Broad Distribution Of Lifetimes Research Articles

Delayed fluorescence of InP:Mn/ZnS nanocrystals

The photoluminescence properties of ZnS-capped Mn-doped InP nanocrystals are shown to be determined by the high ratio of the radiative rate constants of the host and dopant emission, by the polydispersity-induced inhomogeneous broadening, and by the rapid establishment of thermal equilibrium in the distribution of the excitation between the energy levels of the quantum dot and the dopant ion. Because of the combination of these factors, the resulting photoluminescence includes the delayed host fluorescence and the quenched dopant phosphorescence, both having very broad lifetime distributions.

Open-circuit photopotential characterization of photoelectrochemical activities of Au-modified TiO2 nanorods

The open circuit potential (OCP) of a semiconductor electrode can be used to quantify the transient photopotential (Ep), which represents wavelength-dependent charge accumulation and relaxation kinetics of a photoelectrode. Here OCP responses of a plasmonic Au@TiO2 nanorods (NRs) photoelectrode can be quantified without causing electrochemical corrosion of Au. The photogenerated charge accumulation kinetics data based on the wavelength-dependent growth rates of |Ep| can resolve the plasmonic effects on photoelectrochemistry (PEC) of Au@TiO2 NRs. Data fitting with Kohlrausch-Williams-Watts (KWW) stretched exponential kinetics model illustrates the complex charge relaxations at the Au/TiO2 Schottky contact, from which long relaxation lifetimes with broad lifetime distributions can be obtained. This is attributed to the abundant deep defects in the nanostructure TiO2, which has been strongly confirmed by reducing the oxygen vacancies using a post-thermal annealing treatment. Single-particle dark-field scattering (DFS) spectrum is measured with a tunable wavelength light source to support visible light activities of PEC characteristics of Au@TiO2 NRs. Light scattering spectra of >200 single Au@TiO2 NRs particles are collected to compare directly with PEC responses of OCP of the ensemble Au@TiO2 NRs.

The interplay between chromophore and protein determines the extended excited state dynamics in a single-domain phytochrome

Phytochromes are a diverse family of bilin-binding photoreceptors that regulate a wide range of physiological processes. Their photochemical properties make them attractive for applications in optogenetics and superresolution microscopy. Phytochromes undergo reversible photoconversion triggered by the Z ⇄ E photoisomerization about the double bond in the bilin chromophore. However, it is not fully understood at the molecular level how the protein framework facilitates the complex photoisomerization dynamics. We have studied a single-domain bilin-binding photoreceptor All2699g1 (Nostoc sp. PCC 7120) that exhibits photoconversion between the red light-absorbing (Pr) and far red-absorbing (Pfr) states just like canonical phytochromes. We present the crystal structure and examine the photoisomerization mechanism of the Pr form as well as the formation of the primary photoproduct Lumi-R using time-resolved spectroscopy and hybrid quantum mechanics/molecular mechanics simulations. We show that the unusually long excited state lifetime (broad lifetime distribution centered at ∼300 picoseconds) is due to the interactions between the isomerizing pyrrole ring D and an adjacent conserved Tyr142. The decay kinetics shows a strongly distributed character which is imposed by the nonexponential protein dynamics. Our findings offer a mechanistic insight into how the quantum efficiency of the bilin photoisomerization is tuned by the protein environment, thereby providing a structural framework for engineering bilin-based optical agents for imaging and optogenetics applications.

Local Enrichment of Unsaturated Chains around the A2A Adenosine Receptor.

Two 15 μs all-atom simulations of the A2A adenosine receptor were obtained in a ternary mixture of cholesterol, saturated phosphatidylcholine lipids, and unsaturated phosphatidylcholine lipids. An analysis of local lipid solvation is reported on the basis of a Voronoi tessellation of the upper and lower leaflets, identifying first and second solvation shells. The local environments of both the inactive state and the partially active state of the receptor are significantly enriched with unsaturated chains but depleted of cholesterol and saturated chains, relative to the bulk membrane composition. In spite of the local depletion of cholesterol, the partially active receptor binds cholesterol at three locations during the entire simulation trajectory. These long-lived interactions represent the extreme of a very broad distribution of first-solvation shell lipid lifetimes, confounding sharp distinctions between lipid interactions. The broad distributions of lifetimes also make equilibrating the local lipid environment difficult, necessitating long simulation times.

