Rodlike mesogenic molecules consisting of two diacetylenic groups: Mesomorphic behavior and photoimaging

Article abstract of DOI:10.1039/b300743j

A series of mesogenic compounds with two diacetylenic groups were synthesized. The diacetylene groups were connected to a benzene ring via ester linkages, becoming a part of a rigid rod. The thermal behavior of the compounds and their polymerization were

Full text of DOI:10.1039/b300743j

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Rodlike mesogenic molecules consisting of two diacetylenic groups:
mesomorphic behavior and photoimaging
Hyun Ju Cho,a Kyung Seo,a Cheol Ju Lee,a Hoseop Yunb and Ji Young Chang*a
aSchool of Materials Science and Engineering, and Hyperstructured Organic Materials
Research Center, College of Engineering, Seoul National University, ENG445,
Seoul 151-744, Korea. E-mail: jychang@gong.snu.ac.kr
bDepartment of Molecular Science and Technology, Ajou University, Suwon 442-749, Korea
Received 28th January 2003, Accepted 11th March 2003
First published as an Advance Article on the web 24th March 2003
A series of mesogenic compounds with two diacetylenic groups were synthesized. The diacetylene groups were
connected to a benzene ring via ester linkages, becoming a part of a rigid rod. The thermal behavior of the
compounds and their polymerization were investigated. Compounds 7, 8, 9 and 10 having long alkyl tails
showed nematic mesophases. They were thermally polymerized in the isotropic state above 200 uC. Partial
polymerization occurred by annealing in the liquid crystalline state. When the compounds were exposed to UV
light in the liquid crystalline state, the polymerization was completed within 30 min to give amorphous
polydiacetylenes. In the UV-Vis spectra, absorption of the polymers occurred up to about 600 nm, attributable
to the short conjugated polydiacetylene backbones. Compound 9 with an octyl tail was fabricated into a thin
film and the photopolymerization was carried out through a photomask in the liquid crystalline state resulting
in a patterned image.
sites and thereby disrupt aligned molecules in a microdomain.
We report here the polymerization of mesogenic diacetylenes
and their application to photoimaging.
Introduction
Photoreaction of diacetylenes has been extensively studied
due to their unique reactivity and the interesting properties of
the resulting polymers.1,2 The reaction is known to proceed
topochemically via 1,4-addition by irradiation and thus has
been widely used for preparing ordered macromolecules. The
topochemical polymerization of diacetylenes can be carried out
in the solid state. A diacetylene single crystal is polymerized
to give a single crystal of a polymer when a crystal structure
satisfies the requirements for the topochemical polymerization.
Such structural requirements can be also met in Langmuir–
Blodgett films,3–5 self-assembling monolayers,6,7 and vesicles.8
The 1,4-polymerization of diacetylenes in liquid crystalline states
was also reported to give highly ordered polymers.9–13 Liquid
crystalline state polymerization is of great interest because of
its application to the fabrication of stable, optically anisotropic
films. In addition, it has potential imaging applications since
LC molecules can be oriented macroscopically under electric
or magnetic fields and photopolymerization will change their
optical properties.
In our previous communication,14 we reported a photo-
polymerizable liquid crystal molecule with two photoreactive
chalcone units. Photopolymerization by UV irradiation through
a photomask resulted in an excellent patterned image, which
resulted from the [2 1 2] addition reaction of chalcones in
the irradiated area. These results prompted us to investigate
further on photopolymerizable liquid crystals suitable for
photoimaging. The photopolymerizable groups can be intro-
duced either to the end of flexible spacers or into a mesogen,
becoming a part of the rigid rod. The latter will be a better
approach to obtain a high contrast in an image, resulting from
the significant changes of the aligned structures by the poly-
merization and thereby the optical property changes of the
irradiated area.
