Dipole Mechanism Research Articles

Nuclear magnetic relaxation by the dipolar EMOR mechanism: Three-spin systems.

In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. Starting from the stochastic Liouville equation, we have developed a non-perturbative theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole couplings, and Larmor frequencies. Here, we implement the general dipolar EMOR theory for a macromolecule-bound three-spin system, where one, two, or all three spins exchange with the bulk solution phase. In contrast to the previously studied two-spin system with a single dipole coupling, there are now three dipole couplings, so relaxation is affected by distinct correlations as well as by self-correlations. Moreover, relaxation can now couple the magnetizations with three-spin modes and, in the presence of a static dipole coupling, with two-spin modes. As a result of this complexity, three secondary dispersion steps with different physical origins can appear in the longitudinal relaxation dispersion profile, in addition to the primary dispersion step at the Larmor frequency matching the exchange rate. Furthermore, and in contrast to the two-spin system, longitudinal relaxation can be significantly affected by chemical shifts and by the odd-valued ("imaginary") part of the spectral density function. We anticipate that the detailed studies of two-spin and three-spin systems that have now been completed will provide the foundation for developing an approximate multi-spin dipolar EMOR theory sufficiently accurate and computationally efficient to allow quantitative molecular-level interpretation of frequency-dependent water-proton longitudinal relaxation data from biophysical model systems and soft biological tissue.

Novel single-phase white-emitting phosphor Sr6Gd2Na2(PO4)6F2:Eu2+,Mn2+: band gap, luminescence, and energy transfer

White-emitting phosphors Sr6Gd2Na2(PO4)6F2:Eu2+,Mn2+ (SGNPF:Eu2+,Mn2+) were synthesized under a weak reductive atmosphere. Phase purity was examined based on XRD patterns, and results showed that the obtained SGNPF:Eu2+,Mn2+ phosphors were well crystallized with no impurity phase. The emission spectra of SGNPF:Eu2+,Mn2+ phosphor exhibited broad blue and broad yellow emission bands with two peaks at approximately 460 and 560 nm under 300 nm excitation. The energy transfer process from Eu2+ to Mn2+ was verified through luminescence spectra and fluorescence decay curves. The energy transfer from Eu2+ to Mn2+ in SGNPF was demonstrated to be a resonant type via a dipole–dipole mechanism. Adjusting the relative contents of Eu2+ and Mn2+ could change the emitting colors of SGNPF:Eu2+,Mn2+ phosphors from blue and azure to white, and the ideal white light-emitting phosphor SGNPF:0.05Eu2+,0.25Mn2+ with color coordinate (0.29, 0.31) was obtained. All investigations indicated that the SGNPF:Eu2+,Mn2+ phosphors exhibit great potential as a single component white-emitting phosphor for white light-emitting diodes.

On the optimization, and the intensity dependence, of the excitation rate for the absorption of two-photons due to the direct permanent dipole moment excitation mechanism

A model two-level dipolar molecule, and the rotating wave approximation and perturbation theory, are used to investigate the optimization and the laser intensity dependence of the two-photon excitation rate via the direct permanent dipole mechanism. The rate is proportional to the square of the laser intensity I only for small intensities and times when perturbation theory is applicable. An improvement on perturbation theory is provided by a small time RWA result for the rate which is not proportional to I2; rather it is proportional to the square of an effective intensity Ieff. For each laser intensity the optimum RWA excitation rate as a function of time, for low intensities, is proportional to I, not I2, and for high intensities it is proportional to Ieff. For a given two-photon transition the laser-molecule coupling optimizes for an intensity Imax which, for example, leads to a maximum possible excitation rate as a function of time. The validity of the RWA results of this paper, and the importance of including the effects of virtual excited states, are also discussed briefly.

