Liquid crystalline symmetrical 3,6-diaryl-1,2,4,5-tetrazines

Article abstract of DOI:10.1080/15421406.2013.791585

A new series of symmetrical 3,6-diphenyl-1,2,4,5-tetrazines with four alkoxy tails were synthesized along with new examples with two alkoxy tails. Mesogenic properties were studied by differential scanning calorimetry and polarizing optical microscopy. Th

Full text of DOI:10.1080/15421406.2013.791585

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Liquid Crystalline Symmetrical 3,6-
Diaryl-1,2,4,5-Tetrazines
Farid Fouad a , Brett Ellman b , Scott Bunge c , Patrik Miller c &
Robert Twieg c
a Department of Chemistry and Biochemistry , Kent State University
at East Liverpool , East Liverpool , Ohio , USA
b Department of Physics , Kent State University , Kent , Ohio , USA
c Department of Chemistry and Biochemistry , Kent State
University , Kent , Ohio , USA
Published online: 03 Oct 2013.
To cite this article: Farid Fouad , Brett Ellman , Scott Bunge , Patrik Miller & Robert Twieg (2013)
Liquid Crystalline Symmetrical 3,6-Diaryl-1,2,4,5-Tetrazines, Molecular Crystals and Liquid Crystals,
582:1, 34-42, DOI: 10.1080/15421406.2013.791585
To link to this article: http://dx.doi.org/10.1080/15421406.2013.791585
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Mol. Cryst. Liq. Cryst., Vol. 582: pp. 34–42, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.791585
Liquid Crystalline Symmetrical
3,6-Diaryl-1,2,4,5-Tetrazines
FARID FOUAD,1,∗ BRETT ELLMAN,2 SCOTT BUNGE,3
PATRIK MILLER,3 AND ROBERT TWIEG3
1Department of Chemistry and Biochemistry, Kent State University at East
Liverpool, East Liverpool, Ohio, USA
2Department of Physics, Kent State University, Kent, Ohio, USA
3Department of Chemistry and Biochemistry, Kent State University, Kent,
Ohio, USA
A new series of symmetrical 3,6-diphenyl-1,2,4,5-tetrazines with four alkoxy tails were
synthesized along with new examples with two alkoxy tails. Mesogenic properties were
studied by differential scanning calorimetry and polarizing optical microscopy. The
influence of the number and length of alkoxy chains attached to the 3,6-diphenyl-
1,2,4,5-tetrazine core is discussed. Compared with the two-tailed tetrazines, 6 the four-
tailed liquid crystalline tetrazines, 5e–g possess lower smectic-C phase transitions, and
clearing point temperatures.
Keywords: Liquid crystals; smectic phase; Tetrazines; π-stacking
1. Introduction
Heterocyclic liquid crystals possess characteristic structural and electronic properties that
influence the type of mesophase and the phase transition temperatures [1, 2]. For example,
many known heterocyclic liquid crystals are based on 1,4-terphenyl structures in which one
or more benzene rings are replaced by heterocycles [1, 3]. Our X-ray crystallographic study
of 1,4-bis-(4-dodecyloxyphenyl)-1,2,4,5-tetrazine, 6k, revealed a high degree of copla-
narity of the central three-ring system. Moreover, the reported computational simulation
study of 3,6-bis-(4-butylphenyl)-1,2,4,5-tetrazine showed that the molecule is character-
ized by adequate conformational rigidity of the central three-ring aromatic fragment [1].
The calculations revealed that the core diphenyltetrazine fragment in the temperature range
0–500 K remains almost unchanged. A 3,6-diphenyltetrazine-ring system can be flat be-
cause steric interactions among the rings are relieved (since there are no substituent atoms
on the tetrazine ring, unlike the situation in p-terphenyl) and conjugation of the aromatic
π-systems favors a coplanar arrangement of the aromatic rings.
Tetrazines have been reported as nematic LC components in guest-host-type opto-
electronic displays [4, 5], components in field-induced color switching displays [6], and
smectic-C LC in electrooptical display devices [7]. A literature survey of tetrazines revealed
∗Address correspondence to Farid Fouad, Assistant Professor of Chemistry, Kent State University
at East Liverpool, 400 East Fourth Street, East Liverpool, OH, USA. E-mail: ffouad@kent.edu
34