Experimental
Materials and instrumentation
4-Iodophenol, trimethylsilylacetylene, dichlorobis(triphenyl-
phosphine)palladium(II), copper(I) iodide, bromine, terephtha-
loyl dichloride, and isophthaloyl dichloride were purchased
from Aldrich and used as received. Reagent grade solvents
were dried and purified as follows. Pyridine was distilled over
calcium hydride. Tetrahydrofuran (THF) and 1,4-dioxane
were dried over sodium metal and distilled. Methanol was dried
over molecular sieves 4 A and distilled. 1H and 13C NMR
˚
spectra were recorded on a JEOL JNM-LA 300 (300 MHz)
spectrometer or Bruker Avance DPX-300 (300 MHz) at room
temperature. IR spectra were obtained with the use of a Perkin
Elmer Spectrum GX I spectrometer. UV-Vis spectra were
obtained with the use of a Hewlett-Packard HP8452A. Thermal
analyses were performed using a TGA 2050 thermalgravimetric
analyzer and a TA modulated DSC 2920. Optical microscopy
study was performed with a Nikon 264445 equipped with a
Linkam heating stage and Linkam TP 92 controller. Elemental
analyses were performed at the National Center for Inter-
University Research Facilities of Seoul National University,
Seoul, Korea.
1-Bromoheptyne
4-Trimethylsilylethynylphenol and 4-ethynylphenol were pre-
pared according to our previous report.15 Bromine (39.9 g,
0.25 mol) was added to a vigorously stirred solution of potas-
sium hydroxide (37.5 g) in water (100 ml) at 0 uC.16 1-Heptyne
(0.15 mol) in 1,4-dioxane (80 ml) was added dropwise to the
above mixture over 2 h. The reaction mixture was stirred for
40 h at room temperature and the reaction mixture was
extracted with chloroform. The organic layer was dried with
anhydrous MgSO4. Upon filtration and evaporation the
In this work, we prepared rodlike mesogenic molecules
mainly consisting of two diacetylenic groups. Unlike the poly-
merization of diacetylenic compounds with one polymerizable
group to yield polymers with ordered structures, the poly-
merization of these compounds was expected to occur at two
986
J. Mater. Chem., 2003, 13, 986–990
This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b300743j

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product was isolated and used for reaction without further
purification.
terephthaloyl dichloride (0.27 g, 1.23 mmol) in THF (50 ml)
was added. The solution was stirred for 6 h. After evaporation,
the product was isolated by column chromatography on silica
gel (methylene chloride–hexane ~ 5/1 v/v) and further purified
by recrystallization from methylene chloride–hexane (yield
65%).
1H NMR (CDCl3, 300 MHz): d 2.20 (t, 2H, CCH2), 1.54–
0.91 (m, 9H, alkyl chain protons).
1-Bromooctyne, 1-bromononyne and 1-bromodecyne were
prepared using the same method as described above.
Elemental analysis. Calc. for C38H34O4: C, 82.28; H, 6.18.
1
Found: C, 82.24; H, 6.21%. H NMR (CDCl3, 300 MHz): d
1-(4-Hydroxyphenyl)-4-pentylbutadiyne (1)
8.32 (s, 4H, C6H4), 7.57, 7.22 (dd, 8H, C6H4), 2.37 (t, 4H,
CCH2), 1.57–0.90 (m, 18H, alkyl chain protons). 13C NMR
(CDCl3, 300 MHz): d 163.8, 150.8, 133.8, 133.7, 130.3, 121.7,
120.2, 85.2, 74.8, 73.7, 64.9, 31.0, 27.9, 22.1, 19.5, 13.9. IR (KBr
pellet, cm21): 2933, 2866, 2241, 2142, 1739, 1595, 1501, 1249,
1194.
Methanol (35 ml), a solution of hydroxylamine?HCl (0.75 g) in
water (10 ml), a 70% aqueous solution of ethylamine (4.75 g),
and copper(I) chloride (0.11 g) were placed in a flask.17 To
the mixture, 4-ethynylphenol (14.9 mmol) was added in one
portion. Then 1-bromoheptyne (14 mmol) was added over
about 1 h, while keeping the temperature at 35 uC. After
stirring for 6 h at the same temperature, a solution of KCN
(0.37 g) and NH4Cl (1.50 g) in water (50 ml) was added with
vigorous stirring. The mixture was extracted with chloroform,
dried with anhydrous MgSO4, and filtered. After evaporation
of the solvent, the product was isolated by column chroma-
tography on silica gel (25% ethyl acetate in hexane) as an oil
(yield 52%).
