Gelation of 1-alkoxy-4-(2-perfluoroalkyl)ethoxybenzenes in organic solvents

Article abstract of DOI:10.1080/15421400590957107

This paper describes preparation and physico-chemical properties of 1-alkoxy-4-(2-perfluoroalkyl)ethoxybenzenes (FmOOCn). For FmOOCn, 4-F(CF ₂)_mCH₂CH₂O-phenyl-O-(CH ₂)_nH, three homologues,

Full text of DOI:10.1080/15421400590957107

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Gelation of 1-Alkoxy-4-(2-
Perfluoroalkyl)Ethoxybenzenes
in Organic Solvents
Yuki Morita a , Takeyasu Tasaka a , Kosuke Kawabe
a , Hiroaki Okamoto a , Shunsuke Takenaka a
Hidetoshi Kita a
&
a Department of Advanced Materials Science and
Engineering, Faculty of Engineering , Yamaguchi
University , Tokiwadai, Ube, Yamaguchi, Japan
Published online: 31 Aug 2006.
To cite this article: Yuki Morita , Takeyasu Tasaka , Kosuke Kawabe , Hiroaki
Okamoto , Shunsuke Takenaka & Hidetoshi Kita (2005) Gelation of 1-Alkoxy-4-(2-
Perfluoroalkyl)Ethoxybenzenes in Organic Solvents, Molecular Crystals and Liquid
Crystals, 435:1, 153/[813]-162/[822], DOI: 10.1080/15421400590957107
To link to this article: http://dx.doi.org/10.1080/15421400590957107
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Mol. Cryst. Liq. Cryst., Vol. 435, pp. 153=[813]–162=[822], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590957107
Gelation of 1-Alkoxy-4-(2-Perfluoroalkyl)Ethoxybenzenes
in Organic Solvents
Yuki Morita
Takeyasu Tasaka
Kosuke Kawabe
Hiroaki Okamoto
Shunsuke Takenaka
Hidetoshi Kita
Department of Advanced Materials Science and Engineering, Faculty of
Engineering, Yamaguchi University, Tokiwadai, Ube Yamaguchi, Japan
This paper describes preparation and physico-chemical properties of 1-alkoxy-4-(2-
perfluoroalkyl)ethoxybenzenes (FmOOCn). For FmOOCn, 4-F(CF2)mCH2CH2O-
phenyl-O-(CH2)nH, three homologues, m ¼ 8 (n ¼ 1, 2) and m ¼ 10 (n ¼ 1) reveal
monotropic smectic A and unidentified smectic phases, respectively. Simul-
taneously, most of the homologous are possible to gelatinize various organic sol-
vents such as octane, cyclohexane, DMF, ethanol, and so on. The gelation ability
increases with increasing the carbon number of the perfluoroalkyl chain. The
formed gel is transparent, semitransparent or opaque, and the phase transition
between the gel phase and the isotropic fluid is thermally reversible. The images
of gel observed with scanning electron microscope (SEM) show gathered fibrous
aggregates.
Keywords: fibrous aggregate; gelation; perfluoroalkyl chain; SEM; smectic phase
INTRODUCTION
Diblock semifluorinated n-alkanes were studied in recent years in
relation to their interesting peculiarities such as excellent liquid
crystallinity and gel formation ability in organic solvents [1–4].
In our earlier paper, we described liquid crystal properties of alkyl
This work was partially supported by the Mazda Foundation.
Address correspondence to Yuki Morita, Department of Advanced Materials Science
and Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai 2557, Ube,
Yamaguchi 755-8611, Japan. E-mail: y-morita@po.cc.yamaguchi-u.ac.jp
153=[813]