Photophysical Model for Non-Exponential Relaxation Dynamics in Hybrid Perovskite Semiconductors

The photoluminescence (PL) decay of hybrid halide perovskite single crystals (MAPbX3, MA = CH3NH3+, Pb = Pb2+, X = Br–, and I–) is measured over 4 orders of magnitude in intensity over the time scales of 100s of nanoseconds to a few microseconds. This long PL decay is non-exponential, suggesting the presence of a distribution of carrier relaxation times. Spectro-temporal studies show that the emission peak red-shifts with increasing time. The physics of this problem is closely related to donor–acceptor pair recombination in crystalline semiconductors and recombination in a-Si:H. Based on these models, we present a simple model to account for the recombination dynamics in the perovskite systems. This model also accounts for the fluence dependence of the recombination kinetics. In this model, a fraction of the photogenerated electrons and holes are trapped in localized states. The electrons tunnel to the hole sites for recombination. The broad distribution of lifetimes is a consequence of the fact that the ...

Role of Proton Diffusion in the Nonexponential Kinetics of Proton-Coupled Electron Transfer from Photoreduced ZnO Nanocrystals.

Experiments have suggested that photoreduced ZnO nanocrystals transfer an electron and a proton to organic radicals through a concerted proton-coupled electron transfer (PCET) mechanism. The kinetics of this process was studied by monitoring the decay of the absorbance that reflects the concentration of electrons in the conduction bands of the nanocrystals. Interestingly, this absorbance exhibited nonexponential decay kinetics that could not be explained by heterogeneities of the nanoparticles or electron content. To determine if proton diffusion from inside the nanocrystal to reactive sites on the surface could lead to such nonexponential kinetics, herein this process is modeled using kinetic Monte Carlo simulations. These simulations provide the survival probability of a proton hopping among bulk, subsurface, and surface sites within the nanocrystal until it reaches a reactive surface site where it transfers to an organic radical. Using activation barriers predominantly obtained from periodic density functional theory, the simulations reproduce the nonexponential decay kinetics. This nonexponential behavior is found to arise from the broad distribution of lifetimes caused by different types of subsurface and surface sites. The longer lifetimes are associated with the proton becoming temporarily trapped in a subsurface site that does not have direct access to a reactive surface site due to capping ligands. These calculations suggest that movement of the protons rather than the electrons dominate the nonexponential kinetics of the PCET reaction. Thus, the impact of both bulk and surface properties of metal-oxide nanoparticles on proton conductivity should be considered when designing heterogeneous catalysts.

Transient and quasi-permanent networks in xyloglucan solutions

Viscoelastic properties of aqueous solutions of xyloglucan extracted from Hymenaea courbaril seeds (Jatobá gum) were investigated by rheology over a wide range of concentrations and temperatures. The polymer was characterized in dilute solutions by light scattering measurements and size exclusion chromatography. Xyloglucan formed, in semi-dilute solutions (C 0.3wt%), a transient network with cross-links characterized by a broad distribution of lifetimes, independent of the temperature and concentration. Progressively, at higher temperatures (>60°C), a second much weaker quasi-permanent network was formed and attributed to the exchange of intra- to inter-chain bonds. The stiffness of the second network increased with decreasing temperature, but it could be easily broken by applying a relatively weak shear stress and is readily reversible on re-heating, and partially reversible on resting at 20°C.

Analysis of simulated fluorescence intensities decays by a new maximum entropy method algorithm.

A new algorithm for the Maximum Entropy Method (MEM) is proposed for recovering the lifetime distribution in time-resolved fluorescence decays. The procedure is based on seeking the distribution that maximizes the Skilling entropy function subjected to the chi-squared constraint χ 2 ~ 1 through iterative linear approximations, LU decomposition of the Hessian matrix of the lagrangian problem and the Golden Section Search for backtracking. The accuracy of this algorithm has been investigated through comparisons with simulated fluorescence decays both of narrow and broad lifetime distributions. The proposed approach is capable to analyse datasets of up to 4,096 points with a discretization ranging from 100 to 1,000 lifetimes. A good agreement with non linear fitting estimates has been observed when the method has been applied to multi-exponential decays. Remarkable results have been also obtained for the broad lifetime distributions where the position is recovered with high accuracy and the distribution width is estimated within 3 %. These results indicate that the procedure proposed generates MEM lifetime distributions that can be used to quantify the real heterogeneity of lifetimes in a sample.

