Transient Lifetime Research Articles

Radical Cations of Aromatic Selenium Compounds: Role of Se···X Nonbonding Interactions

Selenium centered radical cations in aliphatic selenium compounds are stabilized by formation of two-center-three electron (2c-3e) hemi bonds either with nearby heteroatoms forming monomer radicals or with selenium atoms of the parent molecules forming dimer radicals. Such radicals in aromatic selenium compounds would generally be stabilized as monomers by the delocalization of the spin density along the aromatic ring. To test the assumption if aromatic selenides having Se···X nonbonding interactions can show different types of radical cations, we have performed pulse radiolysis studies of three structurally related aromatic selenium compounds and the results have been substantiated with cyclic voltammetry and quantum chemical calculations. The three aromatic selenium compounds have functional groups like -CH2N(CH3)2 (1), -CH2OH (2), and -CH3 (3) at ortho position to the -SeCH3 moiety. The energy of Se···X nonbonding interactions (E(nb)) for these compounds is in the order 1 (Se···N) > 2 (Se···O) > 3 (Se···H). Radical cations, 1(•+), 2(•+) and 3(•+) were produced by the one-electron oxidation of 1, 2 and 3 by radiolytically generated (•)OH and Br2(•-) radicals. Results on transient spectra, lifetime, and secondary reactions of 1(•+), 2(•+), and 3(•+) indicated that 1(•+) shows a significantly different absorption spectrum, longer lifetime, and less oxidizing power compared to those of 2(•+) or 3(•+). Quantum chemical calculations suggested that 1(•+) is stabilized by the formation of a 2c-3e bond between Se and N atoms, whereas 2(•+) and 3(•+) acquire stability through the delocalization of the spin density on the aromatic ring. These results provide evidence for the first time that stronger nonbonding interactions between Se···N in the ground state, facilitate the formation of stabilized radical cations, which can significantly influence the redox chemistry and the biological activity of aromatic selenium compounds.

Understanding Growth Kinetics of Nanorods in Microemulsion: A Combined Fluorescence Correlation Spectroscopy, Dynamic Light Scattering, and Electron Microscopy Study

Even though nanostructures of various shapes and sizes can be controlled by microemulsions, there is substantial difficulty in understanding their growth mechanism. The evolution of nanostructures from the time of mixing of reactants to their final stage is a heterogeneous process involving a variety of intermediates. To obtain a deeper insight into these kinetic steps, we studied the slow growth kinetics (extending over eight days) of iron oxalate nanorods inside the polar core of water-in-oil microemulsion droplets made of cetyltrimethylammonium bromide/1-butanol/isooctane. Fluorescence correlation spectroscopy (FCS), dynamic light scattering (DLS), and transmission electron microscopy (TEM) have been employed to monitor the nanostructure growth at (near) the single-droplet level and in an ensemble. Analyzing FCS data with suitable kinetic model we obtain transient dimer lifetime (28 μs) and the droplet fusion rates (and fusion tendency) on each day as the reaction proceeds. The droplet fusion rate is found to directly control the nanorod growth in microemulsion solution and attains its maximum value (3.55 × 10(4) s(-1)) on day 6, when long nanorods are found in TEM data, implying that more and more reactants are fed into the growing system at this stage. Combining FCS, DLS, and TEM results, we find three distinct periods in the entire growth process: a long nucleation-dominant nanoparticle growth period which forms nanoparticles of critical (average) size of ∼53 nm, followed by a short period where isotropic nanoparticles switch to anisotropic growth to form nanorods, and finally elongation of nanorods and growth (and shrinking) of nanoparticles.

