Terms Of Energy Transfer Research Articles

Molecular mixtures based on porphyrins and phthalocyanines for application in photovoltaics

This paper presents a study of the photophysical properties of mixtures of magnesium or zinc porphyrins and phthalocyanines dissolved in dimethyl sulfoxide in order to follow deactivation pathways of radiative and non-radiative processes of excited states of molecules in the mixtures and their photocurrent generation when embedded in a photoelectrochemical cell. Interactions between dyes in the mixtures were examined by means of absorption, photoacoustics and laser induced optoacoustic spectroscopy. The changes of the spectral parameters (thermal deactivation, quantum yield of triplet state population, lifetime of the triplet states) for the dyes in the mixtures, when compared to those for the individual dyes, evidently indicate the strong interaction between the dye molecules. The alteration of photocurrent generated in the photoelectrochemical cell based on the dye mixture was observed (when compared to that for the individual dyes) and it was discussed in terms of interaction and energy transfer between dyes. . The Forster radia were estimated and their values are found to be typical for the resonance energy transfer. Energy transfer between dyes can cause the increase of photocurrent in the photoelectrochemical cell.

Ultrafast Energy Redistribution in Photoexcited Sodium‐Ammonia Clusters

AbstractThe dynamics in photoexcited Na(NH3)n clusters up to n = 20 using femtosecond laser pump and probe techniques are investigated. It is found that electronic excitation energy of the metal atom chromophore is transferred to internal vibrations of the surrounding ammonia solvent molecules. The transfer time strongly depends on the number of ammonia molecules bound to the sodium atom, decreasing from 2000 ps when only one ammonia molecules is attached to the metal atom down to a limiting value of about 100 fs for Na(NH3)10. These results can be understood in terms of an internal energy transfer where the rates are governed by the density of states and the Franck‐Condon overlap of the acceptor modes.

Kiloelectronvolt Argon-Induced Molecular Desorption from a Bulk Polystyrene Solid

The particle-induced desorption of polystyrene (PS) tetramers from a molecular solid sample has been studied using molecular dynamics simulations. To identify the specifics of bulk organic sample sputtering better, a PS monolayer on silver and a thick (45 Angstrom) PS sample have been bombarded by 0.5-keV Ar projectiles, at a polar angle of 45degrees. The sputtering yields of molecular species and their kinetic energy distributions were calculated from the final positions and velocities of the atoms in the simulation. One of the main results of this study is that the kinetic energy spectrum of ejected PS tetramers is significantly narrower for the bulk sample than the overlayer on metal. The results are explained in terms of energy transfer in the surface region. For the PS overlayer on silver, the sputtering process is characterized by the quick dissipation of the projectile energy in the metal substrate, accompanied by the fast ejection of molecules (1 ps) with a significant amount of kinetic energy. For the bulk sample, the sputtering process is slow (10 ps), the energy remains localized in the excited molecules (vibrations), and the resulting kinetic energies are comparatively low. Based on a limited series of trajectories using Ar projectiles excited at an accelerating voltage of 5 keV, we also comment on the effect of the primary particle energy on the molecular motion/desorption processes.

Luminescent rhenium(I)–gold(I) hetero organometallics linked by ethynylphenanthrolines

The first luminescent rhenium(I)–gold(I) hetero organometallics, Re{phen Au(PPh 3)}(CO) 3Cl ( 3) and Re{(PPh 3)Au phen Au(PPh 3)}(CO) 3Cl ( 4), have been prepared using the gold(I) complex AuCl(PPh 3) (PPh 3 = triphenylphosphine) and the novel rhenium(I) complexes Re(phen H)(CO) 3Cl ( 5) (phen H = 3-ethynyl-1,10-phenanthroline) or Re(H phen H)(CO) 3Cl ( 6) (H phen H = 3,8-bis(ethynyl)-1,10-phenanthroline). All the present rhenium(I) complexes 3– 6 were revealed to possess a facial configuration ( fac-isomer) with respect to the three carbonyl ligands. The main frameworks for these new gold(I) organometallics were constructed by the Au–C σ-bonding (with the η 1-type coordination) between the ethynylphenanthrolines and the Au(I) phosphine unit. Re(I)–Au(I) heterometallics 3 and 4 have shown single phosphorescence from the 3MLCT excited state and this observation can be interpreted in terms of the efficient intramolecular energy transfer from the Au(I) unit to the Re(I) unit.

