Dipole-dipole Mechanisms Research Articles

Solvent-Driven Self-Organization of Meso-Substituted Porphyrins: Morphological Analysis from Fluorescence Lifetime Imaging Microscopy.

A morphological analysis of different thin films of meso-tetra-p-(di-p-phenylamino)phenylporphyrin, H2T(TPA)4P, was made by fluorescence lifetime imaging microscopy (FLIM) and scanning electron microscopy (SEM). A comprehensive study of H2T(TPA)4P was undertaken through UV/vis absorption and fluorescence techniques in different solvents, solvent mixtures and in thin films. In solution, occurrence of intramolecular energy transfer from the triphenylamine (TPA) moieties to the porphyrin core, with quenching efficiencies in the order of 94-97%, is observed. The energy transfer rate constants are determined assuming Förster's dipole-dipole and Dexter's electron exchange mechanisms. In drop-cast-prepared thin films, from samples with different solvent mixtures, the photoluminescence (PL) quantum yield (ΦPL) decreases ∼1 order of magnitude compared to the solution behavior. FLIM and SEM experiments showed the self-organization and morphology of H2T(TPA)4P in thin films to be highly dependent on the solvent mixture used to prepare the film. In chloroform, the solvent's evaporation results in the formation of elongated and overlapped microrod structures. Introduction of a cosolvent, namely, a polar cosolvent, promotes changes in the morphology of the self-assembled structures, with the formation of three-dimensional spherical structures and hollow spheres. H2T(TPA)4P dispersed in a polymer matrix shows enhanced ΦPL values when compared to the drop-cast films. FLIM images showed coexistence of three different states or domains: aggregated, interface, and nonaggregated or less-aggregated states. This work highlights the importance of FLIM in the morphological characterization of heterogeneous films, together with the photophysical characterization of nano- and microdomains.

Pulse sequences for measuring exchange rates between proton species: From unlocalised NMR spectroscopy to chemical exchange saturation transfer imaging

Within the field of NMR spectroscopy, the study of chemical exchange processes through saturation transfer techniques has a long history. In the context of MRI, chemical exchange techniques have been adapted to increase the sensitivity of imaging to small fractions of exchangeable protons, including the labile protons of amines, amides and hydroxyls. The MR contrast is generated by frequency-selective irradiation of the labile protons, which results in a reduction of the water signal associated with transfer of the labile protons’ saturated magnetization to the protons of the surrounding free water. The signal intensity depends on the rate of chemical exchange and the concentration of labile protons as well as on the properties of the irradiation field. This methodology is referred to as CEST (chemical exchange saturation transfer) imaging. Applications of CEST include imaging of molecules with short transverse relaxation times and mapping of physiological parameters such as pH, temperature, buffer concentration and chemical composition due to the dependency of this chemical exchange effect on all these parameters. This article aims to describe these effects both theoretically and experimentally. In depth analysis and mathematical modelling are provided for all pulse sequences designed to date to measure the chemical exchange rate. Importantly, it has become clear that the background signal from semi-solid protons and the presence of the Nuclear Overhauser Effect (NOE), either through direct dipole-dipole mechanisms or through exchange-relayed signals, complicates the analysis of CEST effects. Therefore, advanced methods to suppress these confounding factors have been developed, and these are also reviewed. Finally, the experimental work conducted both in vitro and in vivo is discussed and the progress of CEST imaging towards clinical practice is presented.

Accurate Evaluation of Real-Time Density Functional Theory Providing Access to Challenging Electron Dynamics.

We demonstrate that electronic excitations and their transition densities can be obtained from real-time density functional theory calculations with great accuracy by relating the data from numerical propagation to the analytical form of the electronic response after a boost excitation. The latter is derived in this article. This approach facilitates quantitatively obtaining oscillator strengths, identifying excitations that carry very small oscillator strengths, and assessing electronic couplings from transition densities based on comparatively short propagation times. These features are of interest in particular for studying light-harvesting systems. For demonstration purposes, we study the Q band excitations of bacteriochlorophyll a (BChl a) and calculate coupling strengths between two BChl a's to check the validity of the dipole-dipole and pure Coulomb coupling mechanisms. For further illustration, we investigate the paradigm test system Na4 and the coupling between two Na2 dimers.

