Effects of molecular configuration of a chiral trimeric phenylene vinylene on its liquid crystalline properties

Article abstract of DOI:10.1016/j.cclet.2013.02.013

Based on an analysis of the ¹H NMR, FT-IR, UV-vis spectra, a chiral trimeric phenylene vinylene derivative (ChTPV) synthesized by a Wittig reaction was determined to have a trans-cis configuration, and it could be isomerized to the trans-trans isomer when treated with iodine. The melting point of ChTPV with a trans-trans configuration increased by 162.1 °C compared to the trans-cis isomer, and the ChTPV with trans-trans configuration exhibited the typical liquid-crystalline texture of the smectic A in the heating process. The results indicated that the molecular configuration can influence the formation of the liquid crystalline phase of ChTPV.

Full text of DOI:10.1016/j.cclet.2013.02.013

Chinese Chemical Letters 24 (2013) 397–400
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Effects of molecular configuration of a chiral trimeric phenylene vinylene on its
liquid crystalline properties
*
Ping Yu, Zhen-Lin Zhang, Lei Wang, Chun-Luan Liu, Shi-Min Liu, Hai-Quan Zhang
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
A R T I C L E I N F O
A B S T R A C T
Based on an analysis of the 1H NMR, FT-IR, UV–vis spectra, a chiral trimeric phenylene vinylene
derivative (ChTPV) synthesized by a Wittig reaction was determined to have a trans–cis configuration,
and it could be isomerized to the trans–trans isomer when treated with iodine. The melting point of
ChTPV with a trans–trans configuration increased by 162.1 8C compared to the trans–cis isomer, and the
ChTPV with trans–trans configuration exhibited the typical liquid-crystalline texture of the smectic A in
the heating process. The results indicated that the molecular configuration can influence the formation
of the liquid crystalline phase of ChTPV.
Article history:
Received 19 December 2012
Received in revised form 27 January 2013
Accepted 31 January 2013
Available online 14 March 2013
Keywords:
Oligomer phenylene vinylene
Configuration
ß 2013 Hai-Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
Liquid crystalline
1. Introduction
effects on liquid crystalline properties, we report the configu-
rational character and the liquid crystalline properties of ChTPV
Oligo(phenylenevinylene)s (OPVs) have received consider-
able attention since they can be used to prepare photoelectronic
devices [1–3]. The molecular order plays a critical role in
photoelectronic device performance, for example, both charge
mobility and luminescent efficiency are influenced by molecular
aggregation [4,5]. One approach to control the molecular order
has been used to design molecules that exhibit thermotropic or
lyotropic liquid crystalline (LC) behaviors [6,7]. In addition, new
liquid crystals also can have potential applications in the fields
other than the photoelectronic devices (separation membrane
and heat-resistant materials) [8,9]. ChTPV is a chiral OPVs
derivative, which is functionalized on both ends with a R-(+)-2-
methylbutyric acid ester group. Synthesis of ChTPV and its self-
assembly properties in H2O/THF were studied in our previous
work [10,11]. The reaction sequence to prepare ChTPV is
illustrated in Scheme 1. However, the ChTPV with a trans–cis
configuration was found unexpectedly as the major isomer
based on a detailed analysis for 1H NMR and FT-IR. The cis-
configuration disturbs the formation of the liquid crystalline
phase because cis-configuration generally leads to more non-
planarity and exists in a twisted molecular conformation [12].
Furthermore, the cis-isomer isomerizes to the trans-isomer
under refluxing in toluene with iodine as a catalyst [13]. In order
to obtain the information of the molecular configuration and its
in this paper.
2. Experimental
The synthesis of the trans–cis ChTPV and trans–trans ChTPV is
outlined in Scheme 1. The detailed procedure of the trans–cis
ChTPV has been reported elsewhere [10]. The trans–trans ChTPV
was purified by column chromatography, using silica gel and
chloroform/petroleum ether (10:1) as the eluents.
Reagents were purchased from commercial sources (Aldrich)
and used without further purification. N,N-dimethyl formamide
(DMF), tetrahydrofuran (THF), carbon tetrachloride and toluene
were distilled and purged with argon before use. 1H NMR spectra
were measured in CDC13 solution at 25 8C on an AVANCZ 500
spectrometer with tetramethylsilane (TMS) as the internal
standard. FT-IR, UV–vis, and fluorescence spectra were measured,
respectively, on an E55-FRA106 FT-IR, UV-3100, and RF-5301PC
spectrophotometers. The differential scanning calorimetry (DSC,
STA449C) analysis was used with a heating and cooling rate of
10 8C/min under a dry nitrogen purge. The optical textures were
observed by polarized optical microscopy (POM, Olympus BX51)
equipped with a hot stage calibrated to an accuracy of Æ0.1 8C
(Linkam THMS600).
3. Results and discussion
The aromatic area of the 1H NMR of the two ChTPV isomers are
shown in Fig. 1. The 1H NMR spectrum of the ChTPV shows that the
* Corresponding author.
E-mail address: hqzhang@ysu.edu.cn (H.-Q. Zhang).
1001-8417/$ – see front matter ß 2013 Hai-Quan Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.02.013

