Liquid crystals obtained from disclike mesogenic diacetylenes and their polymerization

Full text of DOI:10.1021/ja961193m

J. Am. Chem. Soc. 1997, 119, 3197-3198
3197
Liquid Crystals Obtained from Disclike Mesogenic
Diacetylenes and Their Polymerization
Ji Young Chang,* Jung Hyun Baik, Chan Bong Lee, and
Man Jung Han
Department of Applied Chemistry
Ajou UniVersity, Suwon 442-749, Korea
Sung-Kwon Hong
Department of Polymer Science and Engineering
Chungnam National UniVersity, Taejon, Korea
ReceiVed April 11, 1996
Since the topochemical polymerization of diacetylenes by
irradiation or thermal annealing was first reported more than
two decades ago,1 it has become one of the favorite methods
used to obtain the organized macromolecules.2 Various types
of diacetylenes have been prepared and used for Langmuir-
Blodgett films,3 self-assembling monolayers,4 and vesicles.5 The
polymerization of diacetylenes occurs in liquid crystalline (LC)
phases as well as in the solid state.6 In an LC state, molecules
are self-organized into ordered arrays comparable to those in
Langmuir-Blodgett or self-assembling processes. Most di-
acetylenic mesogens found in the literatures consist of rigid and
linear diacetylenic parts and flexible alkyl chains, which exhibit
nematic or smetic phases.6,7 Disclike mesogenic diacetylenes
are of particular interest. They stack in columns, and zipping
of diacetylenes along the column axis by polymerization will
lead to supramolecules with columnar structures. In this paper,
we report novel disclike mesogenic diacetylenes which are
polymerizable in LC states.
Three disclike diacetylenes (1-3) were synthesized as
potential mesogens (Figure 1). The linear diacetylenic side
groups were prepared according to Scheme 1. Ethynylation with
(trimethylsilyl)acetylene in the presence of bis(triphenylphos-
phine)palladium(II) chloride and CuI in triethylamine8 and
subsequent removal of the trimethylsilyl group in a solution of
KOH/MeOH gave compound 4. 4-Ethynylphenol and 4-ethy-
nylaniline, prepared following literatures procedures8a,d were
coupled with 4-(octyloxy)phenylacetylene or 1-octyne in the
presence of copper(II) acetate9 in a solution of pyridine and
methanol (1:1). The unsymmetrical diacetylene products were
isolated by column chromatography on silica gel (ethyl acetate/
Figure 1. Structures of disclike diacetylenes.
Scheme 1
hexane) and further purified by recrystallization from ethyl
acetate/methanol (5, mp 110 °C) or ethyl acetate/hexane (6, mp
103 °C; 7, mp 148 °C). The linear diacetylenes 5-7 were linked
to benzene cores by esterification or amidation reactions. Com-
pounds 1-3 were purified by column chromatography on silica
gel (methylene chloride/hexane) and then recrystallization from
ethyl acetate/methanol (1 and 2) or chloroform/hexane (3).10
Compound 1 showed a monotropic transition. In DSC
analysis an endotherm at 132 °C appeared on the first heating
but disappeared on reheating. A clearing transition was
observed at 146 °C. On heating beyond 200 °C, a strong
exotherm appeared, indicating thermal polymerization took
place. When cooled from the isotropic liquid state to room
temperature, the compound exhibited one exothermic transition
at 137 °C (∆H ) -13.6 kJ/mol). A birefringent phase with
mosaic textures11 began to form at 140 °C and persisted to room
temperature. That no further crystallization occurred is attribut-
able to the high viscosity of the compound in its LC phase.
After the textures (see Figure 2a) were fully developed at 137
°C, the compound was quenched to room temperature and
subjected to XRD experiments. In the small-angle region, at
least three peaks corresponding to d spacings of 30.3, 26.5, and
(1) Wegner, G. Z. Naturforsch. 1969, 24b, 824-832.
(2) AdVances in Polymer Science; Cantow, H.-J., Ed.; Springer-Verlag:
Berlin, 1984; Vol. 63.
