Diphenyltriacetylenes: Novel nematic liquid crystal materials and analysis of their nematic phase-transition and birefringence behaviours

Article abstract of DOI:10.1039/c3tc31658k

Improvements in birefringence properties of LC materials are actively being pursued because of the increase in their applications in optical devices. 1,6-Diphenyl-1,3,5-hexatriyne, i.e., diphenyltriacetylenes (DPTAs; number of acetylene units, n = 3) are

Full text of DOI:10.1039/c3tc31658k

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Diphenyltriacetylenes: novel nematic liquid crystal
materials and analysis of their nematic phase-transition
and birefringence behaviours†
Cite this: J. Mater. Chem. C, 2013, 1,
8094
*
Yuki Arakawa, Sungmin Kang, Shunpei Nakajima, Koichi Sakajiri, Yokan Cho,
*
Susumu Kawauchi, Junji Watanabe and Gen-ichi Konishi
Improvements in birefringence properties of LC materials are actively being pursued because of the increase in
their applications in optical devices. 1,6-Diphenyl-1,3,5-hexatriyne, i.e., diphenyltriacetylenes (DPTAs; number
of acetylene units, n ¼ 3) are expected to have wide nematic mesophases and exhibit extremely high
birefringence properties because of the high aspect ratio of their conjugated structures. Therefore, we
synthesised DPTA derivatives as novel, highly birefringent nematic liquid crystal materials. The derivatives
exhibit stable enantiotropic nematic phases with longer alkoxy carbon chain lengths (i.e., with dodecyloxy
groups, m ¼ 12). High birefringence values of Dn ¼ 0.46 at 550 nm and 140 ꢀC and a high extraordinary
refractive index (ne) of over 2.0 were exhibited for triyne DPTA–OC6. Furthermore, we investigated the
effect of the acetylene bond on the nematic transition behaviours and refractive index parameters of the
conjugated oligoyne or polyyne compounds with two terminal aromatic rings. An increasing number of
acetylene units (n ¼ 1–3), i.e., extended conjugation, led to wider nematic phases and higher birefringence;
the increment in Dn per acetylene unit was estimated to be approximately 0.14.
Received 23rd August 2013
Accepted 14th October 2013
DOI: 10.1039/c3tc31658k
www.rsc.org/MaterialsC
birefringence because of their highly anisotropic molecular
structures. For instance, diphenylacetylenes (DPAs), so-called
Introduction
tolanes,1 and diphenyldiacetylenes (DPDAs),2 possessing one
(n ¼ 1) and two (n ¼ 2) acetylene units, respectively, are widely
employed as conventional liquid crystal (LC) materials.
Numerous reports of these materials exist, exploiting their
nematic phases and high birefringence properties in optoelec-
tronic applications.
Conjugated oligoyne or polyyne compounds incorporating two
terminally attached aromatic rings, as illustrated in Chart 1, are
of great importance in the context of advanced optical and
electrical materials chemistry, including highly birefringent
(Dn) materials,1–3 non-linear optical (NLO) materials,4 and
organic light-emitting diodes (OLEDs),5 based on their rich
optoelectronic structures. As a result of their omnipresence in
the literature with respect to optoelectronics, the number of
reports in synthetic organic chemistry on the synthesis of
conjugated oligoyne compounds has steadily increased, and the
incorporation of acetylenic units into their molecular structures
are actively being reported.6
As the importance of LC materials with high Dn values has
increased with their widespread application in optical devices
such as light-storage devices, laser-emitting lms, telecommu-
nication devices, and LC display technologies, improvements in
birefringence properties of materials are actively being
pursued.4,5,7 The most effective approaches to improving bire-
fringence include the extension of p-conjugation along the
molecular long axis (i.e., the extraordinary index of the refrac-
tion (ne) direction of the ellipsoid).3 Therefore, conjugated oli-
goyne or polyyne compounds, including the above-mentioned
compounds, are promising candidates for such improvements.
To the best of our knowledge, no reports on the liquid crystal
properties of diphenyltriacetylenes (DPTAs; n ¼ 3) appear in the
literature,6a in spite of numerous reports describing the
synthesis of the analogous diphenyl-oligoynes as presented
above. DPTAs would be expected to have wide nematic meso-
phases and exhibit extremely high birefringence properties
because of the high aspect ratio of their conjugated structures.
Ma et al. have reported a mechanical model study of diphenyl-
oligoynes, describing the correlations of their crystal and liquid
In terms of their molecular architectures, these compounds
constitute a class of conjugated rod-like or “calamitic” molecules.
