Synthesis of diphenyl-diacetylene-based nematic liquid crystals and their high birefringence properties

Article abstract of DOI:10.1039/c2jm16002a

We synthesized two series of diphenyl-diacetylene (DPDA)-based materials with alkoxy and alkyl tails of length m (DPDA-OCm and DPDA-Cm, respectively), and measured their nematic-phase birefringence (Δn) as a function of wavelength and temperature. We found that Δn decreases with an increase in m, possibly by a dilution effect of the low-Δn alkyl tail. Further, of the two series, Δn was found to be relatively higher in the DPDA-OCm materials, with the highest value of 0.4 obtained for DPDA-OC1 at 550 nm at 10 °C below the isotropic-to-nematic transition temperature. Further, we observed the temperature dependence for Δn, which is proportional to the order parameter (s). From extrapolation to s = 1 (the perfect orientation state), it is speculated that the DPDA-O moiety has the potential to afford a very large Δn of 0.9. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16002a

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PAPER
Synthesis of diphenyl-diacetylene-based nematic liquid crystals and their high
birefringence properties†
*
Yuki Arakawa, Shunpei Nakajima, Ryohei Ishige, Makoto Uchimura, Sungmin Kang, Gen-ichi Konishi
*
*
and Junji Watanabe
Received 19th November 2011, Accepted 13th February 2012
DOI: 10.1039/c2jm16002a
We synthesized two series of diphenyl-diacetylene (DPDA)-based materials with alkoxy and alkyl tails
of length m (DPDA–OCm and DPDA–Cm, respectively), and measured their nematic-phase
birefringence (Dn) as a function of wavelength and temperature. We found that Dn decreases with an
increase in m, possibly by a dilution effect of the low-Dn alkyl tail. Further, of the two series, Dn was
found to be relatively higher in the DPDA–OCm materials, with the highest value of 0.4 obtained for
DPDA–OC1 at 550 nm at 10 ꢀC below the isotropic-to-nematic transition temperature. Further, we
observed the temperature dependence for Dn, which is proportional to the order parameter (s). From
extrapolation to s ¼ 1 (the perfect orientation state), it is speculated that the DPDA–O moiety has the
potential to afford a very large Dn of 0.9.
the nematic phase was well elucidated as a function of m,
wavelength, and temperature. Our results will be helpful in the
design of final target polymers for application to optical phase
difference films, laser emission films and optical rotation plates.
Introduction
With the strong commercial demand for flat panel displays and
high-speed telecommunication devices, the applications of high-
birefringence (Dn) liquid crystals (LCs) have extended to optical
phase difference films, laser emission films and optical rotation
plates. In these applications, such high-Dn materials offer the
critical advantage of allowing smaller cell gaps and film
thicknesses.
Experimental
Instruments
The 1H NMR and 13C NMR spectra were measured in CDCl3 on
a JEOL LNM-EX 400 at room temperature using tetrame-
thylsilane (TMS) as an internal standard. The transition
behavior was investigated by polarizing optical microscopy
(POM) (Leica DM2500P microscopy with a Mettler FP90 hot
stage) and differential scanning calorimetry (Perkin Elmer
The preferred candidates for high-Dn LCs are compounds
containing a conjugation along the long molecular axis.1–13
Moreover, the presence of an acetylene unit (–C^C–) is
extremely effective as its Dn is approximately identical to that of
phenyl groups. Thus, in the pursuit of high Dn, a range of LC
molecules containing tolane, bistolane, diphenyl-diacetylene,
and thiophenyl-diacetylene moieties1–3,5–7,11–15 have been
prepared thus far. In particular, Wu et al. reported that diphenyl-
diacetylene with alkyl tails having carbon numbers m of 3–5
exhibited high Dn and low viscosity.5,6 Other synthesized
homologues of diphenyl-diacetylene with alkoxy, esters and
fluorine groups have also been reported.1–3,11,16–20 However, to
our knowledge, their optical properties have not been investi-
gated in detail, except for the materials prepared by Wu et al.5,6
In this study, we synthesized two series of diphenyl-diac-
etylenes with p-alkoxy tails (m ¼ 1–7) and p-alkyl tails (m ¼ 4–12)
which form enantiotropic nematic LCs. Their birefringence Dn in
DSC7) with heating and cooling scans performed at 10 ꢀC minꢁ1
.