Microdynamics in stationary complex networks

Many complex systems, including networks, are not static but can display strong fluctuations at various time scales. Characterizing the dynamics in complex networks is thus of the utmost importance in the understanding of these networks and of the dynamical processes taking place on them. In this article, we study the example of the US airport network in the time period 1990-2000. We show that even if the statistical distributions of most indicators are stationary, an intense activity takes place at the local ("microscopic") level, with many disappearing/appearing connections (links) between airports. We find that connections have a very broad distribution of lifetimes, and we introduce a set of metrics to characterize the links' dynamics. We observe in particular that the links that disappear have essentially the same properties as the ones that appear, and that links that connect airports with very different traffic are very volatile. Motivated by this empirical study, we propose a model of dynamical networks, inspired from previous studies on firm growth, which reproduces most of the empirical observations both for the stationary statistical distributions and for the dynamical properties.

Broad lifetime distributions for ordering dynamics in complex networks

We search for conditions under which a characteristic time scale for ordering dynamics toward either of two absorbing states in a finite complex network of interactions does not exist. With this aim, we study random networks and networks with mesoscale community structure built up from randomly connected cliques. We find that large heterogeneity at the mesoscale level of the network appears to be a sufficient mechanism for the absence of a characteristic time for the dynamics. Such heterogeneity results in dynamical metastable states that survive at any time scale.

Origin of the broad lifetime distribution of localized excitons in InGaN/GaN quantum dots

AbstractWe derive an energy‐dependent decay‐time distribution function from the multi‐exponential decay of the ensemble photoluminescence (PL) of Inx Ga1–x N/GaN quantum dots (QDs), which agrees well with recently published single‐QD time‐resolved PL measurements. Using eight‐band k · p modelling, we show that the built‐in piezo‐ and pyroelectric fields within the QDs cause a sensitive dependence of the radiative lifetimes on the exact QD geometry and composition. Moreover, the radiative lifetimes also depend heavily on the composition of the direct surrounding of the QDs. A broad lifetime distribution occurs even for moderate variations of the QD structure. Thus, for unscreened fields a multi‐exponential decay of the ensemble PL is generally expected in this material system. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoexcitation dynamics in an alternating polyfluorene copolymer

The authors used transient photoinduced absorption on femtosecond to nanosecond time scales as well as delayed fluorescence up to microseconds to study the photogeneration and recombination of charges in thin films of the alternating polyfluorene copolymer poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]. The authors interpret the results using a coupled rate equation model and find that the authors can fit all exptl. results with a single set of parameters. The model includes prompt (<0.1 ps) as well as slower (.apprx.0.1-1 ns) charge-pair formation, which the authors attribute to Coulombically bound intra- and interchain polaron pairs, resp. The intrachain polaron pairs are promptly generated from vibronically excited (hot) primary singlet excitons S1* and recombine geminately back to the lowest singlet exciton state S1 with a lifetime distribution having a mean lifetime of .apprx.2.4 ps. The interchain polaron pairs, which can be seen as precursors to free charges, are formed via 2 channels: via singlet excitons being dissocd. with a linear rate const. of .apprx.5 ns as well as via a time-dependent bimol. exciton-exciton annihilation process generating higher-energy exciton states Sn* of which a fraction subsequently dissocs. into interchain polaron pairs. The authors observe a total yield of 12%-23% interchain polaron pairs (a precursor to free polarons), depending on the excitation intensity used. This also defines the upper limit of the free polaron yield at zero elec. field in this material. The long-lived interchain polaron pairs recombine geminately back to the ground state or to singlet excitons with a broad distribution of lifetimes having a mean lifetime of .apprx.0.27 micro s. The fraction of interchain polaron pairs recombining back to singlet excitons, with subsequent radiative decay back to the ground state, gives rise to delayed fluorescence extending to microsecond time scales. [on SciFinder (R)]