Such architectures tend to possess high liquid crystallinity and
Chart 1
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1
O-okayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail: konishi.g.aa@m.titech.ac.jp;
skang@polymer.titech.ac.jp; Fax: +81-3-5734-2888; Tel: +81-3-5734-2321
† Electronic supplementary information (ESI) available: Characterization data,
UV-visible spectral data and birefringence data. See DOI: 10.1039/c3tc31658k
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1-Iodo-4-(methoxymethoxy)benzene (1). To a solution of
4-iodophenol (3.0 g, 13.6 mmol) and N-ethyl-N,N-diisopropyl-
amine (2.5 mL, 14.7 mmol) in THF (20 mL) was added chlor-
omethylmethylether (1.1 mL, 13.6 mmol) at 0 ꢀC, and the
mixture was stirred at room temperature for 48 h. Then, the
mixture was extracted with ethyl acetate, and the organic
extracts were washed with brine, and dried over MgSO4. Aer
removal of the solvent, the residue was puried by silica gel
column chromatography to afford the title compound as a
colorless solid. Yield: 50%. 1H NMR (400 MHz, CDCl3) d 7.56 (d,
J ¼ 9.0 Hz, Ar-H, 2H), 6.82 (d, J ¼ 9.0 Hz, Ar-H, 2H), 5.14 (s, Ar-O-
CH2, 2H), 3.46 (s, O-CH3, 3H) ppm.
Chart 2
crystal phases on the basis of computational studies.8 Although
it was presumed that compounds with n ¼ 1–3 did not possess
liquid crystal phases, the models used in the computations
contained non-alkyl tails. Thus, we initiated an investigation
into incorporation of alkoxy tails to both terminal arene para-
positions of DPTAs to induce liquid crystallinity, in order to
examine the effect of alkoxy length on phase-transition behav-
iours of the resulting materials.
1-[4-(Methoxymethoxy)phenyl]-4-trimethylsilyl-1,3-butadiyne
(2). To a mixture of 1 (1.9 g, 7.2 mmol), 1,4-bis(trimethylsilyl)-1,3-
butadiyne (3.8 g, 19.4 mmol), 4-tert-butylphenol (1.3 g, 8.6
mmol), Pd(PPh3)4 (0.25 g, 0.22 mmol), CuI (41 mg, 0.22 mmol),
K2CO3 (3.2 mg, 23 mmol), THF (20 mL) degassed with argon was
added, and stirred at room temperature for 48 h. Then, the
reaction mixture was ltered, extracted with ethyl acetate,
washed with 2 M HClaq and brine, and dried over MgSO4. Aer
removal of the solvent, the residue was puried by silica gel
column chromatography (eluent: ethyl acetate/hexane ¼ 1/10) to
afford the title compound as colorless oil (1.42 g, 97%). 1H NMR
(400 MHz, CDCl3) d 7.43 (d, J ¼ 8.8 Hz, Ar-H, 2H), 6.97 (d, J ¼ 8.8
Hz, Ar-H, 2H), 5.18 (s, Ar-O-CH2, 2H), 3.47 (s, O-CH3, 3H), 0.23 (s,
Si-(CH3)3, 9H) ppm; 13C NMR (100 MHz, CDCl3) d 158.0, 134.2,
116.2, 114.4, 94.2, 90.0, 88.0, 76.8, 73.2, 56.2, 0.37 ppm; FT-IR
Herein, we report the synthesis of diphenyltriyne derivatives
possessing terminally connected alkoxy tails as a new LC
material via Sonogashira9 and Cadiot–Chodkiewicz10 coupling
chemistry. In addition, we describe their thermal transition
behaviours by differential scanning calorimetry (DSC)
measurements and polarized-light optical microscopy (POM)
observations. Furthermore, the properties of a series of conju-
gated oligoyne compounds containing n acetylene units (n ¼
1–3) were systematically investigated to determine the effects of
acetylene unit number on thermal transition temperatures,
mesophase, and birefringence properties. We also investigated
the birefringence properties of DPTA–OC6 and bistolane–OC6
(Chart 2),11 in which the central acetylene bond of DPTA–OC6 is
replaced with a benzene ring, to evaluate the effects of an arene–
acetylene substitution on birefringence. This report is antici-
pated to serve as a guide for the molecular design of materials
with wide nematic phases and high birefringent properties.
(KBr) 2958, 2203, 2103, 1602, 1507 cmꢁ1
.
1-[4-(Methoxymethoxy)phenyl]-1,3-butadiyne (3). A mixture
of 2 (1.0 g, 3.9 mmol), K2CO3 (1.1 g, 7.7 mmol), MeOH (15 mL)
and THF (15 mL) was stirred at room temperature for 30 min.
