Liquid crystalline conjugated oligomers: Synthesis and mesomorphic properties of laterally and terminally alkyl-substituted oligo(1,4- phenyleneethynylene)s

Article abstract of DOI:10.1039/b808036d

A new series of liquid crystalline conjugated oligomers, namely laterally and terminally alkyl-substituted derivatives of oligo(1,4-phenyleneethynylene)s, were synthesized, and the relationships between the molecular structures and the mesomorphic properties were investigated. The mesomorphic properties of the oligomer derivatives were examined by differential scanning calorimetry and polarizing microscopy. In the alkyl-substituted oligomers with varying core lengths, C₈H₁₇C₆H₄CC[C ₆H₂(C₂H₅)₂CC] _nC₆H₄C₈H₁₇ where n = 2-5, the trimer (n = 2) exhibited a monotropic nematic phase, and the others showed enantiotropic nematic phases. In the alkyl-substituted tetramers, R ¹C₆H₄CC[C₆H₂(R ²)₂CC][C₆H₂(R³) ₂CC][C₆H₂(R²)₂CC]C ₆H₄R¹ where R¹, R², and R³ = alkyl chains of varying lengths and H, most of the tetramer derivatives exhibited nematic phases, whereas the others did not show any mesophases. It was found that the formation of the mesophase and the crystalline-nematic and nematic-isotropic transition temperatures are strongly affected by the structural features of the oligomer molecules: (i) the length of the oligomer core, (ii) the length, number, and position of the lateral chains, and (iii) the length of the terminal chains. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b808036d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Liquid crystalline conjugated oligomers: synthesis and mesomorphic
properties of laterally and terminally alkyl-substituted oligo
(1,4-phenyleneethynylene)s†
*
Tetsuo Yatabe, Yasuzo Suzuki and Yuji Kawanishi
Received 12th May 2008, Accepted 11th July 2008
First published as an Advance Article on the web 13th August 2008
DOI: 10.1039/b808036d
A new series of liquid crystalline conjugated oligomers, namely laterally and terminally alkyl-
substituted derivatives of oligo(1,4-phenyleneethynylene)s, were synthesized, and the relationships
between the molecular structures and the mesomorphic properties were investigated. The mesomorphic
properties of the oligomer derivatives were examined by differential scanning calorimetry and
polarizing microscopy. In the alkyl-substituted oligomers with varying core lengths,
C8H17C6H4CC[C6H2(C2H5)2CC]nC6H4C8H17 where n ¼ 2–5, the trimer (n ¼ 2) exhibited a monotropic
nematic phase, and the others showed enantiotropic nematic phases. In the alkyl-substituted tetramers,
R1C6H4CC[C6H2(R2)2CC][C6H2(R3)2CC][C6H2(R2)2CC]C6H4R1 where R1, R2, and R3 ¼ alkyl chains
of varying lengths and H, most of the tetramer derivatives exhibited nematic phases, whereas the others
did not show any mesophases. It was found that the formation of the mesophase and the crystalline–
nematic and nematic–isotropic transition temperatures are strongly affected by the structural features
of the oligomer molecules: (i) the length of the oligomer core, (ii) the length, number, and position of the
lateral chains, and (iii) the length of the terminal chains.
Introduction
Liquid crystals are functional self-assembling structures that
combine the fluidity and orientational order of the constituent
molecules.1–5 Recently, liquid crystals formed by linear conju-
gated oligomers have attracted considerable attention because of
their anisotropic optical and electrical properties such as photo-
Fig. 1 Schematic representation of a conjugated oligomer with lateral
and electroluminescence, charge carrier mobility, etc.6–18 These
properties depend on both the electrons delocalized over the
oligomer backbone and the liquid crystalline order of the
conjugated molecules.8,11,19,20 However, most of the recent
interest has focused on the optical and electrical properties, and
there have been few systematic studies of the mesomorphic
properties.21,22 The design and synthesis of conjugated oligomer
derivatives for the understanding and control of the mesomor-
phic properties should constitute an important part of oligomer
liquid crystal research.
and terminal chains.
(iii) the length and number of the terminal chains. It has been
reported that some derivatives of conjugated oligomers, for
example, phenylene-,25–27 fluorene-,28–30 phenylenevinylene-,31–34
and phenyleneethynylene-based35,36 oligomers, exhibit liquid
crystalline mesophases. In these oligomer derivatives, however,
few systematic structural variations have been achieved in either
the oligomer core or the lateral and terminal chains. There are
too few examples to allow us to understand the relationships
between the molecular structures and the mesomorphic proper-
ties of liquid crystalline conjugated oligomers.
Low molecular weight mesogens are generally composed of
a rod-like core (e.g. biphenyl) and one or two flexible end-groups
(e.g. alkyl and alkoxy chains).1,23,24 When the core is a conjugated
oligomer backbone, in addition to terminal chains, several lateral
chains are needed to increase the solubility and to reduce the
transition temperatures (Fig. 1). The mesomorphic properties of
the conjugated oligomer derivatives should depend on the
following structural features: (i) the shape and length of the core,
(ii) the length, number, and positions of the lateral chains, and
We chose oligo(1,4-phenyleneethynylene) derivatives as
a suitable family with which to investigate the structure–property
relationships for a number of reasons. First, the tolane (diphe-
nylacetylene) derivatives are representatives of liquid crystals
that form nematic phases.37–40 The mesophase is the simplest
liquid crystal phase, in which the molecules have only orienta-
tional long-range order.1 It has also been reported that the
phenyleneethynylene-based dimers, R(C6H4CC)2C6H4R where
R ¼ alkyl41,42 and alkoxy43,44 chains, and the dimers with a single
lateral ethyl chain, RC6H4CC[C6H3(C2H5)CC]C6H4R where
R ¼ alkyl chains,45,46 exhibit nematic phases. Therefore, it is
expected that laterally and terminally alkyl-substituted deriva-
tives of the longer oligomers will also exhibit nematic phases.
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan.
E-mail: tetsuo-yatabe@aist.go.jp
† Electronic supplementary information (ESI) available: Synthesis
procedures and characterization data for compounds 7c–f, 8b–e, 9b–e,
14a,b and 15a,b. See DOI: 10.1039/b808036d
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Second, the oligomer backbones are linear cores with very little
conformational deviation.47,48 One would expect it to be easier to
understand the structure–property relationships of the alkyl-
substituted derivatives because they have straightforward
molecular structures and the simplest mesophase. Finally,
convenient methods have been developed for synthesizing
monodisperse linear oligomers.49–52 The synthetic procedures
involve the Sonogashira coupling reaction53 of aryl halides with
terminal alkynes and the deprotection of silylalkynes. These
reactions proceed with excellent chemoselectivity under mild
conditions to give the desired products with high yields. There-
fore, various types of alkyl-substituted derivatives of the
oligomers can be easily synthesized.