Photogeneration of charge carriers in blends of conjugated polymers and semiconducting nanoparticles

Composites of nanoparticles and conjugated polymers are studied via photoluminescence (PL), photoinduced absorption spectroscopy and light induced electron spin resonance (LESR) spectroscopy. Two different types of nanoparticles were investigated namely cadmium selenide and indium phosphide. They were available with a tri- n-octylphosphine-oxide–tri- n-octylphosphine shell or pyridine washed. Nanoparticles were mixed with poly(2-methoxy-5-(3-,7-dimethyloctyloxy)-1,4-phenylenevinylene) (OC 1C 10-PPV) in different weight ratios. The quenching of the PL, the formation of polaronic radicals as well as the occurrence of two new absorption bands are explained in the frame of a photoinduced charge transfer between the polymer and the nanocrystal. For a detailed analysis, the lifetime and recombination kinetics are studied by varying the chopper frequency and by changing the laser power. We find a broad distribution of lifetimes, which suggests a dispersive recombination process. With LESR the charge transfer process is confirmed and the g-factor of the polymer cation was determined. Time dependent studies show the involving of long living species in the recombination process.

Photoblinking of Rhodamine 6G in Poly(vinyl alcohol): Radical Dark State Formed through the Triplet

We investigate the fluorescence intensity of rhodamine 6G in poly(vinyl alcohol) as a function of excitation intensity, illumination time, the presence of oxygen, and temperature. The variations in emissivity (or fluorescence brightness) are attributed to a dark state, which shows populating kinetics resembling those of the triplet state, but a much longer lifetime. We simulate the observed kinetics by a four-level model, in which a long-lived dark state is formed through the triplet as an intermediate state. The weak temperature dependence of the lifetime of the dark state points to electron tunneling as the main recovery process. This intermolecular mechanism also explains the observed broad distribution of lifetimes. An electron-spin-resonance experiment confirms the assignment of the dark state to a radical. For the first time, photoinduced charge transfer is identified as a source of blinking in single-molecule measurements.

Dissociation dynamics of propargyl chloride molecular ion near the reaction threshold: manifestation of quantum mechanical tunneling

The Cl loss from the propargyl chloride molecular ion has been investigated using mass-analyzed ion kinetic energy spectrometry (MIKES). The kinetic energy release distribution in the unimolecular dissociation has been determined. The potential energy surface for the mechanistic pathway has been calculated at the B3LYP/6-311G** density functional theory level. The calculated potential energy surface suggested that the threshold dissociation of the propargyl chloride molecular ion produces the C(3)H(3)(+) ion, only with the cyclopropenium structure, and with the release of a large amount of kinetic energy. This is in agreement with experimental results. Also, calculation of the rate constants with statistical rate models predicted that the reaction observed on a microsecond time scale occurs via tunneling through the rate-determining isomerization barrier for H-atom transfer. It has been found that a broad lifetime distribution is a manifestation of quantum mechanical tunneling of a precursor prepared under thermal conditions. Reinterpretation of previous photoelectron-photoion coincidence results taking into account the tunneling effect necessitated raising the critical energy to 0.64 eV from the energy of 0.34 eV reported previously. Copyright 2000 John Wiley & Sons, Ltd.

Glass-like behavior in supercritical fluids: The effect of critical slowing down on solute dynamics

Compressible supercritical fluids exhibit extended density inhomogeneities which, due to their collective many-particle nature, fluctuate only very slowly, a phenomenon known as “critical slowing down.” Our computations demonstrate that these slowly fluctuating inhomogeneities may remain effectively frozen on the time scale of fast dynamic processes such as solute vibrational relaxation. Furthermore, we find that a given solute molecule may experience any of a broad distribution of effectively static local solvent environments and that each solute’s dynamical behavior will be controlled by its own local solvent environment. Consequently, an ensemble of solute molecules will relax with a correspondingly broad distribution of lifetimes. Although such inhomogeneous broadening effects are expected in amorphous solids and glasses, this is the first confirmation of their presence in supercritical fluids.