Excited State Dynamics of Two New Ru(II) Cyclometallated Dyes: Relation to Cells for Solar Energy Conversion and Comparison to Conventional Systems

The preparation, characterization, and photophysical properties of a series of ruthenium(II) complexes possessing the cyclometallating deprotonated 2-phenyl pyridine ligand, phpy–, together with dppn (benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine), a ligand with an extended π-system, are reported. Related complexes have been used as efficient dyes in dye-sensitized solar cells (DSSCs), and the Ru→dppn metal-to-ligand charge transfer (MLCT) absorption of the new complexes [Ru(phpy)(bpy)(dppn)]+ (4) and [Ru(phpy)(dppn)2]+ (5) is red-shifted relative to the Ru→bpy MLCT peak in [Ru(phpy)(bpy)2]+ (3). These new compounds are compared to conventional complexes where phpy– is replaced by 2,2′-bipyridine (bpy), including [Ru(bpy)3]2+, [Ru(bpy)2(dppn)]2+ (1), and [Ru(bpy)(dppn)2]2+ (2). Unlike 1 and 2, with long-lived dppn-centered 3ππ* excited states (τ ∼ 20 μs), the corresponding cyclometallated complexes 4 and 5 exhibit weakly emissive Ru→dppn 3MLCT states with transient absorption lifetimes of 25 and 45 ps, respect...

Time-resolved photoluminescence imaging with electronic shuttering using an image intensifier unit

In contrast to traditional steady-state photoluminescence imaging (PLI), time-resolved photoluminescence imaging (TR-PLI) allows for a calibration-free measurement of the effective transient minority charge carrier lifetime τeff in a silicon sample. For transient photoluminescence measurements, the illumination source as well as the camera signal have to be modulated on a time-scale in the order of τeff. Different approaches for camera signal modulation have been presented, including the use of a complementary metal-oxide-semiconductor (CMOS) camera or a rotating shutter wheel. In this work, the use of an InGaAs-based image intensifier unit as a fast optical shutter for TR-PLI was evaluated. Due to the fast switching times of the image intensifier, effective lifetimes down to 1/50 of the modulation period could be resolved reliably. Measurements under different illumination conditions allow for an injection-dependant analysis of τeff and comparison to photoconductance decay measurements.

Effects of Carbon–Metal–Carbon Linkages on the Optical, Photophysical, and Electrochemical Properties of Phosphametallacycle-Linked Coplanar Porphyrin Dimers

5-(Diphenylphosphanyl)-10,15,20-triarylporphyrins (meso-phosphanylporphyrins) underwent complexations with palladium(II) and platinum(II) salts to afford phosphapalladacycle- and phosphaplatinacycle-fused coplanar porphyrin dimers, respectively, via regioselective peripheral β-C-H activation of the meso-phosphanylporphyrin ligands. The optical and electrochemical properties of these metal-linked porphyrin dimers as well as their porphyrin monomer/dimer references were investigated by means of steady-state UV-vis absorption/fluorescence spectroscopy, cyclic and differential pulse voltammetry, time-resolved spectroscopy (fluorescence and transient absorption lifetimes and spectra), and magnetic circular dichroism spectroscopy. All the observed data clearly show that the palladium(II) and platinum(II) linkers play crucial roles in the electronic communication between two porphyrin chromophores at the one-electron oxidized state and in the singlet-triplet intersystem-crossing process at the excited state. It has also been revealed that the C-Pt-C linkage makes more significant impacts on these fundamental properties than the C-Pd-C linkage. Furthermore, density functional theory calculations on the metal-linked porphyrin dimers have suggested that the antibonding dπ-pπ orbital interaction between the peripherally attached metal and adjacent pyrrolic β-carbon atoms destabilizes the highest occupied molecular orbitals of the porphyrin π-systems and accounts for the observed unique absorption properties. On the basis of these experimental and theoretical results, it can be concluded that the linear carbon-metal-carbon linkages weakly but definitely perturb the optical, photophysical, and electrochemical properties of the phosphametallacycle-linked coplanar porphyrin dimers.

Effect of alicyclic ring size on the photophysical and photochemical properties of bis(arylidene)cycloalkanone compounds

A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone, C4-C7, respectively, with a D-π-A-π-D structure containing the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring size on the photophysical, photochemical and electrochemical properties of these compounds were investigated systematically. We found that an increase of the number of carbons in the central alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and photophysical and photochemical properties. These effects include decreases in the fluorescence quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum yield with the increase of the ring size. The one-photon absorption spectra, the two-photon absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with increasing ring size. The results of this study provide guidance for the design of new cycloketone-based D-π-A-π-D 2PA compounds for photopolymerization and photodynamic therapy applications.

