204 research outputs found
Experimental and molecular dynamics studies of anthraquinone dyes in a nematic liquid-crystal host: a rationale for observed alignment trends
Five anthraquinone dyes with bis(4-propylphenyl) substituents, connected via sulfide or amine linking groups at the 1,5-positions or directly at the 2,6-positions, have been studied as guests in the nematic liquid crystal host, E7. Polarized UV-visible absorption spectra of aligned samples were used to obtain experimental dichroic order parameters, which exhibit values in the range 0.51-0.74. Fully atomistic MD simulations of these guest-host systems were carried out, generally using default parameters but using new force constants derived here for the dyes containing flexible phenyl-sulfide and phenyl-amine linking groups. An analysis of the alignment of the dye molecules in these simulations provides calculated molecular order parameters, which are combined with calculated order parameters for the alignment of the transition dipole moments within the dyes, reported previously, to give calculated dichroic order parameters. The trend in the calculated dichroic order parameters between the dyes shows a good match with the trend in the experimental values, enabling the observed variation to be rationalised primarily by changes in the alignment of the calculated transition dipole moments within the dyes; the calculated molecular order parameters show a relatively small variation between the dyes. The results indicate that this computational approach may be used generally to rationalise trends in the alignment of guest molecules in liquid crystal hosts, suggesting that it may also be able to provide a predictive aid in the design of guest dyes
Hall magnetohydrodynamics of partially ionized plasmas
The Hall effect arises in a plasma when electrons are able to drift with the
magnetic field but ions cannot. In a fully-ionized plasma this occurs for
frequencies between the ion and electron cyclotron frequencies because of the
larger ion inertia. Typically this frequency range lies well above the
frequencies of interest (such as the dynamical frequency of the system under
consideration) and can be ignored. In a weakly-ionized medium, however, the
Hall effect arises through a different mechanism -- neutral collisions
preferentially decouple ions from the magnetic field. This typically occurs at
much lower frequencies and the Hall effect may play an important role in the
dynamics of weakly-ionised systems such as the Earth's ionosphere and
protoplanetary discs.
To clarify the relationship between these mechanisms we develop an
approximate single-fluid description of a partially ionized plasma that becomes
exact in the fully-ionized and weakly-ionized limits. Our treatment includes
the effects of ohmic, ambipolar, and Hall diffusion. We show that the Hall
effect is relevant to the dynamics of a partially ionized medium when the
dynamical frequency exceeds the ratio of ion to bulk mass density times the
ion-cyclotron frequency, i.e. the Hall frequency. The corresponding length
scale is inversely proportional to the ion to bulk mass density ratio as well
as to the ion-Hall beta parameter.Comment: 11 page, 1 figure, typos removed, numbers in tables revised; accepted
for publication in MNRA
The conductivity of dense molecular gas
We evaluate the conductivity tensor for molecular gas at densities ranging
from 10^4 to 10^15 cm^-3 for a variety of grain models. The Hall contribution
to the conductivity has generally been neglected in treatments of the dynamics
of molecular gas. We find that it is not important if only 0.1 micron grains
are considered, but for a Mathis-Rumpl-Nordsieck grain-size distribution (with
or without PAHs) it becomes important for densities between 10^7 and 10^11
cm^-3. If PAHs are included, this range is reduced to 10^9 -- 10^10 cm^-3.
The consequences for the magnetic field evolution and dynamics of dense
molecular gas are profound. To illustrate this, we consider the propagation of
Alfven waves under these conditions. A linear analysis yields a dispersion
relation valid for frequencies below the neutral collision frequencies of the
charged species. The dispersion relation shows that there is a pair of
circularly polarised modes with distinct propagation speeds and damping rates.
We note that the gravitational collapse of dense cloud cores may be
substantially modified by the Hall term.Comment: MNRAS accepted; 9 pp incl 8 figs, LaTeX, uses epsf.sty mn.st
Managing lupin Anthracnose
Anthracnose in lupins was first reported in commercial crops in Western Australia in September 1996. By October 1996, several thousand lupin breeding lines and wild types of 11 lupin species were sown in New Zealand for resistance screening. In 1997, resistance to anthracnose was confirmed in several breeding fines and commercial cultivars of narrow-leafed lupins (I. angustifolius), landraces of albus lupins (I. albus) and wild types of several other lupin species. Important information on critical seed infection levels and fungicide seed treatment has also been determined
Molecular Design Parameters of Anthraquinone Dyes for Guest-Host Liquid-Crystal Applications : Experimental and Computational Studies of Spectroscopy, Structure and Stability
A set of five anthraquinone dyes with bis(4-propylphenyl) substituent groups, connected via sulfide or amine linkages at the 1,5-positions or directly at the 2,6-positions, have been studied in solution by UV-vis spectroscopy and electrochemistry, allied with density functional theory calculations of structures, electronic transitions, and redox potentials. The visible transitions and redox potentials are shown to vary with the HOMO and LUMO energies, with the variation in both color and redox stability between the dyes being attributable principally to variations in the HOMOs located mainly on the substituents and outer anthraquinone rings. The calculated molecular structures and visible transition dipole moments are shown to vary subtly with substituent, giving variations in the molecular aspect ratios, minimum moment of inertia axes, and transition dipole moment vector orientations that can rationalize the alignment trends reported in the literature for such anthraquinone dyes in liquid crystal hosts, showing why 1,5-disulfide and 2,6-diphenyl substituents give better designs than 1,5-diamine substituents. The computational approaches reported here are shown to give good matches with experimental trends, indicating that they may be used more generally to aid the rational molecular design of dyes for applications as guests in liquid crystal hosts
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