1,022 research outputs found
Synthesis, Properties, and Solid-State Structures of a Series of 6,13-Dicyanoheteropentacene Analogues: Towards New Liquid Crystalline Materials
The focus of this thesis is the synthesis of novel heterocyclic pentacene analogs and the investigation of their self-organization for the development of new materials for organic electronics. The thesis consists of two interrelated projects: the first being development of an improved synthesis of a series of liquid crystalline dicyanotetraoxapentacenes (DCTOPs) while the second entails the exploratory synthesis of several novel dicyanoheteropentacene analogues and a preliminary investigation of their photophysical properties and solid-state structures. Both of these projects centre around the use of nucleophilic aromatic substitution reactions on tetrafluoroterephthalonitrile.
Soluble, tetrakis(bis(alkoxy)phenyl)-substituted DCTOPs were originally synthesised via a short synthesis complicated by a tedious purification required in the last step. Despite this, derivatives bearing long alkyl chains were prepared which displayed liquid crystalline properties in addition to aggregation-induced emission. Building upon this success, but with the goal of achieving DCTOPs in an efficient synthetic manner for this thesis, changes were made which eliminated the troublesome fourfold Suzuki coupling by changing the order of reactions, which in turn required a protection-deprotection sequence. Purification in the new synthesis was greatly simplified and the target tetraaryl-DCTOPs were accessed in good overall yields and purities. The synthesis and solid state structures of these DCTOPs are discussed in Chapter 2.
Building on the methods developed in Chapter 2, several novel pentacene analogues containing combinations of nitrogen, oxygen, and sulfur atoms installed within the pentacene core were also synthesised. These compounds were prepared in good yields, and preliminary photophysical studies show that all the compounds displayed luminescence in solution and the solid state. It was also shown that replacement of O with N leads to a red shift in absorption and emission spectra. The X-ray crystal structures show that several of these compounds exhibit Ïâstacking in the solid state, which is an important design element for applications in organic electronics. The synthesis, photophysical properties, and solid-state organization of these novel 6,13-dicyanoheteropentacene analogues are discussed in Chapter 3
Unequal arm space-borne gravitational wave detectors
Unlike ground-based interferometric gravitational wave detectors, large
space-based systems will not be rigid structures. When the end-stations of the
laser interferometer are freely flying spacecraft, the armlengths will change
due to variations in the spacecraft positions along their orbital trajectories,
so the precise equality of the arms that is required in a laboratory
interferometer to cancel laser phase noise is not possible. However, using a
method discovered by Tinto and Armstrong, a signal can be constructed in which
laser phase noise exactly cancels out, even in an unequal arm interferometer.
We examine the case where the ratio of the armlengths is a variable parameter,
and compute the averaged gravitational wave transfer function as a function of
that parameter. Example sensitivity curve calculations are presented for the
expected design parameters of the proposed LISA interferometer, comparing it to
a similar instrument with one arm shortened by a factor of 100, showing how the
ratio of the armlengths will affect the overall sensitivity of the instrument.Comment: 14 pages, 7 figures, REVTeX
On the Critical Behaviour of Heat Conducting Sphere out of Hydrostatic Equilibrium
We comment further on the behaviour of a heat conducting fluid when a
characteristic parameter of the system approaches a critical value.Comment: 4 pages, emTex (LaTex 2.09), submitted to Classical and Quantum
Gravity (Comments and Addenda
Copper(II)- and gold(III)-mediated cyclization of a thiourea to a substituted 2-aminobenzothiazole
Benzothiazole derivatives are a class of privileged molecules due to their biological activity and pharmaceutical applications. One route to these molecules is via intramolecular cyclization of thioureas to form substituted 2-aminobenzothiazoles, but this often requires harsh conditions or employs expensive metal catalysts. Herein, the copper(II)- and gold(III)-mediated cyclizations of thioureas to substituted 2-aminobenzothiazoles are reported. The single-crystal X-ray structures of the thiourea N-(3-methoxyphenyl)-N\u27- (pyridin-2-yl)thiourea, C13H13N3OS, and the intermediate metal complexes aquabis[5-methoxy-N-(pyridin-2-yl-ÎșN)-1,3-benzothiazol-2-amine-ÎșN3]copper(II) dinitrate, [Cu(C13H11N3OS)2(H2O)](NO3)2, and bis{2-[(5-methoxy-1,3-benzothiazol- 2-yl)amino]pyridin-1-ium} dichloridogold(I) chloride monohydrate, (C13H12N3OS)2[AuCl2]Clâ
H2O, are reported. The copper complex exhibits a distorted trigonalâbipyramidal geometry, with direct metal-to-benzothiazoleligand coordination, while the gold complex is a salt containing the protonated uncoordinated benzothiazole, and offers evidence that metal reduction (in this case, AuIII to AuI) is required for the cyclization to proceed. As such, this study provides further mechanistic insight into the role of the metal cations in these transformations
