A Comparison of Density‐Functional Theory and Hartree?Fock Modeling of Organolithium Equilibria

Experimental values for a number of organolithium equilibria in THF solution expressed as pK values are compared with computational results at several theory levels. Solvation effects are considered only by coordination of lithium with two THF molecules. Best results are obtained with the hybrid density‐functional method mPW1PW91, which gives satisfactory results over a range of almost 40 pK units, but explicit thermal corrections to 25° are required.     

Supramolecular Tetrad Featuring Covalently Linked Bis(porphyrin)–Phthalocyanine Coordinated to Fullerene: Construction and Photochemical Studies

A multimodular donor–acceptor tetrad featuring a bis(zinc porphyrin)–(zinc phthalocyanine) ((ZnP–ZnP)–ZnPc) triad and bis‐pyridine‐functionalized fullerene was assembled by a “two‐point” binding strategy, and investigated as a charge‐separating photosynthetic antenna‐reaction center mimic. The spectral and computational studies suggested that the mode of binding of the bis‐pyridine‐functionalized fullerene involves either one of the zinc porphyrin and zinc phthalocyanine (Pc) entities of the triad or both zinc porphyrin entities leaving ZnPc unbound. The binding constant evaluated by constructing a Benesi–Hildebrand plot by using the optical data was found to be 1.17×10⁵ M ^?1, whereas a plot of “mole‐ratio” method revealed a 1:1 stoichiometry for the supramolecular tetrad. The mode of binding was further supported by differential pulse voltammetry studies, in which redox modulation of both zinc porphyrin and zinc phthalocyanine entities was observed. The geometry of the tetrad was deduced by B3LYP/6‐31G* optimization, whereas the energy levels for different photochemical events was established by using data from the optical absorption and emission, and electrochemical studies. Excitation of the zinc porphyrin entity of the triad and tetrad revealed ultrafast singlet–singlet energy transfer to the appended zinc phthalocyanine. The estimated rate of energy transfer (k_ENT) in the case of the triad was found to be 7.5×10¹¹ s^?1 in toluene and 6.3×10¹¹ s^?1 in o‐dichlorobenzene, respectively. As was predicted from the energy levels, photoinduced electron transfer from the energy‐transfer product, that is, singlet‐excited zinc phthalocyanine to fullerene was verified from the femtosecond‐transient spectral studies, both in o‐dichlorobenzene and toluene. Transient bands corresponding to ZnPc ^{? +} in the 850 nm range and C₆₀ ^{? ?} in the 1020 nm range were clearly observed. The rate of charge separation, k_CS, and rate of charge recombination, k_CR, for the (ZnP–ZnP)–ZnPc ^{? +}:Py₂C₆₀ ^{? ?} radical ion pair (from the time profile of 849 nm peak) were found to be 2.20×10¹¹ and 6.10×10⁸ s^?1 in toluene, and 6.82×10¹¹ and 1.20×10⁹ s^?1 in o‐dichlorobenzene, respectively. These results revealed efficient energy transfer followed by charge separation in the newly assembled supramolecular tetrad.     

Distributed strain sensing using frequency-modulated continuous-wave reflectometric optical technique

This paper presents an analysis describing the feasibility of a distributed optical fiber strain sensor whose principle of operation is based on the frequency-modulated continuous-wave (FMCW) reflectometry technique. The system consists of a frequency-swept laser diode and an unbalanced two-beam interferometer. In this system, the test arm comprises a number of single-mode fibers, which act as the sensing fibers, with a mirror at the far end and mechanical splices as the connectors, as well as the reflectors. Theoretical analysis shows that the measuring resolution of the strain is inversely proportional to the length of the sensing fiber. The strain variation of the sensing fiber is measured by demodulating the phase shift of the beat signal using a heterodyne signal processing system, and therefore the requirement of a high precision temperature control of the light source can be eliminated in this way. The comparison between the theoretical and experimental values of the strain resolution showed excellent agreement.     

