Synthesis and liquid crystalline properties of new triazine-based π-conjugated macromolecules with chiral side groups

Article abstract of DOI:10.3906/KIM-1912-51

In this study, we reported the synthesis of a new tribranched macromolecule liquid crystal with triazine in the centre. The central triazine core is bonded via sequences of Sonigashira coupling to 3 triazine unites through acetylenic bridges. The triazines at the periphery are substituted with 2 chiral citronellyloxy side groups. The salt of the resulting star-shaped macromolecule, which was oily at room temperature, with 4-dodecyloxybenzoic acid at 1:1 ratio exhibited a Smectic C (SmC) mesophase. The liquid crystalline properties of organic salt were investigated using DSC (differential scanning calorimetry) and POM (polarizing optical microscopy).

Full text of DOI:10.3906/KIM-1912-51

Turk J Chem
Turkish Journal of Chemistry
(2020) 44: 726 – 735
© TÜBİTAK
http://journals.tubitak.gov.tr/chem/
doi:10.3906/kim-1912-51
Research Article
Synthesis and liquid crystalline properties of new triazine-based π-conjugated
macromolecules with chiral side groups
1
1
1
Nihat AKKURT1,2, Mohammed Hadi Ali AL-JUMAILI , Hale OCAK , Fatih ÇAKAR ,
Lokman TORUN1,3,∗
1
Chemistry Department, Faculty of Science and Art, Yildiz Technical University , Istanbul, Turkey
2
Chemistry Department, Faculty of Science and Art, Kirklareli University, Kirklareli, Turkey
3
TORKIM R&D Chemical Industry, YTÜ Technopark Incubation Centre, Istanbul, Turkey
Received: 27.12.2019
•
Accepted/Published Online: 11.04.2020
•
Final Version: 01.06.2020
Abstract: In this study, we reported the synthesis of a new tribranched macromolecule liquid crystal with triazine in the
centre. The central triazine core is bonded via sequences of Sonigashira coupling to 3 triazine unites through acetylenic
bridges. The triazines at the periphery are substituted with 2 chiral citronellyloxy side groups. The salt of the resulting
star-shaped macromolecule, which was oily at room temperature, with 4-dodecyloxybenzoic acid at 1:1 ratio exhibited a
Smectic C (SmC) mesophase. The liquid crystalline properties of organic salt were investigated using DSC (differential
scanning calorimetry) and POM (polarizing optical microscopy).
Key words: Triazine, ionic liquid crystals, hydrogen bonding, star-shaped triazine, Sonogashira coupling.
1. Introduction
Liquid crystals possessing properties of both liquids and solids have been focus of intensive research activities
from industrial and academic points of views due to their technological importance and wide commercial
application in last few decades. Their widespread use includes in consumer electronic, office equipment,
calculators, watches, stereos, etc. Perhaps the most important applications are found on liquid crystal displays
(LCDs), which have a dominant position in consumer electronics [1-6].
There is a growing interest in the synthesis and investigation of nonconventional liquid crystals to discover
new LC phases and low-temperature applications, which have been regarded as new models in the progression
of LC science and technology as they are capable of exhibiting unique physical properties and uncommon phase
transitions [1,3,4,6].
In the studies on liquid crystal materials, intensive efforts have been devoted to the synthesis of liquid
crystals with star-shaped topologies to investigate their potential applications, as they may possess unique
properties. Many examples of such liquid crystalline compounds, which include benzene and triazine cores and
alkyl chains in the periphery, exhibit interesting LC phases [7-15].
Ionic liquid crystals can be considered as the materials that combine the properties of liquid crystals
and ionic liquids. A growing interest has been devoted to the research activities in the field of ionic liquids