Low Frequency Gravitational Waves from White Dwarf MACHO Binaries
The possibility that Galactic halo MACHOs are white dwarfs has recently
attracted much attention. Using the known properties of white dwarf binaries in
the Galactic disk as a model, we estimate the possible contribution of halo
white dwarf binaries to the low-frequency (10^{-5} Hz} < f < 10^{-1}Hz)
gravitational wave background. Assuming the fraction of white dwarfs in
binaries is the same in the halo as in the disk, we find the confusion
background from halo white dwarf binaries could be five times stronger than the
expected contribution from Galactic disk binaries, dominating the response of
the proposed space based interferometer LISA. Low-frequency gravitational wave
observations will be the key to discovering the nature of the dark MACHO binary
population.Comment: 9 pages, 1 figure, AASTe
Using Binary Star Observations to Bound the Mass of the Graviton
Interacting white dwarf binary star systems, including helium cataclysmic variable (HeCV) systems, are expected to be strong sources of gravitational radiation, and should be detectable by proposed space-based laser interferometer gravitational wave observatories such as LISA. Several HeCV star systems are presently known and can be studied optically, which will allow electromagnetic and gravitational wave observations to be correlated. Comparisons of the phases of a gravitational wave signal and the orbital light curve from an interacting binary white dwarf star system can be used to bound the mass of the graviton. Observations of typical HeCV systems by LISA could potentially yield an upper bound on the inverse mass of the graviton as strong as h/mg=λg\u3e1Ă1015 km (mg\u3c1Ă10-24 eV), more than two orders of magnitude better than present solar system derived bounds
Semiclassical Stability of the Extreme Reissner-Nordstrom Black Hole
The stress-energy tensor of a free quantized scalar field is calculated in
the extreme Reissner-Nordstr\"{o}m black hole spacetime in the zero temperature
vacuum state. The stress-energy appears to be regular on the event horizon,
contrary to the suggestion provided by two-dimensional calculations. An
analytic calculation on the event horizon for a thermal state shows that if the
temperature is nonzero then the stress-energy diverges strongly there.Comment: 10 pages, REVTeX, 4 figures in separate uuencoded compressed fil
Astrophysical Bounds on Global Strings
Global topological defects produce nonzero stress-energy throughout
spacetime, and as a result can have observable gravitational influence on
surrounding matter. Gravitational effects of global strings are used to place
bounds on their cosmic abundance. The minimum separation between global strings
is estimated by considering the defects' contribution to the cosmological
energy density. More rigorous constraints on the abundance of global strings
are constructed by examining the tidal forces such defects will have on
observable astrophysical systems. The small number of observed tidally
disrupted systems indicates there can be very few of these objects in the
observable universe.Comment: 14 pages, REVTe
Null Geodesics in the Alcubierre Warp Drive Spacetime: The View from the Bridge
The null geodesic equations in the Alcubierre warp-drive spacetime are numerically integrated to determine the angular deflection and redshift of photons which propagate through the distortion of the `warp-drive\u27 bubble to reach an observer at the origin of the warp effect. We find that for a starship with an effective warp speed exceeding the speed of light, stars in the forward hemisphere will appear closer to the direction of motion than they would to an observer at rest. This aberration is qualitatively similar to that caused by special relativity. Behind the starship, a conical region forms from within which no signal can reach the starship, an effective `horizon\u27. Conversely, there is also a horizon-like structure in a conical region in front of the starship, into which the starship cannot send a signal. These causal structures are somewhat analogous to the Mach cones associated with supersonic fluid flow
Gamma-Ray Bursts and Quantum Cosmic Censorship
Gamma-ray bursts are believed to result from the coalescence of binary
neutron stars. However, the standard proposals for conversion of the
gravitational energy to thermal energy have difficulties. We show that if the
merger of the two neutron stars results in a naked singularity, instead of a
black hole, the ensuing quantum particle creation can provide the requisite
thermal energy in a straightforward way. The back-reaction of the created
particles can avoid the formation of the naked singularity predicted by the
classical theory. Hence cosmic censorship holds in the quantum theory, even if
it were to be violated in classical general relativity.Comment: Latex File, 6 pages. This essay received the third award from the
Gravity Research Foundation for the year 199
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