[
P. Yu et al. / Chinese Chemical Letters 24 (2013) 397–400
Scheme 1. Synthetic routes of trans–cis ChTPV and trans–trans ChTPV.
chemical shift of the vinylene protons appear at about 6.5 ppm
with a coupling constant of 12 Hz, which is characteristic for the
chemical shift and coupling constant of vinylene protons in the cis-
configuration [12] (Fig. 1a). In addition, the chemical shift of the
trans-vinylene protons appears also at about 7.1 ppm. Further-
more, the integral area of cis-vinylene protons is nearly identical to
that the integral area of trans-vinylene protons. These results
indicate that the cis-configuration and trans-configuration maybe
exist simultaneously in one ChTPV molecule, namely the ChTPV
adopts a trans-cis configuration. The iodine-treated ChTPV did not
have the cis-vinylene protons at about 6.5 ppm, which indicates
that the iodine-treated ChTPV shows an all-trans-configuration.
The reason for the signals of the olefinic protons in trans-ChTPV
shifting downfield compared to those in the cis-configuration is the
decreased electron cloud density on the olefinic segments, because
trans-configuration exhibits more planarity and greater conjugat-
ed effect in the bisstyryl benzene direction than that of the
cis-configuration.
ChTPV and the iodine-treated ChTPV. It is well known that the
FT-IR spectroscopy is one of the most important analytical means
of cis/trans configuration of the vinylene bond. The FT-IR spectrum
of the ChTPV and the iodine-treated ChTPV shows a same
absorption band at about 970 cmÀ1, which is assigned to the
out-of-plane bending mode of the C–H in the trans-vinylene [14].
Here, the intensity of the carbonyl stretch at 1754 cmÀ1 is invariant
in the isomerization process from cis-configuration to trans-
configuration, because the carbonyl group did not participate in
chemical reactions. When FT-IR spectra is normalized using the
wavenumber of 1754 cmÀ1 as the standard, the intensity of other
absorption peak is proportional to the quantity of the groups.
Based on the fact that the intensity of the frequency at 970 cmÀ1
(out-of- plane deformation of the trans-vinylene) doubled for the
iodine-treated ChTPV compared to the original ChTPV, also
indicating that the original ChTPV adopted a trans–cis configura-
tion.
The UV–vis spectra of the ChTPV show that the maximal
absorption peak at 338 nm with a shoulder peak of 275 nm. The
former attributes to the p–p* transition of the delocalized electron
Fig. 2 shows the normalized FT-IR spectra with wavenumber of
1754 cmÀ1 as the standard (a), UV–vis and PL spectrum (b) of the
[(Fig._1)TD$FIG]
Fig. 1. 1H NMR of the ChTPV (a) and the treated ChTPV with iodine (b).

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P. Yu et al. / Chinese Chemical Letters 24 (2013) 397–400
399
a
b
1754
970
300
350
400
450
500
550
600
1800
1600
1400
1200
1000
800
Wavenumber(cm-1)
Wavelength (nm)
Fig. 2. Normalized FT-IR spectra with wavenumber of 1754 cmÀ1 as the standard (a), UV–vis and PL spectrum (b) of the ChTPV (black line) and the iodine-treated ChTPV (red
line). (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
[(Fig._3)TD$FIG]
Fig. 3. DSC traces collected from ChTPV with heating and cooling rates of 10 8C/min ((a) ChTPV and (b) ChTPV treated with iodine) and POM photograph of the ChTPV treated
with iodine ((c) 293 8C and (d) 298 8C).
and the latter is assigned to the
p
–
p
* transition of the isolated
with trans–trans configuration also showed two endothermic
peaks at 231.0 8C and 263.0 8C in the lst heating process. But, clear
point was not observed in the heating process. Here, the
endothermic peak at 263.0 8C is assigned to the melting point
and the peak at 231.0 8C is also for crystal-to-crystal transition. But
the ChTPV with trans–trans configuration exhibited a typical focal
conic sector texture at 293 8C, 298 8C (Fig. 3c and d), which is
assigned to smectic A mesophase.
bisstyryl benzene. In contrast to ChTPV, the maximal absorption
peak of the iodine-treated ChTPV red shifts from 338 nm to
358 nm, indicating the electron delocalizes in all conjugated
system. But their PL spectra show almost the same maximal peak,
which is reasonable due to conversion of the configuration in
excited state.
The liquid crystal properties of the ChTPV and the iodine-
treated ChTPV were investigated using differential scanning
calorimetry (DSC, Fig. 3a and b) and hot stage polarized optical
microscopy (POM, Fig. 3c and d). Two endothermic peaks of the
ChTPV with trans–cis configuration could be observed at 54.4 8C
and 100.9 8C in the 1st heating process. The endothermic peak at
100.9 8C is consistent with the melting point and the peak at
54.4 8C belongs to the crystal-to-crystal transition. The exothermic
peak in the 1st cooling process was found at 44.3 8C, which
attributes to the peak temperature of crystallization. The ChTPV
4. Conclusion
In summary, the ChTPV with trans–cis configuration was
obtained by a Witting coupling reaction, and the isomerization
from cis to trans was realized using iodine as a catalyst. In contrast
to trans–cis configuration, the ChTPV with trans–trans configura-
tion shows higher thermal properties and a typical texture of
smectic A mesophase.

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P. Yu et al. / Chinese Chemical Letters 24 (2013) 397–400
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This work was supported by the National Natural Science
Foundation of China (No. 51173155) and the Hebei Province
Science Foundation of China (No. E2010001182).
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