Phase transition behaviour of amphiphilic supermolecules possessing a semiperfluorinated alkyl chain

Article abstract of DOI:10.1080/15421400701732423

Recently we have found that a novel amphiphilic compound, 1H, 1H, 2H, 2H-heptadecafluoro-1-decyl 3, 4, 5- tris[6-(4'-cyanobiphenyl-4-yloxy)hexyloxy] benzoate, exhibited the phase sequence of isotropic liquid -smectic A-bicontinuous cubic-crystal. In this

Full text of DOI:10.1080/15421400701732423

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Phase Transition
Behaviour of Amphiphilic
Supermolecules Possessing a
Semiperfluorinated Alkyl Chain
Akihisa Yamaguchi a & Atsushi Yoshizawa a
a Department of Frontier Materials Chemistry ,
Faculty of Science and Technology, Hirosaki
University , Bunkyo-cho, Hirosaki, Japan
Published online: 22 Sep 2010.
To cite this article: Akihisa Yamaguchi & Atsushi Yoshizawa (2007) Phase Transition
Behaviour of Amphiphilic Supermolecules Possessing a Semiperfluorinated Alkyl
Chain, Molecular Crystals and Liquid Crystals, 479:1, 181/[1219]-189/[1227], DOI:
10.1080/15421400701732423
To link to this article: http://dx.doi.org/10.1080/15421400701732423
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Mol. Cryst. Liq. Cryst., Vol. 479, pp. 181=[1219]–189=[1227], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701732423
Phase Transition Behaviour of Amphiphilic
Supermolecules Possessing a Semiperfluorinated
Alkyl Chain
Akihisa Yamaguchi
Atsushi Yoshizawa
Department of Frontier Materials Chemistry, Faculty of Science
and Technology, Hirosaki University, Bunkyo-cho, Hirosaki, Japan
Recently we have found that a novel amphiphilic compound, 1H, 1H, 2H, 2H-
heptadecafluoro-1-decyl 3, 4, 5- tris[6-(40-cyanobiphenyl-4-yloxy)hexyloxy]benzoate,
exhibited the phase sequence of isotropic liquid -smectic A-bicontinuous cubic-
crystal. In this study, we have prepared compounds possessing biphenyl groups
or alkyl chains instead of the cyanobiphenyl groups of the amphiphilic compound
and investigated their liquid-crystalline properties using optical microscopy and
differential scanning calorimetry. The compound possessing the biphenyl groups
was found to exhibit a columnar phase, however, the compound possessing the alkyl
chains did not show any liquid-crystalline phase. We discuss the structure-property
relationship in the amphiphilic oligomers.
Keywords: cubic phase; liquid crystals; oligomers; phase transition
1. INTRODUCTION
Supermolecular assemblies with well-defined morphologies (e.g.,
layers, interpenetrating networks, columns, and spheroids) are funda-
mental components for structure formation in biological systems, as
well as for application to production of novel functional materials.
For those reasons, investigation of the driving forces of this self
assembly process is an important contemporary research topic [1,2].
This work was partially supported by Aomori Prefecture Collaboration of Regional
Entities for the Advancement of Technological Excellence, JST and A Grant for Priority
Research Designated by the President of Hirosaki University.
Address correspondence to Atsushi Yoshizawa, Department of Frontier Materials
Chemistry, Faculty of Science and Technology, Hirosaki University, 3, Bunkyo-cho,
Hirosaki, 036-8561, Japan. E-mail: ayoshiza@cc.hirosaki-u.ac.jp
181=[1219]

