3,4-Difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-based tolan liquid crystals

Article abstract of DOI:10.1016/j.tet.2013.04.077

A series of new 3,4-difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-based tolan liquid crystals resulted from palladium-free Sonogashira coupling reactions of 1-(4-iodophenyl)-3,4-difluoropyrrole, 1-(4-iodophenyl)-3,3,4,4- tetrafluoropyrrolidine with terminal phenyl acetylenes. These new compounds exhibit typical nematic and smectic phases, good thermal stabilities, and high clearing points in comparison to the liquid crystal phase of 3,4-difluorobenzene-based tolan. These encouraging results lead us to believe this class of compounds could be used as new liquid crystalline materials. It is possible that the fluorinated N-heterocycles, 3,4-difluoropyrrole and 3,3,4,4-tetrafluoropyrrolidine, could replace the expensive 2,3-difluorobenzene, 3,4-difluorobenzene, and 3,4,5-trifluorobenzene components in the tolan core structure to form promising liquid crystalline materials.

Full text of DOI:10.1016/j.tet.2013.04.077

Tetrahedron xxx (2013) 1e7
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
3,4-Difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-based tolan
liquid crystals
,b,
Hongren Chen a, Peilian Liu a, Huijuan Li a, Stelck Daniel c, Zhuo Zeng a
*
a College of Chemistry & Environment, South China Normal University, 378 Waihuanxi Lu, Guangzhou 510006, China
b Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 345 Lingling Lu, Shanghai 200032, China
c Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new 3,4-difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-based tolan liquid crystals resulted
from palladium-free Sonogashira coupling reactions of 1-(4-iodophenyl)-3,4-difluoropyrrole, 1-(4-
iodophenyl)-3,3,4,4-tetrafluoropyrrolidine with terminal phenyl acetylenes. These new compounds ex-
hibit typical nematic and smectic phases, good thermal stabilities, and high clearing points in compar-
ison to the liquid crystal phase of 3,4-difluorobenzene-based tolan. These encouraging results lead us to
believe this class of compounds could be used as new liquid crystalline materials. It is possible that the
fluorinated N-heterocycles, 3,4-difluoropyrrole and 3,3,4,4-tetrafluoropyrrolidine, could replace the ex-
pensive 2,3-difluorobenzene, 3,4-difluorobenzene, and 3,4,5-trifluorobenzene components in the tolan
core structure to form promising liquid crystalline materials.
Received 18 February 2013
Received in revised form 11 April 2013
Accepted 17 April 2013
Available online xxx
Keywords:
Fluorine
Heterocycles
Liquid crystals
Mesophases
Phase transitions
Ó 2013 Published by Elsevier Ltd.
1. Introduction
fluorobenzene and 3,4-difluorobenzene derivatives as STN and
TFT liquid crystal materials.1,13e16 Some of these reported liquid
The super twisted nematic liquid crystal display (STN-LCD)
product was widely used in cell phones, PDAs, and vehicles that
carry intellectualization alphanumeric display terminals.1e4 How-
ever, their display performances in the area as contrast and re-
sponse rates were insufficient for new electronic product
requirements.5 Liquid crystalline materials need to have large bi-
crystal molecules are shown in Scheme 1. The tolan molecules with
a fluorine atom as a substituent on the benzene ring have large
birefringence values and low viscosities.
C3H7
C4H9
F
refringence (Dn) values, which are essential for liquid crystal dis-
plays with a high response speed. These materials must be also free
(a) C 90 N 189 I
(b) C 57 I
from interference fringes.6 It has been shown that the response
speed can be accelerated with an increase in the birefringence (
Dn)
F
value and a decrease in the viscosity ( ) of liquid crystal materi-
h
als.7e9 Tolan-type liquid crystal compounds with the strong con-
jugation between the benzene rings and an acetylenic bond within
the liquid crystal have some advantages. These include large
thresholds, low viscosity, broader nematic phase ranges, and lower
melting points.10e12 One can modify the liquid crystal birefringence
and clearing points to improve the response speed of a liquid
crystal material.12 Interest in modern high-performance display
materials continues to grow and a large number of tolan-type liq-
uid crystals have been reported.13 There are reports of many 4-
F
Low viscosity and large birefringence value tolan LCD
Scheme 1. Fluorinated tolan liquid crystals.
