Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine alkanoate esters

Article abstract of DOI:10.1080/15421406.2018.1499700

A novel series of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine alkanoate esters were synthesized and their mesogenic properties were studied using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The impact of changing the tail-core linkage from alkyl or alkoxy to ester is profound. Compared to the alkyl or alkoxy linkages, the ester linkage reduced mesogenic properties. Short-tailed compounds are non mesogenic (4a-4e), while long-tailed compounds (4f-4r) exhibit nematic phases. Unlike the alkyl or alkoxy tail series, none of the 18 presented esters in this series exhibits a smectic phase.

Full text of DOI:10.1080/15421406.2018.1499700

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-
tetrazine alkanoate esters
Farid Fouad, Faycal Khabouchi, Alek Nielsen & Robert Twieg
To cite this article: Farid Fouad, Faycal Khabouchi, Alek Nielsen & Robert Twieg (2018)
Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine alkanoate esters, Molecular Crystals and
Liquid Crystals, 665:1, 82-90, DOI: 10.1080/15421406.2018.1499700
To link to this article: https://doi.org/10.1080/15421406.2018.1499700
Published online: 13 Feb 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 665, NO. 1, 82–90
https://doi.org/10.1080/15421406.2018.1499700
Mesogenic 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine
alkanoate esters
Farid Fouada, Faycal Khabouchib, Alek Nielsenb, and Robert Twiegb
aDepartment of Chemistry & Biochemistry, Kent State University, East Liverpool Campus, East Liverpool,
b
Ohio, USA; Department of Chemistry & Biochemistry, Kent State University, Kent, Ohio, USA
KEYWORDS
ABSTRACT
Heterocyles; liquid crystals;
nematic; tetrazines
A novel series of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine alkanoate
esters were synthesized and their mesogenic properties were studied
using differential scanning calorimetry (DSC) and polarizing optical
microscopy (POM). The impact of changing the tail-core linkage from
alkyl or alkoxy to ester is profound. Compared to the alkyl or alkoxy
linkages, the ester linkage reduced mesogenic properties. Short-
tailed compounds are non mesogenic (4a-4e), while long-tailed com-
pounds (4f-4r) exhibit nematic phases. Unlike the alkyl or alkoxy tail
series, none of the 18 presented esters in this series exhibits a smec-
tic phase.
GRAPHICAL ABSTARCT
1. Introduction
Application of Liquid Crystals (LCs) in various display technologies has been widely
pursued. Some new technologies, such as flexible, high resolution, ultra-high definition
(UHD), three-dimensional (3D) liquid crystal displays (LCDs), and fast response touch
panels are now emerging [1–2]. LC technology is now not limited to the fabrication of
CONTACT Farid Fouad
ffouad@kent.edu
Department of Chemistry & Biochemistry, Kent State University, East
Liverpool Campus, East Liverpool, OH 43920, USA.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2018 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
83
flat displays. Various biomedical applications, such as biosensors have also been devel-
oped [3–5]. Because of the current and potential LCs applications, demand for more
LCs with diverse physical properties increasingly rises. The introduction of a polar het-
erocyclic ring may influence the birefringence and dielectric anisotropy and improve
mesogenic properties motivating organic chemists to synthesize different heterocyclic
LCs [6–7]. For example pyridine [8], pyrimidine [9], pyrazine [10], tetrazine [11], 1,3,4-
oxadiazole [12–13], 1,2,4-oxadiazole [14], thiophene, 1,3-thiazole, and 1,3,4-thiadiazole
[15–17] heterocycles have been explored. Many 1,2,4,5-tetrazines are reported as
nematic and/or smectic liquid crystalline materials depending on the structure of the
calamitic core and the number, length, and linkage of the attached tails (Fig. 1). For
example, phenyltetrazines (two aromatic-rings core, Fig. 1a) are mesogenic and have
been successfully used in field-induced color switching displays and guest host systems
[18–19]. Unlike p-terphenyls, 3,6-diaryltetrazines can be coplanar with more extended
conjugation over the three aromatic rings bringing special electrophysical and optical
properties. Thus, a series of 3,6-bis(alkoxyphenyl)tetrazines (Fig. 1b) showed Nematic
and/or Smectic C phases [20]. The bis-alkyl analogues (Fig. 1c) were also synthesized
and were Nematic and/or Smectic C liquid crystals [21]. We had previously reported a
series of symmetrical 3,6-diaryl-1,2,4,5-tetrazines with four alkoxy tails (Fig. 1d) that
showed mesogenic properties only for long tails (R ꢀ C9) [11]. These four-tailed liquid
crystalline tetrazines possessed lower smectic-C phase transition temperatures and melt-
ing points, and better solubility than typical two tail materials.