Bis(4-hexylbutadiynylphenyl) terephthalate (7)
This compound was prepared from terephthaloyl dichloride
(0.97 g, 4.4 mmol) and 1-(4-hydroxyphenyl)-4-hexylbutadiyne
(2 g, 8.83 mmol) as described for compound 6 (yield 75%).
Elemental analysis. Calc. for C40H38O4: C, 82.44; H, 6.5.
1
Found: C, 82.40; H, 6.66%. H NMR (CDCl3, 300 MHz): d
1H NMR (CDCl3, 300 MHz): d 7.38, 6.77 (dd, 4H, C6H4),
5.30 (s, 1H, OH), 2.37 (t, 2H, CCH2), 1.73–0.88 (m, 9H, alkyl
chain protons).
8.31 (s, 4H, C6H4), 7.57, 7.22 (dd, 8H, C6H4), 2.37 (t, 4H,
CCH2), 1.57–0.88 (m, 22H, alkyl chain protons). 13C NMR
(CDCl3, 300 MHz): d 163.8, 150.8, 133.8, 133.7, 130.3, 121.7,
120.2, 85.2, 74.8, 73.7, 64.9, 31.3, 28.5, 28.2, 22.5, 19.6, 14.0. IR
(KBr pellet, cm21): 2930, 2855, 2240, 2144, 1740, 1597, 1499,
1257, 1192.
1-(4-Hydroxyphenyl)-4-hexylbutadiyne (2)
This compound was prepared from 4-ethynylphenol and
1-bromooctyne as described for compound 1 as an oil (yield
56%).
1H NMR (CDCl3, 300 MHz): d 7.38, 6.77 (dd, 4H, C6H4),
5.28 (s, 1H, OH), 2.35 (t, 2H, CCH2), 1.59–0.87 (m, 11H, alkyl
chain protons).
Bis(4-heptylbutadiynylphenyl) terephthalate (8)
This compound was prepared from terephthaloyl dichloride
(0.74 g, 3.4 mmol) and 1-(4-hydroxyphenyl)-4-heptylbutadiyne
(1.64 g, 6.8 mmol) as described for compound 6 (yield 64%).
Elemental analysis. Calc. for C42H42O4: C, 82.59; H, 6.93.
1-(4-Hydroxyphenyl)-4-heptylbutadiyne (3)
1
Found: C, 82.33; H, 7.05%. H NMR (CDCl3, 300 MHz): d
This compound was prepared from 4-ethynylphenol and
1-bromononyne as described for compound 1 as an oil (yield
57%).
1H NMR (CDCl3, 300 MHz): d 7.38, 6.78 (dd, 4H, C6H4),
5.66 (s, 1H, OH), 2.35 (t, 2H, CCH2), 1.59–0.86 (m, 13H, alkyl
chain protons).
8.31 (s, 4H, C6H4), 7.57, 7.22 (dd, 8H, C6H4), 2.37 (t, 4H,
CCH2), 1.54–0.89 (m, 26H, alkyl chain protons). 13C NMR
(CDCl3, 300 MHz): d 163.8, 150.8, 133.8, 133.7, 130.3, 121.7,
120.2, 85.2, 74.8, 73.7, 64.9, 31.7, 28.8, 28.7, 28.2, 22.6, 19.6,
14.0. IR (KBr pellet, cm21): 2926, 2853, 2242, 2145, 1740, 1595,
1503, 1266, 1198.
1-(4-Hydroxyphenyl)-4-octylbutadiyne (4)
Bis(4-octylbutadiynylphenyl) terephthalate (9)
This compound was prepared from 4-ethynylphenol and
1-bromodecyne as described for compound 1 as an oil (yield
66%).