Then, the reaction mixture was extracted with ethyl acetate and
washed with brine. By evaporating the solvent, the title
compound was obtained as a colorless solid without further
purication (0.72 g, >99%). 1H NMR (400 MHz, CDCl3) d 7.45 (d,
J ¼ 8.9 Hz, Ar-H, 2H), 6.98 (d, J ¼ 8.9 Hz, Ar-H, 2H), 5.18 (s, Ar-O-
CH2, 2H), 3.47 (s, O-CH3, 3H), 2.46 (s, C^C–H, 1H) ppm; 13C
NMR (100 MHz, CDCl3) d 158.2, 134.4, 116.2, 114.0, 94.2, 75.4,
72.5, 70.9, 70.7, 56.1 ppm; FT-IR (KBr) 2954, 2898, 2196, 1601,
Experimental
Instruments
The 1H NMR and 13C NMR spectra were measured using a JEOL
LNM-EX 400 at room temperature. CDCl3 and tetramethylsilane
(TMS) were using as a measurement solvent and an internal
standard, respectively. The liquid crystalline textures were
investigated by polarizing optical microscopy (POM) (Leica
DM2500P microscopy with a Mettler FP90 hot stage) and the
transition temperatures and enthalpy changes were measured
by differential scanning calorimetry (Perkin Elmer DSC7) with
heating and cooling scans performed at 10 ꢀC minꢁ1. The
measurements of ne, no and Dn were performed with uniaxially
aligned nematic cells, where the glass cells contain an indium
tin oxide (ITO) layer purchased from EHC. And the cell gaps (d)
of 2–3 mm were measured by the interferometric method. The
transmittance of light under crossed nicol conditions was
observed as a function of wavelength by a microscope spectro-
scopic method using a Nikon LV100 Pol optical microscope
equipped with a USB4000 (Ocean photonics) spectrometer.
1507 cmꢁ1
.
1-(Methoxymethoxy)-4-[2-(trimethylsilyl)ethynyl]benzene (4).
To a mixture of 1 (1.5 g, 5.7 mmol), Pd(PPh3)4 (0.13 g, 0.11
mmol) and CuI (21 mg, 0.11 mmol) was added to a solution
mixture of TEA (20 mL) and THF (20 mL) and trimethylsilyla-
cetylene (0.94 mL, 6.8 mmol) degassed with argon was added,
and stirred at 45 ꢀC for 12 h. The reaction mixture was extracted
with ethyl acetate, and washed with 2 M HClaq and brine, and
dried with MgSO4. Aer removal of the solvent, the residue was
puried by silica gel column chromatography (eluent: ethyl
acetate/hexane ¼ 1/10) to afford the title compound as colorless
oil (1.32 g, 99%). 1H NMR (400 MHz, CDCl3) d 7.39 (d, J ¼ 8.8 Hz,
Ar-H, 2H), 6.95 (d, J ¼ 8.8 Hz, Ar-H, 2H), 5.17 (s, Ar-O-CH2, 2H),
3.46 (s, O-CH3, 3H), 0.24 (s, Si-(CH3)3, 9H) ppm; FT-IR (NaCl)
Material
Unless otherwise noted, all chemicals were commercially 2958, 2898, 2826, 2157, 1605, 1571, 1506 cmꢁ1
.
available and used as received. The spectral data of DPTA-OCm
(m ¼ 6, 7, 8, 10, 11, 12, and 14) are described in the ESI.†
1-Ethynyl-4-(methoxymethoxy)benzene (5). A mixture of 4
(1.3 g, 5.6 mmol), K2CO3 (1.6 g, 11.3 mmol), MeOH (15 mL) and
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THF (15 mL) was stirred at room temperature for 2 h. Then, the pale yellow solid (49 mg, 65%). 1H NMR (400 MHz, CDCl3) d 7.45
reaction mixture was extracted with ethyl acetate and washed (d, J ¼ 8.3 Hz, Ar-H, 4H), 6.83 (d, J ¼ 8.3 Hz, Ar-H, 4H), 3.96 (t, J ¼
with brine. By evaporating the solvent, the title compound was 6.5 Hz, Ar-O-CH2, 4H), 1.79 (tt, J ¼ 6.5 and 7.2 Hz, CH2, 4H), 1.45
obtained as a colorless solid without further purication (0.88 g, (m, CH2, 8H), 0.93 (t, J ¼ 6.8 Hz, CH3, 6H) ppm; 13C NMR (100
97%). 1H NMR (400 MHz, CDCl3) d 7.42 (d, J ¼ 8.8 Hz, Ar-H, 2H), MHz, CDCl3) d 160.3, 134.6, 114.7, 112.6, 78.8, 73.5, 68.1, 66.4,
6.98 (d, J ¼ 8.8 Hz, Ar-H, 2H), 5.18 (s, Ar-O-CH2, 2H), 3.47 (s, 28.8, 28.1, 22.4, 14.0 ppm; FT-IR (KBr) 2961, 2939, 2865, 2243,
O-CH3, 3H), 3.00 (s, C^C–H, 1H) ppm; FT-IR (KBr) 2957, 2932, 2193, 1603, 1561, 1511, 1470, 1291, 1255, 1182 cmꢁ1
.
