Double Exponential Fit Research Articles

Endogenous SNAP-25 Regulates Native Voltage-gated Calcium Channels in Glutamatergic Neurons

In addition to its primary role as a fundamental component of the SNARE complex, SNAP-25 also modulates voltage-gated calcium channels (VGCCs) in various overexpression systems. Although these studies suggest a potential negative regulatory role of SNAP-25 on VGCC activity, the effects of endogenous SNAP-25 on native VGCC function in neurons are unclear. In the present study, we investigated the VGCC properties of cultured glutamatergic and GABAergic rat hippocampal neurons. Glutamatergic currents were dominated by P/Q-type channels, whereas GABAergic cells had a dominant L-type component. Also, glutamatergic VGCC current densities were significantly lower with enhanced inactivation rates and shifts in the voltage dependence of activation and inactivation curves compared with GABAergic cells. Silencing endogenous SNAP-25 in glutamatergic neurons did not alter P/Q-type channel expression or localization but led to increased VGCC current density without changes in the VGCC subtype proportions. Isolation of the P/Q-type component indicated that increased current in the absence of SNAP-25 was correlated with a large depolarizing shift in the voltage dependence of inactivation. Overexpressing SNAP-25 in GABAergic neurons reduced current density without affecting the VGCC subtype proportion. Accordingly, VGCC current densities in glutamatergic neurons from Snap-25(+/-) mice were significantly elevated compared with wild type glutamatergic neurons. Overall, this study demonstrates that endogenous SNAP-25 negatively regulates native VGCCs in glutamatergic neurons which could have important implications for neurological diseases associated with altered SNAP-25 expression.

Free Radical-Induced Redox Chemistry of Nedaplatin and Satraplatin under Physiological Conditions

Temperature-dependent kinetics for the reactions of hydroxyl radicals and hydrated electrons with the anti-cancer drug nedaplatin have been determined using a combination of electron pulse radiolysis and absorption spectroscopy. Under physiological pH and chloride concentrations, the kinetics was well described by the equations [Formula: see text]and [Formula: see text]corresponding to Arrhenius activation energies of 15.88 +/- 1.16 and 14.14 +/- 1.41 kJ mol(-1) for hydroxyl radical oxidation and hydrated electron reduction, respectively. Through a comparison of spectral and kinetic literature it is believed that the oxidation reaction gives predominantly an intermediate Pt(III) species, whereas reduction gives a Pt(I) moiety. Analogous hydrated electron measurements for the Pt(IV) drug satraplatin showed multiple-component decays at higher temperatures (>20 degrees C), indicating that significant thermal degradation of this chemical occurs. From double-exponential curve fitting, the satraplatin reduction kinetics was found to be well described by the equation [Formula: see text]giving an activation energy of 22.78 +/- 1.78 kJ mol(-1) for this reaction. This measured temperature dependence was consistent with several model Pt(IV) compounds also investigated in this study, with all these data suggesting that the metal ion reduction to give Pt(III) was the dominant reaction occurring.

Optical Properties of Fluorescein‐labeled Organoclay

We report the preparation of aminopropyl-functionalized magnesium phyllosilicate (Mg-organoclay) conjugated to fluorescein isothiocyanate (FITC) functionality (FITC-organoclay) by one-pot sol-gel synthesis. The physical characteristics of the Mg- and FITC-organoclays were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), FT-IR and UV-Vis spectrophotometry. X-ray fluorescence and elemental analysis were conducted to confirm the composition of the Mg-organoclay. The FITC-organoclay particles were polydispersed with an average particle size of approximately 50 nm, as determined from SEM images. The XRD patterns of FITC-organoclay exhibited broad peaks and reduced basal d(001) indicating a less condensed and more disordered structure than was observed for Mg-organoclay. The conjugation between FITC- and Mg-organoclay was characterized by FT-IR spectroscopy. Fluorescence excitation and emission spectral data demonstrated the successful conjugation of FITC dye molecules to the Mg-organoclay. The time-resolved fluorescence measurements revealed that FITC had a lifetime of 4.58 ns, whereas the lifetime of FITC-organoclay required a double exponential fit (tau(1,2) = 0.72 and 2.68 ns). As a result, the lifetime of the FITC-organoclay was shorter than that of FITC in ethanol and indicated moderate photostability in the solution state. The cellular uptake of FITC-organoclay in human lung alveolar carcinoma epithelial cells (A549) was quantified and characterized.