Luminescence Properties of Thiol-Stabilized CdTe Nanocrystals

Thiol-capped CdTe nanocrystals with cubic zinc blende structure are synthesized in aqueous solution. Their steady-state and time-resolved luminescence characteristics are studied at room and liquid nitrogen temperatures. A strong exciton luminescence peak at 2.3 eV dominates the emission spectrum of CdTe nanocrystals at room temperature, whereas the trap band centered at 2.0 eV undergoes substantial temperature quenching. Luminescence excitation spectra reveal different channels leading to radiative recombination via either excitons or traps. The mean luminescence decay time of CdTe nanocrystals at room temperature decreases from 120 ns at 1.94 eV to 20 ns at 2.43 eV. Luminescence decay kinetics of CdTe nanocrystals are strongly nonexponential and are described by extremely broad lifetime distributions lying within the range from a few hundred picoseconds to a few hundred nanoseconds.

Reasons for the deviation of I1/ I3 and τ1 from the expected lifetime parameters of positronium annihilation in polymers

High quality positron lifetime spectra were taken for several amorphous polymers. The spectra were analysed using the routine MELT which assumes continuous lifetime distributions, as well as the discrete-term analysis routine LIFSPECFIT. Disagreements between the analysed lifetime parameters and theoretical expectations, I 1 /I 3 > I p- Ps/ I o- Ps=1/3 and τ 1 > τ p- Ps, which are well known, also appear in our results. The disagreements are distinctly larger using a discrete-term routine than when applying MELT (or CONTIN). From the analysis of computer-generated spectra, we found that these disagreements may be well understood by assuming a distribution of e + lifetimes in addition to an o-Ps lifetime distribution, both of which can occur as consequence of size and shape distributions of free-volume holes in amorphous polymers (or of bubbles in molecular liquids). The disagreements appear to be due to the inability of the routines correctly to mirror very broad lifetime distributions. For broad distributions, MELT (and also CONTIN) delivers, as an artefact of the spectrum analysis, a distribution consisting of two narrow subpeaks instead of a single broad one. The lower subpeak of the e + lifetime distribution may mix with the p-Ps lifetime component resulting in artificially large values for τ 1 and I 1. That is the reason for the unphysical increase of τ 1 and I 1 with decreasing I 3 observed in the analysed parameters of simulated as well as experimental spectra.

Dynamics of Long-lived Polarons in Poly( p-phenylene)-Type Ladder Polymers (LPPP)

The X-band spin 1/2 photoluminescence-detected magnetic resonance (PLDMR) of LPPP films and solutions featuring a high degree of intrachain order and high PL quantum yields (24% in films and 84% in solutions) is described. Excitation at 458 run yields a PL-enhancing polaron resonance at g~2.00 of width ΔH 1/2~8G and 4G in films and frozen solutions, respectively. The resonances are assigned to enhanced nonradiative recombination of long-lived polaron pairs, which are singlet exciton quenching centers. Their behavior is consistent with two distinct polaron pair lifetimes or a very broad distribution of lifetimes which are ~10 times longer in solutions than in films. The distinct lifetimes which are ~40 and ~670μs in films and ~370 and ~3300μs in solutions, are discussed in relation to longer-lived intrachain interconjugation segment pairs and shorter-lived interchain pairs. The spectral dependence of the resonance demonstrates that the emission from charge transfer excitons in aggregates of the films is much more spin-dependent than that from single polymer chains of the frozen solutions.

Dynamics of Long-Lived Polarons in Poly(para-phenylene)-Type Ladder Polymers.

The X-band spin- $\frac{1}{2}$ photoluminescence-detected magnetic resonance of poly(para-phenylene)-type ladder polymer films and solutions are described. The photoluminescence-enhancing resonances are assigned to enhanced nonradiative recombination of long-lived polaron pairs, which are singlet exciton quenching centers. Their behavior is consistent with two distinct polaron lifetimes or a very broad distribution of lifetimes; in solutions they are $\ensuremath{\approx}10$ longer than in films. Distinct lifetimes are discussed in relation to longer-lived intrachain interconjugation segment and shorter-lived interchain pairs.