Among these fluorinated tolan molecules, compound (a) has
a relatively high clearing point. However, it has inferior compati-
bility with other liquid crystal compounds when deposited. Hence,
this drawback makes it impossible to take advantage of this feature.
Compound (b) has no liquid crystal phase and hence when it is used
as a component of liquid crystal composition, it adversely reduces
* Corresponding author. Tel.: þ86 20 39310258; fax: þ86 20 39310187; e-mail
addresses: zhuoz@scnu.edu.cn, zhuosioc@yahoo.com.cn (Z. Zeng).
0040-4020/$ e see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2013.04.077
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2
H. Chen et al. / Tetrahedron xxx (2013) 1e7
nematiceisotropic (NeI) phase transitions of the composition. It is
possible that by replacing fluorobenzene or difluorobenzene in the
liquid crystals by fluorinated heterocycles could either improve
their solubility, increase their compatibility with other liquid
crystalline compounds or increase the liquid crystal phase range for
these compounds.
Sonogashira coupling reaction.22 We choose to use the Sonogashira
coupling reaction, which has been extensively used in recent years
and is based on the aryl iodides and terminal acetylenes.23 The aryl
iodide 4F, 3,3,4,4-tetrafluoro-1-(4-iodophenyl)pyrrolidine, was
obtained by the trifluoromethanesulfonic alkyl esters 1a reacted
with 4-iodobenzenamine in the presence of Et3N in CH3CH2OH.
Dehydrofluorination of 4F with excess t-BuONa in DMSO at 90 ꢀC
for 12 h generated the 3,4-difluoro-1-(4-iodophenyl)pyrrole, 2F, in
high yields. The Sonogashira reaction procedure is sensitive to ox-
ygen; traces of oxygen can lead to self-coupling, and utilizes an
expensive palladium catalyst or a variety of homogenous palladium
catalyst, which raises the cost.24 An efficient palladium-free Sono-
gashira reaction procedure has been developed.25 Reaction of
aryl iodides 4F and 2F with phenyl acetylene in the water with
CuI catalyst combining polyethylene glycol as phase transfer cata-
lyst, we synthesized a series new 3,4-difluoropyrrole-, 3,3,4,4-
tetrafluoropyrrolidine-based tolan liquid crystals 4FA1,4FA2,
2FA1,2FA2, 4FB1e4FB3, and 2FB1e2FB3 were obtained in 90%
yield. This synthetic route is of palladium-free, producing high
yield, thus making the industrialization of these tolan-type liquid
crystals possible (Scheme 2). This method appears to be an untried,
but should be a straight forward single step route to these fluori-
nated N-heterocycles tolan-type liquid crystalline materials.
Tolan liquid crystals with a longer alkoxy substituent display
good compatibility properties and can be used as effective com-
ponents to form liquid crystals that have a large birefringence. The
3,4-difluoropyrrole-based tolan liquid crystals with C3eC6 alkoxy
and ester group chains as substituent, 2FA3e2FA6 and 2FC1e2FC3
are shown in Scheme 3.
We have a continuing interest in fluorinated N-heterocycles for
new building blocks in high-performance liquid crystal materi-
als.17e19 New fluorinated liquid crystals (FLCs) bearing fluorinated
N-heterocycles will prove to be highly valuable because of their
advantageous applications. The advantages of 3,3,4,4-tetrafluoro
pyrrolidine and its dehydrofluorinated derivative, 3,4-difluoro
pyrrole, that make them attractive candidates for further in-
vestigation include: (a) they have a large dielectric constant, which
gives the liquid crystal the appropriate dipole moment; (b) linking
3,4-difluoropyrrole with phenyl acetylene compounds will expand
the tolan conjugation system; (c) stronger electron withdrawing
properties relative to fluorobenzene and difluorobenzene-based
tolan liquid crystal giving novel characteristics. Therefore, in this
work, we report the synthesis, characterization, and properties of
some new 3,4-difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-
based tolan liquid crystals.