2954, 2190, 1685, 1603, 1507 cmꢁ1
.
1-(2-Bromoethynyl)-4-(methoxymethoxy)benzene (6). A mixture
of 5 (0.88 g, 5.4 mmol), AgNO3 (92 mg, 0.54 mmol) and NBS (1.2
g, 6.5 mmol) in acetone (10 mL) was stirred at room tempera-
ture for 3 h. Then the reaction mixture was extracted with ethyl
acetate, washed with water and brine, and dried over MgSO4.
Aer removal of the solvent, the residue was puried by silica
gel column chromatography (eluent: ethyl acetate/hexane ¼ 1/
10) to afford the title compound as colorless oil (0.93 g, 71%).
1H NMR (400 MHz, CDCl3) d 7.38 (d, J ¼ 8.8 Hz, Ar-H, 2H), 6.96
(d, J ¼ 8.8 Hz, Ar-H, 2H), 5.17 (s, Ar-O-CH2, 2H), 3.47 (s, O-CH3,
3H) ppm; 13C NMR (100 MHz, CDCl3) d 157.5, 133.4, 116.1,
116.0, 94.2, 79.8, 56.1, 48.2 ppm; FT-IR (KBr) 2957, 2933, 2897,
Results and discussion
Synthesis of DPTA derivatives
Firstly, asymmetric diacetylene 2 was prepared in good yield
by the Sonogashira cross-coupling reaction9 of 1-iodo-4-
(methoxymethoxy)benzene (1) and bis(1,6-trimethylsilyl)-1,3-
butadiyne. Since the protiodesilylation of 2 afforded unstable
terminal acetylene 3, the material was immediately subjected
to the Cadiot–Chodkiewicz reaction10 between 1-[4-(methox-
ymethoxy)phenyl]-1,3-butadiyne (3) and 1-(2-bromoethynyl)-
4-(methoxymethoxy)benzene (6), affording MOM-protected
1,3,6-hexatriyne 7. 1,3,6-hexatriyne (8) was obtained following
deprotection by treatment of 7 with HCl in MeOH/CHCl3. Finally,
1,10-(1,3,5-hexatriyne-1,6-diyl)bis(4-alkyloxybenzene) derivatives
(DPTA–OCm) (m ¼ 5–14) were prepared by Williamson ether-
ication between parent compound 8 and the corresponding
alkyl bromides (Scheme 1).
2059, 1602, 1507 cmꢁ1
.
1,10-(1,3,5-Hexatriyne-1,6-diyl)bis[4-(methoxymethoxy)benzene]
(7). To a solution of 3 (0.73 g, 3.9 mmol), CuCl (7.7 mg, 77 mmol), a
catalyst amount of NH2OH$HCl, 70% ethylamine solution
(0.50 mL), and toluene (2.0 mL) were added in order. 6 in toluene
(2.0 mL) was added dropwise to the mixture. Then, the reaction
mixture was stirred at room temperature, and extracted with ethyl
acetate. The organic layer was washed with water and brine, and
dried with MgSO4. Aer removal of the solvent, the residue was
puried by silica gel column chromatography (eluent: ethyl
acetate/hexane ¼ 1/10) to afford the title compound as a brown
solid (0.74 g, 55%). 1H NMR (400 MHz, CDCl3) d 7.47 (d, J ¼ 8.9 Hz,
Ar-H, 4H), 6.98 (d, J ¼ 8.9 Hz, Ar-H, 4H), 5.19 (s, Ar-O-CH2, 4H),
3.47 (s, O-CH3, 6H) ppm; 13C NMR (100 MHz, CDCl3) d 158.3,
134.6, 116.3, 114.1, 94.1, 78.5, 73.7, 66.3, 56.2 ppm; FT-IR (KBr)
Each compound was puried by recrystallization in hexane,
and characterised by 1H/13C NMR and Fourier transform
2963, 2181, 1603, 1511 cmꢁ1
.