182=[1220]
A. Yamaguchi and A. Yoshizawa
In particular, cubic liquid-crystalline phases, which represent three-
dimensional ordered supermolecular arrangements such as inter-
penetrating networks and spheroids, have recently attracted much
attention [3,4]. Cubic mesophases are common in lyotropic systems,
such as surfactant-solvent systems. The mesophase morphologies of
these amphiphilic systems are governed mainly by the volume fraction
of the two incompatible segments combined in such molecules. The
phase sequence of lamellar (smectic, SmA)-bicontinuous cubic
(Cubv)-hexagonal columnar (Colh)-micellar cubic (CubI), upon increas-
ing the interface curvature between hydrophilic regions and lipophilic
regions [1,5]. In contrast to lyotropic cubic systems, few thermotropic
compounds with cubic phase are known. They have been found for
nitro-substituted and cyano-substituted biphenyl carboxylic acids
[6,7], dibenzoylhydrazides [8], polycatenar compounds [9,10], metal
complexes [11], amphiphilic polyhydroxy compounds [12,13] and
carbohydrate derivatives [14]. In most cases, bicontinuous cubic meso-
phases have been found for these cubic phases. In tetracatenar meso-
gens which comprise an extended, rigid core and four terminal chains,
the phase sequence of SmC-CubV-Col as increasing terminal chains is
apparent. This phase sequence is explained in terms of the curvature
in the interface between the aromatic core and the terminal chains,
and the formation of aggregates of molecules [10e,11f]. In the case of
polyhydroxy derivatives, the same sequence of SmA-CubV-Colh-CubI
as that reported for lyotropic systems was obtained. The mesophase
morphologies are dominated by the interface curvature between
hydrophilic regions and lipophilic regions [2c,d].
Pre-organization [15] or a ‘bottom-up’ approach, is an important
concept in the design of mesogenic molecules [2a–b,16,17]. We have
reported novel pre-organized systems, e.g., U-shaped molecules [18],
binaphthyl derivatives [19] and k-shaped molecules [20]; the pre-
organized compounds were found to induce unusual ordering in the
supramolecular liquid-crystalline phase. Therefore, the introduction
of amphiphilic properties into the oligomeric molecules is expected to
realize a novel type of phase transitions.
Recently, we prepared amphiphilic liquid-crystalline oligomers that
comprise cyanobiphenyl mesogenic moieties and a semiperfluorinated
alkyl chain (Fig. 1) [21]. The amphiphilic liquid-crystalline oligomers
1–3 were found to show a different phase sequence of SmC-Col-CubV
on increasing the cyanobiphenyl mesogenic moieties; the sequence of
Col-CubV is opposite that of the ordinary sequence. In this study, we
have prepared the related compounds possessing biphenyl groups or
alkyl chains instead of the cyanobiphenyl groups of compound 3 and
investigated the structure-property relationships.

Self-Assembly of Amphiphilic Oligomers
183=[1221]
FIGURE 1 Molecular structures and transition temperatures of the
amphiphilic oligomers 1–3.
2. EXPERIMENTAL
2.1. Preparation of Materials
The structures of the final products were elucidated using infrared
(IR) spectroscopy (FTS-30; Bio-Rad Laboratories, Inc.) and proton
nuclear magnetic resonance (1H NMR) spectroscopy (JNM-A400;
JEOL).
2.1.1. 1H, 1H, 2H, 2H-Heptadecafluoro-1-decyl 3, 4, 5-tris[6-
(biphenyl-4-yloxy)hexyloxy]benzoate, 4
4-Hydroxybiphenyl (0.68 g, 4 mmol) and 1, 6-dibromohexane (1.64 g,
6 mmol) were dissolved in cyclohexanone (4 ml). Potassium carbonate
(0.83 g, 6 mmol) was added and the resulting mixture stirred at 70ꢀC
for 8 h. The reaction mixture was filtered and the solvent removed
by evaporation under reduced pressure. The residue was purified
using column chromatography on silica gel using a dichloromethane-
hexane (1:1) mixture as the eluent. Recrystallization from hexane gave
4-(6-bromohexyloxy)biphenyl: yield: 0.84 g (63%).
4-(6-Bromohexyloxy)biphenyl (0.80 g, 2.4 mmol) and ethyl 3, 4,
5-trihydroxybenzoate (0.14 g, 0.7 mmol) were dissolved in cyclohexa-
none (4.5 ml). Then, K2CO3 (0.50 g, 3.6 mmol) and KI (0.06 g,
0.36 mmol) were added and the resulting mixture was stirred at
140ꢀC for 25 h. The reaction mixture was filtered and the solvent
removed by evaporation under reduced pressure. The residue was
purified using column chromatography on silica gel using dichloro-
methane as the eluent and recrystallization from a hexane=chloroform
roform (4=1) mixture, giving ethyl 3, 4, 5-tris[6-(biphenyl-4-yloxy)-
hexyloxy]benzoate: yield: 0.42 g (63%).
Ethyl 3, 4, 5-tris[6-(biphenyl-4-yloxy)hexyloxy]benzoate (0.38 g,
0.4 mmol) obtained was refluxed in ethanol=THF (10 ml=10 ml) and
KOH (0.16 g, 2.9 mmol) for 2 h. The solution was cooled to room