2. Results and discussions
2.1. Synthesis
3,4-Difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-based tolan
liquid crystals, 4FA1,4FA2, 4FB1e4FB3, 2FA1e2FA6, 2FB1e2FB3,
and 2FC1e2FC3 were synthesized as shown in Schemes 2 and 3.
Details of the synthesis and characterization of the materials are
given in Supplementary data.
Demethylation of 4FA1 with BBr3 in dichloromethane at 0 ꢀC for
4 h yielded the 4-(2-(4-(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl)
ethynyl)phenol, 4FOH.26 The dehydrofluorination 4FOH with ex-
cess t-BuONa gave the intermediate 4-(2-(4-(3,4-difluoro-1H-pyr-
rol-1-yl)phenyl)ethynyl)phenol, 2FOH. Alkylation of 2FOH with
alkyl bromide in DMF as solvent at 90 ꢀC for 12 h gives longer
alkoxy substituent tolan liquid crystals, 2FA3e2FA6. When 2FOH is
reacted with a slight molar excess of acrylic acid, trans-40-propyl-
(1,10-cyclohexyl)-4-carboxylic acid (3CA) or trans-40-propyl-(1,10-
bicyclohexyl)-4-carboxylic acid (3CCA), the resulting compounds
are an alkyl ester substituted 3,4-difluoropyrrole-based tolan liquid
crystals, 2FC1e2FC3.
2.2. Liquid crystalline properties
The phase transitions and thermodynamic data of compounds
were investigated by differential scan calorimetry (DSC), thermal
gravimetric analysis (TGA), polarizing optical microscope (POM),
and variable-temperature X-ray diffraction (VTXRD). Three series
of 3,4-difluoropyrrole-, 3,3,4,4-tetrafluoropyrrolidine-based tolan
liquid crystal molecules, which differ in structure especially at the
terminal position will be investigated. Substituents such as alkoxyl,
alkyl, and ester groups in the para positions of arylacetylene unit
were used (Schemes 4e6). There is evidence that these compounds
exhibit diverse thermal behavior. In the following, detailed ac-
counts of all the series of mesophase behaviors are presented and
compared with homologous tolan compounds bearing different
substituents.
The transition temperatures and mesophase behaviors for the
series A liquid crystals are given in Table 1. All the compounds were
found to exhibit mesomorphic behavior. Their enthalpy values are
in the range of 0.28e16.74 J/g by the differential scan calorimetry
(DSC) thermograms.
Scheme 2. Synthesis of the fluorinated N-heterocycles tolan-type liquid crystals.
Three synthetic routes for preparation tolan-type liquid
crystals have been described previously. These include the
FristcheButtenbergeWiechell rearrangement,20 the 1,2-dibromo-
1,2-diarylethane dehydrobromination reaction,21 and the
A mosaic texture of a smectic B (SmB) phase was observed from
polarizing optical microscope (POM) for 3,3,4,4-tetrafluoropyr
rolidine-based tolan compounds 4FA1, 4FA2 at 198.0 ꢀC, and
190.0 ꢀC upon sample cooling. They exhibited a narrow phase
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H. Chen et al. / Tetrahedron xxx (2013) 1e7
3
F
F
F
F
F
F
F
N
N
OCH3
N
OH
F
4FOH
4FA1
F
F
N
OR
F
2FA3-2FA6: R= n-C3H7 to n-C6H13
OH
F
F
O
N
O
R
F
m
2FC1: m=0, R=C2H3
2FC2: m=1, R=C3H7
2FC3: m=3, R=C3H7
Reagents and conditions: ) BBr3, CH2Cl2, 0 0C, 4h. ) t-BuONa, DMSO, N2, 100 0C, 12h. ) DMF, K2CO3, n-
RBr 90 0C, 12h. ) DCC, DMAP, C2H3COOH(3CA or 3CCA), CHCl3, 80 0C.
