Acetylene-bridged triazine π-conjugated structures: Synthesis and liquid crystalline properties

Article abstract of DOI:10.3906/kim-1907-26

We report the synthesis and liquid crystalline properties of a new type of tribranched compound built up from triazine as the central core and electron-rich triazine rings as peripheral groups. An equimolar mixture of the star-shaped compound based on the 1,3,5-triazine core with 4-dodecyloxybenzoic acid produced an organic salt exhibiting a columnar mesophase with a dendritic growing texture at low temperatures. The liquid crystalline properties of the triazine-based compound and organic salt were investigated by differential scanning calorimetry and polarizing optical microscopy.

Full text of DOI:10.3906/kim-1907-26

Turk J Chem
Turkish Journal of Chemistry
(2019) 43: 1436 – 1444
© TÜBİTAK
http://journals.tubitak.gov.tr/chem/
doi:10.3906/kim-1907-26
Research Article
Acetylene-bridged triazine π-conjugated structures: synthesis and liquid
crystalline properties
1
1
Nihat AKKURT1,2,, Mohammed Hadi Ali AL-JUMAILI ,, Belkız BİLGİN ERAN ,,
1
1,3,∗
Hale OCAK ,, Lokman TORUN
,
1
Department of Chemistry, Faculty of Science and Arts, Yıldız Technical University, İstanbul, Turkey
Department of Chemistry, Faculty of Science and Arts, Kırklareli University, Kırklareli, Turkey
2
3
TORKİM ARGE Kimya Sanayi Tic. A. Ş., YTÜ Teknopark, Kuluçka Merkezi, Esenler, İstanbul Turkey
Received: 12.07.2019
•
Accepted/Published Online: 06.09.2019
•
Final Version: 07.10.2019
Abstract: We report the synthesis and liquid crystalline properties of a new type of tribranched compound built up from
triazine as the central core and electron-rich triazine rings as peripheral groups. An equimolar mixture of the star-shaped
compound based on the 1,3,5-triazine core with 4-dodecyloxybenzoic acid produced an organic salt exhibiting a columnar
mesophase with a dendritic growing texture at low temperatures. The liquid crystalline properties of the triazine-based
compound and organic salt were investigated by differential scanning calorimetry and polarizing optical microscopy.
Key words: Triazine, liquid crystals, star-shaped materials, hydrogen bonding
1. Introduction
Liquid crystals (LCs) represent an intriguing state of matter that combines mobility and order at a molecular
and supramolecular level [1]. Many examples of LCs can be found in nature. Proteins, tobacco mosaic virus,
and soap are some of the naturally occurring liquid crystalline materials. An abundant number of synthetically
made LC materials are also reported in the literature [2].
Star-shaped compounds based on the 1,3,5-triazine core with triarmed alkoxy groups on the periphery
represent octupolar structures with long enough flexible side chains and possess fascinating nonlinear optical
(NLO) properties as they may form columnar phase LCs. Alkoxy chains with sufficient length and numbers at
the triazine rings provide liquid crystalline phase Colhd [3,4].
Discotic molecules dominate conventional LCs. Therefore, growing attention is devoted to the synthesis
and study of unconventional LCs to discover new LC phases and low-temperature applications [5–9]. Star-
shaped LCs with small cores and some extended mesogenic units as LC arms provide molecular structures
considered to be discotic LCs. They have flexible spacers and may exhibit nematic LC phases rather than a
columnar phase, which is ideal for electrooptical application LC displays [10,11].
Intensive effort has been devoted to the synthesis of the many kinds of star-shaped LCs to explore their
potential applications. A large number of new LCs have been reported based on 1,3,5-triazine in the core with
various types of substituents and benzoic acid mesogenic arms [1].
A large number of reports in the literature examine the liquid crystalline properties of charged materials
containing anions and cations. Such compounds exhibit low vapor pressure and can replace volatile LCs [12].
∗Correspondence: ltorun@yildiz.edu.tr
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This work is licensed under a Creative Commons Attribution 4.0 International License.

AKKURT et al./Turk J Chem
In the present study, we report the preparation and the liquid crystalline properties of a triarmed organic
salt utilizing 1,3,5-triazine units in the core and on the periphery, which are substituted with n-dodecyloxy
1
13
groups as seen in Schemes 1 and 2. The LC was characterized by HRMS, FTIR, H NMR, and C NMR
analyses. The mesomorphic studies were conducted using polarizing optical microscopy (POM) and differential
scanning calorimetry (DSC).
2. Synthesis
Scheme 1. Synthesis of 2,4,6-tris((4,6-bis(dodecyloxy)-1,3,5-triazin-2-yl)ethynyl)-1,3,5-triazine.
Scheme 2. Synthesis of organic salt.