worldwide [16-18]. One of the important properties of ionic liquids is that these compounds have very low vapor
pressures and can substitute volatile organics. Other applications include the use of solvents and batteries for
∗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
extraction processes, as an electrolyte for fuel cells and dye-sensitive solar cells [18]. A large number of literature
reports are available on the liquid crystalline properties of the charged materials containing anions and cations
[15,19,20].
One approach to obtain ionic liquids is by converting an organic material into its salt. This can be
made by mixing the organic substance, which has hydrogen bond accepting heteroatoms, with a benzoic acid
derivative with long alkyl chain in appropriate ratios. This is a useful and practical way of converting nonliquid
crystal materials into materials with mesomorphic properties [21,22].
In this study, we reported the synthesis and the LC properties of a triarmed organic material utilizing
1,3,5-triazine units in the core and on the periphery, which were substituted with chiral citronellyloxy groups.
The 1:1 mixture of this compound with 4-dodecyloxybenzoic acid presented a liquid crystal material, which
1
13
was characterized by FTIR, H NMR, and C NMR analyses. The mesomorphic properties were investigated
using POM and DSC.
2. Synthesis and characterization
Synthesis of the triarmed macromolecule with chiral citronellyloxy side groups is outlined in Scheme 1 (Synthesis
of 2,4,6-tris((4,6-bis((S)-citronellyloxy)-1,3,5-triazin-2-yl)ethynyl)-1,3,5-triazine (3)).
Scheme 1. Synthesis of 2,4,6-tris((4,6-bis((S)-citronellyloxy)-1,3,5-triazin-2-yl)ethynyl)-1,3,5-triazine (3).
The TMS protected acetylene was treated with 1.3.5-tricloro-2,4,6-triaizen and subsequently with cit-
ronellyloxy groups followed by desililylation afforded intermediate compound 2 (Scheme 1). The HRMS analysis
+
showed m/z [M+H] calculated for C25 H39 N3 O2 : 414,304; found 414.312 for of compound 2. H NMR and
CNMR data for compound 2 are provided in the experimental section (See supporting information).
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A Negishi protocol was applied for the synthesis of RZnCl intermediate, which then coupled with triazine
in the presence Pd catalyst to afford the targeted macromolecule 3 in 34% yield as colourless oily material.
Compound 4 was synthesized according to literature [23]. Converting compound 3 into its alkoxy benzoic acid
salt 5 was necessary in order to obtain a solid material at room temperature, which made it possible to obtain
an observable phase transitions under POM (Scheme 2). Structural characterizations for compounds 3 and
1
13
3+
5 were confirmed by H NMR, C NMR, HRMS, and FTIR. The HRMS analysis showed m/z [M+3Na]
calculated for (C78 H114 N12 Na3 O6) 3+ : 461,289; found 461,292 for compound 3. The formation of organic salt
5 was confirmed by the shift of the carbonyl stretching frequency from 1680 cm
−1
in free acid compound 4
−1
to 1650 cm
in the salt 5. In addition, the difference between the relevant proton and carbon chemical shift
in the free acid 4 and salt 5 provided other confirming interaction between acid for and compound 3, which
resulted the formation of salt 5 (See supporting information).
Scheme 2. Synthesis of organic salt 5.
2.1. Experimental section
Synthesis of 2,4-dichloro-6-((trimethylsilyl)ethynyl)-1,3,5-triazine (1) [24]
Trimethylsilylacetylene (1.73 g, 17.6 mmol) was dissolved in dry THF (15 mL). n-Butyllithium (11.0 ml, 17.6
mmol, 1.6 M in hexane) was then added dropwise by a syringe under argon atmosphere. The reaction mixture
was stirred for 1 h at 0 °C in an ice bath. A solution of 2,4,6-trichloro-1,3,5-triazine (3.24 g, 17.5 mmol) dissolved
in dry THF (8 mL) was added to the mixture slowly and the resulting mixture was stirred for 2 h at 0 °C, and