Light induced intramolecular electron and energy transfer events in rigidly linked borondipyrromethene: Corrole Dyad

We have designed and synthesized a photo-induced energy/electron donor–acceptor conjugate comprising of corrole linked to BODIPY at the 5-position via ester linkage. The dyad was characterized by elemental analysis, MALDI-MS, UV-Visible, 1H NMR fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical methods. A comparison of the UV–visible and 1H NMR spectra of the dyad with those of the corresponding individual model compounds (i.e., BODIPY-CO2H and BPFC-OH) reveal that there exist minimum π–π interactions between BODIPY and corrole π-planes. Quenched emission of BODIPY and corrole part of the dyad has been observed in five different solvents. Excitation spectral data provided evidence for an intramolecular excitation energy transfer (EET) from the singlet BODIPY to the corrole and an intramolecular photoinduced electron transfer (PET) from singlet state of corrole to ground state of BODIPY. Detailed analysis of the data suggests that Forster׳s dipole–dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET.

A new dipole index of the salinity anomalies of the tropical Indian Ocean.

With the increased interest in studying the sea surface salinity anomaly (SSSA) of the tropical Indian Ocean during the Indian Ocean Dipole (IOD), an index describing the dipole variability of the SSSA has been pursued recently. In this study, we first use a regional ocean model with a high spatial resolution to produce a high-quality salinity simulation during the period from 1982 to 2014, from which the SSSA dipole structure is identified for boreal autumn. On this basis, by further analysing the observed data, we define a dipole index of the SSSA between the central equatorial Indian Ocean (CEIO: 70°E-90°E, 5°S-5°N) and the region off the Sumatra-Java coast (SJC: 100°E-110°E, 13°S-3°S). Compared with previous SSSA dipole indices, this index has advantages in detecting the dipole signals and in characterizing their relationship to the sea surface temperature anomaly (SSTA) dipole variability. Finally, the mechanism of the SSSA dipole is investigated by dynamical diagnosis. It is found that anomalous zonal advection dominates the SSSA in the CEIO region, whereas the SSSA in the SJC region are mainly influenced by the anomalous surface freshwater flux. This SSSA dipole provides a positive feedback to the formation of the IOD events.

A clear effect of charge compensation through Na+ co-doping on the luminescence spectra and decay kinetics of Nd3+-doped CaAl4O7

We present a detailed analysis of luminescence behavior of singly Nd3+ doped and Nd3+, Na+ co-doped calcium aluminates powders: Ca1−xNdxAl4O7 and Ca1−2xNdxNaxAl4O7 (x=0.001–0.1). Relatively intense Nd3+ luminescence in IR region corresponding to typical 4F3/2→4IJ (J=9/2–13/2) transitions with maximum located at about 1079nm was obtained in all samples on direct excitation into f–f levels. The effect of dopant concentration and charge compensation by co-doping with Na+ ions on morphology and optical properties were studied. The results show that both, the Nd3+ concentration and the alkali metal co-doping affected the optical properties but had no influence on the powders morphology. The studies of luminescence spectra (298 and 77K) in a function of dopant concentration showed an increasing distortion of the local symmetry of Nd3+with raising activator content due to certain defects created in the crystal lattice. On the other hand Na+ addition led to significant narrowing of absorption and luminescence bands and also a reduction of the number of their components, showing smaller disturbance of Nd3+ ions local symmetries. Consequently, charge compensated by Na+ co-doping materials showed significantly enhanced Nd3+ luminescence. The decrease of emission intensity and luminescence lifetimes with increase of activator concentration was attributed mainly to phonon-assisted cross-relaxation processes between Nd3+ ions. Analysis with Inokuti–Hirayama model indicated dipole–dipole mechanism of ion-ion interaction. Na+ addition led to much smaller concentration quenching due to smaller clustering of dopant ions in CaAl4O7 lattice.