2.1. Experimental section
2.1.1. 2-Chloro-4,6-bis(dodecyloxy)-1,3,5-triazine (1) [13]
A mixture of 2,4,6-trichloro-1,3,5-triazine (1.9 g, 10.3 mmol), dodecan-1-ol (3.839 g, 20.6 mmol), and K2 CO3
(2.84 g, 20.6 mmol) in 10 mL of dioxane was stirred at 50 ◦ C for 5 h. The reaction mixture was cooled. The
solution was poured into ethyl acetate (20 mL) and the organic layer was separated, washed with water twice
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(2 × 10 mL), dried with Na2 SO4 , and the solvent was evaporated under reduced pressure. The material
obtained was purified by column chromatography with hexane/ethyl acetate (19:1) as an eluent to give white
1
solid materials (3.5 g, yield: 70%) H NMR (500 MHz, CDCl3) δ 3.70 (t, J = 6.7 Hz, 4H, OCH2), 1.81–1.51
13
(m, 4H, CH2), 1.58–1.20 (m, 36H, CH2), 0.94 (t, J = 6.9 Hz, 6H CH3).
C NMR (126 MHz, CDCl3)
δ
173.2, 172.2, 68.5, 32.0, 29.7 (2C), 29.6 (2C), 29.4, 29.3, 28.7, 25.9, 22.7, 14.1.
2.1.2. Synthesis of 2,4-bis(dodecyloxy)-6-ethynyl-1,3,5-triazine (2)
Step 1: Trimethylsilylacetylene (0.345 g, 3.51 mmol) was dissolved in dry THF (3 mL). n-Butyllithium (2.20
mL, 3.52 mmol, 1.6 M in hexane) was then added dropwise by syringe under argon gas. The reaction mixture
was stirred for 1 h under argon gas at 0 ◦ C in an ice bath. Solution of 2-chloro-4,6-bis(dodecyloxy)-1,3,5-triazine
(1.704 g, 3.52 mmol) in dry THF (8 mL) was then added to the mixture dropwise. The reaction mixture was
stirred for 2 h under argon gas at 0 ◦ C in an ice bath. The reaction mixture was allowed to warm up to room
temperature. The corresponding compound (2,4-bis(dodecyloxy)-6-((trimethylsilyl)ethynyl)-1,3,5-triazine) was
directly used without isolation and purification in the next step.
Step 2: The mixture of corresponding compound (2,4-bis(dodecyloxy)-6-((trimethylsilyl)ethynyl)-1,3,5-triazine,
900 mg, 1.65 mmol), K2 CO3 (684 mg 4, 95 mmol) in 1:1 MeOH/THF (20 mL) was stirred for 24 h at room
temperature. The solution was poured into ethyl acetate (20 mL) and the organic layer was separated, washed
with water twice (2 × 10 mL), dried with sodium sulfate, filtered, and the solvent was evaporated under re-
duced pressure. The crude product was subjected to purification by column chromatography with hexane/ethyl
1
acetate (6:1) as an eluent to give yellow liquid materials (2) (700 mg, 42%). H NMR (500 MHz, CDCl3)
δ
3.92 (s, 1H), 3.52 (t, J = 6.7 Hz, 4H, OCH2), 1.50–1.40 (m, 4H, CH2), 1.33–1.08 (m, 36H, CH2), 0.77 (t, J =
13
6.8 Hz, 6H, CH3).
C NMR (126 MHz, CDCl3) δ 174.1, 173.7, 74.4, 69.2, 63.5, 32.4, 30.1 (2C), 30.0 (2C),
+
29.8, 29.4, 27.4, 26.2, 23.2, 14.5. HRMS = m/z [M+H] calcd for C29 H52 N3 O2 : 474.40; found: 474.41.
2.1.3. 2,4,6-Tris((4,6-bis(dodecyloxy)-1,3,5-triazin-2-yl)ethynyl)-1,3,5-triazine (3)
n-BuLi (0.65 mL, 1.05 mmol, 1.6 M in hexane) was added dropwise to a stirred solution of 2,4-bis(dodecyloxy)-6-
ethynyl-1,3,5-triazine (2) (0.5 g, 1.05 mmol) in THF (8 mL) at –78 ◦ C The solution was stirred for 1 h, allowed
to warm to room temperature, and stirred for another 20 min. The solution was then cooled to –78 ◦ C and a
solution of cyanuric chloride (1) (0.055 mg, 0.3 mmol) in THF (1 mL) was added dropwise and the mixture was
stirred for 30 min. The mixture was warmed to room temperature and then stirred for an additional 2 h. Water
(10 mL) and dichloromethane (10 mL) were added. The aqueous layer was extracted with dichloromethane
(2 × 10 mL). The combined organic layers were then washed with brine, dried over Na2 SO4 , filtered, and
evaporated under reduced pressure. The resulting crude product was purified by column chromatography (6:1
1
hexane/ethyl acetate) to obtain the target compound 4 (0.09 g, yield 21%). H NMR (500 MHz, CDCl3)
δ
3.63 (t, J = 6.7 Hz, 12H, OCH2), 1.64–1.50 (m, 12H, CH2), 1.44–1.22 (m, 108H, CH2), 0.88 (t, J = 7.0 Hz,
13
18H, CH3). C NMR (126 MHz, CDCl3) δ 173.4, 173.0, 172.4, 92.8, 90.4, 68.8, 32.2, 30.0, 29.9 (2C), 29.8,
+
29.6 (2C), 28.9, 26.1, 23.0, 14.4). HRMS = m/z [M] calcd for C90 H150 N12 O6 : 1495.18; found: 1495.19.
