An ω-functionalized perfluoroalkyl chain: Synthesis and use in liquid crystal design

Article abstract of DOI:10.1039/a700085e

A multiblock polyphilic compound incorporating a perfluoroalkyl spacer and associating groups at both extremities exhibits smectic A and smectic C mesophases.

Full text of DOI:10.1039/a700085e

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An w-functionalized perfluoroalkyl chain: synthesis and use in liquid crystal
design
Sandrine Pensec and François-Gene`s Tournilhac*†
ESPCI (CNRS URA429), Chimie et Electrochimie des Mate´riaux Mole´culaires, 10 rue Vauquelin, 75231 Paris Cedex 05, France
A multiblock polyphilic compound incorporating a perfluo-
roalkyl spacer and associating groups at both extremities
exhibits smectic A and smectic C mesophases.
mesomorphic state, unlike fragments have a tendency to
segregate, hence leading to the formation of smectic lay-
ers.4,5,8–10 Perfluoroalkyl chains have been widely used in the
design of such materials, but always as end-groups. Starting
from benzyl 4-iodoperfluorobutanoate, we have initiated the
synthesis of new polyphilic compounds incorporating a per-
fluoroalkyl spacer in the central part of the molecule. Com-
pound 7 was synthesized in three steps starting from 4
(Scheme 3).
Perfluoroalkyl iodides permit the formation of stable C–C
bonds by the Ullman reaction1 or by free-radical addition to
alkenes followed by dehalogenation.2 They have been widely
used in the design of highly fluorinated mesomorphic mater-
ials.3,4 In the course of our research into polyphilic mesogens,5
the need arose for new molecular architectures incorporating a
central perfluoroalkyl bridge. Symmetric a,w-diiodoperfluoro-
alkanes are available by several routes6,7 but, to our knowledge,
there is no efficient procedure for the synthesis of a per-
fluoroalkyl chain bearing two different reactive groups in the a
and w positions.
Compound 4 was treated with 1-chloroundec-10-ene in the
presence of azoisobutyronitrile (AIBN) to give benzyl
15-chloro-2,2,3,3,4,4-hexafluoro-6-iodopentadecanoate
5
which was in turn reduced to benzyl 15-chloro-
2,2,3,3,4,4-hexafluoropentadecanoate 6 using Zn/HCl. Com-
pound 6 was reacted with 4-cyano-4A-hydroxybiphenyl in the
presence of K2CO3 in DMF to give the polyphilic compound 7.†
Compound 7 melts at 124.5 °C and is not mesomorphic. The
pKa value of 7 is < 2. The potassium salt 8 was prepared by
treatment with an aqueous solution of K2CO3 (Scheme 4).
The mesomorphic properties of 8 have been investigated by
differential scanning calorimetry, optical microscopy and X-ray
diffraction. On cooling from the isotropic phase, birefringence
was detected at 186 °C by the growth of baˆtonnets11 which