154=[814]
Y. Morita et al.
FIGURE 1 Structure of FmOOCn.
4-(2-perfluoroalkyl)ethoxybenzoates, and clarified that 2-(perfluor-
oalkyl)ethoxybenzene is an excellent segment for liquid crystal mate-
rials as compared with the other substituted benzenes such as
perfluoroalkylbenzenes, perfluoroalkylmethoxybenzenes, and so on,
though preparation is not so easy [5,6]. In this connection, we are
interested in the liquid crystal and gelation properties of a new
homologous series of 1-alkoxy-4-(2-perfluoroalkyl)ethoxybenzenes,
FmOOCn, where m and n indicate the carbon numbers of the per-
fluoroalkyl and alkoxy chains, respectively.
The chemical structure is shown in Figure 1.
In this paper, we present liquid crystal (LC) and gelation properties
of FmOOCn, and the results will be discussed in terms of physical
properties of the molecules.
EXPERIMENTAL
Materials
FmOOCn were prepared according to Scheme 1.
1-Benzyloxy-4-(2-perfluorooctyl)ethoxybenzene
2-Perfluorooctylethanol (23.2 g, 50 mmol), 4-benzyloxyphenol (10.0 g,
50 mmol) and triphenylphosphine (13.1 g, 50 mmol) were dissolved in
dry THF (150 mL), and DEAD (8 mL, 51 mmol) was added dropwise
during 15 min under N2 at 0ꢀC. After stirring at 0ꢀC for overnight,
the mixture was quenched with cold water (1 mL). Then, the solvent
was removed in vacuo. The residues were dissolved in ethanol, and
the solution was cooled. White precipitates were collected by
filtration and was purified by column chromatography on silica-gel
(eluent CHCl3) to give 1-benzyloxy-4-(2-perfluorooctyl)ethoxybenzene
as colorless needles, 1.5 g (4.5%).
4-(2-Perfluorooctyl)ethoxyphenol
A
suspension of 1-benzyloxy-4-(2-perfluorooctyl)ethoxybenzene
(2.5 g, 3.9 mmol) and 5% Pd on charcoal (0.32 g) in a mixture of ethanol
(100 mL) and toluene (100 mL) was stirred at room temperature for a

Physico-Chemical Properties of New Gelator
155=[815]
SCHEME 1 Synthesis of FmOOCn.
week under an atmosphere of H2. After the catalyst was filtered off,
the solvent was removed in vacuo. The residues were recrystallized
from toluene to give 4-(2-perfluorooctyl)ethoxyphenol as colorless crys-
tals, 1.2 g (56%).
4-(2-Perfluorooctyl)ethoxy-dodecyloxybenzene (F8OOC12)
4-(2-Perfluorooctyl)ethoxyphenol (0.25 g, 0.45 mmol), anhydrous
potassium carbonate (0.08 g, 0.58 mmol) and 1-bromododecane
(0.13 g, 0.52 mmol) were suspended in 10 mL of 3-pentanone, and the
mixture was refluxed for 24 hours. After the precipitate was filtered
off, the solvent was removed in vacuo, and the residues were purified
by column chromatography on silica-gel (eluent CHCl3) to give
4-(2-perfluorooctyl)ethoxy-dodecyloxybenzene (F8OOC12) as colorless
powders, 0.16 g (50%).
1
The purity of each compound was checked by HPLC and H NMR.
Method
Transition temperature and the mesomorphism in bulk state were
characterized using a Nikon POH polarizing microscope fitted with a

156=[816]
Y. Morita et al.
Mettler thermo-control system (FP-900). A typical procedure for gel-
ation test is as follows [7]: a weighed sample was mixed with an
organic solvent in a sample tube with cap (/10 ꢁ 34 mm) and the mix-
ture was heated until the solid was well dissolved. The resulting sol-
ution was cooled in a refrigerator and placed at room temperature
for 1 hour. Then, the gelation was checked visually. When upon inver-
sion of the sample tube there was no fluid running down the walls, and
it was judged gel. When the samples caused gelation, the gelation abil-
ity was evaluated quantitatively by the critical gel concentration (cgc)
which was the minimum concentrations of gelators necessary for
gelation at room temperature. For preparation of SEM sample,
FmOOCn was dissolved in ethanol or cyclohexane at the cgc in sample
tube and a piece of thin glass was soaked in the solution. Then,
the solvent was removed off by pumping in vacuo for 12 hours and
the gel was loaded on a piece of thin glass. The SEM pictures were
taken using Nihon-denshi JSM 6335 F NT.
RESULTS AND DISCUSSION
Transition temperatures for FmOOCn in bulk state are summarized
in Table 1.
F8OOC1 and F8OOC2 exhibit smectic (Sm) phase having a common
fan texture under a homogeneous alignment, however, the higher
TABLE 1 Transition Temperatures of FmOOCn in Bulk State
Transition temperature=ꢀC
FmOOCn
C
Sm
Sm A
I
F8OOC1
F8OOC2
F8OOC3
F8OOC4
F8OOC5
F8OOC6
F8OOC8
F8OOC12
F8OOC14
F10OOC1
F10OOC5
F10OOC8
F10OOC12
.
.
.
.
.
.
.
.
.
.
.
.
.
60
67
60
70
72
72
78
86
87
88
90
96
95
-
-
-
-
-
-
-
-
(.52)
(.58)
.
.
.
.
.
.
.
.
.
.
.
.
.
-
-
-
-
-
-
-
-
-
-
-
-
(.84)
-
-
-
C, Sm, Sm A and I indicate crystal, unidentified smectic, smectic A and isotropic
phases, respectively.
Parentheses indicate a monotropic transition temperature.