3CCA:
3CA:
COOH
COOH
Scheme 3. Synthesis of the fluorinated N-heterocycles tolan-type liquid crystals with alkoxy and ester group as substituents.
Table 1
Thermal and mesophase behavior of the series A compounds
R
H
F
OCn
2n+1
Compd
n
Phase behavior/ꢀC [
DH]/J/g
4FA1
4FA2
2FA1
2FA2
2FA3
2FA4
2FA5
2FA6
1
2
1
2
3
4
5
6
Cr 199.3 [7.51]
Cr 193.2 [16.14]
Cr 152.5 [6.38]
Cr 166.2 [3.43]
Cr 157.3 [1.90]
Cr 146.9 [0.28]
Cr 187.0 [8.06]
Cr 179.8 [5.80]
SmB 209.0
SmB 200.0
Isob
Isob
Isob
Isob
Isob
Isob
Isob
Isob
n = 1, 2
F
N
series A: 4FA1-4FA2, R =
SmF 171.0 [14.76]a
SmF 182.5 [7.94]a
SmF 188.7 [5.78]a
SmF 179.3 [1.22]a
SmF 196.0
N 190.0
N 207.0
N197.0
F
F
F
n = 1 to 6
2FA1-2FA6, R = N
SmF 183.4 [2.19]a
F
Abbreviation: Cr¼crystalline solid, N¼nematic phase, SmB¼smectic B, SmF¼smec-
tic F, Iso¼isotropic liquid state.
Scheme 4. Fluorinated N-heterocycles tolan-type liquid crystals with an alkoxy as
substituent (series A).
a
Transition temperature and enthalpy change (in square brackets) was de-
termined by DSC (peak temperature, first heating scan, 5 K/minꢁ1).
b
Transition temperatures were determined by POM.
transformation range, e.g., 4FA1 (Cr 199.3 ꢀC SmB 209.0 ꢀC), 4FA2 (Cr
193.2 ꢀC SmB 200.0 ꢀC). The 3,3,4,4-tetrafluoropyrrolidine ring
distorts the N1-envelope conformation. In addition the shorter
methoxyl, ethoxyl substituents in the molecules decrease the
mesophase range.
Replacing the distorted conformation 3,3,4,4-tetrafluoropyr
rolidine ring with aromatic 3,4-difluoropyrrole to give com-
pounds, 2FA1e2FA6, which result an increase in the mesophase
range, e.g., 4FA1 (Cr 199.3 ꢀC SmB 209.0 ꢀC), 2FA1 (Cr 152.0 ꢀC SmF
171.0 ꢀC).
Scheme 5. Fluorinated N-heterocycles tolan-type liquid crystals with an alkyl as
a substituent (series B).
The compounds, 2FA1e2FA6, were observed from polarizing
optical microscope (POM) as a mosaic texture of a smectic F (SmF)
phase (Fig. 1a). Upon further heating of these compounds, the
schlieren texture of a nematic phase (Fig. 1b) was observed for
2FA1e2FA3 with C1eC3 alkoxy substituents as the end groups, e.g.,
2FA1 (Cr 152.5 ꢀC SmF 171.0 ꢀC N 190.0 ꢀC), 2FA2 (Cr 166.2 ꢀC SmF
182.5 ꢀC N 207.0 ꢀC), 2FA3 (Cr 157.3 ꢀC SmF 188.7 ꢀC N 197.0 ꢀC). In
order to ascertain the structures of the two different mesophases
observed, variable-temperature X-ray diffraction (VTXRD) experi-
ments were carried out on 2FA1. The diffraction patterns were
recorded by heating the sample into the isotropic phase from the
crystal. A representative and typical diffraction pattern obtained for
Scheme 6. Fluorinated N-heterocycles tolan-type liquid crystals with an ester as
a substituent (series C).
