Abstract: The rate of energy transfer from B800 to B850 in the peripheral light harvesting complex LH2 is modeled in detail. A method for determining ensemble average energy transfer rates in complex, coupled multichromophoric systems is reported and is employed to investigate the interplay of electron−phonon coupling (fast fluctuations of the protein) and site energy disorder (slow fluctuations) on the spectral overlap between donor and acceptor, and therefore the energy transfer rate. A series of model calculations for Rb. sphaeroides is reported. The disorder is found to have a marked influence on the calculated rate of energy transfer and is responsible for a faster energy transfer time than would occur in its absence and furthermore accounts for the weak temperature dependence observed in experiment. The excitonic nature of the acceptor (albeit dynamically localized) also has impact in terms of how B850 functions as an energy acceptor. These conclusions are further elucidated by calculations of Rps. acidophila...

Abstract: The electrical properties of p-type Mg-doped GaN are investigated through variable-temperature Hall effect measurements. Samples with a range of Mg-doping concentrations were prepared by metalorganic chemical vapor phase deposition. A number of phenomena are observed as the dopant density is increased to the high values typically used in device applications: the effective acceptor energy depth decreases from 190 to 112 meV, impurity conduction at low temperature becomes more prominent, the compensation ratio increases, and the valence band mobility drops sharply. The measured doping efficiency drops in samples with Mg concentration above 2×1020 cm−3.

Abstract: The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve as sensitive indicators of the status of proton donation (“d”) and acceptance (“a”) (i.e., the structural signature of H-bond type) for each water monomer in the cluster. In general, the magnitudes of the O–H frequencies (ν) for each cluster can be arranged in the following order: ν3da (single donor–single acceptor) ≅ν3daa (single donor–double acceptor) >ν3dda (double donor–single acceptor) >ν1dda>ν1da> (or ≅) ν1daa. The increase in the cluster size has a pronounced effect on the decrease of the lower frequencies. However, there are small changes in the higher frequencies (ν3da and ν3daa). The intensities of ν1daa and ν1da are very high, since the increased atomic charges can be correlated to the enhanced H-bond relay effect. On the other hand, the intensities of the ν1dda modes are diminished by more than half. Most of the above data have been compared to the available experimental data. Keeping in view the recent experimental reports of the HOH bending modes, we have also analyzed these modes, which show the following trend: ν2dda>ν2daa≅ν2da. The present study therefore would be useful in the assignments of the experimental O–H stretching and HOH bending modes.

Abstract: The adsorption–desorption equilibria of di-oxygen (O2) on gold cluster anions ( Au N − , N=2–22 ) have been measured in the high-pressure limit by pulsed flow-reactor methods at ambient temperature. Only certain clusters (N=2,4,6,8,10,12,14,18,20) exhibit measurable adsorption, and for most of these the reaction can be driven to completion (>90% conversion to AuNO2−). In no case could secondary adsorption be detected, i.e. a 1:1 reaction stoichiometry is strictly obeyed. The size (N) variation in the adsorption free energies is correlated with the known electron affinities of the gold clusters. Taken with the positive electron affinity of O2, these results suggest that O2 acts as a single-electron acceptor, yielding adsorbed superoxide (O2−), while the gold cluster strives to pair electrons (or close electron shells). This species could serve as an activated form for low-temperature oxygen-atom transfer reactions catalyzed by supported gold clusters, wherein the activation of molecular oxygen is regarded as a crucial step.

Abstract: Both proton transfer and hydrogen bonding play important roles in biological systems. In order to measure hydrogen bond basicity, we are building a new scale that differs significantly from the pKa scale of proton transfer basicity. The strength of hydrogen bond acceptors (HBAs) is measured from the Gibbs energy change ΔGHB for the formation of 1:1 hydrogen bonding complexes between hydrogen bond acceptors (bases) and a reference hydrogen bond donor (4-fluorophenol) in tetrachloromethane at 298 K. The pKHB database (1.364 pKHB =–ΔGHB (kcal mol-1)) comprises ca. 1000 hydrogen bond acceptors. The HBA strength depends on (i) the position of the acceptor atom in the periodic table, (ii) polarizability, field/inductive and resonance effects of substituents around the acceptor atom, and (iii) proximity effects including steric hindrance of the acceptor site, intramolecular hydrogen bonding and lone-pair–lone-pair repulsions. The ranking of oxygen and sp nitrogen bases does not depend very much on the solvent and the reference hydrogen bond donor, but sp2 and sp3 nitrogen bases gain strength in solvents of higher reaction field than CCl4 and lose strength toward CH and weak NH donors. The complete scatter pattern exhibited by the pKa versus pKHB plot demonstrates the non-equivalence of the two scales. The HBA strength scale is applied to the prediction of the hydrogen bonding site in polybasic drugs (e.g strychnine and carbimazole), and to the calculation of octanol–water partition coefficients. A possible relationship between HBA strength and antihistaminic activity is studied for the `push–pull' drugs cimetidine, ranitidine and famotidine.