Physico-Chemical Properties of New Gelator
157=[817]
FIGURE 2 Plots of transition temperatures vs. n of FmOOCn. ꢂ: melting
points of F8OOCn; ꢀ: SmA-I (monotropic) transition temperatures of
F8OOCn; ^: melting points of F10OOCn; 4: Sm-I (monotropic) transition
temperature of F10OOCn.
members (n > 2) do not show any LC phase even in a cooling process.
Therefore, it would be reasonable to assume that formation and ther-
mal stability of the Sm phase are a subtle counterbalance between
fluoro-and hydrocarbon chain lengths. The Sm phase was classified
into the A (SmA) modification, since this phase is miscible with the
SmA one of methyl and ethyl 4-(2-perfluorooctylethoxy)benzoates [5].
The transition temperatures are plotted against hydrocarbon chain
length, n in Figure 2.
Extension of the hydrocarbon chain length causes a monotonous
increase of melting point. These tendencies are also observed in LC
properties of alkyl 4-(2-perfluoroalkyl)ethoxybenzoates [5]. F10OOC1
also exhibits a monotropic smectic (Sm) phase. The Sm phase has a
fan texture and optically uniaxial nature. Considering the fact that
latent heat of the Sm-I transition (18.9 kJ=mol) is larger than those
of F8OOC1 (4.0 kJ=mol) and F8OOC2 (4.6 kJ=mol), the Sm phase is
assumed to have higher ordered molecular arrangement than SmA
phase. The higher homologs of F10OOCn (n > 2) do not exhibit any
LC phase. These results also indicate that the subtle balance between
fluoro- and hydrocarbon chain is important in exhibiting Sm phase.
Values of the cgc of FmOOCn are summarized in Table 2.
The gelation ability of FmOOCn increases with increasing the
carbon numbers of both perfluoroalkyl and alkyl chains. FmOOCn
can gelatinize various solvents, that is, alcohols such as methanol

158=[818]
TABLE 2 Values of the cgc (wt%) of FmOOCn
FmOOC
Y. Morita et al.
Solvent
F8OOC1
F8OOC5
F8OOC14
F10OOC1
F10OOC5
Methanol
1-Octanol
Octane
Cyclohexane
Toluene
Acetonitrile
N,N-dimethyl
formamide
Chloroform
5.3ꢄ
1.3 (op)
1.3 (tr)
4.8 (tr)
4.0 (tr)
11 (se)
1.7 (se)
1.6 (tr)
0.58 (op)
0.46 (tr)
4.1 (op)
4.1 (se)
8.9 (se)
0.20(se)
0.57 (tr)
2.1 (op)
1.5 (se)
5.3 (tr)
3.3 (tr)
11 (op)
3.8 (op)
1.8 (tr)
0.68 (op)
0.39 (tr)
2.7 (se)
2.8 (tr)
5.6 (se)
0.26 (op)
0.69 (tr)
8.0 (op)
14 (tr)
11 (tr)
33 (se)
10 (op)
16 (tr)
23ꢄ
15ꢄ
11ꢄ
13ꢄ
7.5ꢄ
ꢄMixture of gel and crystal.
Abbreviation: op, opaque gel; tr, transparent gel; se, semitransparent gel.
and 1-octanol, hydrocarbons such as octane, cyclohexane, and toluene,
and others such as chloroform, acetonitrile, N,N-dimethylformamide.
The formed gel is transparent, semitransparent or opaque. The phase
transition between the gel phase and isotropic fluid is thermally
reversible, while the transition temperatures on the heating process
are always higher than those on the cooling one. The effective order
for gelation is given by:
alcohols > polarsolvents > hydrocarbons > halogen solvents:
It is considered that the criteria of gel stability are three points: (1)
the critical gel concentration (cgc), (2) the sol-to-gel transition tem-
perature (Tgel), (3) the duration of period that a gel persists in a sealed
tube at room temperature without macroscopic phase separation or
flowing when inverted [8].
Figure 3 shows the plot of the cgc of F8OOCn vs. the carbon num-
ber, n in 1-octanol.
The cgc of F8OOCn steeply decreases on ascending of n when n < 5,
and becomes constant for the higher homologues (n ꢃ 5). For F8OOC1
and F8OOC2, crystallization and gelation simultaneously happen on
the cooling process. For F8OOC3 and the higher members, formed
gel is stable at room temperature for more than one week, without
recrystallization.
The Tgel values of F8OOCn in 1-octanol are plotted vs. gelator con-
centration in Figure 4.
The Tgel increase in proportional to the gelator concentration at lower
region and become plateau at ca. 10 wt% of gelator concentration. The

