Systematic design of single-mode coupled-resonator optical waveguides in photonic crystals

By establishing a direct relation between the dispersion and the field profile of a coupled-resonator optical waveguide (CROW) and those of its constituent cavities, we present a systematic method for the design of a single-mode CROW and for control of its dispersion. The procedure includes the design of a single-mode cavity and control of its frequency by engineering its structure. Then, by chaining these cavities in the proper direction and at an appropriate distance, we achieve the desired dispersion for the CROW.     

Synthese,Charakterisierung und Struktur von Mn3SiO4F2

Synthesis, Characterization, and Structure of Mn₃SiO₄F₂ Mn₃SiO₄F₂ was synthesized by chemical vapour transport in a temperature gradient (800 → 700 °C) using MnF₂ as precursor and iodine as transport agent. SiO₂ was provided from the wall of the used silica tubes. The chemical composition of the crystals was determined by EELS and EDX analysis. The structure of Mn₃SiO₄F₂ was determined and refined to R(|F|) = 0.039, wR(F²) = 0.087, respectively. The orthorhombic phase crystallizes in the space group Pnma (No. 62) with a = 10.758(2) Å, b = 9.145(1) Å, c = 4.850(1) Å and Z = 4. Two crystallographically different Mn‐atoms are surrounded by oxygene and fluorine octahedrally. Si is tetrahedrally surrounded only by oxygen. IR‐measurements proved that in Mn₃SiO₄F₂ no substitution of F^– by OH^– takes place as in the mineral norbergite (Mg₃SiO₄(OH,F)₂).     

An annulene-fused cyclopentadienide. A photochromic cyclopentadienodimethyldihydropyrene where the fused cyclopentadienide group resembles benzene in its effect on the dihydropyrene-metacyclophanediene valence isomerization

The synthesis of the green cyclopentadiene-fused dimethyldihydropyrene 12 was achieved in 36% yield in 7 steps from the parent dihydropyrene 3. Reaction of 12 with KH or LiCH(2)SiMe(3) gave the [14]annulene-fused cyclopentadienide anion quantitatively. In the (1)H NMR spectra, the internal methyl protons of 12 at delta -3.9, change dramatically on formation of anion 5, becoming deshielded to delta -1.82. This is caused by the reduction in diatropicity of the [14]annulene ring on fusion to the 6pi-cyclopentadienide ring. The anion is also a photochromic switch. Irradiation of the closed form 5 with visible light opens it to the open form 5', which reverts to the closed form 5 either with UV light or thermally. The switching behavior is between that of the parent 3/3' and the benzannelated system 4/4' and suggests that in its effect on the photoswitching, cyclopentadienide is behaving chemically similar to benzene.     

Kinetic study of the complex reaction between copper(II) and 2-(2′-hydroxy-3′-methoxyphenyl)benzothiazole

2-(2′-Hydroxy-3′-methoxyphenyl)benzothiazole reacts with copper(II) in an ethanol/water mixture to form an O,S chelate which exhibits the remarkable property of changing the chelation site above a pH of ca. 5.0, to the O,N site. The detailed kinetics of this reaction in an ethanol/water mixture (3:1) at a temperature of 25 °C was investigated using a stopped-flow spectrophotometric technique employing a wavelength of 400 nm. The initial complex, Cu(O,S), is formed via a fast, reversible second-order complex formation step whereupon the formation of the Cu (O,N) follows first order kinetics. The Cu(O,N) complex is, however, unstable towards internal electron exchange and after the reaction is complete, a black polymeric material very slowly precipitates out of solution. Rate and equilibrium constants for the postulated reactions are presented and discussed.     

Synthesis and photochromic properties of molecules containing [e]-annelated dihydropyrenes. Two and three way pi-switches based on the dimethyldihydropyrene-metacyclophanediene valence isomerization

The syntheses of several new simple negative, a simple positive, and multiple negative photochromes containing the dihydropyrene-cyclophanediene photochromic system are described. The photo-openings of the negative photochromes, the [e]-annelated benzo (7), naphtho (9), anthro (11), furano (19), and triphenyleno (15) derivatives of the parent 2,7-di-tert-butyl-trans-10b,10c-dimethyl-dihydropyrene (5), as well as its 4,5-dibromo derivative (13), are described to give the corresponding cyclophanedienes, as well as their photoclosures and thermal closures back to the dihydropyrenes. These are compared to the results obtained for the positive photochrome dibenzo[e,l]dihydropyrene (21) and to the bis(dihydropyreno)chrysene (44) and the (dihydropyrenobenzo)(benzo)metacyclophanediene (47) photochromes, which have more than one photochromic switch present and thus have more than a simple "on-off" state. Thermodynamic data are obtained for the thermal closing reactions. The anthrodihydropyrene (12) has the fastest thermal closing (tau(1/2) = 20 min), while the furanodihydropyrene (19') has the slowest (tau(1/2) = 63 h) at 46 degrees C. An electrochemical readout of the state of the switch is demonstrated for the benzodihydropyrene (7).     