1,10-(1,3,5-Hexatriyne-1,6-diyl)bis[4-hydroxybenzene] (8). A
solution of 7 (0.73 g, 2.1 mmol), 2 M HClaq (5 mL), MeOH (20 mL)
and chloroform (20 mL) was stirred at 45 ꢀC for 12 h. The reaction
mixture was extracted with ethyl acetate, washed with water and
brine, and dried over MgSO4. Aer removal of the solvent, the
residue was puried by silica gel column chromatography
(eluent: ethyl acetate) to afford the title compound as a brown
solid (0.58 g, >99%). 1H NMR (400 MHz, CDCl3) d 7.42 (d, J ¼ 8.8
Hz, Ar-H, 4H), 6.79 (d, J ¼ 8.9 Hz, Ar-H, 4H) ppm, 13C NMR (100
MHz, acetone-d6) d 159.9, 135.3, 116.4, 111.3, 79.7, 72.2, 66.4
ppm; FT-IR (KBr) 3282, 3002, 2934, 2252, 2197, 1607, 1589, 1511,
1447, 1374, 1241 cmꢁ1
.
1,10-(1,3,5-Hexatriyne-1,6-diyl)bis[4-pentyloxybenzene] DPTA–
OC5: general procedure for 1,10-(1,3,5-hexatriyne-1,6-diyl)bis-
[4-alkoxybenzene]. A mixture of 8 (50 mg, 0.19 mmol),
1-pentylbromide (59 mg, 0.39 mmol), K2CO3 (54 mg,
0.39 mmol) and acetonitrile (20 mL) was stirred at reux
temperature for 12 h. Then, the reaction mixture was extracted
with chloroform and washed with water and brine, and dried with
MgSO4. Aer removal of the solvent, the residue was puried by
recrystallization with hexane to afford the title compound as a
Scheme
1
Synthesis of 1,10-(1,3,5-hexatriyne-1,6-diyl)bis(4-alkyloxybenzene)
derivatives (DPTA–OCm).
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infrared (FT-IR) spectroscopy, as well as mass spectrometry
(MS). A representative 13C NMR spectrum of DPTA–OC6 in
CDCl3 is shown in Fig. 1, in which the three characteristic peaks
attributed to the acetylene units are indicated by arrows at d ¼
78.8, 73.5 and 66.4 ppm.
Thermal stability of the diphenyl-oligoynes has been already
discussed in the literature.6h,i The decomposition temperature
of DPTA (diphenyl-1,3,5-hexatriyne) was observed at over
250 ꢀC.6i DPTA-OCm was thermally stable enough to evaluate its
liquid crystallinity.
Fig. 2 Transition temperature plots against the alkoxy tail carbon chain length
(m) for DPTA–OCm.
Liquid crystallinities of DPTA–OCm [1,10-(1,3,5-hexatriyne-1,6-
diyl)bis(4-alkoxybenzene)]
the heating/cooling scans is shown in Fig. 3(a). As observed
from Fig. 3(a), three clear transitions were detected in addition
to sharp exothermic peaks, implying that the possible top-
ochemical reactions did not occur.12 The compound was
therefore identied as being stable in this temperature range.
POM images of DPTA–OC14 are shown for the temperatures
130 ꢀC and 117 ꢀC in Fig. 3(b) and (c), respectively. At 130 ꢀC, a
nematic phase was clearly seen (Fig. 3(b)), exhibiting a schlieren
texture with two and four brushes, which is typical for a nematic
phase. At 117 ꢀC, a typical smectic-C phase with a focalconic
texture was evident (Fig. 3(c)). It is worthy to note that a hexa-
triene derivative with dodecyloxy groups (m ¼ 12) also exhibited
only a nematic phase in the mesophase. This trend is in
contrast to that found in conventional materials. With
increasing length of the alkoxy tail (m), the nematic phase is
usually converted to the smectic phase as a result of micro-
segregation between aromatic and aliphatic components.
As is discussed later, the molecular shapes of the
compounds contribute to their nematic stability; as expected,
an odd–even oscillation was also observed for Tm and Tc values
of DPTA–OCm with relatively short alkoxy tails, i.e., m ¼ 5–10.
The transition temperatures were lower in odd-numbered
members, particularly in the case of the heptyloxy derivative
Phase-transition temperatures and mesophase structures were
determined based on POM observations; the corresponding
enthalpy changes were established using DSC measurements.
All thermotropic phase-transition measurements were obtained
at 10 ꢀC minꢁ1 on the cooling scan; the results are summarised
in Table 1. Fig. 2 illustrates the transition temperatures plotted
against the carbon chain length (m ¼ 5–14) of the alkoxy tail.
As observed in Fig. 2, not only these two compounds but also
the set of compounds from OC5 to OC12 exhibited enantio-
tropic nematic phase (N), whereas DPTA–OC14 showed enan-
tiotropic smectic-C (Sm-C) and nematic phases. A representative
DSC trace of DPTA–OC14 recorded at a rate of ꢂ10 ꢀC minꢁ1 in
ꢀ
(m ¼ 7), which showed a Tc of 93 C in the cooling scan.