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4
H. Chen et al. / Tetrahedron xxx (2013) 1e7
Table 2
Thermal and mesophase behavior of the series B compounds
Compd
n
Phase behavior/ꢀC [
DH]/J/g
4FB1
4FB2
4FB3
2FB1
2FB2
2FB3
2
3
5
2
3
5
Cr 173.5 [9.56]
Cr 176.8 [5.75]
Cr 157.4 [9.15]
Cr 137.1 [2.07]
Cr 159.3 [13.01]
Cr 149.6 [4.76]
SmG 182.0
Isob
Isob
Isob
Isoa
Isob
Isob
SmG 186.0
SmG 165.3
SmE 164.2
SmE 167.0
SmE160.0
Abbreviation: Cr¼crystalline solid, SmE¼smectic E, SmG¼smectic G, Iso¼isotropic
liquid state.
a
Transition temperature and enthalpy change (in square brackets) was de-
termined by DSC (peak temperature, first heating scan, 5 K/minꢁ1).
b
Transition temperatures were determined by POM.
Fig. 1. Optical texture of (a) smectic F phase for 2FA1 upon cooling to 167 ꢀC and (b)
nematic phase for 2FA1 upon cooling to 186 ꢀC.
compound 2FA1 at 167.0 ꢀC is shown in Fig. 2A. The diffuse char-
ꢀ
acter of the wide angle peak at 6.51 A indicates a liquid like in-plane
order. In the small angle region two sharp reflections were seen
ꢀ
with spacing of (d) 19.57 and 9.73A. These reflections are in the
ratio 1:2 or 2:4 indicating a lamellar ordering in the mesophase.
Smectic F shows a diffuse outer ring accompanied by a strong inner
diffraction as observed in previous reports.27 The smectic F (SmF)
phase for 2FA1 was judged by variable-temperature X-ray diffrac-
tion (VTXRD) experiments and the polarizing photomicrographs.
When 2FA1 was heated further to 186.0 ꢀC, there is no diffraction
peak at the small angles, indicating that it does not form a layered
¼20ꢀ, which
Fig. 3. Optical texture of (a) smectic G phase for 4FB1 upon cooling to 180 ꢀC and (b)
smectic E phase for 2FB1 upon cooling to 168 ꢀC.
structure. There is only one diffuse peak, around 2
q
reflects remote disorder of the molecule arrangement in the hori-
zontal direction (Fig. 2B). The nematic phase was characterized at
186.0 ꢀC for 2FA1 by the XRD and POM experiments. With the in-
crease in alkoxy chain length, the nematic phase thermal stability
decreases and the smectic phase thermal stability increases. When
longer alkoxy substituents (C4, C5, C6) were employed, the
2FA4e2FA6 only show the smectic F (SmF) phase and the nematic
phase disappears.
for compound 2FB1 at 168 ꢀC upon cooling. In order to ascertain the
structures of the mesophases observed, variable-temperature X-ray
diffraction (VTXRD) experiments were carried out on 2FB1. In the
small angle region, two sharp reflections were seen with spacing of
ꢀ
(d) 10.23 and 20.60 A. These reflections are in the ratio 1:2 or 2:4
indicating a lamellar ordering in the mesophase, three sharp re-
ꢀ
flections were seen with spacing of (d) 6.78, 4.51, 4.04 A in the wide
angle region. The liquid crystal state for SmE, SmG, SmI shows
several sharp outer diffraction peaks as previously reported.27 The
smectic E (SmE) phase for 2FB1 was determined by the polarizing
photomicrographs (Fig. 3b) and variable-temperature X-ray dif-
fraction (VTXRD) experiments (Fig. 4).
Fig. 2. Variable-temperature X-ray diffraction (VTXRD) obtained in (A) the SmF phase
of 2FA1 at 167.0 ꢀC and (B) the nematic phase of 2FA1 at 186.0 ꢀC.
Thermal stabilities for 4FA1, 2FA1 were determined by thermal
gravimetric analysis (TGA). Data show that the decomposition
temperatures were higher than the clearing point for these com-
pounds. 4FA1, 2FA2 had a Td at 270.2 ꢀC, 275.2 ꢀC, respectively.