Physico-Chemical Properties of New Gelator
159=[819]
FIGURE 3 The cgc of F8OOCn vs. n in 1-octanol.
Tgel’s curves transfer upper with increasing n and their values become
large with increasing n at the same gelator concentration.
In Figure 5 the Tgel values of F8OOC5 in various alcohols and
octane is plotted vs. gelator concentration.
The lower the gelator concentration is at the onset of the plateau
region, the higher the Tgel and the higher the gelation stability are.
At the plateau region at higher gelator concentrations (ꢃ15 wt%) in
alcohols except octane, the Tgel values are almost same. The order of
gel stability is presented by: benzyl alcohol > 1-octanol > 1-butanol >
ethanol > octane.
FIGURE 4 Sol-to-gel phase transition temperatures of F8OOCn, n ¼ 1 (&),
n ¼ 2 (4), n ¼ 5 (ꢂ), and n ¼ 14 (ꢀ) vs. gelator concentration in 1-octanol.

160=[820]
Y. Morita et al.
FIGURE 5 Sol-to-gel phase transition temperatures of F8OOC5 vs. gelator
4
concentration in ethanol (&), 1-butanol (4), 1-octanol (ꢂ), octane ( ), and
benzyl alcohol (ꢀ).
It seems that some solubility of hydrocarbons as well as solvophobic
force derived from perfluoroalkyl chains is one of the important factor
to assemble in polar solvent.
The SEM photographs of gel for F8OOC5 formed in ethanol (a) and
in cyclohexane (b) are shown in Figure 6.
The images in Figures 6(a) and (b) show many gathering fibrous
˚
aggregates, one of whose width is ca. ꢃ 400 A. Those aggregates form
the three-dimensional network to immobilize the organic solvent,
and finally cause physical gelation.
FIGURE 6 Scanning electron micrographs of F8OOC8 dispersed in organic
solvents. (a) Gelator concentration: 1 wt% in ethanol and (b): Gelator concen-
tration 3 wt% in cyclohexane, magnification ꢁ10000.

Physico-Chemical Properties of New Gelator
161=[821]
FIGURE 7 DSC thermograms of F8OOC5. Run-1: the first heating of the gel
sample prepared for SEM observation. Run-2: the cooling of isotropic sample.
The phase transition behavior of the gel samples prepared for SEM
observation was examined by differential scanning calorimeter (DSC),
and the DSC thermograms of F8OOC5 are shown in Figure 7.
On the first heating process (Run-1) the gel sample shows an endo-
thermic peak at 72ꢀC (52.1 kJ=mol), which is comparable with those of
solid formed from isotropic solution. On the cooling process (Run-2) an
exothermic peak is observed at 59ꢀC. The phase transition cycle is com-
pletely reversible. These results suggest that the gel state in Figure 6
is quite similar to the solid state in thermodynamic point of view.
CONCLUSIONS
For 1-alkoxy-4-(2-perfluoroalkyl)ethoxybenzenes (FmOOCn), F8OOC1
and F8OOC2 exhibit monotropic smectic A phase, and F10OOC1 exhibits
a monotropic higher ordered Sm one. As the carbon number of alkoxy
chains increases, FmOOCn (n ꢃ 3) do not show any mesophase.
FmOOCn (n ꢃ 3) can gelatinize various organic solvents, which
especially can gelatinize alcohols and other polar solvents at low concen-
trations (0.5 ꢅ 2 wt%). The formed gel is transparent, semitransparent
or opaque, and the sol-to-gel phase transition is thermally reversible.
The images of gathered fibrous aggregates are observed by SEM.
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