“…In addition, these architectural differences also produce a different curvature on the external surface of the corresponding monodendron. Interfacial curvature plays an important role in the formation of lyotropic and related thermotropic LC phases. − A detailed discussion of the geometrical aspects of lyotropic LC phases which supports the rationalization of the molecular design and the observed phase behavior of the monodendrons reported in this publication is available in the Supporting Information. − In contrast to the abundance and biological significance of lyotropic systems which exhibit cubic symmetry 15b,,, relatively few thermotropic systems which display similar phases − are known. 1 Schematic Representation of (a) the Self-Assembly of 12-ABG into a Supramolecular Cylindrical Dendrimer and the Subsequent Formation of the Columnar Hexagonal LC Supramolecular Assembly and (b) the Self-Assembly of 12G2-AG into a Spherical Supramolecular Dendrimer and the Subsequent Formation of the Cubic LC Supramolecular Assembly …”
The rational design, the synthesis via a new convergent
approach, and the characterization of four generations
of monodendrons based on the AB3 building block methyl
3,4,5-trishydroxybenzoate are described. The first
generation
monodendrons are crystalline. The second generation methyl
3,4,5-tris-3‘,4‘,5‘-[tris(n-dodecyloxy)benzyloxy]benzoate
(12Gn-AG-CH
3
, n = 2),
the corresponding benzyl alcohol
(12G2-AG-CH
2
OH) and
benzoic acid (12G2-AG), and
all higher generation monodendrons with n = 3 and 4
exhibit in addition to the crystal phase an isomorphous
cubic
liquid-crystalline (LC) phase of Pm3̄n space
group. The cubic unit cell parameters of 12G2-AG,
12G3-AG, and
12G4-AG are 68.3, 79.2, and 84.0 Å, respectively.
Computed electron density profiles demonstrated that in
the
cubic phase 12, 6, and 2 monodendrons of generation 2, 3 and 4 are
self-assembled in supramolecular dendrimers
resembling spherical micelles when they are centered at the corners and
the body-center and respectively highly
rounded tetrahedra when they are at 1/4 and
3/4 distance along one of the bisectors of each
face of the cubic cell.
This novel thermotropic LC phase is similar to that of the
lyotropic Pm3̄n phase of biological lipids.
These
supramolecular dendrimers contain a poly(benzyl ether) core
dispersed in an aliphatic matrix of nearly uniform
density which is made up of the melted terminal long alkyl chains of
the monodendrons. The size of the supramolecular
dendrimer and of the entire unit cell increases with generation number
as a result of a disproportionately large
increase in the number of AB3 repeat units relative to the
number of terminal alkyl chains in a monodendron which
decreases its average cone angle. The present experiments provide
the first examples of monodendrons which self-assemble into spherical supramolecular dendrimers forming a thermotropic
cubic LC phase.
“…In addition, these architectural differences also produce a different curvature on the external surface of the corresponding monodendron. Interfacial curvature plays an important role in the formation of lyotropic and related thermotropic LC phases. − A detailed discussion of the geometrical aspects of lyotropic LC phases which supports the rationalization of the molecular design and the observed phase behavior of the monodendrons reported in this publication is available in the Supporting Information. − In contrast to the abundance and biological significance of lyotropic systems which exhibit cubic symmetry 15b,,, relatively few thermotropic systems which display similar phases − are known. 1 Schematic Representation of (a) the Self-Assembly of 12-ABG into a Supramolecular Cylindrical Dendrimer and the Subsequent Formation of the Columnar Hexagonal LC Supramolecular Assembly and (b) the Self-Assembly of 12G2-AG into a Spherical Supramolecular Dendrimer and the Subsequent Formation of the Cubic LC Supramolecular Assembly …”
The rational design, the synthesis via a new convergent
approach, and the characterization of four generations
of monodendrons based on the AB3 building block methyl
3,4,5-trishydroxybenzoate are described. The first
generation
monodendrons are crystalline. The second generation methyl
3,4,5-tris-3‘,4‘,5‘-[tris(n-dodecyloxy)benzyloxy]benzoate
(12Gn-AG-CH
3
, n = 2),
the corresponding benzyl alcohol
(12G2-AG-CH
2
OH) and
benzoic acid (12G2-AG), and
all higher generation monodendrons with n = 3 and 4
exhibit in addition to the crystal phase an isomorphous
cubic
liquid-crystalline (LC) phase of Pm3̄n space
group. The cubic unit cell parameters of 12G2-AG,
12G3-AG, and
12G4-AG are 68.3, 79.2, and 84.0 Å, respectively.
Computed electron density profiles demonstrated that in
the
cubic phase 12, 6, and 2 monodendrons of generation 2, 3 and 4 are
self-assembled in supramolecular dendrimers
resembling spherical micelles when they are centered at the corners and
the body-center and respectively highly
rounded tetrahedra when they are at 1/4 and
3/4 distance along one of the bisectors of each
face of the cubic cell.
This novel thermotropic LC phase is similar to that of the
lyotropic Pm3̄n phase of biological lipids.