This compound was prepared from terephthaloyl dichloride
(0.36 g, 1.6 mmol) and 1-(4-hydroxyphenyl)-4-octylbutadiyne
(0.9 g, 3.5 mmol) as described for compound 6 (yield 45%).
Elemental analysis. Calc. for C44H46O4: C, 82.72; H, 7.26.
1H NMR (CDCl3, 300 MHz): d 7.38, 6.77 (dd, 4H, C6H4),
5.56 (s, 1H, OH), 2.34 (t, 2H, CCH2), 1.58–0.86 (m, 15H, alkyl
chain protons).
1
Found: C, 82.10; H, 7.40%. H NMR (CDCl3, 300 MHz): d
8.31 (s, 4H, C6H4), 7.57, 7.22 (dd, 8H, C6H4), 2.36 (t, 4H,
CCH2), 1.58–0.86 (m, 30H, alkyl chain protons). 13C NMR
(CDCl3, 300 MHz): d 163.8, 150.8, 133.8, 133.7, 130.3, 121.7,
120.2, 85.2, 74.8, 73.7, 64.9, 31.8, 29.1, 29.0, 28.9, 28.2, 22.6,
19.6, 14.1. IR (KBr pellet, cm21): 2923, 2853, 2242, 2145, 1740,
1596, 1501, 1266, 1199.
1-(4-Hydroxyphenyl)-4-decylbutadiyne (5)
To a solution of 4-ethynylphenol (2.88 g, 24.4 mmol) and
1-dodecyne (10 ml, 46.8 mmol) in pyridine–methanol (1 : 1 v/v,
200 ml) was added copper(II) acetate (8.85 g, 48.8 mmol), and
the reaction mixture was refluxed for 6 h under nitrogen.
Insoluble solids were removed by filtration. After evaporation
of the solvent, the product was isolated by column chromato-
graphy on silica gel (ethyl acetate–hexane ~ 1/3 v/v, yield
40%).
1H NMR (CDCl3, 300 MHz): d 7.38, 6.77 (dd, 4H, C6H4),
5.07 (s, 1H, OH), 2.34 (t, 2H, CCH2), 1.58–0.85 (m, 19H, alkyl
chain protons).
Bis(4-decylbutadiynylphenyl) terephthalate (10)
This compound was prepared from terephthaloyl dichloride
(1.0 g, 4.92 mmol) and 1-(4-hydroxyphenyl)-4-decylbutadiyne
(2.78 g, 9.84 mmol) as described for compound 6 (yield 48%).
Elemental analysis. Calc. for C48H54O4: C, 82.96; H, 7.83.
1
Found: C, 82.84; H, 7.85%. H NMR (CDCl3, 300 MHz): d
8.31 (s, 4H, C6H4), 7.57, 7.22 (dd, 8H, C6H4), 2.38 (t, 4H,
CCH2), 1.58–0.86 (m, 38H, alkyl chain protons). 13C NMR
(CDCl3, 300 MHz): d 163.7, 150.8, 133.8, 133.7, 130.3, 121.7,
120.2, 85.2, 74.8, 73.7, 64.9, 31.8, 29.5, 29.4, 29.3, 29.0, 28.8,
28.2, 22.6, 19.6, 14.0. IR (KBr pellet, cm21): 2918, 2850, 2243,
2152, 1737, 1595, 1501, 1279, 1204.
Bis(4-pentylbutadiynylphenyl) terephthalate (6)
To a solution of compound 1 (0.52 g, 2.5 mmol) in THF
(100 ml) was added NaH (72 mg, 3.0 mmol). After the mix-
ture was stirred for 1 h at room temperature, a solution of
J. Mater. Chem., 2003, 13, 986–990
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Bis(4-octylbutadiynylphenyl) isophthalate (11)
reaction23–25 of 1-bromoalkynes with 4-ethynylphenol, whereas
compound 5 was prepared by oxidative coupling reaction of
1-dodecyne with 4-ethynylphenol in the presence of copper(II)
acetate in a solution of pyridine and methanol (1 : 1).26 The
oxidative reaction yielded three different diacetylene products,
and the desired unsymmetric diacetylene was isolated by column
chromatography on silica gel. Linear diacetylenes 6–10 were
obtained by an esterification reaction of the side groups with
terephthaloyl dichloride in THF and bent-shaped compound
11 by an esterification reaction of 4 with isophthaloyl dichl-
oride. The structures of the products were confirmed by 1H
and 13C NMR spectroscopy, and elemental analysis.