Structural and optical investigations on Dy3+doped lithium tellurofluoroborate glasses for white light applications

Dy3+doped lithium tellurofluoroborate glasses were prepared with the chemical composition (40–x)B2O3+15TeO2+15Li2O+15LiF+15NaF+xDy2O3 (BTLNxD; where x=0.05, 0.1, 0.25, 0.5, 1.0 and 2.0 in wt%) following conventional melt quenching technique and characterized through FTIR, optical absorption, luminescence and lifetime measurements. The vibrations of the borate and tellurite network have been identified through FTIR spectral measurements. The optical properties have been explored from the absorption and luminescence spectra. The optical band gap of direct and indirect allowed transitions was determined from Tauc׳s plot and the variations in the band gap energy with structural arrangements in the prepared glasses is also discussed. Judd–Ofelt theory has been used to evaluate the intensity parameters Ω2, Ω4 and Ω6 using the experimental oscillator strengths and absorption energy levels. The radiative parameters such as transition probability (A), stimulated emission cross-section (σPE), branching ratios (βR) and radiative lifetime (τR) corresponding to the 4F9/2→6H11/2, 6H13/2 and 6H15/2 emission transitions were calculated from the luminescence spectra. The yellow-to−blue (Y/B) luminescence intensity ratios and colour chromaticity coordinates of the title glasses have also been estimated to evaluate the white light emission as a function of Dy3+ion concentration. These results have been used to identify the suitability of the prepared glasses for W-LED applications. The decay rates for the 4F9/2 level have been measured and found to deviate from exponential to non-exponential nature with the increase of Dy3+ion concentration. The non-exponential decay rates have been fitted with the Inokuti–Hirayama model for S=6 which indicate that dipole–dipole mechanism is responsible for the energy transfer processes through Dy3+–Dy3+interaction.

Designing a dual-mode broadband solar spectral converter: The example of Bi3+, Cr3+, Yb3+-tridoped perovskite

A promising dual-mode broadband solar spectral converter CaTiO3: Bi3+, Cr3+, Yb3+ was successfully developed by solid-stated reaction. The structure, photoluminescence (PL), photoluminescence excitation (PLE) and diffuse reflectance (DR) spectra in the UV–vis–NIR region have been systematically investigated. The results show that the as-prepared samples simultaneously exhibit two distinct spectral converting patterns, nonlinear quantum-cutting (QC) involving Bi3+–Ti4+ metal-to-metal charge transfer state (BT-MMCTs) → Yb3+: 2F5/2 + Yb3+: 2F5/2 and linear downshift (DS) involving Cr3+: 4T2 → Yb3+: 2F5/2. It deduces that the nonlinear QC is based on a cooperative energy transfer (CET) process while the linear DS belongs to a dipole–dipole mechanism. With the present converter, broadband UV–vis (300–700 nm) photons, which are not fully utilized by the existing c-Si solar cells, can be efficiently harvested and converted into ∼1000 nm NIR photons via the dual-mode mechanism. Moreover, not only the PLE spectrum of CaTiO3: Bi3+, Cr3+, Yb3+ matched well with that of the solar radiation, but also its NIR emission peak position fell well over the spectral response of the commercial crystalline Si (c-Si) solar cells. This as-prepared dual-mode solar spectral converter with multiple advantages can simultaneously realize high quantum yield and broadband conversion, which offers a new and effective way to boost the conversion efficiency of c-Si solar cells. We believe this novel design of dual-mode solar spectral converters can inspire a direction for the synthesis of more advanced UV–vis–NIR phosphors that can be used in Si solar cells.

Nuclear magnetic relaxation by the dipolar EMOR mechanism: General theory with applications to two-spin systems.

In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. We have embarked on a systematic program to develop, from the stochastic Liouville equation, a general and rigorous theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole coupling strengths, and Larmor frequencies. Here, we present a general theoretical framework applicable to spin systems of arbitrary size with symmetric or asymmetric exchange. So far, the dipolar EMOR theory is only available for a two-spin system with symmetric exchange. Asymmetric exchange, when the spin system is fragmented by the exchange, introduces new and unexpected phenomena. Notably, the anisotropic dipole couplings of non-exchanging spins break the axial symmetry in spin Liouville space, thereby opening up new relaxation channels in the locally anisotropic sites, including longitudinal-transverse cross relaxation. Such cross-mode relaxation operates only at low fields; at higher fields it becomes nonsecular, leading to an unusual inverted relaxation dispersion that splits the extreme-narrowing regime into two sub-regimes. The general dipolar EMOR theory is illustrated here by a detailed analysis of the asymmetric two-spin case, for which we present relaxation dispersion profiles over a wide range of conditions as well as analytical results for integral relaxation rates and time-dependent spin modes in the zero-field and motional-narrowing regimes. The general theoretical framework presented here will enable a quantitative analysis of frequency-dependent water-proton longitudinal relaxation in model systems with immobilized macromolecules and, ultimately, will provide a rigorous link between relaxation-based magnetic resonance image contrast and molecular parameters.