Two-Order Targeted Brain Tumor Imaging by Using an Optical/Paramagnetic Nanoprobe across the Blood Brain Barrier

Surgical resection is a mainstay of brain tumor treatments. However, the completed excision of malignant brain tumor is challenged by its infiltrative nature. Contrast enhanced magnetic resonance imaging is widely used for defining brain tumor in clinic. However its ability in tumor visualization is hindered by the transient circulation lifetime, nontargeting specificity, and poor blood brain barrier (BBB) permeability of the commercially available MR contrast agents. In this work, we developed a two-order targeted nanoprobe in which MR/optical imaging reporters, tumor vasculature targeted cyclic [RGDyK] peptides, and BBB-permeable Angiopep-2 peptides are labeled on the PAMAM-G5 dendrimer. This nanoprobe is supposed to first target the α(V)β(3) integrin on tumor vasculatures. Increased local concentration of nanoprobe facilitates the association between BBB-permeable peptides and the low-density lipoprotein receptor-related protein (LRP) receptors on the vascular endothelial cells, which further accelerates BBB transverse of the nanoprobe via LRP receptor-mediated endocytosis. The nanoprobes that have penetrated the BBB secondly target the brain tumor because both α(V)β(3) integrin and LRP receptor are highly expressed on the tumor cells. In vivo imaging studies demonstrated that this nanoprobe not only efficiently crossed intact BBB in normal mice, but also precisely delineated the boundary of the orthotropic U87MG human glioblastoma xenograft with high target to background signal ratio. Overall, this two-order targeted nanoprobe holds the promise to noninvasively visualize brain tumors with uncompromised BBB and provides the possibility for real-time optical-image-guided brain tumor resection during surgery.

Optical Modulation and Selective Recovery of Cy5 Fluorescence

Fluorescence modulation offers the opportunity to detect low-concentration fluorophore signals within high background. Applicable from the single-molecule to bulk levels, we demonstrate long-wavelength optical depopulation of dark states that otherwise limit Cy5 fluorescence intensity. By modulated excitation of a long-wavelength Cy5 transient absorption, we dynamically modulate Cy5 emission. The frequency dependence enables specification of the dark-state timescales enabling optical-demodulation-based signal recovery from high background. These dual-laser illumination schemes for high-sensitivity fluorescence-signal recovery easily improve signal-to-noise ratios by well over an order of magnitude, largely by discrimination against background. Previously limited to very specialized dyes, our utilization of long-lived dark states in Cy5 enables selective detection of this very common single-molecule and bulk fluorophore. Although, in principle, the "dark state" can arise from any photoinduced process, we demonstrate that cis-trans photoisomerization, with its unique transient absorption and lifetime enables this sensitivity boosting, long-wavelength modulation to occur in Cy5. Such studies underscore the need for transient absorption studies on common fluorophores to extend the impact of fluorescence modulation for high-sensitivity fluorescence imaging in a much wider array of applications.

Analysis and optimization approach for the doped amorphous layers of silicon heterojunction solar cells

Comparison of the open-circuit voltage (external voltage Voc,ext) determined by Suns-Voc measurements with the implied voltage Voc,impl determined by transient photoconductance decay lifetime measurements can yield a quick and easy analysis of silicon heterojunction (SHJ) solar cells, especially in regard to finding the optimum doping concentration of the emitter layer [or back surface field (BSF)]. A sufficiently high doping concentration of the emitter and BSF is mandatory to extract the internal Fermi-level splitting and thus the internal voltage, at the solar cell contacts. However increasing the concentration of doping gases during the deposition of doped amorphous silicon layers results in a reduction of the interface passivation quality and Voc,impl. The best trade off is realized when the ratio of Voc,ext to Voc,impl (external/internal Voc-ratio ζ) reaches a saturation value near 1 upon increasing the doping concentration. AFORS-HET (Automat FOR Simulation of HETerostructures) simulations resulted in the conclusion that the characteristics of the external/internal Voc-ratio are mainly determined by the active doping concentration (doping minus defect concentration).