Theory of Frequency-Dependent Polarization of General Planar Electrodes with Zeta Potentials of Arbitrary Magnitude in Ionic Media: 2. Applications and Results from Homogeneous and Array Systems of Electrodes

The expressions obtained in the previous paper for electrode polarization are applied to a homogeneous planar electrode and a planar array of electrodes used in the generation of nonuniform fields. The effective far field experienced outside the double layer is computed for both electrodes, and sample spectra are provided. The effective far field expression contains the electrode impedance and the effects of concentration polarization due to the static double layer on the electrode generated by the ζ potential. The effective far field results are compact and contain simple integrals that can be evaluated numerically.     

Mechanistic insights into the photochromism of trans-10b,10c-dimethyl-10b,10c-dihydropyrene derivatives

A series of dimethyldihydropyrene derivatives was studied to elucidate the photochemical mechanism associated with the switching between the dimethyldihydropyrene (DHP, closed) and metacyclophanediene (CPD, open) forms of the molecule. Quantum yields of ring opening and closure, fluorescence quantum yields and lifetimes, as well as laser flash photolysis studies were performed to establish the effect of substituents on the switching efficiency. Ring opening of the DHPs occurs from the first singlet excited state. The low quantum yields for the ring opening reaction observed (< or =0.042) are a consequence of the low rate constant (< or =1.7 x 10(7) s(-1)) for this process. The quantum yields for ring closure of the CPD were determined for select compounds and were of the order of 0.1-0.4. These results show that the efficiency for ring opening of this class of compounds is intrinsically low, but can be modulated to some extent by the introduction of substituents. These properties should be taken into account when considering what type of photoswitching devices DHPs might be useful for.     

A beta-lactone route to chiral gamma-substituted alpha-amino acids: application to the concise synthesis of (S)-alpha-azidobutyro lactone and a natural amino acid

[reaction: see text] Beta-lactones are useful synthetic intermediates allowing access to a number of functional arrays. In this report, enantiomerically pure 4-trichloromethyl-2-oxetanone is shown to be a versatile amino acid synthon leading to a variety of gamma-substituted alpha-amino acid precursors. The utility of this methodology was demonstrated by the concise synthesis of a protected homoserine equivalent, alpha-azidobutyro lactone, and a naturally occurring alpha-amino acid from the seeds of Blighia unijugata.     

Darstellung und Struktur von U2Ta6O19, einer neuen Verbindung mit „Jahnberg‐Struktur”︁, sowie Anmerkungen zu den ersten Oxidchloriden in den Systemen Th/Nb/O/Cl und Th/Zr(Hf)/Nb/O/Cl

Synthesis and Crystal Structure of U₂Ta₆O₁₉, a New Compound with “Jahnberg‐Structure” and a Note to the First Oxide Chlorides in the Systems Th/Nb/O/Cl and Th/Zr(Hf)/Nb/O/Cl Black crystals of U₂Ta₆O₁₉ with hexagonal shape were obtained (at T₁) by chemical transport using HCl (p (HCl, 298 K) = 1 atm; silica tube) as transport agent in a temperature gradient (T₂ → T₁; 1000 °C → 950 °C) and using a mixture of UO₂, Ta₂O₅, and HfO₂ (or ZrO₂) (1 : 2 : 2) as starting materials (at T₂). For the structure determination the best result was achieved in space group P6₃/mcm (No. 193, a = 6.26(2) Å, c = 19.86(6) Å). U₂Ta₆O₁₉ is isotypical to Th₂Ta₆O₁₉. In the crystal structure each uranium atom is surrounded by oxygen atoms like a bi‐capped trigonal antiprism and tantalum atoms like a pentagonal bipyramid (CN = 7). Like the “Jahnberg Structures” both coordination polyhedra arrange themselves in separate layers (U–O‐polyhedra, in o‐, Ta–O‐polyhedra in p‐layers) so that in the direction of the c‐axis the sequence of layers is p‐p‐o. Using chemical transport it was possible to prepare the compounds Th₁₂Nb₁₆O₆₃Cl₂ and Th₈M₄Nb₁₆O₆₃Cl₂ (M = Zr, Hf), which are the first quaternary and quinquinary examples in these systems. They crystallize isotypically.     

Biosynthetic insights provided by unusual sesterterpenes from the medicinal herb Aletris farinosa

A series of novel sesterterpenes (2–6) have been isolated from the roots of Aletris farinosa and structurally characterized by MS, NMR, and X-ray crystallography in conjunction with computational modeling. Their structures provide new insights into the mechanisms of sesterterpene biosynthesis. Specifically, we propose with support from density functional theory computations that the configuration at a single stereocenter determines the fate of a key tetracyclic carbocationic intermediate, derived from an oxidogeranylfarnesol precursor. Whereas one epimer of the carbocation undergoes H⁺ elimination to give 6, the other undergoes a spectacular cascade of seven 1,2-methyl and hydride migrations leading to the previously unreported carbon skeleton of 5. Theoretical calculations suggest that the cascade is triggered by substrate preorganization in the enzyme active site.