Tunable luminescence and energy transfer properties of KSr4(BO3)3:Dy3+, Eu3+ phosphors for near-UV warm-white LEDs

Dy3+ and Eu3+ codoped KSr4(BO3)3 phosphors were successfully synthesized by solid-state reaction process. Under near-ultraviolet (near-UV) excitation, individual Dy3+ or Eu3+ ion activated sample exhibits characteristic emissions in their respective regions. In KSr4(BO3)3:Dy3+, xEu3+ phosphors, the energy transfer from Dy3+ to Eu3+ was revealed to be resonant type by dipole–dipole mechanism. And the critical distance RDy–Eu was calculated to be 13.95Å. The energy transfer efficiency of KSr4(BO3)3:Dy3+, xEu3+ reached about 20% when the concentration of Eu3+ was 0.035. Moreover, the emitting colors of Dy3+ and Eu3+ codoped samples can be adjusted from the edge of white area to warm white via tuning the activator (Eu3+) doped concentration. More significantly, the chromaticity coordinates (0.334, 0.351) of KSr4(BO3)3:0.005Dy3+, 0.015Eu3+ sample are extremely adjacent to standard white light (0.33, 0.33). The quantum efficiency of the KSr4(BO3)3:0.005Dy3+, 0.015Eu3+ phosphor with 389nm excitation is found to be 42%, demonstrating that the obtained single-component white-emitting phosphor exhibits potential applications for solid state lighting.

Influence of multiphonon and cross relaxations on 3P0 and 1D2 emission levels of Pr3+ doped borosilicate glasses for broad band signal amplification

We discuss non-radiative relaxations of visible–near infrared (Vis–NIR) emissions originating from 3P0 and 1D2 levels of Pr3+ glasses. Thermal stability of host lithium aluminium borosilicate (LABS) glass is estimated from thermogravimetric and differential thermal analysis. The structural analysis of host LABS glass through Fourier transform infrared and Raman spectral profiles provide an insight to understand the effect of OH content and phonon energy on luminescence characteristics of Pr3+ ions. Visible emission spectrum of Pr3+ glass is composed of two prominent emission bands at 493 nm (3P0 → 3H4) and 605 nm (1D2 → 3H4) when excited by 448 nm. In NIR region a narrow emission band at 1.06 μm (1D2 → 3F3) and ultra-broad emission band at 1.49 μm (1D2 → 1G4) are observed for 488 nm excitation. For these emission transitions, emission decay curves are measured and analysed. The lifetime shortening due to non-radiative energy transfer is explained by multiphonon interactions and cross-relaxation routes, and later verified by Dexter's model. Electric dipole–dipole mechanism is identified to be responsible for ion–ion interactions intervening in 3P0 and 1D2 states of Pr3+ causing quenching in emission intensities and lifetimes. The large absorption and stimulated emission cross-sections of Pr3+ ions around 1.49 μm suggests suitability of a host material operating for broadband signal amplifications at low-loss optical telecommunication windows. Besides, emission parameters like stimulated emission cross-section (σemi), effective band width (Δλeff) are calculated for 1.49 μm (1D2 → 1G4) of Pr3+ doped silicate based glasses.

