New fluorescent N-heterocyclic liquid crystals with high birefringence

Article abstract of DOI:10.1016/j.molliq.2016.10.071

A new series of tolane-based liquid crystals with different five-membered N-heterocycle as molecular rigid core have been prepared in good yield. All of these new compounds show mesomorphic behaviors, good thermal stability and intrinsic fluorescence nature. The physical and electro-optical properties of these liquid crystal materials were evaluated. Their properties can be adjusted by varying the alkyl/alkoxyl chain or N-heterocycles. Most of the new compounds exhibit high birefringence, and compounds with propyl chain display stronger fluorescence intensity than those with methoxyl. These tolane-based N-heterocyclic liquid crystals may be used as new high birefringence fluorescent liquid crystal materials.

Full text of DOI:10.1016/j.molliq.2016.10.071

Journal of Molecular Liquids 224 (2016) 909–913
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
New fluorescent N-heterocyclic liquid crystals with high birefringence
Junsheng Jian a, Fengying Hong a, Zhengce Xia a, Dezhao Zhu a, Hongxiang Wu a, Jianhui Liu a, Xiao'e Bu a,
a,
Ming Cui a, Zhuo Zeng a,b, , Hui Wang
⁎
⁎
a
College of Chemistry & Environment, South China Normal University, Guangzhou 510006, China
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 January 2016
Received in revised form 6 September 2016
Accepted 14 October 2016
Available online 15 October 2016
A new series of tolane-based liquid crystals with different five-membered N-heterocycle as molecular rigid core
have been prepared in good yield. All of these new compounds show mesomorphic behaviors, good thermal sta-
bility and intrinsic fluorescence nature. The physical and electro-optical properties of these liquid crystal mate-
rials were evaluated. Their properties can be adjusted by varying the alkyl/alkoxyl chain or N-heterocycles.
Most of the new compounds exhibit high birefringence, and compounds with propyl chain display stronger fluo-
rescence intensity than those with methoxyl. These tolane-based N-heterocyclic liquid crystals may be used as
new high birefringence fluorescent liquid crystal materials.
Keywords:
Fluorescence
Birefringence
Mesophase
© 2016 Elsevier B.V. All rights reserved.
N-heterocycle
Liquid crystal
1. Introduction
pyrazole, imidazole, 1,2,4-triazole are easy to obtain as the commercial
available material with low cost. They are of great importance either as
Liquid crystals (LCs) have attracted considerable attention owing to
their fascinating properties and wide applications [1,2].
chromophore in fluorescent materials or polar terminal group in ther-
motropic liquid crystals.
In microscopy techniques like near-field scanning optical, fluores-
cence and fluorescence confocal polarizing, the LC systems have been
doped with some probe dye molecules to generate fluorescent charac-
teristics. However, the dyes may lead to phase separation or complex
formation with the host molecules, which may alter the properties of
the mixed system. It is valuable to develop LC molecules with intrinsic
fluorescence nature, some of which have been synthesized and reported
in literature [3,4].
Interest in combining the fluorescence and high polarity properties
of N-heterocycle to develop high performance liquid crystal materials
was aroused. In this work, a series of new tolane-type N-heterocyclic
liquid crystals with high birefringence and intrinsic fluorescence nature
have been prepared. Their mesomorphic behaviors, physicochemical
properties such as UV and fluorescence, and thermal stability properties
were investigated. They are expected to be used as new fluorescent liq-
uid crystal materials with high birefringence.
Tolane-type liquid crystal compounds with strong conjugation ex-
hibit high birefringence value [5–7]. Their fluorescent properties can
be modified by introducing the chromophore and/or the auxochrome
on the terminal. Interest in modern high-performance display materials
continues to grow and a large number of tolane-type liquid crystals
have been reported [8]. Some tolane-type liquid crystals are shown in
Fig. 1.
One of the promising approaches to increase birefringence (Δn) and
dielectric anisotropy (Δε) is to introduce a polar ring into the molecule.
Large number of compounds containing heterocyclic unit have been re-
ported [9–11]. A new class of fluorinated N-heterocycle based liquid
crystals were synthesized by our group [12]. These compounds exhibit
broad nematic phase range, good thermal stability and strongly positive
dielectric anisotropy. Five-membered N-heterocycles, such as pyrrole,
2. Results and discussion
2.1. Synthesis
Synthetic routes of novel tolane-type liquid crystals are outlined in
Scheme 1. Details of the synthesis and characterization of the materials
are given in the experimental section.
Compound 2C3H7 or 2CH3O was prepared through Sonogashira cou-
pling reaction reported by Zheng [13] Coupling of 1-bromo-4-
iodobenzene with 1-ethynyl-4-propylbenzene, 1C3H7, or 1-ethynyl-4-
methoxybenzene, 1CH3O, by using PdCl2(PPh3)2/CuI catalyst and Et3N
as a base in THF at room temperature for 12 h, generated compounds,
2C3H7 or 2CH3O, in high yields. And then, compound 2 was reacted
with N-heterocycles by using CuI as the catalyst in the presence of
Cs2CO3 in DMF at 120 °C for 12 h, to give the novel tolane-type N-het-
erocyclic liquid crystals, C3H7-Nhet or CH3O-Nhet, in 45–85% yield.
⁎
Corresponding authors.
E-mail addresses: zhuoz@scnu.edu.cn (Z. Zeng), huiwang@scnu.edu.cn (H. Wang).
http://dx.doi.org/10.1016/j.molliq.2016.10.071
0167-7322/© 2016 Elsevier B.V. All rights reserved.