6a,b, and 7a–f, which are related to tetramer 2, were designed and
synthesized. Tetramers 5a, 6a, and 7a–f exhibited nematic pha-
ses, whereas tetramers 5b,c and 6b did not. Their mesomorphic
properties were found to be strongly affected by the length,
number, and position of the lateral chains. In addition, tetramers
8a–e and 9a–e were synthesized to investigate the effects of the
lateral and terminal chains on the mesomorphic properties. We
found that their transition temperatures are strongly affected by
both the structure of the lateral chains and the length of the
terminal chains.
Experimental
In the present study, we synthesized a series of laterally and
terminally alkyl-substituted oligo(1,4-phenyleneethynylene)s,
C8H17C6H4CC[C6H2(C2H5)2CC]nC6H4C8H17 where n ¼ 2–5
(1–4, Fig. 2), and investigated their mesomorphic properties.
Trimer 154 exhibited a monotropic nematic phase, and tetramer
2, pentamer 3, and hexamer 4 exhibited enantiotropic nematic
phases. The results show that the oligomer core length is an
important factor in mesophase behavior. Next, tetramers 5a–c,
General methods
Column chromatography was performed with Merck silica gel 60
(0.063–0.200 mm). The final products were purified by high
performance liquid chromatography (HPLC), which was carried
out with a preparative chromatograph equipped with a JASCO
PU-2086 HPLC pump, a JASCO RI-2031 refractive index
detector, and two GPC columns in series (Shodex K-2002 and
Fig. 2 Structures of oligomers 1–4, 5a–c, 6a,b, 7a–f, 8a–e, and 9a–e.
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2002.5). Chloroform was used as eluent at a flow rate of 3.0 or
3.5 mL minꢀ1. The chemical structures of the compounds were
characterized by 1H and 13C NMR spectra, recorded in CDCl3 on
a Varian INOVA 400 spectrometer (400 MHz for protons,
100 MHz for carbon) using tetramethylsilane as an internal
standard. The melting points were determined with a Laboratory
Devices Mel-Temp II melting point apparatus and are uncor-
rected. Elemental analyses were carried out on a CE Instruments
EA 1110. Atmospheric pressure chemical ionization mass
spectrometry (APCI-MS) was conducted using a Finnigan AQA
mass spectrometer.
CuI (22 mg, 0.12 mmol) in piperidine (10 mL) was stirred for 18 h
at room temperature. Cyclohexane (50 mL) was added to the
reaction mixture. The mixture was washed with an NH4Cl
aqueous solution. The organic layer was dried over Na2SO4 and
concentrated under reduced pressure. The residue was chroma-
tographed on silica gel with CH2Cl2–hexane (1 : 4). The crude
product was purified by HPLC. Yield: 312 mg (82%) of a white
solid; phase transition (ꢁC): Cr 74 I. 1H NMR (CDCl3): d ¼ 0.88
(t, J ¼ 6.8 Hz, 6H), 1.23–1.36 (m, 32H), 1.57–1.66 (m, 4H), 2.62
(t, J ¼ 7.6 Hz, 4H), 2.86 (q, J ¼ 7.6 Hz, 4H), 2.87 (q, J ¼ 7.6 Hz,
4H), 7.17 (d, J ¼ 8.0 Hz, 4H), 7.39 (s, 4H), 7.45 (d, J ¼ 8.0 Hz,
4H); 13C NMR (CDCl3): d ¼ 14.10, 14.73, 14.97, 22.66, 27.21,
27.26, 29.25, 29.45, 31.29, 31.87, 35.93, 87.57, 92.81, 94.37,
120.55, 122.45, 122.71, 128.51, 131.40, 131.52, 131.65. Elemental
analysis: calc. for C54H66: C, 90.70; H, 9.30; found: C, 90.79; H,
9.22%. APCI-MS: m/z ¼ 715.9 ([M + H]+, calc. 715.5).
The mesomorphic properties of the compounds were examined
by differential scanning calorimetry (DSC) and polarizing
microscopy. The transition temperatures and the associated
enthalpy values were determined on a Perkin-Elmer DSC 7
differential scanning calorimeter equipped with a Perkin-Elmer
TAC 7/DX thermal analysis controller. The apparatus was
operated at a scanning rate of 2 C minꢀ1 for both heating and
Compound 2. Synthesized by the procedure described for 1.
Quantities: 13 (259 mg, 0.524 mmol), 10f (406 mg, 1.28 mmol),
Pd(PPh3)2Cl2 (23 mg, 0.033 mmol), CuI (22 mg, 0.12 mmol), and
piperidine (10 mL). Yield: 368 mg (81%) of a pale yellow solid;
phase transitions (ꢁC): Cr 100 N 137 I. H NMR (CDCl3): d ¼
ꢁ
cooling. Indium (156.60 ꢁC; 28.45 J gꢀ1) was employed as
a standard for calibrating temperature and enthalpy. Phase
identification was performed using polarizing microscopy. The
optical textures in the mesophases were observed using a Leitz
Orthoplan-Pol polarizing microscope equipped with a Mettler
FP82HT hot stage and a Mettler FP80 central processor.
1
0.88 (t, J ¼ 7.0 Hz, 6H), 1.23–1.36 (m, 38H), 1.57–1.66 (m, 4H),
2.62 (t, J ¼ 7.6 Hz, 4H), 2.86 (q, J ¼ 7.6 Hz, 4H), 2.87 (q, J ¼
7.6 Hz, 4H), 2.88 (q, J ¼ 7.6 Hz, 4H), 7.17 (d, J ¼ 8.2 Hz, 4H),
7.39 (s, 4H), 7.41 (s, 2H), 7.45 (d, J ¼ 8.2 Hz, 4H); 13C NMR
(CDCl3): d ¼ 14.10, 14.72, 14.98, 15.00, 22.66, 27.21, 27.27,
27.30, 29.24, 29.26, 29.45, 31.29, 31.87, 35.93, 87.56, 92.77, 92.98,
94.41, 120.55, 122.41, 122.68, 122.75, 128.51, 131.40, 131.53,
131.66, 131.73, 143.23, 143.29, 143.39, 143.52. Elemental
analysis: calc. for C66H78: C, 90.98; H, 9.02; found: C, 90.83; H,
8.85%. APCI-MS: m/z ¼ 872.3 ([M + H]+, calc. 871.6).
Materials
Bis(triphenylphosphine)palladium(II) dichloride and copper(I)
iodide were purchased from Tokyo Chem. Ind. (TCI) and Wako
Pure Chem. Ind. (Wako), respectively. Piperidine and tetrahy-
drofuran were purchased from Wako and used without further
purification for the Sonogashira coupling. Iodobenzene (10a),
4-iodotoluene (10b), and 1-alkyl-4-iodobenzenes where alkyl ¼
ethyl, propyl, and butyl (10c–e) were purchased from Aldrich,
TCI, and Wako, respectively. 1-Iodo-4-octylbenzene (10f) was
prepared by a halogen exchange reaction50b with commercially
available 1-bromo-4-octylbenzene from TCI. Starting with
commercially available 1,4-dibromo-2,5-diethylbenzene from
Junsei Chem., (2,5-diethyl-4-iodophenyl)ethynyltrimethylsilane
(11)55a was prepared in the same way as that described for the
dihexyl-substituted derivative.50b
Compound 3. Synthesized by the procedure described for 1.
