596 research outputs found
Synthesis of 6-thiosubstituted pentacenes and study of an unexpected photorearrangement of 6-phenylthiopentacene dimer
Two new monosubstituted pentacene derivatives, 6-phenylthiopentacene, 26, and 6-acetylthiopentacene, 27, have been prepared using a dehydrogenative aromatization and elimination reaction that utilizes 1,8-diazabicycloundec-7-ene (DBU) as reagent. Although many disubstituted pentacenes have been synthesized and characterized, monosubstituted pentacenes have received much less attention. While studying the photodegradation of 26 under ambient light and air conditions, an unexpected transformation to 6,13-bis(phenylthio)pentacene, 32, was observed. The transformation requires short wavelength (254 nm) UV irradiation. Through a combined experimental and computational study, we have identified this transformation as a multi-step intramolecular process initiated by the photoexcitation of the endo anti dimer of 26. The photoexcited dimer undergoes an intramolecular acid-base reaction which involves two proton transfers and nucleophilic substitution to form the rearranged photodimer which upon dissociation yields bisadduct 32. Using 254 nm light and a quartz reaction vessel, bisadduct 32 has been isolated in 20% yield from monoadduct 26
A decade of dental education: Faculty of Dentistry
Published on the occasion of the 10th anniversary of the faculty.Incudes bibliographical references.published_or_final_versionConservative dentistry: achievements and perspectives 47Congratulations Wang, Gungwu Wang, Gungwu viiForeword Wei, Stephen H.Y. Wei, Stephen H.Y. ixAcknowledgements Wei, Stephen H.Y. Wei, Stephen H.Y. xiThe Prince Philip Dental Hospital 1Departments and units: a decade of activities and progressStudents' life in the faculty 151Alumni 155Faculty of Dentistry - dental graduates 1985-1992 157The next decade and beyond - some prospects in dentistry 175Brief history of the Faculty of Dentistry 1Administration and governance 4Other services 7Children's dentistry and orthodontics: achievemnets and perspective 13Oral and maxillofacial survey: achievements and perspectives 69Periodontology and public health: achievements and perspectives 95Prosthetic dentistry: achievements and perspectives 113Dental technology unit 125Dental materials science unit: achievements and perspectives 131Oral Biology Unit: achievements and perspectives 139Optimizing adhesion with mineralized dental tissues Gwinnett, A. John Gwinnett, A.John 177Conservative approach to esthetic dentistry Suzuki, M. Mike Suzuki, M. Mike 183The wonderful world of glass ionomer cements Mount, Graham J. Mount, Graham J. 189Biocompatibility of dental materials Smith, Dennis C. Smith, Dennis C. 197Esthetic implant restorations Lewis, Steven Lewis, Steven 203Chemicals in the treatment of periodontal diseases Gjermo, Per Gjermo, Per 21
Structural and Physical Properties of CaFe4As3 Single Crystals
We report the synthesis, and structural and physical properties of CaFe4As3
single crystals. Needle-like single crystals of CaFe4As3 were grown out of Sn
flux and the compound adopts an orthorhombic structure as determined by X-ray
diffraction measurements. Electrical, magnetic, and thermal properties indicate
that the system undergoes two successive phase transitions occurring at TN1 ~
90 K and TN2 ~ 26 K. At TN1, electrical resistivities (\rho(b) and \rho(ac))
are enhanced while magnetic susceptibilities (\chi(b) and \chi(ac)) are reduced
in both directions parallel and perpendicular to the b-axis, consistent with
the scenario of antiferromagnetic spin-density-wave formation. At TN2, specific
heat reveals a slope change, and \chi(ac) decreases sharply but \chi(b) has a
clear jump before it decreases again with decreasing temperature. Remarkably,
both \rho(b) and \rho(ac) decrease sharply with thermal hysteresis, indicating
the first-order nature of the phase transition at TN2. At low temperatures,
\rho(b) and \rho(ac) can be described by {\rho} = {\rho}0 + AT^\alpha ({\rho}0,
A, and {\alpha} are constants). Interestingly, these constants vary with
applied magnetic field. The ground state of CaFe4As3 is discussed.Comment: 15 pages, 8 figures, Submitted to Physical Review
Surface and Bulk Structural Properties of Single Crystalline Sr3Ru2O7
We report temperature and thermal-cycling dependence of surface and bulk
structures of double-layered perovskite Sr3Ru2O7 single crystals. The surface
and bulk structures were investigated using low-energy electron diffraction
(LEED) and single-crystal X-ray diffraction (XRD) techniques, respectively.
Single-crystal XRD data is in good agreement with previous reports for the bulk
structure with RuO6 octahedral rotation, which increases with decreasing
temperature (~ 6.7(6)degrees at 300 K and ~ 8.1(2) degrees at 90 K). LEED
results reveal that the octahedra at the surface are much more distorted with a
higher rotation angle (~ 12 degrees between 300 and 80 K) and a slight tilt
((4.5\pm2.5) degrees at 300 K and (2.5\pm1.7) degrees at 80 K). While XRD data
confirms temperature dependence of the unit cell height/width ratio (i.e.
lattice parameter c divided by the average of parameters a and b) found in a
prior neutron powder diffraction investigation, both bulk and surface
structures display little change with thermal cycles between 300 and 80 K.Comment: 25 pages, 5 figures, 5 tables, to appear in Physical Review
Quantum states and intertwining phases in kagome materials
In solid materials, nontrivial topological states, electron correlations, and
magnetism are central ingredients for realizing quantum properties, including
unconventional superconductivity, charge and spin density waves, and quantum
spin liquids. The Kagome lattice, made up of connected triangles and hexagons,
can host these three ingredients simultaneously and has proven to be a fertile
platform for studying diverse quantum phenomena including those stemming from
the interplay of these ingredients. In this review, we introduce the
fundamental properties of the Kagome lattice as well as discuss the complex
observed phenomena seen in several emergent material systems such as the
intertwining of charge order and superconductivity in some Kagome metals,
modulation of magnetism and topology in some Kagome magnets, and symmetry
breaking with Mott physics in the breathing Kagome insulators. We also
highlight many open questions in the field as well as future research
directions of Kagome systems
Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe
We present magnetization, specific heat, resistivity, and Hall effect
measurements on the cubic B20 phase of MnGe and CoGe and compare to
measurements of isostructural FeGe and electronic structure calculations. In
MnGe, we observe a transition to a magnetic state at K as identified
by a sharp peak in the ac magnetic susceptibility, as well as second phase
transition at lower temperature that becomes apparent only at finite magnetic
field. We discover two phase transitions in the specific heat at temperatures
much below the Curie temperature one of which we associate with changes to the
magnetic structure. A magnetic field reduces the temperature of this transition
which corresponds closely to the sharp peak observed in the ac susceptibility
at fields above 5 kOe. The second of these transitions is not affected by the
application of field and has no signature in the magnetic properties or our
crystal structure parameters. Transport measurements indicate that MnGe is
metal with a negative magnetoresistance similar to that seen in isostructural
FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about
0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe
is shown to be a low carrier density metal with a very small, nearly
temperature independent diamagnetic susceptibility.Comment: 16 pages 23 figure
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