These
supramolecular dendrimers contain a poly(benzyl ether) core
dispersed in an aliphatic matrix of nearly uniform
density which is made up of the melted terminal long alkyl chains of
the monodendrons. The size of the supramolecular
dendrimer and of the entire unit cell increases with generation number
as a result of a disproportionately large
increase in the number of AB3 repeat units relative to the
number of terminal alkyl chains in a monodendron which
decreases its average cone angle. The present experiments provide
the first examples of monodendrons which self-assemble into spherical supramolecular dendrimers forming a thermotropic
cubic LC phase.
“…A representative texture of the cubic phase of the complex with 0.3 mol of LiCF 3 SO 3 is shown in Figure . The cubic phase is rare in thermotropic liquid crystalline molecules and has been observed in only a few systems such as biphenyl carboxylic acid derivatives with lateral nitro and cyano groups, , azoxybenzene derivatives, two chain diols, silver thiolates, and several biforked mesogens. , Although molecular models for the structure of the cubic phase have been proposed, , they are still a controversial subject. − 5 Representative optical polarized micrograph (100×) of the texture of the dark area with straight edges exhibited by the optical isotropic cubic phase of the complex of 12-4 with 0.3 mol of LiCF 3 SO 3 at 105 °C on the cooling scan.…”
The rod−coil polymer of ethyl
4-[4‘-oxy-4-biphenylylcarbonyloxy]-4‘-biphenylcarboxylate
with
poly(ethylene oxide) with a degree of polymerization of 12
(12-4) was observed to exhibit a microphase-separated lamellar structure with nanoscale dimension and to melt into a
layered smectic A mesophase.
The complexes of 12-4 with 0.05−0.8 mol of
LiCF3SO3 per ethylene oxide unit of a
molecule were also
prepared in order to investigate mesomorphic phase changes of
12-4 upon complexation. An abrupt
mesophase change was observed for the entire range of salt
concentrations. The complexes with 0.05−0.2 mol of LiCF3SO3 display an
enantiotropic smectic A phase as their highest temperature
mesophase.
From the complex with 0.2 mol of
LiCF3SO3, an enantiotropic cubic phase is
induced and a smectic A
phase disappears at the complex with 0.25 mol of
LiCF3SO3, which displays a cubic mesophase
only.
The complex with 0.3 mol of LiCF3SO3
also exhibits an enantiotropic cubic phase; however, it exhibits
a
cylindrical micellar mesophase as its highest temperature mesophase,
contrary to the complexes with up
to 0.25 mol of LiCF3SO3. The
complexes with 0.4−0.7 mol of LiCF3SO3
exhibit only a cylindrical micellar
mesophase, and the complex with higher salt concentration becomes
amorphous. These results,
characterized by a combination of differential scanning calorimetry,
optical polarized microscopy, and
X-ray scattering experiments, are discussed.
“…Thermotropic cubic mesophases have recently received keen interest because they are certainly liquid crystalline but optically isotropic. − The D phase is one of the cubic phases that has been found in two higher homologues ( n = 16 and 18) of 4‘- n -alkoxy-3‘-nitrobiphenyl-4-carboxylic acids (ANBC- n , Chart , where n is the carbon number of the alkoxy group) 4-6 and the corresponding 3‘-cyano analogues (ACBC-16 and -18). , From the X-ray studies, ,,,, several structural models have been proposed for the D phase; these models have proposed that micelles are arranged in harmony with a cubic symmetry to make the D phase optically isotropic, but are different in shape and packing of the micelles from each other. In these X-ray studies, the number of diffractions detected was too small, reflecting the fact that the unit cell of the D phase contains about 1000 molecules whose positions and directions are thermally fluctuating.…”
Infrared (IR) spectra were measured for three
4‘-n-alkoxy-3‘-nitrobiphenyl-4-carboxylic acid
homologues
[ANBC-n, where the carbon number of the alkoxy group
(n) is 14, 16, and 22] as a function of
temperature.
The phase sequences for ANBC-14, -16, and -22 are crystal
(C)−smectic C (SC)−smectic A
(SA)−“structured”
liquid (I1)−isotropic liquid (I2), crystal 2
(C2)−crystal 1 (C1)−SC−cubic
(D)−SA−I1−I2, and
C−SC−D−I1−I2, respectively. The molar fraction
of dimeric COOH groups was determined by using the
absorbances
of two bands at ∼1696 and ∼1734 cm-1,
which are assigned to the dimer and monomer carbonyl
stretching
vibrations, respectively. In the crystalline phase all the COOH
groups form hydrogen-bonded dimers, and
the dissociation of the dimers into monomers occurs above the melting
point. It is noted that about 85% of
ANBC molecules are still in the dimer state in the D phase, and the D
phase transforms into SA/I1 phase,
when the fraction of dimeric COOH is ∼0.74 mol %, for both
n = 16 and 22. The temperature
dependence
of the CH2 stretching bands for these two compounds
suggests that the alkoxy chains are rather linear-shaped
to hold some local order in the SC phase, and the
conformational structure becomes liquidlike above the
SC−D phase transition temperature. On the basis of
the present IR results, we deduce a possible structure
inside micelles in the D phase and the molecular state in the
I1 phase.
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