184=[1222]
A. Yamaguchi and A. Yoshizawa
temperature and acidified using aq. HCl. Water (20 ml) was added to
the mixture and the aqueous phase was extracted with dichloro-
methane (2 ꢁ 20 ml). The organic extracts were combined, dried over
Na2SO4, filtered and evaporated, giving 3, 4, 5-tris[6-(biphenyl-4-
yloxy)hexyloxy]benzoic acid: yield: 0.36 g (99%).
A mixture of 3, 4, 5-tris[6-(biphenyl-4-yloxy)hexyloxy]benzoic acid
(0.3 g, 0.32 mmol) and thionyl chloride (2 ml) was stirred at room tem-
perature for 4 h. Thionyl chloride was then removed by evaporation
under reduced pressure, leaving an off-white solid. The product was
dissolved 1, 4-dioxane (2 ml). DMAP (0.06 g, 0.5 mmol) and 1H, 1H,
2H, 2H-heptadecafluoro-1-decanol dissolved in 1, 4-dioxane (3 ml)
were added and the resulting mixture was stirred at 80ꢀC for 4 h.
The reaction mixture was filtered and the solvent removed by evapor-
ation under reduced pressure. The residue was purified using column
chromatography on silica gel using a dichloromethane-hexane (4:1)
mixture as the eluent and reprecipitated with chloroform=methanol
(1=10), giving the desired product: yield: 0.30 g (69%). 1H NMR
(400 MHz, CDCl3): d ¼ 7.55–7.45 (m, 15H, Ar-H), 7.40–7.36 (m, 6H,
Ar-H), 7.27 (s, 2H, Ar-H), 6.95–6.91 (m, 6H, Ar-H), 4.58 (t, J ¼ 6.4 Hz,
Hz, 2H, CH2OCO), 4.05–3.93 (m, 12H, OCH2), 2.65–2.51 (m, 2H,
CF2CH2), 1.87–1.74 (m, 12H, CH2), 1.56–1.50 (m, 12H, CH2); IR
(KBr): 2942, 2867, 1718, 1600, 1250, 1211, 819 cmꢂ1
.
2.1.2. 1H, 1H, 2H, 2H-Heptadecafluoro-1-decyl 3, 4,
5- tridodecyloxybenzoate, 5
1-Bromododecane (1.5 g, 6 mmol) and ethyl 3, 4, 5-trihydroxybenzoate
(0.4 g, 2 mmol) were dissolved in cyclohexanone (15 ml). Then, K2CO3
(0.83 g, 6 mmol) and KI (0.1 g, 0.6 mmol) were added and the resulting
mixture was stirred at 140ꢀC for 7 h. The reaction mixture was filtered
and the solvent removed by evaporation under reduced pressure. The
residue was purified using column chromatography on silica gel using
a toluene-ethyl acetate (29:1) mixture as the eluent, giving ethyl 3, 4,
5-tridodecyloxybenzoate: yield: 0.57 g (41%).
Ethyl 3, 4, 5-tridodecyloxybenzoate (0.5 g, 0.7 mmol) was refluxed in
ethanol (95%, 15 ml) and KOH (0.16 g, 2.9 mmol) for 2 h. The solution
was cooled to room temperature and acidified using aq. HCl. Water
(20 ml) was added to the mixture and the aqueous phase was extracted
with dichloromethane (3 ꢁ 20 ml). The organic extracts were com-
bined, dried over Na2SO4, filtered and evaporated. Recrystallization
from ethanol gave 3, 4, 5-tridodecyloxybenzoic acid: yield: 0.46 g
(96%).
A mixture of 3, 4, 5-tridodecyloxybenzoic acid (0.2 g, 0.3 mmol) and
thionyl chloride (2.5 ml) was stirred at room temperature for 4 h.