Modifying the substituent in the para-position of arylacetylene
unit influences the liquid crystal state. Changing the substituent of
series A from alkoxy to alkyl forms the series B liquid crystals,
4FB1e4FB3, 2FB1e2FB3. These compounds form the smectic G,
4FB1e4FB3, and smectic E phase, 2FB1e2FB3 (Table 2). Compound
4FB1 displayed a mosaic texture of SmG at 180 ꢀC upon cooling
(Fig. 3a). A mosaic texture of a smectic E (SmE) phase was observed
Fig. 4. Variable-temperature X-ray diffraction (VTXRD) of smectic E (SmE) phase of
2FB1 upon cooling to 160.0 ꢀC.
A comparison of the series B liquid crystals with the analogous
series A, shows an increase the amount of order from SmB to SmE,
e.g., 4FA2 (Cr 193.2 ꢀC SmB 200.0 ꢀC), 4FB1 (Cr 176.8 ꢀC SmG
186.0 ꢀC), 2FA2 (Cr 152.5 ꢀC SmF 171.0 ꢀC N 190.0 ꢀC), and 2FB1 (Cr
137.1 ꢀC SmE 164.2 ꢀC), respectively.
The decomposition temperatures were higher than the clearing
point for these compounds, e.g., 4FB1, 2FB2 had a Td at 259.3 ꢀC,
269.2 ꢀC, respectively.
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H. Chen et al. / Tetrahedron xxx (2013) 1e7
5
The series C compounds (Scheme 6), 2FC1e2FC3, with the ester
as the terminal group were synthesized by combination the 2FOH
with the liquid crystal building block (acrylic acid, 3CA, 3CCA)
giving high yields (Table 3). The 2FC1 with acrylic ester as the
terminal group displayed a typical schlieren texture of the nematic
phase (Fig. 5), 2FC1 (Cr 183.3 ꢀC N 220.0 ꢀC). Variable-temperature
X-ray diffraction (VTXRD) experiments were carried out on 2FC1,
only a wide diffraction peak was observed at 180.0 ꢀC in the small
angle region, which indicates that no lamellar ordering exists in the
mesophase (Fig. 6). Though the use of a combination the polarizing
photomicrographs of compound 2FC1 and variable-temperature X-
ray diffraction (VTXRD) experiments, the nematic (N) phase for
2FC1 was identified.
Fig. 7. Optical texture of smectic A phase for 2FC2 upon cooling to 177.0 ꢀC.
2.3. Discussion
Table 3
Thermal and mesophase behavior of the series C compounds
2.3.1. Evaluation of the influence of the alkyl, alkoxy, and ester
chain. The terminal alkyl, alkoxy, and ester chains have an in-
fluence on the mesophase of these new compounds. Increasing the
length of the carbon chain of the terminal alkoxy group increases
the polarization and thus the lateral attraction between molecules.
When the lateral attraction is greater than the terminal attraction,
the smectic phase forms preferentially. Compounds with short or
medium carbon chain lengths as the alkoxy substituent (OCH3 to
OC6H13), 2FA1e2FA6, always show a smectic phase and a nematic
phase, e.g., 2FA1 (Cr 152.5 ꢀC SmF 171.0 ꢀC N 190.0 ꢀC), 2FA2 (Cr
166.2 ꢀC SmF 182.5 ꢀC N 207.0 ꢀC), 2FA3 (Cr 157.3 ꢀC SmF 188.7 ꢀC N
197.0 ꢀC), 2FA4 (Cr 146.9 ꢀC SmF 179.3 ꢀC), 2FA5 (Cr 187.0 ꢀC SmF
196.0 ꢀC), 2FA6 (Cr 179.8 ꢀC SmF 183.4 ꢀC).
Compd
Phase behavior/ꢀC [
DH]/J/g
2FC1
2FC2
2FC3
Cr 183.3 [8.14]
Cr 160.3 [9.42]
Cr 172.1 [3.71]
N 220.0
Sm A 208.0
Sm A 180.0
Isoa
Isoa
Isoa
Abbreviation: Cr¼crystalline solid, SmA¼smectic A, Iso¼isotropic liquid state.
a
Transition temperature was determined by POM, DSC and enthalpy change (in
square brackets) was determined by DSC (peak temperature, first heating scan, 5 K/
minꢁ1).
