Induced Charge Transfer Research Articles

Dual-Signaling Fluorescent Chemosensors Based on Conformational Restriction and Induced Charge Transfer.

Twist and shout! By restricting conformation, 2,6-biarylpyridines can be induced to "speak" to us through increased fluorescence emission. In combination with induced charge transfer, this allows these simple fluorophores to reveal otherwise silent binding events to alkali metal and alkaline earth cations through brightening and color change (see picture; from left to right: no metal, Li+ , Mg2+ , and Ca2+ ).

Dual-Signaling Fluorescent Chemosensors Based on Conformational Restriction and Induced Charge Transfer

Dual-Signaling Fluorescent Chemosensors Based on Conformational Restriction and Induced Charge Transfer

Photo-EPR of a Fe 3+ doped BaTiO 3 single-crystal at 15 K under copper vapor laser illumination

Photo-EPR investigation of the Fe 3+ ion in a BaTiO 3 single-crystal at 15 K using in situ copper vapor laser (CVL) illumination has shown that the electron transfer at the Fe site is symmetry dependent. It was seen that relaxation of the photo-induced electron transfer to the pre-illumination condition occurred even at 15 K very quickly. The growth and decay kinetics were found to be nearly identical. These investigations have shown that the history of the sample and the wavelength of light have an important role in the photo induced charge transfer process.

Studies on the early detection of wastewater’s toxicity using a microbial sensing system

A microbial sensing system with the built-in special processing algorithm was developed for detecting the toxicity or over-nutritivity of physically or chemically pretreated wastewater before been dispensed into a reservoir for further biological treatment. The electrical current is the output of the system varied according to both bio-chemical and electro-chemical reactions between the bacteria-attached electrode and wastewater. The toxic substances may inhibit the oxygen uptake of bacteria distinctively sampled from the activated sludge. The formation of reduction–oxidation reactions on the membrane was considered owing to bio-chemical activities of the oxygen uptake rate, which induce charge transfer effects. Present microbial sensing system is capable of measuring different degrees of toxicity correlated with the inhibition rates of the bacteria. The system is a portable and compact design primarily including a sensor, a signal amplifier, a low pass filter, and a data acquisition system. The wastewater reacted with the modified electrode that contained approximately 20 millions of bacteria controlled under flow culture condition at an optimized interval of ca. 9 h. Experimental results obtained from three varied concentrations of 40 ml specified toxic solutions indicated that the degree of toxicity in different wastewater was roughly correlated with their electro-induced bioactivities within 8 min. Only data acquired between 1 and 500 s were empirically found to be sufficient for regression calculation and computation to grade the degrees of wastewater’s toxicity. Each testing result measured by the newly developed microbial sensing system was verified with the simultaneous measurement using an oxygen-dissolved analyzer. A highly unified relation between measurements using the microbial sensing system and oxygen-dissolved analyses was obtained according to slope of the fitted first order linear equation to be 0.9921±0.0433. The correlation coefficient is 0.9767.

Photoinduced formation of ferromagnetic clusters inLa0.9Ca0.1MnO3thin films

Microwave losses due to conductivity and magnetic permeability are investigated in La0 9Ca0 jMn0 3 films illuminated with photons in the energy range o f E = 0 .5-2 eV. Growth of photoinduced ferromagnetic hysteresis is observed when the magnetic field is swept between —15 and 15 mT at temperatures below 60 K. The time required for saturation of the opening of the hysteresis loop depends on the photon energy having the minimum of =40 s at the illumination intensity 7 = 3.5X10 photons/cnrs. Both photoinduced magnetization and the increase of microwave photoconductivity can be well explained with a model assuming that small ferromagnetic regions exist within an insulating ferromagnetic phase of the sample and that these regions are expanded by optically induced charge transfer between Jahn-Teller split e states of neighboring Mn + ions.

The mechanism of the photo-induced magnetic transition in Co–Fe cyanide with ab initio calculations

Following our previous band structure calculation for stoichiometric crystal, ab initio cluster calculations with partially ligand substitution were carried out to clarify the mechanism of the heat-induced charge transfer observed in a Co–Fe cyanide complex exhibiting a photo-induced magnetic transition system. The existence of iron vacancies is found to be another reason for the stabilization of the cobalt high-spin (HS) state as compared with the low-spin (LS) state rather than the crystal structure expansion evidenced by the previous band calculation. Doped electrons are localized on the nearest Co sites of vacancies inducing charge transfer from Fe to Co around them. We suggest that this plays a role of domain nucleation in the heat/photo-induced LS–HS transition.

Laser excited charge transfer processes in oxygen organic molecules mixtures: O( 3P j) formation

Laser induced charge transfer (CT) processes have attracted attention for many years. These reactions play an important role in chemistry as well as biophysics and biology. When the reaction is induced by laser on a complex formed in a supersonic beam, the impact geometry is well defined and reaction details are more evident. This paper deals with features relevant to O 2 photodissociation induced in clusters of oxygen molecules and rare gases, water or organic molecules. Our studies, performed in a supersonic beam by using lasers of different wavelength to induce the reaction and to detect the reaction products by multiphoton ionization, show that production of atomic oxygen is strongly dependent on the ionization potential of the electron donor.