3,6-Diaryl-1,2,4,5-Tetrazines
35
that they possess nematic and/or smectic liquid crystalline phases, a high inherent color
(red or violet), and a marked dichroism in the visible area [4–7]. Studies of a number of
3,6-bis(alkoxyphenyl)tetrazines (6, Rꢀ = C-1 to C-10) showed that the type of mesogenic
phases depended on the alkoxy chain length. Molecules with C-1 through C-5 alkoxy
chains possess nematic phases while molecules with C-6 through C-9 chains, possess both
smectic-C and nematic phases. The C-10 derivative has only a smectic-C mesogenic phase
[8]. However, their relatively high phase transition temperatures (smectic-C phase transi-
tion temperatures range from 163◦C to 111◦C, Table 3) and poor solubility are undesirable
for practical uses. Thus, this work aims to improve the physical and mesogenic properties
of 3,6-diaryltetrazines by installing four alkoxy groups at positions 3 and 4 in the terminal
aryl groups with variable alkoxy chain lengths.
2. Experimental and Methods
1
The H and 13C NMR spectra were recorded using a Bruker Avance 400 MHz spectro-
meter with tetramethylsilane as the internal standard. Mass spectra was performed at The
Ohio State University, Mass Spectrometry & Proteomics Facility. Differential scanning
calorimetry (TA Instruments DSC, scanning rate of 5◦C/min) and polarizing microscopy
(POM, Mettler FP90 and FP28HP hot stage) were used to study the phase transitions.
The texture of mesogenic phases was determined by POM and compared with 3,6-bis(4-
dihexyloxyphenyl)-1,2,4,5-tetrazine (6, Rꢀ = C-6) for which phase assignments exist in the
literature [8]. Reagents and solvents were commercially available and used without further
purification.
2.1. General Procedure for the Synthesis of 3,4-Dialkoxybenzonitriles 2a–g
In a 50 mL round-bottom flask with stirbar was charged with 3,4-dihydroxybenzonitrile
(2.70 g, 20.0 mmol, 1.0 eq), alkyl bromide (52.0 mmol, 2.6 eq), potassium carbonate
(11.06 g, 80.0 mmol, 4.0 eq), potassium iodide (5% of alkyl bromide by weight), and
DMF (20 mL). This mixture was stirred at 70◦C for 15 hr, cooled to room temperature and
transferred to a separatory funnel with 200 mL ethyl acetate and 100 mL water. The organic
phase was separated, washed with water (5 × 100 mL), and dried over magnesium sulfate.
The solvents were removed under reduced pressure and the crude product was purified
by recrystallization from hexanes. The 1H NMR data perfectly fit the proposed structures
[9–11].
2.2. General Procedure for the Synthesis of Tetrazoles, 3a–g and 8k–l
An oven dried 50 mL two-neck flask with stirbar was charged with the benzonitrile, 2a–g
or 7k–l (10.0 mmol, 1.0 eq), sodium azide (1.30 g, 20.0 mmol, 2.0 eq), lithium chloride
(1.27 g, 30.0 mmol, 3.0 eq), NMP (11 mL), and then fitted with a reflux condenser. The
mixture was heated at 130◦C for 3 days. The mixture was then cooled to room temperature
and poured into 50 mL water and acidified to pH 2. The off white precipitate formed was
1
filtered off, washed with water (3 × 50 mL) and recrystallized from 1-propanol. The H
NMR data perfectly fit the proposed structures [12, 13].