910
J. Jian et al. / Journal of Molecular Liquids 224 (2016) 909–913
Table 1
C3H7
C3H7
C3H7
OC2H5
Thermal behavior of the new compounds.
Compound
Phase transition temperature/°Ca enthalpies of
Td/°Cb
A1 Δn=0.23
A2 Δn=0.30
transition [kJmol−1
]
C3H7-N1
Cr 173.1 [26.83]
Cr 130,4 [24.65]
Cr 128.5 [3.66]
Cr 128.6 [20.99]
Cr 193.4 [9.99]
Cr 95.6 [27.23]
Cr 171.1 [16.38]
Cr 163.3 [46.94]
N
203c
I
I
I
I
I
I
I
I
267.8
250.5
248.3
280.8
221.7
272.7
251.7
287.6
Fig. 1. Some tolane-type liquid crystal molecules.
C3H7-N12
C3H7-N13
C3H7-N124
CH3O-N1
CH3O-N12
CH3O-N13
CH3O-N124
SmB
SmB
SmE
N
SmA
N
145.5c
160.6c [17.6]
139c
2.2. Liquid crystalline properties
207c
158.6c
193c
The mesomorphic behaviors and thermodynamic data of the new
compounds were investigated by a combination of hot stage polarizing
optical microscopy (POM), differential scanning calorimetry (DSC)
and thermal gravimetric analysis (TGA). The phase transitions and ther-
modynamic 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. 2.
N
179c
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 = crys-
talline solid, N = nematic phase, SmB = smectic B; SmC = smectic C; SmE = smectic E;
I = isotropic liquid state.
b
Td = decomposition temperature.
Transition temperatures were determined by POM.
c
All the new compounds were found to exhibit mesomorphic behavior.
The enthalpy values are in the range of 3.66–46.94 kJ/mol by the differen-
tial scan calorimetry (DSC) thermograms. These liquid crystal molecules
differ in structure at the N-heterocycle rigid core or terminal alky chain.
The new compounds with different five-membered N-heterocyclic
rigid core exhibit different mesophases. Compounds C3H7-N1 and
CH3O-N1 with pyrrole as molecular rigid core display narrow nematic
phase, which was evidenced by their schlieren texture shown in Fig. 2.
The other factor that affects the mesophase is the propyl or methoxyl
substituent. Compound C3H7-N12 exhibits SmB phase, while CH3O-N12
displays SmA phase. The propyl causes stronger molecular packing ar-
rangement which lead to the formation of smectic B phase.
2.3. Thermal stability
The novel compounds are thermally stable up to 221.7–287.6 °C de-
termined 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 new tolane-type compounds with N-heterocycles as end
group exhibit good thermal stability. The stability of compounds with
methoxyl substituent increases as the number of the N atom increases,
e.g., CH3O-N124 (287.6 °C), CH3O-N12 (272.7 °C), CH3O-N1 (221.7 °C).
The compounds prefer to form smectic phase as the number of the N
atom increases. The melting point and clearing point are decreased,
caused by molecular packing is lowering, e.g., C3H7-N1 (Cr 173.1 °C,
N 203 °C), C3H7-N12 (Cr 130.4 °C, SmB 145.5 °C), C3H7-N124
(Cr 128.6 °C, SmE 139 °C).
The transition points of most the compounds with methoxyl group
are higher than those with propyl, e.g., CH3O-N1 (Cr 193.4 °C, N
207 °C), C3H7-N1 (Cr 173.1 °C, N 203 °C); CH3O-N13 (Cr 171.1 °C, N
193 °C), C3H7-N13 (Cr 128.5 °C, SmB 160.6 °C). The transition point of
the new compounds decrease when changing the N-heterocyclic rigid
core from pyrrole to 1,2,4-triazole.
2.4. UV/Vis absorption, photoluminescence spectroscopy and birefringence
The UV absorption spectra data for solutions of the new tolane-type
N-heterocyclic compounds in dichloromethane are presented in
The shorter methoxyl substituent decreases the mesophase range.
The longer propyl provides an appropriate length to width ratio,
which is propitious to the stability of mesomorphism.
I
Br
R
R
Br
a
1R
2R
R=C3H7, CH3O Yield 95-96%
Fig. 2. Optical texture (a) for CH3-N1 schlieren of nematic phase upon cooling to 197 °C,
(b) for CH3O-N1 schlieren of nematic phase upon cooling to 204 °C.
R = C3H7, CH3O
N-het
b
R
N
het
Table 2
R-Nhet
UV–visible, fluorescence spectra analysis data and birefringence of new compounds.
R = C3H7, CH3O
N-het = N1, N12, N13, N124 Yield 45-85%
Compounds
λ
max [nm]a
λ
exc [nm]a
λ
em [nm]a
Birefringenceb
N
N
C3H7-N1
305
309
298
302
307
309
302
307
310
310
310
310
310
310
310
310
338
336
330
336
340
346
349
359
0.37
0.31
0.20
0.27
N-het =
C3H7-N12
C3H7-N13
C3H7-N124
CH3O-N1
CH3O-N12
CH3O-N13
CH3O-N124
N
N
N
H
N
H
N
H
N
H
c
–
N12
N1
N13
N124
0.35
0.33
0.34
Reagents and cooditions: a) PdCl2(PPh3)2, CuI, Et3N, THF, rt, 12h;
b) CuI, Cs2CO3, DMF, 120 , 12h.
a
The spectra were recorded from a dilute solution in dichloromethane (1.5 × 10−5 mol/L).
Extrapolated from 5 wt% solution in nematic host mixture 1C40200-000 (Δn = 0.131).
Poor solubility.
b
c
Scheme 1. Synthesis of novel tolane-type N-heterocyclic liquid crystals.