Aside from their mesogenic properties, the use of tetrazines in bioimaging is a rapidly
growing field. For example, tetrazine-fluorophore conjugates can undergo inverse-elec-
tron-demand [4 þ 2] cycloaddition with trans-cyclooctene modified protein, DNA or
RNA accompanied by a substantial increase in the fluorescence intensity of the pyrida-
zine product [22–26].
2. Experimental
2.1. Materials
Chemicals were procured from suppliers including Sigma Aldrich, Acros Organics, and
Frontier Scientific. Silica gel (Silicycle SiliaFlash F60, 230-400 mesh) was used for
Figure 1. Some examples of reported liquid crystalline tetrazines: a) nematic LC for display devices
based on the guest-host effect [18]; b) S-dialkoxyphenyltetrazines, N and/or SmC [19]; c) S-dinonylphe-
nyltetrazines, N and SmC [20]; and d) S-diphenyltetrazines with four alkoxy tails, SmC [11].

84
F. FOUAD ET AL.
column chromatography, and silica gel plates (Scientific Adsorbents Inc.) were used for
thin layer chromatography (TLC).
2.2. Characterization and measurements
NMR experiments were performed using a Bruker Advance 400 nuclear magnetic reson-
ance spectrometer, using various solvents as noted individually. Differential scanning
calorimetry was performed using a TA Instruments DSC 2920 Modulated DSC.
Polarized light microscopy was used to identify phase transitions, using a Nikon Eclipse
E600 POL polarizing optical microscope, a Mettler Toledo FP82HT hot stage, and a
Mettler Toledo FP90 Central Processor temperature controller.
2.3. Synthesis
2.3.1. Preparation of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine, 3 [27-28]
The 4-hydroxybenzonitrile (1, 6.0 g, 50.37 mmol) was dissolved in hydrazine hydrate
(25 mL), and the mixture was heated at 90ꢁC for 16 hr. The yellow precipitate was
filtered and washed with water (300 mL) and ethanol (20 mL). The crude dihydrotetra-
zine intermediate (2) was obtained as a yellow powder (3.94 g, 58.4%) and was used in
the next oxidation step without further purification.
An aqueous solution of NaNO2 (25 mL, 10%) was added dropwise to a solution of 2
(3.8 g, 14.18 mmol) in glacial acetic acid (40 mL). After stirring for 3 hours at room tem-
perature, the red precipitate was filtered, washed with water (200 mL) and ethanol
(20 mL), and recrystallized from 1-propanol to afford 3 as dark red crystals (2.96 g,
1
79%). H NMR (DMSO-d6): d ¼ 10.35 (br. s, 2H), 8.36 (d, 4H), 7.02 (d, 4H) ppm.
2.3.2. Synthesis of target products 4a-4e via esterification of 3 using acid
anhydrides and pyridine
To a solution of 3 (1.0 eq, 0.5 g, 1.88 mmol) in DMF (5 mL) was added dry pyridine
(6.6 eq, 1 mL, 0.981 g, 12.40 mmol), and a solution of the respective acid anhydride
(4 eq, 7.52 mmol) in dry DMF (2 mL). The mixture was heated to 90ꢁC in an oil bath
under nitrogen atmosphere for two hours. After cooling the purple precipitate was
filtered off, washed with DMF (10 mL) and toluene (2 mL) and recrystallized
from toluene.