it was allowed to warm up to room temperature with stirring for additional 1 h. The solution was poured into
ethyl acetate (25 mL) and the organic layer was washed with water 2 times (2 × 25 mL). The organic layer
was dried with sodium sulphate, filtered and the solvent was rotary evaporated affording compound 1 (1.43 g,
13
33%), which was used without purification. C NMR δ 172.4, 162.3, 107.0, 99.7, and –0.01.
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AKKURT et al./Turk J Chem
Synthesis of 2,4-bis((S)-citronellyloxy)-6-ethynyl-1,3,5-triazine (2)
Step 1: A mixture of 2,4-dichloro-6((trimethylsilyl)ethynyl)-1,3,5-triazine (compound 1, 1.23 g, 5 mmol), (S)-
citronellol (1.72 g, 11.0 mmol), and K2 CO3 (1.67 g, 11.0 mmol) in 25 mL THF was stirred at 70 °C for 3 h.
The reaction mixture was allowed to warm up to room temperature. The corresponding reaction mixture was
poured into ethyl acetate (25 mL) and the organic layer was washed with water 2 times (2 ×15 mL), dried
with sodium sulphate and the solvent evaporated under reduced pressure. The corresponding compound was
directly used without isolation and purification in the next step.
Step 2: The mixture of corresponding compound 2 (2,4-bis((S)-citronellyloxy)-6-((trimethylsilyl)ethynyl)-1,3,5-
triazine) (1.39 g, 3 mmol), K2 CO3 (0.46 g, 3.30 mmol) in 1:1 MeOH/THF (30 mL) was stirred 24 h at room
temperature. The solution was poured into ethyl acetate (25 mL) and the organic layer was washed with
water (2 ×10 mL) and dried with Na2 SO4 , filtered. The solvent was evaporated under reduced pressure. The
residue was purified by column chromatography with hexane/ethyl acetate (5:1) as an eluent to give yellow oily
1
materials (2) with yield (0.65 g, 47%). H NMR (500 MHz, CDCl3) δ 5.16–5.04 (m, 2 H CH=C(CH3)2 ),
4.03 (s, 1H,HC≡C), 3.76–3.59 (m, 4 H), 2.05 –1.92 (m, 4H), 1.71–1.57 (m, 16H), 1.44–1.30 (m, 2H), 1.24 –
13
1.14 (m, 4H), 0.93–0.88 (m, 6H). C NMR (126 MHz, CDCl3) δ 173.5, 171.2, 131.2, 124.7, 84.7, 77.3, 77.0,
+
76.8, 70.7, 61.1, 39.8, 37.2, 31.6, 29.2, 25.7, 19.5, 17.6. HRMS = (M+H)calc for C25 H39 N3 O2 = 413.30.
+
(M+H)found = 414.31.
Synthesis of 2,4,6-tris((4,6-bis (S)-citronellyloxy)ethynyl)-1,3,5-triazine (3) [25]
n-BuLi (0.45 mL, 1.20 mmol) was added dropwise to a stirred solution of compound 2 (0.49 g, 1.20 mmol) in dry
THF under argon atmosphere at –78 °C, and then anhydrous ZnCl2 (0.16 g, 1.20 mmol) in THF was dropwise
added into the solution and the mixture was stirred at –78 °C for about 1 h, and then warmed up to room
temperature. A solution of cyanuric chloride (0.05 g, 0.30 mmol), Pd(PPh3)4 (0.02 g, 5% eq ) in 10 mL of THF
was added dropwise and the mixture was stirred 65 °C for 24 h. The mixture was cooled to room temperature.
Water (10 mL) and chloroform (10 mL) were added. The aqueous layer was extracted with chloroform (2 ×10
mL). The combined organic layers were then washed with brine, dried over Na2 SO4 , filtered and evaporated
under reduced pressure. The residue was purified by column chromatography hexane/ethyl acetate (6:1) (0.04
1
g oily material, yield 34%). H NMR (500 MHz, CDCl3) δ 5.11–4.93 (m, 6H), 4.15–3.97 (m, 12H), 1.94–1.84
13
(m, 12H), 1.64–1.47 (m, 48H), 1.38–1.26 (m, 12H), 1.15–1.06 (m, 6H), 0.83 (dd, J = 7.8, 4.1 Hz, 18H).
C
NMR (126 MHz, CDCl3) δ 172.5, 172.1, 170.0, 130.1, 123.7, 108.9, 102.6, 76.4, 76.1, 75.9, 61.9, 35.9, 28.4,
+
+
24.6, 24.3, 19.9, 18.3, 16.5. HRMS Mcalc for C78 H114 N12 O6 = 1315.8 Mfound = 1315.9.
Synthesis of 4-(dodecyloxy) benzoic acid (4-DBA) (4) [23]
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, 2.8 eq) in ethanol (50 mL) was heated under reflux for 72 h. The resulting mixture was
hydrolysed with 10% aqueous potassium hydroxide (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 to give the pure product 4-dodecyloxybenzoic acid white solid (4), 4.55 g 91 %