The effect of chemical doping and hydrostatic pressure on Tc of Y 1− yCa yBa 2Cu 3O x single crystals

We performed susceptibility measurements on Y 1− y Ca y Ba 2Cu 3O x single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on T c( x), d T c/d p( x) and T c,max. Starting from the parabolic T c( n h) behavior we calculated n h values from T c and T c,max for each sample. It is shown that in the overdoped region d T c/d p can be described by a pressure induced charge transfer with d n h/d p≈3.7 10 −3 GPa −1 and a d T c,max/d p value of 0.8 K/GPa, irrespective of the Ca content. In the underdoped region additional pressure effects lead to a peak in d T c/d p at ≈0.11 holes/CuO 2 plane. However, with increasing Ca content this peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in d T c/d p at very low n h values can be assigned to the ortho II ordering in the CuO chain system.

Laser excited N 2+ in a 22-pole ion trap: : Experimental studies of rotational relaxation processes

The laser induced charge transfer N 2 + + Ar → Ar + + N 2 has been employed to measure rotationally resolved excitation spectra of N 2 +, held in a 22-pole ion trap. Detection of the Ar + product ions proved to facilitate an almost background free spectroscopic method. Rotational temperatures and Doppler temperatures determined from these spectra show that the internal and translational degrees of freedom of the parent ions are very well coupled to the trap temperature via collisions with the Ar buffer gas. Laser excitation starts a complex reaction kinetics of the finite ensemble of N 2 + ions. Various elementary reaction steps have been distinguished and followed in detail by varying laser duration, storage time, and target gas density. For a fixed temperature (90 K), specific rate coefficients have been determined. Pumping only one particular quantum state of ortho-N 2 + ( X 2∑ g , v″ = 0, J″ = 6.5) shows that rate coefficients for the mixing of fine structure states ( F 1/ F 2) as well as nuclear spin states (ortho/para) in collisions with Ar are negligible (smaller than 10 −12 cm 3 s −1). The rate coefficient for transitions between the rotational states of one subset, e.g. ortho-N 2 + ( F 1), are surprisingly small. The results are discussed in the framework of possible dynamical constraints, e.g. imposed by the potential energy surface.

Core-hole induced charge transfer and Coster-Kronig enhanced fluorescence at the3dthreshold of lanthanum studied by resonant inelastic scattering

Soft x-ray emission near the 3d threshold of La is studied with synchrotron radiation. At excitation energies above the ${M}_{\mathrm{IV}}$ edge we observe Coster-Kronig enhanced fluorescence that refills the 3d hole via 5p and 4f electrons. We find a new delayed onset of the fluorescence as the 7.5-eV inelastic scattering evolves into 3d-4f emission about 8 eV above the absorption threshold, which we attribute to the energy required to produce a charge-transfer hole. The 16.3-eV Raman-scattering loss is due to a net transition of a 5p electron into a 4f orbit. The results are compared with our calculations.

Site dependence of large oxygen isotope effect in Y0.7Pr0.3Ba2Cu3O6.97.

We report site-selective oxygen isotope effects on {ital T}{sub {ital c}} and penetration depth in Y{sub 0.7}Pr{sub 0.3}Ba{sub 2}Cu{sub 3}O{sub 6.97} and in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. In Pr-substituted (underdoped) samples with {sup 18}O in only the CuO{sub 2} plane-sites, the {ital T}{sub {ital c}} shift was {minus}1.46 K vs {minus}1.7 K for {sup 18}O at all sites; the {alpha}{sub O} values are 0.2 and 0.24. The corresponding {ital T}{sub {ital c}} shifts in pure YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} were {minus}0.24 K and {minus}0.3 K. The shift is dominated by CuO{sub 2} planar oxygen mass in both compounds, and not by the apical sites. Thus, apical site isotope induced charge transfer to CuO{sub 2} planes does not seem a viable explanation for the isotope effects in YBCO. Our results indicate that the oxygen related phonons in the CuO{sub 2} planes play a significant role in the pairing mechanism. {copyright} {ital 1996 The American Physical Society.}

Influence of Si/Al ratio on Auger line intensities of zeolites

The dependence of the O K L2,3 L2,3 and Na K L2,3 L2,3 Auger line intensities and the binding energy Eb of the O 1s photoelectron line of zeolites A, X, Y and mordenite on the structure induced charge transfer effects is demonstrated. Intensity of the O K L2,3 L2,3 Auger line and the Eb of the O 1s line are increased by shortening the T-O bond. Chemical shifts of the O 1s line are thus influenced predominantly by rehybridization of valence orbitals of the zeolites. Further experiments are need to explain the influence of structure-induced charge transfer effects on Auger line intensities and core level binding energies of ion-exchanged sodium in zeolites.

Oxygen vacancy related defects in batio3

Abstract Depending on the acceptor content of BaTiO3 crystals, two types of Ti3+ ESR signals are observed. Acceptor-poor samples, which thus also have low concentrations of compensating oxygen vacancies, Vo, contain isolated Ti3+ free small polarons. Acceptor-rich crystals show signals to be attributed to Ti3+ next to Vo—alkali-acceptor complexes. This model is supported by a comparison of the optical absorptions and light induced charge transfer transitions of both types of specimens.