Quantities: 14b (253 mg, 0.389 mmol), 10f (378 mg, 1.20 mmol),
Pd(PPh3)2Cl2 (18 mg, 0.026 mmol), CuI (14 mg, 0.074 mmol),
and piperidine–THF (1 : 1, 40 mL). Yield: 304 mg (76%) of a pale
1
yellow solid; phase transitions (ꢁC): Cr 122 N 201 I. H NMR
(CDCl3): d ¼ 0.88 (t, J ¼ 6.8 Hz, 6H), 1.23–1.36 (m, 44H),
1.57–1.66 (m, 4H), 2.62 (t, J ¼ 7.6 Hz, 4H), 2.86 (q, J ¼ 7.6 Hz,
4H), 2.87 (q, J ¼ 7.6 Hz, 4H), 2.88 (q, J ¼ 7.6 Hz, 8H), 7.17
(d, J ¼ 8.2 Hz, 4H), 7.39 (s, 4H), 7.41 (s, 4H), 7.45 (d, J ¼ 8.2 Hz,
4H); 13C NMR (CDCl3): d ¼ 14.10, 14.72, 14.99, 15.01, 22.67,
27.22, 27.27, 27.30, 29.24, 29.26, 29.45, 31.29, 31.87, 35.93, 87.56,
92.76, 92.94, 93.02, 94.42, 120.55, 122.40, 122.65, 122.74, 122.77,
128.51, 131.40, 131.53, 131.66, 131.75, 143.24, 143.31, 143.40,
143.53. Elemental analysis: calc. for C78H90: C, 91.17; H, 8.83;
found: C, 91.10; H, 8.67%. APCI-MS: m/z ¼ 1028.6 ([M + H]+,
calc. 1027.7)
Tetraethyl-substituted dimer 12 was prepared from iodo-
benzene 11 according to the procedure described for the related
dimer without lateral chains.51a Hexaethyl-substituted trimer
13 was prepared by Sonogashira coupling of 1,4-diethyl-2,5-
diethynylbenzene55b with iodobenzene 11 and then desilylation
according to the procedure described for the hexahexyl-
substituted trimer.52a Alkyl-substituted trimers 16a–c, 17a,b, and
18a–f were also prepared by Sonogashira coupling and then
desilylation using a previously described procedure.50,52a
Compound 4. Synthesized by the procedure described for 1.
Quantities: 15b (259 mg, 0.524 mmol), 10f (406 mg, 1.28 mmol),
Pd(PPh3)2Cl2 (23 mg, 0.033 mmol), CuI (22 mg, 0.12 mmol), and
piperidine–THF (1 : 1, 40 mL). Yield: 387 mg (76%) of a yellow
solid; phase transitions (ꢁC): Cr 159 N 258 I. 1H NMR (CDCl3):
d ¼ 0.89 (t, J ¼ 6.8 Hz, 6H), 1.23–1.36 (m, 50H), 1.57–1.66
(m, 4H), 2.62 (t, J ¼ 7.6 Hz, 4H), 2.83–2.93 (m, 20H), 7.17 (d, J ¼
8.2 Hz, 4H), 7.40 (s, 4H), 7.41 (s, 6H), 7.45 (d, J ¼ 8.2 Hz, 4H);
13C NMR (CDCl3): d ¼ 14.10, 14.73, 14.99, 15.02, 22.67, 27.22,
Synthesis
Synthetic procedures and characterization data for intermediates
14a,b and 15a,b and tetramers 7c–f, 8b–e, and 9b–e are described
in the ESI.†
Compound 1. A mixture of 12 (180 mg, 0.533 mmol), 10f
(417 mg, 1.32 mmol), Pd(PPh3)2Cl2 (22 mg, 0.031 mmol), and
4470 | J. Mater. Chem., 2008, 18, 4468–4477
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27.27, 27.30, 29.26, 29.45, 31.29, 31.87, 35.93, 87.56, 92.76, 92.93,
92.97, 93.02, 94.43, 120.55, 122.40, 122.64, 122.70, 122.75,
122.77, 128.51, 131.40, 131.53, 131.66, 131.76, 143.24, 143.32,
143.41, 143.53. Elemental analysis: calc. for C90H102: C, 91.32; H,
8.68; found: C, 91.31; H, 8.49%. APCI-MS: m/z ¼ 1184.7
([M + H]+, calc. 1184.8).
Pd(PPh3)2Cl2 (13 mg, 0.019 mmol), CuI (13 mg, 0.068 mmol),
and piperidine–THF (2 : 1, 15 mL). Yield: 208 mg (71%) of a pale
yellow solid; phase transitions (ꢁC): Cr 97 N 155 I. H NMR
1
(CDCl3): d ¼ 0.88 (t, J ¼ 6.8 Hz, 6H), 1.23–1.36 (m, 32H), 1.57–
1.66 (m, 4H), 2.62 (t, J ¼ 7.6 Hz, 4H), 2.85 (q, J ¼ 7.6 Hz, 8H),
7.17 (d, J ¼ 8.2 Hz, 4H), 7.39 (s, 4H), 7.45 (d, J ¼ 8.2 Hz, 4H),
7.51 (s, 4H); 13C NMR (CDCl3): d ¼ 14.10, 14.68, 14.73, 22.67,
27.17, 29.24, 29.26, 29.45, 31.29, 31.87, 35.93, 87.52, 90.25, 93.70,
94.50, 120.52, 121.96, 122.96, 123.26, 128.51, 131.41, 131.43,
131.50, 131.55, 143.39, 143.45, 143.55. Elemental analysis: calc.
for C62H70: C, 91.35; H, 8.65; found: C, 91.25; H, 8.45%. APCI-
MS: m/z ¼ 816.2 ([M + H]+, calc. 815.6).
Compound 5a. Synthesized by the procedure described for 1.