(3) For example, see: (a) Tieke, B.; Graf, H. J.; Wegner, G.; Naegele,
B.; Ringsdorf, H.; Banerjie, A.; Day, D.; Lando, J. B. Colloid Polym. Sci.
1977, 255, 521-531. (b) Tieke, B.; Wegner, G.; Naegele, B.; Ringsdorf,
H. Angew. Chem. 1976, 88, 805-806. (c) Kruchinin, V. N.; Repinsky, S.
M.; Sveshnikova, L. L. Thin Solid Films 1994, 240, 131-137.
(4) (a) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.;
Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050-1053. (b)
Kim, T.; Crooks, R. M. Tetrahedron Lett. 1994, 35, 9501-9504.
(5) (a) O’Brien, D. F.; Ramaswami, V. In Encyclopedia of Polymer
Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger,
C. G., Menges, G., Eds.; John Wiley & Sons: New York, 1985; Vol. 17,
pp 108-135. (b) Peek, B. M.; Callahan, J. H.; Namboodiri, K.; Singh, A.;
Gaber, B. P. Macromolecules 1994, 27, 292-297.
(6) (a) Tsibouklis, J. AdV. Mater. 1995, 7, 407-408. (b) Garito, A. F.;
Teng, C. C.; Wong, K. Y.; Khamiri, O. Z. Mol. Cryst. Liq. Cryst. 1984,
106, 219-258. (c) Tsibouklis, J.; Campbell, C.; Werninck, A. R.; Shand,
A. J.; Milburn, G. H. W. Polym. Bull. 1992, 29, 661-668. (d) Schen, M.
A.; Kotowski, K.; Cline, J. Polymer 1991, 32, 1843-1850. (e) Izuoka,
A.; Okuno, T.; Ito, T.; Sugawara, T.; Sato, N.; Kamei, S.; Tohyama, K.
Mol. Cryst. Liq. Cryst. 1993, 226, 201-205. (f) Hammond, P. T.; Rubner,
M. F. Macromolecules 1995, 28, 795-805.
x
20.4 Å appeared. They are in the ratio of 1: 3/2:2/3, which is
in good agreement with a hexagonal lattice. The calculated
diameter of the disc is 52 Å by simple molecular modeling,
and the peak with a d spacing of 26.5 Å likely corresponds to
the radius. Wide-angle XRD showed a peak with a d spacing
of 3.5 Å atop a diffuse peak, which probably resulted from
stacking of the discs.
Compound 2 exhibited a clearing transition at a much higher
temperature than compound 1 because of hydrogen bondings
(7) (a) Tsibouklis, J.; Shand, A. J.; Werninck, A. R.; Milburn, G. H. W.
Liq. Cryst. 1988, 3, 1393-1400. (b) Attard, G. S.; West, Y. D. Liq. Cryst.
1990, 7, 487-494. (c) Okuno, T.; Fukada, M.; Izuoka, A.; Sato, N.;
Sugawara, T. Mol. Cryst. Liq. Cryst. 1992, 217, 59-64. (d) Ozcayir, Y.;
Blumstein, A. Mol. Cryst. Liq. Cryst. 1986, 135, 237-246. (e) Grant, B.;
Clecak, N. J.; Cox, R. J. Mol. Cryst. Liq. Cryst. 1979, 51, 209-214. (f)
Plachetta, C.; Rau, N. O.; Schulz, R. C. Mol. Cryst. Liq. Cryst. 1983, 96,
141-151.
(8) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627-630. (b) Austin, W. B.; Bilow, N.; Kellaghan, W. J.;
Lau, K. S. Y. J. Org. Chem. 1981, 46, 2280-2286. (c) Chang, J. Y.; Rhee,
S. B.; Cheong, S.; Yoon, M. Macromolecules 1992, 25, 2666-2670. (d)
Chang, J. Y.; Ji, H. J.; Han, M. J. Bull. Korean Chem. Soc. 1995, 16, 674-
677.
(9) Campbell, I. D.; Eglinton, G. In Organic Syntheses; John Wiley and
Sons: New York, 1973; Collect. Vol. 5, pp 517-520.