J. Jian et al. / Journal of Molecular Liquids 224 (2016) 909–913
911
Fig. 3. Absorption spectra (left) and fluorescence spectra (right) of tolane-type liquid crystals.
Table 2. The position of N atom in N-heterocyclic rigid core influences
the max absorption wavelength, for example, changing the position
from 1,3 to 1,2, gives a red shift, e.g., C3H7-N13, λmax = 298 nm, C3H7-
data of new compounds are shown in Table 2. All spectra were recorded
from a dilute solution in dichloromethane (1.5 × 10−5 mol/L).
As shown in Fig. 3, the intensities of compounds with propyl group
are stronger than that of compounds with methoxyl group. The double
peak phenomenon of fluorescence spectra for compounds with propyl
group result from the marketed difference between the excited state
and the ground state [15]. Compound C3H7-N12 exhibited the strongest
N12, λmax = 309 nm, CH3O-N13, λmax = 302 nm, CH3O-N12, λmax
=
309 nm.
Recently, adding the property of liquid crystallinity to emissive ma-
terials is of increasing interest [14]. The emission fluorescence spectra
Fig. 4. The optimized geometry of C3H7-N1 (a), C3H7-N12 (b), C3H7-N13 (c), C3H7-N124 (d).
Fig. 5. HOMO of C3H7-N12 (a) and LUMO of C3H7-N12 (b); HOMO of CH3O-N12 (c) and LUMO of CH3O-N12 (d).

