Micro Far-Infrared Reflectivity of CaNb(2)O(6) Single Crystal Fibers Grown by the Laser-Heated Pedestal Growth Technique

CaNb(2)O(6) single crystal fibers were grown by the laser-heated pedestal growth technique, directly from the starting reagents. Optically transparent fibers were obtained in the form of rods with elliptical cross-section, free from cracks, impurities, and secondary phases, with an average diameter of 0.4 mm and about 20 mm of length. The fibers grew within the orthorhombic Pbcn columbite structure, with the growth axis nearly parallel to the crystallographic a-direction. The parameters b and c were parallel to the shorter and larger ellipsis axes. A special setup using a microscope was developed to obtain the far-infrared reflectivity spectra of these micrometer-sized fibers, allowing the identification and assignment of 34 of the 38 polar phonons foreseen for the material. From these phonons, the intrinsic dielectric constant ( = 18.2) and quality factor ( of 185 THz) could be estimated, showing the potential of the material for applications in microwave circuitry. These results, along with previous polarized Raman data (Cryst. Growth Des. 2010, 10, 1569), allow us to present a comprehensive set of optical phonon modes and to discuss the potential use of designed CaNb(2)O(6) microcrystals in compact optical devices.

fac-/mer-[RuCl(3)(NO)(P-N)] (P-N = [o-(N,N-dimethylamino)phenyl]diphenylphosphine): Synthesis, characterization and DFT calculations

Complex fac-[RuCl(3)(NO)(P-N)] (1) was synthesized from the reaction of [RuCl(3)(H(2)O)(2)(NO)] and the P-N ligand, o-[(N,N-dimethylamino)phenyl]diphenylphosphine) in refluxing methanol solution, while complex mer,trans-[RuCl(3)(NO)(P-N)] (2) was obtained by photochemical isomerization of (1) in dichloromethane solution. The third possible isomer mer, cis-[RuCl(3)(NO)(P-N)] (3) was never observed in direct synthesis as well as in photo-or thermal-isomerization reactions. When refluxing a methanol solution of complex (2) a thermally induced isomerization occurs and complex (1) is regenerated. The complexes were characterized by NMR ((31)P{(1)H}, (15)N{1H} and 1H), cyclic voltammetry, FTIR, UV-Vis, elemental analysis and X-ray diffraction structure determination. The (31)P{(1)H} NMR revealed the presence of singlet at 35.6 for (1) and 28.3 ppm for (2). The (1)H NMR spectrum for (1) presented two singlets for the methyl hydrogens at 3.81 and 3.13 ppm, while for (2) was observed only one singlet at 3.29 ppm. FTIR Ru-NO stretching in KBr pellets or CH(2)Cl(2) solution presented 1866 and 1872 cm(-1) for (1) and 1841 and 1860 cm(-1) for (2). Electrochemical analysis revealed a irreversible reduction attributed to Ru(II)-NO(+) -> Ru(II)-NO(0) at -0.81 V and -0.62 V, for (1) and (2), respectively; the process Ru(II) -> Ru(III), as expected, is only observed around 2.0 V, for both complexes. Studies were conducted using (15)NO and both complexes were isolated with (15)N-enriched NO. Upon irradiation, the complex fac-[RuCl(3)(NO)(P-N)] (1) does not exchange (14)NO by (15)NO, while complex mer, trans-[RuCl(3)(NO)(P-N)] (2) does. Complex mer, trans-[RuCl(3)((15)NO)(P-N)] (2`) was obtained by direct reaction of mer, trans-[RuCl(3)(NO)(P-N)] (2) with (15)NO and the complex fac-[RuCl(3)((15)NO)(P-N)] (1`) was obtained by thermal-isomerization of mer, trans-[RuCl(3)((15)NO)(P-N)] (2`). DFT calculation on isomer energies, electronic spectra and electronic configuration were done. For complex (1) the HOMO orbital is essentially Ru (46.6%) and Cl (42.5%), for (2) Ru (57.4%) and Cl (39.0%) while LUMO orbital for (1) is based on NO (52.9%) and is less extent on Ru (38.4%), for (2) NO (58.2%) and Ru (31.5%). (C) 2011 Elsevier B. V. All rights reserved.

On the generalized eigenvalue method for energies and matrix elements in lattice field theory

We discuss the generalized eigenvalue problem for computing energies and matrix elements in lattice gauge theory, including effective theories such as HQET. It is analyzed how the extracted effective energies and matrix elements converge when the time separations are made large. This suggests a particularly efficient application of the method for which we can prove that corrections vanish asymptotically as exp(-(E(N+1) - E(n))t). The gap E(N+1) - E(n) can be made large by increasing the number N of interpolating fields in the correlation matrix. We also show how excited state matrix elements can be extracted such that contaminations from all other states disappear exponentially in time. As a demonstration we present numerical results for the extraction of ground state and excited B-meson masses and decay constants in static approximation and to order 1/m(b) in HQET.

