Cascade Energy Transfer Research Articles

White Emission from a Polymer Blend Light-Emitting Diode by Incomplete Cascade Energy Transfer

White light emission was obtained from a polymer light-emitting diode (LED) made of a ternary polymer blend comprised of poly(9-vinylcarbazole) (PVK), poly(9,9′-dihexlyfluorene-2,7-divinylene-mphenylenevinylene-stat-p-phenylenevinylene) (CPDHFPV), and poly[2-methoxy-5-(2′-(ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) (19:1:1 by wt.). Incomplete cascade energy transfer was employed to obtain white emission from the polymer blend LED. Compatibility between PVK and CPDHFPV facilitated the Förster-type energy transfer from PVK to CPDHFPV. However, poor compatibility between CPDHFPV and MEH-PPV resulted in a partial energy transfer between the polymers causing the blend to emit two colors simultaneously. Consequently, pure white color near CIE coordinate (0.33, 0.33) could be obtained with a low sensitivity to the drive voltage.

Charge separation in a novel artificial photosynthetic reaction center lives 380 ms.

An extremely long-lived charge-separated state has been achieved successfully using a ferrocene-zincporphyrin-freebaseporphyrin-fullerene tetrad which reveals a cascade of photoinduced energy transfer and multistep electron transfer within a molecule in frozen media as well as in solutions. The lifetime of the resulting charge-separated state (i.e., ferricenium ion-C(60) radical anion pair) in a frozen benzonitrile is determined as 0.38 s, which is more than one order of magnitude longer than any other intramolecular charge recombination processes of synthetic systems, and is comparable to that observed for the bacterial photosynthetic reaction center. Such an extremely long lifetime of the tetrad system has been well correlated with the charge-separated lifetimes of two homologous series of porphyrin-fullerene dyad and triad systems.

White emission from a ternary polymer blend by incomplete cascade energy transfer

White light emission was obtained from a light-emitting diode (LED) prepared from a ternary polymer blend (19:1:1 by weight) consisting of poly(9-vinylcarbazole) (PVK), poly(9,9′-dihexlyfluorene-2,7-divinylene- m-phenylenevinylene-stat- p-phenylenevinylene) (CPDHFPV), and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), where the order of bandgap energy is PVK>CPDHFPV>MEH-PPV. The major component PVK acted not only as the matrix or diluent but also as the excitation energy donor to help the blend generate white light with high efficiency. Good miscibility between PVK and CPDHFPV facilitated the Förster-type excitation energy transfer from PVK to CPDHFPV enhancing the quantum efficiency. However, poor miscibility between CPDHFPV and MEH-PPV resulted in partial energy transfer between the polymers causing the blend to emit two colors simultaneously. Consequently, the incomplete cascade energy transfer in the blend generated a pure white color near CIE coordinate (0.33, 0.33) and the emissive color of this system showed a low sensitivity to the drive voltage.

Analytic conditions for targeted energy transfer between nonlinear oscillators or discrete breathers

It is well known that any amount of energy injected in a harmonic oscillator which is resonant and weakly coupled with a second harmonic oscillator, tunnels back and forth between these two oscillators. When the two oscillators are anharmonic, the amplitude dependence of their frequencies breaks, in general, any eventual initial resonance so that no substantial energy transfer occurs unless, exceptionally, an almost perfect resonance persists. This paper considers this interesting situation more generally between two discrete breathers belonging to two weakly coupled nonlinear systems, finite or infinite. A specific amount of energy injected as a discrete breather in a nonlinear system (donor) which is weakly coupled to another nonlinear system (acceptor) sustaining another discrete breather, might be totally transferred and oscillate back and forth between these donor and acceptor breathers. The condition is that a certain well-defined detuning function is bounded from above and below by two coupling functions. This targeted energy transfer is selective, i.e., it only occurs for an initial energy close to a specific value. The explicit calculation of these functions in complex models with numerical techniques developed earlier for discrete breathers, allows one to detect the existence of possible targeted energy transfer, between which breathers, and at which energy. It should also help for designing models having desired targeted energy transfer properties at will. We also show how extra linear resonances could make the energy transfer incomplete and irreversible. Future developments of the theory will be able to describe more spectacular effects, such as targeted energy transfer cascades and avalanches, and energy funnels. Besides rather short-term applications for artificially built devices, this theory might provide an essential clue for understanding puzzling problems of energy kinetics in real materials, chemistry, and bioenergetics.

