Effects of surface morphology on the anchoring and electrooptical dynamics of confined nanoscale liquid crystalline films

The orientation and dynamics of two 40-nm thick films of 4-n-pentyl-4'-cyanobiphenyl (5CB), a nematic liquid crystal, have been studied using step-scan Fourier transform infrared spectroscopy (FTIR). The films are confined in nanocavities bounded by an interdigitated electrode array (IDA) patterned on a zinc selenide (ZnSe) substrate. The effects of the ZnSe surface morphology (specifically, two variations of nanometer-scale corrugations obtained by mechanical polishing) on the initial ordering and reorientation dynamics of the electric-field-induced Freedericksz transition are presented here. The interaction of the 5CB with ZnSe surfaces bearing a spicular corrugation induces a homeotropic (surface normal) alignment of the film confined in the cavity. Alternately, when ZnSe is polished to generate fine grooves along the surface, a planar alignment is promoted in the liquid crystalline film. Time-resolved FTIR studies that enable the direct measurement of the rate constants for the electric-field-induced orientation and thermal relaxation reveal that the dynamic transitions of the two film structures are significantly different. These measurements quantitatively demonstrate the strong effects of surface morphology on the anchoring, order, and dynamics of liquid crystalline thin films.     

Synthesis and properties of waterborne self‐crosslinkable sulfo‐urethane silanol dispersions

A novel type of crosslinkable waterborne polyurethane ionomer was prepared by the acetone process. Two new types of sulfonated diols compatible with this process were synthesized from dimethyl 5‐sodium sulfo isophthalate using a one‐ or two‐stage method. Isocyanate‐terminated polyurethane oligomers were prepared from the sulfonated diols with various combinations of diols and diisocyanates and subsequently reacted with amino silane derivatives. Stable, low‐volatile organic chemical, waterborne dispersions of the sulfo‐urethane silanol polymers spontaneously crosslink upon drying without extra additives or processing steps. Despite the lack of organic coalescing solvents, the dispersions have minimum film‐forming temperatures below 10 °C, regardless of glass‐transition temperature. Tensile strengths up to 6000 psi with elongations between 300 and 600% were obtained for the crosslinked films. The hard‐segment content of the films can be controlled to produce films with a Sward–Rocker hardness value up to 42. Through silane end‐group modification, the crosslinking density of the films can also be modified to produce polyurethanes with a wide range of physical properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3037–3045, 2002     

Bases, filtrations and module decompositions of free Lie algebras

We use the technique known as elimination to devise some new bases of the free Lie algebra which (like classical Hall bases) consist of Lie products of left normed basic Lie monomials. Our bases yield direct decompositions of the homogeneous components of the free Lie algebra with direct summands that are particularly easy to describe: they are tensor products of metabelian Lie powers. They also give rise to new filtrations and decompositions of free Lie algebras as modules for groups of graded algebra automorphisms. In particular, we obtain some new decompositions for free Lie algebras and free restricted Lie algebras over fields of positive characteristic.     

The Structure,Characterization, and Magnetic Properties of Ca4–xNixIrO6 (x = 0.25, 0.5)

Ca_4–xNi_xIrO₆ (x = 0.25, 0.5) crystallizes with trigonal (rhombohedral) symmetry in the space group R3¯c, Z = 6, for Ca_3.75Ni_0.25IrO₆ a = 9.3013(1) Å, c = 11.1554(1) Å; for Ca_3.5Ni_0.5IrO₆ a = 9.2723(1) Å, c = 11.0825(1) Å. Ca_3.75Ni_0.25IrO₆ and Ca_3.5Ni_0.5IrO₆ are isotypic to compounds of the Sr₄PtO₆ structure type. The structure of Ca_3.75Ni_0.25IrO₆ has been solved by means of single crystal X-ray diffraction data analysis with the reliability factors of R = 0.019 and R_w = 0.022. Also, both structures have been determined by Rietveld refinement of powder X-ray diffraction data. The structure consists of chains of alternating face-sharing IrO₆ octahedra and (Ca/Ni)O₆ trigonal prisms. The chains are separated by the calcium cations which are in a distorted square antiprismatic coordination. Magnetic measurements revealed that both Ca_3.75Ni_0.25IrO₆ and Ca_3.5Ni_0.5IrO₆ follow Curie-Weiss behavior at high temperatures. Ca_3.75Ni_0.25IrO₆ undergoes a single antiferromagnetic transition at T_N = 5 K whereas Ca_3.5Ni_0.5IrO₆ undergoes two antiferromagnetic transitions at T_N1 = 13 K and T_N2 = 4 K.     