Luminescence characteristics of YAP:Ce scintillator powders and composites

Cerium doped yttrium aluminate perovskite YAlO(3) (YAP) powders are pursued as interesting alternatives to bulk crystals for application in scintillating devices. The emissions of these materials in the near-UV and visible spectral regions originate from electric dipole transitions between 4f and 5d energy levels of Ce(3+) and largely depend on the environment occupied by the ion. In search for improved synthesis conditions that can lead to phase pure powders with optimized structural and spectroscopic characteristics, in this work we have employed the polymeric precursor (Pechini) method to prepare crystalline and amorphous YAP:Ce powders doped with 1-10 mol% Ce(3+). Interesting composite materials were also obtained by dispersing some of the YAP:Ce powders in silica xerogels. A comparative structural and spectroscopic study of all the samples was done by XRD, FT-IR, emission, excitation and excited state lifetime measurements. In agreement with previous reports, excitation at 296 nm results in intense emission in the range 315-425 nm with an average decay time of 30 ns. (c) 2010 Elsevier B.V. All rights reserved.

L-(2-Quinolyl)-2-naphthol: A new intra-intermolecular photoacid-photobase molecule

Photochemical and photophysical properties of 1-(2-quinolyl)-2-naphthol (2QN) in water and organic solvents, as well in glassy media were studied to investigate the occurrence of intramolecular excited state prototropic reactions between the naphthol and quinoline rings. Spectral data show the two chromophores apparently behaving independently. However, in acid aqueous media or in low polarity solvents a new electronic transition red shifted band with respect to that of the parent compounds assigned to an intramolecular H-bond and to a quinoid form, respectively, shows up. Model calculations and R-X data lend support to a minimum energy conformer having a dihedral angle of similar to 39 degrees between the two groups. Singlet excited state properties (S-1) show a high suppressive effect of one ring over the other, resulting in very low emission yields at room temperature. The occurrence of excited state intramolecular proton transfer is observed in water (zwitter ion form) and in low polarity media (quinoid form) and originates from a previously CT H-bonded state. Phosphorescence data allowed a reasonable description of the electronic states of 2QN. In addition two new derivatives were prepared having the N atom blocked by methylation and both the N and O groups blocked by a CH2 bridge. The spectral data of these two compounds confirmed the attributions made for 2QN. (C) 2007 Elsevier B.V. All rights reserved.

On the use of 2,1,3-benzothiadiazole derivatives as selective live cell fluorescence imaging probes

Newly designed 2,1,3-benzothiadiazole-containing fluorescent probes with four excited state intramolecular proton transfer (ESIPT) sites were successfully tested in live cell-imaging assays using a confluent monolayer of human stem-cells (tissue). All tested dyes were compared with the commercially available DAPI and gave far better results. (c) 2010 Elsevier Ltd. All rights reserved.

Solvatochromic Shift of the pi -> pi* Transition in All-Trans, Cis-13, Cis-11, Cis-9, and Cis-7 Retinal Isomers Induced by Water and Methanol

The solvatochromic shift of the lowest singlet it pi -> pi* electronic transition in the all-trans, cis-13, cis-11, cis-9, and cis-7 retinal isomers were computed under the influence of water, methanol, and benzene solvents. Excitation energies were calculated in gas phase and in solution. The calculations in solution were performed considering the sequential Monte Carlo (MC) /Quantum Mechanical approach. The MC simulations were performed considering the full retinal isomer molecules and 900 water molecules, 900 methanol, or 400 benzene ones. The OPLS/AA parametrization was chosen for retinal, methanol, and benzene molecules and the SPC model was used for water one. From the MC calculations 100 independent configurations were selected, with 100 solvent molecules in thermodynamical equilibrium at T = 298.15 K. Average point-charges were obtained from those independent configurations for water, methanol, and benzene solvent. TDDFT and CASSCF//CASPT2 methodologies were used to compute the vertical excitation energy of the retinal isomers in different environment. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110: 2076-2087, 2010

Photophysics and photostability of adenine in aqueous solution: A theoretical study

The sequential Monte Carlo/CASPT2 approach was employed to investigate deactivation and emission processes from the lowest-lying pi pi * and n pi * excited states of 9H-adenine in aqueous solution. It is found that conical intersections connecting the pi pi* and n pi* states with the ground state are also present in solution, whereas the barriers for the deactivation paths are significantly smaller on solvated conditions. The large destabilization of the n pi* state found in solution possibly prevents its involvement in the deactivation photophysics and explains the change from a bi- to a mono-exponential decay for the molecule in the gas phase and solution, respectively. (C) 2010 Elsevier B.V. All rights reserved.