2.3.3. Synthesis of target products 4f-4i via esterification of 3 using carboxylic acids
and DCC
A solution of 3 (1.0 eq, 0.5 g, 1.88 mmol) and the respective carboxylic acid (2.6 eq,
4.89 mmol) in dry THF (10 mL) was treated with DMAP (0.15 eq, 34 mg, 0.28 mmol,)
and DCC (2.6 eq, 1.0 g, 4.89 mmol) and heated at reflux overnight. The mixture was
cooled, the solvent was removed under reduced pressure and the solid mixture was dis-
solved in DCM and cooled to 0ꢁC for 2 hours. Dicyclohexylurea was removed by

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
85
filtration and DCM was evaporated under vacuum to yield a light purple solid which
was then purified by column chromatography. (SGC, 1:1 DCM/EA).
2.3.4. Synthesis of target products 4j-4r via esterification of 3 using carboxylic acids
and oxalyl chloride
A solution of oxalyl chloride in dry DCM (12 eq, 11.3 mL, 2M, 22.6 mmol) was added
dropwise to a solution of the respective carboxylic acid (4 eq, 7.52 mmol) in dry DCM
(20 mL) at 0ꢁC (ice bath) under nitrogen. The resulting mixture was stirred and allowed
to reach room temperature over two hours. DCM was removed under vacuum and the
acid chloride was dissolved in dry DMF (2mL) and cannulated into a solution of 3 (1.0
eq, 0.5 g, 1.88 mmol) and dry pyridine (6.6 eq, 1 mL, 0.981 g, 12.40 mmol) in DMF
(5 mL). The mixture was heated to 90 ꢁC in an oil bath under a nitrogen atmosphere
for two hours. After cooling the purple precipitate was filtered, washed with DMF
(10 mL) and toluene (2 mL) and recrystallized from toluene.
2.3.5. Spectral data of compounds 4a-4r
1
Representative compound 4j is fully characterized by H NMR,13C NMR and HRMS.
Compound 4a [29]:
Yield 72%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.74 (d, 4H,
J ¼ 8.8 Hz); 7.44 (d, 4H, J ¼ 8.8 Hz); 2.42 (s, 6H).
Compound 4b:
Yield 87.2%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (dd, 4H,
J ¼ 5.2, 1.6 Hz); 7.43 (dd, 4H, J ¼ 5.2, 1.6 Hz); 2.68 (q, 4H, J ¼ 7.6 Hz); 1.34 (t,
6 H, J ¼ 7.6 Hz).
Compound 4c [30]:
Yield 54.5%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (d, 4H,
J ¼ 8.8); 7.44 (d, 4H, J ¼ 8.8 Hz); 2.67 (t, 4H, J ¼ 7.2); 1.87(m, 4H); 1.14 (t, 6H,
J ¼7.6 Hz).
Compound 4d:
Yield 53.6%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.72 (d, 4H,
J ¼ 8.8); 7.41(d, 4H, J ¼ 8.8); 2.68 (t, 4H, J ¼ 7.2); 1.83 (m, 4H); 1.54 (m, 4H); 1.06 (t,
6H, J ¼ 7.2).
Compound 4e:
1
Yield 89%; H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (d, 4H, J ¼ 8.8);
7.42(d, 4H, J ¼ 8.8); 2.67 (t, 4H, J ¼ 7.2); 1.83 (m, 4H); 1.46-1.45(m, 8H); 1.01
(t, 6H, J ¼ 7.2).
Compound 4f:
Yield 86%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.71 (d, 4H,
J ¼ 9.2 Hz), 7.37 (d, 4H, J ¼ 9.2 Hz), 2.65 (t, 4H, J ¼ 7.2 Hz), 1.82 (m, 4H), 1.51-1.36 (m,
12H), 0.95 (t, m, 6H, J ¼ 7.1 Hz); 13C NMR: 171.9, 163.3, 154.5, 129.4, 129.2, 122.7,
34.5, 31.5, 28.8. 24.3, 22.1, 14.0.
