N-heterocyclic carbene Pt(II) complexes from caffeine: synthesis, structures and photoluminescent properties

The caffeine-derived N-heterocyclic carbene (NHC) complex [Pt(II)(C^N)(NHC)Cl] (C^N = 2-phenylpyridine), 4 has the opposite stereochemistry and a shorter Pt-C(carbene) bond compared to that of an analogous benzimidazole-derived N,N-heterocyclic carbene (NNHC) Pt complex 2. These suggest a lower trans influence of pyridyl N compared to cyclometallated carbon and an increased Pt-NHC π-backbonding because of decreased π-donation resulting from conjugation to the electron deficient pyrimidine of caffeine. Complex 4 has a lower emission quantum yield (Φ) and is blue-shifted into the green region of the visible spectrum relative to non-carbene Pt(II) cyclometalated complex 5.     

Synthesis, characterization, and single-molecule metamagnetism of new Co(II) polynuclear complexes of pyridine-2-ylmethanol

The reaction between pyridine-2-ylmethanol (HL), anhydrous CoCl(2) and NaH afforded polynuclear Co(II) complexes [Co(7)(L)(12)]Cl(2) (1), [Co(6)Na(L)(12)]Cl (2) and [Co(4)Cl(2)(L)(6)] (3), depending on the HL:CoCl(2) ratio set in the reaction. The core structures of the centrosymmetric complexes 1 and 2 are of the M@Co(6) type (M = Co or Na, respectively) with a coplanar arrangement of the metals whereas that of centrosymmetric 3 is of an incomplete dicubane type. The experimental conditions allowing interconversions between these polynuclear complexes have been determined, which provides a more rational control of their synthesis. Thus, 1 transforms to 3 when reacted with CoCl(2) in a 1 : 1 ratio, whereas the same reaction performed with a large excess of CoCl(2) gave the tetranuclear pseudo-cubane complex [Co(4)(L)(4)Cl(2)(MeOH)(4)] upon recrystallization. Conversely, 1 was isolated from the reaction of 3 with HL and NaH. The crystal structure of these compounds is reported, along with the magnetic behaviour of 1 and 3. The analysis of the magnetism using the effective spin-1/2 Hamiltonian approach revealed single-molecule metamagnetic behavior in 3.     

Magnetically recoverable magnetite/gold catalyst stabilized by poly(N-vinyl-2-pyrrolidone) for aerobic oxidation of alcohols

Fe(3)O(4):PVP/Au nanocomposite synthesized via a two-step procedure was tested as a quasi-homogenous alcohol oxidation catalyst. It was found that the nanocomposite was able to carry out aerobic oxidation of alcohols in water at room temperature. Studies show rapid magnetic recoverability and reusability characteristics.     

Rapid injection NMR reveals η3 'π-allyl' Cu(III) intermediates in addition reactions of organocuprate reagents

By using rapid injection NMR, it has now been possible to prepare and characterize the η(3) 'π-allyl' copper(III) intermediate that has been proposed for addition reactions of organocopper(I) reagents and α,β-unsaturated carbonyl compounds.     

Milliwatt-level mid-infrared (10.5-16.5 μm) difference frequency generation with a femtosecond dual-signal-wavelength optical parametric oscillator

We demonstrate the generation of mid-infrared radiation using a femtosecond dual-signal-wavelength optical parametric oscillator and difference frequency generation in an extracavity gallium selenide or silver gallium diselenide crystal. This system generates up to 4.3?mW of average mid-infrared power. Its spectra can be tuned to between 10.5?μm and 16.5?μm wavelength (952 cm^-1-606 cm^-1) with more than 50 cm^-1 spectral bandwidth. We demonstrate that the power and spectra of this system are temporally very stable.     

Multimode lasing from the microcavity of an octagonal quasi-crystal based on holographic polymer-dispersed liquid crystals

An eightfold photonic quasi-crystal (PQC) sample is fabricated holographically using two-beam interference with multi-exposure based on polymer-dispersed liquid crystals. The transmission spectra from the finite-difference time-domain (FDTD) simulation prove the photonic stop band of the rotational symmetry structure of the sample. The resonant mode of the circular microcavity formed in the PQC is calculated. Amplified spontaneous emission and multimode lasing action are demonstrated from the pumped laser-dye-doped PQC microcavity using a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) pulse laser.     

Current induced resistance change of magnetic tunnel junctions with ultra-thin MgO tunnel barriers

Ultra-thin magnetic tunnel junctions with low resistive MgO tunnel barriers are prepared to examine their stability under large current stress. The devices show magnetoresistance ratios of up to 110% and an area resistance product of down to . If a large current is applied, a reversible resistance change is observed, which can be attributed to two different processes during stressing and one relaxation process afterwards. Here, we analyze the time dependence of the resistance and use a simple model to explain the observed behavior. The explanation is further supported by numerical fits to the data in order to quantify the timescales of the involved phenomena.     

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

Using a two-dimensional multiple-quantum (MQ) double rotation (DOR) experiment the contributions of the chemical shift and quadrupolar interaction to isotropic resonance shifts can be completely separated. Spectra were acquired using a three-pulse triple-quantum z-filtered pulse sequence and subsequently sheared along both the ν₁ and ν₂ dimensions. The application of this method is demonstrated for both crystalline (RbNO₃) and amorphous samples (vitreous B₂O₃). The existence of the two rubidium isotopes (⁸⁵Rb and ⁸⁷Rb) allows comparison of results for two nuclei with different spins (I = 3/2 and 5/2), as well as different dipole and quadrupole moments in a single chemical compound. Being only limited by homogeneous line broadening and sample crystallinity, linewidths of approximately 0.1 and 0.2 ppm can be measured for ⁸⁷Rb in the quadrupolar and chemical shift dimensions, enabling highly accurate determination of the isotropic chemical shift and the quadrupolar product, P_Q. For vitreous B₂O₃, the use of MQDOR allows the chemical shift and electric field gradient distributions to be directly determined—information that is difficult to obtain otherwise due to the presence of second-order quadrupolar broadening.     

Multi-objective optimisation of positively homogeneous functions and an application in radiation therapy

Unconstrained multi-objective optimisation problems with p$p$ positively homogeneous objective functions are considered. We prove that such problems reduce to multi-objective optimisation problems with p−1$p-1$ objectives and a single equality constraint. Thus, problems with two objectives can be solved with standard single objective optimisation methods and, for problems with p>2$p>2$ objectives, we can compute infinitely many efficient solutions by solving a finite number of single objective problems. The proposed procedure is applied on radiotherapy for cancer treatment.