13 research outputs found
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
space group symmetry and has the remarkable
feature of exhibiting hybrid donor layers with a kagome topology which
sustain metallic conductivity. We report a detailed study of the structural
evolution of the system as a function of temperature and pressure.
This rhombohedral phase is maintained on cooling down to 220 K or
up to 0.7 GPa pressure, beyond which a symmetry-breaking transition
to a triclinic <i>P</i>1Ģ
phase drives a metal to
insulator transition. Band structures calculated from the structural
data lead to a clear description of the effects of temperature and
pressure on the structural and electronic properties of this system.
Linear energy dispersion is calculated at the zero-gap Fermi level
where valence and conduction bands touch for the rhombohedral phase.
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] thus exhibits a regular (right circular) Dirac-cone
like that of graphene at the Fermi level, which has not been reported
previously in a molecular solid. The Dirac-cone is robust over the
stability region of the rhombohedral phase, and may result in exotic
electronic transport and optical properties
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
space group symmetry and has the remarkable
feature of exhibiting hybrid donor layers with a kagome topology which
sustain metallic conductivity. We report a detailed study of the structural
evolution of the system as a function of temperature and pressure.
This rhombohedral phase is maintained on cooling down to 220 K or
up to 0.7 GPa pressure, beyond which a symmetry-breaking transition
to a triclinic <i>P</i>1Ģ
phase drives a metal to
insulator transition. Band structures calculated from the structural
data lead to a clear description of the effects of temperature and
pressure on the structural and electronic properties of this system.
Linear energy dispersion is calculated at the zero-gap Fermi level
where valence and conduction bands touch for the rhombohedral phase.
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] thus exhibits a regular (right circular) Dirac-cone
like that of graphene at the Fermi level, which has not been reported
previously in a molecular solid. The Dirac-cone is robust over the
stability region of the rhombohedral phase, and may result in exotic
electronic transport and optical properties
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
space group symmetry and has the remarkable
feature of exhibiting hybrid donor layers with a kagome topology which
sustain metallic conductivity. We report a detailed study of the structural
evolution of the system as a function of temperature and pressure.
This rhombohedral phase is maintained on cooling down to 220 K or
up to 0.7 GPa pressure, beyond which a symmetry-breaking transition
to a triclinic <i>P</i>1Ģ
phase drives a metal to
insulator transition. Band structures calculated from the structural
data lead to a clear description of the effects of temperature and
pressure on the structural and electronic properties of this system.
Linear energy dispersion is calculated at the zero-gap Fermi level
where valence and conduction bands touch for the rhombohedral phase.
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] thus exhibits a regular (right circular) Dirac-cone
like that of graphene at the Fermi level, which has not been reported
previously in a molecular solid. The Dirac-cone is robust over the
stability region of the rhombohedral phase, and may result in exotic
electronic transport and optical properties
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
space group symmetry and has the remarkable
feature of exhibiting hybrid donor layers with a kagome topology which
sustain metallic conductivity. We report a detailed study of the structural
evolution of the system as a function of temperature and pressure.
This rhombohedral phase is maintained on cooling down to 220 K or
up to 0.7 GPa pressure, beyond which a symmetry-breaking transition
to a triclinic <i>P</i>1Ģ
phase drives a metal to
insulator transition. Band structures calculated from the structural
data lead to a clear description of the effects of temperature and
pressure on the structural and electronic properties of this system.
Linear energy dispersion is calculated at the zero-gap Fermi level
where valence and conduction bands touch for the rhombohedral phase.
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] thus exhibits a regular (right circular) Dirac-cone
like that of graphene at the Fermi level, which has not been reported
previously in a molecular solid. The Dirac-cone is robust over the
stability region of the rhombohedral phase, and may result in exotic
electronic transport and optical properties
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
space group symmetry and has the remarkable
feature of exhibiting hybrid donor layers with a kagome topology which
sustain metallic conductivity. We report a detailed study of the structural
evolution of the system as a function of temperature and pressure.
