Development of novel bistolane-based liquid crystalline molecules with an alkylsulfanyl group for highly birefringent materials

Article abstract of DOI:10.1039/c5ra25122b

In order to generate high-birefringence liquid crystal (LC) materials, the introduction of highly polarisable groups into the terminal positions of the mesogen represents one of the most important design strategies. Even though the alkylsulfanyl group is

Full text of DOI:10.1039/c5ra25122b

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Development of novel bistolane-based liquid
crystalline molecules with an alkylsulfanyl group
Cite this: RSC Adv., 2016, 6, 16568
for highly birefringent materials†
Yuki Arakawa,ab Sungmin Kang, Hideto Tsuji,b Junji Watanabea
a
*
a
*
and Gen-ichi Konishi
In order to generate high-birefringence liquid crystal (LC) materials, the introduction of highly polarisable
groups into the terminal positions of the mesogen represents one of the most important design
strategies. Even though the alkylsulfanyl group is a potentially interesting option for this purpose, it has
so far been very difficult to obtain enantiotropic mesophases for LCs containing this group. Herein, we
report novel high-birefringence LC molecules based on 1,4-bis(2-phenylethynyl)benzene (bistolane) with
alkylsulfanyl groups in the terminal positions. The incorporation of a fluorine atom into the central
benzene ring of an alkylsulfanyl-substituted bistolane led to the formation of
a well-defined
enantiotropic nematic phase. In contrast, the nonfluorinated analogue with alkylsulfanyl groups did not
exhibit a mesophase. In comparison with an alkoxy-substituted derivative, the birefringence of the
alkylsulfanyl-substituted analog was substantially higher (Dn ¼ 0.42 at 550 nm). Furthermore, the
birefringence properties of the alkylsulfanyl-substituted derivative were observed to be proportional to
the order parameter and largely temperature-dependent over the entire temperature range. These
results thus provide not only fundamental insights into structure–reactivity relations, but also furnish
practical design principles for the synthesis of new sulfur-containing, rod-shaped LC materials with
optical applications.
Received 26th November 2015
Accepted 26th January 2016
DOI: 10.1039/c5ra25122b
www.rsc.org/advances
synthesised.3,5,6 Oen, highly polarisable groups such as cyano
and isothiocyanate groups are attached to the terminal posi-
Introduction
tions of such materials, in order to accomplish large molecular
polarisability anisotropies.6c,d,g,h
Recently, liquid-crystalline (LC) materials with high birefrin-
gence (Dn) have been introduced in a variety of optical appli-
cations, including LC display (LCD) technology,1 cholesteric
(Ch) LC lms,2 holographic materials,3 LC lenses,4 and polar-
isers.5 In these applications, nematic liquid crystals are the
most important type of LC materials, owing to their desirable
uidity and the ability to easily control their alignment.