Luminescence spectroscopy of matrix-isolated atomic manganese: Site size and orbital occupancy dependence of crystal field splitting

Narrow linewidth emission features observed in the near-UV following y (6)P state excitation of atomic manganese isolated in the solid rare gases are assigned to b (4)D and a (4)P states. These states arise from the 3d(5)4s(2) electronic configuration, identical to that of the (6)S ground state, and the origin of the narrow linewidths. Two thermally stable sites, labeled blue and red on the basis of their position in absorption spectra, are occupied by atomic Mn in Ar and Kr while a single site is present in Xe. The red site produces a single, narrow line emission for the b (4)D state at 329 nm. In contrast, a lineshape analysis of the complex blue site b (4)D state emission between 331 and 332 nm reveals the occurrence of three zero phonon lines (ZPLs). Millisecond emission decay curves recorded for these features are found to be complex, requiring double and triple exponential fit functions. The origins of the complex decays and multiple ZPLs are shown to arise from weak crystal field splitting (CFS) of the J=7/2 spin-orbit level of the b (4)D state of atomic Mn isolated in the blue site of the solid rare gases. Fields of cubic symmetry are capable of inducing splitting for J>3/2 so atoms isolated in both single vacancy and tetravacancy sites in the fcc lattices of the solid rare gases are prone to this effect. b (4)D state emission is also produced following y (6)P excitation for Mn atoms occupying the red sites in Ar and Kr. However, Mn atoms isolated in the larger tetravacancy sites have small matrix shifts and do not exhibit any CFS. The magnitudes of the weak CF splittings are shown to depend on both the excited state electronic configurations 3d(5)4s(2) b (4)D and 3d(6)4s(1) a (4)D states and the size of the matrix site occupied by atomic Mn.

Effect of Lipid Type on the Binding of Lipid Vesicles to Islet Amyloid Polypeptide Amyloid Fibrils

Amyloid deposits, composed primarily of the 37-residue islet amyloid polypeptide (IAPP), are observed near pancreatic beta-cells of type II diabetics, with their presence strongly correlating with a loss of beta-cell mass and decreased pancreatic function. Although beta-cell membranes have been implicated as the likely target of amyloidogenic IAPP toxicity, interactions between membranes and IAPP in the fibrillar state have not been well characterized. In this study, turbidity measurements were conducted to provide a detailed description of the binding reaction between IAPP fibrils and lipid vesicles made from phosphatidylcholine. The kinetic data representing the rate and extent of the fibril-vesicle binding reaction are described well by an empirical double-exponential equation. The extent of binding was found to increase with increasing amyloid fibril concentration. Modification of the vesicle composition significantly altered the observed binding reaction kinetics, with the change quantified using the parameters obtained from the double-exponential fitting analysis. When the vesicles contained a significant amount of the lipid phosphatidylglycerol, substantial sedimentation of the vesicles under gravity was detected following the initial binding reaction. To rationalize the observed kinetic binding data, we developed a mesoscopic simulation model based on a hard particle representation of the species involved. In light of the observed data and simulation predictions, the potential roles of IAPP amyloid fibrils in membrane binding are discussed.