A theory of nonvertical triplet energy transfer in terms of accurate potential energy surfaces: the transfer reaction from pi,pi* triplet donors to 1,3,5,7-cyclooctatetraene.

Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion parameter gamma=(2g(2)/kappa(1))(1/2) in which g stands for the norm of the energy gradient in the PES of the acceptor triplet state and kappa(1) is a combination of vibrational force constants of the ground-state acceptor in the gradient direction. The application of the model to existing experimental data for the triplet energy transfer reaction to COT from a series of pi,pi(*) triplet donors, provides a detailed interpretation of the parameters that determine the transfer rate constant. In addition, the model shows that the observed decrease of the acceptor electronic excitation energy is due to thermal activation of C=C bond stretchings and C-C bond torsions, which collectively change the ground-state COT bent conformation (D(2d)) toward a planar triplet state (D(8h)).

Plasma waves and electromagnetic radiation at Venus and Mars

Plasma waves play a significant role in the interaction of the solar wind with the ionospheres of the unmagnetized planets, Venus and Mars. Upstream of the planet plasma oscillations are generated by electrons energized and reflected at the bow shock. The spatial extent of the plasma oscillations is indicative of the amount of energy gained at the shock. Comparison with the Earth's bow shock suggests that the electron energization is controlled by shock curvature. At the dayside ionopause of the unmagnetized planets low frequency waves are observed. These waves were originally identified as whistler-mode waves at Venus, but were subsequently identified as lower hybrid or ion acoustic waves. The waves appear to have an important role in mediating coupling between the solar wind and planetary ionosphere, in terms of either energy or momentum transfer. Plasma waves are also observed in the nightside ionosphere of the planets, and whistler-mode waves have been observed at low altitudes at Venus. These waves have been identified with planetary lightning, although recent observations by the Cassini spacecraft during its Venus flybys suggest that lightning at Venus must have different characteristics than terrestrial lightning. The waves are also subject to collisional Joule dissipation, and considerations of the predicted heating may help constrain the possible source mechanisms for the waves. Given the rich plasma wave environment at the unmagnetized planets, and the importance of plasma waves in the dynamics of the plasma, future aeronomy missions should include some form of wave instrumentation.

Plasma waves and electromagnetic radiation at Venus and Mars

Plasma waves play a significant role in the interaction of the solar wind with the ionospheres of the unmagnetized planets, Venus and Mars. Upstream of the planet plasma oscillations are generated by electrons energized and reflected at the bow shock. The spatial extent of the plasma oscillations is indicative of the amount of energy gained at the shock. Comparison with the Earth's bow shock suggests that the electron energization is controlled by shock curvature. At the dayside ionopause of the unmagnetized planets low frequency waves are observed. These waves were originally identified as whistler-mode waves at Venus, but were subsequently identified as lower hybrid or ion acoustic waves. The waves appear to have an important role in mediating coupling between the solar wind and planetary ionosphere, in terms of either energy or momentum transfer. Plasma waves are also observed in the nightside ionosphere of the planets, and whistler-mode waves have been observed at low altitudes at Venus. These waves have been identified with planetary lightning, although recent observations by the Cassini spacecraft during its Venus flybys suggest that lightning at Venus must have different characteristics than terrestrial lightning. The waves are also subject to collisional Joule dissipation, and considerations of the predicted heating may help constrain the possible source mechanisms for the waves. Given the rich plasma wave environment at the unmagnetized planets, and the importance of plasma waves in the dynamics of the plasma, future aeronomy missions should include some form of wave instrumentation.

Rapid isolation and purification of photosystem I chlorophyll-binding protein from Chlamydomonas reinhardtii.

