Thin film voltammetry of spinach photosystem II. Proton-gated electron transfer involving the Mn4 cluster

Thin film voltammetry was used to obtain direct, reversible, electron-transfer peaks between electrodes and the spinach photosystem II (PS II) reaction center in lipid films for the first time. Three well-defined pairs of reduction-oxidation peaks were found using cyclic and square wave voltammetry at 4 degrees C at pH 7.5, reflecting direct, reversible electron transfer involving cofactors of PS II. These peaks were assigned to the oxygen-evolving complex (OEC) tetramanganese cluster (Em = 0.2 V vs NHE), quinones (Em = -0.29 V), and pheophytin (Em = -0.72 V). PS II that was depleted of the OEC did not give the peak at 0.2 V. Observed Em values, especially for the OEC, may be influenced by protein-lipid interactions and electrode double-layer effects. Voltammetry at pH 6 and at pH 7.5 with a time window of >100 ms revealed that the manganese cluster oxidation is gated by slow deprotonation of a reduced form. Additional rapid protonation/deprotonation steps are also involved in the electrochemical reduction-oxidation pathways.     

Exploring the electron transfer pathways in photosystem I by high-time-resolution electron paramagnetic resonance: observation of the B-side radical pair P700(+)A1B(-) in whole cells of the deuterated green alga Chlamydomonas reinhardtii at cryogenic temperatures

Crystallographic models of photosystem I (PS I) highlight a symmetrical arrangement of the electron transfer cofactors which are organized in two parallel branches (A, B) relative to a pseudo-C2 symmetry axis that is perpendicular to the membrane plane. Here, we explore the electron transfer pathways of PS I in whole cells of the deuterated green alga Chlamydomonas reinhardtii using high-time-resolution electron paramagnetic resonance (EPR) at cryogenic temperatures. Particular emphasis is given to quantum oscillations detectable in the tertiary radical pairs P700(+)A1A(-) and P700(+)A1B(-) of the electron transfer chain. Results are presented first for the deuterated site-directed mutant PsaA-M684H in which electron transfer beyond the primary electron acceptor A0A on the PsaA branch of electron transfer is impaired. Analysis of the quantum oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the geometry of the B-side radical pair. The orientation of the g tensor of P700(+) in an external reference system is adapted from a time-resolved multifrequency EPR study of deuterated and 15N-substituted cyanobacteria (Link, G.; Berthold, T.; Bechtold, M.; Weidner, J.-U.; Ohmes, E.; Tang, J.; Poluektov, O.; Utschig, L.; Schlesselman, S. L.; Thurnauer, M. C.; Kothe, G. J. Am. Chem. Soc. 2001, 123, 4211-4222). Thus, we obtain the three-dimensional structure of the B-side radical pair following photoexcitation of PS I in its native membrane. The new structure describes the position and orientation of the reduced B-side quinone A1B(-) on a nanosecond time scale after light-induced charge separation. Furthermore, we present results for deuterated wild-type cells of C. reinhardtii demonstrating that both radical pairs P700(+)A1A(-) and P700(+)A1B(-) participate in the electron transfer process according to a mole ratio of 0.71/0.29 in favor of P700(+)A1A(-). A detailed comparison reveals different orientations of A1A(-) and A1B(-) in their respective binding sites such that formation of a strong hydrogen bond from A1(-) to the protein backbone is possible only in the case of A1A(-). We suggest that this is relevant to the rates of forward electron transfer from A1A(-) or A1B(-) to the iron-sulfur center F(X), which differ by a factor of 10. Thus, the present study sheds new light on the orientation of the phylloquinone acceptors in their binding pockets in PS I and the effect this has on function.     