Abstract: The cyclophane-type molecular dyads 1 x 2H and 1 x Zn, in which a doubly bridged porphyrin donor adopts a close, tangential orientation relative to the surface of a fullerene acceptor, were prepared by Bingel macrocylization. The porphyrin derivatives 2 x 2H and 2 x Zn with two appended, singly linked C60 moieties were also formed as side products. NMR investigations revealed that the latter compounds strongly prefer conformations with one of the carbon spheres nesting on the porphyrin surface, thereby taking a similar orientation to that of the fullerene moiety in the doubly bridged systems. Cyclic voltammetric measurements showed that the mutual electronic effects exerted by the fullerene on the porphyrin and vice versa are only small in all four dyads, despite the close proximity of the donor and acceptor components. The steady-state and time-resolved absorption and luminescence properties of 1 x Zn and 2 x Zn were investigated in toluene solution and it was shown that, upon light excitation, both the porphyrin- and the fullerene-centered excited states are deactivated to a lower-lying CT state, emitting in the IR spectral region (lambda max = 890 and 800 nm at 298 and 77 K, respectively). In the more polar solvent benzonitrile, this CT state is still detected but, owing to its very low energy (below 1.4 eV), is not luminescent and shorter-lived than in toluene. The remarkable observation of similar photophysical behavior of 1 x Zn and 2 x Zn suggests that a tight donor-acceptor distance cannot only be established in doubly bridged cyclophane-type structures but also in singly bridged dyads, by taking advantage of favourable fullerene-porphyrin ground-state interactions.

Abstract: The spontaneous assembly of aromatic cation−radicals (D+•) with the parent donor (D) to afford the paramagnetic dimer (D)2+• is accompanied by a dramatic color change. For example, spectral (UV−vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation−radical with its neutral counterpart to produce the mixed-valence or dimeric cation−radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation−radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation−radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation−radicals (D)2+• (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of M...

Abstract: Acceptor and donor doped SrTi1−xFexO3−δ materials for novel temperature independent resistive oxygen sensors for lean-burn engine exhaust gases were prepared and characterized by X-ray diffraction. Their electrical resistance, R, was investigated in the oxygen partial pressure range from 10−4 to 1 bar between 700°C and 1000°C. Doped and undoped samples with x=0.3 obey an R∝pO2−1/5 power law. Undoped samples show negligible temperature dependence in a small oxygen partial pressure (pO2) range between 10−2 and 3×10−2 bar. Acceptor (Ga) doping shifts the pO2 range of negligible temperature dependence to lower pO2, whereas donor (La) doping results in a right-hand shift to higher pO2. Combined acceptor and donor doping leads to an extended pO2 range of negligible temperature dependence. At pO2=10−1 bar the temperature coefficient of resistance (TCR) of this material composition is always below 250 ppm/K in the complete investigated temperature range. At about 725°C and 975°C TCR=0 can be found.

Abstract: Self-assembled monolayers (SAMs) of α-helical peptides carrying an N-ethylcarbazolyl (ECz) group were prepared. The helical peptide SAMs on a gold surface were characterized by quartz crystal microbalance measurements, cyclic voltammetry, and impedance spectroscopy, showing that a tridecapeptide SAM with the N-terminal binding to gold was packed more densely than that with the C-terminal binding. The helix tilt angles from the surface normal in these peptide SAMs were found to be about 40° on the basis of Fourier transform infrared reflection−absorption spectroscopy. Photocurrent generation of these peptide SAMs in an aqueous solution was investigated by photoexcitation of ECz groups either in the presence of an electron donor or acceptor. In the presence of methyl viologen (electron acceptor), electron donation from a gold surface to an ECz group was observed upon photoexcitation. On the other hand, in the presence of triethanolamine or ethylenediaminetetraacetic acid (electron donor), the direction of p...