Quantities: 16a (210 mg, 0.402 mmol), 10f (356 mg, 1.13 mmol),
Pd(PPh3)2Cl2 (28 mg, 0.040 mmol), CuI (33 mg, 0.17 mmol), and
piperidine (15 mL). Yield: 261 mg (72%) of a pale yellow solid;
1
phase transition (ꢁC): Cr 103 I. H NMR (CDCl3): d ¼ 0.88
(t, J ¼ 6.8 Hz, 6H), 1.03 (t, J ¼ 7.6 Hz, 6H), 1.23–1.36 (m, 32H),
1.57–1.66 (m, 4H), 1.70–1.81 (m, 4H), 2.62 (t, J ¼ 7.6 Hz, 4H),
2.82 (t, J ¼ 7.6 Hz, 4H), 2.86 (q, J ¼ 7.6 Hz, 4H), 2.87 (q, J ¼
7.6 Hz, 4H), 7.17 (d, J ¼ 8.2 Hz, 4H), 7.386 (s, 2H), 7.389 (s, 4H),
7.45 (d, J ¼ 8.2 Hz, 4H); 13C NMR (CDCl3): d ¼ 14.06, 14.10,
14.69, 14.93, 22.66, 23.91, 27.20, 29.24, 29.26, 29.45, 31.29, 31.87,
35.93, 36.28, 87.57, 92.85, 93.02, 94.41, 120.55, 122.45, 122.73,
122.77, 128.51, 131.40, 131.47, 131.66, 132.56, 141.76, 143.13,
143.38, 143.52. Elemental analysis: calc. for C68H82: C, 90.81;
H, 9.19; found: C, 91.12; H, 9.09%. APCI-MS: m/z ¼ 900.3
([M + H]+, calc. 899.6).
Compound 6b. Synthesized by the procedure described for 1.
Quantities: 17b (179 mg, 0.362 mmol), 10f (294 mg, 0.930 mmol),
Pd(PPh3)2Cl2 (18 mg, 0.026 mmol), CuI (17 mg, 0.089 mmol),
and piperidine (20 mL). Yield: 221 mg (70%) of a pale yellow
solid; phase transition (ꢁC): Cr 94 I. 1H NMR (CDCl3): d ¼ 0.88
(t, J ¼ 7.0 Hz, 6H), 1.01 (t, J ¼ 7.2 Hz, 6H), 1.02 (t, J ¼ 7.2 Hz,
6H), 1.23–1.36 (m, 20H), 1.57–1.66 (m, 4H), 1.69–1.80 (m, 8H),
2.62 (t, J ¼ 7.6 Hz, 4H), 2.80 (t, J ¼ 7.6 Hz, 8H), 7.17 (d, J ¼
8.2 Hz, 4H), 7.36 (s, 4H), 7.44 (d, J ¼ 8.2 Hz, 4H), 7.50 (s, 4H);
13C NMR (CDCl3): d ¼ 14.05, 14.10, 22.66, 23.72, 23.77, 29.23,
29.26, 29.45, 31.28, 31.87, 35.93, 36.16, 87.73, 90.47, 93.57, 94.37,
120.55, 122.07, 123.07, 123.28, 128.52, 131.38, 131.40, 132.31,
132.37, 141.93, 142.00, 143.52. Elemental analysis: calc. for
C66H78: C, 90.98; H, 9.02; found: C, 90.98; H, 8.89%. APCI-MS:
m/z ¼ 872.3 ([M + H]+, calc. 871.6).
Compound 5b. Synthesized by the procedure described for 1.
Quantities: 16b (204 mg, 0.336 mmol), 10f (285 mg, 0.901 mmol),
Pd(PPh3)2Cl2 (21 mg, 0.030 mmol), CuI (20 mg, 0.11 mmol), and
piperidine (20 mL). Yield: 223 mg (68%) of a pale yellow solid;
1
phase transition (ꢁC): Cr 91 I. H NMR (CDCl3): d ¼ 0.89 (t,
J ¼ 6.8 Hz, 12H), 1.23–1.37 (m, 40H), 1.47–1.38 (m, 4H), 1.57–
1.66 (m, 4H), 1.66–1.76 (m, 4H), 2.62 (t, J ¼ 7.6 Hz, 4H), 2.83
(t, J ¼ 7.6 Hz, 4H), 2.86 (q, J ¼ 7.6 Hz, 4H), 2.87 (q, J ¼ 7.6 Hz,
4H), 7.17 (d, J ¼ 8.2 Hz, 4H), 7.38 (s, 2H), 7.39 (s, 4H), 7.45
(d, J ¼ 8.2 Hz, 4H); 13C NMR (CDCl3): d ¼ 14.10, 14.70, 14.88,
22.66, 27.19, 29.24, 29.26, 29.34, 29.45, 30.81, 31.29, 31.81, 31.87,
34.31, 35.93, 87.58, 92.84, 93.04, 94.40, 120.55, 122.48, 122.71,
122.73, 128.51, 131.40, 131.44, 131.66, 132.45, 142.00, 143.12,
143.37, 143.52. Elemental analysis: calc. for C74H94: C, 90.37; H,
9.63; found: C, 90.39; H, 9.53%. APCI-MS: m/z ¼ 984.6
([M + H]+, calc. 983.7).
Compound 7a. Synthesized by the procedure described for 1.
Quantities: 18a (156 mg, 0.408 mmol), 10f (311 mg, 0.984 mmol),
Pd(PPh3)2Cl2 (15 mg, 0.021 mmol), CuI (17 mg, 0.089 mmol),
and piperidine–THF (1 : 1, 20 mL). Yield: 231 mg (75%) of
1
a white solid; phase transitions (ꢁC): Cr 158 N 261 I. H NMR
(CDCl3): d ¼ 0.88 (t, J ¼ 6.8 Hz, 6H), 1.23–1.36 (m, 26H), 1.57–
1.66 (m, 4H), 2.62 (t, J ¼ 7.6 Hz, 4H), 2.86 (t, J ¼ 7.6 Hz, 4H),
7.17 (d, J ¼ 8.0 Hz, 4H), 7.39 (s, 2H), 7.45 (d, J ¼ 8.0 Hz, 4H),
7.50 (s, 8H); 13C NMR (CDCl3): d ¼ 14.10, 14.69, 22.66, 27.15,
29.23, 29.25, 29.44, 31.23, 31.86, 35.93, 88.51, 89.99, 91.60, 93.97,
120.10, 122.44, 122.98, 123.40, 128.51, 131.39, 131.50, 131.53,
131.60, 143.50, 143.73. Elemental analysis: calc. for C58H62: C,
91.77; H, 8.23; found: C, 91.43; H, 8.00%. APCI-MS: m/z ¼ 760.0
([M + H]+, calc. 759.5).
Compound 5c. Synthesized by the procedure described for 1.