Electroactive core-shell nanocluster films of heme proteins, polyelectrolytes, and silica nanoparticles

Novel protein core-shell nanocluster films were assembled layer by layer on solid surfaces. In the first step, positively charged heme protein hemoglobin (Hb) or myoglobin (Mb) and negatively charged poly(styrenesulfonate) (PSS) were alternately adsorbed on the surface of SiO2 nanoparticles, forming core-shell SiO2-(protein/PSS)m nanoclusters. In the second step, the SiO2-(protein/PSS)m nanoclusters and polycationic poly(ethylenimine) (PEI) were assembled layer by layer on various solid substrates, forming [[SiO2-(protein/PSS)m]/PEI]n films. Various techniques were used to characterize the nanoclusters and monitor the film growth. [[SiO2-(protein/PSS)m]/PEI]n films at pyrolytic graphite (PG) electrodes exhibited well-defined, chemically reversible cyclic voltammetric reduction-oxidation peaks characteristic of the heme Fe(III)/Fe(II) redox couples. The proteins in the films retained near native conformations in the medium pH range, and the films catalyzed electrochemical reduction of oxygen and hydrogen peroxide. Advantages of the nanocluster films over the simple [SiO2/protein]n layer-by-layer films include a larger fraction of electroactive protein and higher specific biocatalytic activity. Using this approach, biocatalytic activity can be tailored and controlled by varying the number of bilayers deposited on the nanoparticle cores and the number of nanocluster layers on electrodes.     

Transmembrane charge transfer in photosynthetic reaction centers: some similarities and distinctions

This mini review presents a general comparison of structural and functional peculiarities of three types of photosynthetic reaction centers (RCs)--photosystem (PS) II, RC from purple bacteria (bRC) and PS I. The nature and mechanisms of the primary electron transfer reactions, as well as specific features of the charge transfer reactions at the donor and acceptor sides of RCs are considered. Comparison of photosynthetic RCs shows general similarity between the core central parts of all three types, between the acceptor sides of bRC and PS II, and between the donor sides of bRC and PS I. In the latter case, the similarity covers thermodynamic, kinetic and dielectric properties, which determine the resemblance of mechanisms of electrogenic reduction of the photooxidized primary donors. Significant distinctions between the donor and acceptor sides of PS I and PS II are also discussed. The results recently obtained in our laboratory indicate in favor of the following sequence of the primary and secondary electron transfer reactions: in PS II (bRC): Р(680)(Р(870)) → Chl(D1)(В(А)) → Phe(bPhe) → Q(A); and in PS I: Р(700) → А(0А)/A(0B) → Q(A)/Q(B).     

Afterglow thermoluminescence measured in isolated chloroplasts

The thermoluminescence afterglow (AG) measured in plant leaves originates from the S(2)/S(3)Q(B)(-) charge pair recombination in photosystem II (PSII) initiated by reverse electron flow from stromal reductants to PQ and then to the Q(B) site in PSII centers that are in the S(2)/S(3)Q(B) state. In this study, we show that this luminescence, absent in isolated thylakoid membranes, can be measured in intact chloroplasts that retain their stromal content including the electron acceptor pool (oxidized ferredoxin/NADP(+)) of photosystem I. The properties of the chloroplasts AG emission is similar to the AG in leaves in terms of temperature maximum, period-four modulation, far-red light stimulation, and antimycin A inhibition.     

Cyclic perfluorocarbon radicals and anions having high global warming potentials (GWPs): structures, electron affinities, and vibrational frequencies

Adiabatic electron affinities, optimized molecular geometries, and IR-active vibrational frequencies have been predicted for small cyclic hydrocarbon radicals C(n)H(2)(n)(-)(1) (n = 3-6) and their perfluoro counterparts C(n)F(2)(n)(-)(1) (n = 3-6). Total energies and optimized geometries of the radicals and corresponding anions have been obtained using carefully calibrated (Chem. Rev. 2002, 102, 231) density functional methods, namely, the B3LYP, BLYP, and BP86 functionals in conjunction with the DZP++ basis set. The predicted electron affinities show that only the cyclopropyl radical tends to bind electrons among the hydrocarbon radicals studied. The trend for the perfluorocarbon (PFC) radicals is quite different. The electron affinities increase with expanding ring size until n = 5 and then slightly decrease at n = 6. Predicted electron affinities of the hydrocarbon radicals using the B3LYP hybrid functional are 0.24 eV (C(3)H(5)/C(3)H(5)(-)), -0.19 eV (C(4)H(7)/C(4)H(7)(-)), -0.15 eV (C(5)H(9)/C(5)H(9)(-)), and -0.11 eV (C(6)H(11)/C(6)H(11)(-)). Analogous electron affinities of the perflurocarbon radicals are 2.81 eV (C(3)F(5)/C(3)F(5)(-)), 3.18 eV (C(4)F(7)/C(4)F(7)(-)), 3.34 eV (C(5)F(9)/C(5)F(9)(-)), and 3.21 eV (C(6)F(11)/C(6)F(11)(-)).     