Luminescence properties of Eu2+-activated NaCaBeSi2O6F for white light-emitting diode applications

A series of blue-emitting NaCaBeSi2O6F:Eu2+ phosphors were obtained via a solid state reaction. NaCaBeSi2O6F:Eu2+ exhibit a broad absorption from 250 to 450nm when monitored at 467nm and display a broad emission band from 400 to 600nm when excited at 397nm. The concentration quenching was obtained when the concentration of Eu2+ was increased to 0.8mol%. Energy transfer interactions between Eu2+ in host is due to dipole-dipole mechanisms and the critical distance was calculated to be 31.96Å. The results obtained in this study highly indicates that NaCaBeSi2O6F:Eu2+ can be a potential blue-emitting phosphor for light quality improvement in white-LED applications.

Optimization of photoluminescence of GdAl3(BO3)4:Sm3+ phosphors for solid state lighting devices

The GdAl3(BO3)4:Sm3+ phosphors prepared by solid-state reaction method were characterized through thermal, structural and photoluminescence studies at room temperature only. The observed X-ray diffraction peaks were well consistent with JCPDS No. 83-1907. When excited with 406 nm wavelength, the studied phosphors exhibit orange-red luminescence through 4G5/2 → 6H5/2, 4G5/2 → 6H7/2 and 4G5/2 → 6H9/2 transitions. The concentration of Sm3+ ions was optimized to be 0.01 mol% for intense luminescence in GdAl3(BO3)4:Sm3+ phosphors. Beyond 0.01 mol% of Sm3+ ions concentration, luminescence quenching was observed due to energy transfer among the excited Sm3+ ions through cross-relaxation and dipole-dipole interaction mechanisms. The GdAl3(BO3)4:0.01 mol% Sm3+ phosphor was identified as a notable host material to emit intense orange-red luminescence for various solid state lighting devices under 406 nm excitation.

Band inversion amplifies 31 P-31 P nuclear overhauser effects: Relaxation mechanism and dynamic behavior of ATP in the human brain by 31 P MRS at 7 T.

To develop an improved method to measure the 31 P nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (τc ), and contribution of dipole-dipole (DD) and chemical shift anisotropy (CSA) mechanisms to T1 relaxation of ATP in human brain. The NOE of ATP in human brain was evaluated by monitoring changes in magnetization in the β-ATP signal following a band inversion of all downfield 31 P resonances. The magnetization changes observed were analyzed using the Bloch-McConnell-Solomon formulation to evaluate the relaxation and motion dynamic parameters that describe interactions of ATP with cellular solids in human brain tissue. The maximal transient NOE, observed as a reduction in the β-ATP signal, was 24 ± 2% upon band inversion of γ- and α-ATP, which is 2-3-fold higher than achievable by frequency-selective inversion of either γ- or α-ATP. The rate of 31 P-31 P cross relaxation (0.21 ± 0.02 s-1 ) led to a τc value of (9.1 ± 0.8) × 10-8 s for ATP in human brain. The T1 relaxation of β-ATP is dominated by CSA over the DD mechanism (60%: 40%). The band inversion method proved effective in amplifying 31 P NOE, and thus facilitating ATP τc and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. Magn Reson Med 77:1409-1418, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Host-sensitized luminescence properties in CaNb2O6:Ln(3+) (Ln(3+) = Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+)) phosphors with abundant colors.