Effect of hysteresis of dipole on remnant polarization in ferroelectrics

Decrease in remnant polarization at lower temperature, or low temperature degradation of polarization, in some ferroelectrics has attracted much attention. To investigate the mechanism of the decrease, phenomenological theory of ferroelectrics and the relevant mechanism of dipole in alternating electric field are used to develop a model of hysteresis-frozen effect of dipole in electric hysteresis loop measurement. Within the frame of Landau-Ginzburg-Devonshire theory, Ising model is used to derive the relationship among remnant polarization, coercive field, and saturated polarization strength. Then, two aspects are investigated: response of a dipole and thermodynamic properties of ferroelectric. Response of a dipole in an electric field is often described by relaxation time, on the assumption that Debye equation is satisfied. Potential barrier in the Debye equation is the Gibbs free energy barrier from one ferroelectric state, +P, to another ferroelectric state, -P. Increase in the Gibbs free energy barrier with temperature decreasing will prolong the relaxation time. As ferroelectrics can be taken as a capacitor, first order response function is used to introduce a hysteresis factor with measuring frequency and relaxation time into the expression of remnant polarization. In the aspect of thermodynamic properties of ferroelectric, the variation of compositions is a significant reason. In numerical simulation based on the derived formula the remnant polarization exhibits a frequency related peak, and shift of the peak depends on some other reasons: the increase of soft-mode coefficient in phase transition shifts the peak towards high temperature; the increases of coercive field, temperature-polarization coefficient (a concept defined in the present paper to indicate increase in polarization with increasing temperature) and saturated electric field shift the peak toward low temperature. Compared with the reported experimental results of BaTiO3/BiScO3 compound ceramics, the results show a good coincidence with numerical simulations. The parameter values of numerical simulation indicate that a large shift toward high temperature in peak of remnant polarization with increasing BiScO3 composition ratio is due to the increase in soft-mode coefficient with only small decrease in the Curie temperature. The soft-mode coefficient and temperature-polarization coefficient are closely related to polarization characteristic, ferroelectric, dielectric and mechanical properties. Therefore, the decrease in remnant polarization at low temperatures, ascribed to the hysteresis of dipole to a constant measuring frequency, may have an influence on changes in various properties, but freezing effect of dipole at low temperature can help ferroelectrics to save data longer.

Long-wave UVA radiation excited warm white-light emitting NaGdTiO4: Tm3+/Dy3+/Eu3+ ions tri-doped phosphors: Synthesis, energy transfer and color tunable properties

NaGdTiO4 (NGT) phosphors doped with different activator ions (Tm3+, Dy3+, and Eu3+) were synthesized by a conventional solid-state reaction method in an ambient atmosphere. These phosphors were characterized by scanning electron microscope images, transmission electron microscope images, X-ray diffraction patterns, Fourier transform infrared spectra, and photoluminescence spectra. All the samples were crystallized in an orthorhombic phase with a space group of Pbcm (57). In Tm3+/Dy3+ ions co-doped samples, white-light emission was observed under near-ultraviolet (NUV) excitation. In addition, the energy transfer between Tm3+ and Dy3+ ions was proved to be a resonant type via an electric dipole–dipole mechanism and the critical distance of energy transfer was calculated to be 19.91 Å. Furthermore, Tm3+/Dy3+/Eu3+ ions tri-doped NGT phosphors demonstrated warm white-light emission by appropriately tuning the activator content, based on the principle of energy transfer. These NUV wavelength excitable phosphors exhibit great potential as a single-phase full-color emitting phosphor for white light-emitting diode applications.

Photoluminescence properties and energy transfer of a color tunable phosphor: Dy3+ and Tm3+ co-activated SrCaAl2SiO7

A series of single-phased emission-tunable SrCaAl2SiO7:Dy3+, Tm3+ phosphors were successfully synthesized by solid-state reaction process. By controling the energy transfer, the emission colors could be tuned from blue to yellow, including the white light region. It was discovered that the energy is transferred from Tm3+ to Dy3+ by luminescence spectra and decay curves. The resonance-type energy transfer from Tm3+ to Dy3+ was demonstrated to be via the dipole–dipole mechanism. By changing the doping concentration of Dy3+ and Tm3+ in SrCaAl2SiO7, the sample with corresponding color coordinates close to the white color chromaticity coordinates (x=0.33, y=0.33) could be realized.