Materials
Unless otherwise noted, all chemicals were commercially avail-
able and used as received. Trimethylsilylacetylene, 4-iodophenol,
Pd(PPh3)4, 4-bromoanisole (1-OC1) and 4-bromophenetole
(1-OC2) (TCI), and alkyl bromide, PPh3, CuI, 1,8-diazabicyclo
[5,4,0]-7-undecene (DBU) and N,N,N0,N0-tetramethylethylene-
diamine (TMEDA) (Wako), and CuCl (Nacalai tesque) were
purchased, respectively. 1,4-Bis(4-hexyloxyphenyl)buta-1,3-
diynes were prepared according to Scheme 1. The chemical
structures of 1-iodo-4-hexyloxybenzene (1-OC6),21 1-hexyloxy-4-
[2-(trimethylsilyl)ethynyl]benzene (2-OC6),22 and 1-ethynyl-4-
hexyloxybenzene (3-OC6)22 were confirmed by comparison with
the corresponding literature. Our procedures for their synthesis
are cited in the ESI†. Other compounds (DPDA–OC1–7) were
synthesized in a similar manner as BPDA-OC6.
Department of Organic and Polymeric Materials, Tokyo Institute of
Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
E-mail: jwatanab@polymer.titech.ac.jp; 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: Typical DCS
chart and spectral data. See DOI: 10.1039/c2jm16002a
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the cooling scan are listed in Table 1. All the compounds exhibit
an enantiotropic nematic phase.2,5,6
When the transition temperatures, Ti (isotropic to nematic)
and Tc (nematic to crystalline), are plotted against m (Fig. 1), we
can see at a glance that the LC temperature region of the DPDA–
OCm series is higher than that of the DPDA–Cm series. This is
due to the dipolar interaction of the ether linkage. Further, as
expected, the transition temperatures oscillate with even/odd m
for both series. In DPDA–Cm, the transition temperatures for
odd-m members are higher than those for even-m members,
whereas the DPDA–Om series exhibits the opposite trend; this
opposite trend is due to the additional oxygen atom in the ether
linkage.
The distinct feature in the LC behaviour of these materials is
that the nematic phase formation is maintained when m is
increased. This trend is in contrast to that in conventional
materials, in which the nematic phase usually changes into the
smectic phase because of the micro-segregation of the aromatic
and aliphatic components. This obstinate nematic LC formation
may stem from the good miscibility of the DPDA mesogenic
moiety and the aliphatic tail.
Scheme 1 Synthesis of dialkoxy-diphenyldiacetylene (DPDA–OCm).
1,4-Bis(4-hexyloxyphenyl)buta-1,3-diyne (4-OC6)
DBU (0.52 mL, 3.5 mmol), TMEDA (7.0 mL, 0.052 mmol), CuCl
(6.9 mg, 0.07 mmol) and acetonitrile (10 mL) were bubbled with
oxygen for 5 min, then 4-ethynyl-1-hexyloxybenzene (0.7 g,
3.5 mmol) was added to the mixture. The reaction was stirred at
room temperature for 3 h. The solvent was removed under
reduced pressure, and the obtained residue was extracted with
diethylether, washed with water and dried over MgSO4. After
removing the solvent under reduced pressure and purifying by
silica gel column chromatography (eluent: hexane), recrystalli-
zation with methanol was carried out to give 4-OC6 as a colorless
Table
1
Phase transition temperatures (ꢀC) and enthalpies
(DH, kJ molꢁ1) for DPDA–OCm and DPDA–Cm obtained from cooling
DSC thermograms at a rate of 10 ꢀC minꢁ1
Material
Transition temperature/ꢀC (enthalpy/kJ molꢁ1
)
DPDA–OC1
DPDA–OC2
DPDA–OC3
DPDA–OC4
DPDA–OC5
DPDA–OC6
DPDA–OC7
DPDA–C4
DPDA–C5
DPDA–C6
DPDA–C7
DPDA–C8
DPDA–C9
DPDA–C10
DPDA–C12
Cr 98.6 (23.8) N 183.6 (1.65) Iso Tm: 141.9
Cr 187.3 (40.8) N 208.8 (1.66) Iso Tm: 192.0
Cr 123.4 (17.5) N 175.2 (1.69) Iso Tm: 139.5
Cr 149.3 (42.7) N 173.6 (2.26) Iso Tm: 157.5
Cr 110.1 (33.3) N 153.1 (1.90) Iso Tm: 119.1
Cr 115.3 (41.8) N 147.5 (2.01) Iso Tm: 123.5
Cr 92.7 (23.7) N 134.6 (1.71) Iso Tm: 104.6
Cr 65.1 (12.8) N 96.6 (0.80) Iso Tm: 76.3
Cr 79.0 (11.5) N 105.6 (0.79) Iso Tm: 86.2
Cr 55.3 (20.5) N 84.8 (0.74) Iso Tm: 60.1
Cr 81.0 (20.6) N 87.1 (1.24) Iso Tm: 89.7
Cr 59.0 (18.2) N 74.9 (1.08) Iso Tm: 66.3
Cr 69.6 (20.1) N 73.1 (0.44) Iso Tm: 78.5
Cr 51.5 (24.9) N 74.8 (1.59) Iso Tm: 57.7
Cr 57.3 (5.45) N 67.5 (2.21) Iso Tm: 62.8
1
solid (0.65 g, 93%). H NMR (400 MHz, CDCl3) d 7.44 (d, J ¼
9.0 Hz, Ar–H, 4H), 6.84 (d, J ¼ 9.0 Hz, Ar–H, 4H), 3.96 (t, J ¼
6.6 Hz, Ar–O–CH2, 4H), 1.78 (tt, J ¼ 6.6, 7.6 Hz, CH2, 4H),
1.50–1.31 (m, CH2, 12H), 0.91 (t, J ¼ 7.1 Hz, CH3, 6H) ppm.