Comparative bioelectrochemical study of core-shell nanocluster films with ordinary layer-by-layer films containing heme proteins and CaCO3 nanoparticles

Negatively charged heme protein hemoglobin (Hb) or myoglobin (Mb) at pH 9.0 and positively charged poly(diallyldimethylammonium) (PDDA) were alternately adsorbed on the surface of CaCO(3) nanoparticles, forming core-shell CaCO(3)-[PDDA/(protein/PDDA)(m)] ([protein-m]) nanoclusters. Oppositely charged [protein-m] and poly(styrenesulfonate) (PSS) were then assembled layer by layer on various solid substrates, forming {[protein-m]/PSS}(n) films. In the meantime, ordinary layer-by-layer films of heme proteins with CaCO(3) nanoparticles ({protein/CaCO(3)}(n)) were also grown on solid surfaces. Transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) spectroscopy, quartz crystal microbalance (QCM), and cyclic voltammetry (CV) were used to characterize the nanoclusters and monitor the growth of the two types of films. Both kinds of protein films assembled on pyrolytic graphite (PG) electrodes exhibited well-defined, nearly reversible CV reduction-oxidation peaks, characteristic of heme Fe(III)/Fe(II) redox couples, and were used to catalyze the electrochemical reduction of hydrogen peroxide. The {[protein-m]/PSS}(n) films demonstrate distinct advantages over the {protein/CaCO(3)}(n) films due to their larger fraction of electroactive proteins, higher catalytic efficiency, and better thermostability. The penetration experiments of the electroactive probe into these films indicate that the {[protein-m]/PSS}(n) nanocluster films possess more pores or channels than the simple {protein/CaCO(3)}(n) films, which may be beneficial to counterion transport in the charge-hopping mechanism and helpful for the diffusion of catalysis substrates into the films. In addition, the electrochemical and biocatalytic activity of protein nanocluster films can be tailored by controlling the number of bilayers assembled on the nanoparticle cores (m) as well as the film thickness or the number of nanocluster layers on the electrodes (n).     

Electron transfer kinetics in photosynthetic reaction centers embedded in polyvinyl alcohol films

The coupling between electron transfer and protein dynamics has been studied at room temperature in isolated reaction centers (RCs) from the photosynthetic bacterium Rhodobacter sphaeroides by incorporating the protein in polyvinyl alcohol (PVA) films of different water/RC ratios. The kinetic analysis of charge recombination shows that dehydration of RC-containing PVA films causes reversible, inhomogeneous inhibition of electron transfer from the reduced primary quinone acceptor (Q(A)(-)) to the secondary quinone Q(B). A more extensive dehydration of solid PVA matrices accelerates electron transfer from Q(A)(-) to the primary photooxidized electron donor P(+). These effects indicate that incorporation of RCs into dehydrated PVA films hinders the conformational dynamics gating Q(A)(-) to Q(B) electron transfer at room temperature and slows down protein relaxation which stabilizes the primary charge-separated state P(+)Q(A)(-). A comparison with analogous effects observed in trehalose-coated RCs suggests that protein motions are less severely reduced in PVA films than in trehalose matrices at comparable water/RC ratios.     

