N-terminal strategy (N1-N4) toward high performance liquid crystal materials

Article abstract of DOI:10.1016/j.tet.2015.11.013

Liquid crystal materials have a variety of applications in many fields such as display techniques as well as photonics and optics. However, only few design principles have been disclosed on liquid crystal materials with different N-heterocycles as the terminal groups, which hinder the development of the heterocyclic liquid crystals. Here, a strategy of molecular design for N-heterocyclic liquid crystal materials is reported. On the basis of this strategy, a series of convenient N-heterocycles such as pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, 1,2,3,4-tetrazole were applied to synthesize the novel liquid crystals. Most of them have proved to exhibit good mesomorphic behaviors which make them excellent components in the mixture of the LCDs materials. The simple attachment of N-heterocyclic units to the liquid crystal molecules through a mild reaction condition will provide a good prospect for the design of N-heterocyclic liquid crystals.

Full text of DOI:10.1016/j.tet.2015.11.013

Accepted Manuscript
N-terminal Strategy (N1-N4) Toward High Performance Liquid Crystal Materials
Fengying Hong, Zhengce Xia, Dezhao Zhu, Hongxiang Wu, Jianhui Liu, Zhuo Zeng
PII:
S0040-4020(15)30197-6
10.1016/j.tet.2015.11.013
TET 27269
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 September 2015
Revised Date: 1 November 2015
Accepted Date: 9 November 2015
Please cite this article as: Hong F, Xia Z, Zhu D, Wu H, Liu J, Zeng Z, N-terminal Strategy (N1-N4)
Toward High Performance Liquid Crystal Materials, Tetrahedron (2016), doi: 10.1016/j.tet.2015.11.013.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical Abstract
N-terminal Strategy (N1-N4) Toward High Performance Liquid
Crystal Materials
Fengying Hong,a Zhengce Xia,a Dezhao Zhu,a Hongxiang Wu,a Jianhui Liu,a Zhuo Zeng*,a,b
a
College of Chemistry & Environment South China Normal University, Guangzhou, China,
510006
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, Shanghai, China, 200032
A series of N-heterocyclic liquid crystal materials were synthesized. The
properties of new compounds can be adjusted by varying the bridge and/or the
terminal N-heterocycles. The compounds with difluoromethylene linker displayed
excellent physical and electro-optical characteristics which offer a new approach for
the development of high performance liquid crystal materials.

ACCEPTED MANUSCRIPT
N-terminal Strategy (N1-N4) Toward High Performance
Liquid Crystal Materials
Fengying Hong,[a] Zhengce Xia,[a] Dezhao Zhu,[a] Hongxiang Wu,[a] Jianhui Liu,[a]
Zhuo Zeng*[a,b]
a College of Chemistry & Environment, South China Normal University, Guangzhou,
China, 510006
b
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai, China, 200032
Corresponding author. Tel.: 0086-20-3931-0258; fax: 0086-20-3931-0187; e-mail
address: zhuoz @scnu.edu.cn (Z. Zeng).
Abstract: Liquid crystal materials have a variety of applications in many fields such
as display techniques as well as photonics and optics. However, only few design
principles have been disclosed on liquid crystal materials with different
N-heterocycles as the terminal groups, which hinder the development of the
heterocyclic liquid crystals. Here, a strategy of molecular design for N-heterocyclic
liquid crystal materials is reported. On the basis of this strategy, a series of convenient
N-heterocycles such as pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole,
1,2,3,4-tetrazole were applied to synthesize the novel liquid crystals. Most of them
have proved to exhibit good mesomorphic behaviors which make them excellent
components in the mixture of the LCDs materials. The simple attachment of
N-heterocyclic units to the liquid crystal molecules through a mild reaction condition
will provide a good prospect for the design of N-heterocyclic liquid crystals.
Keywords: heterocycles • liquid crystals • mesophases • phase transitions •
electro-optical properties
1. Introduction
In recent years, as a consequence of the display technology boom, liquid crystals
(LCs) have attracted considerable attention due to their fascinating properties and
1

ACCEPTED MANUSCRIPT
successful commercial applications.[1,2] It is worth noting that TFT-LCDs have been
shown tremendous promise in the field of display for their remarkable features,
including low power consumption, light weight, a flickerless image, thin profile,
low-voltage operation and large dynamic range of screen luminance, etc.[3]
Designing novel liquid crystals as advanced functional materials involves the
selection of a suitable core fragment, linking group, and terminal functionality.
Esters are commonly used as linking group because they extend the molecular
length, and enhance the polarizability anisotropy.[4] One of the solutions to reduce
driving voltage is to increase the dielectric anisotropy (∆ε). To insert a
difluorooxymethylene bridge into the liquid crystal molecules can increase the
dielectric anisotropy without raising the viscosity.[5] Some commercial liquid crystal
molecules are shown in Scheme 1,[5a,6] which display high dielectric anisotropy, low
rotational viscosity, high specific resistivity or voltage holding ratio[7] e.g. A1, and A2.
Although these compounds show some comparative advantageous parameters, the
raw material, polyfluorophenol, is difficult to obtain for complicated synthetic route
which will lead to a higher expenditure in production. The reduction of production
cost has become a major technology driving force in the severe competitive
environment of a saturating market for LCDs.[8]
Another promising approach to increase ∆ε and polarity is to introduce a polar ring
into the molecule. Large number of compounds containing heterocyclic unit have
been reported.[9-11] A new class of N-heterocycle based liquid crystals that employed
3,4-difluoropyrrole, 3,3,4,4-tetrafluoropyrrolidine, 3,3,4,4,5,5-hexafluoropiperidine
and pyrrolidine as terminal group, were synthesized by our group.[12] These
compounds exhibit broad nematic phase range, good thermal stability and strongly
positive dielectric anisotropy. The interest in development of N-heterocyclic liquid
crystal materials was aroused. Five-membered N-heterocycles, such as pyrrole,
pyrazole, imidazole, 1,2,3-triazole, 1,2,4-trizole, 1,2,3,4-tetrazole, are of great
importance as building block in thermotropic liquid crystals. These strong polar
heterocycles will influence lateral or longitudinal dipoles along with changes in the
molecular shape.[13,14] Moreover, the heterocycles are also easy to obtain through
2