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AKKURT et al./Turk J Chem
2.1.4. Synthesis of 4-(dodecyloxy) benzoic acid (4-DBA) (4) [14]
A solution of 4-hydroxy benzoic acid (16.43 mmol), 1-bromododecane (11 mL, 46 mmol, 2.8 eq), and KOH (2.58
g, 46 mmol) in ethanol (50 mL) was heated under reflux for 3 days. The resulting material was hydrolyzed with
10% aqueous KOH (25 mL) under refluxing overnight, after which the mixture was cooled to room temperature
and acidified with HCl (6 M). The precipitate was filtered, washed with water, and recrystallized from ethanol
1
to give the pure product 4-dodecyloxybenzoic acid as a white solid (4), 4.55 g, 91% yield. H NMR (500 MHz,
CDCl3)δ 8.05 (d, J = 8.8 Hz, 2H, Ar-H), 6.93 (d, J = 6.8 Hz, 2H, Ar-H), 4.05 (t, J = 6.4 Hz, 2H, OCH2),
1.96–1.68 (m, 2H, CH2), 1.52–1.27 (m, 18H, CH2), 0.89 (t, J = 7.0 Hz, 3H, CH3).
2.1.5. Synthesis of organic salt (5)
4-DBA mesogenic unit (4) with a carboxyl group was added to 2,4,6-tris((4,6-bis(dodecyloxy)-1,3,5-triazin-2
yl)ethynyl)-1,3,5-triazine (3) in one to one ratio. Sonication was performed in dry THF for 10 min until a
transparent solution was obtained. Then the solvent was removed in a vacuum.
3. Result and discussion
The interaction between the triarmed π-conjugated system and the mesogenic carboxyl group was mainly
studied by FTIR. The carboxylic peak at 1680 cm−1 in 4-(dodecyloxy) benzoic acid shifted to 1520 cm−1 in
the salt material 5. However, the C=N peaks were observed in both compound 3 and organic salt at 1505 as
seen in Figure 1 [15].
Figure 1. FT-IR spectra of compound 3, organic salt, and benzoic acid (4-DBA).
The interaction between the s-triazine and a carboxyl group of the mesogenic unit was also examined
using NMR spectroscopy. The signals corresponding to the aromatic hydrogens of the benzoate unit shift from
8.05 ppm and 6.95 ppm to 7.96 ppm and 6.84 ppm, respectively. The shift was attributed due to an increase
in electron density of the aromatic ring. Likewise, the signals of –O–CH2 – hydrogens of 4-DBA in the complex
shifted to a higher field, 3.95 ppm. The corresponding signals of pure 4-DBA were observed at 4.05 ppm (Figure
2). Additionally, the triplet signals at 4.27 ppm of compound 3 show no shifting by complexation because their
electronic environment did not change.
13
In addition, the C NMR spectra show that the carbonyl carbon shifted from 171.6 to 173.10 ppm.
This change in chemical shift is due to a decrease in electron density after the ionic interaction, whereas the
aromatic carbon next to the alkoxy group shifts slightly from 163.69 to 163.57 ppm (Figure 3).
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AKKURT et al./Turk J Chem
1
Figure 2. Comparison of the H NMR spectra (in CDCl3) of compound 3 (triazine compound of organic salt), organic
salt 5, and benzoic acid (4-DBA).
13
Figure 3.
C NMR spectra (in CDCl3 ) of benzoic acid (4-DBA) (bottom) and organic salt 5 (top).
5+
5+
Additionally, the HRMS spectrum of the salt indicates the presence of M
at 360.47 and (M+5Na)
at
383.4788 peaks, which correspond to C109 H180 N12 O9 (1802.71, molecular weight of 5) and C109 H180 N1 2O9 Na5
(1917.394, pentasodium salt of compound 5), respectively (Figure 4).