The enthalpy changes of transitions between the nematic
phase and isotropic liquids were found to be relatively small
compared with the changes in the enthalpies of fusion in
transitions from the crystalline state to the nematic phase; these
values were much higher for all compounds. Odd–even oscil-
lations were also observed for enthalpy changes; however, with
increasing alkoxy chain lengths above decyloxy, odd–even
oscillations were not observed for transition temperatures and
enthalpy changes, especially for Tm as shown in Fig. 4. This is
thought to be due to the long alkoxy tails enhancing molecular
interactions. The refractive index parameters therefore also
Iso increased as the alkoxy tails increased in length from decyloxy
Fig. 1 13C NMR spectrum of DPTA-OC6 (CDCl3, 100 MHz).
Table 1 Thermotropic phase-transition results for DPTA–OCm (cooling scan rate
of ꢁ10 ꢀC minꢁ1
)
Cooling (1st)
Sample Cr Ttr/ꢀC
SmC Ttr/ꢀC
N
Ttr/ꢀC
to dodecyloxy.
OC5
OC6
OC7
OC8
$
$
$
$
$
$
$
$
$
128.9
133.1
93.1
107.43
94.2 (25.2)
103.2 (32.0)
104.3 (34.1)
109.7 (38.3)
104.6 (42.9)
$
$
$
$
$
$
$
$
$
154
174
171
169
158.8 (1.3)
151.2 (2.2)
148.1 (1.6)
146.6 (2.0)
138.2 (3.0)
$
$
$
Effects of acetylene units on transition temperatures
$
In order to determine the effect of acetylene unit number (n) on
the mesophases and transition temperatures, we compared
OC9
$
OC10
OC11
OC12
OC14
$
$
diphenyl-oligoyne compounds with n ¼ 1–3 while keeping the tail
length constant, i.e., hexyloxy (m ¼ 6); the rst two compounds
$
$
116.6 (3.0)
$
are denoted DPA–OC6 (n ¼ 1) and DPDA–OC6 (n ¼ 2). The
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Fig. 3 (a) DSC trace of DPTA–OC14 (heating/cooling scan rate of ꢂ10 ꢀC minꢁ1). POM images of the nematic phases of DPTA–OC14 at (b) 130 ꢀC and at (c) 117 ꢀC in
the cooling scan.
transition temperatures of these compounds were identied
using POM observations and DSC measurements; the results are
listed in Table 2 and are plotted against the nuꢁm1ber of acetylene
ꢀ
units during the cooling process at 10 C min (Fig. 5). As the
results show, the diphenyl-oligoyne analogues exhibited similar
overall phase behaviour, in which only a nematic phase was
found. The nematic regimes were 40.9, 32.2 and 8.2 ꢀC for n ¼ 3,
2, and 1, following the order n ¼ 3 > 2 > 1; thus, the nematic
ranges for the diphenyl-oligoynes became larger with an increase
in the number of acetylene units. The variation in the range from
n ¼ 2 to n ¼ 1 was particularly large, approximately 24 ꢀC, which
is greater than the variation of approximately 9 ꢀC observed from
n ¼ 3 to n ¼ 2. As described previously, DPTA derivatives maintain
a wide nematic phase, even for longer alkoxy tails, eventually
exhibiting a smectic phase in the case of tetradecyloxy derivatives.
According to the literature, smectic phases begin to appear for
nonyloxy analogues of tolanes (DPAs) and DPDAs. Thus, the alkyl
tail lengths of diphenyl-oligoyne compounds that maintain their
Table 2 Transition temperatures (ꢀC) and corresponding enthalpies (kJ molꢁ1
for diphenyl-oligoynes at a rate of 10 ꢀC minꢁ1 on the cooling scan
)
Tc (DH)/
TI–N (DH)/
N phase
range/ꢀC
kJ molꢁ1
kJ molꢁ1
a
Tm /ꢀC
n ¼ 1
n ¼ 2
n ¼ 3
94.8 (18.3)
115.3 (41.8)
133.1 (36.6)
98.8 (1.7)
147.5 (2.0)
174b (ꢁ)
8.1
32.2
40.9
98.1
123.5
143.2
Fig. 4 Enthalpy change plots against the alkoxy tail carbon chain length (m) for
DPTA–OCm. (a) Between the crystal phase and nematic phase. (b) Between the
nematic phase and isotropic liquid.
a
b
Determined upon the heating process.
observation.
Determined by POM
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Fig. 6 Wavelength dispersion of refractive index parameters at 140 ꢀC for
DPTA–OC6. Dashed line shows ne, dotted line shows no and solid line shows Dn.
Fig. 5 Transition temperature plots as a function of acetylene unit number (n)
for OC6 during the cooling process at 10 ꢀC minꢁ1
.
nematic phase increases with increasing number of acetylene
units.
decomposition of the compound; it was therefore not
possible to measure the temperature dependence of the
birefringence for DPTA–OC6 (Fig. 6).