36
F. Fouad et al.
Table 1. Percent yields, melting points, MS, and HRMS data for tetrazines 5a–g and 6k–l
MS (ESI, m/z)
HRMS calcd.
Compound
% Yield
m.p.
[M + Na]+
[M + Na]+/found
5a
5b
5c
5d
5e
5f
5g
6k
6l
27
26
22
17
26
17
12
18
18
162◦C
143◦C
144◦C
135◦C
134◦C
131◦C
129◦C
180◦C
116◦C
601.4
657.5
713.5
769.6
825.6
881.7
993.8
625.5
793.6
601.3730/601.3718
657.4356/657.4359
713.4982/713.4965
769.5608/769.5596
825.6234/825.6226
881.6860/881.6876
993.8112/993.8109
625.4457/625.4468
793.6335/793.6332
2.3. General Procedure for the Synthesis of Trityltetrazoles, 4a–g and 9k–l
The respective tetrazole, 3a–g or 8k–l (4.0 mmol, 1.0 eq), was dissolved in anhydrous
DMF (15 mL) and then trityl chloride (1.35 g, 4.83 mmol, 1.2 eq) was added followed
by potassium tert-butoxide (0.54 g, 4.83 mmol, 1.2 eq). The resulting mixture was stirred
under nitrogen at room temperature for 15 hr. The reaction mixture was poured into
100 mL ice-water with stirring, whereupon a white precipitate formed. The precipitate was
1
vacuum filtered, washed with water, and then recrystallized from ethanol. The H NMR
data perfectly fit the proposed structures.
2.4. General Procedure for the Synthesis of Tetrazines, 5a–g and 6k–l
A solution of respective trityltetrazole (2.0 mmol) in benzonitrile (8 mL) was stirred at
150◦C under nitrogen for 7 hr. The red precipitate formed upon cooling was isolated by
vacuum filtration and recrystallized from hot toluene to yield the final tetrazine product.
1
The H and 13C NMR data perfectly fit the proposed structures. Percent yields, melting
point (m.p.), MS, and HRMS data are summarized in Table 1.
3. Result and Discussion
3.1. Syntheses
The synthesis of 3,6-bis(3,4-dialkoxyphenyl)-1,2,4,5-tetrazines 5a–g and 3,6-bis(4-
alkoxyphenyl)-1,2,4,5-tetrazines 6k–l was carried out as outlined in Scheme 1. Among
the available synthetic routes to make s-tetrazines [14], the method involving thermolysis
of 5-aryl-2-trityl tetrazoles was selected for convenience [15]. Thus, commercially available
3,4-dihydroxybenzonitrile and 4-hydroxybenzonitrile were alkylated and then converted to
their corresponding tetrazoles by reaction with lithium chloride and sodium azide in NMP.
Tritylation of the tetrazoles with triphenylmethyl chloride and potassium t-butoxide in
DMF at room temperature was found to be more efficient than the reported method [1]
using triphenylmethyl chloride and sodium hydroxide under conditions of phase transfer
catalysis. The 2-trityltetrazole products obtained under phase transfer catalysis were always
contaminated with unreacted starting materials. Heating the N-trityl tetrazoles in benzoni-
trile at 150◦C leads to a formation of the target tetrazines 5a–g and 6k–l. While yields

3,6-Diaryl-1,2,4,5-Tetrazines
37
R
R
O
O
R
OH
O
O
R
O
R
OH
O
R
a
c
b
N
N
N
NH
CN
CN
N
N
N
N
Tr
2a-g
1
3a-g
4a-g
d
a, R = -C5H11
b, R = -C6H13
c, R = -C7H15
d, R = -C8H17
e, R = -C9H19
f, R = -C10H21
g, R = -C12H25
RO
N
N
N
N
RO
OR
OR
5a-g
OR'
OR'
OR'
N
N
N
c
d
b
R'O
OR'
N
CN
6k-l
N
N
N
NH
k, R' = -C12 H25
l, R' = -C18 H37
7k-l
N
N
N
N
Tr
9k-l
8k-l
Scheme 1. Synthesis of target tetrazines 5a–g and 6k–l. Reagents and conditions: (a) RBr, K2CO3,
KI, DMF, 70◦C, 12 hr; (b) NaN3, LiCl, NMP, 130◦C, 3 days; (c) TrCl, KOtBu, DMF; and (d)
benzonitrile, 150◦C, 7 hr.
were low in this final step, pure products could be isolated very easily. It is noteworthy
that the nitriles and tetrazoles, 2, 3, 7, and 8 can be important intermediates for the syn-
thesis of other nitrogen heterocyclic compounds with potential applications in materials
and pharmaceutical sciences, e.g., 1,3-oxazoles [16], 2-thiazolines and 2-oxazolines [17],
imidazoles [18], isoquinolines [19], and quinazolines [20].
3.2. Mesomorphic Properties
Liquid crystal properties of tetrazines 5a–g and 6k–l were studied using POM as well
as using DSC. It turned out that the number and length of alkoxy chains attached to the
3,6-diphenyl-1,2,4,5-tetrazine core influence mesomorphism. For the four-tailed series, no
mesophases were observed for compounds 5a–d that have chains of eight or less carbon
atoms. A smectic-C phase was observed for compounds 5e–g (chains of 9, 10, and 12
carbon atoms). In contrast, all the two-tailed tetrazines 6a–l examined possess liquid crystal
properties [8].