The available procedures for isolation and purification of photosystem I (PSI) from Chlamydomonas reinhardtii are time consuming and usually require several hours of sucrose gradient ultracentrifugation steps. This may lead to structural and functional impairment, including release of pigments and/or dissociation of protein subunits. Moreover, it is difficult to isolate intact complexes from thylakoids containing mutated PSI that accumulate to lower levels. Hence, isolation of intact PSI core complex depends on the speed of the procedure and the mildness of the extraction and purification. We have, therefore, modified the procedure for PSI isolation to both increase the yield of PSI and to reduce contamination by other pigment protein complexes. The modified procedure involves dodecyl maltoside solubilization of crude-thylakoid membranes followed by single-step column chromatography using a weak anion-exchanger. PSI eluted from the column between 13 mM and 15 mM Mg S04. This new rapid purification procedure yielded pure PSI preparations with a Chl/P700 ratio of approx 90 and showing typical absorption difference spectra with a maximum bleaching occurring at 696 nm. Femtosecond transient absorption spectroscopy of purified PSI complex revealed a high degree of similarity in terms of excitation energy transfer within the PSI core to observations in cyanobacterial PSI.

Effect of particle inertia on turbulence in a suspension.

We propose a one-fluid analytical model for a turbulently flowing dilute suspension, based on a modified Navier-Stokes equation with a k-dependent effective density of suspension rho(eff)(k) and an additional damping term proportional, variant gamma(p)(k), representing the fluid-particle friction (described by Stokes law). The statistical description of turbulence within the model is simplified by a modification of the usual closure procedure based on the Richardson-Kolmogorov picture of turbulence with a differential approximation for the energy transfer term. The resulting ordinary differential equation for the energy budget is solved analytically for various important limiting cases and numerically in the general case. In the inertial interval of scales, we describe analytically two competing effects: the energy suppression due to the fluid-particle friction and the energy enhancement during the cascade process due to decrease of the effective density of the small-scale motions. An additional suppression or enhancement of the energy density may occur in the viscous subrange, caused by the variation of the extent of the inertial interval due to the combined effect of the fluid-particle friction and the decrease of the kinematic viscosity of the suspensions. The analytical description of the complicated interplay of these effects supported by numerical calculations is presented. Our findings allow one to rationalize the qualitative picture of the isotropic homogeneous turbulence of dilute suspensions as observed in direct numerical simulations.

Energy transfer processes in TPP-doped polyfluorene

The energy transfer processes between polyfluorene and tetraphenyl porphyrin have been investigated through steady state and time resolved measurements. Radiative transfer is found to be negligible and so the results are interpreted in terms of Forster resonance energy transfer. Rate equations are fitted to the results to obtain a concentration dependent transfer rate. From this we calculate a value for the Forster radius of 36A. This agrees well with the values we calculate from the spectral overlap, taking a wavelength dependent refractive index into account.

Impulse dispersion in a tapered granular chain

This paper describes a unique way to disperse an impulse in granular material. A number of chrome steel balls are arranged in a chain formation with monotonically decreasing size. The incoming impulse is received by the largest ball at one end and propagates through the chain. Due to translation symmetry breaking, the classical solitary wave known to develop in a chain of particles of the same size quickly lose its signature, disperses itself throughout the balls in the chain, and finally manifests itself as a collection of smaller impulses. This paper presents experimental confirmation of this mechanism and a comparison with recent theoretical predictions of impulse dispersion in a chain of hard inelastic spheres. Some discrepancies with theory are interpreted in terms of energy transfer to rotational degrees of freedom.

Lagrangian Particle Approach to Large Eddy Simulations of Hydrodynamic Turbulence

The flux of energy from large to small scales in hydrodynamic turbulence controls the dissipation of energy at a given scale in the fluid. An accurate parametrization of this flux is a prerequisite in order to devise reliable methods to simulate turbulent flows without resolving all the scales of motion. This problem is discussed in the context of a particle method based on the Smooth Particles Hydrodynamics algorithm. Motivated by the von Karman–Howarth–Kolmogorov exact relation for the energy flux, and by Lagrangian dynamics considerations we postulate an energy transfer term which is quadratic in the velocity and formally time reversal invariant. The numerical simulation of the model however is observed to spontaneously break the time reversal symmetry, demonstrating that the proposed term acts on average as the desired eddy damping.

Singlet and triplet energy transfer in a benzil-doped, light emitting, solid-state conjugated polymer

Abstract The luminescence emitted from pure and benzil-doped thin films of the conjugated polymer polyfluorene [PF2/6] are compared. The prompt fluorescence from the first singlet-excited state of the polymer is quenched by 90% in the presence of 10% per weight benzil. In addition to the prompt fluorescence, time-resolved spectroscopy at low temperature also allows the detection of phosphorescence and delayed fluorescence from the host polymer. Again the delayed fluorescence is strongly quenched but the phosphorescence is enhanced in doped samples. An explanation of the results is given in terms of singlet energy transfer from the host to benzil and triplet energy transfer from the dopant back to PF2/6. We have applied this to enable better understanding of the photophysics in PF2/6 doped with a platinum porphyrin complex.