gradually merge to form the well known focal conic texture.
This mesophase was identified as smectic A since homeotropic
domains were also evident in the same temperature range. The
focal conic texture is stable down to 135 °C, the temperature at
which the focal domains become divided into subdomains
showing different interference colours (broken fan texture12,13).
At the same temperature, the schlieren texture appears in the
previously homeotropic domains. These texture changes are
When reacting perfluoroglutaryl chloride with potassium
iodide at 200–400 °C, McLoughlin noticed the formation of
some 4-iodoperfluorobutyryl chloride,7 which was isolated in
the form of the free acid (Scheme 1).
In this original work,7 relatively high yields (50–60%) of 3
were obtained when using high temperatures and long reaction
times. In our experiments, all precautions were taken: (i) to stop
the progression of the reaction at the optimum time; (ii) to avoid
the hydrolysis of 2 into the corresponding acid; and (iii) to form
a versatile derivative of 2 which may be easily purified and
characterized.
The starting material (hexafluoroglutaryl chloride), obtained
from hexafluoroglutaric acid and oxalyl chloride, was vaporized
at 220 °C with a nitrogen flow and passed at 350 °C through a
glass tube packed with finely ground potassium iodide. The
reaction time was determined by the flow of nitrogen. The
products were trapped at 278 °C. The degree of conversion
estimated by the relative intensities of the 1793 cm21 (CNO
stretch) and 1128 cm21 (CF stretch) IR bands; was taken as a
criterion for optimizing the reaction time. The products 1, 2 and
3 were immediately treated with benzyl alcohol in toluene in the
presence of pyridine and benzyl-4-iodoperfluorobutanoate 4
was isolated by liquid chromatography with an overall yield of
16% (Scheme 2).‡
Bifunctional spacer units are useful in the chemistry of
covalently grafted surfaces, artificial membranes, liquid crystal-
line polymers, etc. Compound 4 allows the preparation of new
mesomorphic materials as will be exemplified below.
Polyphilic compounds are made up of a sequence of
molecular fragments differing in their chemical nature. In the
(CH2)9Cl
CH2
O
O
4
(CF2)3 CH2 CHl (CH2)9Cl
i
5
H+ / Zn
ii
CH2
O
O
(CF2)3 (CF2)11Cl
6
HO
CN
iii
HO2C(CF2)3(CH2)11
O
CN
Cl
Cl
O
Cl
Kl
Kl
(CF2)3
(CF2)3
I
I
(CF2)3
I
350 °C
350 °C
7
O
O
1
2
3
Scheme 3 Reagents and conditions: i, AIBN, heptane, 90 °C, 4 h, 81%; ii,
isooctane + acetic acid, 110 °C, 2 h, 72%; iii, K2CO3, DMF, 90 °C, 3 h, then
HCl, 68%
Scheme 1
C6H5CH2OH
CH2
O
O
K2CO3
2
K+ / –O2C(CF2)3(CH2)11
O
CN
(CF2)3
I
7
4
8
Scheme 2
Scheme 4
Chem. Commun., 1997
441