Abstract: Ab initio calculations on over fifty hydrogen bond donor molecules, and their complexes with a reference hydrogen bond acceptor, are reported. Properties calculated for the molecules and complexes are assessed for their ability to correlate and predict experimentally derived values of hydrogen bond donor capacity, α. Two such properties stand out as excellent predictors of α: both the electrostatic potential at the donor H nucleus and the hydrogen bond stabilization energy correlate α to close to the estimated experimental error. Several other calculated properties, including atomic charges and multipoles on the donor H, the extent of charge transfer from acceptor to donor, and topological properties of the electron density, are also correlated with α.

Abstract: Described herein are methods and reagents for the ligation of a peptide acceptor to an RNA, as well as the RNA-peptide acceptor products.

Abstract: The question as to whether the CH⋯O interaction constitutes a true H-bond is examined from the perspective of calculated NMR chemical shieldings Fluorinated methane derivatives F n H 3− n CH are taken as proton donors; H 2 O, CH 3 OH and H 2 CO serve as acceptors The shifts in the isotropic chemical shieldings of the bridging proton that occur upon formation of the complex are all within the fairly narrow range of −10 to −15 ppm These shifts become more negative as the donor is made more acidic, following the order H 3 CH 2 CH 2 HCH The carbonyl oxygen of the H 2 CO acceptor induces the most negative shift, followed in order by H 2 O and then the CH 3 OH The magnitudes of the anisotropic shifts are much larger, indicating that the tensor components perpendicular to the H-bond axis are more sensitive to formation of an H-bond than is the parallel analog The isotropic shift of the donor atom is quite sensitive to its separation from the acceptor molecule, even reversing sign as this distance changes The carbonyl and hydroxyl acceptor O atoms exhibit shifts of their isotropic shielding of opposite sign, a distinction due to the perpendicular component The behavior of the bridging proton in these CH⋯O complexes is quite similar to the water dimer with its conventional H-bond On the other hand, the classification of the CH⋯O interaction on the basis of the donor and acceptor atoms, is less conclusive, as these atoms undergo more complex behavior, and are more sensitive to the specific level of theory applied

Abstract: The structures of benzyl alcohol, its 1:1 water complex, and its dimer have been investigated by R2PI spectroscopy and IR−UV ion dip spectroscopy, combined with ab initio computation. The sole molecular conformer observed in the jet has a gauche arrangement of the gauche arrangement of the OH group relative to the C1−Cα bond, but the extent of π-type intramolecular H-bonding is small. Analysis of its rotational band contours suggests the incidence of vibronic coupling involving motion of the side chain and also leads to an estimate for the dihedral angle τ1(OCCC) lying in the range 35°−60°, in good agreement with the values (50°−60°) indicated by high-level ab initio calculations. The 1:1 water complex is assigned to a structure in which water binds as a proton acceptor to the alcohol group, and as a weak proton donor to the π-system of the aromatic ring. The arrangement of H-bonds is similar within the dimer: the OH of one molecule acts as both acceptor to the alcohol group and as donor to the π-system ...

Abstract: phase the anode and for the acceptor phase the cathode. This is not quite compatible with the more-or-less sharp interfaces found in bilayer photodiodes where a fullerene or other acceptor is evaporated on top of a polymer layer. If we wanted to make stratified but inter-diffused structures, a common solvent would hinder the utilization of these preparation methods for the fabrication of sharp multilayers. The application of special conditions during the spin-coating, however, makes it possible to obtain diffuse bilayer structures from soluble organic molecules and polymers. In this letter we present our attempts to combine the advantages of the interpenetrating network morphology in blends with the advantages of the bilayer structures, in order to obtain high fill-factor values in devices. This reflects the good conditions for charge collection and transport, which are essential for the realization of efficient

Abstract: Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport propert...