A series of Ln(3+) (Ln(3+) = Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+)) ion doped CaNb2O6 (CNO) phosphors have been prepared via the conventional high-temperature solid-state reaction route. The X-ray diffraction (XRD) and structure refinement, diffuse reflection, photoluminescence (PL), and fluorescent decay curves were used to characterize the as-prepared samples. Under UV radiation, the CNO host present a broad emission band from about 355 to 605 nm centered around 460 nm originating from the NbO6 octahedral groups, which has spectral overlaps with the excitation of f-f transitions of Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+) in CNO:Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+) samples. They show both host emission and respective emission lines derived from the characteristic f-f transitions of activators, which present different emission colors owing to the energy transfer from the NbO6 group in the host to Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+) with increasing activator concentrations. The decreases of decay lifetimes of host emissions in CNO:Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+) demonstrate the energy transfer from the hosts to Eu(3+)/Tb(3+)/Dy(3+)/Sm(3+). The energy transfer mechanisms in CNO:Eu(3+)/Tb(3+)/Dy(3+) phosphors have been determined to be a resonant type via dipole-dipole mechanisms. For CNO:Sm(3+), the metal-metal charge transfer transition (MMCT) might contribute to the different variations of decay lifetimes and emission intensity from CNO:Eu(3+)/Tb(3+)/Dy(3+) samples. The best quantum efficiency is 71.2% for CNO:0.01/0.02Dy(3+). The PL properties of as-prepared materials indicate the promising application in UV-pumped white-emitting lighting diodes field.

High resolution magic angle spinning NMR as a tool for unveiling the molecular enantiorecognition of omeprazole by amylose-based chiral phase

Polysaccharide-based chiral stationary phases (CSP) demonstrate great versatility and higher chiral selectivity for a variety of chiral compounds in multimodal elution modes (normal, reverse and polar organic). The main role of CSP phenyl carbamate based derivatives as chiral selectors is the formation of diastereoisomeric complexes by means of π-π interaction, dipole-dipole, hydrogen bonding and/or inclusion complex mechanisms. Nevertheless, the mechanism behind their enantioselectivity requires clarification. High resolution magic angle spinning nuclear magnetic resonance spectroscopy ((1)H HR/MAS NMR) has provided key information on the recognition process at the binding sites of the CSP surface. Herein we report the results obtained using omeprazole as a probe for these investigations.

Sensitivity enhancement and low-field spin relaxation in singlet NMR

The singlet states of nuclear spin-1/2 pairs often display extended lifetimes that can be an order of magnitude longer than conventional relaxation times. We show that, in favourable circumstances, acquisition of the NMR signal during an extended multiple spin-echo train, followed by suitable data processing, enhances the signal-to-noise ratio of singlet NMR by up to an order of magnitude. The achievable enhancement depends on the transverse relaxation time constant, the magnetic field inhomogeneity, and the acceptable degradation in digital spectral resolution. We use the combination of singlet NMR and multiple spin-echo data acquisition to study the low-field nuclear relaxation processes of (15)N-labelled nitrous oxide ((15)N(2)O) in solution. A general relaxation theory for coupled 2-spin-1/2 systems in low magnetic field is developed. Experimental trajectories of the nuclear spin observables are compared with theoretical expressions, including dipole-dipole and spin-rotation relaxation mechanisms. The estimated values of the spin-rotation tensors are compared with previous estimations from NMR and molecular beam electric resonance.

Spectral and spatial transfer and diffusion of excitons in multiple quantum dot structures

A formalism is developed for resonant and nonresonant spectral and spatial energy transfer of excitons in disordered semiconductor multiple-quantum-dot structures. Dipole-dipole and photon-exchange energy-transfer mechanisms are considered. For nonresonant transfer, we study two-site transfer rates in a disordered system as a function of the energy mismatch, the temperature, and the distance. The total time-dependent decay rate of the initial spectral intensity excited at a given energy in the inhomogeneous spectral profile is calculated. For resonant transfer, two-site transfer rates are studied as a function of the distance. The diffusion constant is calculated exactly in a regular quantum dot lattice in order to assess the upper limit of the diffusion constant of a disordered system. We find that the total time-dependent spectral decay rate and the diffusion constant are dominated by the weak long-range photon-exchange interaction mechanism over the standard short-range F\"orster (dipole-dipole) mechanism in a uniform macroscopic multi-quantum-dot system due to the long mean-free path of the photons.