Direct electron transfer and electrocatalysis of hemoglobin in layer-by-layer films assembled with Al-MSU-S particles

In this paper, layer-by-layer (LBL) {MSU/Hb}(n)/PDDA films assembled by alternate adsorption of positively charged hemoglobin (Hb) and negatively charged mesoporous molecular sieves of Al-MSU-S onto a glassy carbon electrode (GCE) were reported. Al-MSU-S was synthesized by the precursor of zeolite Y and ionic liquids 1-hexadecane-3-methylimidazolium bromide (CMIMB) as a template in basic medium. It exhibited larger pore diameter, pore volume and surface area. Direct electrochemical and electrocatalytic properties of Hb in these layer-by-layer films were investigated. A pair of well-defined nearly reversible cyclic voltammetric peaks was observed and the formal potential of the heme Fe(III)/Fe(II) redox couple was found to be -0.295V (vs. SCE). The influences of layer's number and the pH of the external solution to the electron transfer behavior of Hb in {MSU/Hb}(n)/PDDA films were also estimated by cyclic voltammetry and a set of optimized conditions for film fabrication was inferred. The hemoglobin in{MSU/Hb}(n)/PDDA films displayed a good electrocatalytic activity to the reduction of hydrogen peroxide, which had linear current responses from 1.0 x 10(-6) to 1.86 x 10(-4)mol/L with the detection limit of 5.0 x 10(-7)mol/L (S/N=3). The apparent Michaeli-Menten constant (K(m)(app)) was 0.368 mmol/L. Thus, this methodology shows potential application of the preparation of third-generation biosensors.     

Direct electron transfer for hemoglobin in biomembrane-like dimyristoyl phosphatidylcholine films on pyrolytic graphite electrodes

Stable thin films made from dimyristoyl phosphatidylcholine (DMPC) with incorporated hemoglobin (Hb) on pyrolytic graphite (PG) electrodes were characterized by electrochemical and other techniques. Cyclic voltammetry (CV) of Hb-DMPC films showed a pair of well-defined and nearly reversible peaks at about -0.27 V vs. saturated calomel electrode (SCE) at pH 5.5, characteristic of Hb heme Fe(III)/Fe(II) redox couple. The electron transfer between Hb and PG electrodes was greatly facilitated in DMPC films. Apparent heterogeneous rate constants (ks) were estimated by fitting square wave voltammograms of Hb-DMPC films to a model featuring thin layer behavior and dispersion of formal potentials for redox center. The formal potential of Hb heme Fe(III)/Fe(II) couple in DMPC films shifted linearly between pH 4.5 to 11 with a slope of -48 mV pH-1, suggesting that one proton is coupled to each electron transfer in the electrochemical reaction. Soret absorption band positions suggest that Hb retains a near native conformation in DMPC films at medium pH. Differential scanning calorimetry (DSC) showed the phase transition for DMPC and Hb-DMPC films, suggesting DMPC has an ordered multibilayer structure. Trichloroacetic acid (TCA) was catalytically reduced by Hb-DMPC films with significant decreases in the electrode potential required.     

Tuning of redox potentials for the design of ruthenium anticancer drugs -- an electrochemical study of [trans-RuCl(4)L(DMSO)](-) and [trans-RuCl(4)L(2)](-) complexes, where L = imidazole, 1,2,4-triazole, indazole

The electrochemical behavior of [trans-RuCl(4)L(DMSO)](-) (A) and [trans-RuCl(4)L(2)](-) (B) [L = imidazole (Him), 1,2,4-triazole (Htrz), and indazole (Hind)] complexes has been studied in DMF, DMSO, and aqueous media by cyclic voltammetry and controlled potential electrolysis. They exhibit one single-electron Ru(III)/Ru(II) reduction involving, at a sufficiently long time scale, metal dechlorination on solvolysis, as well as, in organic media, one single-electron reversible Ru(III)/Ru(IV) oxidation. The redox potential values are interpreted on the basis of the Lever's parametrization method, and particular forms of this linear expression (that relates the redox potential with the ligand E(L) parameter) are proposed, for the first time, for negatively (1-) charged complexes with the Ru(III/II) redox couple center in aqueous phosphate buffer (pH 7) medium and for complexes with the Ru(III/IV) couple in organic media. The E(L) parameter was estimated for indazole showing that this ligand behaves as a weaker net electron donor than imidazole or triazole. The kinetics of the reductively induced stepwise replacement of chloride by DMF were studied by digital simulation of the cyclic voltammograms, and the obtained rate constants were shown to increase with the net electron donor character (decrease of E(L)) of the neutral ligands (DMSO < indazole < triazole < imidazole) and with the basicity of the ligated azole, factors that destabilize the Ru(II) relative to the Ru(III) form of the complexes. The synthesis and characterization of some novel complexes of the A and B series are also reported, including the X-ray structural analyses of (Ph(3)PCH(2)Ph)[trans-RuCl(4)(Htrz)(DMSO)], [(Ph(3)P)(2)N][trans-RuCl(4)(Htrz)(DMSO)], (H(2)ind)[trans-RuCl(4)(Hind)(DMSO)], and [(Hind)(2)H][trans-RuCl(4)(Hind)(2)].     