Structural aging and stiction dynamics in confined liquid films

The static friction (stiction) of the molecularly thin films of an irregularly shaped molecule 1,3-dimethylbutyl octyl ether (DBOE) confined between mica surfaces was investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P. The results show two relaxation processes, one on stopping and one on starting, where each process has a fast and a slow time constant. For stopping mode, there is no stiction spike when t is shorter than a characteristic nucleation time, tau(n) (fast time constant). When t exceeds tau(n), stiction spike appears whose height increases logarithmically with t. With regard to starting, the relaxation behavior was evaluated by a double exponential fit of the slipping regime (force decay) of the spike and two time constants (tau(1) and tau(2)) were obtained. The fast time constant on starting tau(1) is almost equal to that on stopping tau(n). To the best of our knowledge, this is the first direct observation of the agreement of the time constant on stopping and that on starting, indicative of a reversible structural transition (solid-liquid transition) in the stop-start stiction dynamics. The two fast time constants exhibit exponential dependence on P, which implies a glasslike nature of the transition. Comparison with the stick-slip friction reveals that the solid-liquid transition involved in stiction and that in stick-slip dynamics is different for DBOE; first-order-like discontinuous transition is suggested for stick-slip friction. Origins of the different solid-liquid transition dynamics in stiction and in stick-slip friction are discussed by comparing with the dynamics of other confined liquid systems.

Structural relaxation of SiO2 at elevated temperatures monitored by in situ Raman scattering

Raman scattering spectra of high-purity synthetic silica glass were measured in situ in the temperature range from 950 to 1200 °C by means of an experimental approach which gives very low thermal background. The temperature dependence of the scattering permits an analysis of the spectra into the first-order and overtone contributions in a consistent manner. For low-OH content the dynamics of relaxation of the D2 defect follow a single exponential decay, but more complex relaxation is found when OH content is high. In the latter case a double-exponential fit describes the observed relaxation well. The activation energies found are: for creation of D2 defects 0.53 ± 0.07 eV; for single exponential relaxation in low-OH material, 6.0 ± 0.3 eV; for high-OH material, primary relaxation 5.1–5.2 ± 0.3 eV, secondary relaxation 2.4–2.5 ± 0.5 eV.

Recombination dynamics of deep defect states in zinc oxide nanowires

The recombination dynamics of defect states in zinc oxide nanowires has been studied bydeveloping a general expression for time-resolved photoluminescence intensity based on asecond-order approximation for the radiative and non-radiative recombination rates. Themodel allows us to determine the parameters that characterize the recombination fromdeep defect states (defect concentration, unimolecular lifetime and bimolecular coefficient)through multi-fitting analysis of time-resolved photoluminescence measurements. Analysesconducted on zinc oxide nanowires gave deep state concentrations of the order of1018 cm−3 and unimolecular lifetimes and bimolecular recombination coefficient comparable to thosetypical of interband recombination in direct gap semiconductors. The consistency of a‘two-channel decay’ model (double exponential decay) has been tested by means of asimilar analysis procedure. The results suggest that double exponential fitting oftime-resolved photoluminescence data of zinc oxide nanowires may be just a merephenomenological tool which does not reflect the real recombination dynamics of the visibleemission band.

Fluorescence Lifetimes of Tyrosine Residues in Cytochrome c′′ as Local Probes to Study Protein Unfolding

Time-resolved fluorescence spectroscopy was used to show that multiple tyrosine residues of a protein can serve as localized probes of structural changes during thermal unfolding. Cytochrome c'' from Methylophilus methylotrophus, which has four tyrosine residues, was chosen as a model protein. The procedure involved, first, the assignment of the experimental decay times to the tyrosine residues, followed by the interpretation of the changes in the decay times and pre-exponential coefficients with temperature. We found that the fluorescence decays of cytochrome c'' are double-exponential from 23 to 80 degrees C, with decay times much shorter than those of the parent compound N-acetyl-tyrosinamide; this quenching was ascribed to dipole-dipole energy transfer from the tyrosine residues to the heme. The tyrosine-heme distances (R) and theoretical decay times, tau(comp), were estimated for each tyrosine residue. The analysis of the simulated decay generated with tau(comp), showed that a double-exponential fit is sufficient to describe the four decay times with two pre-exponential coefficients close to values observed from the experimental decay. Therefore, the decay times at 23 degrees C could be assigned to the individual tyrosine residues as tau(1) to Tyr-10 and Tyr-23 (at 20.3 A) and tau(2) to Tyr-12 and Tyr-115 (at 12-14 A). On the basis of this assignment and MD simulations, the temperature dependence of the decay times and pre-exponential coefficients suggest that upon unfolding, Tyr-12 is displaced from the heme, with loss of the structure of alpha-helix I. Moreover, Tyr-115 remains close to the heme and the structure in this region of the protein is not altered significantly. Altogether the data support the view that the protein core, comprising the heme and the four alpha-helices II to V, is clearly more stable than the remaining region that includes alpha-helix I and the loop between residues 19-27.