Compound 4g:
Yield 87%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.72 (d, 4H,
J ¼ 9.6 Hz), 7.38 (d, 4H, J ¼ 9.2 Hz), 2.64 (t, 4H, J ¼ 7.2 Hz), 1.82 (m, 4H), 1.51-1.24 (m,

86
F. FOUAD ET AL.
16H), 0.96 (t, 6H, J ¼ 7.2 Hz); 13C NMR: 171.8, 163.3, 154.5, 129.4, 129.1, 122.6, 34.4,
31.6, 29.1, 28.9, 24.8, 22.6, 14.1.
Compound 4h:
Yield 80%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.72 (d, 4H,
J ¼ 8.8 Hz), 7.38 (d, 4H, J ¼ 9.6 Hz), 2.64 (t, 4H, J ¼ 6.8 Hz), 1.83-1.78 (m, 4H), 1.52-1.26
(m, 20H), 0.96 (t, 6H, J ¼ 7.3 Hz).
Compound 4i:
1
Yield 84%; H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.72 (d, 4H, J¼ 8Hz), 7.37
(d, 4H, J ¼ 8 Hz), 2.65(t, 4H, J ¼ 8 Hz), 1.85-1.78 (m, 4H), 1.52-1.23 (m, 24H), 0.96 (t, 6H, J ¼ 7.1 Hz).
Compound 4j:
Yield 66%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (d, 4H,
J ¼ 7.0 Hz), 7.42 (d, 4H, J ¼ 7.0 Hz), 2.69 (t, 4H, J ¼ 7.6 Hz), 1.82-1.72 (m, 8H), 1.47-1.45
(m, 24H), 0.96 (t, 6H, J ¼ 6.4); 13C NMR: 172.2, 163.3, 154.5, 129.4, 129.3, 122.9, 34.5,
32.0, 29.7,29.6, 29.4,29.3, 29.2, 25.0, 22.9, 14.4. HRMS: calculated for C36H51N4O4,
603.3910 [M þ H]þ, found 603.3905.
Compound 4k:
Yield 66%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (d, 4H,
J ¼ 7.1 Hz), 7.42 (d, 4H, J ¼ 7.1 Hz), 2.67 (t, 4H, J ¼ 7.2 Hz), 1.83-1.82 (m, 4H), 1.47-1.45
(m, 32H), 0.94 (t, 6H, J ¼ 6.8 Hz); 13C NMR: 172.2, 163.3, 154.5, 129.5, 129.2, 122.9,
34.5, 32.1, 29.8, 29.7, 29.6 (2C), 29.5, 29.4, 24.9, 22.8, 14.4.
Compound 4l:
1
Yield 69%; H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (dd, 4H, J ¼ 6.8,
2 Hz), 7.42 (dd, 4H, J ¼ 6.8, 2 Hz), 2.68 (t, 4H, J ¼ 7.2), 1.86-1.84(m, 4H), 1.48-1.46 (m,
36H), 0.95 (t, 6H, J ¼ 6.8).
Compound 4m:
Yield 72%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.73 (d, 4H,
J ¼ 8.8 Hz), 7.43 (d, 4H, J ¼ 8.8 Hz), 2.70 (t, 4H, J ¼ 7.6 Hz), 1.85-1.84 (m, 4H), 1.48-1.46
(m, 40H), 0.95 (t, 6H, J ¼ 6.8 Hz); 13C NMR: 172.2, 163.3, 154.5, 129.5, 129.4, 122.9,
34.5, 32.1, 29.9, 29.8 (2C), 29.7, 29.6, 29.5, 29.4, 29.2, 25.0, 22.9, 14.4.