38
F. Fouad et al.
Table 2. Phase transition temperatures of 3,6-bis(3,4-dialkoxyphenyl)-1,2,4,5-tetrazines,
5a–g (the four-tailed tetrazines)
Transition temperature and enthalpy changes
Second heating/◦C
First cooling/◦C
Compound
R =
(ꢀH, KJ/mol)
(ꢀH, KJ/mol)
5a
5b
5c
5d
5e
−C5H11
−C6H13
−C7H15
−C8H17
−C9H19
Cr 162 (62.39) I
Cr 143 (7.92) I
Cr 144 (77.53) I
Cr 135 (81.66) I
Cr 109 (12.04) Sc 134
(89.64) I
I 143 (72.59) Cr
I 136 (10.49) Cr
I 126 (79.23) Cr
I 128 (84.50) Cr
I 131 (92.21) Sc 108
(10.84) Cr
5f
−C10H21
−C12H25
Cr 104 (18.53) Sc 131
(105.78) I
Cr1 36 (5.84) Cr2 98
(22.33) Sc 129
(125.33) I
I 121 (109.13) Sc 102
(18.67) Cr
I 120 (129.92) Sc 96
(22.14) Cr2 26
(4.01) Cr1
5g
Compared with the two-tailed structures, it is obvious that introducing two more tails
(four-tailed tetrazines, 5e–g) changes the mesogenic properties of the molecule in sev-
eral ways, including (1) depression of m.p. and clearing points, (2) loss of the nematic
phase prior to melting, (3) depression of the smectic-C phases formation temperatures,
and (4) decrease in the mesomorphic range (Tables 2 and 3). The new two-tailed deriva-
tives with C-12 chains—6k, and interestingly, the C-18 chains—6l retain the smectic-C
phases.
3.3. Crystal Structure of 1,4-bis-(4-Dodecyloxyphenyl)tetrazine, 6k
We have determined the single crystal structure of 1,4-bis-(4-dodecyloxyphenyl)tetrazine,
6k. Some relevant structure parameters and X-ray experimental data for compound 6k are
briefly summarized in Table 4.
The crystal is built of slip-stacked molecules with coplanar cores (Fig. 1). Adjacent
molecules in the stacks are offset by one unit cell length in the aˆ direction. There are no Van
der Waals close contacts between molecules within a stack, though there are VdW C–H
short contacts between adjacent stacks. The closest intrastack C–C networks of contacts are
3.48 Å (between carbons on neighboring tetrazine rings), with comparable C–C interactions
between tetrazine nitrogens and carbons within the phenyl groups of adjacent molecules.
We define a natural orthogonal coordinate system based on the tetrazine moiety (with
origin at the ring centroid) with x-axis spans the two tetrazine carbons and a z-axis projects
perpendicular to the ring (Fig. 2). In this system, the vector between molecular centroids

