Diacetyl Donor‐Acceptor Pair

For the carbazol–diacetyl donor‐acceptor pair, the features of exothermal triplet‐triplet (T‐T) transfer of electron excitation energy in the gas phase upon excitation of carbazol by the nitrogen laser radiation which is not absorbed by diacetyl have been investigated. The luminescence spectra of carbazol in the gas phase have been studied. It is shown that carbazol phosphorescence in the gas phase is absent. Therefore, for estimating the T‐T transfer efficiency and its temperature dependence, in the kinetic approximation the dependences of the intensity of time‐resolved delayed fluorescence of carbazol and sensitized phosphorescence of diacetyl on diacetyl pressure have been analyzed. It has been found that in the gas phase the features of the electronic structure of the molecules under investigation are responsible for the low efficiency of transfer (2·10−4 at 453 K), which increases by an order of magnitude with increasing temperature in the 453–513 K range despite the increasing influence of the back transfer.

Electronic excitation energy transfer in solid solution of 7-diethylamino-3,4-benzophenoxazine-2-one (Nile Red dye) in bis[N-2-(oxybenzylidene)-4-tert-butylaniline]zinc

Transfer of electronic excitation energy from the host material to guest molecules was studied taking the solid-phase system bis[N-(2-oxybenzylidene)-4-tert-butylaniline]zinc + 7-diethylamino-3,4-benzophenoxazine-2-one (the guest compound) as an example. The luminescence excitation spectra and the concentration quenching of green fluorescence of the host compound and the excitation of sensitized red fluorescence of the guest molecules were studied. Among the exciton mechanism of energy migration and Forster"s long-range, single-jump mechanism, the former dominates. The number of jumps (m) and the mean diffusion length of the singlet exciton (l), calculated in the framework of the model of partially delocalized, randomly hopping exciton, were estimated at ≈1.5·103 and ≈450 A, respectively.

Interaction of Pr3+ optical centers in the Y2SiO5 crystal

It is shown on the basis of studies of the optical spectra and luminescence decay of nonequivalent Pr3+ optical centers in the Y2SiO5 crystal that the occupation of the nonequivalent cation sites of the crystal lattice by the activator ions is irregular. For excitation in the region of the optical transitions 3H4→1D2in the temperature interval 6–80 K there is no transfer of electronic excitation energy between Pr3+ ions localized in different cation sites. The luminescence decay law of the Pr3+ optical centers is determined by the concentration of activator ions and has a complicated non-single-exponential character; the quenching of the luminescence of the Pr3+ ions is due to the formation of dimers of these ions. © 2002 American Institute of Physics.

Analysis of the Structure of Thin Films of Zn‐Porphyrins by the Kinetics of the Fluorescence Anisotropy

The structure of thin films of Zn‐tetraoctylphenylporphyrin (ZnTOPP) obtained by the spinning method is investigated. The kinetics of the decay of the fluorescence anisotropy of the films is analyzed in the form of a sum of exponents and by simulating the orientation of ZnTOPP complexes on the substrate (quartz) surface with allowance for the processes of the electron excitation energy transfer. The ZnTOPP films have a lamellar structure where individual layers form ordered domains. Within a domain, linear nonintersecting stacks of molecules are formed. In each stack the planes of the molecules are collinear, oriented perpendicularly to the surface of the substrate base, and form an angle of 45° with the directing axis of a stack.