This rhombohedral phase is maintained on cooling down to 220 K or
up to 0.7 GPa pressure, beyond which a symmetry-breaking transition
to a triclinic <i>P</i>1Ģ
phase drives a metal to
insulator transition. Band structures calculated from the structural
data lead to a clear description of the effects of temperature and
pressure on the structural and electronic properties of this system.
Linear energy dispersion is calculated at the zero-gap Fermi level
where valence and conduction bands touch for the rhombohedral phase.
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] thus exhibits a regular (right circular) Dirac-cone
like that of graphene at the Fermi level, which has not been reported
previously in a molecular solid. The Dirac-cone is robust over the
stability region of the rhombohedral phase, and may result in exotic
electronic transport and optical properties
Robust Dirac-Cone Band Structure in the Molecular Kagome Compound (EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>]
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] is a molecular solid
with <i>R</i>3Ģ
space group symmetry and has the remarkable
feature of exhibiting hybrid donor layers with a kagome topology which
sustain metallic conductivity. We report a detailed study of the structural
evolution of the system as a function of temperature and pressure.
This rhombohedral phase is maintained on cooling down to 220 K or
up to 0.7 GPa pressure, beyond which a symmetry-breaking transition
to a triclinic <i>P</i>1Ģ
phase drives a metal to
insulator transition. Band structures calculated from the structural
data lead to a clear description of the effects of temperature and
pressure on the structural and electronic properties of this system.
Linear energy dispersion is calculated at the zero-gap Fermi level
where valence and conduction bands touch for the rhombohedral phase.
(EDT-TTF-CONH<sub>2</sub>)<sub>6</sub>[Re<sub>6</sub>Se<sub>8</sub>(CN)<sub>6</sub>] thus exhibits a regular (right circular) Dirac-cone
like that of graphene at the Fermi level, which has not been reported
previously in a molecular solid. The Dirac-cone is robust over the
stability region of the rhombohedral phase, and may result in exotic
electronic transport and optical properties
Reversible Control of Crystalline Rotors by Squeezing Their Hydrogen Bond Cloud Across a Halogen Bond-Mediated Phase Transition
We report on a crystalline rotor
that undergoes a reversible phase transition at 145 K. Variable-temperature
X-ray and <sup>1</sup>H spinālattice relaxation experiments,
and calculations of rotational barriers, provide a description (i)
of the way in which the rotatorsā dynamics changes back and
forth at the onset of the phase transition and (ii) of the mechanism
responsible for the abrupt switching of the crystalline rotors from
a very low-energy 4-fold degenerate equilibrium state, in which the
rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy
state associated with a slower motion (2.3 GHz at 145 K). Our results
provide evidence that the reversible change observed in the rotational
barriers at the transition is due to a cooperative modulation of the
CāH<sub>rotator</sub>Ā·Ā·Ā·I<sub>stator</sub> hydrogen
bond cloud across a CāI<sub>stator</sub>Ā·Ā·Ā·I<sub>stator</sub>āC halogen bond-mediated phase transition. In
addition, we report evidence for second-harmonic generation from this
material, thereby confirming with a second example the benefit of
using polarized light to probe the torsional degree of freedom of
chiral helix blades, as well as symmetry and dimensionality of large
collections of chiral rotors in the solid state
Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations
The
rod-like molecule bisĀ((4-(4-pyridyl)Āethynyl)Ābicyclo[2.2.2]Āoct-1-yl)Ābuta-1,3-diyne, <b>1</b>, contains two 1,4-bisĀ(ethynyl)Ābicyclo[2.2.2]Āoctane
(BCO) chiral rotators linked by a diyne fragment and self-assembles
in a one-dimensional, monoclinic <i>C</i>2/<i>c</i> centrosymmetric structure where two equilibrium positions with large
occupancy imbalance (88% versus 12%) are identified on a single rotor
site. Combining variable-temperature (70ā300 K) proton spinālattice
relaxation, <sup>1</sup>H <i>T</i><sub>1</sub><sup>ā1</sup>, at two different <sup>1</sup>H Larmor frequencies (55 and 210 MHz)
and DFT calculations of rotational barriers, we were able to assign
two types of Brownian rotators with different activation energies,
1.85 and 6.1 kcal mol<sup>ā1</sup>, to the two <sup>1</sup>H spinālattice relaxation processes on the single rotor site.