3,6-Diaryl-1,2,4,5-Tetrazines
39
Table 3. Phase transition temperatures of 3,6-bis(4-alkoxyphenyl)-1,2,4,5-tetrazines, 6a–l
(the two-tailed series)∗
Compound
R =
Transition temperatures ◦C
Cr 247 N 253 I
Cr 238 N 268 I
Cr 203 N 224 I
6a
6b
6c
6d
6e
6f
6g
6h
6i
−CH3
−C2H5
−C3H7
−C4H9
Cr 190 N 226 I
Cr 181 N 210 I
−C5H11
−C6H13
−C7H15
−C8H17
−C9H19
−C10H21
−C12H25
Cr 163 Sc 181 N 207 I
Cr 146 Sc 183 N 197 I
Cr 131 Sc 187 N 195 I
Cr 120 Sc 188 N 190 I
Cr 111 Sc 189 I
Cr2 96 (40.71) Cr1 112 (29.01) Sc
180 (14.64) I 178 (15.69) Sc 109
(29.61) Cr
6j
6k
6l
−C18H37
Cr2 54 (16.45) Cr1 107 (35.11) Sc
116 (75.16) I 107 (49.28) Sc
(25.23) 44 Cr
∗Compounds 6k and 6l are new and transitions are reported for heating and cooling. Data for
compounds 6a–j were obtained from reference [8].
(aˆ) in a stack is (3.1 Å, –0.59 Å, 3.40 Å). This is a substantial offset, and implies that the
π-orbitals of identical atoms on adjacent cores do not substantially overlap. The overall
π-stacking may still be significant, however, as reflected by the relative close C–N and C–C
contact networks mentioned above.
Table 4. Structure parameters and experimental data for compound 6k
CCDC deposit number
Empirical formula
Formula weight
894997
C38 H58 N4 O2
602.88
Temperature
150 (2) K
Wavelength
0.71073 A
Crystal system, space group
Unit cell dimensions
Triclinic, P-1
a = 4.6394 (4) Å α = 83.8450(10)◦
b = 8.0165 (7) Å β = 88.1550(10)◦
c = 23.897 (2) Å γ = 75.9730(10)◦
857.28 (13) Å3
Volume
Z, Calculated density
Absorption coefficient
1, 1.168 Mg/m3
0.072 mm−1

40
F. Fouad et al.
Figure 1. Stacking in compound 6k. The slip-stacking of the cores (along the horizontal axis of the
figure) as well as the interdigitated tails are evident.
In the crystal, the diphenyltetrazine core is highly coplanar with torsion angles less
than 1◦ among the respective atoms (i.e., no atom deviates by more than 0.22 Å from the
tetrazine-ring plane). The start of the alkoxy tails (the oxygen and first carbon in the tail)
also lies in the ring plane with torsion only about 1◦. The tails as a whole, however, project
Figure 2. The x-ray structure of a molecule of 6k showing the orthogonal coordinate system based
on the molecular core as well as the angles characterizing the tail geometry.

3,6-Diaryl-1,2,4,5-Tetrazines
41
sharply out of the plane of the molecular core. Indeed, in our orthogonal coordinate system,
the tails are rotated by θ = 38◦ out of the xy (or ring)-plane and by ϕ = 36◦ about the z-axis
(Fig. 2). Of course, in any phase other than crystalline the spatial relationships of the core
units and the conformations of the tails are dynamic and deviate from the “frozen” crystal
structure.
Therefore, the overall length of the molecule is substantially smaller than if the tails
were lying along the y-axis (the length along yˆ is 34.3 Å vs. about 46 Å for the case of the
tails lying along y). This will presumably reduce the layer spacing in the smectic phase.
4. Conclusion
We have synthesized a series of symmetrical 3,6-diaryl-1,2,4,5-tetrazines with four or
two alkoxy chains with various lengths and investigated their thermotropic liquid crystal
properties. Unlike the two-tailed tetrazines, the four-tailed tetrazines with less than 9 carbon
chains are not mesogenic. Four-tailed tetrazines with C-9, C-10, and C-12 alkoxy chains
possess smectic-C phases. Two-tailed tetrazines with C-12 and C-18 alkoxy chains possess
smectic C phases as well. Compared with the two-tailed tetrazines, the four-tailed liquid
crystalline tetrazines possess lower smectic-C phase transition temperatures and m.p. and
better solubility. A single crystal structure of one of the two-tailed tetrazines was obtained,
which is a progenitor to the Sc phase obtained on heating.
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