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Water oxidation is a critical step in water splitting to make hydrogen fuel. We report the solution synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) electrocatalytic properties of ∼2-3 nm thick films of NiOx, CoOx, NiyCo1-yOx, Ni0.9Fe0.1Ox, IrOx, MnOx, and FeOx. The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochemical properties of the oxides. Ni0.9Fe0.1Ox was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm-2 at an overpotential of 336 mV with a Tafel slope of 30 mV dec-1 with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrOx control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni0.9Fe0.1OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, NiyCo1-yOx thin films showed decreasing activity relative to the pure NiOx films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis. [ABSTRACT FROM AUTHOR]

Analyses combining X-ray powder diffraction (XRD) and solid-state NMR (SSNMR) data can now provide crystal structures in challenging powders that are inaccessible by traditional methods. The flavonoid catechin is an ideal candidate for these methods, as it has eluded crystallographic characterization despite extensive study. Catechin was first described nearly two centuries ago, and its powders exhibit numerous levels of hydration. Here, synchrotron XRD data provide all heavy-atom positions in (+)-catechin 4.5-hydrate and establish the space group as C2. SSNMR data (13C tensor and 1H/13C correlation) complete the conformation by providing catechin's five OH hydrogen orientations. Since 1903, this phase has been erroneously identified as a 4.0 hydrate, but XRD and density data establish that this discrepancy is due to the facile loss of the water molecule located at a Wyckoff special position in the unit cell. A final improvement to heavy-atom positions is provided by a geometry optimization of bond lengths and valence angles with XRD torsion angles held constant. The structural enhancement in this final structure is confirmed by the significantly improved fit of computed 13C tensors to experimental data. [ABSTRACT FROM AUTHOR]

The synthesis, structure, and photophysical properties of a new family of trinuclear CuRe2 chromophore-quencher complexes having the general form [Cu(pyacac)2(Re(bpy')(CO)3)2](OTf)2 (where pyacac = 3-(4-pyridyl)-acetylacetonate and bpy' = 4,4'-5,5'-tetramethyl-2,2'-bipyridine (tmb, 1), 4,4'-dimethyl- 2,2'-bipyridine (dmb, 2), 2,2'-bipyridine (bpy, 3), 4,4'-dichloro-2,2'-bipyridine (dclb, 4), and 4,4'-diethylester- 2,2'-bipyridine (deeb, 5)) are reported. Time-resolved emission data acquired in room-temperature CH2CI2 solutions revealed excited-state lifetimes of 14.9 ± 0.7, 8.1 * 0.4, 8.2 ± 0.4, 5.6 ± 0.3, and 5.0 ± 0.3 ns for complexes 1-5, respectively. The emission in each case is assigned to decay of the Re'-based 3MLCT excited state; the lifetimes are all significantly less than the corresponding BeRe2 model complexes, which were also prepared and characterized. Electron transfer was found to be significantly endothermic for all five CuRe2 complexes: this fact, coupled with the ca. 10 Å donor-acceptor distance and favorable spectral overlap between the 3MLCT emission profile and absorptions of the Cu" center, implicates dipolar energy transfer as the dominant quenching pathway in these compounds. Gaussian decorivolution of the ground- state absorption spectrum of Cu(phacac)2 (phacac = 3-phenylacetylacetonate) allowed for a differential analysis of the spectral overlap between the donor emission spectra and the two observed ligand-field transitions of the Cu" ion. Energy transfer was found to occur preferentially to the lower energy ligand-field band due primarily to more favorable dipole orientation as measured by the κ2 term from Färster theory. These results, supported by time-dependent DFT calculations on Cu(phacac)2, indicate enhanced dipolar coupling to the dxz→dxy, transition of the Cu" center and thus represent an orbitally specific energy-transfer process occurring in this system. [ABSTRACT FROM AUTHOR]

The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4′-(X)2-bpy)2)](PF6)3 (where M = MnII or ZnII, X = CH3 or CF3, mcb is 4′-methyl-4-carboxy-2,2′-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a RuII polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)]-(PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the ³MLCT excited state of the RuII chromophore but with a lifetime (τobs = 5.0 ± 0.1 ns) and radiative quantum yield (Φr ≈ 7 × 10-4) that are significantly attenuated relative to the ZnII model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (τobs = 730 ± 30 ns and Φr = 0.024, respectively). Quenching of the ³MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (τobs = 45 ± 5 ps) is >104 shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (τobs = 1.31 ± 0.05 µs). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form knr = k0 + A·exp{-ΔE7kBT} with k0 = 1.065 ± 0.05 × 107 s-1, A = 3.7 ± 0.5 × 1010 s-1, and ΔE= 1230 ± 30 cm-1. Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of λ 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn2II core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (ΔG0ET = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be ~10 cm-1 and ~0.15 cm-1 in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude.… [ABSTRACT FROM AUTHOR]