Light-harvesting and photocurrent generation by gold electrodes modified with mixed self-assembled monolayers of boron-dipyrrin and ferrocene-porphyrin-fullerene triad.

Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron-dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron-dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet-singlet energy transfer from the pyrene (or boron-dipyrrin) to the porphyrin on the gold surface. The boron-dipyrrin has also been combined with a reaction center model, ferrocene-porphyrin-fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 +/- 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolayer-modified metal electrodes and across artificial membranes using donor-acceptor linked molecules. The incident photon-to-current efficiency (IPCE) of the photoelectrochemical cell at 510 and 430 nm was determined as 0.6% and 1.6%, respectively. Thus, the present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.

Energy transfer between collagen-dye molecules as a probe of higher-lying electronic states following multiquantum excitation

Under high-intensity nanosecond 1064 nm irradiation of a collagen-rhodamine 6G (R6G) system, multiquantum excitation of the R6G molecules to their S2 state was facilitated. The observation of this effect was achieved through associated energy transfer to the resonant collagen excited state band. Analogous cascaded energy transfer phenomena, resulting from the S2 electronic state of R6G, were observed in an R6G-collagen-coumarin system.

A tightly coupled linear array of perylene, bis(porphyrin), and phthalocyanine units that functions as a photoinduced energy-transfer cascade

We have prepared a linear array of chromophores consisting of a perylene input unit, a bis(free base porphyrin) transmission unit, and a free base phthalocyanine output unit for studies in artificial photosynthesis and molecular photonics. The synthesis involved four stages: (1) a rational synthesis of trans-AB2C-porphyrin building blocks each bearing one meso-unsubstituted position, (2) oxidative, meso,meso coupling of the zinc porphyrin monomers to afford a bis(zinc porphyrin) bearing one phthalonitrile group and one iodophenyl group, (3) preparation of a bis(porphyrin)-phthalocyanine array via a mixed cyclization involving the bis(free base porphyrin) and 4-tert-butylphthalonitrile, and (4) Pd-mediated coupling of an ethynylperylene to afford a perylene-bis(porphyrin)-phthalocyanine linear array. The perylene-bis(porphyrin)-phthalocyanine array absorbs strongly across the visible spectrum. Excitation at 490 nm, where the perylene absorbs preferentially, results in fluorescence almost exclusively from the phthalocyanine (phi(f) = 0.78). The excited phthalocyanine forms with time constants of 2 ps (90%) and 13 ps (10%). The observed time constants resemble those of corresponding phenylethyne-linked dyads, including a perylene-porphyrin (< or = 0.5 ps) and a porphyrin-phthalocyanine (1.1 ps (70%) and 8 ps (30%)). The perylene-bis(porphyrin)-phthalocyanine architecture exhibits efficient light-harvesting properties and rapid funneling of energy in a cascade from perylene to bis(porphyrin) to phthalocyanine.

Cascade Energy Transfer in CeF3 Crystals

The emission spectra of the CeF3 crystals in UV-visible region and their temperature dependence in the range of 6-453 K under UV, VUV excitation as well as their excitation spectra (4-25 eV) have been studied. The 350 nm emission band from perturbed Ce3+ can be excited by 295.2 and 302.4 nm emissions from the regular Ce3+. The 350 nm emission band, in turn, can excite the emission bands centered at 470 and 535 nm caused by some defects below room temperature. These results show that there is a cascade energy transfer process from regular Ce3+ to quenching centers via perturbed Ce3+ and defect centers in the CeF3. Meanwhile, thermoluminescence, giving peaks at about 240 and 300°C has been discovered, and the corresponding deep traps with depth ∼ 1 eV are found to be a kind of quenching center for CeF3 luminescence at room temperature.