Food Webs and Container Habitats: The Natural History and Ecology of Phytotelmata

Roger Kitching’s bookcould be regarded asthe first field guide tothe fascinating worldof life in ephemeralbodies of water.Phytotelmata,aquatic organismsliving in containers ofplant origin, whichrange from flatworms to crustaceans toaquatic larvae of insects and frogs, occurnot only in tree holes, bamboo internodesand basins of buttressed trees, but also inbromeliad tanks, palm fronds and otherwatertight axils of leaves and floral bract,and even in the highly acidic fluid in thepitchers of pitcher plants. How unique arethe assemblages of such organisms inphytotelm and do they function as acommunity, showing the emergentcharacters, such as stable food webs, thatare seen in other communities?This book presents evidence andtantalizing facts about the speciesdiversity, trophic relations and evolution ofphytotelmata. Based on his own extensivework, Kitching constructs species-basedfood webs of the saprophagous and thepredatory animals found in tree holes andpitcher plants, and examines global,continental and regional patterns in theseand others reported in the literature.Disregarding microbes, rotifers andnematodes, he singles out competition andpredation as the key forces that structurethe phytotelm community and shows howbiogeographical and ecological factorsinfluence the pattern and process inthese communities.A drawback to this approach is that it isentirely binary in linking species (havingor not having a connection) and suffersfrom a lack of quantitative information onthese links. It is difficult to assess the rolesof individual species in these habitatsbecause nutrient input will differdepending on both the host plant and thenature of its container (pans or rot holes).All species of saprophytes need to beknown and their competitive andcomplementary functions in nutrient andenergy consumption worked out toproduce comprehensive food webs forthese systems. However, the top-downeffects of predators are easier to assessand here an experimental approach can beused to manipulate the food-webstructure. Not surprisingly, impacts ofpredators are more dramatic in smallerphytotelm units than in larger ones, butnot all predators, even those occurring inhigh density, have a keystone role incontrolling the overall food-web structure.Since 1960 (Ref. 1), ecologists havefocused on the analysis of food webs inmicrocosms, such as small springs,decaying logs and cowpats. This was anattempt to produce an accurate descriptionof the entire community in a semi-closedsystem so that all species present in thatsystem could be accounted for in terms ofenergy transfer and trophic relations. ThisLindeman-style approach to ecosystemanalysis

Infrared and visible emission from organic electroluminescent devices based on praseodymium complex

Praseodymium(dibenzoylmethanato) 3 (bathophenanthroline ) [Pr(DBM)3bath] was employed as an emitting and electron transport layer, and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1, 1′-biphenyl-4,4′-diamine (TPD) as a hole transport layer in organic electroluminescent (EL) devices. Bilayer device TPD/Pr(DBM)3bath and trilayer devices TPD/TPD:Pr(DBM)3bath/Pr(DBM)3bath with a different ratio of TPD to the Pr-complex were fabricated. Emission bands at 608 nm (1D2→3H6), 890 nm (1D2→3F2), 1015 nm (1D2→3F3), 1065 nm (1D2→3F4) and 1550 nm (1D2→1G4) originating from the internal f–f transitions of a Pr3+ ion were observed from EL devices using both bilayer and trilayer structures. Decreasing the ratio of TPD to the Pr-complex, the emission of the D21→H63 transition was promoted and that from the exciplex suppressed, which was explained in terms of energy transfer from the ligand to the central ion.

Ultrafast Energy-Transfer Dynamics between Block Copolymer and π-Conjugated Polymer Chains in Blended Polymeric Systems

The blending of polymers has been conventionally employed to increase the quantum efficiencies of polymeric electroluminescence (EL) devices via the energy transfer process, which was interpreted in terms of the Forster-type energy transfer based on dipole−dipole interactions. The detailed analysis of various time-resolved spectroscopic data in the blended polymer between MEH-PPV (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) and DSiPV (poly[1,3-propanedioxy-1,4-phenylene-1,2-ethylene(2,5-bis(trimethylsilyl)-1,4-phenylene)-1,2-ethylene-1,4-phenylene]) provides some evidence that the ultrafast energy-transfer channel via the shortest interchain distance between the adjacent constituent polymer chains should be considered to account for the dynamics of stimulated emission (SE) and photoinduced absorption (PA) in the blended polymeric systems. These interchain interactions are also responsible for the diminishment of SE of the blended polymer in the transient absorption spectra, because PA is en...