Synchrotron X-ray powder diffraction and solid-state 13C NMR shift tensor data are combined to provide a unique path to structure in microcrystalline organic solids. Analysis is demonstrated on ambuic acid powder, a widely occurring natural product, to provide the complete crystal structure. The NMR data verify phase purity, specify one molecule per asymmetric unit, and provide an initial structural model including relative stereochemistry and molecular conformation. A refinement of X-ray data from the initial model establishes that ambuic acid crystallizes in the P21 space group with unit cell parameters a = 15.5047(7), b = 4.3904(2), and c = 14.1933(4) Å and β = 110.3134(3)°. This combined analysis yields structural improvements at two dihedral angles over prior NMR predictions with differences of 103° and 37° found. Only minor differences of ±5.5°, on average, are observed at all remaining dihedral angles. Predicted hydroxyl hydrogen-bonding orientations also fit NMR predictions within ±6.9°. This refinement corrects chemical shift assignments at two carbons and reduces the NMR error by ∼16%. This work demonstrates that the combination of long-range order information from synchrotron powder diffraction data together with the accurate shorter range structure given by solid-state NMR measurements is a powerful tool for studying challenging organic solids. [ABSTRACT FROM AUTHOR]

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited- state dynamics associated with formation of the ²E excited state in a CrIII transition metal complex. Cr- (acac)3 (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with τ = 1.1 ± 0.1 Ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire 4A2 → 4T2absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (kvib) in the ²E state. The data on Cr(acac)3 indicate that intersystem crossing associated with the 4T2→ ²E conversion occurs at a rate kISC > 1013 S-1 and furthermore competes effectively with vibrational relaxation in the initially formed 4T2 state. Excitation into the higher energy 4LMCT state (&lemda;ex = 336 nm) gives rise to biphasic kinetics with τ1 = 50 ± 20 fs and τ2 2 = 1.2 ± 0.2 Ps. The slower component is again assigned to vibrational cooling in the ²E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of CrIII, these results reinforce the notion that the conventional picture of excited-state dynamics in which kvib > kIC > kISC does not generally apply when describing excited-state formation in transition metal complexes. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of nanoparticle-cored dendrimers (NCDs), consisting of a metal core capped by arylpolyethers terminated with ester or carboxylate groups, are reported. These NCDs, comprising nanometer-sized gold clusters at the core and organic dendrons radially connected to the gold core by gold-sulfur bonds, were analyzed by TEM, TGA, UV, IR, and NMR spectroscopies. The density of the branching units connected to the core decreased from 1.90/nm[SUP2] for a first-generation NCD (Au-G1 (CO[SUB2]Me)) to 0.80/nm[SUB2] for a fourth-generation NCD (Au-G4(CO[SUB2]Me)). Although the ester-terminated NCDs were stable and resisted aggregation, they were easily hydrolyzed to the corresponding watersoluble sodium salts. Aqueous solutions of (Au-Gn(CO[SUB2]Na)) exhibited micellar properties. Since these NCDs possess a relatively unpassivated metal core and an organic aryl ether shell with micellar and dendritic properties, they are expected to have important potential applications in catalysis. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a group of new dendrimers—namely, nanoparticie-cored dendrimers (NCDs)—are described. These materials were obtained by the reduction of hydrogen tetrachloroaurate phase-transferred into toluene in the presence of Fréchet-type polyaryl ether dendritic disulfide wedges of generation 1-5. These materials, possessing nanometer-sized gold clusters at the core and dendritic wedges radially connected to the core by Au-S bonds, were analyzed by TEM and TGA, and by UV, IR, and NMR spectroscopies. The number of branching units connected to the core decreased with the generation of the dendritic wedge, and this number changed from 2.18/nm² for Au- G-2 to 0.27/nm² for Au-G-5. This result suggests that, in the higher-generation NCDs, a large fraction of the surface area of the metal cluster is not passivated and is therefore available for catalytic activity. [ABSTRACT FROM AUTHOR]

Reports on the measurement of carbon nuclear magnetic resonance (NMR) chemical shift tensors for two forms of solid 10-deacetyl baccatin III to investigate the origins of solid-state NMR. Finding that lattice interactions cannot fully account for the shift differences in the two forms; Possible appearance of the observed tensor differences from conformational variations in ring substituents.