Compound 4n:
Yield 82%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.74 (d, 4H,
J ¼ 6.4 Hz), 7.43 (d, 4H, J ¼ 6.4 Hz), 2.67 (t, 4H, J ¼ 6.1 Hz), 1.83-1.82 (m, 4H), 1.48-1.32
(m, 48H), 0.93 (t, 6H, J ¼ 6.6 Hz).
Compound 4o:
Yield 80%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.74 (d, 4H,
J ¼ 8.8 Hz), 7.51(d, 4H, J ¼ 8.4 Hz), 2.72 (t, 4H, J ¼ 7.2 Hz), 1.91-1.80 (m, 4H), 1.54-
1.51(m, 56H), 0.99 (t, 6H, J ¼ 5.6 Hz).
Compound 4p:
Yield 77%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.75 (d, 4H,
J ¼ 8.4 Hz), 7.46 (d, 4H, J ¼ 8.4 Hz), 2.70 (t, 4H, J ¼ 7.2 Hz), 1.92-1.91 (m, 4H), 1.55-1.47
(m, 64H), 1.00 (t, 6H, J ¼ 5.6 Hz).
Compound 4q:
Yield 72%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.75 (d, 4H,
J ¼ 8.4 Hz), 7.47 (d, 4H, J ¼ 8.4 Hz), 2.70 (t, 4H, J ¼ 7.2 Hz), 1.93-1.91 (m, 4H), 1.54-1.38
(m, 68H), 0.99 (t, 6H, J ¼ 6.8 Hz).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
87
Compound 4r:
Yield 46%; 1H NMR (1,1,2,2-Tetrachloroethane-d2, 400MHz) d: 8.74 (d, 4H,
J ¼ 8.8 Hz), 7.45 (d, 4H, J ¼ 6.8 Hz), 2.69 (t, 4H, J ¼ 6.4 Hz), 1.93-1.87 (m, 4H), 1.49-1.38
(m, 76H), 0.99 (t, 6H, J ¼ 6.8 Hz).
Table 1. Phase transition temperatures of compounds 4a-4r determined by DSC.
Compound
R
Phase Transition temperatures (ꢁC)
4a
4b
4c
4d
4e
4f
4g
4h
4i
CH3
Cr 261.6 Iso
Cr 260.2 Iso
Cr 252.1 Iso
Cr 238.8 Iso
Cr1 51.5 Cr2 228.4 Iso
Cr 219.1 N 220.2 Iso
Cr 210.2 N 213.3 Iso
Cr 200.4 N 207.5 Iso
C2H5
C3H7
C4H9
C5H11
C6H13
C7H15
C8H17
C9H19
Cr 196.2 N 207.4 Iso
4j
4k
4l
4m
4n
4o
4p
4q
4r
C10H21
C11H23
C12H25
C13H27
C15H31
C17H35
C19H39
C20H41
C22H45
Cr1 41.7 Cr2 141.4 Cr3 192.1 N 205.5 Iso
Cr1 36.6 Cr2 142.4 Cr3 187.5 N 202.4 Iso
Cr1 60.5 Cr2 143.1 Cr3 183.5 N 199.0 Iso
Cr1 54.8 Cr2 143.1 Cr3 180.1 N 196.0 Iso
Cr1 68.5 Cr2 142.9 Cr3 173.2 N 188.2 Iso
Cr1 77.7 Cr2 142.7 Cr3 167.9 N 181.4 Iso
Cr1 85.9 Cr2 142.1 Cr3 163.6 N 174.4 Iso
Cr1 96.9 Cr2 142.8 Cr3 161.1 N 171.6 Iso
Cr1 99.9 Cr2 141.3 Cr3 146.3 N 167.9 Iso
Scheme 1. Synthesis of 3,6-bis(4-hdroxyphenyl)-1,2,4,5-tetrazine alkanoate esters, compounds 4a-r.

88
F. FOUAD ET AL.
Figure 2. Representative DSC heating and cooling traces of compounds 4k (a), 4m (b), and 4o (c) at
a rate of 10 ꢁC/mi.