912
J. Jian et al. / Journal of Molecular Liquids 224 (2016) 909–913
Table 3
Geometrical parameters of new compounds calculated with Gaussian 03.
Compounds
Molecular length (10−1 nm)
L/D
Energy (a.u.)
HOMO (ev)
LUMO (ev)
EHL (ev)
Dipole (D)
C3H7-N1
18.48
18.48
18.33
18.13
17.47
17.44
17.50
17.30
4.30
4.30
4.26
4.21
4.06
4.05
4.07
4.02
−866.17
−882.19
−882.20
−893.49
−862.74
−878.75
−878.79
−894.78
−0.048
−0.051
−0.056
−0.047
−0.045
−0.048
−0.053
−0.059
−0.202
−0.202
−0.211
−0.234
−0.197
−0.197
−0.205
−0.205
0.154
0.151
0.155
0.187
0.152
0.149
0.152
0.146
2.04
2.09
5.17
4.81
2.86
3.68
5.43
5.18
C3H7-N12
C3H7-N13
C3H7-N124
CH3O-N1
CH3O-N12
CH3O-N13
CH3O-N124
intensity at 336 nm, which makes it new liquid crystal materials with
intrinsic fluorescene nature possible.
Their melting points, decomposition temperatures, UV/Vis absorption,
photoluminescence spectroscopy and birefringences were measured.
Their properties can be adjusted by varying the terminal chain or N-het-
erocycles. The melting points of the compounds with methoxyl are
higher than those with propyl. As the number of the N-atom increases,
compound prefers to form smectic phase. The longer propyl substituent
causes the liquid crystal compounds to have stronger fluorescene inten-
sity. Compound C3H7-N12 exhibited the strongest fluorescene intensity
and high Δn value (Δn = 0.31), hence it was suggested as new fluores-
cent liquid crystal materials with high birefringence.
As shown in Fig. 3, the intensities of compounds with propyl group
are stronger than that of compounds with methoxyl group. The double
peak phenomenon of fluorescence spectra for compounds with propyl
group result from the marketed difference between the excited state
and the ground state [15]. Compound C3H7-N12 exhibited the strongest
intensity at 336 nm, which makes it new liquid crystal materials with
intrinsic fluorescene nature possible.
New tolane-type N-heterocyclic compounds have high Δn value,
which range from 0.20 to 0.37. The N-heterocyclic rigid core has influ-
ence on Δn value. As the number of N atoms increases, the Δn value of
compound with propyl decreases, e.g., C3H7-N1 (Δn = 0.37), C3H7-
N12 (Δn = 0.31), C3H7-N124 (Δn = 0.27). It can be illustrated by
the polarity that N-atom brings. The propyl and methoxyl substituent
also influence the Δn value, e.g., C3H7-N12 (Δn = 0.31), CH3O-N12
(Δn = 0.35). Compound C3H7-N1 containing molecular rigid core pyr-
role ring has the largest Δn value (0.37). It can be employed as new fluo-
rescent liquid crystal materials with high birefringence.
4. Experimental section
The experimental details, synthesis, structural characterization of
new compounds and their liquid crystal properties, geometrical param-
eters 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.
2.5. Theoretical study
The geometry optimization of the structures were performed by
density functional theory (DFT) Becke's three-parameter hybrid func-
tion with non-local correlation of Lee-Yang-Parr (B3LYP) method in
gas phase [16]. The corresponding frequency analyses were computed
at the same level of theory to characterize them as minima (no imagi-
nary frequencies) with help of Gaussian03 (Revision D.01) suite of pro-
grams [17]. All of above calculations used a 6 − 31 + G basis set [18].
The computed structures were visualized using the GaussView pro-
gram. The optimized geometry of compounds with propyl were
shown in Fig. 4, and the HOMO, LUMO of compounds C3H7-N12 and
CH3O-N12 were shown in Fig. 5. The HOMO, LUMO of other compounds
were shown in the Supplementary data.
As shown in Table 3, the length to width ratios (L/D) of the new com-
pounds is larger than 4, which is an important factor that affects the for-
mation of liquid crystal.
The energy of compound C3H7-N124 is the lowest among com-
pounds with propyl, and that of compound CH3O-N124 is also the low-
est among compounds with methoxyl. This indicates that compounds
with 1,2,4-triazole as end group is more stable than those with other
five-membered N-heterocycles. It may result from the intrinsic dipole
moment of 1,2,4-triazole which lead to the increase of the molecular
polarity.
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the
online version, at http://dx.doi.org/10.1016/j.molliq.2016.10.071.
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