Picosecond Dynamics of the Prototropic Reactions of 7-Hydroxyflavylium Photoacids Anchored at an Anionic Micellar Surface

Three water-insoluble, micelle-anchored flavylium salts, 7-hydroxy-3-octyl-flavylium chloride, 4`-hexyl-7-hydroxyflavylium chloride, and 6-hexyl-7-hydroxy-4-methyl-flavylium chloride, have been employed to probe excited-state prototropic reactions in micellar sodium dodecyl sulfate (SDS). In SDS micelles, the fluorescence decays of these three flavylium salts are tetraexponential functions in the pH range from 1.0 to 4.6 at temperatures from 293 to 318 K. The four components of the decays are assigned to Four kinetically coupled excited species in the micelle: specifically, promptly deprotonable (AH(+)*) and nonpromptly deprotonable (AH(h)(+)*) orientations of the acid in the micelle. the base-proton geminate pair (A*center dot center dot center dot H(+)), and the free conjugate base (A*). The initial prompt deprotonation to form the germinate pair occurs at essentially the same rate (k(d) similar to 6-7 x 10(10) s(-1)) for all three photoacids. Recombination of the germinate pair is similar to 3-fold faster than the rate of proton escape from the pair (k(rec) similar to 3 x 10(10) s(-1) and k(diss) similar to 1 x 10(10) s(-1)), corresponding to an intrinsic recombination efficiency of the pair of similar to 75%. Finally, the reprotonation of the short-lived free A* (200-350 ps, depending oil the photoacid) has two components, only one of which depends oil the proton concentration in the intermicellar aqueous phase. Ultrafast transfer of the proton to water and substantial compartmentalization of the photogenerated proton at the micelle surface Oil the picosecond time scale strongly suggest preferential transfer of the proton to preformed hydrogen-bonded water bridges between the photoacid and the anionic headgroups. This localizes the proton in the vicinity of the excited base much more efficiently than ill bulk water, resulting ill the predominance of geminate re reprotonation at the micelle surface.

Picosecond Dynamics of Proton Transfer of a 7-Hydroxyflavylium Salt in Aqueous-Organic Solvent Mixtures

The intermediacy of the geminate base proton pair (A*center dot center dot center dot H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base proton pair A*center dot center dot center dot H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*center dot center dot center dot H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec) of A*center dot center dot center dot H(+)) from the diffusion controlled rates (dissociation, k(diss) and formation, k(diff)[H(+)], of A*center dot center dot center dot H+), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).

Modulation of trans-to-cis photoisomerization and photoluminescence of 1,2-bis(4-pyridyl)ethylene or 4-styrylpyridine coordinated to fac-tricarbonyl(5-chloro-1,10-phenathroline)rhenium(I)

Photochemical and photophysical properties of fac-[Re(CO)(3)(Clphen)(trans-L)](+) complexes, Clphen = 5-chloro-1,10-phenathroline and L = 1,2-bis(4-pyridyl)ethylene, bpe, or 4-styrylpyridine, stpy, were investigated to complement the understanding of intramolecular energy transfer process in tricarbonyl rhenium(I) complexes having an electron withdrawing group attached to polypyridyl ligands. These new compounds were synthesized, characterized and the photoisomerization quantum yields were accurately determined by (1)H NMR spectroscopy. The true quantum yields for fac-[Re(CO)(3)(Clphen) (trans-bpe)](+) were constant (Phi = 0.55) at all investigated irradiation wavelengths. However, for fac-[Re(CO)(3)(Clphen)(trans-stpy)](+), similar true quantum yields were observed only at higher energy irradiation (Phi(313 nm) = 0.53 and Phi(365 nm) = 0.57), but it decreased significantly at 404 nm (Phi = 0.41). These results indicated different deactivation pathways for the trans-stpy complex photoisomerization. Quantum yields decreased as the (3)IL(trans-L) and (3)MLCT(Re -> NN) excited states become closer and the behavior was discussed in terms of the excited state energy gaps. Additionally, luminescence properties of photoproducts, fac-[Re(CO)(3)(Clphen)(cis-L)](+), were also investigated in different environments to analyze the relative energy of the (3)MLCT(Re -> Clphen) excited state for each compound. (C) 2011 Elsevier B.V. All rights reserved.