Dependence of radiative lifetimes of porous silicon on excitation wavelength and intensity

Excited state radiative lifetime measurements are made on porous silicon as a function of excitation wavelength and excitation intensity. The results indicate that a simple quantum confinement model for the light absorption and emission mechanism is not suitable. We support our results by suggesting that a cascading energy transfer process among surface molecule-like states is most likely active and we provide a general indication of the density of energy transfer states.

Coupled sea surface‐atmosphere model: 2. Spectrum of short wind waves

A physical model of the short wind wave spectrum in the wavelength range from a few millimeters to few meters is proposed. The spectrum shape results from the solution of the energy spectral density balance equation. Special attention is paid to the description of the capillary range of the short wave spectrum. It is assumed that in this range the spectrum shape is determined mainly by the mechanism of generation of parasitic capillaries. This is described as the cascade energy transfer from the gravity to the capillary waves. Thus the capillary wave spectrum results through the balance between generation of capillaries and their viscous dissipation. The short gravity wave spectrum results through the balance between wind input and dissipation due to wave breaking. A parameterization of wind input is obtained in part 1 of the present paper. To describe the dissipation due to wave breaking, the approach developed by Phillips [1985] is used. The spectral rate of energy dissipation is presented in the form of a power dependence of the ratio of the saturation spectrum to some threshold level. It is further shown that the threshold level depends on the drift current shift in the water viscous sublayer, which affects the energy losses by wave breaking. To obtain a short wave spectrum which is valid in the whole wavenumber domain, the capillary and the short gravity wave spectra are patched in the vicinity of the wavenumber corresponding to the minimum phase velocity. This short wave spectrum is incorporated into the wind over waves coupled model developed in part 1 of the present paper. The measured statistical properties of the sea surface related to the short waves, such as the spectral shape of omnidirectional and up‐wind spectra, their wind speed dependence and angular spreading, and the wind speed dependence of integral mean square slope and skewness parameters, are well reproduced by the model. Also the model well reproduces the measured wind speed dependence of the drag coefficient and of the coupling parameter.

Toward transparent molecular wires: electron and energy transfer in transition metal derivatized conducting polymers

A great deal of research has concentrated on long range electron and energy transport in transition metal-based systems, including molecular donor-acceptor assemblies, electron and energy transfer cascades, dendrimers, and derivatized polymer systems. In an effort to improve efficiencies for electron and energy transport over large distances, several groups have now turned to conjugated systems. Several challenges exist to incorporating conducting materials/polymers in the study of photoinduced electron and energy transfer: solubility and processibility of the materials, thermal stability and limitations on direct spectroscopic characterization due to band gap absorptions. We have prepared a new series of conducting materials that provides for direct incorporation of chromophores and electrophores within the backbone of a conducting polymer. Energy transfer dynamics between conducting polymer bridges and porphyrin or metal-to-ligand charge transfer (MLCT) chromophores can be controlled through intermolecular interactions in solid vs solution samples. We have also developed a methodology to incorporate transmissive benzothiophene-type polymers such as polyisothianaphthene (PITN) within a copolymer assembly. These new materials are now being used to investigate long range electronic coupling and have potential applications that range from artificial photosynthesis to light emitting diodes.

Energy Transfer from Luminescent Porous Silicon to Adsorbed Osmium(II) and Ruthenium(II) Polypyridyl Complexes

Surface adsorption of transition-metal polypyridyl complexes on highly luminescent, anodized porous silicon (p-Si) wafers leads to dramatic quenching of the emission. Facile energy transfer occurs from the surface to adsorbed complexes of RuII and OsII. Photoluminescence (PL) from underivatized p-Si is excitation- and monitoring-wavelength-dependent. A phenomenological model is proposed to explain these observations in which intercluster energy transfer occurs by an energy-transfer cascade, and there is weak kinetic coupling to even lower-energy surface emitters. Adsorption of [Zn(dmb)3](PF6)2 (dmb is 4,4‘-dimethyl-2,2‘-bipyridine) or fac-[Re(bpy)(CO)3(4-Etpy)](PF6)2 (bpy is 2,2‘-bipyridine; 4-Etpy is 4-ethylpyridine), possibly by ion-exchange, results in quenching of the low-energy surface emitters. For adsorbed [MII(bpy)2(4-CO2H-4‘Mebpy)](PF6)2 (M = Ru, Os; 4-CO2H-4‘Mebpy is 4-carboxylic acid-4‘-methyl-2,2‘-bipyridine), p-Si* → MII energy transfer occurs to low-lying metal-to-ligand charge-transfer (MLC...