肌红蛋白在双十二烷基二甲基溴化铵-粘土多双层复合薄膜电极上的电化学与电催化

将双十二烷基二甲基溴化铵(DDAB)-粘土(Clay)复合物的水分散系与肌红蛋白(Mb)水溶液的混合物涂布到热解石墨(PG)电极表面,可制得Mb-DDAB-Clay薄膜电极.在pH5.5的缓冲溶液中,该薄膜电极在-0.25V(vs.SCE)处有一对可逆的循环伏安还原氧化峰,为Mb血红素辅基Fe(Ⅲ)/Fe(Ⅱ)电对的特征峰.在DDAB-Clay薄膜的微环境中,Mb与PG电极之间的电子传递得到极大促进,并显示了很好的稳定性.Soret吸收带的位置表明,在适中的pH范围内,Mb在薄膜中保持了其原始构象.X射线衍射实验结果表明,Mb的嵌入并未对薄膜的有序多层结构有很大影响.在DDAB-Clay薄膜环境中,Mb血红素Fe(Ⅲ)/Fe(Ⅱ)电对的式量电位在pH4.5～11.0范围内与溶液pH值成线性关系,表明Mb的电化学还原很可能是一个质子伴随一个电子的电极过程.Mb-DDAB-Clay薄膜可以用于催化还原溶解氧和三氯乙酸.     

Bulk heterojunction photoelectrochemical cells consisting of oxotitanyl phthalocyanine nanoporous films and I(3)(-)/I(-) redox couple

Photoelectrochemical cells based on oxotitanylphthalocyanine (TiOPc) films and an I(3)(-)/I(-) redox couple have been constructed. The TiOPc films were prepared on an indium-tin oxide coated glass plate (ITO) by the micellar disruption method and characterized by their unique nanoporous structure. A photocurrent action spectrum for input radiation directed through the ITO/TiOPc film, film-thickness dependence, and morphological investigation revealed that the cells consisted of a bulk heterojunction formed between the nanoporous TiOPc films and the liquid I3-/I- electrolyte, resulting in a larger short-circuit current (J(sc)= 2.1 mA/cm(2)), open-circuit voltage (V(oc)= 0.11 V), fill factor (ff= 0.31), and hence a larger energy conversion efficiency (eta= 0.13% for an incident white-light intensity of 53 mW/cm2) than the bilayer structure composed of the vaccum-evaporated TiOPc compact film and the I(3)(-)/I(-) electrolyte (J(sc)= 0.16 mA/cm(2), V(oc)= 0.018 V, ff = 0.27, and eta = (1.5 x 10(-3)%).     

New degenerate metal-oxide electrodes for nearly reversible direct electron transfer to the redox proteins

Heavily doped cadmium tin oxide (CTO) film electrodes were developed for fast electron exchange with redox proteins. The metal oxide films showed nearly reversible electron transfer for the [2Fe–2S] proteins spinach ferredoxin (Sp fd) and putidaredoxin (Pdx), and the well-studied heme protein horse heart cytochrome c. These represent a family of proteins that are of comparable size, but vary significantly in overall charge, formal redox potential, and type of metal center. The unmediated electron exchange was achieved through variation of metal oxide film synthesis parameters that led to an increase of the charge carrier concentration up to the levels typical for degenerate semiconductors. In addition, the flat band potential of the films was shifted close to or more positive of the formal redox potentials of proteins such that the semiconductor electrodes would be utilized in an accumulation mode. The rates and sustainability of electron transfer for the two ferredoxins obtained on these cadmium tin oxide electrodes are as high or higher than previously reported.     

