Synthesis, characterization, and photochemical and computational investigations of Ru(II) heterocyclic complexes containing 2,6-dimethylphenylisocyanide (CNx) ligand

The isocyanide ligand forms complexes with ruthenium(II) bis-bipyridine of the type [Ru(bpy)(2)(CNx)Cl](CF(3)SO(3)) (1), [Ru(bpy)(2)(CNx)(py)](PF(6))(2) (2), and [Ru(bpy)(2)(CNx)(2)](PF(6))(2) (3) (bpy = 2,2'-bipyridine, py = pyridine, and CNx = 2,6-dimethylphenylisocyanide). The redox potentials shift positively as the number of CNx ligands increases. The metal-to-ligand charge-transfer (MLCT) bands of the complexes are located at higher energy than 450 nm and blue shift in proportion to the number of CNx ligands. The complexes are not emissive at room temperature but exhibit intense structured emission bands at 77 K with emission lifetimes as high as 25 micros. Geometry optimization of the complexes in the singlet ground and lowest-lying triplet states performed using density functional theory (DFT) provides information about the orbital heritage and correlates with X-ray and electrochemical results. The lowest-lying triplet-state energies correlate well with the 77 K emission energies for the three complexes. Singlet excited states calculated in ethanol using time-dependent density functional theory (TDDFT) and the conductor-like polarizable continuum model (CPCM) provide information that correlates favorably with the experimental absorption spectra in ethanol.     

Optical Spectroscopy of Hybrid Sol-Gel Coatings Doped with Noble Metals

Sol-gel coatings in the xM (100-x) SiO₂ system, (M = Cu, Ag and Au) x =0.1–10 mol%), are deposited on soda lime glass slides by using silicon tetramethoxide Si(OCH₃)₄) and methyltriethoxysilane (SiCH₃[OCH₂CH₃]₃) as silica precursors. Anhydrous CuCl, CuCl₂ 2H₂O, Cu(NO₃)₂ 3H₂O, CuSO₄ 5H₂O, AgNO₃ and HAuCl₄ 3H₂O are used as copper, silver and gold sources. Coatings with thicknesses ranging from 100 to 900 nm are deposited on the subs trates by dip-coating and subsequently densified at 500°C for 1 h in air. Spectroscopic studies of the coatings as a function of the thicknesses and the metal concentration are carried out by photoluminescence (PL) and optical absorption (OA). In addition, direct observations of some gold coatings were performed by transmission electron microscopy (TEM). Results indicate that for silver and copper containing coatings the excitation and emission spectra arise from electronic transitions in Ag⁺ and Cu⁺ ions and no significant absorption bands due to colloidal precipitation are observed. Gold containing coatings show purple coloration due to an absorption peaking in the 520–560 nm range, which is characteristic of gold colloids. The presence of these colloids is confirmed by TEM observations.     

Aza-Wittig reaction of fluoroalkylated N-vinylic phosphazenes with carbonyl compounds. Usefulness of 2-azadienes for the preparation of fluoroalkyl pyridine derivatives

A method for the preparation of 3-fluoroalkyl substituted 2-aza-butadienes 6 by aza-Wittig reaction of 3-fluoroalkyl-N-vinylic phosphazenes 4 and aldehydes 5 is reported. [4+2] Cycloaddition reaction of these heterodienes 6 with enamines 9 gives fluoroalkyl substituted pyridine 15, 16, 24-27 and isoquinoline 12-14, 20 derivatives.     

Interface double-exchange ferromagnetism in the Mn-Zn-O system: new class of biphase magnetism

In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of Mn(3+) and Mn(4+) via a double-exchange mechanism. The presence of the ZnO around MnO(2) modifies the kinetics of MnO(2)-->Mn(2)O(3) reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.     

Aqueous photopolymerization with visible‐light photoinitiators: Acrylamide polymerization photoinitiated with a phenoxazine dye/amine system

We studied the photoinduced electron‐transfer polymerization of acrylamide with, as a visible‐light initiator, the heterocycle‐N‐oxide resazurin in the presence of triethanolamine. The irradiation of resazurin produces resorufin, which also absorbs in the visible region. Both phenoxazine dyes in the presence of the amine are efficient photoinitiators of acrylamide polymerization in an aqueous medium. The polymerization rates were measured at several amine concentrations. These values increase with the amine concentration, reaching a maximum value; further amine addition slightly decreases the polymerization rate. Time‐resolved photolysis studies of the dyes were carried out under the polymerization conditions. The quenching of the dye excited states by the amine was analyzed with static‐fluorescence and laser‐flash photolysis. These data were used for fitting curves of the polymerization rate versus the amine concentration, and it was concluded that the interaction of triplet excited dyes with the amine leads to acrylamide polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4074–4082, 2001     

Topology of character varieties and representations of quivers

In Hausel et al. (2008) [10] we presented a conjecture generalizing the Cauchy formula for Macdonald polynomial. This conjecture encodes the mixed Hodge polynomials of the character varieties of representations of the fundamental group of a punctured Riemann surface of genus g. We proved several results which support this conjecture. Here we announce new results which are consequences of those in Hausel et al. (2008) [10].     

Correlation length of quasiperiodic vortex lattices

We investigated vortex-lattice dynamics in superconducting Nb thin films with different quasiperiodic arrays of magnetic pinning centers. The mixed-state magnetoresistance exhibits minima for well-defined applied fields, related to matching effects between the vortex lattice and those arrays. The results show that critical matching can originate at a local scale. For fractal arrays, the vortex-lattice correlation length is longer and the minima are deeper, close to those of periodic arrays.     

Role of manganese oxide octahedral molecular sieves in styrene epoxidation

Manganese oxide octahedral molecular sieves having 2 x 2 tunnel structure (OMS-2) and synthesized by different methods were used for studying styrene oxidation with tert-butyl hydroperoxide (TBHP) as the oxidant. The catalytic activity of the as-synthesized OMS-2 materials was investigated. The physical and chemical properties of the OMS-2 materials are related to their activity in styrene oxidation. This particular study emphasizes the acid-base properties and the porous nature of these materials, and their role in styrene oxidation. Results of styrene oxidation reveal that acidity coupled with high porosity play a crucial role in these catalytic reactions. A desired acidity coupled with pore volume found in OMS-2 synthesized by reflux methods (OMS-2(R)) and high-temperature methods (OMS-2(HT)) produces materials with higher styrene conversion and styrene oxide selectivity when compared with OMS-2 synthesized by solvent free (OMS-2(S)), microwave (OMS-2(MW)), or hydrothermal methods (OMS-2(HY)). Transition metal doped OMS-2 catalysts show better selectivity of styrene oxide when compared to their undoped catalysts.