ACCEPTED MANUSCRIPT
simple synthetic route with low cost.
F
F
F
F
C3H7
C3H7
CF2O
F
CF2O
F
F
A1 C 40.91 N 42.01 I
A2 C 42.9 N 105.5 I
∆ε=9.8 ∆n=0.07
∆ε=16.8 ∆n=0.135
Scheme 1. Some commercial liquid crystal molecules
In this paper, combining both the advantages of five-memberd N-heterocycles
(N1-N4), ester linking group and difluorooxymethylene bridge, we report the
synthesis, characterization, and the mesomorphic properties of three series of novel
liquid crystals with various five-membered N-heterocycles as terminal groups. We
also expect to elucidate a structure-mesogenic behavior relationship in order to guide
the synthesis of new liquid crystal materials.
2. Results and Discussion
2.1. Synthesis
Synthetic routes of novel liquid crystals are outlined in Scheme 2-6. Details of the
synthesis and characterization of the materials are given in the experimental section.
Varieties of the synthetic routes of difluoromethyleneoxy moiety have been
reported previously.[15] In this paper, the intermediates, 3CCABr, 3CCAFBr and
3CCANO2, with difluorooxymethylene bridge, were synthesized through oxidative
fluorodesulfuration reported by P. Kirch et.al. (Scheme 2)[16]
O
S
S
a
C3H7
CF3SO3
C3H7
OH
3CCA
HO
3CCB
R1
b
HO
NO2
b
Br
R1
Br
F
F
F
O
F
C3H7
C3H7
NO2
O
3CCANO2 67% yield
3CCABr: R1=H 65% yield
3CCAFBr: R1=F 73% yield
Reagents and conditions: a) 1. HS(CH2)3SH, CF3SO3H, toluene/isooctane
1:1; azeotropic removal of water, 102 oC. 2. crystallization by addition of
methyl tert-butyl ether at 0 oC. b) 1. CH2Cl2, -70 oC; 2. Et3N, CH2Cl2, 1 h;
3. Et3N 3HF, 15min; 4. Br2, CH2Cl2, -70 oC, 0 oC.
3

ACCEPTED MANUSCRIPT
Scheme 2. Synthesis of compounds 3CCABr, 3CCAFBr, 3CCANO2
Ullmann-type coupling with transition-metal catalyst is one of the most important
methods for the formation of C(aryl)-N bond.[17] In this paper, the 3CCABr was
reacted with N-heterocycles by using CuI as the catalyst in the presence of Cs2CO3 in
DMF at 120 °C for 24 h, to give the difluoromethyleneoxy bridged CF2ON series
compounds with different five-membered N-heterocycles (N1-N4) as the terminal in
80-85% yield. (Scheme 3) Reaction of 3CCABr and 1,2,3-triazole at the same
condition gave the nucleophilically-substituted product CF2ON-123 and CF2ON-125,
resulting from the tautomeric change of 1,2,3-triazole into its isomer 1,2,5-triazole as
nucleophilic reagent.
F
F
F
O
F
O
C3H7
N-het
C3H7
Br
het
N
3CCABr
CF2ON 84-87% yield
N
N
N-heterocycle=
N
N
N
N
H
N
H
N
H
H
N-1
N-12
N-13
N-124
CF2ON-1: N-het = N-1
CF2ON-12: N-het = N-12
N
CF2ON-13: N-het = N-13
CF2ON-124: N-het = N-124
F
F
O
C3H7
N
N
F
N
H
N
F
N
C3H7
CF2ON-123 49% yield
O
Br
3CCABr
F
F
O
N
C3H7
N
N
N
CF2ON-125 46% yield
N
N
F
F
F
N
F
F
C3H7
C3H7
H
O
Br
O
Br
CF2ON-13-Br 83% yield
3CCAFBr
Reagents and conditions: CuI, Cs2CO3, DMF,120 oC,24 h.
Scheme 3. Synthesis of new liquid crystals CF2ON series
To introduce a lateral fluorine atom on benzene can decrease the melting point of
the liquid crystals. 3CCAFBr was synthesized in two steps from 3CCA, and then
4

ACCEPTED MANUSCRIPT
treated with imidazole under the same Ullmann-type coupling condition to give the
unexpected product CF2ON-13-Br in 75% yield, resulting from the substitution of
fluorine atom in 3CCAFBr.
The compound 3CCANO2 can be reduced by Zn and NH4Cl in the solution of
CH3OH/THF to gain 3CCANH2 in 93% yield (Scheme 4). Synthesis of CF2ON-1234
by Ullmann coupling reaction was not successful. 3CCABr was treated with
1,2,3,4,-tetrazole to yield an inseparable mixture and the compound CF2ON-1234 was
hard to isolate. Therefore, the CF2ON-1234 was prepared by the synthetic routine
reported by K. Luo et.al at 91% yield.[18 ] (Scheme 4)
N N
N
F
N
H
a
F
O
C3H7
inseparable mixture
Br
3CCABr
F
F
O
F
F
O
b
C3H7
C3H7
NO2
NH2
3CCANO2
c
3CCANH2 94% yield
F
F
O
C3H7
N
N
N
N
CF2ON-1234 89% yield
Reagents and conditions: a) CuI, Cs2CO3, DMF, 120 oC, 24 h. b) Zn, NH4Cl,
CH3OH/THF 2:1, at room temperature, 24 h. c) NaN3, (EtO)3CH, AcOH, 80 oC.
Scheme 4. Synthesis of novel liquid crystal CF2ON-1234
The synthesis of COON series with different five-membered N-heterocycles as
terminal groups bridged with ester was shown in scheme 5. The esterification
reactions were conducted in dried trichloromethane using dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP) as catalyst/base at 80 °C for 12 h.
5