Power dependence of luminescence of Tb 3+-doped KYb(WO4) 2 crystal

The effect of excitation intensity on luminescence properties of Tb 3+-doped KYb(WO 4) 2 crystal is reported. In particular, emissions following the Stokes and anti-Stokes excitations were investigated. It was found that, in addition to the Tb 3+ luminescence, an intense Yb 3+ luminescence at room temperature was observed after a direct excitation of Tb 3+ ions in spite of the fact that both ions are far off-resonance. The intensities of Tb 3+ and Yb 3+ luminescences were dependent on the excitation power and changed by a sublinear function (slope<1). The mechanism of the non-resonant energy transfer from Tb 3+ to Yb 3+ is discussed in terms of the phonon-assisted cross-relaxation and the down-conversion contributing to the process. The up-conversion emission of Tb 3+ (after a direct excitation of Yb 3+ ions) was observed at room temperature and its intensity varied with the slope <2. It was found that the emission decay times were strongly dependent on the excitation power. The mechanism of this behaviour is discussed in terms of many-body energy transfer.

The Effect of Sudden Failure of a Rotary Blood Pump on Left Ventricular Performance in Normal and Failing Hearts

We investigated the hemodynamic effect of regurgitation (or back-flow) due to sudden failure of a rotary blood pump (diagonal pump). Seven healthy sheep (Group C) and 7 with chronic heart failure (Group F) were studied. Chronic heart failure was obtained by intracoronary injection of microspheres several weeks earlier. Left ventricular function and ventricular efficacy were assessed by the pressure-volume relationship. The back-flow through the stopped pump was significantly lower in Group F (2.3 +/- 0.34 L/min) than in Group C (2.8 +/- 0.33 L/min). Mean aortic blood pressure dropped significantly from 68.3 +/- 9.65 to 61.9 +/- 9.75 mm Hg in Group C and from 62.5 +/- 9.12 to 51.5 +/- 9.08 in Group F but remained stable during the 15 min period of pump stop. Parameters of left ventricular contractility (preload recruitable stroke work) dropped significantly in both groups, remained stable during the pump stop, and returned to baseline values 30 min after the end of back-flow. The ventricular efficacy (in terms of energy transfer) was tolerant against this acute volume overload even in the failing hearts. Sudden pump failure of a rotary blood pump leads to an acute depression of the hemodynamic state and myocardial contractility. However, this depression remained stable over 15 min, did not lead to further deterioration of the animals, and was completely reversible.

Picosecond dynamics of excitations studied in three generations of new 4,4′-bis(diphenylamino)stilbene-based dendrimers

Picosecond fluorescence decay is observed in a new family of dendrimers constructed from 4,4′-bis(diphenylamino)stilbene repeat units. The rate of the decay is shown to depend on detection wavelength and the generation number in such a way that the energy of accepting states gradually lowers with the generation number. These observations can be consistently described in terms of the fast energy transfer from initially created localized states to more delocalized ones.

Monolayer-Protected Clusters with Fluorescent Dansyl Ligands

Monolayer-protected Au cluster (MPC) molecules with mixed monolayers of alkanethiolate and ω-carboxylalkanethiolate ligands were functionalized with the fluorescent label dansyl cadaverine. The emission intensity per MPC-attached dansyl group varies with (a) the number of dansyl sites per MPC (i.e., loading, 4 to 24), (b) with the linker chainlength between Au core and dansyl site, and (c) with chainlengths of alkanethiolates in the surrounding MPC monolayer coating. Emission intensity per MPC-attached dansyl group increases with loading and with the length of core−dansyl linker chains when the surrounding alkanethiolate monolayers have chain lengths equal to or greater than the linker. These results are interpreted in terms of distance-dependent energy transfer from the excited dansyl label to the metal-like MPC core and various steric constraints on thermal fluctuations of linker chains that influence the distance of approach of the label to the core. Correction for self-absorption by the highly colored...