60 research outputs found
Functionalized Helical Building Blocks for Nanoelectronics
Molecular building blocks are designed
and created for the <i>cis</i>- and <i>trans</i>-dibrominated perylenediimides.
The syntheses are simple and provide these useful materials on the
gram scale. To demonstrate their synthetic versatility, these building
blocks were used to create new dimeric perylenediimide helixes. Two
of these helical dimers are twistacenes, and one is a helicene. Crucially,
each possesses regiochemically defined functionality that allows the
dimer helix to be elaborated into higher oligomers. It would be very
difficult to prepare these helical PDI building blocks regioselectively
without the methods described
Single-Walled Carbon Nanotubes: Mimics of Biological Ion Channels
Here
we report on the ion conductance through individual, small diameter
single-walled carbon nanotubes. We find that they are mimics of ion
channels found in natural systems. We explore the factors governing
the ion selectivity and permeation through single-walled carbon nanotubes
by considering an electrostatic mechanism built around a simplified
version of the Gouy–Chapman theory. We find that the single-walled
carbon nanotubes preferentially transported cations and that the cation
permeability is size-dependent. The ionic conductance increases as
the absolute hydration enthalpy decreases for monovalent cations with
similar solid-state radii, hydrated radii, and bulk mobility. Charge
screening experiments using either the addition of cationic or anionic
polymers, divalent metal cations, or changes in pH reveal the enormous
impact of the negatively charged carboxylates at the entrance of the
single-walled carbon nanotubes. These observations were modeled in
the low-to-medium concentration range (0.1–2.0 M) by an electrostatic
mechanism that mimics the behavior observed in many biological ion
channel-forming proteins. Moreover, multi-ion conduction in the high
concentration range (>2.0 M) further reinforces the similarity
between single-walled carbon nanotubes and protein ion channels
Synthesis and Self-Assembly of Photonic Materials from Nanocrystalline Titania Sheets
We describe the use of benzyl alcohols
in a solvothermal/alcoholysis
reaction to form nanocrystalline sheets of anatase titania. By tuning
the reaction conditions, we adjust the size of the nanosheets. The
type and density of benzyl groups that decorate the basal plane of
the titania sheets control the self-assembly into layered structures.
These layered materials can be grown from solid substrates to create
iridescent thin films that reflect specific wavelengths of visible
light
Altering the Polymorphic Accessibility of Polycyclic Aromatic Hydrocarbons with Fluorination
Substituting hydrogen
with fluorine is an extensively employed
strategy to improve the macroscopic properties of compounds for use
in fields as diverse as pharmaceutics and optoelectronics. The role
fluorine substitution plays on polymorphismthe ability of
a compound to adopt more than one crystal structurehas not
been previously studied. Yet, this understanding is important as different
polymorphs of the same compound can result in drastically different
bulk properties (e.g., solubility, absorptivity, and conductivity).
Strategies to either promote or suppress the crystallization of particular
polymorphs are thus desired. Here, we show that substituting hydrogen
with fluorine affects the polymorphic behavior of contorted hexabenzocoronene
(cHBC). A polycyclic aromatic hydrocarbon and molecular semiconductor,
cHBC exhibits two polymorphs (i.e., <i>P</i>2<sub>1</sub>/<i>c</i> crystal structure which we refer to as polymorph
I and a triclinic crystal structure which we refer to as polymorph
II) that are accessible through postdeposition processing of amorphous
films. While the same two polymorphs remain accessible in fluorinated
derivatives of cHBC, fluorination appears to favor the formation of
polymorph I, with progressively smaller energy barrier for transformation
from polymorph II to polymorph I with fluorination
Conductive Molecular Silicon
Bulk silicon, the bedrock of information technology,
consists of
the deceptively simple electronic structure of just Si–Si σ
bonds. Diamond has the same lattice structure as silicon, yet the
two materials have dramatically different electronic properties. Here
we report the specific synthesis and electrical characterization of
a class of molecules, oligosilanes, that contain strongly interacting
Si–Si σ bonds, the essential components of the bulk semiconductor.
We used the scanning tunneling microscope-based break-junction technique
to compare the single-molecule conductance of these oligosilanes to
those of alkanes. We found that the molecular conductance decreases
exponentially with increasing chain length with a decay constant β
= 0.27 ± 0.01 Å<sup>–1</sup>, comparable to that
of a conjugated chain of CC π bonds. This result demonstrates
the profound implications of σ conjugation for the conductivity
of silicon
Conductive Molecular Silicon
Bulk silicon, the bedrock of information technology,
consists of
the deceptively simple electronic structure of just Si–Si σ
bonds. Diamond has the same lattice structure as silicon, yet the
two materials have dramatically different electronic properties. Here
we report the specific synthesis and electrical characterization of
a class of molecules, oligosilanes, that contain strongly interacting
Si–Si σ bonds, the essential components of the bulk semiconductor.