Lasing wavelength dependence of gain-clamped EDFA performance with different optical feedback schemes

Performance of gain-clamped erbium-doped fibre amplifier with different optical feedback schemes has been studied as a function of lasing wavelength. An inverse relation of the signal gains with the amplified spontaneous emission spectrum is observed for the entire laser tuning range as a result of the population variation caused by the different strength of the oscillating laser. The noise figure also shows a feature of the lasing wavelength dependence. However, co-feedback scheme exhibits behaviour opposite to that of the counter-feedback.     

Solubilization of Immiscible Liquids Into Mesophases Formed from Amphiphilic Block Copolymers

Optical microscopy and X-ray diffraction studies reveal the presence of highly swollen mesophases in four-component systems containing amphiphilic triblock copolymers. According to the composition of the system, lamellar phases or hexagonal structures are produced. The polymeric mesophases exhibit textures very similar to those encountered in low molecular weight liquid crystalline phases.     

Organometallic clusters as precursors for metallic nanoparticles: effect of cluster size, ligand set, and decomposition method

The pyrolysis of organometallic clusters containing osmium and/or ruthenium on a silica support leads to metallic nanoparticles in the 1 to 10 nm size range. The particle size obtained is correlated to the ligand set, and there are also indications that the shape of the cluster may have some effect on the shape of the nanoparticles obtained. In comparison, the size of nanoparticles obtained through thermolysis in a coordinating solvent are fairly independent of the nature of the organometallic precursor and tends to lead to aggregation of smaller, spherical nanoparticles into larger particles.     

Surface tension-driven self-folding polyhedra

We discuss finite element simulations and experiments involving the surface tension-driven self-folding of patterned polyhedra. Two-dimensional (2D) photolithographically patterned templates folded spontaneously when solder hinges between adjacent faces were liquefied. Minimization of interfacial free energy of the molten solder with the surrounding fluidic medium caused the solder to ball up, resulting in a torque that rotated adjacent faces and drove folding. The simulations indicate that the folding process can be precisely controlled, has fault tolerance, and can be used to fold polyhedra composed of a variety of materials, ranging in size from the millimeter scale down to the nanometer scale. Experimentally, we have folded metallic, arbitrarily patterned polyhedra ranging in size from 2 mm to 15 microm.     

Au(i)-benzimidazole/imidazole complexes. Liquid crystals and nanomaterials

Three series of Au(I)-imidazole complexes with stoichiometries of [Au(Cn-bim)Cl], [Au(Cn-im)Cl], and [Au(Cn-im)2][NO3] x 2H2O (Cn-bim = N-CnH2n+1 -substituted benzimidazole and Cn-im = N-CnH2n+1-substituted imidazole) together with the compound of [Au(C18-bim)2][NO3] are synthesiszed. Typical structures of each series are determined by single crystal X-ray diffraction. The last series of compounds, are liquid crystals, and exhibit a wider mesophase range than their Ag(I) analogues. These Au(I) complexes form Au nanostructures both through chemical reduction or thermolysis. For the first time, N-long chain imidazole is utilized to stabilize colloidal Au in solution. Also for the first time, unique examples of simple thermolysis to produce large Au plates of nanothickness are demonstrated. Formation of a plate-like morphology through fusion of sphere-like nanoparticles at an early stage is evidenced by TEM images.     

Pricing vulnerable European options with stochastic default barriers

CF Lo and KC Ku Institute of Theoretical Physics and Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China Email: cho-hoi_hui{at}hkma.gov.hk Received on 31 July 2006. Accepted on 15 March 2007. This paper develops a valuation model of European options incorporatinga stochastic default barrier, which extends a constant defaultbarrier proposed in the Hull–White model. The defaultbarrier is considered as an option writer's liability. Closed-formsolutions of vulnerable European option values based on themodel are derived to study the impact of the stochastic defaultbarriers on option values. The numerical results show that negativecorrelation between the firm values and the stochastic defaultbarriers of option writers gives material reductions in optionvalues where the options are written by firms with leverageratios corresponding to BBB or BB ratings.