Organometallic uranium(V)-imido halide complexes: from synthesis to electronic structure and bonding

Reaction of (C5Me5)2U(=N-2,4,6-(t)Bu3-C6H2) or (C5Me5)2U(=N-2,6-(i)Pr2-C6H3)(THF) with 5 equiv of CuX(n) (n = 1, X = Cl, Br, I; n = 2, X = F) affords the corresponding uranium(V)-imido halide complexes, (C5Me5)2U(=N-Ar)(X) (where Ar = 2,4,6-(t)Bu3-C6H2 and X = F (3), Cl (4), Br (5), I (6); Ar = 2,6-(i)Pr2-C6H3 and X = F (7), Cl (8), Br (9), I (10)), in good isolated yields of 75-89%. These compounds have been characterized by a combination of single-crystal X-ray diffraction, (1)H NMR spectroscopy, elemental analysis, mass spectrometry, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and variable-temperature magnetic susceptibility. The uranium L(III)-edge X-ray absorption spectrum of (C5Me5)2U(=N-2,4,6-(t)Bu3-C6H2)(Cl) (4) was analyzed to obtain structural information, and the U=N imido (1.97(1) A), U-Cl (2.60(2) A), and U-C5Me5 (2.84(1) A) distances were consistent with those observed for compounds 3, 5, 6, 8-10, which were all characterized by single-crystal X-ray diffraction studies. All (C5Me5)2U(=N-Ar)(X) complexes exhibit U(V)/U(IV) and U(VI)/U(V) redox couples by voltammetry, with the potential separation between these metal-based couples remaining essentially constant at approximately 1.50 V. The electronic spectra are comprised of pi-->pi* and pi-->nb(5f) transitions involving electrons in the metal-imido bond, and metal-centered f-f bands illustrative of spin-orbit and crystal-field influences on the 5f(1) valence electron configuration. Two distinct sets of bands are attributed to transitions derived from this 5f(1) configuration, and the intensities in these bands increase dramatically over those found in spectra of classical 5f(1) actinide coordination complexes. Temperature-dependent magnetic susceptibilities are reported for all complexes with mu(eff) values ranging from 2.22 to 2.53 mu(B). The onset of quenching of orbital angular momentum by ligand fields is observed to occur at approximately 40 K in all cases. Density functional theory results for the model complexes (C5Me5)2U(=N-C6H5)(F) (11) and (C5Me5)2U(=N-C6H5)(I) (12) show good agreement with experimental structural and electrochemical data and provide a basis for assignment of spectroscopic bands. The bonding analysis describes multiple bonding between the uranium metal center and imido nitrogen which is comprised of one sigma and two pi interactions with variable participation of 5f and 6d orbitals from the uranium center.     

Synthesis and characterization of six new quaternary actinide thiophosphate compounds: Cs(8)U(5)(P(3)S(10))(2)(PS(4))(6)(,) K(10)Th(3)(P(2)S(7))(4)(PS(4))(2), and A(5)An(PS(4))(3), (A = K, Rb, Cs; An = U, Th)