Hydration Equilibrium Constants of Aldehydes, Ketones and Quinazolines

SPARC chemical reactivity and physical processes models were coupled and extended to calculate hydration equilibrium constants for aldehydes, ketones, quinazoline and substituted quinazolines compounds from molecular structure. The energy differences between the initial (anhydrible) and the final (hydrated) states in the gas phase for a molecule of interest were calculated using SPARC mechanistic perturbation models. These perturbations models quantify the interactions of the appended perturber (P) with the carbonyl reaction center of the aldehydes or ketones or with the imine reaction center of the quinazolines. The perturbations of the reaction center were factored into mechanistic components of electrostatic, resonance and steric effects in the gas phase. The solvation energy (Henry's constant) of the anhydrible and the hydrated states on going from the gas phase to the aqueous phase were calculated using SPARC physical processes models. These physical processes models quantify the intermolecular interactions between the solute and the solvent molecules upon placing a solute molecule in the aqueous phase. The intermolecular interactions are factored into dispersion, induction, dipole-dipole and H-bonding interaction mechanisms. The RMS deviation error was 0.36 pK h y d r a t i o n units for 36 aldehyde and ketone compounds and 0.43 pK h y d r a t i o n units for quinazoline and 31 substituted quinazoline compounds.

An NMR experiment for the accurate measurement of heteronuclear spin-lock relaxation rates.

Rotating-frame relaxation rates, R(1)(rho), are often measured in NMR studies of protein dynamics. We show here that large systematic errors can be introduced into measured values of heteronuclear R(1)(rho) rates using schemes which are usually employed to suppress cross-correlation between dipole-dipole and CSA relaxation mechanisms. For example, in a scalar-coupled two-spin X-H spin system the use of (1)H WALTZ16 decoupling or (1)H pulses applied at regularly spaced intervals leads to a significant overestimation of heteronuclear R(1)(rho) values. The problem is studied experimentally and theoretically for (15)N-(1)H and (13)C-(1)H spin pairs, and simple schemes are described which eliminate the artifacts. The approaches suggested are essential replacements of existing methodology if accurate dynamics parameters are to be extracted from spin-lock relaxation data sets.

Automated NMR determination of protein backbone dihedral angles from cross-correlated spin relaxation.

The simultaneous interpretation of a suite of dipole-dipole and dipole-CSA cross-correlation rates involving the backbone nuclei 13Calpha, 1Halpha, 13CO, 15N and 1HN can be used to resolve the ambiguities associated with each individual cross-correlation rate. The method is based on the transformation of experimental cross-correlation rates via calculated values based on standard peptide plane geometry and solid-state 13CO CSA parameters into a dihedral angle probability surface. Triple resonance NMR experiments with improved sensitivity have been devised for the quantification of relaxation interference between 1Halpha(i)-13Calpha(i)/15N(i)-1HN(i) and 1Halpha(i-1)-13Calpha(i-1)/15N(i)-1HN(i) dipole-dipole mechanisms in 15N, 13C-labeled proteins. The approach is illustrated with an application to 13C, 15N-labeled ubiquitin.

Measurement of 3hJNC' connectivities across hydrogen bonds in a 30 kDa protein.

A method is described which permits detection of 3hJNC' scalar couplings across hydrogen bonds in larger, perdeuterated proteins. The experiment is demonstrated for the uniformly 2H/13C/15N-enriched 30 kDa ribosome inactivating protein MAP30. The 3hJNC' interactions are smaller than 1 Hz, but their detection in an HNCO experiment is made possible through the use of constructive interference between the 15N chemical shift anisotropy and 1H-15N dipole-dipole relaxation mechanisms in a manner similar to that of recently proposed TROSY schemes. Sensitivity of the HNCO experiment depends strongly on the 15N transverse relaxation rate of the downfield 15N multiplet component and on the amide proton T1. In perdeuterated MAP30 at 40 degrees C, the average TROSY T2 was 169 ms at 750 MHz 1H frequency, and a wide range of longitudinal relaxation rates was observed for the amide protons.

Interaction of Tm3+ ions in calcium-niobium-gallium and yttrium-aluminum garnet laser crystals

The cross relaxation of Tm3+ ions (on the 3H4→3F4, 3H6→3F4 transitions) has been studied in CNGG and YAG laser crystals. The radiationless transfer of energy from Cr3+ ions to Tm3+ ions has been studied in YAG crystals. The interaction between these ions is shown to be due to dipole-dipole and shorter-range quadrupole-quadrupole mechanisms.