HRMS-DART (m/z): [M] calcd for C28H34O2, 402.2558; found,
402.2559.
Results and discussion
Synthesis and characterization
As shown in Scheme 1, the synthetic route to dialkoxy-diphe-
nyldiacetylene (DPDA–OCm) began with the synthesis of 4-
alkyloxy-1-iodobenzenes with m ¼ 1–7 by the Williamson ether
reaction. Subsequently, 4-alkyloxy-1-ethynylbenzenes were
prepared by palladium-catalyzed Sonogashira coupling, fol-
lowed by base-induced hydrolysis.
Finally, 1,4-bis(4-alkoxyphenyl-1-yl)buta-1,3-diynes were
obtained by Glaser coupling. The structures of the obtained
compounds were confirmed by 1H-NMR spectroscopy (see
ESI†).
DPDA–Cm (m ¼ 4–12) were synthesized from 4-alkyl-1-
ethynylbenzenes via a similar route.
Thermal properties
The thermal behaviors of DPDA–OCm (m ¼ 1–7) and DPDA–
Cm (m ¼ 4–12) were investigated by polarizing optical micros-
copy (POM) and differential scanning calorimetry (DSC). The
representative DSC curves are shown in Fig. S1†, and the phase
transition temperatures and enthalpy changes measured during
Fig. 1 Transition temperatures plotted as a function of the carbon
number m of the alkyl tail in DPDA–OCm (closed symbols) and DPDA–
Cm (open symbols). The circles and squares are Ti and Tc, respectively.
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The typical UV-vis spectra for DPDA–OC6 and DPDA–C6
(Fig. 2) show no dependence on m. Further, it can be observed
that the longest absorption edges of the DPDA–Cm and DPDA–
OCm series are at 340 nm and 350 nm, respectively. This indi-
cates that the DPDA moiety is an appropriate mesogenic unit
since it is transparent in the visible wavelengths.5,6
The Dn values of the standard 5CB material and DPDA–C5
measured here (see Fig. 4a) correspond to those reported in the
literature,5,23 which supports the credibility of our measurement.
All the compounds exhibited higher Dn than 5CB, which is
attributable to the long conjugated DPDA. Further, the bire-
fringence of DPDA–OCm is relatively higher than that of
Wavelength dependence of birefringence
In much of the literature,1,2,7–10 the reported Dn values for high-
Dn candidates are not actual measurements but extrapolations of
values measured in a mixture with nematic LC compounds such
as 5CB, etc. However, here, we measured actual values for the
nematic LCs of pure target compounds.
The Dn measurement was performed for a homogeneously
aligned nematic LC kept in a thin cell. Nematic alignment was
achieved via the conventional method in which the nematic LC
was sandwiched between two polyimide-coated glass plates that
were rubbed to promote a homogeneous molecular orientation.
The cell gap (4–6 mm) was precisely determined by the interfer-
ometric method. After filling the cell with the LC material,
perfect alignment was confirmed under a polarized microscope.
The transmittance of light under cross-polarization conditions
was then observed as a function of wavelength by a microscope
spectroscopic method using a Nikon LV100 Pol optical micro-
scope equipped with a USB4000 (Ocean photonics) spectrom-
eter. The transmitted light intensity I was modeled with the
following equation:
Fig. 3 Wavelength dependence of light intensity (open circles) trans-
mitted through a homogeneous nematic cell of DPDA–OC1 under cross-
polarization conditions. Here, the orientation direction of the nematic
phase was set at 45ꢀ to the polarization axis. The solid curve is the fitting
of eqn (1).
ꢀ
ꢁ
pDnd
I ¼ I0 sin2
sin2q
(1)
(2)
l
b
c
Dn ¼ a þ
þ
l2 l4
where I0, d, l and q represent the light intensity without a cell, cell
thickness, wavelength and angle between extraordinary axis and
polarization direction, respectively. Then, the observed trans-
mittance was fitted with the calculated value by adopting Cau-
chy’s equation (eqn (2)) for the wavelength dependence of Dn.