The conventional design of high-Dn materials is based on
rod-shaped cores, which are generated by a combination of
aromatic rings and unsaturated bonds. This strategy usually
provides an improved refractive index (n) value along the
molecular long axis, the so-called extraordinary refractive index
(ne). To date, a number of high-Dn materials, wherein multiple
aromatic rings such as benzene, naphthalene, thiophene, or
pyridine are joined by alkane, alkene, or azo bonds, have been
In the context of the aforementioned molecular design
strategy, we believe that the alkylsulfanyl (alkylthio or thioether)
group should also represent a promising option, due to its high
polarisability7 and the exibility and/or small bend angle of its
S–C bond,8 which lead to low transition temperatures relative to
alkoxy groups. Hird et al. reported that LC biphenyl derivatives,
bearing alkylsulfanyl groups with varying length of the carbon
chain, exhibited high Dn, albeit that liquid crystallinity was
scarcely observed.7a–d Moreover, Seed et al. reported that
naphthalene-based, rod-like materials with butylsulfanyl
groups exhibited high Dn and nematic phases.7e,f Very recently,
we reported novel hydrogen-bonded, tolane-based LC
compounds with hexylsulfanyl groups as high-Dn materials,
which exhibited enantiotropic mesophases induced by the
carboxylic dimeric form.9 Compared to derivatives with alkyl
and alkoxy groups, these materials exhibited high Dn and highly
correlated mesophases on account of the high polarisability of
the sulfur atoms and the S–S interactions. However, additional
smectic phases were eventually observed at relatively high
temperatures (T > 200 ꢀC), just below the nematic LC regime. It
is generally assumed that sulfur-containing rod-like molecules,
aDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo
152-8552, Japan. E-mail: skang@polymer.titech.ac.jp; konishi.g.aa@m.titech.ac.jp;
Fax: +81-3-5734-2321
bDepartment of Environmental and Life Sciences, Graduate School of Engineering,
Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
† Electronic supplementary information (ESI) available: DSC charts and POM
images. See DOI: 10.1039/c5ra25122b
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due to their strong molecular attractions, can generate stable a Perkin Elmer DSC7, whereby heating/cooling scans were
nematic phases only with difficulty. Accordingly, reports on rod- carried with a temperature gradient of 10 C minꢁ1. Measure-
ꢀ
shaped LC materials with alkylsulfanyl groups are scarce,6–10 ment of ne, no and Dn was achieved with uniaxially aligned
while an abundance of literature related to the mesomorphic nematic cells, which were purchased from EHC and contained
properties of discotic-11 and bend-type7,12 LC materials is an indium tin oxide (ITO) layer. Cell gaps (d) of 2–3 mm were
available.
measured according to the interferometric method.14 The
Herein, we report the synthesis and characterisation of the transmittance of light under crossed nicol conditions was
novel, conjugated, rod-like nematic LC material 6, which is recorded as a function of wavelength by a spectroscopic
based on 1,4-bis(2-phenylethynyl)benzene (bistolane), bearing microscopy method using a Nikon LV100 Pol optical micro-
alkylsulfanyl groups at the para-positions of the terminal scope equipped with
benzene rings and a uorine atom at the central benzene ring spectrometer.
(Fig. 1). The use of bistolane offers two main advantages: (a)
a
USB4000 (Ocean photonics)
a high Dn due to the large anisotropic molecular structure, and
(b) the opportunity to readily implement molecular derivatisa-
Materials
tion on account of 14 potential substitution positions on the
three benzene rings.6a,b,e,q In this study, we focused on bistolane
derivatives with a uorine atom on the central benzene ring (4–
6). Such a uorination should effectively lower the transition
temperatures and expand the nematic phase.13 Furthermore, we
speculated that the uorination of bistolane derivatives with
alkylsulfanyl groups could also lead to the formation of enan-
tiotropic nematic phases. Indeed, by introducing uorine at the
central benzene moiety of alkylsulfanyl-substituted bistolane 3
that did not exhibit a mesophase, an enantiotropic nematic
phase was successfully generated for uorinated 6. Its phase
transition behaviour and Dn were examined in detail by
comparison with uorinated (4 and 5) and nonuorinated
analogues (1–3) that contained alkyl, alkoxy, or alkylsulfanyl
end groups. Moreover, the Dn, no, and ne values in this study are
actual values for the nematic phases of pure LC target
compounds, obtained from our previous report.6k In contrast,
Dn values in the literatures represent almost always extrapola-
tions of measurements on mixtures that contain nematic LC
compounds, such as e.g. 5-cyanobiphenyl (5CB).
1-Ethynyl-4-hexylbenzene, 4-iodophenol, 4-bromobenzenethiol,
trimethylsilylacetylene, 4-bromoiodobenzene, 4-bromo-2-
uoroiodobenzene, and Pd(PPh3)4 were purchased from TCI
(Tokyo, Japan), while 6-bromohexane, triethylamine (TEA), and
PPh3 were purchased from Wako Pure Chem (Tokyo, Japan).