This compound was prepared from isophthaloyl dichloride
(0.29 g, 1.4 mmol) and 1-(4-hydroxyphenyl)-4-octylbutadiyne
(0.72 g, 2.8 mmol) as described for compound 6 (yield 35%).
Elemental analysis. Calc.for C44H46O4: C, 82.72; H, 7.26.
1
Found: C, 82.74; H, 7.47%. H NMR (CDCl3, 300 MHz): d
8.97 (t, 1H, C6H4), 8.46, (q, 2H, C6H4), 7.69 (t, 1H, C6H4), 7.56,
7.21 (dd, 8H, C6H4), 2.36 (t, 4H, CCH2), 1.60–0.86 (m, 30H,
alkyl chain protons). 13C NMR (CDCl3, 300 MHz): d 163.7,
150.8, 135.1, 133.8, 131.8, 130.0, 129.2, 121.8, 120.2, 85.2, 74.7,
73.7, 64.9, 31.8, 29.1, 29.0, 28.9, 28.2, 22.6, 19.6, 14.0. IR (KBr
pellet, cm21): 2923, 2853, 2246, 1749, 1606, 1504, 1299, 1237.
Mesomorphic behaviors
UV irradiation
We examined morphological changes of the linear ester com-
pounds by differential scanning calorimetry (DSC) and pola-
rizing optical microscopy. The transition temperatures and
corresponding enthalpy values of the compounds are summar-
ized in Table 1 and their DSC thermograms are depicted in
Fig. 1. Compound 6 with a pentyl tail showed no mesophase.
Only the melt transition on heating and crystallization on
cooling were observed in DSC and polarizing microscopy
analysis. By lengthening an alkyl tail to a hexyl, a liquid crys-
talline phase began to form. Compound 7 showed a mono-
tropic transition. In DSC analysis, on heating, the peak for
the crystal to isotropic liquid transition appeared at 135.4 uC
and a strong exotherm was observed above 200 uC, indicat-
ing that thermal polymerization took place. When this com-
pound was cooled from the isotropic liquid state to room
temperature, only the peak for crystallization occurred at
107.5 uC. Nevertheless, we could observe a mesophase between
119 and 107 uC by polarizing microscopy. This compound
Thin films of the compounds prepared on glass plates by melt
casting were placed on a Linkam heating stage equipped with a
Linkam TP 92 controller under an UV lamp (100 W short-arc
mercury lamp). Photopolymerization was carried out under
nitrogen at desired temperatures.
Results and discussion
Monomer synthesis
Linear diacetylenic side groups were synthesized according
to Scheme 1. 4-Trimethylsilylethynylphenol was prepared by the
coupling reaction of 4-iodophenol with trimethylsilylacetylene
in the presence of a palladium catalyst.15,18–22 The trimethyl-
silyl group was removed easily under basic conditions to give
4-ethynylphenol. The unsymmetric diacetylene side groups
(1–4) were prepared by the Cadiot–Chodkiewicz coupling
Scheme 1
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Table 1 Phase transition temperatures and corresponding enthalpy
values of the diacetylenic compounds
disappearance of the peaks for diacetylenic groups in the IR
spectrum. The resulting polymer was dark red and insoluble
in common organic solvents such as chloroform, which is a
good solvent for the monomer.
T/uC (DH/kJ mol21 a
)
Thermal polymerization also occurred in the liquid crystal-
line state. When compound 9 was annealed in the liquid crystal-
line state (170 uC) for 16 h, it was partially polymerized. With
a polarizing microscope, liquid crystalline phases were still
observed on heating and cooling along with the amorphous
polymer domains. About 30% of diacetylenic groups of the
compound were found to be consumed when compared the
absorbances of the diacetylene peaks in the IR spectra obtained
before and after polymerization.
