Photophysical characterizations of 2-(4-Biphenylyl)-5 phenyl-1,3,4- oxadiazole in restricted geometry

Langmuir and Langmuir-Blodgett (LB) films of nonamphiphilic 2-(4-Biphenylyl)-5 phenyl-1,3,4- oxadiazole (abbreviated as PBD) mixed with stearic acid (SA) as well as also with the inert polymer matrix poly(methyl methacrylate) (PMMA) have been studied. Surface pressure versus area per molecule (-A) isotherms studies suggest that PBD molecules very likely stand vertically on the air-water interface and this arrangement allows the PBD molecules to form stacks and remain sandwiched between SA/PMMA molecules. At lower surface pressure phase separation between PBD and matrix molecules occur resulting due to repulsive interaction. However at higher surface pressure PBD molecules form aggregates. The UV-Vis absorption and Steady state fluorescence spectroscopic studies of the mixed LB films of PBD reveal the nature of the aggregates. H-type aggregates predominates in the mixed LB films whereas I-type aggregates predominates in the PBD-PMMA spin coated films. The degree of deformation produced in the electronic levels are largely affected by the film thickness and the surface pressure of lifting.Comment: 15 pages, 6 figure

Monopolar and dipolar relaxation in spin ice Ho2_2Ti2_2O7_7

When degenerate states are separated by large energy barriers, the approach to thermal equilibrium can be slow enough that physical properties are defined by the thermalization process rather than the equilibrium. The exploration of thermalization pushes experimental boundaries and provides refreshing insights into atomic scale correlations and processes that impact steady state dynamics and prospects for realizing solid state quantum entanglement. We present a comprehensive study of magnetic relaxation in Ho$_2$Ti$_2$O$_7$ based on frequency-dependent susceptibility measurements and neutron diffraction studies of the real-time atomic-scale response to field quenches. Covering nearly ten decades in time scales, these experiments uncover two distinct relaxation processes that dominate in different temperature regimes. At low temperatures (0.6K<T<1K) magnetic relaxation is associated with monopole motion along the applied field direction through the spin-ice vacuum. The increase of the relaxation time upon cooling indicates reduced monopole conductivity driven by decreasing monopole concentration and mobility as in a semiconductor. At higher temperatures (1K<T<2K) magnetic relaxation is associated with the reorientation of monopolar bound states as the system approaches the single-spin tunneling regime. Spin fractionalization is thus directly exposed in the relaxation dynamics

AND/R: Advanced neutron diffractometer/reflectometer for investigation of thin films and multilayers for the life sciences

An elastic neutron scattering instrument, the advanced neutron diffractometer/reflectometer (AND/R), has recently been commissioned at the National Institute of Standards and Technology Center for Neutron Research. The AND/R is the centerpiece of the Cold Neutrons for Biology and Technology partnership, which is dedicated to the structural characterization of thin films and multilayers of biological interest. The instrument is capable of measuring both specular and nonspecular reflectivity, as well as crystalline or semicrystalline diffraction at wave-vector transfers up to approximately 2.20 Å(-1). A detailed description of this flexible instrument and its performance characteristics in various operating modes are given.D. J. M. is supported through a NSF NIRT grant Contract No. 0304062

Multilayer formation in an azacrown [18]N6 Langmuir film

A neutron reflectivity study of a deuterated azacrown [18]N6 at the air-water interface shows that it forms multilayers upon compression, with monolayers and trilayers being more stable than bilayers.Peer reviewedChemistr

Structural studies of Langmuir-Blodgett films of discogenic molecules

Thin films of disk-shaped molecules are expected to display anisotropic optical and transport properties, leading to applications in optical display or sensor technologies. A number of disk-shaped mesogens exhibiting columnar liquid crystalline phases have been characterized at the air-water interface and in ultrathin films on solid supports. At the air-water interface, there are two conformations generally adopted by monomeric systems: one in which the plane of the core is parallel, or face-on, to the interface, and another in which it is perpendicular, or edge-on. Mono- and multi-layer films of molecules of each type have been prepared and studied using combined techniques of atomic force microscopy and x-ray diffraction in reflection and in grazing incidence. In each case it was found that the presumed conformation at the air-water interface is preserved upon transfer to a solid support, and that in the case of the latter, columnar structures aligned along the direction of LB film deposition are formed and exhibit a quantifiable mosaic spread of orientations. Oligomeric systems exhibiting features common to both types of organization are also presented

Formation of a hexagonal columnar mesophase by N-acylated Poly(ethylenimine)

Both discotic molecules and certain linear polymers form columnar hexagonal mesophases. Whereas the dislike structure of the macrocycle 1 is dictated by the molecular shape, in polymer 2 it is presumably enforced by a helical folding of the polymer's main chain induced by the side chains (bottom right)

Acellular low-potassium dextran preserves pulmonary function after 48 hours of ischemia

Background. We previously have shown that extracellular preservation solutions provide superior pulmonary protection after 18 hours of cold ischemia at 4 degrees C in an isolated, whole-blood-perfused, rabbit lung model. We also reported that the addition of 20% whole blood to a low-potassium dextran solution (BLPD) conferred no discernible advantage over low-potassium dextran (LPD) alone in this same model. Our current study was aimed at documenting the importance of blood in buffering extracellular preservation solutions during 24 to 48 hours of hypothermic ischemia

Self-organization of discogenic molecules at the air-water interface

The behavior of several species of discogenic molecules at the air-water interface has been determined by measuring surface pressure $\nu s$, molecular area isotherms. Alkylthiotriphenylene derivatives do not appear to form monolayers at the interfaces, due to lack of amphiphilic character. Two truxene derivatives were found to display molecular areas on the order of 65 Å$^{2}$, most likely due to the molecules lying “edge-on” to the water surface. Hexacyclens generally lie with the core group parallel to the surface of the water with the tail groups extending away from the interface; a higher density phase may coorespond to conformational changes within the molecule