CuI was purchased from Kanto Chemical (Tokyo, Japan). Unless
otherwise noted, all other chemicals were purchased from
common commercial suppliers and used as received. 1-Ethynyl-
4-hexyloxybenzene,6k 1-bromo-4-hexylsulfanylbenzene,9 1-hex-
ylsulfanyl-4-[2-(trimethylsilyl)ethynyl]benzene,9 and 1-ethynyl-4-
hexylsulfanylbenzene9 were synthesized according to previously
reported procedures.
The synthetic routes to 1–6 are shown in Scheme 1, and the
procedures and characterisation data, including 1H and 13C
NMR spectra, as well as high-resolution mass spectroscopy
(HRMS) data, are described below. Compounds 1–6 were
prepared in a yield of ꢂ50% by Pd-catalyzed Sonogashira
coupling reactions.
1,4-Bis[4-(hexyl)phenyl]ethynylbenzene (1) (general proce-
dure for the Sonogashira coupling). In a two-neck ask,
a mixture of 1-ethynyl-4-hexylbenzene (3.11 mmol, 0.58 g), TEA
(5 mL), and THF (5 mL) was degassed with argon bubbling.
Another two-way ask was charged with 1-bromo-4-
Experimental
General
1H NMR and 13C NMR spectra were measured in CDCl3 on
a JEOL LNM-EX 400 or a Bruker DPX300S spectrometer using
tetramethylsilane (TMS) as an internal standard. The transition
behavior was investigated by polarizing optical microscopy
(POM) using a Leica DM2500P microscope with a Mettler FP90
hot stage, and by differential scanning calorimetry (DSC) using
Fig. 1 Molecular structure of bistolane derivatives 1–6.
Scheme 1 Synthetic route to 1–6.
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iodobenzene (1.41 mmol, 0.40 g), CuI (71 mmol, 85 mg), and described Sonogashira coupling method. 1-Ethynyl-4-
Pd(PPh3)4 (71 mmol, 14 mg) and lled with argon, before the hexyloxybenzene (0.630 g, 3.11 mmol), 1-bromo-3-uoro-4-
aforementioned liquid mixture was added. The resulting iodobenzene (0.420 g, 1.41 mmol), Pd(PPh3)4 (81.0 g, 71.0
mixture was stirred for 12 h at 60 ꢀC. Subsequently, 2 M HCl aq. mmol), CuI (14.0 mg, 71.0 mmol), TEA (5 mL) and THF (5 mL)
was added to quench any remaining TEA, and all inorganic salts were used. Pale yellow solid (0.101 g, 72%). 1H NMR (400 MHz,
were removed by ltration. Then, the reaction mixture was CDCl3) d 7.52–7.39 (m, Ar–H, 5H), 7.28–7.18 (m, Ar–H, 2H), 6.87
extracted with ethyl acetate, and the combined organic fractions (d, J ¼ 8.8 Hz, 4H), 3.97 (t, J ¼ 6.6 Hz, –OCH2–, 4H), 1.79 (tt, J ¼
were washed with water and brine, before being dried over 6.6 and 7.6 Hz, –OCH2CH2–, 4H), 1.52–1.24 (m, –CH2CH2CH2-
MgSO4. Aer ltration and evaporation of the solvents, the CH3, 12H), 0.91 (t, J ¼ 6.6 Hz, –CH3, 6H) ppm. HRMS-FAB+ (m/z):
crude product was puried by column chromatography on silica [M] calcd for C34H37FO2, 496.2778; found, 496.2772.
gel (eluent: hexane), and recrystallization from methanol.
1,4-Bis[4-(hexylsulfanyl)phenyl]ethynyl-2-uorobenzene (6).