Six new actinide metal thiophosphates have been synthesized by the reactive flux method and characterized by single-crystal X-ray diffraction: Cs(8)U(5)(P(3)S(10))(2)(PS(4))(6) (I), K(10)Th(3)(P(2)S(7))(4)(PS(4))(2) (II), K(5)U(PS(4))(3) (III), K(5)Th(PS(4))(3) (IV), Rb(5)Th(PS(4))(3) (V), and Cs(5)Th(PS(4))(3) (VI). Compound I crystallizes in the monoclinic space group P2(1)/c with a = 33.2897(1) A, b = 14.9295(1) A, c = 17.3528(2) A, beta = 115.478(1) degrees, Z = 8. Compound II crystallizes in the monoclinic space group C2/c with a = 32.8085(6) A, b = 9.0482(2) A, c = 27.2972(3) A, beta = 125.720(1) degrees, Z = 8. Compound III crystallizes in the monoclinic space group P2(1)/c with a = 14.6132(1) A, b = 17.0884(2) A, c = 9.7082(2) A, beta = 108.63(1) degrees, Z = 4. Compound IV crystallizes in the monoclinic space group P2(1)/n with a = 9.7436(1) A, b = 11.3894(2) A, c = 20.0163(3) A, beta = 90.041(1) degrees, Z = 4, as a pseudo-merohedrally twinned cell. Compound V crystallizes in the monoclinic space group P2(1)/c with a = 13.197(4) A, b = 9.997(4) A, c = 18.189(7) A, beta = 100.77(1) degrees, Z = 4. Compound VI crystallizes in the monoclinic space group P2(1)/c with a = 13.5624(1) A, b = 10.3007(1) A, c = 18.6738(1) A, beta = 100.670(1) degrees, Z = 4. Optical band-gap measurements by diffuse reflectance show that compounds I and III contain tetravalent uranium as part of an extended electronic system. Thorium-containing compounds are large-gap materials. Raman spectroscopy on single crystals displays the vibrational characteristics expected for [PS(4)](3)(-), [P(2)S(7)](4-), and the new [P(3)S(10)](5)(-) building blocks. This new thiophosphate building block has not been observed except in the structure of the uranium-containing compound Cs(8)U(5)(P(3)S(10))(2)(PS(4))(6).     

Pressure-induced formation of molecular B2X4(μ-X)2 (X = Cl, Br, I) species

Boron(III) halides (BX(3), where X = F, Cl, Br, I) at ambient pressure conditions exist as strictly monomeric, trigonal-planar molecules. Using correlated ab initio calculations, the three heavier halides (X = Cl, Br, I) are shown to possess B(2)X(4)(μ-X)(2) local minima, isostructural with the diborane molecule. The calculated dissociation barrier of the B(2)I(4)(μ-I)(2) species [≈14 kJ/mol with CCSD(T)/cc-pVTZ] may be high enough to allow cryogenic isolation. The remaining dimer structures are more labile, with dissociation barriers of less than 6 kJ/mol. All three dimer species may be stabilized by application of external pressure. Periodic density functional theory calculations predict a new dimer-based P1 solid, which becomes more stable than the P6(3)/m monomer-derived solids at 5 (X = I) to 15 (X = Cl) GPa. Metadynamics simulations suggest that B(2)X(4)(μ-X)(2)-based solids are the kinetically preferred product of pressurization of the P6(3)/m solid.     

FA(I):A(+) and FA(II):Cu(+) laser activity and photographic sensitization at the low coordinated surfaces of AgBr ab initio calculations

The twofold potentials of F(A)(I):Au(+) and F(A)(II)Cu(+) color centers at the low coordinated surfaces of AgBr thin films in providing tunable laser activity and photographic sensitization were investigated using ab initio methods of molecular electronic structure calculations. Clusters of variable size were embedded in simulated Coulomb fields that closely approximated the Madelung fields of the host surfaces, and the nearest neighbor ions to the F(A) defect site were allowed to relax to equilibrium in each case. Based on the calculated Stokes shifted optical transition bands and horizontal shifts along the configuration coordinate diagrams, both F(A)(I):Au(+) and F(A)(II):Cu(+) color centers were found to be laser active. The laser activity faded quickly as the bromide ion coordination decreased from 5 (flat) to 4 (edge) to 3 (corner) and as the size of the impurity cation increased from Cu(+) to Au(+). The latter relation was explainable in terms of the axial perturbation of the impurity cation. The smallest calculated Stokes-shift at the corner surface suggested that emission had the same oscillator strength as absorption. All relaxed excited states RESs of the defect containing surfaces were deep below the lower edges of the conduction bands of the defect free ground state surfaces, indicating that F(A)(I):Au(+) and F(A)(II):Cu(+) are suitable laser defects. The probability of orientational destruction of the two centers attributed to the assumed RES saddle point ion configurations along the <110> axis was found to be directly proportional to the size of the impurity cation, with activation energy barriers of about 0.655-3.294 eV for Cu(+), and about 1.887-3.404 eV for Au(+). The possibility of exciton (energy) transfer from the sites of higher coordination to those of lower coordination is demonstrated. The more laser active F(A)(II):Cu(+) center was more easily formed than the less laser active F(A)(I):Au(+) center. The Glasner-Tompkins empirical relation was generalized to include F(A) centers at the low coordinated surfaces of silver bromide thin film. As far as color photographic sensitization is concerned, the lowest unoccupied molecular orbitals of the selected dye molecules in the excited states were high enough for electron injection. F(A) defect formation and rotational diffusion of silver clusters reduced the energy gaps between the excited dye molecules and the lower edges of the conduction bands and allowed for hole injection. About 54-60% of the reduction of silver ions at the flat surface of AgBr was attributed to the host anions and F(A) defect formation, leaving about 40-46% for the reduction of photoelectrons as well as the electrons of the developer or dye molecules. The unrelaxed rotational diffusions of the central Ag(4) by 90 degrees decreased the latter percentage, but were severely hindered by activation energy barriers.     