ACCEPTED MANUSCRIPT
N-het
b
het
O
N
het
N
H3CO
HO
H3CO
Br
a
c
C3H7
O
N het
COON 87-90% yield
N
N
N
N
N-het =
N
H
N
H
N
H
N
H
N-1
N-12
N-13
N-124
COON-1: N-het = N-1
COON-13: N-het = N-13
COON-124: N-het = N-124
COON-12: N-het = N-12
H3CO
Br
N
N
a
N
H
N
H3CO
N
H3CO
HO
N
N
N
N
N
b
c
b
N
N
HO
O
N
N
N
c
3CCA
3CCA
C3H7
O
C3H7
N
N
O
N
O
N
N
N
COON-123 90% yield
COON-125 89% yield
Reagents and conditions: a) CuI, Cs2CO3, DMF, 120 oC,24 h. b) BBr3,
CH2Cl2, 0 oC, 4 h. c) DCC, DMAP, CHCl3, 80 oC.
Scheme 5. Synthesis of new compounds COON series
Liquid crystals with ethyl as linker display low viscosity and can be used as
effective additives to decrease the viscosity and melting point of liquid crystal mixture.
The synthesis of liquid crystals with ethyl as linker between CF2O, or COO and
N-heterocycle, CF2OEtN and COOEtN series, was shown in Scheme 6.
3CCB (dithianylium triflate) was treated with 2-(1H-imidazol-1-yl)ethanol by
oxidative fluorodesulfuration procedure to synthesize the CF2OEtN. But the mixture,
containing 8 products monitored by TLC, was hard to isolate by flash chromatography.
The COOEtN series was easily obtained by a two-step reaction in good yield.
6

ACCEPTED MANUSCRIPT
OH
HO
Cl
N-het
het
N
a
c
3CCB
3CCA
b
F
F
O
C3H7
C3H7
O
O
het
N
CF2OEtN
het
N
COOEtN 88-89% yield
N
N
COOEtN-12: N-het = N-12
COOEtN-13: N-het = N-13
COOEtN-124: N-het = N-124
N
N
N
N-het =
N
H
N
H
H
N-12
N-13
N-124
Reagents and conditions: a) EtONa/EtOH, reflux, 24 h. b) DCC, DMAP, CHCl3, 80 oC. c)
1. HS(CH2)3SH, CF3SO3H, toluene/isooctane 1:1; azeotropic removal of water, 102 oC; 2.
crystallization by addition of methyl tert-butyl ether at 0 oC; 3. CH2Cl2, -70 oC, Et3N,
CH2Cl2, 1 h; 4. Et3N 3HF, 15min; 5. Br2, CH2Cl2, -70 oC, 0 oC.
Scheme 6. Synthesis of new compounds COOEtN series
A strategy to synthesize novel high performance liquid crystal materials with
different five-membered N-heterocycles as terminal groups was developed. The new
compounds containing the convenient N- heterocycles, such as pyrrole, pyrazole,
imidazole, 1,2,3-triazole, 1,2,4-triazole and 1,2,3,4-tetrazole, are the first examples of
terminal N-heterocyclic liquid crystals. This simple synthetic routine will lead to easy
preparation of these novel liquid crystal molecules and enhance the future prospects
for the design of N-heterocyclic liquid crystals.
2.2. Liquid Crystalline Properties
As a preliminary investigation, the mesomorphic behaviors of the new compounds
were characterized and studied by a combination of hot stage polarizing optical
microscopy (POM) and differential scanning calorimetry (DSC). The phase
transitions and thermodynamic data are presented in Table 1. The assignment of the
mesophases was made based on their optical texture, some of which are presented in
Fig. 1.
There are three factors that will affect the compounds’ mesomorphic behavior,
including: 1) the structure of the N-heterocycles; 2) the linking group; 3) the nature of
the core fragment.
7

ACCEPTED MANUSCRIPT
Table 1. Thermal behavior of the new compounds
Compound
3CCABr
Phase transition temperature/ °C[a]
Enthalpies of transition [Jg-1]
Td/°C[c]
248.2
286.7
254.8
313.1
295.2
286.0
273.6
Cr 63.7 [51.50]
Cr 113.2 [30.56]
Cr 127.3 [72.91]
Cr 124.5 [29.71]
Cr 176.3 [66.51]
Cr 160.6 [27.11]
Cr 125.3 [49.06]
N
N
N
N
N
N
N
162.8[1.47]
240.7 [1.98]
247.5 [3.48]
233.0 [1.37]
246.0 [1.37]
241.7 [1.10]
242.5 [2.13]
I
I
I
I
I
I
I
CF2ON-1
CF2ON-12
CF2ON-13
CF2ON-123
CF2ON-124
CF2ON-125
N 244
[5.39]
CF2ON-1234 Cr 154.0 [18.76]
CF2ON-13-Br Cr 81.9 [23.92]
Cr’ 169 [16.8]
SmB 219 [28.94]
I
270.7
I
I
I
I
I
I
I
I
I
I
291.8
279.6
299.8
251.2
261.3
240.2
202.4
232.4
219.3
252.4
COON-1
COON-12
Cr 124.4 [20.48]
Cr 142.5 [55.12]
Cr 115.4 [21.56]
Cr 88.4 [ 8.89]
Cr 94.4 [ 19.9]
Cr 146[b]
Cr’ 150.3 [47.09]
SmB 170.6 [8.99]
Cr’ 140[38.76]
N
N
N
246.0[1.24]
280.8[1.46]
187.3 [2.99]
COON-13
COON-123
COON-124
COON-125
COOEtN-12
COOEtN-13
COOEtN-124
Cr’ 126.7 [7.62]
Cr’ 128.7 [4.44]
SmB 170.7[71.6]
SmB 155.3[42.67]
SmB
153[b]
N
190[b]
Cr 57.8 [81.64]
Cr 83.5 [69.81]
Cr 89.0 [121.9]
[a]Transition temperature and enthalpy change (in square brackets) were determined by DSC (peak temperature,
first heating scan, 10 Kmin-1) and confirmed by POM. Cr = crystalline solid, N= nematic phase, SmB=smectic B;
SmC=smectic C; I= isotropic liquid state; [b] Transition temperatures were determined by POM; [c] Td=
decomposition temperature.
The CF2ON series, with various five-membered heterocycles as the terminal groups
and difluorooxymethylene linkage chain, displayed nematic phase, except
CF2ON-1234 exhibited SmB between 169 °C and 219 °C. The nematic phase was
deduced, as evidenced by its schlieren texture, some of which can be seen in Fig. 1.
All these compounds have wide nematic ranges, the widest of which for CF2ON-1,
(Cr 113.2 °C, N 240.7 °C), is 127.5 °C. As the number of the N atom increases, the
nematic phase range decreases, and the melting point increases, e.g., CF2ON-1 (Cr
113.2 °C, N 240.7 °C), CF2ON-12 (Cr 127.3 °C, N 247.5 °C), CF2ON-123 (Cr
176.3 °C, N 246.0 °C), CF2ON-1234 (Cr 154.0 °C, Cr’ 169 °C, SmB 219 °C, N
244 °C). The increase in the polarity of molecule and intermolecular force is
attributable to the increased number of N atom.
8