Colorless solid (0.35 g, 55%). 1H NMR (400 MHz, CDCl3) d 7.51– Compound 6 was synthesised according to the previously
7.48 (brs, Ar–H, 4H), 7.45 (d, J ¼ 8.0 Hz, Ar–H, 4H), 7.17 (d, J ¼ described Sonogashira coupling method. 1-Ethynyl-4-
8.0 Hz, Ar–H, 4H), 2.62 (t, J ¼ 7.8 Hz, –PhCH2–, 4H), 1.62 (tt, J ¼ hexylsulfanylbenzene (1.0 mmol, 0.30 g), 1-bromo-3-uoro-4-
7.0 and 7.8 Hz, –PhCH2CH2–, 4H), 1.39–1.22 (m, –CH2CH2- iodobenzene (2.1 mmol, 0.46 g), Pd(PPh3)4 (50 mmol, 58 mg),
CH2CH3, 12H), 0.89 (t, J ¼ 6.8 Hz, –CH3, 6H) ppm.13C NMR (100 CuI (50 mmol, 10 mg), TEA (5 mL) and THF (5 mL) were used.
MHz, CDCl3) d 143.7, 131.5, 131.4, 128.5, 123.1, 120.1, 91.4, Colorless solid (0.45 g, 85%). 1H NMR (400 MHz, CDCl3) d 7.49–
88.5, 35.9, 31.7, 31.2, 28.9, 22.6, 14.1 ppm.
7.41 (m, Ar–H, 5H), 7.29–7.22 (m, Ar–H, 6H), 2.95 (t, J ¼ 7.4 Hz,
1,4-Bis[[4-(hexyloxy)phenyl]ethynylbenzene (2). Compound 2 –SCH2–, 4H), 1.68 (tt, J ¼ 7.4 and 7.4 Hz, –SH2CH2–, 4H), 1.44 (tt,
was synthesised according to the previously described Sonoga- J ¼ 6.6 and 7.2 Hz, –SCH2CH2–, 4H), 1.23–1.27 (m, –CH2CH2-
shira coupling method. 1-Bromo-4-iodobenzene (0.400 g, 1.41 CH2CH3, 12H), 0.89 (t, J ¼ 6.8 Hz,–CH3, 6H) ppm. HRMS-FAB+
mmol), Pd(PPh3)4 (81.0 g, 71.0 mmol), CuI (14.0 mg, 71.0 mmol), (m/z): [M] calcd for C34H37FS2, 528.2321; found, 528.2330.
1-ethynyl-4-hexyloxybenzene (0.630 mg, 3.11 mmol), TEA (5 mL)
1
and THF (5 mL) were used. Colorless solid (0.371 g, 55%). H
Results and discussion
NMR (400 MHz, CDCl3) d 7.43 (d, J ¼ 8.4 Hz, Ar–H, 8H), 6.85 (d, J
Thermal properties of 1–6
¼ 8.4 Hz, Ar–H, 4H), 3.97 (t, J ¼ 6.6 Hz, –OCH2–, 4H), 1.83 (tt, J ¼
6.6 and 7.4 Hz, –OCH2CH2–, 4H), 1.50–1.32 (m, –CH2CH2CH2- The phase transition behaviour of the bistolane derivatives (1–
CH3, 12H), 0.91 (t, J ¼ 6.6 Hz, –CH3, 6H) ppm. 13C NMR (100 6) was characterised by polarising optical microscopy (POM)
MHz, CDCl3) d 13.9, 22.6, 25.7, 29.2, 31.6, 68.3, 88.0, 91.4, 114.7, and differential scanning calorimetry (DSC). The POM images
115.2, 123.3, 131.3, 133.1, 159.5 ppm. HRMS-FAB+ (m/z): [M] and DSC curves of all compounds, except those described in the
calcd for C34H38O2, 478.2872; found, 478.2878.