Effects of Eu[sup 2+] Co-Doping on VUV Photoluminescence Properties of BaMgAl[sub 10]O[sub 17]:Mn[sup 2+] Phosphors for Plasma Display Panels

The effects of co-doping were examined on vacuum ultraviolet (VUV) photoluminescence properties of phosphors. The phosphors were synthesized with various Eu concentrations ranging from 0 to 2 atom %. When Eu ions were added, the blue luminescence appeared with the decrease in the green luminescence. With increasing Eu concentration, the decay time gradually decreased from (0 atom %) to (2 atom %). The energy-transfer efficiencies were analyzed with the average-decay time acquired by a double-exponential fitting, and the 2 atom % Eu sample showed the energy-transfer efficiency of 19%.

INTERRESPONSE TIME STRUCTURES IN VARIABLE‐RATIO AND VARIABLE‐INTERVAL SCHEDULES

The interresponse-time structures of pigeon key pecking were examined under variable-ratio, variable-interval, and variable-interval plus linear feedback schedules. Whereas the variable-ratio and variable-interval plus linear feedback schedules generally resulted in a distinct group of short interresponse times and a broad distribution of longer interresponse times, the variable-interval schedules generally showed a much more continuous distribution of interresponse times. The results were taken to indicate that a log survivor analysis or double exponential fit of interresponse times may not be universally applicable to the task of demonstrating that operant behavior can be dichotomized into bouts of engagement and periods of disengagement.

Smooth Muscle Myosin Phosphorylated at Single Head Shows Sustained Mechanical Activity

Smooth muscle contraction is regulated by the phosphorylation of myosin. It is well known that tonic smooth muscles can maintain force with low energy consumption (latch state); however, the molecular mechanism underlying this phenomenon is unresolved. Here we show that single-head phosphorylated smooth myosin (SHPMII) exhibits fast ( approximately 24 s(-1)) and slow prolonged ( approximately 1 s(-1)) actin interactions, whereas double-head phosphorylated myosin (DHPMII) predominantly exhibits the fast ( approximately 29 s(-1)) interaction, suggesting that the phosphorylated head of SHPMII is mechanically as active as that of DHPMII. Both the fast and the slow actin interactions of SHPMII support the positive net mechanical displacement of actin. The actin translocating velocity of SHPMII was much slower than that of DHPMII, which is consistent with the slow actin interaction of SHPMII. We propose that the "latch state" can be explained by the motor characteristics of SHPMII that is present during the sustained phase of contraction.

Kinetic Analysis of Interaction of Eukaryotic Release Factor 3 with Guanine Nucleotides