The mesomorphic properties of the star-shaped compound 3 with triazine, 4-DBA, and organic salt
(5) were investigated using POM and DSC. The phase transition of the corresponding molecules is given in
the table. The peak temperatures are given in degrees Celsius and the numbers in parentheses indicate the
transition enthalpy (∆H) in kJ mol−1 . As shown in Figure 5, compound 3 shows one endotherm corresponding
to crystal to isotropic (Iso) transition in the heating DSC thermogram. Nonmesomorphism of compound 3 was
also confirmed by optical polarizing microscope (PM) observation.
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AKKURT et al./Turk J Chem
Figure 4. HRMS result of organic salt 5.
Figure 5. DSC thermograms of (left) compound 3, (center) 4-DBA, and (right) organic salt (5) on 1st heating and
cooling (10 ¡C minꢀ1 ).
Compound 4-DBA, with an n-dodecyloxy alkyl chain, shows enantiotropic liquid crystalline properties
in agreement with the behavior observed for analogous benzoic acids carrying an alkoxy chain with different
numbers of carbon atoms at position 4 of the aromatic ring [16–18]. Upon heating, 4-DBA showed three peaks
corresponding to Cr–SmC–N–Iso transitions. On cooling from the isotropic phase, the same behavior of reverse
transitions was observed. Furthermore, a calorimetric peak corresponding to Cr–Cr transition at 65.86 ◦ C was
detected in the cooling DSC thermogram. A typical texture of SmC mesophase observed for 4-DBA is shown
in Figure 6 as a representative case.
The organic salt 5 exhibits a phase transition sequence of Cr–Col–Iso, which is in agreement with two
endotherms in the DSC heating curves. On cooling from the isotropic liquid, the dendritic growing texture
was observed at 67–20 ◦ C (Figure 7). The dendritic growing was composed of a disc-like shaped π-conjugated
system based on 1,3,5-triazine central core and the mesogenic carboxyl group, referred to here as the Col phase,
which could be considered columns stacked in either rectangular or hexagonal 2D lattices [19–22]. Below about
T = 20.0 ◦ C, the dendritic texture of the Col phase completely disappears and the crystalline phase starts to
appear.
The formation of the columnar mesophase is explained by ionic interaction between the triarmed π-
conjugated system and the mesogenic carboxyl group. In this noncovalent intermolecular interaction, the
star-shaped compound 3 based on the 1,3,5-triazine acts as a proton acceptor and COOH of 4-DBA with the
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AKKURT et al./Turk J Chem
Figure 6. A typical texture of smectic C mesophase of compound 4-DBA T = 118.0 ¡C as observed between crossed
polarizers in an ordinary glass plate on cooling.
Figure 7. Optical textures of organic salt 5 as observed between crossed polarizers in an ordinary glass plate on cooling;
(a) dendritic texture of Col phase at 52 ◦ C and (b) 45 ◦ C; (c) crystalline phase at 20 ◦ C.
Table 1. Mesophases and phase transition temperatures as observed on heating (H→) and cooling (←C) and corre-
sponding transition enthalpies of the compounds 3, 4-DBA, and organic salt 5.
a
Comp.
3
4-DBA
OS (5)
T/◦C [∆H kJ/mol]
H →: Cr 51.66 [238.86] Iso
b
H →: Cr 99.98 [39.01] SmC 132.43 [2.39] N 138.42 [2.05] Iso
H →: Cr 46.16 [160.46] Col 84.75 [18.17] Iso Cr 12.30 [137.59] Col 65.93 [9.61] Iso: ← C
a
PerkinElmer DSC-6; enthalpy values in italics in brackets taken from the 1st heating and cooling scans at a
rate of 10 ◦ C min−1 ; Abbreviations: Cr = crystalline, SmC = tilted smectic phase, N = nematic phase; Col
b
= columnar mesophase, Iso = isotropic liquid phase. [16,17] Cr 95.1 SmC 128.9 N 137.2 Iso [18] Cr 92.4 SmC
131.5 N 142.0 Iso
n-dodecyloxy terminal chain as the proton donor. The resultant ionic interaction has a striking influence on
mesogenic properties such as melting and clearing temperatures as well as inducing a columnar phase at lower
temperatures.
4. Conclusion
We prepared and characterized a π-conjugated macromolecular structure composed of a 1,3,5-triazine central
core and 1,3,5-triazine rings carrying dodecyloxy chains, which are positioned at the peripheries of the central
core by acetylenic bridges, and investigated the properties of the LC of its benzoate salt. The organic salt
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AKKURT et al./Turk J Chem
that was made between the π-conjugated system and 4-dodecyloxy benzoic acid exhibited an enantiotropic Col
phase with a dendritic texture at lower temperatures close to room temperature.
Acknowledgments
This work was supported by TÜBİTAK with project no. 114Z722. We also thank Yıldız Technical University
(project no. 2015-01-01DOP04).
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