1
yield. H NMR (500 MHz, CDCl3) δ 8.05 (d, J = 8.8 Hz 2 Ar-H), 6.93 (d, J = 6.8 Hz 2 Ar-H), 4.01 (t, J =
6.4 Hz, 2H OCH2), 1.96–1.68 (m, 2H), 1.52–1.27 (m, 18H), 0.89 (t, J = 7.0 Hz, 3H).
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AKKURT et al./Turk J Chem
Synthesis of organic salt (5)
4-Hydroxy benzoic acid (4-DBA, 4) mesogenic unit was added into 2,4,6-tris((4,6-bis(S)-citronellyloxy-1,3,5-
triazin-2-yl)ethynyl)-1,3,5-triazine (3) with one to one ratio. The resulting solution in dry THF was sonicated
for 10 min at room temperature until the solution has become transparent. Then, the solvent was removed in
1
vacuum. H NMR (500 MHz, CDCl3) δ 7.98 (d, J = 8.8 Hz, 2H), 6.86 (d, J = 8.9 Hz, 2H), 5.08–4.95
(m, 6H), 4.06–3.99 (m 12H), 3.95 (t, J = 6.6 Hz, 2H), 2.00–1.84 (m, 14H), 1.77–1.70 (m, 3H), 1.65–1.52 (m,
13
30H), 1.51–1.44 (m, 8H), 1.43–1.32 (m, 12H), 1.31–1.17 (m, 26H), 1.16– 1.06 (m, 6H), 0.91–0.78 (m, 21H).
C
NMR (126 MHz, CDCl3) δ 173.1, 171.2, 171.0, 167.0, 163.5,132.2, 131.3, 124.5, 121.3, 114.1, 109.8, 103.89,
68.1, 63.0, 36.9, 35.3, 34.2, 31.9, 30.3, 29.7, 29.6, 29.4, 29.1, 29.0, 25.9, 25.7, 25.3, 22.7, 21.0, 21.04, 19.3, 17.6,
14.1.
3. Result and discussion
The formation of the salt 5 between the macromolecule 3 and the mesogenic carboxyl group was mainly studied
by FTIR, H NMR, and C NMR. The comparison of FTIR spectra of macromolecule 3, free acid 4, and salt 5 are
provided in Figure 1. A sharp peak belonging to the asymmetric stretching of carbonyl peak of the carboxylic
acid appeared at 1680 cm−1 , which shifted in the salt form to 1650 cm−1 [26].
Figure 1. FTIR spectra of macromolecule 3, organic salt 5, and benzoic acid 4-DBA.
The NMR spectroscopic analysis indicated the interaction between macromolecule 3 and 4-DBA. Evi-
dences were found on the chemical shift of the aromatic hydrogens of 4-DBA shifted from 8.05 ppm and 6.95
ppm to 7.98 ppm and 6.86 ppm, indicating a change in the electron density after the formation of salt 5. Like-
wise, the signals of –OCH2 –protons of 4-DBA in the complex shifted to a higher ?eld at 3.95 ppm compared
to the signals of pure 4-DBA at 4.05 ppm (Figure 2). As expected, the multiplet signals in H NMR spectra at
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AKKURT et al./Turk J Chem
4.15–3.97 ppm of compound 3 showed no shifting after the complexation since their electronic environment was
not influenced.
Figure 2. 1H NMR spectra of macromolecule 3, organic salt 5, and benzoic acid 4.
13
Moreover, the C NMR spectra showed a shift of the peak of carbonyl carbon from 171.6 in compound
3 to 172.5 ppm in the salt 5, while the aromatic carbon bounded to the alkoxy group shifted slightly from 163.6
to 163.5 ppm, respectively (Figure 3).
Figure 3. A section of 13C NMR spectra of benzoic acid 4-DBA organic salt 5.
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AKKURT et al./Turk J Chem
The mesomorphic properties of the compound 4-DBA and organic salt 5 were investigated by using POM
and DSC. The phase transitions of the corresponding molecules are given in Table.
Table . Mesophases and phase transition temperatures as observed on heating (H→) and cooling (←C) and corre-
sponding transition enthalpies of the 4-DBA and organic salt 5. (The peak temperatures were expressed in degree Celsius
and the numbers in parentheses referred to the transition enthalpy (∆H) in kJ mol−1).
a
Comp.
4-DBA
OS(5)
T/°C [∆H kJ/mol]
b
H →: Cr 99.98 [39.01] SmC 132.43 [2.39] N 138.42 [2.05] Iso
c
H →: Cr 93.64 [98.52] SmC 115.45 Iso Cr1 65.82 [19.19] Cr2 81.22 [27.03] SmC 102.5 [5.1] Iso: ← C
aPerkin-Elmer DSC-6; enthalpy values in italics in brackets taken from the 2nd heating and cooling scans at a rate of 10 °C min−1
;
abbreviations: Cr = crystalline, SmC= tilted smectic phase, N= nematic phase; Iso = isotropic liquid phase.