Quantities: 16c (233 mg, 0.385 mmol), 10f (348 mg, 1.10 mmol),
Pd(PPh3)2Cl2 (21 mg, 0.030 mmol), CuI (22 mg, 0.12 mmol), and
piperidine (10 mL). Yield: 281 mg (76%) of a pale yellow solid;
phase transition (ꢁC): Cr 102 I. 1H NMR (CDCl3): d ¼ 0.88 (t, J ¼
6.8 Hz, 6H), 1.01 (t, J ¼ 7.6 Hz, 6H), 1.017 (t, J ¼ 7.6 Hz, 6H),
1.021 (t, J ¼ 7.6 Hz, 6H), 1.23–1.36 (m, 20H), 1.57–1.66 (m, 4H),
1.70–1.81 (m, 12H), 2.62 (t, J ¼ 7.6 Hz, 4H), 2.77–2.85 (m, 12H),
7.17 (d, J ¼ 8.2 Hz, 4H), 7.37 (s, 4H), 7.38 (s, 2H), 7.44 (d, J ¼ 8.2
Hz, 4H); 13C NMR (CDCl3): d ¼ 14.01, 14.07, 14.10, 22.66,
23.75, 23.79, 29.24, 29.26, 29.45, 31.29, 31.87, 35.93, 36.15, 36.20,
87.79, 92.89, 93.07, 94.26, 120.59, 122.56, 122.81, 122.83, 128.52,
131.37, 132.27, 132.55, 141.61, 141.64, 141.91, 143.50. Elemental
analysis: calc. for C72H90: C, 90.51; H, 9.49; found: C, 90.58; H,
9.39%. APCI-MS: m/z ¼ 956.5 ([M + H]+, calc. 955.7).
Compound 7b. Synthesized by the procedure described for 1.
Quantities: 18b (157 mg, 0.383 mmol), 10f (328 mg, 1.04 mmol),
Pd(PPh3)2Cl2 (14 mg, 0.020 mmol), CuI (17 mg, 0.089 mmol),
and piperidine–THF (1 : 1, 20 mL). Yield: 221 mg (73%) of
ꢁ
1
a white solid; phase transitions ( C): Cr 140 N 209 I. H NMR
(CDCl3): d ¼ 0.88 (t, J ¼ 7.0 Hz, 6H), 1.01 (t, J ¼ 7.6 Hz, 6H),
1.23–1.36 (m, 20H), 1.57–1.66 (m, 4H), 1.75 (tq, J ¼ 7.6 Hz, 8H),
2.61 (t, J ¼ 7.6 Hz, 4H), 2.80 (t, J ¼ 7.6 Hz, 4H), 7.17
(d, J ¼ 8.2 Hz, 4H), 7.37 (s, 2H), 7.45 (d, J ¼ 8.2 Hz, 4H), 7.47–
7.52 (m, 8H); 13C NMR (CDCl3): d ¼ 14.04, 14.10, 22.67, 23.74,
29.24, 29.26, 29.44, 31.24, 31.87, 35.93, 36.14, 88.51, 90.21, 91.60,
93.84, 120.10, 122.56, 123.01, 123.28, 128.51, 131.35, 131.51,
131.53, 132.43, 142.04, 143.73. Elemental analysis: calc. for
C60H66: C, 91.55; H, 8.45; found: C, 91.52; H, 8.27%. APCI-MS:
m/z ¼ 788.1 ([M + H]+, calc. 787.5).
Compound 6a. Synthesized by the procedure described for 1.
Quantities: 17a (158 mg, 0.361 mmol), 10f (258 mg, 0.816 mmol),
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Compound 8a. Synthesized by the procedure described for 1.
Quantities: 13 (158 mg, 0.320 mmol), 10a (214 mg, 1.05 mmol),
Pd(PPh3)2Cl2 (15 mg, 0.021 mmol), CuI (15 mg, 0.079 mmol),
and piperidine (15 mL). Yield: 155 mg (75%) of a pale yellow
solid; phase transition (ꢁC): Cr 177 I. 1H NMR (CDCl3): d ¼ 1.32
(t, J ¼ 7.6 Hz, 6H), 1.325 (t, J ¼ 7.6 Hz, 6H) 1.327 (t, J ¼ 7.6 Hz,
6H), 2.87 (q, J ¼ 7.6 Hz, 4H), 2.877 (q, J ¼ 7.6 Hz, 4H), 2.884
(q, J ¼ 7.6 Hz, 4H), 7.33–7.39 (m, 6H), 7.405 (s, 4H), 7.411
(s, 2H), 7.52–7.56 (m, 2H); 13C NMR (CDCl3): d ¼ 14.74, 14.97,
15.00, 27.21, 27.27, 27.30, 88.21, 92.86, 92.93, 94.11, 120.50,
122.64, 122.68, 123.45, 128.28, 128.38, 131.50, 131.61, 131.69,
131.75, 143.26, 143.31, 143.49. Elemental analysis: calc. for
C50H46: C, 92.83; H, 7.17; found: C, 92.76; H, 6.82%. APCI-MS:
m/z ¼ 647.6 ([M + H]+, calc. 647.4).
chains and two terminal acetylenes, and 1-iodo-4-octylbenzene
10f gave trimer 1 with an 82% yield. The cross-coupling reaction
of laterally hexaethyl-substituted trimer 13 with iodobenzene 10f
afforded tetramer 2 with an 81% yield. Trimer 1 is a white solid,
whereas tetramer 2 is a pale yellow solid. These oligomers are
soluble in common organic solvents such as hexane, dichloro-
methane, and THF.
Silyl-protected iodophenylacetylene 11 is a key building block
for a stepwise approach to oligomers 3 and 4 by Sonogashira
coupling. Thus, the cross-coupling between the building block
and dimer 12 gave tetramer 14a with a 79% yield, which was then
desilylated to give tetramer 14b with terminal acetylenes with
a 97% yield. The final cross-coupling between tetramer 14b and
iodobenzene 10f afforded the desired product 3 with a 76% yield.
Hexamer 4 was synthesized in the same way starting from trimer
13. Pentamer 3 is a pale yellow solid, and hexamer 4 is a yellow
solid. These oligomers are soluble in cyclohexane, dichloro-
methane, THF, and hot hexane.
Compound 9a. Synthesized by the procedure described for 1.
Quantities: 18f (170 mg, 0.309 mmol), 10a (157 mg, 0.770 mmol),
Pd(PPh3)2Cl2 (17 mg, 0.024 mmol), CuI (19 mg, 0.10 mmol), and
piperidine–THF (1 : 2, 15 mL). Yield: 157 mg (72%) of a white
solid; phase transition (ꢁC): Cr 119 I. 1H NMR (CDCl3): d ¼ 0.87
(t, J ¼ 7.0 Hz, 6H), 1.21–1.45 (m, 20H), 1.66–1.75 (m, 4H), 2.81
(t, J ¼ 7.6 Hz, 4H), 7.33–7.38 (m, 8H), 7.48–7.56 (m, 12H); 13C
NMR (CDCl3): d ¼ 14.11, 22.66, 29.28, 29.50, 29.56, 30.66,
31.90, 34.13, 89.11, 90.32, 91.29, 93.76, 122.50, 123.02, 123.10,
123.25, 128.38, 128.47, 131.37, 131.56, 131.63, 132.36, 142.33.
Elemental analysis: calc. for C54H54: C, 92.26; H, 7.74; found: C,
91.82; H, 7.39%. APCI-MS: m/z ¼ 703.8 ([M + H]+, calc. 703.4).