Self-Assembly of Amphiphilic Oligomers
185=[1223]
Thionyl chloride was then removed by evaporation under reduced
pressure, leaving an off-white solid. The product was dissolved 1,
4-dioxane (2 ml). DMAP (0.06 g, 0.5 mmol) and 1H, 1H, 2H, 2H-hepta-
decafluoro-1-decanol dissolved in 1, 4-dioxane (3 ml) were added and
the resulting mixture was stirred at 80ꢀC for 5 h. The reaction mixture
was filtered and the solvent removed by evaporation under reduced
pressure. The residue was purified using column chromatography on
silica gel using a dichloromethane-hexane (2:3) mixture as the eluent
and recrystallized from ethanol, giving the desired product: yield: 0.2 g
(60%). 1H NMR (400 MHz, CDCl3): d ¼ 7.27 (s, 2H, Ar-H), 4.58
(t, J ¼ 6.4 Hz, 2H, CH2OCO), 4.03–3.93 (m, 6H, OCH2), 2.65–2.51
(m, 2H, CF2CH2), 1.87–1.30 (m, 60H, CH2), 0.96 (t, 9H, CH3); IR
(KBr): 2942, 2862, 1721, 1604, 1248, 1211, 822 cmꢂ1
.
2.2. Liquid-Crystalline and Physical Properties
The initial phase assignments and corresponding transition tempera-
tures for the final products were determined by optical polarized light
microscopy using a polarizing microscope (Optiphot-pol; Nikon Corp.)
equipped with a hot stage and FP80 control processor (FP82; Mettler
Inst. Corp.). Temperatures and enthalpies of transition were investi-
gated using differential scanning calorimetry (DSC) with a calor-
imeter (DSC6200; Seiko Instruments Inc.). The materials were
studied at a scanning rate of 5ꢀC minꢂ1 after being encapsulated in
aluminum pans.
3. RESULTS AND DISCUSSION
Transition temperatures and transition enthalpies of novel com-
pounds 4 and 5 determined by optical microscopy and differential
scanning calorimetry (DSC) are listed in Table 1. Those of compound
3 are also shown in Table 1 [21].
Compound 3 possessing three cyanobiphenyl mesogenic moieties
showed the phase sequence of Iso-SmA-CubV-Cr [21]. However, com-
pound 4 possessing biphenyl groups instead of three cyanobiphenyl
groups of compound 3 showed the phase sequence of Iso-Col-X, but
not the SmA and CubV phases. Figure 2 shows the optical textures
of the Col and X phases. In the Col phase, a spherulitic texture typical
for a hexagonal columnar phase was observed (Fig. 2a). In the X
phase, a mosaic texture was observed (Fig. 2b). A large enthalpy
change (23.8 kJmolꢂ1) of the Col to X transition suggests that the X
phase has a highly ordered three dimensional structure. X-ray mea-
surements are necessary to clear phase structures of Col and X phases.