Unit mass baseline resolution for an intact 148 kDa therapeutic monoclonal antibody by Fourier transform ion cyclotron resonance mass spectrometry.

Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) provides the highest mass resolving power and mass measurement accuracy for unambiguous identification of biomolecules. Previously, the highest-mass protein for which FTICR unit mass resolution had been obtained was 115 kDa at 7 T. Here, we present baseline resolution for an intact 147.7 kDa monoclonal antibody (mAb), by prior dissociation of noncovalent adducts, optimization of detected total ion number, and optimization of ICR cell parameters to minimize space charge shifts, peak coalescence, and destructive ion cloud Coulombic interactions. The resultant long ICR transient lifetime (as high as 20 s) results in magnitude-mode mass resolving power of ~420,000 at m/z 2,593 for the 57+ charge state (the highest mass for which baseline unit mass resolution has been achieved), auguring for future characterization of even larger intact proteins and protein complexes by FTICR MS. We also demonstrate up to 80% higher resolving power by phase correction to yield an absorption-mode mass spectrum.

Decaying turbulence: What happens when the correlation length varies spatially in two adjacent zones

We have imagined a numerical experiment to explore the onset of turbulent intermittency associated with a spatial perturbation of the correlation length. We place two isotropic regions, with different integral scales, inside a volume where the turbulent kinetic energy is initially uniform and leave them to interact and evolve in time. The different length scales produce different decay rates in the two regions. Since the smaller-scale region decays faster, a transient turbulent energy gradient is generated at the interface between the two regions. The transient is characterized by three phases in which the kinetic energy gradient across the interface grows, peaks and then slowly decays. The transient lifetime is almost proportional to the initial ratio of the correlation lengths. The direct numerical simulations also show that the interface width grows in time. The velocity moments inside this interaction zone are seen to depart from their initial isotropic values and, with a certain lag, the anisotropy is seen to spread to small scales. The longitudinal derivative moments also become anisotropic after a few eddy turnover times. This anisotropic behaviour is different from that observed in sheared homogeneous turbulent flows, where high transverse derivative moments are generated, but longitudinal moments almost maintain the isotropic turbulence values. Apart from the behaviour of the energy gradient transients, the results also show the timescaling of the interface diffusion width, and data on the anisotropy of the large and small scales, observed through one-point statistics determined inside the intermittency sublayer, which is associated with the interaction zone.

Coordination Mode Dependent Excited State Behavior in Group 8 Phosphino(terthiophene) Complexes

The ground and excited state behavior of four Ru(II) and Os(II) bipyridyl complexes containing the 3'-(diphenylphosphino)-2,2':5',2''-terthiophene (PT(3)) ligand in two different coordination modes (P,S and P,C) is reported. The complexes are generally stable under extended photoirradiation, except for [Ru(bpy)(2)PT(3)-P,S](PF(6))(2) which decomposes. Emission lifetimes and transient absorption spectra and lifetimes have been obtained for all the complexes. These data support a PT(3) ligand based lowest excited state in the case of both P,S bound complexes, and a charge separated lowest excited state in both P,C bound complexes, conclusions supported by Density Functional Theory (DFT) calculations.

Formalizing Game-play

Current computer conflict simulation games, or wargames, are opaque in the sense that most of the game mechanisms are not directly visible to the players and are frequently not described in user accessible documentation, have a transient lifetime that is mainly shaped by the evolution of graphics hardware and processor speed, and do not, in contrast with, for example, the well-known abstract board games CHESS and GO, have the technical prerequisites for critical intellectual discussion that the thought-intensive and knowledge-rewarding character of these games seems to warrant. The main reason for this state of affairs is that many of the mechanisms of the games, and in particular the details of how the game state changes over time, are directly expressed in computer code. This is purely a technical problem, and it has a straightforward solution, namely, treating this information as data by creating a formalism for describing not just the game map and playing pieces but also all the game rules including the “sequence of play.” The article suggests such a formalism and shows a complete specification of a simple, but complicated enough for present purposes, “introductory” board wargame. This formalism, with tools that support it, can provide an unambiguous authoritative definition of the rules, accessible by both human and computer players; would allow existing board wargames to be played on a computer, without any simplifications or sacrifices of rule details; and may allow construction of more advanced computer players, since a complete formal specification of the game rules is available as input to them.