Eukaryotic translation termination is mediated by two release factors: eRF1 recognizes stop codons and triggers peptidyl-tRNA hydrolysis, whereas eRF3 accelerates this process in a GTP-dependent manner. Here we report kinetic analysis of guanine nucleotide binding to eRF3 performed by fluorescence stopped-flow technique using GTP/GDP derivatives carrying the fluorescent methylanthraniloyl (mant-) group, as well as thermodynamic analysis of eRF3 binding to unlabeled guanine nucleotides. Whereas the kinetics of eRF3 binding to mant-GDP is consistent with a one-step binding model, the double-exponential transients of eRF3 binding to mant-GTP indicate a two-step binding mechanism, in which the initial eRF3.mant-GTP complex undergoes subsequent conformational change. The affinity of eRF3 for GTP (K(d), approximately 70 microM) is about 70-fold lower than for GDP (K(d), approximately 1 microM) and both nucleotides dissociate rapidly from eRF3 (k(-1)(mant-GDP) approximately 2.4 s(-1); k(-2)(mant-GTP) approximately 3.3 s(-1)). Whereas not influencing eRF3 binding to GDP, association of eRF3 with eRF1 at physiological Mg(2+) concentrations specifically changes the kinetics of eRF3/mant-GTP interaction and stabilizes eRF3.GTP binding by two orders of magnitude (K(d) approximately 0.7 microM) due to lowering of the dissociation rate constant approximately 24-fold (k(-1)(mant-GTP) approximately 0.14s(-1) approximately 0.14 s(-1)). Thus, eRF1 acts as a GTP dissociation inhibitor (TDI) for eRF3, promoting efficient ribosomal recruitment of its GTP-bound form. 80 S ribosomes did not influence guanine nucleotide binding/exchange on the eRF1 x eRF3 complex. Guanine nucleotide binding and exchange on eRF3, which therefore depends on stimulation by eRF1, is entirely different from that on prokaryotic RF3 and unusual among GTPases.

Loss of CNGB1 Protein Leads to Olfactory Dysfunction and Subciliary Cyclic Nucleotide-gated Channel Trapping

Olfactory receptor neurons (ORNs) employ a cyclic nucleotide-gated (CNG) channel to generate a receptor current in response to an odorant-induced rise in cAMP. This channel contains three types of subunits, the principal CNGA2 subunit and two modulatory subunits (CNGA4 and CNGB1b). Here, we have analyzed the functional relevance of CNGB1 for olfaction by gene targeting in mice. Electro-olfactogram responses of CNGB1-deficient (CNGB1-/-) mice displayed a reduced maximal amplitude and decelerated onset and recovery kinetics compared with wild-type mice. In a behavioral test, CNGB1-/- mice exhibited a profoundly decreased olfactory performance. Electrophysiological recordings revealed that ORNs of CNGB1-/- mice weakly expressed a CNG current with decreased cAMP sensitivity, very rapid flicker-gating behavior and no fast modulation by Ca2+-calmodulin. Co-immunoprecipitation confirmed the presence of a CNGA2/CNGA4 channel in the olfactory epithelium of CNGB1-/- mice. This CNGA2/CNGA4 channel was targeted to the plasma membrane of olfactory knobs, but failed to be trafficked into olfactory cilia. Interestingly, we observed a similar trafficking defect in mice deficient for the CNGA4 subunit. In conclusion, these results demonstrate that CNGB1 has a dual function in vivo. First, it endows the olfactory CNG channel with a variety of biophysical properties tailored to the specific requirements of olfactory transduction. Second, together with the CNGA4 subunit, CNGB1 is needed for ciliary targeting of the olfactory CNG channel.

Aurora Kinase Promotes Turnover of Kinetochore Microtubules to Reduce Chromosome Segregation Errors

Merotelic kinetochore orientation is a misattachment in which a single kinetochore binds microtubules from both spindle poles rather than just one and can produce anaphase lagging chromosomes, a major source of aneuploidy. Merotelic kinetochore orientation occurs frequently in early mitosis, does not block chromosome alignment at the metaphase plate, and is not detected by the spindle checkpoint. However, microtubules to the incorrect pole are usually significantly reduced or eliminated before anaphase. We discovered that the frequency of lagging chromosomes in anaphase is very sensitive to partial inhibition of Aurora kinase activity by ZM447439 at a dose, 3 microM, that has little effect on histone phosphorylation, metaphase chromosome alignment, and cytokinesis in PtK1 cells. Partial Aurora kinase inhibition increased the frequency of merotelic kinetochores in late metaphase, and the fraction of microtubules to the incorrect pole. Measurements of fluorescence dissipation after photoactivation showed that kinetochore-microtubule turnover in prometaphase is substantially suppressed by partial Aurora kinase inhibition. Our results support a preanaphase correction mechanism for merotelic attachments in which correct plus-end attachments are pulled away from high concentrations of Aurora B at the inner centromere, and incorrect merotelic attachments are destabilized by being pulled toward the inner centromere.

