Ansa‐Bridged Bis(benzene) Titanium Complexes

Taking advantage of an improved synthesis of [Ti(η⁶‐C₆H₆)₂], we report here the first examples of ansa‐bridged bis(benzene) titanium complexes. Deprotonation of [Ti(η⁶‐C₆H₆)₂] with nBuLi in the presence of N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (pmdta) leads to the corresponding 1,1′‐dilithio salt [Ti(η⁶‐C₆H₅Li)₂] ? pmdta that enables the preparation of the first one‐ and two‐atom‐bridged complexes by simple salt metathesis. The ansa complexes were fully characterized (NMR spectroscopy, UV/Vis spectroscopy, elemental analysis, and X‐ray crystallography) and further studied electrochemically and computationally. Moreover, [Ti(η⁶‐C₆H₆)₂] is found to react with the Lewis base 1,3‐dimethylimidazole‐2‐ylidene (IMe) to give the bent sandwich complex [Ti(η⁶‐C₆H₆)₂(IMe)].     

Reversible Assembly of a Supramolecular Cage Linked by Boron–Nitrogen Dative Bonds

Boron–nitrogen dative bonds provide a suitable motif for reversible, yet strong and directed interactions, leading to the highly efficient self‐assembly of small organic building blocks into supramolecular cage structures. A bipyramidal [2+3] assembly, as the first example of a supramolecular cage mediated by B?N dative bonds that exists as a discrete species in solution, is quantitatively obtained from a tribenzotriquinacene‐based trisboronate ester and 1,4‐diazabicyclo[2.2.2]octane. Thermodynamic equilibria of cage formation are investigated by isothermal titration calorimetry and fully reversible cage opening can be observed at elevated temperatures.     

Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

Blinking of the photoluminescence (PL) emitted from individual conjugated polymer chains is one of the central observations made by single‐molecule spectroscopy (SMS). Important information, for example regarding excitation energy transfer, can be extracted by evaluating dynamic quenching. However, the nature of trap states, which are responsible for PL quenching, often remains obscured. We present a detailed investigation of the photon statistics of single poly(3‐hexylthiophene) (P3HT) chains obtained by SMS. The photon statistics provide a measure of the number and brightness of independently emitting areas on a single chain. These observables can be followed during blinking. A decrease in PL intensity is shown to be correlated with either 1) a decrease in the average brightness of the emitting sites; or 2) a decrease in the number of emitting regions. We attribute these phenomena to the formation of 1) shallow charge traps, which can weakly affect all emitting areas of a single chain at once; and 2) deep traps, which have a strong effect on small regions within the single chains.     

Chelation of vanadium(V) by difluoromethylene bisphosphonate, a structural analogue of pyrophosphate

The structural and functional analogy between difluoromethylene bisphosphonate (CF2PP) and pyrophosphate (PPi) is investigated in a reaction with V(V) in the form of vanadate. The reaction of CF2PP with vanadate was investigated using 1.00 M KCl as supporting electrolyte over the ranges 3 < or = [CF2PP] < or = 60 mM and 2.06 < or = pH < or = 11.80. 51V, 19F, and 31P NMR spectroscopic studies showed that a 1:1 species was formed with an H+-dependent formation constant of 110 M-1 at pH 7.22. Results of solution experiments and ab initio calculations are consistent with CF2PP coordinating V(V) in a bidentate manner, as previously reported for PPi. Below pH 4, a minor complex forms, which is consistent with a 1:2 stoichiometry. This complex was also observed with pyrophosphate. The X-ray crystal structure of the monoprotonated difluoromethylene bisphosphonate anion (H[CF2PP]3-)-toludine complex is presented. The H[CF2PP]3- anion crystallized in the triclinic space group P with a = 12.7629(7) A, b = 13.3992(7) A, c = 17.1002(9) A, and V = 2584.4(2) A3, and Z = 2. Sheets of the layers of anions are connected through a network of H-bonds and separated by a layer of toludine cations. The structural features are investigated, and the CF2PP anion was found to be longer and wider than the corresponding PPi. Given the larger size of this anion compared to PPi, the chelation affinity upon CF2 substitution was found to be 4-5-fold reduced at neutral pH.     

Chemoenzymatic approaches toward dechloroansamitocin P-3

[reaction: see text] The enantioselective total synthesis of proansamitocin, a key biosynthetic intermediate of the highly potent antitumor agent ansamitocin P-3, is described which bears a diene-ene RCM as the key macrocyclization step. Feeding of proansamitocin to an AHBA block mutant Actinosynnema pretiosum (HGF073) yielded ansamitocin P-3 as well as dechloroansamitocin P-3, the latter also being formed upon fermentation in the presence of 3-amino-5-methoxybenzoic acid.     

A new mathematical approach for calculating the contribution of anammox, denitrification and atmosphere to an N2 mixture based on a 15N tracer technique

Denitrification and anaerobic ammonium oxidation (anammox) have been identified as biotic key processes of N2 formation during global nitrogen cycling. Based on the principle of a 15N tracer technique, new analytical expressions have been derived for a calculation of the fractions of N2 simultaneously released by anammox and denitrification. An omnipresent contamination with atmospheric N2 is also taken into account and is furthermore calculable in terms of a fraction. Two different mathematical approaches are presented which permit a precise calculation of the contribution of anammox, denitrification, and atmosphere to a combined N2 mixture. The calculation is based on a single isotopic analysis of a sampled N2 mixture and the determination of the 15N abundance of nitrite and nitrate (simplified approach) or of ammonium, nitrite, and nitrate (comprehensive approach). Calculations are even processable under conditions where all basal educts of anammox and denitrification (ammonium, nitrite, and nitrate) are differently enriched in 15N. An additional determination of concentrations of dissolved N compounds is unnecessary. Finally, the presented approach is transferable to studies focused on terrestrial environments where N2 is formed by denitrification and simultaneously by codenitrification or chemodenitrification.     