Optical bistability in mesoporous silicon microcavity resonators

We report on the observation of significant optical bistability in the transmission and reflection properties of mesoporous silicon microcavities when illuminated with a 150 ns pulsed laser at 532 nm. The observed optical hysteresis is shown to be transient in nature and the properties are strongly dependent on the porosity of the cavity layer. The onset and damage threshold intensity are also shown to be porosity dependent. Our modeling suggests that the observed effects are due to changes in the nonlinear refractive index where the transient lifetime increases with increasing porosity. We investigate the role of surface states on influencing the bistable process by passivating the internal porous surface with hydrosilylation chemistry.

Charge carrier trapping at passivated silicon surfaces

Surface passivation has become an essential factor for translating high-efficiency crystalline silicon solar cell concepts into industrial production schemes. In photovoltaics, a widespread method to determine the surface recombination is to measure the effective charge carrier lifetime from the photoconductance of symmetrically passivated silicon wafers in transient or quasi-static mode. In this work, it is shown how the injection history at the surface influences the transient effective lifetime measurement for several passivation layers. This dependence leads to systematic differences between quasi-static and transient measurements. The influence can be explained by charge trapping in slow surface states at the surface passivation layer. A model including slow surface states is used in a fit procedure to evaluate the capture cross sections of these traps. The observed effect is well-pronounced for silicon-rich passivation layers like a-Si:H, a-SiCx:H or silicon-rich a-SiOxNy:H. For PECVD-AlOx layers and for thermally grown SiO2 layers as well, however, the effective lifetime measurement could be influenced.

Light-Induced Antibacterial Activity of Symmetrical and Asymmetrical Oligophenylene Ethynylenes

The light-induced antibacterial activity of symmetric and asymmetric oligophenylene ethynylenes (OPEs) was investigated against Gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) and Gram-negative (Escherichia coli) bacteria. To understand the light-induced biocidal effect better, the transient absorption and triplet lifetime of OPEs were studied in methanol and water. A higher triplet lifetime was observed for OPE samples in water than in methanol. The magnitudes of the changes in optical density (ΔOD) of the S-OPE-n(H) series of symmetric oligomers are much higher than that of the asymmetric OPE-n series in water and are generally correlated with the singlet oxygen yield. It was found that the antibacterial activity against both Gram-positive and Gram-negative bacteria is size-, concentration-, and time-dependent. The light-induced antibacterial activity may result from the coordinated interactions of membrane disruption and interfacial or intracellular singlet oxygen generation, and the dominant factor is most likely the latter. The results obtained in this study will aid in the design of more efficient biocides in the future.

Single-Molecule Fluorescence Photoswitching of a Diarylethene−Perylenebisimide Dyad: Non-destructive Fluorescence Readout

Single-molecule fluorescence photoswitching plays an essential role in ultrahigh-density (Tbits/inch(2)) optical memories and super-high-resolution fluorescence imaging. Although several fluorescent photochromic molecules and fluorescent proteins have been applied, so far, to optical memories and super-high-resolution imaging, their performance is unsatisfactory because of the absence of "non-destructive fluorescence readout capability". Here we report on a new molecular design principle of a molecule having non-destructive readout capability. The molecule is composed of acceptor photochromic diarylethene and donor fluorescent perylenebisimide units. The fluorescence is reversibly quenched when the diarylethene unit converts between the open- and the closed-ring isomers upon irradiation with visible and UV light. The fluorescence quenching is based on an electron transfer from the donor to the acceptor units. The fluorescence photoswitching and non-destructive readout capability were demonstrated in solution (an ensemble state) and at the single-molecule level. Femtosecond time-resolved transient and fluorescent lifetime measurements confirmed that the fluorescence quenching is attributed to the intramolecular electron transfer.