ACCEPTED MANUSCRIPT
(a)
(b)
Fig. 1. Optical texture (a) for CF2ON-12 schlieren of Nematic Phase upon cooling to 231 °C, (b)
for CF2ON-1234 Smectic B phase upon cooling to 189 °C.
The position of N atom in N-heterocycle also plays a crucial role in determining the
nematic ranges, e.g., CF2ON-12 (Cr 127.3 °C, N 247.5 °C), CF2ON-13 (Cr 124.5 °C,
N 233.0 °C), CF2ON-123 (Cr 176.3 °C, N 246.0 °C), CF2ON-124 (Cr 160.6 °C, N
241.7 °C), and CF2ON-125 (Cr 125.3 °C, N 242.5 °C).
The compound CF2ON-13-Br melted into isotropic liquid at 81.9 °C directly with
no texture of liquid crystal being observed. The lateral N-heterocycle on benzene ring
causes the absence of mesomorphic behaviors. It can be attributed to the fact that
N-heterocycle protrude from the side of the mesogenic core cause a disruption in the
intermolecular force of attraction and molecular packing.
Compounds COON series with ester as linking group show three endothermic
peaks in the heating cycle, e.g., COON-1 (Cr 24.4 °C, Cr’ 150.3 °C, N 246.0 °C),
COON-13 (Cr 115.4 °C, Cr’ 140 °C, N 187.3 °C), COON-123 (Cr 88.4 °C, Cr’
126.7 °C, SmB 170.7 °C); a Cr-Cr’ transition was seen before the appearance of a
mesophase. The result was verified by POM as there was no mesohphase between the
Cr-Cr’ transition temperature ranges. Though the ladder - shaped ester molecule
remains linear structure, the ester linking group increases the width of the molecule
compared with CF2O linker, which result in the formation of smectic phase and
decrease of nematic phase temperature range.
The nature of the core fragment also has a great influence on the mesomorphic
properties. The nematic phase range and the melting point increase as the terminal
9

ACCEPTED MANUSCRIPT
substituent was replaced by N-heterocycle. e.g., 3CCABr (Cr 63.7 °C, N 162.8 °C),
CF2ON-1 (Cr 113.2 °C, N 240.7 °C). The terminal N-heterocycle elongates the
molecule to give an appropriate length to width ratio, which is propitious to the
formation and stability of mesomorphism. The melting point markedly decreased
when replacing benzene with ethyl, e.g. the melting point of the compound
COOEtN-12, COOEtN-13, COOEtN-124 is 57.8 °C, 82.5 °C and 89 °C, respectively.
These molecules just melted into isotropic liquids directly with no texture of liquid
crystal being observed. The rigidity of the molecules was destroyed when the core
changed from benzene to ethyl, which causes the loss of the ordered arrangement.
The nematic phase ranges of liquid crystals with difluorooxymethylene bridge are
broader than those with ester, e.g., CF2ON-1 (Cr 113.2 °C, N 240.7 °C), COON-1 (Cr
24.4 °C, Cr’ 150.3 °C, N 246.0 °C), CF2ON-13 (Cr 124.5 °C, N 233.0 °C), COON-13
(Cr 115.4 °C, Cr’ 140 °C, N 187.3 °C).
The
novel
five-membered
N-heterocycles
liquid
crystals
with
difluorooxymethylene linker all exhibited nematic phase, while those with ester or
ethyl ester showed different or no mesomorphic behaviors, which illustrates
mesomorphic behaviors was greatly influenced by the linker. The CF2ON series
exhibited high clearing points, broad nematic phase ranges. It makes them good
candidates as the components to expand the operating temperature range for LCD,
and stabilize the temperature ranges of liquid crystals mixture.
2.3. X-ray diffraction
Variable temperature X-ray diffraction (VTXRD) experiments were performed on
COON-12, COON-123 and COON-124 to investigate the molecular arrangements in
the mesophase.
10