S0002-7863(96)01193-6 CCC: $14.00 © 1997 American Chemical Society

3198 J. Am. Chem. Soc., Vol. 119, No. 13, 1997
Communications to the Editor
The DSC thermogram of compound 3 showed only a second-
order transition at 80 °C and a strong exotherm above 200 °C.
The compound became shearable at approximately 120 °C and
the birefringent texture began to form. We presume that
compound 3 was either in glassy state or as very small crystals.
Wide-angle XRD also showed a broad peak. The texture did
not develop further, since the fluidity decreased rapidly due to
thermal polymerization. The higher susceptibility of compound
3 to thermal polymerization compared with compounds 1 and
2 is ascribed to the absence of the second phenyl ring in the
side group.2
When annealed in LC states at 197 °C under nitrogen,
compound 2 turned to a dark reddish solid in 24 h. Optical
microscopy showed the material was still birefringent. Although
the IR spectroscopy showed only a 35% absorbance decrease
for the acetylene stretching bands at 2149 and 2216 cm-1, no
unreacted monomer was extracted with chloroform13 and the
endothermic melt transition for the monomer was not seen in
the DSC thermogram. The XRD diffractogram showed a peak
with a d spacing of 35.0 Å, a broad peak at 2θ ) 24° as was
observed for the monomer, and an additional broad peak at 2θ
) 12.0° (d ) 7.4 Å). The IR and XRD results suggest that
only one of three diacetylenic groups of each monomer
participated in polymerization without much altering the liquid
crystalline structure.
Figure 2. Optical polarized micrographs of compound 1 at 137 °C
obtained on cooling from the isotropic liquid (a, 200 magnification)
and of compound 2 obtained at 194 °C on heating (b, 500 magnifica-
tion).
Heating of compound 1 in the LC state at 137 °C did not
induce polymerization. When exposed to UV light (254 nm,
12 W) for 40 h at the same temperature under nitrogen, it turned
deep yellow. The phase was still birefringent, and mosaic
textures from unreacted monomers were seen. In the IR
spectrum of compound 1, two weak bands at 2149 and 2223
cm-1 from symmetric and asymmetric stretching vibrations of
carbon-carbon triple bonds appeared. After polymerization,
the absorbances of two bands decreased in the same proportion
by 25% and a weak band at 2186 cm-1 for C-C triple bonds
showed up. The band at 1630 cm-1 became broader and
stronger by overlapping with the band from C-C double bonds.
This result suggests that the polymerization proceeded by 1,4-
addition. In striking contrast to the polymerization of compound
2, monomer conversion determined from the melt enthalpy
decrease in the DSC thermogram was almost consistent with
the IR result. The polymerized sample showed an XRD pattern
similar to that of the monomer. Noticeable was that the peak
with a d spacing of 26.5 Å shifted to 25.7 Å and of 18.5 Å
shifted to 18.0 Å, and a broad peak appeared at 7.2 Å. No
significant change was observed in the wide-angle region. Since
the stacking distance of the discs was suitable for the to-
pochemical polymerization in the hexagonal columnar structure,
the polymerization likely proceeded in a vertical direction with
respect to the disc plane, which would allow more than one
diacetylenic group of the monomer to participate in polymer-
ization. In fact, the IR spectrum of the insoluble polymer
prepared by the monomer extraction with chloroform showed
that about 85% of the diacetylenic groups were consumed.
between amide groups. DSC analysis showed a endothermic
clearing transition at 204 °C, followed by the strong exotherm
corresponding to thermal polymerization. Although no peak
other than the clearing transition peak appeared in DSC
thermogram, an ordered fluid phase formed above 190 °C.12
A
highly viscous phase with a needle-like texture was first
observed by polarizing optical microscopy and became thread-
like by shearing (see Figure 2b). A wide-angle XRD diffrac-
togram obtained after quenching from the LC state to room
temperature showed only a broad peak around 2θ ) 24°. In
the small angle region, a peak with a d spacing of 34.6 Å was
observed, and thus we believe that compound 2 was organized
into discotic nematic phases. Since compound 2 was polym-
erized immediately after clearing, phase transitions on cooling
could not be investigated.