Light amplification in organic thin films using cascade energy transfer

There is currently renewed interest in the development of lasers using solid-state organic and polymeric materials as the gain media. These materials have a number of properties that make them good candidates for such applications — for example, emission bands that are displaced (via a Stokes shift) from absorption bands, and the ease with which the emitting species can be embedded in a suitable host material1,2,3,4,5. But despite these advantages, the threshold power densities required for light amplification that have been reported so far have been high6,7,8. Here we describe an approach, based on energy transfer between molecular species, that can lower the threshold for stimulated emission and laser action while improving markedly the waveguiding properties of the active material. In our materials, an initial molecular excited state is generated in the host compound by absorption of light; this state is then resonantly and non-radiatively transferred down in energy (through one or more steps) between suitably matched dye molecules dispersed in the host, so ensuring that the absorption losses at the final emission wavelengths are very small. Such composite gain media provide provide broad tunability of the emission wavelength, and also decouple the optical emission properties from the transport properties, so providing greater flexibility for the design of future electrically driven device structures.

Picosecond energy transfer and trapping kinetics in living cells of the green bacterium Chloroflexus aurantiacus

The excitation energy transfer and trapping processes in intact cells of Chloroflexus aurantiacus were studied by picosecond time-resolved fluorescence spectroscopy. The fluorescence decay kinetics is investigated over the near infrared emission range between 730 nm and 920 nm using various excitation wavelengths and excitation intensities. The data were analyzed by global decay analysis and are presented as decay-associated spectra (DAS). The specific dependence of the decay kinetics on the excitation wavelength and on the photochemical redox state of the reaction center (RC) allows the identification of the energy transfer and trapping components. The DAS provide evidence for two chlorosomal energy transfer processes. The first one occurs between the chlorosomal bacteriochlorophyll (BChl)- c and the BChl- a 792 complex (B 792) in the chlorosomal baseplate with an equilibration time constant of 15–16 ps, while the second one occurs from the B 792 pigments to the BChl- a 806 pigments in the B 806–866 complex with a time constant of 35–40 ps. The overall energy trapping process in whole cells is mainly determined by the kinetics of the primary charge separation process in the RCs. With open RCs (Q A oxidized) the trapping time constant is 70–90 ps, while the trapping process with closed RCs (Q A reduced) takes as long as 180–200 ps. The results on whole cells reported here are interpreted in conjunction with those reported earlier for the various isolated complexes, i.e., two different chlorosome preparations (Holzwarth, A.R., Müller, M.G. and Griebenow, K. (1990) J. Photochem. Photobiol. B 5, 457–465), the B 806–866 complex (Griebenow, K., Müller, M.G. and Holzwarth, A.R. (1991) Biochim. Biophys. Acta 1059, 226–232) and isolated reaction centers (Müller, M.G., Griebenow, K. and Holzwarth, A.R. (1991) Biochim. Biophys. Acta 1098, 1–12). Based on these data, a unified and self-consistent scheme for the primary processes in the whole photosynthetic system of C. aurantiacus is presented. The BChl antenna pigment groups are arranged to form a linear energy transfer cascade with four energy transfer steps from shorter-wavelength- to longer-wavelength-absorbing antenna pools. The overall fluorescence decay kinetics of the photosynthetic system of C. aurantiacus turns out to be ‘trap-limited’ by the reaction center rather than ‘diffusion-limited’ by the energy transfer processes.

Stimulated Raman Scattering in Hydrogen Using Capilary Cell

Abstract Stimulated Raman Scattering processes have been studied and intense Stokes and anti—Stokes laser lines have been observed in a capillary Raman cell filled with molecular hydrogen and pumped by the third harmonic of the Nd:YAG laser at wavelength lD = 355 nm. Various parametric studies have been performed to establish an optimum condition for the best operation of the Raman laser. The observation of higher—order Stokes and anti—Stokes has been explained on the basis of four—wave mixing and/or cascade energy transfer processes.