ACCEPTED MANUSCRIPT
Fig. 2. X-ray diffraction pattern of compound COON-12 at (a) 231 °C, (b) 150 °C, and (c) 120 °C
(before heating). (*) The diffraction peak at 2θ = 20.16° is from the glass sample holder.
Fig. 2 presents the temperature-dependent X-ray diffraction diagrams obtained
from samples of COON-12 at 231 °C, 150°C, and 120 °C. At 231 °C (Fig. 2 a), the
plot shows only a wide diffraction peak in the wide angle region and no diffraction
peak was found in the small angle region. Combined with POM, nematic (N) phase
for COON-12 at 231 °C was identified. When the measuring temperature was cooled
to 150 °C, only one sharp reflection at 18.38° was seen with corresponding d spacings
of 4.82 Å in the wide-angle region (Fig. 2 b), which indicates the formation of smectic
B phase. The liquid crystal state for SmB shows a single sharp outer diffraction peak,
whereas SmE, SmI, SmG, and SmH exhibit several sharp outer rings, as previously
reported.[19] When the measuring temperature was further cooled to 120 °C, a
crystalline substance was obtained. The X-ray diffraction analysis data of COON-12,
COON-123, COON-124 were shown in Table 2.
Table 2. X-ray diffraction analysis data of COON-12，COON-123，COON-124
Compound
T
d1
d2
d3
Molecular
Ratio
Phase
(°C)
(Å)
(Å)
(Å)
length(l,Å)[a] d1/l
COON-12
150
14.96
11.68
16.47
11.77
9.32
11.74
4.82
16.96
8.48
23.06
23.19
22.76
0.65
0.50
0.72
SmB
SmB
SmB
COON-123[b] 137
COON-124[b] 136
[a] Length l was calculated for the fully extended conformation (estimated with Gaussian03).
[b] See the Supporting Information.
11

ACCEPTED MANUSCRIPT
2.4. Thermal Stability
The novel compounds are thermally stable up to 202 - 313 °C determined by
thermal gravimetric analysis (TGA). The decomposition temperatures of the new
compounds are shown in Table 1. Data shows the decomposition temperatures are
higher than the clearing points. The compounds with difluorooxymethylene-bridge
exhibit the higher decomposition temperatures than that of ester or ethyl ester as
linker.
2.5. Solubility
Solubility is a very important physical property for these new compounds used as
liquid crystal materials. They are immiscible with petrol ether, hexane, diethyl ether,
and miscible with dichloromethane, methanol, ethyl acetate, toluene, DMSO.
The solubilities of CF2ON series were measured by adding 10 % w/w compound to
the nematic host mixture. The results are shown in Table 3. The terminal
N-heterocycle exhibit strong influence on solubility, e.g., CF2ON-1, CF2ON-12,
CF2ON-13, CF2ON-124 and CF2ON-125 can dissolve in commercially available Bayi
nematic host mixture 1C40200-000 at 10 % w/w, while CF2ON-123 and CF2ON-1234
are immiscible.
Table 3. The solubilities of CF2ON series in commercially available Bayi nematic host mixture
1C40200-000, 1C40300-000 and BHR20400
Compounds
3CCABr
host1[a]
10%
10 %
10 %
10 %
-
host2[a]
10%
10 %
5 %
10 %
-
host3[a]
10%
10 %
10 %
10 %
10 %
-
CF2ON-1
CF2ON-12
CF2ON-13
CF2ON-123
CF2ON-124
CF2ON-125
CF2ON-1234
10 %
10 %
-
10 %
5 %
-
10 %
-
[a]commercially available Bayi nematic host mixture: host1:
1C40200-000; host2: 1C40300-000; host3: BHR20400
When the terminal substituent is 1,2,3-triazole or 1,2,3,4,-tetrazole in CF2ON-123
or CF2ON-1234, they show poor solubility or are immiscible in the nematic host
mixture. It can be seen that with one-, two-, three- or four- nitrogen atoms substituent
12

ACCEPTED MANUSCRIPT
on the five-membered rings, the polar character of the compounds increases markedly,
and the solubilities decrease as well.
The nature of the host mixture also has an uneven influence on the solubilities of
the compounds, e.g., CF2ON-12 can dissolve in commercially available Bayi nematic
host mixture 1C40200-000 at 10 % w/w, BHR20400 at 10 % w/w, while in
1C40300-000 at 5 %.
Compound 3CCABr, without N-heterocycle, can dissolve in commercially
available Bayi nematic host mixture 1C40200-000, BHR20400 and 1C40300-000 at
10 % w/w, respectively. The polarity of the molecule increases as the number of N
atom increases, which leads to a poor solubility.
In this case, the solubility of compounds depends on both the nature of the
compounds and the host mixture. Compounds CF2ON-1 and CF2ON-13 show good
solubility in all three host liquid crystal mixtures 1C40200-000, 1C40300-000 and
BHR20400, which make them good components to adjust the physical and chemical
properties.
2.6. Physical and Electro-Optical Properties
The physical and electro-optical properties of liquid crystal materials are of
importance in evaluating the performance of the LCDs. The parameters include
nematic phase range, dielectric anisotropy (∆ε), birefringence (∆n), rotational
viscosity (γ1).
The physical and electro-optical properties of the new compounds are listed in
Table 4. ∆n, γ1 and ∆ε were determined by linear extrapolation from a 5 % or 10 %
w/w solution in the commercially available Bayi mixture 1C40300-000.
13