We used the scanning tunneling microscope-based break-junction technique
to compare the single-molecule conductance of these oligosilanes to
those of alkanes. We found that the molecular conductance decreases
exponentially with increasing chain length with a decay constant β
= 0.27 ± 0.01 Å<sup>–1</sup>, comparable to that
of a conjugated chain of CC π bonds. This result demonstrates
the profound implications of σ conjugation for the conductivity
of silicon
Altering the Polymorphic Accessibility of Polycyclic Aromatic Hydrocarbons with Fluorination
Substituting hydrogen
with fluorine is an extensively employed
strategy to improve the macroscopic properties of compounds for use
in fields as diverse as pharmaceutics and optoelectronics. The role
fluorine substitution plays on polymorphismthe ability of
a compound to adopt more than one crystal structurehas not
been previously studied. Yet, this understanding is important as different
polymorphs of the same compound can result in drastically different
bulk properties (e.g., solubility, absorptivity, and conductivity).
Strategies to either promote or suppress the crystallization of particular
polymorphs are thus desired. Here, we show that substituting hydrogen
with fluorine affects the polymorphic behavior of contorted hexabenzocoronene
(cHBC). A polycyclic aromatic hydrocarbon and molecular semiconductor,
cHBC exhibits two polymorphs (i.e., <i>P</i>2<sub>1</sub>/<i>c</i> crystal structure which we refer to as polymorph
I and a triclinic crystal structure which we refer to as polymorph
II) that are accessible through postdeposition processing of amorphous
films. While the same two polymorphs remain accessible in fluorinated
derivatives of cHBC, fluorination appears to favor the formation of
polymorph I, with progressively smaller energy barrier for transformation
from polymorph II to polymorph I with fluorination
Molecular Materials for Nonaqueous Flow Batteries with a High Coulombic Efficiency and Stable Cycling
This
manuscript presents a working redox battery in organic media
that possesses remarkable cycling stability. The redox molecules have
a solubility over 1 mol electrons/liter, and a cell with 0.4 M electron
concentration is demonstrated with steady performance >450 cycles
(>74 days). Such a concentration is among the highest values reported
in redox flow batteries with organic electrolytes. The average Coulombic
efficiency of this cell during cycling is 99.868%. The stability of
the cell approaches the level necessary for a long lifetime nonaqueous
redox flow battery. For the membrane, we employ a low cost size exclusion
cellulose membrane. With this membrane, we couple the preparation
of nanoscale macromolecular electrolytes to successfully avoid active
material crossover. We show that this cellulose-based membrane can
support high voltages in excess of 3 V and extreme temperatures (−20
to 110 °C). These extremes in temperature and voltage are not
possible with aqueous systems. Most importantly, the nanoscale macromolecular
platforms we present here for our electrolytes can be readily tuned
through derivatization to realize the promise of organic redox flow
batteries
Superatom Fusion and the Nature of Quantum Confinement
Quantum
confinement endows colloidal semiconducting nanoparticles
with many fascinating and useful properties, yet a critical limitation
has been the lack of atomic precision in their size and shape. We
demonstrate the emergence of quantum confined behavior for the first
time in atomically defined Co<sub>6</sub>Se<sub>8</sub>(PEt<sub>3</sub>)<sub>6</sub> superatoms by dimerizing [Co<sub>6</sub>Se<sub>8</sub>] units through direct fusion. To accomplish this dimerization, we
install a reactive carbene on the [Co<sub>6</sub>Se<sub>8</sub>] core
to create a latent fusion site. Then we transform the reactive carbene
intermediate into a material with an expanded core, [Co<sub>12</sub>Se<sub>16</sub>], that exhibits electronic and optical properties
distinct from the parent monomer. The chemical transformation presented
herein allows for precise synthetic control over the ligands and size
of these clusters. We show by cyclic voltammetry, infrared spectroscopy,
single crystal X-ray diffraction, and density functional theory calculations
that the resulting fused [Co<sub>12</sub>Se<sub>16</sub>] material
exhibits strong electronic coupling and electron delocalization. We
observe a bandgap reduction upon expanding the cluster core, suggesting
that we have isolated a new intermediate in route to extended solids.
These results are further corroborated with electronic structure calculations
of a monomer, fused dimer, trimer, and tetramer species. These reactions
will allow for the synthesis of extended highly delocalized wires,
sheets, and cages
Spectroscopic Study of Anisotropic Excitons in Single Crystal Hexacene
The linear optical response of hexacene
single crystals over a
spectral range of 1.3–1.9 eV was studied using polarization-resolved
reflectance spectroscopy at cryogenic temperatures. We observe strong
polarization anisotropy for all optical transitions. Pronounced deviations
from the single-molecule, solution-phase spectra are present, with
a measured Davydov splitting of 180 meV, indicating strong intermolecular
coupling. The energies and oscillator strengths of the relevant optical
transitions and polarization-dependent absorption coefficients are
extracted from quantitative analysis of the data
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