Compounds, 4FB1e4FB3 and 2FB1e2FB3, with nonpolar
alkyl terminal groups only show a smectic phase. The compound,
2FC1, with an unsaturated acrylic ester as ending group shows
the nematic phase with the highest clearing point. The terminal
unsaturated acrylic ester group for 2FC1 incorporates the con-
jugation effect into the molecule. This effect increases the po-
larizability anisotropy and rigidity of the molecule, hence
influencing the mesophase behavior and the clearing point. The
introduction the cyclohexyl group at the 4-positions for liquid
crystal molecules can increase the order parameter by increasing
the linearity of the entire structure. It can be seen the
2FC2e2FC3 with mono or bicyclohexyl group show the smectic A
phase.
Fig. 5. Optical texture of nematic phase for 2FC1 upon cooling to 200.0 ꢀC.
2.3.2. Influence of the terminal fluorinated heterocycle. The melting
points and clearing points of the new liquid crystals with a fluo-
rinated pyrrolidine as core structure are higher than those with
fluorinated pyrrole structure with the same terminal substituent,
e.g., 4FA1 (Cr 199.3 ꢀC SmB 209.0 ꢀC), 2FA1 (Cr 152.0 ꢀC SmF
171.0 ꢀC N 190.0 ꢀC), 4FB1 (Cr 176.8 ꢀC SmG 186.0 ꢀC), 2FB1 (Cr
137.1 ꢀC SmE 164.2 ꢀC). The liquid crystal compounds having the
expensive 2,3-difluorobenzene, 3,4-difluorobenzene, and 3,4,5-
trifluorobenzene structures already have been disclosed to have
a high dielectric aniostropy and intermediate viscosity.1,13e16 Our
introduction of a convenient and easy synthetic route into fluo-
rinated 3,4-difluoropyrrole as the core structure of liquid
crystal molecules should open the door to more investigation. The
fluorinated core structure will increase the symmetrical charac-
teristic and van der Waals interactions within the molecules,
which should enhance the liquid crystal properties. The 3,3,4,4-
tetrafluoropyrrolidine ring adopts a distorted N1-envelope con-
formation, which greatly impacts the mesophase behavior. It also
exhibits a narrow phase transformation range. Comparatively,
compounds 2FA1e2FA6, 2FB1e2FB3, 2FC1e2FC3, having a five-
membered fluorinated pyrrole, increase the mesophase range re-
markably. The results indicate that having a symmetrical 3,4-
difluoropyrrole as the core structure building block elongates
Fig. 6. Variable-temperature X-ray diffraction (VTXRD) of nematic phase of 2FC1 upon
cooling to 180 ꢀC.
The ester bond is a polar moiety, when the terminal ester chain is
long enough, it will facilitate lamellar stacking and generate
a smectic phase. The compounds 2FC2e2FC3, with trans-40-propyl-
(1,10-mono or bicyclohexyl)-4-carboxylic (3CA, 3CCA) ester as the
terminal group, were observed to form smectic A. A typical focal
conic texture of a SmA phase was identified for compound 2FC2
with 3CA as ending group between 160.3 and 208.0 ꢀC (Fig. 7). Re-
placement of the 3CA with the longer 3CCA liquid crystal building
block resulted in a change in the mesophase behavior. A fan-shaped
texture of a SmA phase for compound 2FC3 was observed. Data
show that the decomposition temperatures were higher than the
clearing point for the compound 2FC2 had a Td at 288.7 ꢀC.
Please cite this article in press as: Chen, H.; et al., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.04.077

6
H. Chen et al. / Tetrahedron xxx (2013) 1e7
the structure and impacts the liquid crystal properties
was purified by chromatography on silica gel to give the target
significantly.
product.