As shown in Scheme 2, tetramers 5a–c, 6a,b, and 7a–f, which
have lateral alkyl chains of different lengths and two terminal
octyl chains, were synthesized by Sonogashira coupling. The
cross-coupling between trimers 16a–c and iodobenzene 10f gave
tetramer 5a–c with six lateral chains with 68–76% yields. Tetra-
mers 6a,b with four lateral chains were prepared from trimers
17a,b with 71 and 70% yields, respectively. Tetramers 7a–f with
two lateral chains were prepared from trimers 18a–f with 71–80%
yields. The synthesis of tetramers 8a–e, with six ethyl lateral
chains, and tetramers 9a–e, with two octyl lateral chains, is
shown in Scheme 3. Sonogashira coupling between trimer 13 and
iodobenzene 10a–e gave tetramers 8a–e with 75–81% yields.
Tetramers 9a–e were prepared from trimer 18f in the same way
with 67–72% yields. All the alkyl-substituted tetramers are
soluble in common organic solvents such as hexane, dichloro-
methane, and THF.
Results and discussion
Synthesis
The synthesis of laterally ethyl- and terminally octyl-substituted
oligomers 1–4 is shown in Scheme 1. Sonogashira coupling
between phenyleneethynylene dimer 12, with four lateral ethyl
Scheme 1 Synthesis of alkyl-substituted oligo(1,4-phenyleneethynylene)s 1–4 with different core lengths.
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Table 1 Phase transition temperatures (ꢁC) and transition enthalpies (in
square brackets, kJ molꢀ1) of compounds 1–4
Compound
Phase transitionsa
1
2
3
4
h: Cr 73.9 [66.6] I
c: I 71.6 [ꢀ2.1] N 35.0 [ꢀ50.6] Cr
h: Cr 99.9 [43.2] N 136.5 [2.9] I
c: I 135.1 [ꢀ2.9] N 83.1 [ꢀ41.6] Cr
h: Cr 122.1 [56.4] N 200.9 [4.0] I
c: I 199.3 [ꢀ3.4] N 107.6 [ꢀ57.0] Cr
h: Cr 159.1 [52.1] N 258.4 [5.4] I
c: I 246.2 [ꢀ3.6] N 150.3 [ꢀ51.5] Cr
a
h ¼ on heating; c ¼ on cooling; Cr ¼ crystalline phase; N ¼ nematic
phase; I ¼ isotropic liquid.
Scheme 2 Synthesis of tetramers 5a–c, 6a,b, and 7a–f with lateral alkyl
chains of various lengths.
Fig. 3 Photomicrographs of the nematic phases of (a) trimer 1, (b)
tetramer 2, (c) pentamer 3, and (d) hexamer 4.
Scheme 3 Synthesis of laterally hexaethyl-substituted tetramers 8a–e
and laterally dioctyl-substituted tetramers 9a–e.
In oligomers 2–4 both the Cr–N and N–I transition tempera-
tures increased dramatically with increases in the number of
diethyl-substituted phenyleneethynylene units (Cr–N: 100, 122,
ꢁ
ꢁ
and 159 C; N–I: 137, 201, and 258 C for 2, 3, and 4, respec-
Mesomorphic properties
tively). In addition, a significant increase in the mesophase range
ꢁ
The length of the oligomer core. Thermotropic data for oligo-
mers 1–4 with different core lengths are summarized in Table 1.
Trimer 1 exhibited a monotropic nematic phase. The Cr–I
transition occurred at 74 ꢁC on heating, and on cooling from the
isotropic liquid a nematic Schlieren texture56 was observed at
temperatures between 72 and 35 ꢁC (Fig. 3(a)). Slow crystalli-
zation occurred while the temperature was kept at 35 ꢁC. Unlike
trimer 1, oligomers 2–4 exhibited enantiotropic nematic phases;
the optical texture characteristics of the mesophase were
observed both on heating and cooling (Fig. 3(b)–(d))ꢁ. For
tetramer 2 the mesophase was observed from 100 to 137 C on
was observed (37, 79, and 99 C for 2, 3, and 4, respectively).
These results show that the oligomer core length is an
important factor as regards mesophase behavior. When the
number of repeating units is increased, both the core length
and the number of lateral chains increase. The lateral chains
can disrupt the molecular packing required for mesophase
generation.1 However, the elongation of the conjugated core
causes a large increase in the attractive p–p interaction
between the cores, and the improved p–p interaction gives rise
to increases in both the transition temperatures and the mes-
ophase range.
ꢁ
heating and from 135 to 83 C on cooling. For pentamer 3 the
Since the lateral chain of oligomers 1–4 is the shortest flexible
chain, it is interesting to investigate the structure–property
relationships of the oligomers with longer lateral chains. The
mesomorphic properties of the tetramers with lateral alkyl chains
of various lengths are discussed in the next section.
mesophase was observed from 122 to 201 ꢁC on heating and from
199 to 108 ꢁC on cooling. For hexamer 4 the mesophase occurred
from 159 to 258 ꢁC on heating and from 246 to 150 ꢁC on
cooling.
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Table 2 Phase transition temperatures (ꢁC) and transition enthalpies (in
square brackets, kJ molꢀ1) of compounds 5a–c, 6a,b, and 7a–f
Compound
Phase transitionsa
5a
5b
5c
6a
6b
7a
7b
7c
7d
7e
7f
h: Cr 102.5 [58.2] I
c: I 100.5 [ꢀ2.7] N 87.1 [ꢀ56.5] Cr
h: Cr 91.4 [48.6] I
c: I 69.1 [ꢀ45.6] Cr
h: Cr 101.5 [48.3] I
c: I 84.4 [ꢀ46.3] Cr
h: Cr 97.1 [39.4] N 154.7 [2.8] I
c: I 153.4 [ꢀ2.8] N 80.6 [ꢀ43.2] Cr
h: Cr 94.3 [46.9] I
c: I 75.5 [ꢀ47.1] Cr
h: Cr 157.8 [37.7] N 261.1 [3.8] I
c: I 258.1 [ꢀ2.5] N 151.9 [ꢀ35.4] Cr
h: Cr 140.2 [40.5] N 208.6 [3.5] I
c: I 207.4 [ꢀ3.4] N 135.1 [ꢀ39.9] Cr
h: Cr 149.7 [66.8] N 182.9 [3.5] I
c: I 181.8 [ꢀ3.6] N 132.7 [ꢀ67.1] Cr
h: Cr 156.4 [54.9] N 166.9 [3.2] I
c: I 165.7 [ꢀ3.2] N 148.9 [ꢀ55.7] Cr
h: Cr 121.5 [44.9] N 155.1 [3.1] I
c: I 154.1 [ꢀ3.1] N 111.7 [ꢀ44.6] Cr
h: Cr 96.3 [49.4] N 135.0 [2.4] I
c: I 134.0 [ꢀ2.4] N 73.3 [ꢀ53.4] Cr
Fig. 4 Transition temperatures vs. number of carbon atoms (n) in the
lateral chain of laterally dialkyl-substituted tetramers 7a–f.
a
h ¼ on heating; c ¼ on cooling; Cr ¼ crystalline phase; N ¼ nematic
phase; I ¼ isotropic liquid.