聚苯乙烯-co-聚(2,3,4,5,6-五氟苯乙烯)共聚物: 合成及其多孔薄膜的制备

利用原子转移自由基聚合(ATRP)方法, 分别在三氟甲苯、含氟离子液体以及三氟甲苯/含氟离子液体混合溶剂体系中合成了聚苯乙烯-co-聚(2,3,4,5,6-五氟苯乙烯)(PS-co-PPFS)共聚物, 通过¹H NMR、¹⁹F NMR、元素分析以及凝胶渗透色谱法(GPC)对所得聚合物的分子链结构和组成进行了分析和表征. 随后, 利用静态呼吸图法分别在CS₂, CHCl₃ 和CH₂Cl₂ 中制备了有序多孔薄膜, 用扫描电子显微镜(SEM)观察其表面形貌, 并与利用分子量大小相当的聚苯乙烯均聚物(PS)制备的多孔薄膜进行了对比. 研究结果表明: 在三氟甲苯和含氟离子液体溶剂体系中, 均可利用ATRP 聚合方法获得窄分子量分布的PS-co-PPFS 共聚物(M_n＝5200～7900 g·mol^-1, i>M_w/M_n＝1.12～1.22). 对聚合物薄膜的扫描电子显微镜(SEM)观察和分析显示: 分别以CS₂, CHCl₃ 和CH₂Cl₂ 作为溶剂, 利用静态呼吸图法均可制备出PS-co-PPFS 共聚物多孔薄膜. 然而, 与在CHCl₃ 和CH₂Cl₂ 中制备的PS 均聚物多孔薄膜的表面形貌不同的是, PS-co-PPFS 共聚物多孔薄膜呈现出无序排列、平均孔径大小不同的两种孔结构; 在CHCl₃ 中制备所得薄膜的孔结构有序性相对较好, 两种孔的平均孔径分别为0.75 和0.37 μm.     

Conductivity of Poly(methyl methacrylate)/Polystyrene/Carbon Black and Poly(ethyl methacrylate)/Polystyrene/Carbon Black Ternary Composite Films

Poly(methyl methacrylate)(PMMA)/polystyrene(PS)/carbon black(CB)and poly(ethyl methacrylate)(PEMA)/PS/CB ternary composite films were obtained using solution casting technique to investigate double percolation effect.In both PMMA/PS/CB and PEMA/PS/CB ternary composite films,the CB particles prefer to locate into PS phase based on the results of calculating wetting coefficient,which is also confirmed by SEM images.The conductivity of the films was investigated,and the percolation threshold(￠c)of both ternary composite films with different polymer blend ratios was determined by fitting the McLachlan GEM equation.Conductivity of PMMA/PS/CB ternary composite films showed a typical double percolation effect.However,due to the double emulsion structure of PEMA/PS polymer blends,the PEMA/PS/CB ternary composite films(PEMA/PS=50/50)showed a higher￠c,even CB only located in PS phase,which conflicts with the double percolation effect.A schematic diagram combined with SEM images was proposed to explain this phenomenon.