(10) For 1: 1H NMR (CDCl3, 200 MHz) δ 9.25 (s, 3H, benzene ring
protons), 7.65, 7.25 (dd, 12H, benzene ring protons), 7.50, 6.85 (dd, 12H,
benzene ring protons), 3.95 (t, 3H, OCH2), 1.90-0.80 (m, 45H, alkyl chain
protons); 13C NMR (CDCl3) δ 162.8, 160.1, 150.8, 136.2, 134.2, 133.8,
131.1, 121.8, 120.4, 114.7, 113.2, 82.4, 79.9, 74.8, 72.5, 68.2, 31.8, 29.3,
29.2, 29.1, 26.0, 22.6, 14.1; IR (KBr pellet, cm-1) 3088, 2934, 2860, 2223,
2149, 1754. Anal. Calcd for C81H78O9: C, 81.37; H, 6.57. Found: C,
80.90; H, 6.71. For 2: 1H NMR (DMSO-d6, 200 MHz) δ 10.90 (s, 3H,
NH), 8.85 (s, 3H, benzene ring protons), 7.90, 7.65 (dd, 12H, benzene ring
protons), 7.55, 7.00 (dd, 12H, benzene ring protons), 4.05 (t, 6H, OCH2),
1.80-0.80 (m, 45H, alkyl chain protons); 13C NMR (DMSO-d6) δ 165.0,
160.1, 138.2, 135.6, 134.5, 133.5, 120.5, 120.4, 118.9, 114.8, 113.8, 82.5,
80.9, 75.1, 73.3, 68.4, 32.1, 30.0, 29.7, 29.5, 26.3, 22.9, 14.4; IR (KBr
pellet, cm-1) 3436, 2940, 2860, 2216, 2149, 1673. Anal. Calcd for
C81H81N3O6: C, 81.58; H, 6.85; N, 3.52. Found: C, 81.98; H, 6.86; N,
3.69. For 3: 1H NMR (DMSO-d6) δ 10.8 (s, 3H, NH), 8.70 (s, 3H, benzene
ring protons), 7.90, 7.60 (dd, 12H, benzene ring protons), 2.41 (t, 6H, CCH2),
1.71-0.92 (m, 33H, alkyl chain protons); 13C NMR (DMSO-d6) δ 164.2,
137.7, 135.6, 133.3, 129.5, 120.1, 118.8, 85.2, 81.5, 74.1, 65.1, 31.3, 28.6,
28.2, 22.5, 19.6, 14.0; IR (KBr pellet, cm-1) 3292, 3103, 2935, 2864, 2243,
2156, 1667. Anal. Calcd for C57H57N3O3: C, 82.27; H, 6.91; N, 5.05.
Found: C, 82.07; H, 6.90; N, 5.25.
Acknowledgment. This work was supported by Non Directed
Research Fund, Korea Research Foundation, and in part by Korea
Science and Engineering Foundation. We thank Dr. Dong Young Kim
at KIST for helpful discussions.
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JA961193M
(11) Mosaic textures were reported to appear in smetic phases of
hexagonal structures. See: Demus, D. In Liquid Crystals: Application and
Uses; Bahadur, B. Ed.; World Scientific: Singapore, 1990; Vol. 1, p 1.
(12) The reason for the absence of the transition peak at 190 °C in DSC
thermogram is not clear at this point. One possible explanation is that
molecules were ordered slowly into LC states from the disordered glassy
state. In fact, wide-angle XRD of compound 2 before heating did not show
any prominent peak indicating a crystal lattice.
(13) The polymerized sample was not soluble in chloroform, which
dissolved the monomer easily. It was slightly soluble in N,N-dimethylfor-
mamide (DMF). In the UV-vis spectra obtained in DMF, the monomer
showed λmax and the absorption limit at 359 and 420 nm, respectively, while
the polymer showed λmax below 280 nm and some absorption above 550
nm.