Cr–N transition temperature (158, 140, 150, 156, 122, and 96 ꢁC
for 7a–f, respectively) is not as simple as that in the N–I
transition temperature. Although with tetramers 7a, 7c, 7e, and
7f, which have even-numbered carbon atoms in the lateral chain,
the Cr–N transition temperature decreased with increasing
carbon atom number, the transition temperature of pentyl
derivative 7d was higher than that of propyl derivative 7b. In
addition to these observations, the change in the mesophase
range is interesting. The mesophase range decreased with
increasing lateral chain length from ethyl to pentyl (103, 68, 33,
The length, number, and position of the lateral chains. The
thermotropic data of tetramers 5a–c, 6a,b, and 7a–f are listed in
Table 2. Although tetramer 2 with six ethyl lateral chains
exhibited the enantiotropic mesophase, tetramer 5a, in which two
propyl and four ethyl chains are attached to the central and outer
phenyl rings, respectively, showed a monotropic nematic phase.
ꢁ
The Cr–I transition occurred at 103 C and the mesophase was
ꢁ
ꢁ
observed between 101 and 87 C on cooling. Tetramer 5b, with
and 11 C for 7a–d, respectively). However, the range of hexyl
two hexyl and four ethyl chains, and tetramer 5c, with six propyl
chains, both contrasted clearly with tetramers 2 and 5a in that no
mesophases were found. The Cr–I transitions occurred at 91 and
derivative 7e (34 ꢁC) was almost the same as that of butyl
ꢁ
derivative 7c. Also the range of octyl derivative 7f (39 C) was
slightly greater than that of hexyl derivative 7e.
ꢁ
102 C for 5b and 5c, respectively. Tetramer 6a with four ethyl
These results indicate that the transition temperatures of the
tetramers with two lateral chains can be controlled by varying the
lateral chain length. The increase in the chain length from ethyl
to octyl causes a significant decrease in the mesophase stability
but does not destroy the mesophase. This is quite different from
the tetramers with six lateral chains; for example, tetramer 5b,
with hexyl chains attached to the central phenyl ring, exhibits no
mesophases.
chains exhibited an enantiotropic nematic phase; the mesophase
was observed from 97 to 155 ꢁC on heating and from 153 to 81 ꢁC
on cooling. However, tetramer 6b with four propyl chains
ꢁ
showed no mesophases; the Cr–I transition occurred at 94 C.
These observations show that, with the tetramers with four
and six lateral chains, the formation of the mesophase is greatly
affected by the length of the lateral chains. The replacement of
the ethyl chains of tetramers 2 and 6a with propyl chains
considerably reduces the mesophase stability (N–I transition
temperature) or destroys the mesophase. As a result, it appears
difficult to investigate further the effects of the lateral chains on
the mesomorphic properties of these systems. For a better
understanding of the structure–property relationships, it will be
necessary to investigate the mesomorphic properties of the longer
oligomer derivatives, for example, the hexamers with several
lateral alkyl chains.
Comparisons between tetramers 7a, 6a, and 2, with two, four,
and six ethyl lateral chains, respectively, are of particular
interest. Both the Cr–N and N–I transition temperatures of
tetramer 7a were considerably higher than those of tetramers 6a
and 2 (Cr–N: 100, 97, and 158 ꢁC; N–I: 137, 155, and 261 ꢁC for
2, 6a, and 7a, respectively). The N–I transition temperature of
tetramer 6a was somewhat higher than that of tetramer 2,
whereas their Cr–N transition temperatures were very similar. In
addition, the mesophase range increased significantly as the
number of lateral chains decreased (37, 58, and 103 ꢁC for 2, 6a,
and 7a, respectively).
Laterally dialkyl-substituted tetramers 7a–f all exhibited
enantiotropic nematic phases. As shown in Fig. 4, the N–I
transition temperature decreased monotonically with increasing
carbon atom number in the lateral chain (261, 209, 183, 167, 155,
These observations show that the transition temperatures and
the mesophase range are strongly affected by the number and
position of the lateral chains. The lateral attachment of ethyl
ꢁ
and 135 C for 7a–f, respectively). However, the change in the
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chains to the two phenyl rings, located separately between the
central and terminal phenyl rings, causes a dramatic decrease in
the transition temperatures. In addition, the use of ethyl lateral
chains leads to a wide mesophase range. Therefore, such attach-
ment should be a useful method for controlling the mesomorphic
properties of alkyl-substituted derivatives of the longer oligomers.
Tetramers 8c–e and 9c–e exhibited enantiotropic nematic
phases, but the transition temperatures of tetramers 8c–e were
much higher than those of the corresponding tetramers 9c–e. For
tetramers 8c and 9c with ethyl terminal chains, the mesophases
were observed in the 180–234 and 123–158 ꢁC ranges, respec-
tively. For propyl derivatives 8d and 9d the mesophases were
observed in the 149–240 and 109–171 ꢁC ranges, respectively. For
butyl derivatives 8e and 9e the mesophases occurred in 142–211
and 101–158 ꢁC ranges, respectively. In addition, the mesophase
ranges of tetramers 8c–e were consistently larger than those of
the corresponding tetramers 9c–e (54 and 34 ꢁC for 8c and 9c, 91
and 62 ꢁC for 8d and 9d, and 69 and 57 ꢁC for 8e and 9e,
respectively).
The length of the terminal chains. Tetramer 2 has six ethyl
lateral and two octyl terminal chains, whereas in tetramer 7f four
octyl chains are attached to the central and terminal phenyl rings.
Although the lateral chains of tetramer 2 are quite different from
those of tetramer 7f, their transition temperatures are very
similar (2: Cr 100 N 137 I; 7f: Cr 96 N 135 I (ꢁC)). To investigate
the effects of the lateral and terminal chains on the mesomorphic
properties, the transition temperatures of laterally hexaethyl-
substituted tetramers 8a–e were compared with those of laterally
dioctyl-substituted tetramers 9a–e. Tetramers 8b–e and 9b–e
possess short alkyl terminal chains of various lengths (C1–C4),
whereas tetramers 8a and 9a have no alkyl endgroups.