Abstract: We present a laser-based direct write technique termed matrix-assisted pulsed-laser evaporation direct write (MAPLE DW). This technique utilizes a laser transparent fused silica disc coated on one side with a composite matrix consisting of the material to be deposited mixed with a laser absorbing polymer. Absorption of laser radiation results in the decomposition of the polymer, which aids in transferring the solute to an acceptor substrate placed parallel to the matrix surface. Using MAPLE DW, complex patterns consisting of metal powders, ceramic powders, and polymer composites were transferred onto the surfaces of various types of substrates with <10 micron resolution at room temperature and at atmospheric pressure without the use of masks.

Abstract: A semiconductor device is provided having n-type device layers of III-V nitride having donor dopants such as germanium (Ge), silicon (Si), tin (Sn), and/or oxygen (0) and/or p-type device layers of III-V nitride having acceptor dopants such as magnesium (Mg), beryllium (Be), zinc (Zn), and/or cadmium (Cd), either simultaneously or in a doping superlattice, to engineer strain, improve conductivity, and provide longer wavelength light emission.

Abstract: To understand the influence of substituents on intramolecular charge transfer (ICT), absorption and emission spectra were measured for several derivatives of 2-styrylquinoline (2-StQ), 2-styrylpyridine (2-StP) and 2-styrylthiophene (2-StT) systems. A strong donor, such as an N,N-dimethylamino group can produce an ICT compound in these systems, and the excited state dipole moments were measured using a solvatochromic method. Quinoline and pyridine can only act as acceptors. Depending on substituents, the thiophene ring could be a donor or an acceptor. For same donor in our systems, the strength order of different acceptors is quinoline>naphthalene>pyridine>thiophene. Besides electronic effect, steric conformation is also an important factor in ICT molecules. Protonation and hydrogen bonding at acceptor site show red-shift in absorption maxima. The degree of shift depends on the substituents. Interaction at a donor site gives the opposite result. There is a linear relation between excited state dipole moments and absorption energy.

Abstract: The deep nature of the Mg acceptor will have important implications for the performance of high-speed GaN-based bipolar devices In this work, the effect of the deep acceptor on the band bending within the depletion region is examined in detail The width of the transition region, which separates the mobile holes from the space-charge edge, is carefully investigated High-frequency modulation of the depletion region is discussed for both the large- and small-signal cases For the small-signal case, calculated results are compared to experimental measurements of frequency-dependent capacitance which have been performed on Mg-doped GaN samples

Abstract: A novel molecular electron donor−acceptor (DA) dyad, composed of a phytochlorin donor and a [60]fullerene acceptor, was used for the preparation of solid molecular films capable of performing vectorial photoinduced electron transfer (VPET). Being mixed with octadecylamine at concentrations of 50 mol % and lower, the DA compounds form a stable monolayer, which can be transferred onto a solid substrate. Thus prepared Langmuir−Blodgett (LB) monolayer films are characterized by uniform orientation of the DA molecules and, consequently, can undergo VPET. This was confirmed by time-resolved Maxwell displacement charge (TRMDC) measurements. The rate constant for the electron transfer was ca. 109 s-1 as estimated from the fluorescence lifetime measurements. The majority of the charge transfer states of the DA molecules (>60%) recombined with a time constant of ca. 30 ns, being almost independent of the DA concentration in the concentration range from 2 to 50 mol %, as revealed from TRMDC experiments. Therefore, V...

Abstract: The layer-by-layer alternate assemblies incorporating two kinds of cyanine dyes have been fabricated by alternately adsorbing a cationic polyelectrolyte and anionic cyanine dyes on the quartz plate. A thiacyanine dye (dye I) was employed as the donor and two kinds of thiacarbocyanine dye having a meso-alkyl groupm-ethyl (dye II), m-methyl (dye III)as the acceptor. The mole fraction of the acceptor in the mixed J-aggregate, χ, was varied from 0 to 1. It is confirmed that these dye combinations form the mixed J-aggregate in the alternate assemblies. From steady-state fluorescence spectra of the molecular assemblies, excitation energy transfer from the donor J-aggregate to the acceptor J-aggregate is observed, whose kinetics obeys the Stern−Volmer relationship. The experimentally determined rate constant of energy transfer, kET, is fairly large, indicating efficient energy transfer due to exciton migration through the donor J-aggregate. The relative fluorescence quantum yield and the fluorescence lifetime of...