Based on previous reports and the present investigation of
diphenyl-oligoyne compounds,2g we considered that the
nematic stabilities of these materials might stem from their
molecular shapes. Diphenyl-oligoyne derivatives have a struc-
ture in which two phenyl rings are linked by acetylene units,
giving them dumbbell-like shapes. Since these rod-like mole-
cules have no aromatic rings in the conjugated linkage between
the arene moieties, their p–p interactions would be weaker than
those in molecules with aromatic rings such as bistolanes. The
capping phenyl rings in the molecules laying side-by-side
molecules have difficulty aligning with each other. In fact, the
aromatic cores and alkoxy tails have less opportunity for micro-
segregation to form smectic phases; this effect would be
enlarged by increasing the number of acetylene units. The main
reason for the large observed increase in the nematic range
from n ¼ 2 to n ¼ 1 is that tolane has the shortest acetylenic
linkage; it is believed that the two phenyl rings would be in a
better position to stack with adjacent molecules and increase
intermolecular interactions. The nematic range is therefore
further narrowed. The p–p interactions between aromatic cores
expanded by an acetylene unit are also enhanced, and their
transition temperatures increase with increasing number of
acetylene units.
Additionally, the optimised structures and polarisation of
diphenyl-oligoynes with long alkoxy groups replaced by
methoxy groups were calculated using density functional theory
(DFT) at the uB97X-D/6-31 G(d,p) level.13–15 The results of
measurements and calculated values are shown in Table 3.
The wavelength dispersions of ne, no, and Dn at 140 ꢀC for
DPTA–OC6 are shown in Fig. 6 in the range 450–700 nm (refer to
ESI for the corresponding absorption spectra of these
compounds in THF†). The diphenyl-oligoynes examined dis-
played well-dened oscillating structures and exhibited red-
shis of the absorption edge wavelengths as the p-conjugation
length increased. Finally, the absorption edge of DPTA–OC6
reached the visible region, which is at approximately 400 nm
(ESI, Fig. S1†).
The measured Dn values at 550 nm were 0.45, 0.31, and 0.18
for DPTA–OC6, DPDA–OC6, and DPA–OC6, respectively i.e., the
birefringence increases as the number of acetylene units
increases, as expected (Table 3). It should be noted that Dn ¼
0.46 for DPTA–OC6 is considered high for a birefringent mate-
rial without polar groups such as cyano and isothiocyanato
moieties. In addition, the Dn values were close to that of a high-
birefringence dinaphthyldiacetylene with hexyloxy groups
(DNDA–OC6; Dn ¼ 0.46 at 550 nm, 145 ꢀC), produced by
introduction of an acetylene unit to DPDA, which we previously
reported.2g The ne values increased linearly with increasing
number of acetylene units, eventually reaching a value above
Effects of acetylene units on birefringence properties for –OC6
To investigate the effects of acetylene units on the birefrin-
gence properties of conjugated oligoynes, we measured their
extraordinary and ordinary refractive indices (ne and no,
respectively) and their associated birefringence (Dn
¼
Table 3 Measured refractive index parameters and polarisabilities for methoxy
diphenyl-oligoyne analogues calculated using DFT (uB97X-D/6-31 G(d,p) level)
ne ꢁ no). The ne and no values were measured using a
microscopic spectroscopy method based on the theory of
multiple-beam interference (MBI),3 which has been
described in a previous report.2g,3 The measurements were
performed at the lowest measurable LC temperature for each
compound, i.e., the values indicated at their maxima in the
nematic ranges: 140 ꢀC for DPTA–OC6, 121 ꢀC for DPDA–OC6,
and 96 ꢀC for DPA–OC6. DPTA–OC6 (n ¼ 3) displayed the
highest transition temperature, which was accompanied by
Sample
ne
no
Dna n2
axx (ayy + azz)/2 ab
Dac
n ¼ 1
n ¼ 2
n ¼ 3
1.69 1.50 0.18 2.45 383 140
1.84 1.53 0.31 2.70 516 149
2.02 1.57 0.45 2.99 666 157
221 243
271 368
326 510
315 421
Bistolane-OC6 1.94 1.56 0.38 2.89 595 175
a
c
b
Microscopic spectroscopy with MBI theory. a ¼ (axx + ayy + azz)/3.
Da ¼ axx ꢁ (ayy + azz)/2.
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2.0, whereas changing the acetylene unit number had minimal respective values of no remained at approximately 1.6. There-
inuence on the no values (refer to ESI for the corresponding fore, the birefringence value of DPTA–OC6 was larger than that
values and ttings†). Hence, the birefringence was gradually of bistolane–OC6, which was an expected result when
enhanced (Fig. 7). According to the slope of the linear t to the exchanging the arene ring for an acetylene unit.
respective birefringence values, the increment in Dn per acety-
lene unit was estimated to be approximately 0.14 (Fig. 8).