Fig. 5 shows plots of the transition temperatures vs. the
number of carbon atoms in the terminal chain of the laterally
hexaethyl- and dioctyl-substituted tetramers. The transition
temperatures of the two tetramer derivatives decrease with
increasing carbon atom number, but their changes in transition
temperature are different. The transition temperatures of hex-
aethyl derivatives 8b–e and 2 decreased more sharply than those
of dioctyl derivatives 9c–e and 7f. With tetramers 8b–e, both the
Cr–N and N–I transition temperatures decreased in a zigzag
manner with increasing terminal chain length (Cr–N: 179, 180,
149, and 142 ꢁC; N–I: 243, 234, 240, and 21 ꢁC for 8b–e,
respectively). With tetramers 9c–e, the Cr–N transition temper-
ature decreased monotonically with increasing terminal chain
length (123, 109, and 101 ꢁC for 9c–e, respectively). The N–I
transition temperatures of tetramers 9c and 9e were the same,
and were lower than that of tetramer 9d (158, 171, and 158 ꢁC for
9c–e, respectively).
The thermotropic data of tetramers 8a–e and 9a–e are given in
Table 3. Interestingly, the two tetramer derivatives are almost the
same in terms of phase transitions but very different as regards
transition temperatures. Tetramers 8a and 9a exhibited no mes-
ophases. The Cr–I transition temperature of tetramer 8a (177 ꢁC)
was much higher than that of tetramer 9a (119 ꢁC). In clear
contrast to these derivatives, tetramers 8b and 9b with methyl
endgroups exhibited nematic phases, but their phase transitions
on heating were different. Tetramer 8b showed an enantiotropic
nematic phase; the mesophase was observed from 179 to 243 ꢁC
on heating and from 241 to 162 ꢁC on cooling. Tetramer 9b,
however, exhibited a monotropic nematic phase; the Cr–I tran-
sition occurred at 156 ꢁC and the mesophase was found at
temperatures between 153 and 104 ꢁC on cooling. The iso-
tropization temperature of tetramer 8b was considerably higher
than that of tetramer 9b.
These results indicate that, with the laterally hexaethyl- and
dioctyl-substituted tetramers with short terminal chains, the
Table 3 Phase transition temperatures (ꢁC) and transition enthalpies
(in square brackets, kJ molꢀ1) of compounds 8a–e and 9a–e
Compound
Phase transitionsa
8a
8b
8c
8d
8e
9a
9b
9c
9d
9e
h: Cr 177.0 [53.4] I
c: I 137.8 [ꢀ50.6] Cr
h: Cr 179.2 [50.7] N 243.0 [2.7] I
c: I 240.5 [ꢀ2.1] N 162.3 [ꢀ47.5] Cr
h: Cr 179.6 [47.7] N 233.6 [3.1] I
c: I 232.1 [ꢀ2.3] N 165.8 [ꢀ46.3] Cr
h: Cr 149.3 [41.5] N 239.8 [3.9] I
c: I 237.5 [ꢀ3.5] N 131.5 [ꢀ38.6] Cr
h: Cr 142.4 [42.1] N 211.1 [3.4] I
c: I 209.5 [ꢀ3.3] N 128.4 [ꢀ40.7] Cr
h: Cr 119.1 [55.0] I
c: I 103.4 [ꢀ55.7] Cr
h: Cr 155.5 [68.1] I
c: I 153.3 [ꢀ1.3] N 104.4 [ꢀ63.3] Cr
h: Cr 123.2 [47.6] N 157.5 [1.8] I
c: I 156.5 [ꢀ1.7] N 113.9 [ꢀ38.9] Cr
h: Cr 109.4 [46.5] N 171.0 [2.1] I
c: I 170.1 [ꢀ2.0] N 88.3 [ꢀ48.7] Cr
h: Cr 100.9 [48.4] N 157.5 [2.1] I
c: I 156.4 [ꢀ2.1] N 76.4 [ꢀ36.0] Cr
Fig. 5 Transition temperatures vs. number of carbon atoms (n) in the
terminal chain of laterally hexaethyl-substituted tetramers 8b–e and 2,
and laterally dioctyl-substituted tetramers 9c–e and 7f.
a
h ¼ on heating; c ¼ on cooling; Cr ¼ crystalline phase; N ¼ nematic
phase; I ¼ isotropic liquid.
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1555; (c) M. P. Aldred, A. E. A. Contoret, S. R. Farrar,
S. M. Kelly, D. Mathieson, M. O’Neill, W. C. Tsoi and P. Vlachos,
Adv. Mater., 2005, 17, 1368.
transition temperatures are strongly affected by both the struc-
ture of the lateral chains and the length of the terminal chains.
The transition temperatures of the hexaethyl derivatives are
much higher than those of the corresponding dioctyl derivatives.
This difference is attributed to the bulkiness of the lateral chains.
When octyl chains are attached terminally, however, the transi-
tion temperatures of the two derivatives are very similar. It
appears that the molecular packing and intermolecular interac-
tion are very similar in the two tetramers.
10 (a) H. Okumoto, T. Yatabe, M. Shimomura, A. Kaito, N. Minami
and Y. Tanabe, Adv. Mater., 2001, 13, 72; (b) H. Okumoto,
T. Yatabe, A. Richter, J. Peng, M. Shimomura, A. Kaito and
N. Minami, Adv. Mater., 2003, 15, 716.
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(b) M. Funahashi and J. Hanna, Adv. Mater., 2005, 17, 594; (c)
M. Funahashi and N. Tamaoki, ChemPhysChem, 2006, 7,
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12 (a) S. R. Farrar, A. E. A. Contoret, M. O’Neill, J. E. Nicholls,
G. J. Richards and S. M. Kelly, Phys. Rev. B, 2002, 66, 125107; (b)
K. L. Woon, M. P. Aldred, P. Vlachos, G. H. Mehl, T. Stirner,
S. M. Kelly and M. O’Neill, Chem. Mater., 2006, 18, 2311.
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Conclusion
A new class of conjugated oligomers forming nematic phases,
namely laterally and terminally alkyl-substituted derivatives of
oligo(1,4-phenyleneethynylene)s, were synthesized and the rela-
tionships between the molecular structures and the mesomorphic
properties investigated. The results showed that both the
formation of the mesophase and the transition temperatures
are strongly affected by the length of the oligomer core, the
structure of the lateral chains, and the length of the terminal
chains. However, the effects of the lateral and terminal chains on
the mesomorphic properties have not yet been thoroughly
explained. This will necessitate future work on the crystal
structures. Nevertheless, the structure–property relationships of
the newly synthesized oligomers should be useful for the
molecular design of mesomorphic alkyl-substituted derivatives
of the longer oligomers. We believe that systematic studies of
these mesomorphic oligomers will play an important role in
oligomer liquid crystal research.
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Acknowledgements
We are grateful to Mr S. Yoshikawa of Kochi University of
Technology and Mr T. Sukegawa of Ibaraki University for the
synthesis of compounds 9a, 9b, and 9e, and to Ms S. Miyamoto
of AIST for the APCI-MS measurements. We also thank the
AIST Technical Center for the provision of analytical services
and AIST for financial support.
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