“…It is important to note that the ratio of 1 and 9 was not significantly altered by repeated fractional precipitation from ethanol, which is the purification method described by Xie et al. Furthermore, the Π− A isotherms we obtained using our (impure) sample of 1 were inconsistent with those previously reported. − 1 …”
Section: Introductioncontrasting
confidence: 68%
“…We observed no signs of a first-order lying-down to standing-up transition as reported by Wei et al − Although a definitive explanation for the difference in behaviors of our sample and theirs is at present unknown, it is highly likely that the crown ether liquid crystal samples prepared and studied by Wei et al were contaminated with the same crown ether liquid crystal impurity 9 that we encountered in our initial syntheses.…”
Section: Discussionsupporting
confidence: 46%
“…These features were explained as arising from a first-order transition from a lying-down configuration, with the central core and carbonyl groups in contact with the water subphase but the chains extending away from the subphase (as in Figure b), to a condensed phase with the alkyl chains and carbonyl groups directed away from the interface (Figure c). They also reported that monolayer Langmuir−Blodgett films of 1 could be used as alignment layers for liquid crystals. − If the films were deposited with low spreading pressure, they obtained homogeneous alignment of the liquid crystal, while with high spreading pressure (presumably the conformation in Figure c) homeotropic alignment was obtained. Wei et al inferred that the differing tail conformations in the Langmuir film were transferred to the solid substrate and provided a basis for an altered interaction with the liquid crystal molecules.…”
Section: Introductionmentioning
confidence: 99%
“…Wei et al − studied 1 (as prepared by Xie et al ) at the air−water water interface and reported pressure−area (Π− A ) isotherms with a stepped structure, with an initial rise from Π = 0 to 27 mN/m at approximately 240 Å 2 /molecule, a horizontal plateau, and then a second sharp rise below 76 Å 2 , leading to collapse at 65 mN/m. These features were explained as arising from a first-order transition from a lying-down configuration, with the central core and carbonyl groups in contact with the water subphase but the chains extending away from the subphase (as in Figure b), to a condensed phase with the alkyl chains and carbonyl groups directed away from the interface (Figure c).…”
We have used X-ray reflectivity and grazing-incidence X-ray diffraction to study amphiphilic crown ethers 1 and 2 at the air-water interface. A new synthetic route was employed, which substantially reduced the level of impurities. Unlike previous studies of 1, we saw no evidence of a first-order lying-down to standingup transition of the alkyl tails. We did observe in both compounds a gradual reorientation of the tails away from the interface as the film was compressed, followed by an irreversible collapse to a multilayer structure. The monolayer films have a fluid-like inplane structure, while the multilayer films are crystalline.
“…It is important to note that the ratio of 1 and 9 was not significantly altered by repeated fractional precipitation from ethanol, which is the purification method described by Xie et al. Furthermore, the Π− A isotherms we obtained using our (impure) sample of 1 were inconsistent with those previously reported. − 1 …”
Section: Introductioncontrasting
confidence: 68%
“…We observed no signs of a first-order lying-down to standing-up transition as reported by Wei et al − Although a definitive explanation for the difference in behaviors of our sample and theirs is at present unknown, it is highly likely that the crown ether liquid crystal samples prepared and studied by Wei et al were contaminated with the same crown ether liquid crystal impurity 9 that we encountered in our initial syntheses.…”
Section: Discussionsupporting
confidence: 46%
“…These features were explained as arising from a first-order transition from a lying-down configuration, with the central core and carbonyl groups in contact with the water subphase but the chains extending away from the subphase (as in Figure b), to a condensed phase with the alkyl chains and carbonyl groups directed away from the interface (Figure c). They also reported that monolayer Langmuir−Blodgett films of 1 could be used as alignment layers for liquid crystals. − If the films were deposited with low spreading pressure, they obtained homogeneous alignment of the liquid crystal, while with high spreading pressure (presumably the conformation in Figure c) homeotropic alignment was obtained. Wei et al inferred that the differing tail conformations in the Langmuir film were transferred to the solid substrate and provided a basis for an altered interaction with the liquid crystal molecules.…”
Section: Introductionmentioning
confidence: 99%
“…Wei et al − studied 1 (as prepared by Xie et al ) at the air−water water interface and reported pressure−area (Π− A ) isotherms with a stepped structure, with an initial rise from Π = 0 to 27 mN/m at approximately 240 Å 2 /molecule, a horizontal plateau, and then a second sharp rise below 76 Å 2 , leading to collapse at 65 mN/m. These features were explained as arising from a first-order transition from a lying-down configuration, with the central core and carbonyl groups in contact with the water subphase but the chains extending away from the subphase (as in Figure b), to a condensed phase with the alkyl chains and carbonyl groups directed away from the interface (Figure c).…”
We have used X-ray reflectivity and grazing-incidence X-ray diffraction to study amphiphilic crown ethers 1 and 2 at the air-water interface. A new synthetic route was employed, which substantially reduced the level of impurities. Unlike previous studies of 1, we saw no evidence of a first-order lying-down to standingup transition of the alkyl tails. We did observe in both compounds a gradual reorientation of the tails away from the interface as the film was compressed, followed by an irreversible collapse to a multilayer structure. The monolayer films have a fluid-like inplane structure, while the multilayer films are crystalline.
“…Little focus has been directed at the intralayer packing of alkyl tails when a significant cross-sectional mismatch occurs between the headgroup of the molecule and/or when multiple tails are attached to the same focal point. Bulky crown heads attached to hydrophobic tails have been used in several cases. , A phase transformation has been observed at higher pressures and was associated with a structural transition from planar to folded conformation of side chains . Changing the orientation of crown heads from edge-on to face-on at the air−water interface has been reported for multi-chain molecules.…”
Grazing incident X-ray diffraction and X-ray reflectivity have been performed on Langmuir monolayers of
low generation monodendrons containing a crown-ether polar group, azobenzene spacer, and varying number
of peripheral alkyl chains of 1, 2, 4, and 8. We observe that the cross-sectional mismatch between the bulky
polar head and the alkyl tails has a profound effect on the local ordering of the alkyl tails. It is found that the
alkyl chains in a single-tail molecule are significantly tilted away from the surface normal. The tilt is eliminated
in molecules with two or more alkyl chains where the cross-sectional mismatch is in favor of the peripheral
tails. The molecule with one tail possesses a supercell orthorhombic packing caused by structural
nonequivalency on the neighboring tails. The two- and four-tail molecules form a mixed structure best described
by a quasi-hexagonal unit cell, and the eight-tail molecule forms a more stable hexagonal unit cell. Peripheral
tails for these molecules are in standing-off orientation. We suggest that the steric constraints cause lower
correlations and a staggered packing structure of monolayers from the eight-tail molecule. We suggest that
branching alkyl tails off the same phenyl ring and the presence of the phenyl rings in the vicinity of the
branching are limiting factors on the chain packing at the air−water interface in monodendrons with multiple
peripheral tails. We conclude that a significant portion of the molecules is submerged in the water subphase
and possesses a “kink” shape.
The synthesis and characterization of novel Schiff base liquid crystalline crown ethers prepared from the intermediates 4-(4'-aLkoxylbiphenyl4-~arbony~)bed&hyde, cis-and tmnr-4,4'-diaminodibenzo-18crown-6 are described. The structure of these compounds have been well characterized by elemental analysis, IR, 'H N M R and MS spectroscopy. The liquidcrystalline behavior of these compounds was also inverstigated by differential scanning calorimetq (DSC), polarizing optical microscopy (POM) with a heating stage and polarimetric analysis.
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