Photochromic and fluorescence properties of coumarin fulgimides

Four new fulgimides possessing a fluorescent coumarin unit were synthesized from the corresponding fulgides, and their photochromic as well as fluorescence properties were investigated. The open-ring forms of coumarin fulgimides were found to exhibit fluorescence in the visible region. Upon exposure to UV light, the fulgimides were transformed into the nonfluorescent closed-ring forms, which can be reverted to the initial fluorescent open-ring forms on exposure to visible light. The efficiency of quenching of fluorescence was as high as 95% at the photostationary state of UV irradiation.     

The 9-Borataphenanthrene Anion

The 9-borataphenanthrene anion is easily accessed by deprotonation of a 9,10-dihydro-9-boraphenanthrene and its diverse reactivity is investigated. Alkylation occurs at the carbon atom adjacent to boron, and room temperature hydroboration occurs across the B=C bond. The π-manifold of the central BC₅ ring coordinates to chromium in an η⁶ fashion while only the B=C unit binds η² to gold, indicating versatility of the 9-borataphenanthrene anion as a ligand. Supporting calculations rationalize the reactivity and aromaticity is corroborated by nucleus-independent chemical shift (NICS) indices.     

On clique covers and independence numbers of graphs

Relationships between the following graph invariants are studied: The node clique cover number, θ₀(>G); the clique cover number θ₁(G); node independence number, β₀(G); and an edge independence number, β′₁(G), We extend a theorem of Choudum, Parthasarathy and Ravindra with further statements equivalent to θ₀(G) = θ₁(G). More general results regarding the inequality θ₀(G) ? 0₁(G) are presented.     

Electric field dependence of recombination kinetics in reaction centers of photosynthetic bacteria

Time-resolved charge recombination has been measured by reflectance/absorption spectroscopic analysis of Langmuir-Blodgett films of reaction centers of the photosynthetic bacterium, Rhodopseudomonas sphaeroides over a wide range of applied electric field strengths. The field dependence of the recombination kinetics has been deduced from the time-course of the reduction of the flash-oxidized bacteriochlorophyll dimer [(BChl)⁺₂] recorded at different applied field strengths. Measurements were performed under two different electric field biasing conditions: a constant bias and a high-frequency bipolar square-wave bias. The additional data obtained from bipolar biasing enabled the use of a new deconvolution method to obtain the field dependence of the rate constants from the experimental curves. The deconvolution shows that the rates for charge recombination from the flash-generated state back to the ground state (BChl)₂Q_A approximate exponential functions of the applied electric field. Correlation of the recombination kinetics data with photoinduced electrical response measurements on films with asymmetric up and down populations of reaction centers reveals that fields opposing charge separation result in faster rates of recombination. Although other possibilities are considered, the main source of the effect is believed to be a result of field-induced changes in the free energy gap between and (BChl)₂Q_A. The results presented here are compared to those obtained in experiments with solubilized reaction centers in which the free energy gap between and (BChl)₂Q_A has been changed by quinone replacement.     

Designing Light‐Activated Charge‐Separating Proteins with a Naphthoquinone Amino Acid

The first principles design of manmade redox‐protein maquettes is used to clarify the physical/chemical engineering supporting the mechanisms of natural enzymes with a view to recapitulate and surpass natural performance. Herein, we use intein‐based protein semisynthesis to pair a synthetic naphthoquinone amino acid (Naq) with histidine‐ligated photoactive metal–tetrapyrrole cofactors, creating a 100 μs photochemical charge separation unit akin to photosynthetic reaction centers. By using propargyl groups to protect the redox‐active para‐quinone during synthesis and assembly while permitting selective activation, we gain the ability to employ the quinone amino acid redox cofactor with the full set of natural amino acids in protein design. Direct anchoring of quinone to the protein backbone permits secure and adaptable control of intraprotein electron‐tunneling distances and rates.     

Synthetic hydrogenases: incorporation of an iron carbonyl thiolate into a designed peptide

[FeFe] hydrogenases catalyze reversible hydrogen oxidation at an unusual organometallic active site. Neither enzymatic studies nor synthesis of small molecule models has managed to elucidate the mechanisms of these enzymes. In this paper, we demonstrate the incorporation of an iron carbonyl thiolate mimic of the hydrogenase active site into a de novo artificial peptide, creating the first peptide-based model system for hydrogenases.     

Redox reactions between phosphines (R3P; R = nBu, Ph) or carbene (iPr2IM) and chalcogen tetrahalides ChX4 (iPr2IM = 2,5-diisopropylimidazole-2-ylidene; Ch = Se, Te; X = Cl, Br)

The reactions between chalcogen tetrahalides (ChX(4); Ch = Se, Te; X = Cl, Br) and the neutral donors (n)Bu(3)P, Ph(3)P, or the N-heterocyclic carbene, 2,5-diisopropylimidazole-2-ylidene ((i)Pr(2)IM), have been investigated. In cases involving a phosphine, the chemistry can be understood in terms of a succession of two-electron redox reactions, resulting in reduction of the chalcogen center (e.g., Se(IV) --> Se(II)) and the oxidation of phosphorus to the [R(3)P-X] cation (P(III) --> P(V)). The stepwise reduction of Se(IV) --> Se(II) --> Se(0) --> Se(-II) occurs upon the successive addition of stoichiometric equivalents of Ph(3)P to SeCl(4), which can readily be monitored by 31P{(1)H} NMR spectroscopy. In the case of reacting SeX(4) with (i)Pr(2)IM, a similar two-electron reduction of the chalcogen is observed and there is the concomitant production of a haloimidazolium hexahaloselenate salt. The products have been comprehensively characterized, and the solid-state structures of [R(3)PX][SeX(3)] (9), [Ph(3)PCl](2)[TeCl(6)] (10), (i)Pr(2)IM-SeX(2) (11), and [(i)Pr(2)IM-Cl](2)[SeCl(6)] (12) have been determined by X-ray diffraction analysis. These data all support two electron redox reactions and can be considered in terms of the formal reductive elimination of X2, which is sequestered by the Lewis base.     

A method for measuring recompression shock unsteadiness applied to two supersonic wakes

The recompression shock system in blunt-base cylinder wakes at 0° and 10° angles-of-attack to an M=2.46 freestream is visualized by planar laser scattering, allowing the instantaneous position of the shock to be determined over a wide region of the flow. The recompression shock at =0° is highly branched and appears to be quite weak. The shock appears to be stronger at =10°, with far less branching. Correlation analysis for the =10° wake indicates that fluctuations in the shock position tend to be correlated over relatively long streamwise distances. Analysis of the shock angle for the angle-of-attack wake shows a significant variation in the local angle of the shock, with trends similar to those seen for the unsteadiness in the shock position.