Evidence for an intramolecular charge transfer state in 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al: influence of solvent polarity and temperature

The ultrafast excited-state dynamics of two carbonyl-containing carotenoids, 12'-apo-beta-caroten-12'-al and 8'-apo-beta-caroten-8'-al, have been investigated by transient absorption spectroscopy in a systematic variation of solvent polarity and temperature. In most of the experiments, 12'-apo-beta-caroten-12'-al was excited at 430 nm and 8'-apo-beta-caroten-8'-al at 445 or 450 nm via the S0 --> S2 (11Ag- --> 11Bu+) transition. The excited-state dynamics were then probed at 860 nm for 12'-apo-beta-caroten-12'-al and at 890 or 900 nm for 8'-apo-beta-caroten-8'-al. The temporal evolution of all transient signals measured in this work can be characterized by an ultrafast decay of the S2 --> SN absorption at early times followed by the formation of a stimulated emission (SE) signal, which subsequently decays on a much slower time scale. We assign the SE signal to a low-lying electronic state of the apocarotenals with intramolecular charge-transfer character (ICT --> S0). This is the first time that the involvement of an ICT state has been detected in the excited-state dynamics of a carbonyl carotenoid in nonpolar solvents such as n-hexane or i-octane. The amplitude ratio of ICT-stimulated emission to S2 absorption was weaker in nonpolar solvents than in polar solvents. We interpret the results in terms of a kinetic model, where the S1 and ICT states are populated from S2 through an ultrafast excited-state branching reaction (tau2 < 120 fs). Delayed formation of a part of the stimulated emission is due to the transition S1 --> ICT (tau3 = 0.5-4.1 ps, depending on the solvent), which possibly involves a slower backward reaction ICT --> S1. Determinations of tau1 were carried out for a large set of solvents. Especially in 12'-apo-beta-caroten-12'-al, the final SE decay, assigned to the nonradiative relaxation ICT --> S0, was strongly dependent on solvent polarity, varying from tau1 = 200 ps in n-hexane to 6.6 ps in methanol. In the case of 8'-apo-beta-caroten-8'-al, corresponding values were 24.8 and 7.6 ps, respectively. This indicates an increasing stabilization of the ICT state with increasing solvent polarity, resulting in a decreasing ICT-S0 energy gap. Tuning the pump wavelength from the blue wing to the maximum of the S0 --> S2 absorption band resulted in no change of tau1 in acetone and methanol. Additional measurements in methanol after excitation in the red edge of the S0 --> S2 band (480-525 nm) also show an almost constant tau1 with only a 10% reduction at the largest probe wavelengths. The temperature dependence of the tau1 value of 12'-apo-beta-caroten-12'-al was well described by Arrhenius-type behavior. The extracted apparent activation energies for the ICT --> S0 transitions were in general small (on the order of a few times RT), which is in the range expected for a radiationless process.     

Microstructure and electrical properties of BaTiO<Subscript>3</Subscript> and (Ba,Sr)TiO<Subscript>3</Subscript> ferroelectric thin films on nickel electrodes

Nickel thin films have been sputtered on standard Si/SiO₂ substrates with TiO₂ as an adhesive layer. The thermal stability of these substrates was analyzed. SEM images show an increase in grain size with annealing temperature. They were found to be stable till 800°C, beyond which the nickel layer disintegrated. These substrates were used for deposition of BaTiO₃ and (Ba,Sr)TiO₃ dielectric thin films under a reducing atmosphere. The dielectric thin films were processed with various pyrolysis and annealing temperatures in order to optimize the dielectric properties. Increased pyrolysis temperatures showed an increase in the grain size. Results on these nickelised substrates were finally compared with dielectric films deposited on platinized silicon substrates under identical conditions but crystallized in an oxygen atmosphere.     

Highly Emissive 9-Borafluorene Derivatives: Synthesis,Photophysical Properties and Device Fabrication

A series of 9-borafluorene derivatives, functionalised with electron-donating groups, have been prepared. Some of these 9-borafluorene compounds exhibit strong yellowish emission in solution and in the solid state with relatively high quantum yields (up to 73.6 % for FMesB-Cz as a neat film). The results suggest that the highly twisted donor groups suppress charge transfer, but the intrinsic photophysical properties of the 9-borafluorene systems remain. The new compounds showed enhanced stability towards the atmosphere, and exhibited excellent thermal stability, revealing their potential for application in materials science. Organic light-emitting diode (OLED) devices were fabricated with two of the highly emissive compounds, and they exhibited strong yellow-greenish electroluminescence, with a maximum luminance intensity of >22 000 cd m⁻². These are the first two examples of 9-borafluorene derivatives being used as light-emitting materials in OLED devices, and they have enabled us to achieve a balance between maintaining their intrinsic properties while improving their stability.