ACCEPTED MANUSCRIPT
Table 4. Dielectric anisotropy (∆ε), birefringence (∆n) and the viscosity (γ1) of some novel liquid
crystals
Compound
∆ε
∆n
γ1 /mPa s
TN-I/°C
Dipole/D
CF2ON-1[a]
CF2ON-12[b]
CF2ON-13[a]
CF2ON-123[c]
CF2ON-124[a]
CF2ON-125[b]
A2
3.22
1.22
7.22
-
0.1551
0.1671
0.1381
-
75.2
74.5
184.4
-
127.5
120.2
108.5
69.7
3.07
2.56
3.92
6.24
4.39
1.37
4.06
6.02
2.42
9.8
0.1491
0.1681
0.07
220.4
93.4
212.3
81.1
117.2
62.6
[a]Extrapolated from 10 wt % solution in nematic host mixture 1C40300-000 (the nature of the
host matrix: ∆ε=4.62, ∆n=0.13, γ1 =29.10 mPa s), [b] Extrapolated from 5 wt % solution in
nematic host mixture 1C40300-000, [c] Compound could not be fully characterized due to its
insufficient solubility.
Liquid crystal compounds CF2ON-1, CF2ON-12, CF2ON-13, CF2ON-124 and
CF2ON-125 all show positive anisotropy, especially the ones with immidazole and 1,
2, 4-triazole as terminal groups, e.g., CF2ON-13, CF2ON-124, show strongly positive
anisotropy (∆ε 7.22, ∆ε 6.02). This can be attributed to that their permanent dipole
moments are larger than other compounds.
Optical anisotropy or birefringence (∆n) is another key performance parameter for
the application of liquid crystals.[20] Compounds CF2ON-12 and CF2ON-125 exhibit
high optical anisotropy (∆n 0.1671, ∆n 0.1681), whereas CF2ON-1, CF2ON-13 and
CF2ON-124 have more moderate ∆n values of 0.1551, 0.1381 and 0.1491,
respectively. While the ∆n value of commercially available liquid crystalline
compound A2 is only 0.07, which is much lower than the novel compounds that
mentioned above.
Compounds CF2ON-1, CF2ON-12, CF2ON-125 showed far lower viscosities, (75.2
mPa s, 74.5 mPa s, 93.4 mPa s, respectively ), than compound A2, which make
them excellent components in reducing the switching time of the display.
CF2ON-13 exhibited strongly positive anisotropy, moderate ∆n value, high clearing
point and broad nematic phase range and that makes it the best candidate for the
replacement of difluorobenzene in future development of novel liquid crystal
14

ACCEPTED MANUSCRIPT
material.
2.7. UV/Vis Absorption and Photoluminescence Spectroscopy
The UV absorption spectra data for solutions of CF2ON series in dichloromethane
are presented in Table 5. The position of N atom in terminal N-heterocycle influences
the max absorption wavelength, for example, changing the position from 1,3 to 1,2,
gives a red shift, e.g., CF2ON-13, λmax= 244 nm, CF2ON-12, λmax= 255 nm.
Recently, adding the property of liquid crystallinity to emissive materials is of
increasing interest.[21] The emission fluorescence spectra data of CF2ON series are
shown in Table 5.
Table 5. UV-visible and fluorescence spectra analysis data of CF2ON series
Compound
CF2ON-1
λmax [nm]
254
λexc [nm]
282
λem [nm]
321
CF2ON-12
CF2ON-13
CF2ON-123
CF2ON-124
CF2ON-125
CF2ON-1234
255
290
316
244
286
313
252
286
345
245
282
308
270
282
343
243
280
306
Compounds CF2ON series exhibit emission fluorescence from 306 to 345 nm. The
compound CF2ON-124 displayed the strongest emission fluorescence at 308 nm
(shown in Fig 3). The positions of N atoms in terminal heterocycle result in the
different emission fluorescence, e.g., CF2ON-123, λem= 345 nm CF2ON-124, λem= 308,
,
CF2ON-125, λem= 343 nm. This arises from the fact that with the N atoms are close
together, the conjugated effect is strengthening, which contributes to red shift in
fluorescence.
15

ACCEPTED MANUSCRIPT
Fig. 3. Fluorescence spectra of CF2ON series
The CF2ON series liquid crystal compounds with various five-membered
N-heterocycles as terminal groups can be used as the ligands to synthesize the new
luminescent liquid crystals for the development of profitable application in the
marketplace.
2.8. Theoretical Study
The geometry optimization of the structures were performed by density functional
theory (DFT) Becke’s three-parameter hybrid function with non-local correlation of
Lee-Yang-Parr (B3LYP) method in gas phase.[22] The corresponding frequency
analyses were computed at the same level of theory to characterize them as minima
(no imaginary frequencies) with help of Gaussian03 (Revision D.01) suite of
programs.[23] All of above calculations used a 6-31+G basis set.[24] The computed
structures were visualized using the GaussView program. The optimized geometry of
CF2ON-13 was shown in Fig. 4. The optimized geometry of COON-13 and
COOEtN-13 can be seen in Supplementary data.
16

ACCEPTED MANUSCRIPT
Fig. 4. The optimized geometry of CF2ON-13.
The conformation expression is mainly a thermodynamic problem, and the stability
of the structure can be reflected by the total energy of different conformation. As
shown in Table 6, the single energy points were calculated at the DFT/6-31+G level.
The energy of CF2ON-13 is lower than COON-13 and COOEtN-13, e. g., CF2ON-13
(E = -1357.215467 a.u.), COON-13 (E = -1232.780928 a.u.), and COOEtN-13 (E =
-1080.386874 a.u.), which indicates compound CF2ON-13 is more stable than
COON-13 and COOEtN-13 from a thermodynamic point of view.
Fig. 5. The frontier orbitals COOEtN-13 calculated at the B3LYP/6-31+G level. Upper orbital
indicates LUMO and bottom orbital indicates HOMO
17

ACCEPTED MANUSCRIPT
Table 6. Geometrical parameters of CF2ON-13, COON-13 and COOEtN-13 calculated with
Gaussian 03
Molecular
length (10-1nm)
18.84
HOMO LUMO
EHL
(ev)
Compounds
L/D
Energy (a.u.).
(ev)
(ev)
CF2ON-13
COON-13
COOEtN-13
4.35
5.33
4.85
-1357.215467
-1232.780928
-1080.386874
-0.236
-0.231
-0.235
-0.033
-0.038
-0.013
0.203
0.193
0.222
23.08
20.99
The length to width ratio (L/D) of CF2ON-13 is larger than COON-13, which is
propitious to the formation and stabilization of mesomorphism. Though COOEtN-13
has the largest length to width ratio, the free rotation of the C-C bond in the ethyl
bridge causes the molecule to exist unstable arrangement, and therefore, the
mesomorphic behavior disappears. The HOMO–LUMO gap of CF2ON-13 (0.203 eV)
is larger than that of COON-13 (0.193 eV), which indicates that compound CF2ON-13
is more stable than compound COON-13. The HOMO–LUMO gap of compound
COOEtN-13 is the largest. The conjugation system between the ester group and
heterocycle was prevented by the ethyl linker in COOEtN-13, which can be illustrated
by the frontier orbitals of compound COOEtN-13 (Fig. 5).
18