A stable mixed disulfide between thioredoxin reductase and its substrate, thioredoxin: preparation and characterization.

The flavoenzyme thioredoxin reductase (TrR) catalyzes the reduction of the small redox protein thioredoxin (Tr) by NADPH. It has been proposed that a large conformational change is required in catalysis by TrT in order to visualize a complete pathway for reduction of equivalents. The proposal is based on the comparison of the crystal structures of TrR and glutathione reductase, the latter being a well-understood member of the enzyme family [Waksman, G., et al. (1994) J. Mol. Biol. 236, 800-816]. Bound NADPH is perfectly positioned for electron transfer to the FAD in glutathione reductase, but in TrR, these two components are 17 angstroms apart. In order to provide evidence for the proposed conformational change, a complex between TrR and its substrate Tr involving a mixed disulfide between TrR and Tr was prepared. The redox active disulfide of TrR is composed of Cys135 and Cys138, and the redox active disulfide of Tr is made up of Cys32 and Cys35. The complex C135S-C32S is prepared from forms of TrR and Tr altered by site-directed mutagenesis where Cys138 and Cys35 are remaining in TrR and Tr, respectively. The purified C135S-C32S presents a band on a nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis responding to a molecular weight sum of one subunit of TrR and one of Tr. Several observations indicate that C135S-C32S can adopt only one conformation. It was reported previously that TrR C135S can form a charge transfer complex in the presence of ammonium cation in which the donor is the remaining thiolate of Cys138 [Prongay, A.J., et al., (1989) J. Biol. Chem. 264, 2656-2664], while titration of C135S-C32S with NH4Cl does not induce charge transfer, presumably because Cys138 is participating in the mixed dissulfide. Reduction of C135S-C32S with dithiothreitol (DTT) results in a decrease of epsilon454 to a value similar to that of TrR C135S, and subsequent NH4Cl titration leads to charge transfer complex formation in the nascent TrR C135S. Reductive titrations show that approximately 1 equiv of sodium dithionite or NADPH is required to fully reduce C135S-C32S, and treatment with NH4Cl and DTT demonstrates that the mixed disulfide between Cys138 of TrR C135S and Cys35 of TrC32S that locks the structure in a conformation where FAD can be reduced by NADPH, but electrons cannot flow from FADH2 to the mixed disulfide bond.

Time-dependent second-harmonic generation from the Si–SiO_2 interface induced by charge transfer

We have observed that the second-harmonic signal generated from oxidized Si(001) varies on a time scale of several seconds in experiments involving a fundamental beam of lambda = 770 nm, 110-fs pulses at 76 MHz. We suggest that the temporal behavior arises from absorption of weak (<100-fW average power) third-harmonic light generated in air or in the sample, inducing charge transfer across the Si-SiO(2) interface and trapping in the oxide layer. Detrapping has been determined to take several minutes.

Intercalation in C60: High resolution 13C NMR results

Abstract Solid state high resolution 13C NMR has been used to investigate the physical properties of various phases obtained after intercalation of given chemical species in C60. We present here the case of molecular oxygen, which do not induce charge transfer with the host molecules. We show that, via the magnetic characteristics of the intercalant, information can be gained on its position and movement in the structure. In particular, molecular oxygen is found to occupy the middle of the octahedral site in the fcc structure of solid C60.

Laser field induced charge transfer: Para-nitroaniline coupled to a quantum mechanical radiation field

We present a preliminary model for describing a solvated intramolecular charge transfer reaction coupled to a quantum mechanical radiation field. Actual calculations of energies and couplings were performed with a recently developed self-consistent reaction field response method. The representation of dressed molecular states is used for calculating state populations for various laser fields. The state populations are sensitive to the properties of the laser field.

Optically induced charge transfer paths between defects in BaTiO 3 containing rhodium

An optical method is presented which allows to identify at room temperature the photoinduced charge transfers between different defects in photorefractive crystals. The analysis is based on the labelling of the opticfal spectra with the corresponding microscopic defect models, obtained from simultaneously recording light induced ESR and optical absorption changes at low temperatures. The investigation of as-grown BaTiO 3 crystals containing Rh, using this method, shows that illumination with light of energies between 1.2 eV and 2.5 eV transfers holes from Rh 4+ to Fe 3+. At E > 2.5 eV holes are photoionized from Fe 4+ to Rh 3+ and to an unidentified shallower hole trap.

Laser field induced charge transfer: Para-nitroaniline coupled to a quantum mechanical radiation field

Laser field induced charge transfer: Para-nitroaniline coupled to a quantum mechanical radiation field

Cu and Ba nuclear quadrupole resonance study of Y 2Ba 4Cu 7O 15 at high pressure

We have measured the pressure dependence of 63Cu and 137Ba NQR frequencies at 100K and 300K. Estimates of the pressure induced charge transfer from CuO-chains to CuO 2-planes are calculated.