Reports on the development of a method for the introduction of 5-fluoro group for easy access to a wide variety of 5-fluoro glycosides and glycosyl phosphates. Fluoridolysis of C-5, 6 epoxides; Synthesis of two fluorinated carbohydrates.

Examines the synthesis and spectroscopic properties of elongated ruthenium dihydrogen complexes. Temperature and isotope dependence of dihydrogen ligand distances; Dihydrogen/dihydride equilibria; Isotope exchange and H-D coupling.

Examines the characterization of stereochemistry and molecular conformation using solid-state nuclear magnetic resonance tensors. Comparison of [sup 13]C tensor principal values with ab initio computed values for possible computer-generated stereoisomers; Advantages of chemical shift tensor over isotropic shift; Sensitivity of [sup 13]C tensor principal values to stereochemical differences in the terrain.

Examines the capacity of pyrylium and thiopyrylium ions to act as combined quenchers of photoexcited redox dyes and transmembrane charge carriers to generate long-lived charged separation across bilayer membranes. Rate of diffusion measurement of dihexadecyl phosphate-associated ions; Proton permeabilities of planar lipid bilayers and liposomes.

Examines the structural characterization of higher oxidation states of the water oxidation catalyst. Use of Resonance Raman (RR) and optical spectroelectrochemical titrations; Estimation of rate constants; Structure of frozen ion solutions; Consistency of the RR spectrum with the coordination of terminal oxo ligands by cerium4+.

Examines excited-state dynamics in an iron(II) polypyridyl complex using static, nanosecond and femtosecond spectroscopies. Formation of metal-to-ligand charge transfer excited; Identification of spectroscopic signatures for the initial and long-lived excited states; Analysis of single-wavelength kinetics data; Interpretation of ultrafast intersystem crossing.

Examines the solid-state 13C chemical shift tensors in terpenes using nuclear magnetic resonance (NMR) analyses. Evidence of mixed polymorphic forms in solid verbenol; Structural predictions from computed and experimental NMR shift tensors; Characterization of the minor polymorphic form of verbenol.

Presents a study which described the application of PHORMAT and FIREMAT solid-state nuclear magnetic resonance (NMR) techniques in analyzing shift values in terpene parthenolide. Explanation of the NMR analysis techniques; Methodology used; Results and discussion; Conclusion.

Explores the hydroxyl radical formation during peroxynitrous acid decomposition. Formation of oxygen during decomposition of peroxynitrous acid (ONOOH); Formation of OH and NO2 radicals as oxidizing intermediates; Determination of the rate constants of the reactions; Inhibition of oxygen formation by radical scavengers.

Presents an ab-initio molecular orbital study on the chemistry of 2-carbenabicyclo[3.2.1]octadiene. Synthesis of endo-6-ethynylbicyclooctane from the conversions of 2-carbenabicycloocta-3,6-diene, 3-carbenatetracyclooctane and bicycloocta-2,3,6-triene; Equilibrating C8H8; Carbene-to-carbene rearrangement process.

Cites the relative orientations of molecular properties within crystals. High statistical preference for organics in general; Approximate crystallographic symmetry; Electron-donor/acceptor properties; Net directionality of polarizability vectors; Full-matrix least-squares methods.

Presents a laser photolysis study of the 5T2 *** 1A1 intersystem crossing in the ligand [Fe(t-tpchxn)](ClO4)2. Crystal structure of the complex; Relaxation kinetics; Molecular mechanism of 5T2 *** 1A1 relaxation processes.

Examines two different intersystem crossing processes of low-spin polypyridyl ferrous complexes. 5T2 state resulting from photoexcitation as a general feature; Rate of formation; Reasons for fast intersystem crossing; Mechanistic implications.

Examines the transesterification of RNA using chimeric DNA/RNA molecules containing RNA nucleotides embedded in DNA sequences. Description of synthesis, purification and activity.

Reports on the synthesis of a class of sterically hindered dendrimer-metalloporphyrins for use as shape-selective oxidation catalysts. Instrumentation; Synthesis process; Dendrimer-porphyrin manganese(III) complexes; Epoxidation reactions.