main text, are shown in the ESI.† Phase transition temperatures
1,4-Bis[4-(hexylsulfanyl)phenyl]ethynylbenzene (3). Compound and enthalpy changes were determined and mesophases clas-
3 was synthesised according to the previously described Sonoga- sied; a summary can be found in Table 1. Nonuorinated
shira coupling method. 1-Ethynyl-4-hexylsulfanylbenzene (0.33 bistolanes with alkyl (1) and alkoxy tails (2) were observed to
mmol, 1.50 g), 1-bromo-4-iodobenzene (0.71 mmol, 0.20 g), TEA form enantiotropic nematic (N) phases. However, alkylsulfanyl-
(5 mL), THF (5 mL), Pd(PPh3)4 (75 mmol, 87 mg) and CuI (75 mmol, substituted 3 did not exhibit mesophases, but displayed
14 mg) were used. Colorless solid (0.34 g, 47%). 1H NMR (400 MHz, a melting point (Tm ¼ 171.7 ꢀC). Fluorinated 4 and 5, which
CDCl3) d 7.51–7.46 (brs, Ar–H, 4H), 7.43 (d, J ¼ 8.0 Hz, Ar–H, 4H), contain alkyl and alkoxy tails, respectively, also exhibited
7.29–7.24 (m, Ar–H, 4H), 2.95 (t, J ¼ 7.2 Hz, –SCH2–, 4H), 1.68 (tt, enantiotropic N phases, similar to nonuorinated 1 and 2. It is
J ¼ 7.2 and 7.8 Hz, –SCH2CH2–, 4H), 1.44 (tt, J ¼ 6.8 and 7.8 Hz, noteworthy that uorinated 6, with an alkylsulfanyl tail, even-
–SCH2CH2CH2–, 4H), 1.36–1.24 (m, –CH2CH2CH3, 8H), 0.89 (t, J ¼ tually exhibited an enantiotropic N phase.
7.0 Hz,–CH3, 6H) ppm. 13C NMR (75 MHz, CDCl3) d 139.0, 133.9,
A representative DSC curve and POM image of 6 are shown in
132.2, 131.8, 128.5, 120.5, 91.5, 89.8, 33.7, 31.7, 29.4, 28.8, 22.8, Fig. 2. Here, Tm and Ti (the transition temperature from the N to
14.2 ppm.
the isotropic phase) values of 112.2 ꢀC and 143.6 ꢀC were
1,4-Bis[4-(hexyl)phenyl]ethynyl-2-uorobenzene (4). Compound observed, respectively. These values, which were determined
4 was synthesised according to the previously described Sonoga- from the heating scan, indicated a relatively wide N (DT > 30 ꢀC).
ꢀ
shira coupling method. 1-Ethynyl-4-hexylbenzene (3.11 mmol, 0.58 The Tm value of 6 was found to be approximately 60 C lower
g), 1-bromo-3-uoro-4-iodobenzene (1.41 mmol, 0.40 g), TEA (5 than that of 3. Accordingly, the introduction of a uorine atom
mL), THF (5 mL), Pd(PPh3)4 (71 mmol, 85 mg) and CuI (71 mmol, 14 at the central benzene ring may weaken the intermolecular
mg). Colorless solid (0.30 g, 46%). 1H NMR (400 MHz, CDCl3) interactions due to the expanded rotational volume, and may
d 7.47 (d, J ¼ 8.0 Hz, Ar–H, 2H), 7.44 (d, J ¼ 8.0 Hz, Ar–H, 2H), 7.44 thus contribute to the formation of mesophases in 6. This result
(s, Ar–H, 1H), 7.29–7.22 (m, Ar–H, 2H), 7.17 (d, J ¼ 8.0 Hz, Ar–H, suggests that uorine substitution at the central benzene ring of
4H), 2.62 (t, J ¼ 7.8 Hz, –PhCH2–, 4H), 1.61 (tt, J ¼ 7.0 and 7.8 Hz, conjugated, rod-like molecules effectively contributes to the
–PhCH2CH2–, 4H), 1.38–1.21 (m, –CH2CH2CH2CH3, 12H), 0.88 (t, J formation of mesophases. During the cooling scan, crystalliza-
¼ 6.6 Hz, –CH3, 6H) ppm.