시분해 레이저 유도 백열법을 이용한 매연 입자 크기에 관한 수치적 연구

Temporal behavior of the laser induced incandescence (LII) signal is often used for soot particle sizing, which is possible because the cooling behavior of a laser heated particle is dependent on the particle size. In present study, LII signals of soot particles are modeled using two non-linear coupled differential equations deduced from the energy- and mass-balance of the process. The objective of this study is to obtain an appropriate calibration curve for determining primary particle size by comparing the gated signal ratio and double-exponential curve fitting methods. Not only the effects of laser fluence and gas temperature on the cooling behavior but also heat transfer mechanisms of heated soot particle have been investigated. The second-order exponential curve fitting showed better agreements with the LII signals than the gated signal ratio method which was based on the lust-order exponential curve fit. And the temporal decay rate of the LII signal and primary particle size showed nearly linear relationship, which was little dependent on the laser fluence. And it also could be reconfirmed that vaporization was dominant process of heat loss during first loons after laser pulse, then heat conduction played most important role while thermal radiation had little influence all the time.

Production of K0s, ϕ, Λ and Ξ from 200 GeV d+Au collisions at RHIC

We present the preliminary measurement of K0s, ϕ, Λ and Ξ transverse momentum (pt) distribution from GeV d+Au collisions at RHIC. The measured spectra can be described by using a double exponential fit. The centrality dependence of the particle yield with respect to the number of binary collision scaling (RCP) will be presented for a pt up to 6 GeV/c. The particle dependence of the RCP among K0s, ϕ, Λ and Ξ is consistent with expectations from parton recombination models for hadron formation. The Cronin effect and its particle dependence appear not due to initial parton scatterings alone as previously assumed in models.

Kinetics of muscle contraction and actomyosin NTP hydrolysis from rabbit using a series of metal–nucleotide substrates

Mechanical properties of skinned single fibres from rabbit psoas muscle have been correlated with biochemical steps in the cross-bridge cycle using a series of metal-nucleotide (Me.NTP) substrates (Mn(2+) or Ni(2+) substituted for Mg(2+); CTP or ITP for ATP) and inorganic phosphate. Measurements were made of the rate of force redevelopment following (1) slack tests in which force recovery followed a period of unloaded shortening, or (2) ramp shortening at low load terminated by a rapid restretch. The form and rate of force recovery were described as the sum of two exponential functions. Actomyosin-Subfragment 1 (acto-S1) Me.NTPase activity and Me.NDP release were monitored under the same conditions as the fibre experiments. Mn.ATP and Mg.CTP both supported contraction well and maintained good striation order. Relative to Mg.ATP, they increased the rates and Me.NTPase activity of cross-linked acto-S1 and the fast component of a double-exponential fit to force recovery by approximately 50% and 10-35%, respectively, while shortening velocity was moderately reduced (by 20-30%). Phosphate also increased the rate of the fast component of force recovery. In contrast to Mn(2+) and CTP, Ni.ATP and Mg.ITP did not support contraction well and caused striations to become disordered. The rates of force recovery and Me.NTPase activity were less than for Mg.ATP (by 40-80% and 50-85%, respectively), while shortening velocity was greatly reduced (by approximately 80%). Dissociation of ADP from acto-S1 was little affected by Ni(2+), suggesting that Ni.ADP dissociation does not account for the large reduction in shortening velocity. The different effects of Ni(2+) and Mn(2+) were also observed during brief activations elicited by photolytic release of ATP. These results confirm that at least one rate-limiting step is shared by acto-S1 ATPase activity and force development. Our results are consistent with a dual rate-limitation model in which the rate of force recovery is limited by both NTP cleavage and phosphate release, with their relative contributions and apparent rate constants influenced by an intervening rapid force-generating transition.