On the basis of DFT calculations, the low-energy process has been
assigned to adjacent rotors in a well-correlated synchronous motion,
whereas the high-energy process is the manifestation of an abrupt
change in their kinematics once two blades of adjacent rotors are
seen to rub together. Although crystals of <b>1</b> should be
second harmonic inactive, a large second-order optical response is
recorded when the electric field oscillates in a direction parallel
to the unique rotor axle director. We conclude that conformational
mutations by torsional interconversion of the three blades of the
BCO units break space-inversion symmetry in sequences of mutamers
in dynamic equilibrium in the crystal in domains at a mesoscopic scale
comparable with the wavelength of light used. A control experiment
was performed with a crystalline film of a similar tetrayne molecule,
1,4-bisĀ(3-((trimethylsilyl)Āethynyl)ĀbicycloĀ[1.1.1]Āpent-1-yl)Ābuta-1,3-diyne,
whose bicyclopentane units can rotate but are achiral and produce
no second-order optical response
Design and Evaluation of a Crystalline Hybrid of Molecular Conductors and Molecular Rotors
Combining recent concepts from the fields of molecular
conductivity
and molecular machinery we set out to design a crystalline molecular
conductor that also possesses a molecular rotor. We report on the
structures, electronic and physical properties, and dynamics of two
solids with a common 1,4-bisĀ(carboxyethynyl)Ābicyclo[2.2.2]Āoctane (BABCO)
functional rotor. One, [<i>n</i>Bu<sub>4</sub>N<sup>+</sup>]<sub>2</sub>[BABCO]Ā[BABCO<sup>ā</sup>]<sub>2</sub>, is a
colorless insulator where the dicarboxylic acid cocrystallizes with
two of its monoanionic conjugated bases. The other is self-assembled
by electrocrystallization in the form of black, shiny needles, with
highly conducting molecular slabs of (EDT-TTF-CONH<sub>2</sub>)<sub>2</sub><sup>+</sup> (EDT-TTF = ethylenedithiotetrathiafulvalene)
and anionic [BABCO<sup>ā</sup>] rotors. Using variable-temperature
(5ā300 K) proton spinālattice relaxation, <sup>1</sup>H T<sub>1</sub><sup>ā1</sup>, we were able to assign two types
of Brownian rotators in [<i>n</i>Bu<sub>4</sub>N<sup>+</sup>]<sub>2</sub>[BABCO]Ā[BABCO<sup>ā</sup>]<sub>2</sub>. We showed
that neutral BABCO groups have a rotational frequency of 120 GHz at
300 K with a rotational barrier of 2.03 kcal mol<sup>ā1</sup>. Rotors on the BABCO<sup>ā</sup> sites experience stochastic
32 GHz jumps at the same temperature over a rotational barrier of
2.72 kcal mol<sup>ā1</sup>. In contrast, the BABCO<sup>ā</sup> rotors within the highly conducting crystals of (EDT-TTF-CONH<sub>2</sub>)<sub>2</sub><sup>+</sup>[BABCO<sup>ā</sup>] are essentially
ābrakedā at room temperature. Notably, these crystals
possess a conductivity of 5 S cm<sup>ā1</sup> at 1 bar, which
increases rapidly with pressure up to 50 S cm<sup>ā1</sup> at
11.5 kbar. Two regimes with different activation energies <i>E</i><sub>a</sub> for the resistivity (180 K above 50 and 400
K below) are observed at ambient pressure; a metallic state is stabilized
at ca. 8 kbar, and an insulating ground state remains below 50 K at
all pressures. We discuss two likely channels by which the motion
of the rotors might become slowed down in the highly conducting solid.