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characteristic of the smectic A ? smectic C transition.13 This
second fluid mesophase is monotropic, it may be observed for a
few minutes in the 135–110°C range, whereupon crystallization
rapidly occurs.
The DSC trace shows, on heating, two endotherms at 135 and
190 °C with enthalpies of 8.9 and 4.3 kJ mol21 respectively,
corresponding to the K ? SA and SA ? I transitions. On
cooling the reverse transitions take place at 186 and 123 °C.
Thus, whilst 7 is isotropic in the 135–190 °C range,
the appearance of lamellar order in 8 may be related to the
segregation of the ionic head from all other hydrophobic
moieities.
X-Ray diffraction experiments were carried out on unorien-
ted samples using Cu-Ka radiation. Attempts to investigate 8 in
the smectic C phase failed due to its metastable character. The
X-ray diffraction pattern recorded at 150 °C in the smectic A
phase of 8 is presented in Fig. 1.
At large angles, a broad diffuse ring with a maximum at 2p/q
= 4.9 Å (q = 4p sinq/l) confirms the identification as smectic
A and gives an estimate of the average intermolecular distance
(ca. 5.7 Å). At narrow angles, the diagram shows three 00l
reflections corresponding to an interlayer distance of 42.2 Å.
The distance is intermediate between once and twice the
molecular length (32 Å); a partially bilayered packing of the
smectic Ad type14 may therefore be proposed. A similar
observation was reported for other rodlike mesogens with ionic
head-groups.15 In Na+ and K+ long-chain carboxylates, a lipidic
sublayer thickness 1.3 times greater than the alkyl chain length
was interpreted by the formation of a double layer with strongly
disordered alkyl chains.16 In our case, the rigid cyanobiphenyl
cores cannot be accommodated in any conformation, however,
these fragments usually tend to optimize their packing by partial
interdigitation. As already mentioned for polyphilic mesogens
with cyanobiphenyl cores, the appearance of the smectic Ad and
C phases can be related to the close packing of lengthwise
dimers associated through the cyano groups.9
The potentialities of hydrogen-bond associated systems in the
design of new mesomorphic architectures have been evidenced
by several authors.17 The synthetic scheme described in this
paper permits the grafting of hydrogen-bond donor and acceptor
groups at each molecular end of a polyphilic compound. It is
noteworthy that polar rows of molecules may be formed in the
liquid phases of such materials (Fig. 2).
J. Simon is thanked for helpful and stimulating discussions,
D. Lelie`vre for her help in X-ray measurements.
Footnotes
† E-mail: fgt@cis.espci.fr
‡ Selected analytical data: 4: MS (EI, m/z): 412 [M]+, 277 [M 2
CO2CH2C6H5]+, 227 [M
2
CF2CO2CH2C6H5]+, 177 [M
2
(CF2)2CO2CH2C6H5]+, 127 [M 2 (CF2)3CO2CH2C6H5]+, 91 [M 2
I(CF2)3CO2]+. IR (KBr, cm21) 1778.9 (CNO), 1190.1, 1145.0 (C–F), 770
(C–H arom., out of plane). 1H NMR (CDCl3, Me4Si, 300 MHz, d) 7.43 (s,
5 H, C6H5), 5.4 (s, 2 H, CH2). 13C NMR (CDCl3, 75 MHz, d) 158.45 (t, J
29.9 Hz, CF2COOR), 133.23 (s, arom.), 129.11 (s, arom.), 128.73 (s, arom.),
128.48 (s, arom.), 108.45 (m, J 265.5, 31.75 Hz, CF2CF2CF2COOR),
106.94 (m, J 267.95, 33.6 Hz, CF2CF2COOR), 93.39 (m, J 319.8, 41.5 Hz,
ICF2CF2CF2), 69.75 (s, COCH2). 7: MS (CI, m/z): 561 [M + NH4]+, 543
[M]+. Anal. Found: C, 61.29; H, 5.87; N, 2.61. Calc. C, 61.87; H, 5.74; N,
2.57. IR (KBr, cm21): 3649 (OH), 2921.5 (C–H asym.), 2850.3 (C–H sym.),
2246.1 (CN), 1781.8 (CNO), 1603.7 (CNC), 1178.1, 1142.5 (C–F), 827.0
(C–H, out of plane). 1H NMR ([2H6]acetone, 300 MHz, d) 8.19 (s, 1 H,
COOH), 7.79 (4 H, arom.), 7.67 (2 H, J 9.18 Hz, arom.), 7.05 (2 H, J 9.18
Hz, arom.), 4.03 (t, 2 H, J 6.6 Hz, OCH2), 2.13 (m, 2 H, CH2CF2), 1.78 (qnt,
2 H, J 6.6 Hz, CH2CH2CF2), 1.2 (m, alkyl). 13C NMR ([2H6]acetone, 75
MHz, d) 160.86 (s, arom.), 160.75 (t, J 28.7 Hz, CF2COOH), 145.87 (s,
arom.), 133.46 (s, arom.), 131.80 (s, arom.), 129.17 (s, arom.), 127.86 (s,
arom.), 119.61 [m, J 251.4, 31 Hz, (CH2)11CF2CF2CF2], 119.51 (s, CN),
115.94 (s, arom.), 112.06 (m, J 263 Hz, CF2CF2CF2COOH), 110.84 (s,
arom.), 109.57 (m, J 263, 32 Hz, CF2CF2COOH), 68.72 (s, OCH2), 31.43
[t, J 22 Hz, CH2(CF2)3], 29.8 (CH2), 26.76 [s, OCH2CH2CH2(CH2)8], 20.94
[s, (CH2)9CH2CH2(CF2)3].
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Fig. 2 Polar rows of molecules expected from polyphilic compounds
bearing hydrogen-bond donor and acceptor groups at the extremities
Received, 3rd January 1997; Com. 7/00085E
442
Chem. Commun., 1997