CEPA calculations on open-shell molecules. III. Potential curves for the six lowest excited states of He2 in the vicinity of their equilibrium distances

CEPA-PNO and PNO-CI calculations have been performed for the potential energy curves of the He ₂ ⁺ ground state and the six lowest excited states of He₂ in the range of 1.4 a₀ ≤R ≤ 3.5 a₀. The calculated equilibrium distances as well as the spectroscopic constants are in very good agreement with molecular constants as derived experimentally from the rotation-vibration spectrum of He₂ by Ginter, except for thec ³∑ _g ⁺ state. This latter discrepancy is probably due to an “obligatory” hump in thec ³∑ _g ⁺ state occurring at 3.5 a₀ which cannot be properly treated in our calculation. The relative energetic positions of the six lowest states and their ionization energies are reproduced by our calculations with an accuracy of 0–400 cm⁻¹. Extrapolation of our results to infinite basis sets leads to estimates of the dissociation energies of He₂ excited states which cannot be measured spectroscopically because of the humps in all these states.     

Unprecedented catalytic asymmetric reduction of N-H imines

[Reaction: see text] Addition of lithium bis(trimethylsilyl)amide to perfluorinated ketones 1a-j affords (E)-N-TMS-ketimines 2a-j that are reduced in situ to afford racemic perfluoromethylated amine hydrochloride salts 3a-j in 54-97% yields. Solvolysis of the N-Si bond in MeOH leads to formation of bench-stable, isolable N-H imine Z/E isomer mixtures along with a methanol adduct. Enantioselective reduction of these three-component mixtures provides the first catalytic asymmetric synthesis of trifluoromethylated amines in 72-95% yields and 75-98% ee.     

A Soft Grip: Magnesium Complexes with a Phosphine‐Modified Phosphonium Diylidic Lewis Base

Simple strategies to obtain magnesium complexes with the soft chelating diylidic ligand [Ph₂PCHPPh₂(fluorenylidene)]^? (dppmflu^?) were developed to evaluate the influence of the hard acid (cation) and soft base (anion) mismatch on the stability and reactivity of the formed derivatives. Deprotonation of the precursor Ph₂PCH₂PPh₂(flu) (dppmfluH) by an alkylmagnesium derivative or magnesium amide provided access to [{Mg(dppmflu)(μ‐nBu)}₂], [Mg(dppmflu){N(SiMe₃)₂}], and [{Mg(dppmflu)(μ‐Me)}₂], which were used as starting materials for further investigations. The reaction of [{Mg(dppmflu)(μ‐nBu)}₂] with PhSiH₃ in the presence of THF allowed isolation of the magnesium hydride complex [{Mg(dppmflu)(μ‐H)(thf)}₂] without a stabilizing nitrogen donor ligand. Prolonged heating enforced ligand redistribution and [{Mg(dppmflu)(μ‐H)(thf)}₂] was converted to [Mg(dppmflu)₂] and MgH₂. The homoleptic derivative [Mg(dppmflu)₂], in which the magnesium center is in a very soft ligand environment, can open a THF molecule by frustrated Lewis pair reactivity to give [{Mg(dppmflu)(μ‐OC₄H₈dppmflu)}₂].     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—1. THE PYRIMIDINES: THYMINE, URACIL, N, N-DIMETHYL DERIVATIVES AND THYMIDINE

Abstract— Low-temperature (and some room temperature) absorption and emission, fluorescence and phosphorescence, data including quantum yields and lifetimes have been obtained from the title pyrimidine bases as a function of the nature of the solvent environment. Modest vibrational resolution has been observed for the first time in the absorption spectra, particularly for thymine and uracil. The excitation spectra also show structure. The quantum yields of fluorescence (φ_F) and phosphorescence are independent of the excitation wavelength. Thymine, thymidine and uracil have profoundly different photophysical properties in polar-aprotic vs polar-protic solvents. The N, N-dimethyl substitution of thymine and uracil produces photophysical changes comparable to the solvent change for the unsubsti-tuted bases. The species involved in the emission processes is the keto (lactam) form. It is probable that ^1,3(n,π*) state(s) has(have) changed order relative to a lowest ¹(π,π*) state as a consequence of both the solvent change and N, N-dimethyl substitution. The lowest triplet state is assigned as ³(n π*). We propose that an important factor contributing to the previously reported excitation wavelength dependence of φ_F and φ_T1 (φ_isc) for nucleic-acid components is the equilibrium coexistence of H-bonded and non-H-bonded forms each having different photophysical properties. Consideration is given of the impact of the significantly different photophysical properties of nucleic-acid bases as a function of the nature of the solvent upon the photochemical properties.