ACCEPTED MANUSCRIPT
Lengthen bridge
Mesophase disappear
low viscocity
C3H7
COO
ethyl ester linker
R2= N-het
R2
Replace benzene
with ethyl
or
N-1
N-13
Cr-Cr'
SmB
Nematic
Nematic
SmB or
R1
R2
R1= H, R2= N-het
or
N-12
N-125
N-124
C3H7
COO
modify N number
N-123
or
Change linker COO
to CF2O
Cr-Cr'
SmC
R1
R1= N-het, R2= Br
Lateral N-het
Mesophase disappear
R2
C3H7
CF2O
broad nematic
phase ranges
high clearing points
N1
N3
N2
N4
N-1
N-12
N-124
N-125
N-1234
N-13
N-123
Highest
∆n=0.1681
Biggest
u=7.45 D
Longest
λem=345 nm
Lowest
γ=74.5 mPa s
Broadest
TNI=127.5
Hightest
∆ε=7.22
High
∆ε=6.02
N
N
N
N
N
R2=
N
N
N
N
N
N
N
N
H
N
H
N
N
H
N
H
N
H
H
H
N-1
N-12
N-13
N-123
N-124
N-125
N-1234
Scheme 7. The structure–property relationship of N-heterocyclic liquid crystals
3. Conclusions
A new class of five-membered N-heterocycle-based (N1-N4) liquid crystals
bridged with difluorooxymethylene or ester has been prepared by efficient route in
high yield.
The physical and electro-optical properties of these liquid crystal materials were
evaluated. Their properties can be adjusted by varying the linker chains and/or
terminal N-heterocycles. The structure–property relationship of N-heterocyclic liquid
crystals was shown in Scheme 7. In general, an increase in the number of N atom on
five-membered N-heterocycle (from N1 to N4) results in higher dipole moment,
melting point and poorer solubility. The difluorooxymethylene bridged liquid crystals
exhibit high clearing points and broad nematic mesophase lie in the ranges from
69.7 °C to 127.5 °C with the exception of CF2ON-1234 displaying Smetic B and
Nematic phases. With more than two nitrogen atoms on the heterocycle, the sematic
phase character of the ester liquid crystal greatly increases. While ethyl ester linker
19

ACCEPTED MANUSCRIPT
will cause the mesophic behaviors disappear. The number and position of N atom on
five-membered N-heterocycle also influence the dielectric anisotropy (∆ε),
birefringence (∆n), rotational viscosity (γ1). Compounds containing two or three
nitrogen atoms at 1,3- or 1,2,4- position on heterocycle exhibit high dielectric
anisotropy (∆ε), e.g., compound CF2ON-13, CF2ON-124 (∆ε=7.22, ∆ε=6.02)
respectively. The CF2ON-125 exhibits the highest birefringence (∆n=0.1681).
The readily and commercial available five-membered N-heterocycles could be used
as new terminal groups to form high performance liquid crystal materials.
N-heterocyclic liquid crystals containing difluorooxymethylene bridge show excellent
integrated performance, such as high clearing point, broad nematic phase range,
strongly positive anisotropy, moderate ∆n value, high intensity of luminescence and
good solubility, which offer a new approach for the development of high performance
heterocyclic liquid crystal materials.
Experimental Section
The experimental details, synthesis, structural characterization of new compounds and
their liquid crystal properties, geometrical parameters and frontier orbitals were seen
in the Supplementary data.
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). The Scientific
Research Foundation for the Returned Overseas Chinese Scholars, State Education
Ministry.
Supplementary data
Supplementary data associated with this article can be found in the online version, at
http://dx.doi.org/10.1016/j.tet.(?)
20