Compound
Heating
Cooling
6
7
8
9
10
11
K 145.2 (27) I
K 135.4 (25) I
I 128.4 (27) K
I 119b N 107.5 (26) K
I 196b N 105.5 (26) K
I 210b N 95.5 (25) K
I 204b N 87.3 (27) K
I 94.4 (35) K
K 120.9 (27) N 196b
K 105.0 (26) N 210b
K 103.7 (28) N 214b
K 106.2 (35) I
I
I
I
aK ~ crystalline; N ~ nematic; I ~ isotropic. bDetermined by
polarizing optical microscopy.
When exposed to UV light in the liquid crystalline state
polymerization proceeded rapidly. Compound 9 was fabricated
into a film on a glass plate by melt casting and then poly-
merized with UV light (100 W short-arc mercury lamp) for
30 min at 170 uC. After polymerization, compound 9 turned
into an amorphous dark red polymer which was not soluble
in common organic solvents. In the IR spectrum (Fig. 2), two
bands at 2145 and 2242 cm21 corresponding to the diacetylenic
groups decreased by more than 90% and a weak band at
2203 cm21 emerged. In addition, the band around 1600 cm21
became broader and stronger due to the formation of C–C
double bonds. In the UV-Vis spectra (Fig. 3), the monomer
had an absorption limit of about 400 nm, whereas the absorp-
tion of the polymer occurred up to about 600 nm, arising from
the short conjugated polydiacetylene backbones.
Fig. 1 DSC thermograms of the diacetylenic compounds obtained with
a heating rate of 10 uC min21 and a cooling rate of 5 uC min21 under
nitrogen. For all compounds the second cooling and third heating
traces are shown.
exhibited a Schlieren texture that was characteristic of a
nematic phase. Compound 8 with a heptyl tail showed an
enantiotropic transition. DSC analysis showed the peak for the
crystal to liquid crystal transition at 120.9 uC on heating and
in the cooling scan, the peak for crystallization appeared at
105.5 uC. We could also detect a broad mesophase range by
polarizing optical microscopy. A Schlieren texture correspond-
ing to a nematic phase was observed from 120.9 to 196 uC
on heating. Upon cooling from the isotropic state, the same
texture emerged at 196 uC. On further cooling to 105 uC
crystals began to form. Although the transition of the liquid
crystal to the isotropic liquid was observed by polarizing
optical microscopy, interestingly the peak for the corresponding
transition did not appear in the DSC thermogram. This was
ascribed to the fact that the thermal polymerization began to
take place immediately after the isotropization. Compounds 9
and 10 also showed enantiotropic transitions and the temper-
ature ranges of the liquid crystalline phases were broader than
that of compound 8. This shows that the mesophase became
more stable as the flexible alkyl chain length was increased.
Isophthalate 11, a bent-shaped molecule with an octyl tail
showed the melt transition at almost the same temperature
as the corresponding terephthalate derivative (9), but did not
form any mesophases.
Fig. 2 IR spectra of (a) compound 9 (KBr) and (b) the polymer obtained
upon liquid crystalline state polymerization. Inset: IR spectra of the
diacetylene vibration range.
Polymerization
All compounds were thermally polymerized in their isotropic
states. DSC showed strong exotherms corresponding to the
polymerization around 260 uC. For compound 9 the poly-
merization was completed on heating on a glass plate under
nitrogen at 220 uC for 20 min, as determined by the
Fig. 3 UV spectra of (a) compound 9 and (b) the polymer as a film
cast on a glass plate.
J. Mater. Chem., 2003, 13, 986–990
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Research Center at Seoul National University is gratefully
acknowledged. This paper is dedicated to Prof. Won-Jei Cho
on the occasion of his retirement.
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Photoimaging
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8
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photoimaging materials. These compounds were thermally
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be used for photoimaging.
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