186=[1224]
A. Yamaguchi and A. Yoshizawa
TABLE 1 Transition Temperatures (ꢀC) and Transition Enthalpies
(kJ molꢂ1) (in Square Brackets) of Compounds 3–5
Compound
Transition temperatures (ꢀC) [DH=kJmolꢂ1
]
3
Iso 155.0 [5.4] SmA 135.9 [3.5] CubV 9.3 G
Iso 87.9 [2.7] Col 73.3 [23.8] Xb
Iso 20.1 [50.4] Cr
mp 83.2
mp 104.7
mp 39.0
4a
5
aRecrystallization was not observed on cooling to ꢂ30ꢀC by differential scanning
calorimetry at a scanning rate of 5ꢀC minꢂ1 bX: an unidentified higher ordered phase.
.
Appearance of the columnar phase of compound 4 can be explained in
terms of the interface curvature between the mesogenic groups and
the semiperfluorinated alkyl chain. Furthermore, compound 5 posses-
sing only the alkyl chains without rigid biphenyl cores showed no
mesogenic properties.
Let us discuss structural effects of the amphiphilic oligomers on
appearance of the CubV phase. Replacing the cyanobiphenyl groups
of compound 3 by the biphenyl groups or the alkyl chains was found
to vanish the SmA and CubV phases, suggesting the cyanobiphenyl
groups of compound 3 play an important role in the appearance of
the SmA and CubV phases. In our previous study, results of the XRD
study suggested that the SmA phase of compound 3 has a fluctuated
layer structure [21]. The competition of the steric effect attributable
FIGURE 2 Optical textures of a) the Col phase at 86.2ꢀC and b) the X phase at
72.2ꢀC for compound 4.

Self-Assembly of Amphiphilic Oligomers
187=[1225]
FIGURE 3 Molecular arrangements suggested for compound 3 in the SmA
phase.
to the pronounced taper shape and the segregation effect attributable
to the incompatibility between the fluorinated alkyl chains and the cya-
nobiphenyl groups of compound 3 might give rise to that fluctuation.
Figure 3 shows possible molecular arrangements for the SmA phase
of compound 3. A typical nematogen, 40-hexyloxybiphenyl-4-carboni-
trile (6OCB) is known to dimerize by antiparallel cyano-cyano dipole
interactions in the nematic phase. Furthermore, numerous examples
of the same phenomena are observed in cyanobiphenyl SmA materials
[22,23]. In the fluctuated layer structure, partial intercalation and
strong antiparallel dipole-dipole interactions between the cyanobiphe-
nyl groups of adjacent layers are considered to exist and stabilize the
fluctuated layer structure. Furthermore, the fluctuations in the SmA
phase might engender the formation of the CubV phase. Therefore,
compound 4 without strong dipole moments of the cyano groups could
show no SmA and CubV phases.
On the other hand, an undulatory deformation mode for a nonequili-
brium lamellar structure is known to exist as the unstable transient state
between the lamellar and the Cub (Gyroid with a space group Ia3d)
phases in some diblock copolymers [24]. Recently, fluctuations of lamellar
structure prior to a lamellar to gyroid transition in a nonionic surfactant
system were observed [25]. In compound 3, however, the fluctuated
lamellar structure in the SmA phase is thought to be stabilized by compe-
tition and cooperation of the steric effect attributable to the pronounced
taper shape, the segregation effect attributable to the incompatibility
between the fluorinated alkyl chains and the cyanobiphenyl groups,
and the dimerization effect attributable to strong dipole-dipole inter-
actions between the cyanobiphenyl groups, which can produce the
CubV phase.

188=[1226]
A. Yamaguchi and A. Yoshizawa
4. CONCLUSION
In summary, the structural modifications of amphiphilic liquid-
crystalline oligomer 3 were performed, and the structure-property
relationships indicate that: 1) formation of the columnar phases is domi-
nated by the interface curvature between the mesogenic groups and the
semiperfluorinated alkyl chain, and 2) strong dipole-dipole interactions
between the cyanobiphenyl groups stabilize the fluctuation of the layer
structure in the SmA, which can produce the CubV phase.
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