Nonradiative recombination of excitons in semimagnetic quantum dots

The mechanisms of the nonradiative recombination of excitons in neutral and charged quantum dots based on II–VI semimagnetic semiconductors are investigated. It is shown that, along with the dipole–dipole and direct-exchange mechanisms, there is one more mechanism referred to as the indirect-exchange mechanism and related to sp–d mixing. The selection rules for nonradiative recombination by exchange mechanisms are subsequently derived. The dependence of the efficiency of all recombination mechanisms on the quantum-dot size is studied. The experimentally observed growth in the intracenter photoluminescence intensity with decreasing size of dots and nanocrystals is accounted for. Methods for experimental determination of the contributions of different mechanisms to nonradiative recombination are discussed.

Impedance and dielectric properties of mercury cuprate at nonsuperconducting state

In this paper, impedance and dielectric properties of nonsuperconducting state of the mercury-based cuprate have been investigated by impedance measurements within the frequency interval of 10 Hz–10 MHz for the first time. The dielectric loss factor [Formula: see text] and ac conductivity [Formula: see text] parameters have also been calculated for non-superconducting state. According to impedance spectroscopy analysis, the equivalent circuit of the mercury cuprate system manifests itself as a semicircle in the Nyquist plot that corresponds to parallel connected resistance–capacitance circuit. The oscillation frequency of the circuit has been determined as approximately 45 kHz which coincides with the low frequency radio waves. Moreover, it has been revealed that the mercury-based cuprate investigated has high dielectric constants and hence it may be utilized in microelectronic industry such as capacitors, memory devices etc., at room temperature. In addition, negative capacitance (NC) effect has been observed for the mercury cuprate regardless of the operating temperatures at nonsuperconducting state. Referring to dispersions in dielectric properties, the main contribution to dielectric response of the system has been suggested as dipolar and interfacial polarization mechanisms.

PEGylated NaHoF4 nanoparticles as contrast agents for both X-ray computed tomography and ultra-high field magnetic resonance imaging

It is well-known that multimodal imaging can integrate the advantages of different imaging modalities by overcoming their individual limitations. As ultra-high field magnetic resonance imaging (MRI) will be inevitably used in future MRI/X-ray computed tomography (CT) scanner, it is highly expected to develop high-performance nano-contrast agents for ultra-high field MR and CT dual-modality imaging, which has not been reported yet. Moreover, specific behavior of nano-contrast agents for ultra-high field MRI is a challenging work and still remains unknown. Herein, a novel type of NaHoF4 nanoparticles (NPs) with varied particle sizes were synthesized and explored as high-performance dual-modality contrast agents for ultra-high field MR and CT imaging. The specific X-ray absorption and MR relaxivity enhancements with varied nanoparticle diameters (3 nm, 7 nm, 13 nm and 29 nm) under different magnetic field (1.5/3.0/7.0 T) are investigated. Based on experimental results and theoretical analysis, the Curie and dipolar relaxation mechanisms of NaHoF4 NPs are firstly separated. Our results will greatly promote the future medical translational development of the NaHoF4 nano-contrast agents for ultra-high field MR/CT dual-modality imaging applications.

Synthesis and optical characterization of host sensitized color tunable Tb2−xEux(MoO4)3 nanophosphors for optoelectronic applications