Fluorescence spectroscopic studies to characterize the internal environment of tetraethyl-orthosilicate derived sol–gel bulk and thin films with aging

Characterization of the internal environment of a sol–gel matrix is an important area of investigation in optical biosensors. In the present study, different sol–gel compositions were prepared by varying the water (H 2O) to tetraethyl-orthosilicate (TEOS) ratio ( R) from 1 to 16 and the changes in the internal environment of the sol–gel both in bulk and thin films as a function of aging (storage) were investigated using fluorescence spectroscopy. We focussed on the fluorescence characteristics , viz. emission and excited state lifetime of Hoechst 33258 (H258), a bisbenzimidazole derivative, which was used as fluorescence probe entrapped in the TEOS derived sol–gel bulk and thin films. These sols were prepared at a low pH (∼2.0) and the thin films were coated by dip coating technique at withdrawal speeds of 1 cm/min and 0.1 cm/min. Usually, uniform thin films were obtained at a high speed (1 cm/min) and partially cracked film at a low speed (0.1 cm/min) as observed by fluorescence microscope. These observations did not change during aging. On the contrary, three months long observations on steady-state fluorescence emission measurements on H258 depicted a blue shift from 535 nm to 508 nm at R = 1 in the sol–gel bulk, whereas at higher ratios this was not prominent. At all ratios, dual emission bands were observed in thin films. This may be due to faster sol–gel to xerogel transition during aging depending on the ratio ( R). Analysis of the excited state decay profiles of H258 revealed a double exponential fitting having a short ( τ 1) and a long ( τ 2) component in both fresh and during aging, in the sol–gel bulk and thin films, indicating heterogeneity in the internal environment. The value of τ 1 increased from 0.4 ns to 1.2 ns whereas τ 2 attained a value from 3.0 ns to 3.6 ns at R = 1 upon aging in the sol–gel bulk. The corresponding values of τ 1 and τ 2 in thin films were 0.3 ns and 3.5 ns, respectively. The values of these decay components in thin films did not alter much due to storage, but their relative contributions showed more systematic changes in the thin films. The observed changes could be correlated to rigidification in the bulk depending on the ratio ( R). This process was very slow at R ≥ 4. The heterogeneity in the internal environment of bulk and thin films upon aging appeared to be different as revealed from analysis of excited-state lifetime. Thus, the bisbenzimidazole derivative H258 appears to be very useful probe for characterizing the internal environment of both the sol–gel bulk and thin films.

Optical absorption and photoluminescence in the defect-chalcopyrite-type semiconductorZnIn2Te4

Optical absorption and photoluminescence (PL) spectra have been measured on the defect-chalcopyrite-type semiconductor ${\mathrm{ZnIn}}_{2}{\mathrm{Te}}_{4}$ in the 1.1--1.6 eV photon-energy range at temperatures between 11 and 300 K. The temperature dependence of the direct-gap energy of ${\mathrm{ZnIn}}_{2}{\mathrm{Te}}_{4}$ has been determined from the optical absorption spectra and fit using the Varshni equation and an analytical four-parameter expression developed for the explanation of the band-gap shrinkage effect in semiconductors. The PL spectra show an asymmetric emission band at $\ensuremath{\sim}1.4\mathrm{eV},$ which is attributed to donor-acceptor pair recombination between quasicontinuously distributed donor states and acceptor levels. At temperatures above 90 K, the band-to-band emission begins to appear at the high-energy tail of the donor-acceptor pair recombination. A double-exponential fit analysis of the PL spectra suggests an acceptor level of 64 meV and an unidentified shallow level of 9 meV. An energy-band scheme has been proposed for the explanation of PL emission in the defect-chalcopyrite-type semiconductors.