The 2-benzoyl xanthone/triethylamine system as a type II photoinitiator: A laser flash photolysis and computational study

2-Benzoylxanthone ( BzX) was synthesized, characterized and used as type II photoinitiator ( PI) in combination with triethylamine for the polymerization of methylmethacrylate ( MMA). The photophysical/photochemical behaviour of the photoinitiator, the involved excited state and the reaction with the amine co-initiator was studied by means of absorption and nanosecond time-resolved absorption spectroscopy. Upon irradiation with 266 or 355 nm laser light, the triplet state 3 BzX* ( λ max = 355 nm and 530 nm) was generated as the only transient (lifetime of 22.7 μs) in nitrogen saturated MeCN solution. 3 BzX* was confirmed through quenching experiments with oxygen, 2-methylbutadiene, perylene and MMA and spectral similarity to benzophenone triplet ( 3 BP*). The quantum yield of its formation ( Φ T = 0.8) and the molar absorption coefficient ( ɛ = 7500 L mol −1 cm −1) was measured in MeCN. The triplet was photoreduced by triethylamine (TEA) via photoinduced electron/proton transfer giving the corresponding ketyl and α-amino ethyl radical ( CHMe–NEt 2). From the reduction potential of BzX measured via cyclic voltametry (two cathodic peaks at −1.63 V and −1.91 V vs. Ag/AgCl), an exergonic electron transfer reaction results. The ketyl radical resembles the well-known benzophenone ketyl. In conclusion, the triplet 3 BzX* corresponds to an n → π* transition localized on the benzoyl substituent and resembles the benzophenone triplet 3 BP* but deviates from the triplet state of xanthone ( 3 X*). This is supported through DFT/B3LYP calculations, viz., (i) fully ground and triplet state geometry optimizations show that charge and spin densities are localized on the benzoyl group, and (ii) calculation of the electronic transitions via TD-DFT at B3LYP/631G+(d) and PB1BPE/631G+(d) levels of theory shows the local character. Agreement with experiment is better by applying the conductor like polarized continuum model (CPCM) to consider the solvent effect (MeCN). The effectiveness of BzX as photoinitiator for the polymerization of MMA is found to be double that of the unsubstituted xanthone ( X). The photopolymerization rates ( R p ) were found to be 7.06 × 10 −4 mol L −1 s −1 in the case of BzX and 3.04 × 10 −4 mol L −1 s −1 in the case of BX. This is attributed to the fact that triplet 3 BzX* behaves like the benzophenone triplet 3 BP* and deviates from that of 3 X*, i.e., it is the localization of the triplet excitation on the benzoyl subunit which renders BzX a good type- II photoinitiator.

Influence of Slow Surface States on Effective Lifetime Measurements

Abstract In a recent work, the effect of surface states following a change of the Fermi level with a characteristic relaxation time τ relax on transient effective charge carrier lifetime measurements has been described. The working condition for solar cells, however, is a steady-state low-injection at the passivated surface. In the work presented here, the effect of such surface states in steady-state low-injection conditions is discussed and the considerations are compared with quasi-steady-state lifetime measurements on a sample exhibiting such surface states. Careful comparison of transient and quasi-steady-state lifetime measurements leads to an evaluation of the characteristic relaxation time of the surface traps.

Statistics of rupture in relation to the stability of thin liquid films with different size

We study the stability and the thinning behavior of foam films in the presence of ionic surfactant. Using the method of capillary cell at low pressures (20–50 Pa), combined with interference microscopy, we measured the film thickness vs. time, the critical thickness of rupture, and the lifetime of films with different sizes. The film behavior is characterized by two separate stages: at first, there is a hydrodynamic thinning without rupture; afterwards, film rupture occurs as a stochastic process. The measured lifetimes in an ensemble of films are scattered in a certain range. We found that this statistical behavior is well described by a specific distribution, with cumulative probability 1 − exp(− βt 2/2). By means of theoretical considerations, this distribution is derived from the time dependent differential probability for film rupture. Fitting of experimental data for the statistics of the film lifetimes is performed. This permits one to find the average transient lifetime at the stochastic stage; thus, thinning and rupture are distinguished. The size dependence of the drainage time is shown to comply with theories which describe the behavior of films with irregular thickness. The time for thinning, and the inverse drainage rate coefficient, scale with the film radius as r f 4 / 5 . This work may be relevant to understanding the stability of fluid dispersions in dependence of the particle size.