Photoswitches and Luminescent Rigidity Sensors Based on fac-[Re(CO)(3)(Me(4)phen)(L)](+)

The fac-[Re(CO)(3)(Me(4)phen)(trans-L)](+) complexes, Me(4)phen = 3,4,7,8-tetramethyl-1,10-phenanthroline and L = 4-styrylpyridine, stpy, or 1,2-bis(4-pyridyl)ethylene, bpe, were synthesized and characterized by their spectroscopic, photochemical, and photophysical properties. The complexes exhibit trans-to-cis isomerization upon 313, 334, 365, and 404 nm irradiation, and the true quantum yields can be efficiently determined by absorption changes combined with (1)H NMR data. For fac-[Re(CO)(3)(Me(4)phen)(trans-bpe)](+) similar quantum yields were determined at all wavelengths investigated. However, a lower value (phi(true) = 0.35) was determined for fac-[Re(CO)(3)(Me(4)phen)(trans-stpy)](+) at 404 nm irradiation, which indicates different pathways for the photoisomerization process. The photoproducts, fac-[Re(CO)(3)(Me(4)phen)(cis-L)](+), exhibit luminescence at room temperature with two maxima ascribed to the (3)IL(Me4phen) and (3)MLCT(Re -> Me4phen) excited states. The luminescence properties were investigated in different media, and the behavior in glassy EPA at 77 K showed that the contribution of each emissive state is dependent on the excitation wavelength. The photochemical and photophysical behavior of the complexes were rationalized in terms of the energy gap of excited states and can be exploited in photoswitchable luminescent rigidity sensors.

2-Aminopurine non-radiative decay and emission in aqueous solution: A theoretical study

The minimum energy path along the lowest-lying pi pi* excited state of 2-aminopurine was calculated to elucidate the mechanisms of radiationless decay and emission in water. The sequential Monte Carlo quantum mechanics approach with a multiconfigurational and perturbative description of the wave function was employed to compute the minimum, transition state, and conical intersection. It was found that the barrier in the potential energy surface to access the conical intersection funnel increases in aqueous environment, making the system prone to enlarge the emission yield. These results rationalize the observed enhancement of emission in 2-aminopurine upon increasing of the solvent polarity. (c) 2008 Elsevier B.V. All rights reserved.

A theoretical study of the binding and electronic spectrum of the Mo-2 molecule

Multiconfigurational SCF and second-order perturbation theory have been employed to study seven low-lying singlet and triplet electronic states of the Mo-2 molecule. The bond order of the ground state has been analyzed based on the effective bond order (EBO), indicating that a fully developed sextuple bond is formed between the two Mo atoms. The experimentally observed excited states a(3)Sigma(+)(u) and A(1)Sigma(+)(u) have been determined and the so-called (3)Lambda excited state identified as the b(3)Sigma(+)(u) state, in agreement with experimental expectations. (C) 2007 Elsevier B.V. All rights reserved.

(1)H NMR spectroscopy as a tool to determine accurate photoisomerization quantum yields of stilbene-like ligands coordinated to rhenium(I) polypyridyl complexes

In this work, the use of proton nuclear magnetic resonance, (1)H NMR, was fully described as a powerful tool to follow a photoreaction and to determine accurate quantum yields, so called true quantum yields (Phi(true)), when a reactant and photoproduct absorption overlap. For this, Phi(true) for the trans-cis photoisomerization process were determined for rhenium(I) polypyridyl complexes, fac-[Re(CO)(3)(NN)(trans-L)](+) (NN = 1,10-phenanthroline, phen, or 4,7-diphenyl-1,10-phenanthroline, ph(2)phen, and L = 1,2-bis(4-pyridyl) ethylene, bpe, or 4-styrylpyridine, stpy). The true values determined at 365 nm irradiation (e. g. Phi(NMR) = 0.80 for fac-[Re(CO)(3)(phen)(trans-bpe)](+)) were much higher than those determined by absorption spectral changes (Phi(UV-Vis) = 0.39 for fac-[Re(CO)(3)(phen)(trans-bpe)](+)). Phi(NMR) are more accurate in these cases due to the distinct proton signals of trans and cis-isomers, which allow the actual determination of each component concentration under given irradiation time. Nevertheless when the photoproduct or reactant contribution at the probe wavelength is negligible, one can determine Phi(true) by regular absorption spectral changes. For instance, Phi(313) nm for free ligand photoisomerization determined both by absorption and (1)H NMR variation are equal within the experimental error (bpe: Phi(UV-Vis) = 0.27, Phi(NMR) = 0.26; stpy: Phi(UV-Vis) = 0.49, Phi(NMR) = 0.49). Moreover, (1)H NMR data combined with electronic spectra allowed molar absorptivity determination of difficult to isolate cis-complexes. (C) 2009 Elsevier B. V. All rights reserved.