In the present work, host sensitized multicolor emitting Tb2−xEux(MoO4)3 nanophosphors was synthesized by sol–gel method for the first time. The X-ray diffraction, transmission electron microscopy (TEM), photoluminescence studies and fluorescent lifetime measurements were employed to characterize the samples. The X-ray diffraction and TEM analysis revealed the orthorhombic structure and nanosize of the phosphors. Photoluminescence spectra, concentration effect, fluorescence lifetime and chromaticity variations were performed to study the luminescence properties. These studies suggest that, these phosphors could be effectively excited in a wavelength range from 300 to 485 nm, which matches well with that for nUVLED chips. The sensitization of Eu3+ emission through MoO4 2− → Eu3+ and Tb3+ to Eu3+ energy transfer schemes and energy transfer from MoO4 2− → Tb3+ are investigated in detail. The energy transfer process between Tb3+ and Eu3+ was resonant type via dipole–dipole mechanism, and the critical distance calculated by the concentration quenching method was found to be 17.61 A. The stark splitting of Eu3+ emission lines and luminescence asymmetric ratio reveals the occupation of highly asymmetric site by the Eu3+ in the orthorhombic lattice. The variation in the emission intensities of the two strong emission peaks at 545 nm (green) and 612 nm (red) induces the multi-color emission in the samples by which the emission color of the phosphor be fine-tuned from green through white and eventually to red by adjusting the doping concentration of Eu3+ through energy transfer. The results indicated that Tb2−xEux(MoO4)3 nanophosphor may serve as a multicolor-tunable phosphor and find potential application in smart optoelectronic devices.

(31)P-MRS of healthy human brain: ATP synthesis, metabolite concentrations, pH, and T1 relaxation times.

The conventional method for measuring brain ATP synthesis is (31)P saturation transfer (ST), a technique typically dependent on prolonged pre-saturation with γ-ATP. In this study, ATP synthesis rate in resting human brain is evaluated using EBIT (exchange kinetics by band inversion transfer), a technique based on slow recovery of γ-ATP magnetization in the absence of B1 field following co-inversion of PCr and ATP resonances with a short adiabatic pulse. The unidirectional rate constant for the Pi → γ-ATP reaction is 0.21 ± 0.04 s(-1) and the ATP synthesis rate is 9.9 ± 2.1 mmol min(-1) kg(-1) in human brain (n = 12 subjects), consistent with the results by ST. Therefore, EBIT could be a useful alternative to ST in studying brain energy metabolism in normal physiology and under pathological conditions. In addition to ATP synthesis, all detectable (31)P signals are analyzed to determine the brain concentration of phosphorus metabolites, including UDPG at around 10 ppm, a previously reported resonance in liver tissues and now confirmed in human brain. Inversion recovery measurements indicate that UDPG, like its diphosphate analogue NAD, has apparent T1 shorter than that of monophosphates (Pi, PMEs, and PDEs) but longer than that of triphosphate ATP, highlighting the significance of the (31)P-(31)P dipolar mechanism in T1 relaxation of polyphosphates. Another interesting finding is the observation of approximately 40% shorter T1 for intracellular Pi relative to extracellular Pi, attributed to the modulation by the intracellular phosphoryl exchange reaction Pi ↔ γ-ATP. The sufficiently separated intra- and extracellular Pi signals also permit the distinction of pH between intra- and extracellular environments (pH 7.0 versus pH 7.4). In summary, quantitative (31)P MRS in combination with ATP synthesis, pH, and T1 relaxation measurements may offer a promising tool to detect biochemical alterations at early stages of brain dysfunctions and diseases.

Europium Luminescence: Electronic Densities and Superdelocalizabilities for a Unique Adjustment of Theoretical Intensity Parameters.

We advance the concept that the charge factors of the simple overlap model and the polarizabilities of Judd-Ofelt theory for the luminescence of europium complexes can be effectively and uniquely modeled by perturbation theory on the semiempirical electronic wave function of the complex. With only three adjustable constants, we introduce expressions that relate: (i) the charge factors to electronic densities, and (ii) the polarizabilities to superdelocalizabilities that we derived specifically for this purpose. The three constants are then adjusted iteratively until the calculated intensity parameters, corresponding to the 5D0→7F2 and 5D0→7F4 transitions, converge to the experimentally determined ones. This adjustment yields a single unique set of only three constants per complex and semiempirical model used. From these constants, we then define a binary outcome acceptance attribute for the adjustment, and show that when the adjustment is acceptable, the predicted geometry is, in average, closer to the experimental one. An important consequence is that the terms of the intensity parameters related to dynamic coupling and electric dipole mechanisms will be unique. Hence, the important energy transfer rates will also be unique, leading to a single predicted intensity parameter for the 5D0→7F6 transition.