2.4. Phase characterization
The transition temperatures of compounds 4a-4r (18 compounds) were measured by
differential scanning calorimetry, DSC (Tables 1). The mesomorphism exhibited by
compounds 4a-4r was characterized by a polarizing optical microscope (POM) equipped
with a hot stage.
3. Results and Discussion
3.1. Chemistry
The goal of this work is to investigate the mesogenic properties of novel two-tailed s-
diphenyltetrazines with ester linkages. Following the literature procedure, we were able
to prepare multigram quantities of the key intermediate 3,6-bis(4-hydroxyphenyl)-
1,2,4,5-tetrazine (compound 3, Scheme 1) [27–29]. Standard esterification protocols
were utilized to make target esters (compounds 4a-4r). For short tailed esters (4a-4e,
Scheme 1), esterification using alkanoic acid anhydrides and pyridine was used. For lon-
ger tails (4f-4r, Scheme 1), where the acid anhydride is either expensive or not commer-
cially available, carboxylic acids and DCC or oxalyl chloride were successfully employed.
3.2. Mesomorphic properties
DSC was used to determine the phase transition temperatures at a scanning rate of
10ꢁC/min (Fig. 2, examples of obtained DSC plots). The phase transition temperatures
of compounds 4a-4r are listed in Table 1. The liquid crystalline properties were investi-
gated by POM. Unlike the reported dialkoxy analogues [18], which exhibit nematic
phases for short tails (Fig. 1b, R ¼ C1-C5), short tailed esters (4a-4e, R ¼ C1-C5) are
nonmesogenic, indicating that the ester linkage (compared to the alkoxy, or alkyl)
decreases mesophase stability. In our previous work, the same core structure with four
alkoxy tails (Fig. 1d, R ¼ C5-C8) were also nonmesogenic [11]. However, these com-
pounds do not exactly follow the traditional model of calamitic liquid crystals flexible
tail ꢂ rigid core ꢂ flexible tail as they have four tails.
Longer tailed esters (4f-4r) are all mesogenic exhibiting nematic phases (Fig. 3, exam-
ples of the observed nematic textures). None of the compounds studied here posesses a

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
89
Figure 3. Microphotographs (crossed polarizers) of representative textures: a) Nematic phase of 4m;
b) Nematic Phase of 4p; and c) Nematic phase of 4q.
smectic phase, regardless of the length of the ester tails. This behavior deviates from the
normal pattern of many calamitic liquid crystals which form nematic phases for struc-
tures with short tails, followed by both nematic and smectic for average length tails, and
only smectic for still longer tails (for example the series of compounds shown in Fig. 1b
[19]). Long tailed structures, with C10-C22 tails, have similar DSC pattern, showing two
crystal-crystal transitions before the crystal-nematic transition (Table 1 & Fig. 2). As the
length of the tail increases, the clearing points decreases and the nematic range increases
(4l has the widest nematic range, 16.5 ꢁC).
While lower temperature nematic liquid crystals are desired for the display technol-
ogy, the development of high-temperature liquid crystals for use as stationary phases in
gas-liquid chromatography (GLC) is known [31–33]. Also, high-temperature liquid crys-
tals can be used in liquid crystalline binary mixtures at desirable temperature
ranges [34].
4. Conclusions
A new series of calamitic liquid crystalline s-diphenyl tetrazines with alkylcarboxy
tails is synthesized. The short tailed structures (4a-4e) are nonmesogenic. Compounds
with longer tails (4f-4r) exhibit nematic phases. The study highlights the fact that in
designing calamitic LCs, the core structure, number of branches, length of branches
and type of core-branch linkage are all factors that contribute to the mesogenic stability,
type of LC phase developed, transition temperatures, clearing points, and LC
phase width.
Funding
This work was supported by the National Science Foundation, Research Experiences for
Undergraduates, NSF–REU grant (REU Site at Kent State University: Liquid Crystals
and Advanced Materials) [NSF–1263087].

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F. FOUAD ET AL.
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