tion temperatures (Tc), which indicate a phase transition from
(5). the N to the crystalline phase, were found to be signicantly
1,4-Bis[4-(hexyloxy)phenyl]ethynyl-2-uorobenzene
Compound 5 was synthesised according to the previously decreased, and a nematic temperature range of ꢂ40 ꢀC was
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Table 1 Phase transition temperaꢁtu1 res and enthalpy changes from the 2nd heating (top line) and 1st cooling (bottom line) in DSC thermograms
(temperature gradient: 10 ꢀC min
)
Compound
Cr
T (ꢀC)
DH (kJ molꢁ1
)
N
T (ꢀC)
DH (kJ molꢁ1
)
Iso
LC range (ꢀC)
1
2
3
4
5
6
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
155.3
148.9
179.5
173.3
171.7
165.8
87.9
19.6
18.8
27.3
25
25.9
25.7
37.9
18.7
30.2
30.3
32.6
32.6
ꢃ
ꢃ
ꢃ
ꢃ
187.9
183.8
239.4
235.6
1.5
1.6
2.5
2.7
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
32.6
34.9
59.9
62.3
—
—
77
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
165
1.5
1.6
2.6
2.7
1.3
1.4
66
161.2
216.7
211.9
143.6
139.6
95.2
93.3
101.3
31.3
39.6
123.4
110.6
112.2
100
refers to the measured temperature (T) as well as to the reduced
temperature (TIN ꢁ T) values, wherein TIN represents the tran-
sition temperature from isotropic to namatic phase upon
cooling process.
To evaluate the effect of the introduction of a sulfur atom on
the refractive index, the ordinary (no) and extraordinary refrac-
tive index (ne), as well as the birefringence (Dn ¼ ne ꢁ no) of 5
and 6 were compared.‡.
The actual method for determining the ne, no, and Dn values
of single-component systems is described in our previous
reports.6k,14 The dependence of the obtained refractive indices
(ne and no) and Dn at 550 nm on the reduced temperature (TIN
ꢁ
T) for 5 and 6 are compared in Fig. 3. With an increase of the TIN
ꢁ T values, ne values were found to increase for both
compounds, while no values were found to decrease (Fig. 3(a)).
This is commensurate with an increase of Dn upon decreasing
the temperature (Fig. 3(b)), which is due to the enhanced order
parameters at lower temperatures. Over the entire reduced
temperature region, the ne values of 6 were higher than those of
5, while the corresponding no values remained comparable.
This should be predominantly attributed to the signicantly
greater polarisability of the sulfur atom relative to that of the
oxygen atom. Therefore, Dn values of 6 are clearly higher than
those of 5 at each reduced temperature over the entire reduced
temperature region (Fig. 3(b)). The ne, no, and Dn values of 5 and
6 were compared at TIN ꢁ T ¼ 26 ꢀC in Table 2. The ne value of 6
(1.99) was greater than that of 5 (1.93), resulting in a higher Dn
value of 6 (0.41) relative to that of 5 (0.34). Furthermore, the
temperature dependence of the Dn values for sulfur-containing
6 was found to be higher than that for 5 (cf. the slope of both
curves in Fig. 3(b)), which is consistent with the trend that we
observed in our recent report.9 This may be attributed to the
strong attractive interactions derived from the high polar-
isability of sulfur atoms and the S–S interactions, and may be
a universal trend for sulfur-containing, rod-like nematic LC
molecules. Even though the nematic region of 6 is much
Fig. 2 DSC curve and POM image of 6 at 137 ꢀC (N phase).
observed for 6 (Table 1). In general, the introduction of a uo-
rine atom on the central benzene ring resulted in a signicant
decrease of the Tm values relative to the Ti values, i.e. a wider
mesophase temperature range was observed for each uori-
nated bistolanes relative to its corresponding nonuorinated
analog.13 The lowest Tm value (88 ꢀC) was observed for 4.
Interestingly, a slightly lower Tm value was observed for 6 (112
ꢀ
ꢀC) relative to 5 (123 C). This is consistent with previously re-
ported results, which implies that this might be a universal
trend (cf. 2 and 3 in Table 1).7a,9 The observed results should be
ascribed to the increased exibility of the S–C bond in the
alkylsulfanyl group compared to the C–O bond in the alkoxy
group.8 The DH values for the transition from mesophase to
isotropic phase (DHi) increased in the order of 6 < 4 < 5.