With respect to the polarisabilities of the DPTA compounds
calculated by the DFT method, the contribution of an acetylene
With respect to the calculated polarisability components, unit to the polarizability of axx was larger than that of the
the polarisabilities along the principle axis (x-axis, axx) and benzene ring for the bistolane compound (the corresponding
the resulting anisotropies of the polarisabilities (Da) are values were 666 and 595, respectively), while the contribution of
increased by introducing an acetylene unit, i.e., expanding the the molecular transverse direction was nearly unchanged
p-conjugation. Although another polarizability perpendicular between DPTA and bistolane. These results for the calculated
to the principal axis ((axx + ayy)/2) also increases on addition of molecular polarizabilities are reasonable and provide support
an acetylene unit, this variation is much smaller than that of for the experimentally determined birefringence values.
axx. Thus, the no values exhibit only a minimal increase, and Therefore, the birefringence value of DPTA–OC6 was regarded
the resulting Dn values increase linearly with increasing as larger than that of bistolane–OC6, an expected result with the
number of acetylene units.
To evaluate the effect of the acetylene unit on the birefrin- of 0.07.
gence, we next compared Dn of DPTA–OC6 with that of bisto- The transition temperatures of bistolane–OC6 were deter-
benzene-to-acetylene unit exchange, with an estimated Dn value
ꢀ
lane–OC6 having enantiotropic nematic phases, in which the mined to be 173.3 and 235.6 C for the nematic-to-crystal and
central acetylene unit of DPTA–OC6 is replaced by an aryl unit. isotropic-to-nematic transitions, respectively, in the cooling
The change induced in ne through the substitution of the scan; the melting point in the heating scan was 179.5 ꢀC. As
aromatic ring to the acetylene bond was 0.08; however, described above, DPTA–OC6 exhibited lower transition
temperatures as compared to bistolane–OC6. Thus, DPTA–OC6
was found to possess a higher birefringence and lower transi-
tion temperatures than bistolane–OC6, providing further indi-
cation of the suitability of the DPTA core as a novel LC material.
Conclusions
We have synthesised diphenyltriacetylenes (DPTA–OCm) with
terminal alkoxy carbon chain lengths ranging from pentyloxy to
tetradecyloxy (m ¼ 5–14). The DPTA derivatives reported here
represent a novel, highly birefringent nematic material, pos-
sessing a stable enantiotropic nematic phase for m ¼ 5–12, with
DPTA–OC14 exhibiting
a smectic-C phase. Birefringence
measurements of DPTA–OC6 yielded a high Dn of 0.45 (at 550
nm, 140 ꢀC), and an extraordinary refractive index (ne) of greater
than 2.0. We also investigated the effect of acetylene unit
incorporation on the nematic transition behaviour and bire-
fringence of hexyloxy-based diphenyl-oligoyne analogues of n ¼
1–3, namely, DPA–OC6, DPDA–OC6, and DPTA–OC6, respec-
tively. Firstly, the nematic LC regimes were observed to increase
with increasing numbers of acetylene units. DPA–OC6 (the so-
called conventional tolane LC material) showed a range of only
Fig. 7 Measured refractive index parameters for diphenyl-oligoynes.
8
ꢀC; DPTA–OC6 on the other hand exhibited a range of
approximately 41 ꢀC. A reason for the greater stability and more
widespread LC behaviour of DPTAs is their characteristic
dumbbell molecular shape. Secondly, with respect to birefrin-
gence, although the ne values increased linearly with increasing
acetylene unit number, this variation had very little inuence on
no values. Thus, birefringence gradually became larger, and the
increment in Dn per acetylene unit was estimated to be
approximately 0.14. In addition, to estimate the different
inuences of conjugated acetylene–arene systems on the bire-
fringence, we compared the Dn values of DPTA–OC6 and bis-
tolane–OC6, in which the central acetylenic moiety of DPTA–
OC6 is replaced with a benzene ring. The value for DPTA–OC6
Fig. 8 Linearly fitted line from birefringence values as a function of acetylene
unit number (y ¼ 0.135x + 0.0433, r2 ¼ 0.999).
8100 | J. Mater. Chem. C, 2013, 1, 8094–8102
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was found to be larger than that of bistolane–OC6, with an
estimated Dn value of 0.07, an outcome generally expected with
the benzene-to-acetylene switch. The synthetic approach
described here comprises a valuable method for the improve-
ment of the nematic behaviours and birefringence of LC
materials with interesting optical properties.
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