4.3. General procedure for the preparation of 3,4-difluoro
3. Conclusion
pyrrole-based longer alkoxy substituent tolan
A
series of new 3,4-difluoropyrrole-, 3,3,4,4-tetrafluoro
2FOH (1 mmol, 295.3 mg), alkyl bromide (1.2 mmol), K2CO3
(10 mmol,1380.0 mg), and DMF (15 ml) were placed in a Pyrex glass
tube, sealed, heated at 90 ꢀC for 12 h, and then allowed to cool to
room temperature. The crude product was added to 50 mL
dichloromethane and then washed with water (3ꢂ30 mL), and
dried over anhydrous sodium sulfate. The solvent was removed
under vacuum, and the residue was purified by chromatography on
silica gel to give the target product.
pyrrolidine-based tolan liquid crystals were synthesized by a pal-
ladium-free Sonogashira coupling reaction at high yields. The
new fluorinated compounds 4FA1,4FA2, 2FA1e2FA6, 4FB1e4FB3,
2FB1e2FB3, 2FC1e2FC3 are the first examples of tolan-based liq-
uid crystal with fluorinated N-heterocycles as the core structure.
They have displayed good liquid crystalline properties. Their
melting points, decomposition temperatures, clearing points, and
mesomorphism type were determined. The properties of these
liquid crystals can be adjusted by using different N-heterocycles,
alkyl, alkoxy, and ester substituents as the ending groups. The
mesophase behavior can be affected by altering the N-heterocycle
core structure. Compounds 4FA1,4FA2, 4FB1e4FB3 with the ter-
4.4. General procedure for the preparation of alkyl ester
substituted 3,4-difluoropyrrole-based tolan
minal 3,3,4,4-tetrafluoropyrrolidine ring adopted
a distorted
2FOH (1 mmol, 295.3 mg), alkyl acid (1.2 mmol), 4-dimethyla-
miopryidine (0.1 mmol, 12.2 mg), and N,N0-dicyclohex-
ycarbodiimide (1 mmol, 206.3 mg) were placed in a Pyrex glass
tube, sealed, heated at 90 ꢀC for 12 h, and then cooled to room
temperature. The crude product was added to 50 mL dichloro-
methane and then washed with water (3ꢂ30 mL), and dried over
anhydrous sodium sulfate. The solvent was removed under vac-
uum, and the residue was purified by chromatography on silica gel
to give the target product.
N1-envelope conformation showing an oblique molecular axis
smectic B and G phase. Comparatively, compounds 2FA1e2FA6,
2FB1e2FB3, and 2FC2e2FC3 with a 3,4-difluoropyrrole as terminal
group exhibit vertical molecular axis smectic F, E, A phase, re-
spectively. It can be seen that with the acrylic ester substituent on
the terminal acetylenes, compound 2FC1 exhibits a nematic phase.
The 3,4-difluoropyrrole can replace the expensive 2,3-difluoro
benzene, 3,4-difluorobenzene, and 3,4,5-trifluorobenzene in the
LCs to form promising new liquid crystalline materials. These new
N-heterocycles liquid crystals exhibit high clearing points and
thermal stabilities, meeting one of the criteria for high-perfor-
mance liquid crystals.
Acknowledgements
The authors gratefully acknowledge the support of National
Natural Science Foundation of China (21272080), Department of
Science and Technology, Guangdong Province (2010A020507001-
76, 5300410, FIPL-05-003), and The Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education
Ministry.
4. Experimental section
4.1. General considerations
All the reagents used were analytical reagents purchased from
commercial sources and used as received. 1H and 19F were recorded
on a 400 MHz nuclear magnetic resonance spectrometer operating
at 400 and 376 MHz, respectively. Chemical shifts were reported
relative to Me4Si for 1H, and CCl3F for 19F. The solvent was CDCl3
unless otherwise specified. Thermal gravimetric analysis (TGA)
measurements were performed at a heating rate of 10 ꢀC/min with
a Netzsch STA409PC (Germany) instrument. The DSC was recorded
at a scan rate of 2 ꢀC/min on a Netzsch DSC200PC apparatus. Optical
micrographs were observed with a polarizing optical microscope
(POM) (Nikon LINKAM-THMSE600) equipped with a heating plate
(HCS601). Variable-temperature X-ray diffraction (XRD) experi-
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2013.04.077.
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