Accordingly, the Ti vꢀalue for 6 (144 ꢀC) is considerably lower
ꢀ
than those of 4 (165 C) and 5 (217 C), resulting in a compar-
ꢀ
ꢀ
atively small LC range for 6 (ꢂ30 C upon heating and ꢂ40 C
upon cooling).
Optical properties of 5 and 6
As the optical anisotropy depends both on the polarisability and
the order parameters, the understanding of the temperature
dependence of LC molecules is important for developing
‡ As the refractive indices and the birefringence of alkoxy-substituted derivatives
are signicantly higher than those of alkyl-derivatives,6k those of 4 are omitted
advanced optical materials. Therefore, the following discussion from this comparison.
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Extrapolated Dno values of 0.69 and 0.59 were estimated for 6
and 5, respectively. These results suggest that an improvement
of the Dno values should be expected upon replacement of the
oxygen with sulfur atoms, due to the large temperature depen-
dence of the perfectly aligned sulfur-containing molecules, as
well as on account of the dependence of Dn on (TIN ꢁ T).9
Conclusion
In this article, we describe the design and synthesis of
bistolane-based LC molecules with alkylsulfanyl groups as novel
sulfur-containing, conjugated, rod-like LC molecules for mate-
rials with high birefringence. Nonuorinated alkylsulfanyl
derivative 3 did not exhibit a mesophase, which is in marked
contrast to the nonuorinated analogues with alkyl (1) and
alkoxy groups (2) that exhibit well-dened enantiotropic
nematic phases. Nevertheless, we succeeded in obtaining an
enantiotropic nematic phase for alkylsulfanyl-substituted 6 by
introducing uorine atoms at the lateral position on the central
benzene ring. A comparison of the refractive index properties
revealed higher ne and Dn values for 6 relative to alkoxy-
substituted 5. Furthermore, a calculation of Dno for 5 and 6
suggested a higher optical performance for the latter with
respect to the former, given that the same order parameter is
achieved. The birefringence is signicantly inuenced by
temperature, due to the large intermolecular interactions
arising from the greater polarisability of the sulfur atoms and
the S–S interactions, which is consistent with previously re-
ported results.9 Consequently, the maximum Dn value of 0.42,
which was observed for 6 at 550 nm implies that alkylsulfanyl-
substituted 6 may nd applications in novel materials with
high birefringence. Compared to our previous reports,6l the
obtained Dn values are moderate, which is due to the moderate
Dn of the bistolane mesogen itself. The observed meso-
morphism and optical performance of 6 might in conclusion be
attributed to the combined effects of the uorination, resulting
in the formation of a nematic phase, and the introduction of
terminal alkylsulfanyl groups, which should contribute to the
molecular interactions and polarizability. Thus, the uorina-
tion of the central benzene ring, combined with the incorpo-
ration of polar terminal groups seems to represent a desirable
solution for the generation of highly birefringent LC materials.
Fig. 3 (a) Temperature dependence of the ne and no values of 5 (blue
squares) and 6 (red circles). (b) Temperature dependence of Dn for 5
(blue squares) and 6 (red circles).
Table 2 Experimental ne, no, and Dn values, as well as extrapolated
parameters for 5 and 6 at TIN ꢁ T ¼ 26 ꢀ
C
a
a
b
b
Compound
ne
no
Dna
Dno
To
bb
5
6
1.93
1.99
1.60
1.58
0.34
0.41
0.59
0.69
218.1
142.8
0.20
0.20
a
At TIN ꢁ T ¼ 26 ꢀC. b Extrapolated by tting to Haller's equation.
narrower than that of 5, the maximum Dn value of 6 (0.42) was
higher than that of 5 (0.41) when measured at the respective
minimal temperature. This result indicates that alkylsulfanyl
groups are suitable substituents for the generation of novel
highly birefringent nematogens.
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