The article reports that during the [Ru(bpy)2(OH2)]2O4+-catalyzed water oxidation, the detection and mechanistic relevance of transient ligand radicals has been formed. According to the author, the kinetic analyses of isotope evolution rates indicate that water exchange on {5,5} and intermediary oxidation states is very slow relative to O2 evolution.

Examines the photochemical reaction of ozone and benzene using infrared matrix isolation spectroscopy. Isolation and characterization of the benzene/ozone atom reaction; Photolysis of benzene/ozone mixtures in argon matrix; Identification of butadienylketene; Deuterium isotopic shifts; Phenol/butadienylketene branching ratio.

The article discusses the ability of UDP-(5F)-N-acetylglucosamine (GlcNac) to act as a slow-binding, competitive inhibitor of the retaining glycosyltransferase (GT) MshA from Corynebacterium glutamicum (CgMshA ). It reveals the consistency of the inhibition of CgMshA by the flouro-substituted compound with the oxocarbenium transition-state structure. It analyzes the mechanism of GT enzymes that catalyze the transfer of a sugar moiety to an acceptor molecule and involve in cellular pathways.

Reports a modification of a strategy for ligand binding. Preparation of short peptides; Target receptor; Signal transduction pathways.

Examines the biological implications of the rapid reaction between peroxonitrite ion and carbon dioxide. Peroxonitrite and hydrogen oxoperoxonitrate (ONO2H) as oxidants; Rate constant for reaction between ONO2- and carbon dioxide.

Examines the synthesis of sequence specific phenylacetylene oligomers on an insoluble solid support. Fragment condensation approach investigated; Etherification of chloromthylated polystyrene.

Reports on the preparation and activity of a functional mimic of ribozymes. Control of the position and coordination environment of the catalytic metal ion through chemical synthesis; Derivative suitable for incorporation into a DNA probe; RNA target; Cleavage reactions.

Fe plays a critical, but not yet understood, role in enhancing the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts. We report electrochemical, in situ electrical, photoelectron spectroscopy, and X-ray diffraction measurements on Ni1-xFex(OH)2/Ni1-xFexOOH thin films to investigate the changes in electronic properties, OER activity, and structure as a result of Fe inclusion. We developed a simple method for purification of KOH electrolyte that uses precipitated bulk Ni(OH)2 to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni(OH)2/NiOOH reveals new Ni redox features and no significant OER current until >400 mV overpotential, different from previous reports which were likely affected by Fe impurities. We show through controlled crystallization that β-NiOOH is less active for OER than the disordered γ-NiOOH starting material and that previous reports of increased activity for β-NiOOH are due to incorporation of Fe-impurities during the crystallization process. Through-film in situ conductivity measurements show a >30-fold increase in film conductivity with Fe addition, but this change in conductivity is not sufficient to explain the observed changes in activity. Measurements of activity as a function of film thickness on Au and glassy carbon substrates are consistent with the hypothesis that Fe exerts a partial-charge-transfer activation effect on Ni, similar to that observed for noble-metal electrode surfaces. These results have significant implications for the design and study of Ni1-xFexOOH OER electrocatalysts, which are the fastest measured OER catalysts under basic conditions. [ABSTRACT FROM AUTHOR]

Investigates water exchange rates in the diruthenium micro-Oxo ion cis,cis-[(bpy)[sub 2]Ru(OH[sub 2])][sub 2]O[sup 4+]. Dynamics of water exchange; Basis of global kinetic analyses of optical spectra; Measurement of rate constant for anation of ions.

Reviews the computer software `The Merck Index, 12th Edition, CD-ROM (Macintosh): An Encyclopedia of Chemicals, Drugs and Biologicals,' edited by S. Budavari, M. O'Neill, et al. Content of software; Cost of software.

Reviews the computer software `MacSpartan,' by Wavefunction Incorporated.

The article presents information on a hard-and-soft hybrid nanosystem whose fluorescence intensity can be reversibly modulated using specific bandwidths of light. There is considerable current interest within the rapidly expanding field of nanotechnology in integrating hard and soft structures into complex functional nanosystems that have stimuli responsive properties. The study specifically attaches a spiropyran via a thiol-metal linkage to a core-shell semiconductive CdSe/ZnS nanocrystal. Preparation of both the spiropyran and the core-shell nanocrystals is a straightforward procedures.