ACCEPTED MANUSCRIPT
References
[1] (a) Lin, K.-T.; Lee, G.-H.; Lai C.-K. Tetrahedron. 2015, 71, 4352. (b) Garcia-Amoro´s, J.; Reig,
M.; Castro, M. C. R.; Cuadrado, A.; Raposob, M. M. M.; Velasco, D. Chem. Commun. 2014,
50, 6704; (c) Bisoyi, H. K.; Kumar, S. Chem. Soc. Rev. 2011, 40 306; (d) Wu, J.; Usui, T.;
Hanna, J. J. Mater. Chem. 2011, 21, 8045.
[2] (a) Li, Y.-L.; Lee, G.-H.; Lai C.-K. Tetrahedron. 2015, 71, 5296. (b) Hirose, T.; Takai, H.;
Watabe, M.; Minamikawa, H.; Tachikawa, T.; Kodama, K.; Yasutake, M. Tetrahedron. 2014,
70, 5100; (c) Bisoyi, H. K.; Kumar, S. Chem. Soc. Rev. 2010, 39, 264; (d) Goodby, J. W.; Saez,
I. M.; Cowling, S. J.; Görtz, V.; Draper, M.; Hall, A. W.; Sia, S.; Cosquer, G.; Lee, S. E.;
Raynes, E. P.; Angew. Chem. Int. Ed. 2008, 47, 2754.
[3] Wu, I.-W. J. Soc. Inf. Display. 1994, 2, 1.
[4] Hird, M. Chem. Soc. Rev. 2007, 36, 2070.
[5] Kirsch, P.; Bremer, M. Angew. Chem. Int. Ed. 2000, 39, 4216.
[6] Matsui, S.; Kondo, T.; Sago, K. Mol. Cryst. Liq. Cryst. 2004, 411, 127.
[7] Bremer, M.; Naemura, S.; Tarumi, K. Jpn. J. Appl. Phys. 1998, 37, 88.
[8] Bremer, M.; Kirsch, P.; Memmer, M. K. ; Tarumi, K. Angew. Chem. Int. Ed. 2013, 52, 8880.
[9] (a) Lin, K.-T.; Lee, G.-H.; Lai, C.-K. Tetrahedron. 2015, 71, 4352; (b) Garcia-Amoro´s, J.;
Reig. M.; Castro, M. C. R.; Cuadrado, A.; Raposob, M. M. M.; Velasco, D. Chem. Commun.
2014, 50, 6704; (c) Kyosuke, I.; Tomonori, A.; Makoto, T. Chem. -- Asian. J. 2013, 8, 2951.
[10] (a) Yeh, M.-C.; Su, Y.-L. Tzeng, M.-C.; Ong, C.-W.; Kajitani, T.; Enozawa, H.; Takata, M.;
Koizumi, Y.; Saeki, A.; Seki, S. Angew. Chem. Int. Ed. 2013, 52, 1031; (b) Govindaswamy, S.;
Mamatha, N.; Antoni, K.; Jagdish, V. K.; Marko, P.; Carsten, T. Adv. Funct. Mater. 2012, 22,
1671; (c) Brindaban, R.; Nirupam, D.; Krishna, M. C. Chem.--Eur. J. 2012, 18, 14560; (d)
Seed, A. Chem. Soc. Rev. 2007, 36, 2046.
[11] (a) Cristiano, R.; Santos, D. M. P. O.; Gallardo, H. Liq. Cryst. 2005, 32, 7; (b) Milkhaleva, M.
A. Chem. Heterocycl. Comp. 2003, 39, 1032; (c) Paraskos, A. J.; Swager, T. M.; Chem. Mater.
2002, 14, 4543; (d) Karda, D.; Mieczkowski, J.; Pociecha, D.; Szydloska, J.; Gorcecka, E.; J.
Mater. Chem. 2001, 11, 741.
[12] (a) Liu, P.-L.; Chen, H.-R.; Stelck, D.; Hang, D.-Y.; Zhao, L.; Wang, H.; Zeng, Z. J. Fluorine
Chem. 2013, 156, 327; (b) Chen, R.-H.; Liu, P.-L.; Li, H.-J.; Zhang, H.; Daniel, S.; Zeng, Z.
Eur. J. Org. Chem. 2013, 33, 7517; (c) Chen, H.-R.; Liu, P.-L.; Li, H.-J.; Stelck, D.; Zeng, Z.
Tetrahedron. 2013, 69, 5129; (d) Chen, H.-R.; Hong, F.-Y.; Shao, G.; Hang, D.-Y.; Zhao, L.;
Zeng, Z. Chem. Asian. J. 2014, 9, 3418; (e) Hong, F.-Y.; Li, H,-J.; Zhu, D.-Z.; Xia, Z.-C.;
Zhang, H.; Wang, H.; Zeng, Z. J. Fluorine Chem. 2014, 168, 61; (f) Zhu, D.-Z.; Hong, F.-Y.;
Zhang, H.; Xia, Z.-C.; Wu, H.-X.; Jiang, Q.-W.; Wang, H.; Zeng, Z. J. Mol. Liq. 2015, 204, 84.
[13] (a) Matharu, A. S.; Chambers-Asman, D.; Liq Cryst. 2007, 34, 1317; (b) Gallardo, H.; Zucco,
C.; Da Silva, L.; Mol. Cryst. Liq. Cryst. 2002, 373, 181.
[14] Gallardo, H.; Magnago, R. F.; Bortoluzzi, A. J. Liq. Cryst. 2001, 28, 1343.
[15] (a) Haas, A.; Spitzer, M.; Lieb, M. Chem. Ber. 1988, 121, 1329; (b) Bartmann, E. Adv. Mater.
1996, 8, 570.
[16] Kirsch, P.; Bremer, M.; Taugerbeck, A.; Wallmichrath, T. Angew. Chem. Int. Ed. 2001, 40,
1480.
21

ACCEPTED MANUSCRIPT
[17] Goeb, S.; Ziessel, R. Org. Lett. 2007, 9, 737.
[18] Luo, K.; Meng, L.-G.; Zhang, Y.-C.; Zhang, X.-L.; Wang, L. Adv. Synth. Catal. 2013, 355,
765.
[19] (a) Krigbaum, W. R.; Watanabe, J.; Ishilawa, T. Macromolecules. 1983, 16, 1271; (b) Vries, A.
D. Mol. Cryst. Liq. Cryst. 1973, 24, 337.
[20] (a) Pauluth, D.; Tarumi, K. J. Soc. Inf. Display. 2005, 13, 693; (b) Gray, G. W.; Kelly, S. M.
J. Mater. Chem. 1999, 9, 2037.
[21] Prokhorov, A. M.; Santoro, A.; Gareth Williams, J. A.; Bruce, D. W. Angew. Chem. 2012, 124,
99.
[22] (a) Feller, D.; J. Chem Phys. 1990, 93, 579; (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648;
(c) Lee, C.; Yang, W.-T.; Parr, R. G. Phys Rev B. 1988, 37, 785; (d) Kirsch, P.; Bremer, M.
ChemPhysChem. 2010, 11, 357.
[23] Gaussian 03, Revision D.01 Gaussian, Inc. Wallingford CT, 2004.
[24] Dennington, R.; Keith, T.; Millam, J.; Eppinnett, K.; Hovell W. L.; Gilliland, R. 2003,
GaussView, Version 3.09, Semichem, Inc., Shawnee M.
22