One is defined as a low-velocity viscous regime inherent to a noncovalent,
physical coupling induced by the cooperativity between five C<sub>sp3</sub>āHĀ·Ā·Ā·O hydrogen bonds engaging any
rotor and five BABCO units in its environment. The rotational barrier
calculated with the effect of this set of hydrogen bonds amounts to
7.3 kcal mol<sup>ā1</sup>. Another is quantum dissipation,
a phenomenon addressing the difference of dynamics of the rotors in
the two solids with different electrical properties, by which the
large number of degrees of freedom of the low dimensional electron
gas may serve as a bath for the dissipation of the energy of the rotor
motion, the two systems being coupled by the Coulomb interaction between
the charges of the rotors (local moments and induced dipoles) and
the charges of the carriers
Design and Evaluation of a Crystalline Hybrid of Molecular Conductors and Molecular Rotors
Combining recent concepts from the fields of molecular
conductivity
and molecular machinery we set out to design a crystalline molecular
conductor that also possesses a molecular rotor. We report on the
structures, electronic and physical properties, and dynamics of two
solids with a common 1,4-bisĀ(carboxyethynyl)Ābicyclo[2.2.2]Āoctane (BABCO)
functional rotor. One, [<i>n</i>Bu<sub>4</sub>N<sup>+</sup>]<sub>2</sub>[BABCO]Ā[BABCO<sup>ā</sup>]<sub>2</sub>, is a
colorless insulator where the dicarboxylic acid cocrystallizes with
two of its monoanionic conjugated bases. The other is self-assembled
by electrocrystallization in the form of black, shiny needles, with
highly conducting molecular slabs of (EDT-TTF-CONH<sub>2</sub>)<sub>2</sub><sup>+</sup> (EDT-TTF = ethylenedithiotetrathiafulvalene)
and anionic [BABCO<sup>ā</sup>] rotors. Using variable-temperature
(5ā300 K) proton spinālattice relaxation, <sup>1</sup>H T<sub>1</sub><sup>ā1</sup>, we were able to assign two types
of Brownian rotators in [<i>n</i>Bu<sub>4</sub>N<sup>+</sup>]<sub>2</sub>[BABCO]Ā[BABCO<sup>ā</sup>]<sub>2</sub>. We showed
that neutral BABCO groups have a rotational frequency of 120 GHz at
300 K with a rotational barrier of 2.03 kcal mol<sup>ā1</sup>. Rotors on the BABCO<sup>ā</sup> sites experience stochastic
32 GHz jumps at the same temperature over a rotational barrier of
2.72 kcal mol<sup>ā1</sup>. In contrast, the BABCO<sup>ā</sup> rotors within the highly conducting crystals of (EDT-TTF-CONH<sub>2</sub>)<sub>2</sub><sup>+</sup>[BABCO<sup>ā</sup>] are essentially
ābrakedā at room temperature. Notably, these crystals
possess a conductivity of 5 S cm<sup>ā1</sup> at 1 bar, which
increases rapidly with pressure up to 50 S cm<sup>ā1</sup> at
11.5 kbar. Two regimes with different activation energies <i>E</i><sub>a</sub> for the resistivity (180 K above 50 and 400
K below) are observed at ambient pressure; a metallic state is stabilized
at ca. 8 kbar, and an insulating ground state remains below 50 K at
all pressures. We discuss two likely channels by which the motion
of the rotors might become slowed down in the highly conducting solid.
One is defined as a low-velocity viscous regime inherent to a noncovalent,
physical coupling induced by the cooperativity between five C<sub>sp3</sub>āHĀ·Ā·Ā·O hydrogen bonds engaging any
rotor and five BABCO units in its environment. The rotational barrier
calculated with the effect of this set of hydrogen bonds amounts to
7.3 kcal mol<sup>ā1</sup>. Another is quantum dissipation,
a phenomenon addressing the difference of dynamics of the rotors in
the two solids with different electrical properties, by which the
large number of degrees of freedom of the low dimensional electron
gas may serve as a bath for the dissipation of the energy of the rotor
motion, the two systems being coupled by the Coulomb interaction between
the charges of the rotors (local moments and induced dipoles) and
the charges of the carriers