Covalent organic frameworks (COFs) usually crystallize as insoluble powders, and their processing for suitable devices is thought to be limited. We demonstrate that COFs can be mechanically pressed into shaped objects having anisotropic ordering with preferred orientation between hk0 and 00l crystallographic planes. Five COFs with different functionality and symmetry exhibited similar crystallographic behavior and remarkable stability, indicating the generality of this processing. Pellets prepared from bulk COF powders impregnated with LiClO4 displayed room temperature conductivity up to 0.26 mS cm-1 and high electrochemical stability. This outcome portends use of COFs as solid-state electrolytes in batteries. [ABSTRACT FROM AUTHOR]

At high concentrations, nitrate ion alters the dynamics of ruthenium "blue dimer"-catalyzed water oxidation by Ce(IV) such that the oxidation rate is enhanced and a unique reaction intermediate accumulates. This intermediate is characterized by distinct EPR, optical, and resonance Raman (RR) spectra, with the appearance in the latter of a new oxygen isotope-sensitive band. Both Ce(IV) and nitrate are required to generate this intermediate, which suggests ceric-nitrate complexes as the causative agents. Use of 18O-labeled and 15N-labeled materials has established that (1) the new RR band is not an O-O stretching mode (for example, as might be associated with a peroxo species) but involves the O atom coordinated to a Ru center, and (2) the O2 product does not contain an O atom derived from nitrate, eliminating several plausible pathways involving O-atom transfer to oxidized dimer. Although these results are surprising, similar phenomena have been reported for water oxidation catalyzed by monomeric Ru complexes. The dramatic effects observed for the "blue dimer" make it an ideal candidate for further study. [ABSTRACT FROM AUTHOR]

Growth of graphene on copper (100) single crystals by chemical vapor deposition has been accomplished. The atomic structure of the graphene overlayer was studied using scanning tunneling microscopy. A detailed analysis of moiré superstructures present in the graphene topography reveals that growth occurs in a variety of orientations over the square atomic lattice of the copper surface. Transmission electron microscopy was used to elucidate the crystallinity of the grown graphene. Pristine, defect-free graphene was observed over copper steps, corners, and screw dislocations. Distinct protrusions, known as "flower" structures, were observed on flat terraces, which are attributed to carbon structures that depart from the characteristic honeycomb lattice. Continuous graphene growth also occurs over copper adatoms and atomic vacancies present at the single-crystal surface. The copper atom mobility within vacancy islands covered with suspended graphene sheets reveals a weak graphene-substrate interaction. The observed continuity and room-temperature vacancy motion indicates that copper mobility likely plays a significant role in the mechanism of sheet extension on copper substrates. Lastly, these results suggest that the quality of graphene grown on copper substrates is ultimately limited by nucleation at the surface of the metal catalyst. [ABSTRACT FROM AUTHOR]

Key to single-molecule electronics is connecting functional molecules to each other using conductive nanowires. This involves two issues: how to create conductive nanowires at designated positions, and how to ensure chemical bonding between the nanowires and functional molecules. Here, we present a novel method that solves both issues. Relevant functional molecules are placed on a self-assembled monolayer of diacetylene compound. A probe tip of a scanning tunneling microscope is then positioned on the molecular row of the diacetylene compound to which the functional molecule is adsorbed, and a conductive polydiacetylene nanowire is fabricated by initiating chain polymerization by stimulation with the tip. Since the front edge of chain polymerization necessarily has a reactive chemical species, the created polymer nanowire forms chemical bonding with an encountered molecular element. We name this spontaneous reaction "chemical soldering". First-principles theoretical calculations are used to investigate the structures and electronic properties of the connection. We demonstrate that two conductive polymer nanowires are connected to a single phthalocyanine molecule. A resonant tunneling diode formed by this method is discussed. [ABSTRACT FROM AUTHOR]

Solution-phase photoinduced low-spin to high-spin conversion in the FeII polypyridyl complex [Fe(tren(py)3)]2+ (where tren(py)3 is tris(2-pyridylmethyliminoethyl)amine) has been studied via picosecond soft X-ray spectroscopy. Following ¹A1 → 1MLCT (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L2- and L3-edges were observed concomitant with formation of the transient high-spin 5T2 state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of ∼1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 Å increase in Fe-N bond length upon formation of the high-spin state. The overall occupancy of the Fe-3d orbitals remains constant upon spin crossover, suggesting that the reduction in s-donation is compensated by significant attenuation of π-back-bonding in the metal-ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft X-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations. [ABSTRACT FROM AUTHOR]