Energy transfer between F-type defects in CaO studied by optical-microwave double-resonance spectroscopy

Cascade energy transfer between photo-excited F+, F+A, F and FA point defects in CaO is reported utilizing highly selective optical-microwave double-resonance methods. Time-resolved MIDP experiments show a shortening of the lifetime of the Tx, y spin sublevels in the photo-excited triplet state of F defects in CaO: Mg. The lifetime shortening is attributed to the effect of energy transfer to the FA centers in this host crystal. A rate constant of 53 s−1 characterizes the transfer process at T = 1.2 K.

Long-range energy transfer in a soluble polymer by an energy-transfer cascade

In the study of intramolecular energy or electron transfer, soluble polymers offer the advantage of having multiple sites which can be used to concentrate combinations of chromophores and quenchers within the same molecular framework. We have prepared a functionalized 1:1 copolymer of styrene:m,p (chloromethyl)styrene that contains (1) a polypyridyl complex of Ru{sup II} which upon excitation gives a high-energy metal to ligand charge transfer (MLCT) excited state, (2) a polypyridyl complex of Os{sup II} which has a lower energy excited state, and (3) an anthryl derivative based on 9-anthracenemethanol which has a triplet state of intermediate energy.

Induced resonant absorption of electromagnetic waves by a microparticle

Light absorption by a microparticle in the presence of an atom with the optical transition frequency being resonant with the light frequency is theoretically investigated. The possibility of cascade energy transfer from light to an atom and then to a microparticle is found. The efficiency of cascade-type energy transfer to the microparticle essentially depends on the irradiation condition and the distance between the microparticle and the atom.

Infrared–microwave double resonance studies of collison-induced transitions and energy transfer processes between vibration–rotation–inversion levels of NH3

When the 14NH3 molecules in the upper inversion level of the J=8, K=7 (para) rotational state are pumped by using the N2O P (13) laser line, many microwave inversion lines change their intensities because of collision-induced populational transfer. Pressure dependence of this effect has been studied for several tens of inversion transitions in the ground vibrational state. The non-Boltzmann distribution introduced into the pumped levels is transfered to other levels by two mechanisms: (i) collision-induced transitions into or out of the pumped levels and succeeding cascading processes and (ii) selective energy transfer mechanisms to collision partners. In the former mechanism the transfer of populational change is caused by the transition of pumped molecules whereas in the latter it is caused by collision partners. Although the first mechanism is more efficient than the second, the second mechanism is not negligible and causes the populational change in also ortho rotational state. We used mixtures of 15NH3 and 14NH3 and monitored the 15NH3 inversion lines to discriminate between the two mechanisms. Since 15NH3 is not pumped by the laser line, the double resonance signal is caused only by the second process. The result on 15NH3 is similar to that for the ortho-14NH3, which clearly indicates that the signal previously observed for the ortho-14NH3 is not due to the collision-induced ortho–para conversion but due to the second mechanism. Double resonance signals for para-14NH3 show different pressure dependence from those of ortho-14NH3 and 15NH3. The results are analyzed consistently by taking account of various collision mechanisms. In the course of this analysis it was found that the V–V energy transfer becomes much more inefficient when the energy discrepancy becomes several tens of wavenumbers.

Phenomenology of the “explosive” crystallization of sputtered non-crystalline germanium films

Certain non-crystalline germanium films (> 10 μm in thickness) prepared by rf-sputtering crystallize “explosively” at room temperature when initiated by pricking the surface with a sharp point (or certain other methods). The propagation velocity of the crystallization at room temperature was found to be as fast a 1200 mm sec−1 depending somewhat on the conditions of film preparation, thickness, etc. The density of several such crystallizable films was determined as 5.05 g cm−3±1%. The crystallite size in the crystallized films, measured by X-ray broadening, was typically larger than 500 A. A model for the crystallization process invokes a cascade energy transfer process, basically thermal in nature.