b[14,15] Cr 95.1 SmC 128.9 N 137.2 Iso [16] Cr 92.4 SmC 131.5 N 142.0 Iso confirmed by POM.
c
As shown in Figure 4, compound 4-DBA with n-dodecyloxy alkyl chain showed enantiotropic liquid
crystalline properties that were in agreement with the behaviour observed for the analogous benzoic acids
carrying an alkoxy chain with different numbers of carbon atoms at the 4-position of the aromatic ring [21-
27,29]. The organic salt 5 exhibited a phase transition sequence of Cr1 -Cr2 –SmC-Iso which was in agreement
with 3 endotherms in DSC cooling curves (Figure 4 centre). Upon heating from the crystal, the SmC texture
was observed at 93–115 °C by POM (Figure 5). The texture 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 Smectic C
phase [30-33].
Figure 4. DSC thermograms of heating (left), cooling (centre) organic salt, and 4-DBA (right) on 2nd heating and
cooling (10 °C min-1).
Figure 5a shows Schlieren texture of SmC mesophase. The inset in clearly shows Schlieren regions of the
texture.
The creation of SmC mesophase is explained by the ionic interaction between the triarmed π-conjugated
system and the mesogenic carboxyl group. In this noncovalent intermolecular interaction, the star-shaped
compound 3 acts as proton acceptor and the benzoic acid with n-dodecyloxy group acts as the proton donor.
The ionic interactions between macromolecule 3 and 4-DBA have striking influence on the mesogenic properties
such as melting and clearing temperatures as well as prompting SmC phase at lower temperatures.
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Figure 5. Optical textures of organic salt 5 as observed between crossed polarizers in an ordinary glass plate; (a) texture
of SmC phase at 94 °C heating and (b) 89 °C cooling (c) texture of crystal phase at 77 °C, d) texture of crystal phase
at 74 °C (magnification ×100).
4. Conclusion
A new triazine-based macromolecular π-conjugated system carrying chiral citronellyloxy chains positioned at
the peripheries was synthesized via Sonogashira and Negishi cross coupling reactions. Triazines at the periphery
bearing citronellyloxy groups were connected to the central triazine unit by 3 acetylenic bridges. Its organic
salt was also prepared, which was resulted from hydrogen bonding interaction between macromolecule 3 and
1
13
4-DBA mixed with 1:1 ratio in THF. The resulting organic salt was characterized by H NMR, C NMR, and
FTIR. The organic salt exhibited enantiotopic SmC phase texture at lower temperatures.
The presence of chiral citronellyloxy groups on the periphery lowered the mesophase transitions from
SmC 132.43 °C for linear alkyl chain substituted analogue structure [13] to SmC 115.45 °C in the organic salt
5. Low temperature mesophase transitions are desirable for low temperature applications of liquid crystals.
Acknowledgements
Authors thank to Professor Belkis Bilgin Eran for valuable discussions on POM studies. This work was supported
by TÜBİTAK with the project no 114Z722. We also thank Yıldız Technical University with the project no 2015-
01-01-DOP04.
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Synthesis and liquid crystalline properties of new triazine-based ꢀ-conjugated
macromolecules with chiral side groups
Nihat AKKURT,1,2 Mohammed Hadi Ali AL-JUMAILI,1 Hale OCAK,1 Fatih ÇAKAR,1
Lokman TORUN1,3
Supporting Information
Figure 6. 13C NMR result of compound 1.
1

Figure 7. 1H NMR result of compound 2.
2

Figure 8. 13C NMR result of compound 2.
3

Figure 9. Q-TOF result of compound 2.
4

Figure 10. 1H NMR result of compound 3.
5

Figure 11. 13C NMR result of compound 3.
6

Figure 12. Q-TOF result of compound 3.
7

Figure 13. 1H NMR result of compound 4.
8

4-DBA-OL
90
80
70
60
50
40
30
20
10
0
4000
3000
2000
1000
Wavenumbers (cm-1)
Figure 14. FTIR result of 4-DBA.
9

Figure 15. 1H NMR result of organic salt (5).
10

Figure 16. 13C NMR result of organic salt (5).
11