Embryogenesis is regulated by genetic programs that are dynamically executed in a stereotypic manner, and deciphering these molecular mechanisms requires the ability to control embryonic gene function with similar spatial and temporal precision. Chemical technologies can enable such genetic manipulations, as exemplified by the use of caged morpholino (cMO) oligonucleotides to inactivate genes in zebrafish and other optically transparent organisms with spatiotemporal control. Here we report optimized methods for the design and synthesis of hairpin cMOs incorporating a dimethoxynitrobenzyl (DMNB)-based bifunctional linker that permits cMO assembly in only three steps from commercially available reagents. Using this simplified procedure, we have systematically prepared cMOs with differing structural configurations and investigated how the in vitro thermodynamic properties of these reagents correlate with their in vivo activities. Through these studies, we have established general principles for cMO design and successful- ly applied them to several developmental genes. Our optimized synthetic and design methodologies have also enabled us to prepare a next-generation cMO that contains a bromohydroxyquinoline (BHQ)-based linker for two-photon uncaging. Collectively, these advances establish the generality of cMO technologies and will facilitate the application of these chemical probes in vivo for functional genomic studies. [ABSTRACT FROM AUTHOR]

Polymer nanoparticles of 40-400 nm diameter with spiropyran-merocyanine dyes incorporated into their hydrophobic cavities have been prepared; in contrast to their virtually nonfluorescent character in most environments, the merocyanine forms of the encapsulated dyes are highly fluorescent. Spiro-mero photoisomerization is reversible, allowing the fluorescence to be switched "on" and "off" by alternating UV and visible light. Immobilizing the dye inside hydrophobic pockets of nanoparticles also improves its photostability, rendering it more resistant than the same dyes in solution to fatigue effects arising from photochemical switching. The photophysical characteristics of the encapsulated fluorophores differ dramatically from those of the same species in solution, making nanoparticle-protected hydrophobic fluorophores attractive materials for potential applications such as optical data storage and switching and biological fluorescent labeling. To evaluate the potential for biological tagging, these optically addressable nanoparticles have been delivered into living cells and imaged with a liquid nitrogen-cooled CCD. [ABSTRACT FROM AUTHOR]

Amide-linkedspiropyran—anthraquinone (SP—AQ) conjugates were shown to mediate ZnTPPS4- photosensitized transmembrane reduction of occluded Co(bpy)33+ within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30–35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields: increased efficiencies of fluorescence quenching of ¹ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and decay of two reaction intermediates, identified as 3ZnTPpS4 and a species with the optical properties of a semiquinone radical. Kinetic profiles for Co(bpy)33+ reduction under continuous photolysis in the presence and absence of added ionophores indicated that transmembrane redox mediated by SP-AQ was electroneutral, but reaction by the other quinone-containing mediators was electrogenic. The minimal reaction mechanism suggested from the combined studies is oxidative quenching of vesicle-bound ³ZnTPPS4- by the anthraquinone unit, followed by either H+/e- cotransport by transmembrane diffusion of SP-AQH or, for the other redox mediators, semiquinone anion-quinone electron exchange leading to net transmembrane electron transfer, with subsequent one-electron reduction of the internal Co(bpy)33+. Thermal one-electron reduction of Co(bpy)33+ by EDTA is energetically unfavorable; the photosensitized reaction therefore occurs with partial conversion of photonic energy to chemical and transmembrane electrochemical potentials. [ABSTRACT FROM AUTHOR]

The synthesis of poly(hydridocarbyne), one of a class of carbon-based random network polymers and a structural isomer of polyacetlyene, is reported. The network backbone of this polymer is primarily composed of tetrahedrally hybridized carbon atoms, each bearing one hydride substituent and linked via three carbon-carbon single bonds into a three-dimensional random network of fused rings. This atomic-level carbon network backbone confers unusual properties on the polymer, including facile thermal decomposition to form diamond or diamond-like carbon high-quality films at atmospheric pressure, by direct deposition or by chemical vapor deposition (CVD), without the use of hydrogen or any other reagent. [ABSTRACT FROM AUTHOR]

Describes the derivation of electron transfers (ET) from multiple photon absorptions within a synthetic dendrimer framework containing a viologen marker that serves as an electron trap. Dependence of the yield of photoinduced double ET on laser intensity; Dependence of photochemical behavior on a multiphoton kinetic scheme.

Investigates the electron-transfer reactions in Frechet-type dendrimers with biphenyl peripheral groups and a ruthenium core. Use of pulse radiolysis techniques; Preparation of benzyl alcohol dendrons; Measurement of reaction kinetics.