ChemInform Abstract: NMR Relaxation Studies on Testosterone in Solution: Magnetic Field Dependence of 13C T1 and Anisotropies in the Chemical Shift.

The magnetic field dependence of NMR 13C relaxation times have been measured for testosterone in CDCl3. The data were analysed by a computer-assisted method based on the molecular reorientational motion. For the quaternary carbons, contributions from other than the dipole-dipole relaxation mechanisms are separated into the two terms of chemical-shift anisotropy (T1CA) and spin rotation interaction (T1SR). Anisotropies in the chemical shift are derived from T1CA thus separated. These anisotropies are comparable to the existing data, supporting this method of treatment. The barrier to internal rotation is determined for the two methyl groups; it is higher for the 19-methyl by ca. 2 kJ mol–1 than for the 18-methyl.

NMR relaxation studies on testosterone in solution: magnetic field dependence of 13C T 1 and anisotropies in the chemical shift

The magnetic field dependence of NMR 13C relaxation times have been measured for testosterone in CDCl3. The data were analysed by a computer-assisted method based on the molecular reorientational motion. For the quaternary carbons, contributions from other than the dipole-dipole relaxation mechanisms are separated into the two terms of chemical-shift anisotropy (T1CA) and spin rotation interaction (T1SR). Anisotropies in the chemical shift are derived from T1CA thus separated. These anisotropies are comparable to the existing data, supporting this method of treatment. The barrier to internal rotation is determined for the two methyl groups; it is higher for the 19-methyl by ca. 2 kJ mol–1 than for the 18-methyl.

The influence of cross correlation on multiplet patterns in nuclear overhauser effect spectra

In the presence of cross correlation between the chemical-shift anisotropy and dipole-dipole relaxation mechanisms the individual lines of a weakly coupled multiplet can receive different NOE enhancements. The resulting multiplet asymmetry is due to the presence of antiphase terms which arise solely as a result of relaxation, rather than their more usual origin through selective population transfer effects. The enhancements measured from the overall integrated multiplet intensity are not substantially affected by cross correlation. Experimentally, the multiplet asymmetry is especially pronounced in three spin {H} 19FH systems, when observed at high-field strengths. Similar effects are predicted for cross correlation between separate dipole-dipole relaxation pathways. The generation of these antiphase terms is related to the appearance of “forbidden” cross peaks in multiple-quantum-filtered COSY spectra.

Models for intramolecular triplet exciton migration applied to vinyl aromatic polymers

Models for intramolecular triplet exciton migration have been developed for vinyl aromatic polymers. A one-dimensional model which allows only neighbor-to-neighbor migrations yields frequencies which are several orders of magnitude smaller than those predicted either by exchange or dipole-dipole mechanisms. An intramolecular model permitting three translational degrees of freedom predicts triplet exciton hop frequencies on the order of 10 4 s − in reasonable agreement with either exchange or dipole-dipole mechanisms.

Nuclear spin relaxation by paramagnetic ions in crystals: diffusionless and rapid diffusion limits

The theory of nuclear spin relaxation due to paramagnetic ions in crystals can be treated rigorously in the rapid diffusion and dimensionless limits for all concentrations of a random distribution of paramagnetic ions if the ion-nucleus interaction is known. Some approximations commonly made in the literature to evaluate the relaxation functions in these limits are critically examined for the dipole-dipole and indirect RKKY relaxation mechanisms. If spherically averaging the dipole-dipole mechanism is avoided the resulting relaxation rates are anisotropic. The anisotropy is calculated for cubic crystals and is large enough to be observed experimentally. It is shown that the usual approximations of evaluating the lattice summation in the rapid diffusion limit as an integral and of averaging the RKKY range functions can introduce appreciable errors. The form of the magnetisation recovery in the diffusionless limit is calculated for the dipole-dipole mechanism and the free electron form of the Benoit-de Gennes-Silhouette (BGS) RKKY mechanism without making these approximations. Difficulties arise for the Giovannini-Heeger (GH) RKKY mechanism in that it can yield negative results for the relaxation rates if the free electron form is used.