The measured values (Fig. 3) were successfully fitted with the
theoretical curve fitting, which allowed the determination of the
wavelength dependence of Dn.
Fig. 4 Birefringence dispersion in the (a) DPDA–OCm and (b) DPDA–
Cm series. The birefringence measurements were performed at tempera-
tures 10 ꢀC below Ti.
Fig. 2 UV-vis absorption spectra of DPDA–OC6 (solid curve) and
DPDA–C6 (dashed curve) in THF.
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DPDA–Cm because of the longer conjugation afforded by the
coupling of the ether oxygen with the DPDA core.
Fig. 4a and b further demonstrate that when m is increased, Dn
reduces in both systems. This trend is more remarkable in
DPDA–OCm at lower m. Several reasons are considered: the
dilution effect,24 order parameter effect,25 and decreased optical
anisotropy.24 The analysis shown later will indicate that the
reduction is produced by the dilution effect by the alkyl tails,
which do not contribute to the molecular polarizability as
effectively as the mesogen. Thus, it is concluded that DPDA–
OC1, with shortest tail, exhibits the highest Dn of 0.4 at 550 nm,
making it a candidate high-Dn material.
Temperature effect
Fig. 6 Plot of n0 (circles) and b (triangles) from eqn (6) determined by
fitting against the carbon number m of the alkyl tail in DPDA–OCm and
DPDA–Cm. The solid curve for the values of Dn0 in DPDA–OCm is the
fitting of eqn (6).
The value of Dn is proportional to the order parameter s,
Dn ¼ Dn0s
and s depends on the temperature as,26
s ¼ (1 ꢁ T/Ti)b
(3)
(4)
(5)
The temperature dependence of Dn was examined at 550 nm,
as shown for DPDA–OCm in Fig. 5. In all the compounds, the
fitting is very good, with the determined values of Dn0 and b listed
in Table 2. Further, by plotting Dn0 and b against m (Fig. 6), we
see an obvious increase in Dn0 with a decrease in m but an almost
constant b of around 0.2. The value of Dn0 expected for the
DPDA moiety that corresponds to the extrapolated value for
m ¼ 0 was found to be extremely large, about 0.9 at 550 nm.
The decrease in Dn with an increase in m can be explained by
the dilution effect of the alkyl tail, since the contribution of the
alkyl tail group to Dn is relatively small or negligible in
comparison with that of the O–DPDA–O moiety. Indeed, it is
probably less than 0.1 in the LC system. Thus, the Dn value can
be approximated as
Then, the temperature dependence of Dn is given by
Dn ¼ Dn0 (1 ꢁ T/Ti)b
Here Dn0 is the birefringence for the nematic LC perfectly
oriented (s ¼ 1), and b is a constant that is characteristic of the
nematic material.
Dn ¼ (Dn1 + 2mv Dn2)/(1 + 2mv) z 0.9/(1 + 2mv)
(6)
here, v is the ratio of the molar volume of methylene units in the
alkyl tail to that of the O–DPDA–O moiety; and Dn1 and Dn2 are
the birefringence values expected for the mesogenic moiety and
alkyl tail part, respectively. The best fitting is attained when v ¼
0.077, as seen in Fig. 6. This volume ratio may be reasonable,
since it is nearly equal to the weight ratio of 0.06.
Conclusions
Fig.
5 Temperature dependence of birefringence (open circles)
We synthesized two series of DPDA-based LCs, DPDA–OCm
(m ¼ 1–7) and DPDA–Cm (m ¼ 4–12), and measured their
nematic-phase birefringence as a function of the wavelength and
temperature. The birefringence of the nematic LCs decreases
with an increase in the alkyl tail length, which is considered to be
the result of a dilution effect caused by the low-birefringent alkyl
tail. Further, of the two series, the DPDA–OCm materials
exhibited higher birefringence, with the highest value of 0.4
obtained for DPDA–OC1 at 550 nm at 10 ꢀC below Ti. The
birefringence, which is proportional to the order parameter s,
depends significantly on the temperature. From an extrapolation
to the perfect orientation state (s ¼ 1), the O–DPDA–O moiety
was found to have a potential to afford a huge birefringence of
0.9, which will be useful for optical phase difference films, laser
measured at a wavelength of 550 nm for DPDA–OCm. The solid curves
are the fittings of eqn (5).
Table 2 Values of Dn0 and b in eqn (5) estimated by curve fitting for
DPDA–OCm
Material
Dn0
b
DPDA–OC1
DPDA–OC2
DPDA–OC3
DPDA–OC4 (eqn (6))
DPDA–OC5
DPDA–OC6
0.77
0.68
0.60
0.55
0.50
0.46
0.44
0.20
0.17
0.18
0.15
0.15
0.16
0.16
DPDA–OC7
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