Tetrakis(Oxadiazolylphenyl)pyrazines: New st. Andrew’s cross-shaped liquid crystals

Article abstract of DOI:10.1002/cphc.201800936

π-Conjugated molecules with the shape of St. Andrew’s cross have been synthesized via fourfold Huisgen reaction. Four 2,5- diaryl-1,3,4-oxadiazol arms are attached to a central pyrazine nucleus. These fluorescent stars, when decorated with a rim of eight alkoxy side chains are discotic liquid crystals. Depending on the substitution pattern, the width of the liquid phase varies within a broad range of 25°C to 250°C. In their liquid crystalline phase, the molecules assemble in a typical hexagonal columnar supramolecular arrangement.

Full text of DOI:10.1002/cphc.201800936

Accepted Article
Title: Tetrakis(oxadiazolylphenyl)pyrazines: New St. Andrew's Cross-
Shaped Liquid Crystals
Authors: Nico Röder, Tomasz Marszalek, Daniel Limbach, Wojchiech
Pisula, and Heiner Detert
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.201800936
Link to VoR: http://dx.doi.org/10.1002/cphc.201800936
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10.1002/cphc.201800936
ChemPhysChem
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Tetrakis(oxadiazolylphenyl)pyrazines: New St. Andrew's Cross-
Shaped Liquid Crystals
Nico Röder,[a] Tomasz Marszalek,[b,c] Daniel Limbach,[a] Wojciech Pisula,*[b,c] and Heiner Detert*[a]
Dedicated to Prof. Gerhard Wenz on the occasion of his retirement
diodes and as non-linear optical materials.[13] Their electron
Abstract: π-Conjugated molecules with the shape of St.
deficient character, high photoluminescence quantum yield,
excellent thermal and chemical stability are good prerequisites
Andrews cross have been synthesized via fourfold Huisgen
reaction. Four 2,5-diaryl-1,3,4-oxadiazol arms are attached
to a central pyrazine nucleus. These fluorescent stars, when
decorated with a rim of eight alkoxy side chains are discotic
liquid crystals. Depending on the substitution pattern, the
width of the liquid phase varies within a broad range of 25 °C
to 250 °C. In their liquid crystalline phase, the molecules
assemble in a typical hexagonal columnar supramolecular
arrangement.
for application of these compounds as fluorescent sensors for
transition metal ions,[14] biologically active agents,[15] as
scintillators or emitters in OLEDs.[16] For
a long time,
1,3,4-oxadiazoles were not considered to be a building block of
liquid crystalline compounds because it was generally accepted
that the 134° angle between the substituents on the oxadiazole
inhibits the formation of mesophases.[17] In 1989, Chudgar et al.
published a series of calamitic mesogens with this structural
unit.[18] The first DLC with 1,3,4-oxadiazole units was discovered
by Park in 2001.[19] The star-shaped mesogen with 1,3,5-
triethinylbenzene as the nucleus and three identical 2,5-
diphenyloxadiazole branches forms a narrow (10 °C) discotic-
nematic phase. Mesomorphism was attributed to result
predominantly from π-stacking between the arms and less from
core-core interactions.
Here we present a novel group of St. Andrew's cross-shaped,
symmetrical discotic mesogens, composed of electron deficient
heterocycles. Four alkoxyphenyl-oxadiazolyl wing groups are
attached to the tetraphenylpyrazine nucleus (TOPP). The central
ring is sterically analogous to a 1,2,4,5-tetrasubstituted benzene
but with a higher electron affinity. Together with the synthesis of
TOPPs, we report the structures of the molecules, their optical
properties, the influence of side chain length on thermotropic
behavior, and the structures of the mesophases.
Introduction
Thermotropic liquid crystals (LC) are encountered in many areas
of everyday life and have revolutionized display technology.[1]
This important category of soft matter possesses an additional
mesomorphic state between crystalline and liquid phases. Most
LC are based on a rigid, rod-shaped nucleus[2] with flexible side
chains in the periphery. As early as 1910, Tutin reported the
mesomorphous properties of 2,5-di(p-methoxyphenyl)pyrazine,
the first LC with a pyrazine center.[3] In addition to the
mesomorphism of calamitic LCs, Chandrasekhar reported in
[4]
1977 that also disc-shaped molecules can form LC phases.
Discotic LCs (DLC) are usually composed of a rigid, mostly
aromatic core and several aliphatic side chains on the rim.[5] The
center is typically a condensed (hetero)aromatic nucleus like
(hexaaza)triphenylene,[6]
hexabenzocoronene,[7]
triazine,[8]
Results and Discussions
triazatruxene, or tistriazolotriazine.[9] Small central rings with
three to six conjugated arms are a second successful strategy to
DLCs.[10] Tetraphenylpyrazine, first reported in 1845[11] has been
explored as building block for electronic materials, e.g.
photoconductive materials for electrophotography, as electron-
transmitting layers in OLEDs or in high-temperature functional
fluids. However, in comparison to the cruciforms with benzene
nuclei, only a few mesogens are known.[12]
Structure-property correlations of liquid crystals are often based
on variation of the flexible periphery of a rigid core molecule.
Retrosynthetic analysis of the TOPPs reveals a p-substituted
tetraphenylpyrazine as central unit, either with tetrazoles or
carboxylic acids. These functional groups are easily converted to
1,3,4-oxadiazoles via Huisgen reaction of activated acid
derivatives and tetrazoles.[20] Tetracarboxylic acid 3 is the
product of a benzoin condensation[21] of methyl p-formylbenzoate
1 and condensation/oxidation in an ammonium acetate melt
followed by hydrolysis led directly to the fourfold carboxylic acid
3 (33% over 4 steps, Scheme 1). The fourfold acid chloride 4,
central building block for all TOPPs, was prepared from the
thoroughly dried acid and thionyl chloride immediately before
reaction with tetrazoles.
Materials based on 2,5-diaryl-1,3,4-oxadiazoles have been
successfully used as semiconductors in electroluminescent
[a]
Dr. Nico Röder, Dipl.-Chem. Daniel Limbach, Prof. Heiner Detert
Institute for Organic Chemistry
Johannes Gutenberg University
Duesbergweg 10-14, 55118 Mainz, Germany
E-mail: detert@uni-mainz.de
[b]
[c]
Dr. Tomasz Marszalek, Prof. Wojciech Pisula
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz, Germany
E-mail: pisula@mpip-mainz.mpg.de
Department of Molecular Physics,
Faculty of Chemistry, Lodz University of Technology, Zeromskiego
116, 90-924 Lodz, Poland.
Supporting information for this article is given via a link at the end of
the document
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10.1002/cphc.201800936
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FULL PAPER
ClOC
ClOC
COCl
COCl
O
H
Table
1.
Substitution
patterns
and
yields
of
Tetrakisoxadiazolylphenylpyrazines
N
i - v
TOPP
12a
12b
12c
12d
12e
12f
R1
R2
R3
Yield [%][a]
N
1
4
COOCH3
H
OC3H7
OC3H7
OC4H9
OC6H13
OC7H15
OC8H17
OC10H21
OC12H25
OC14H29
OC16H33
H
H
49
70
73
81
24
36
32
71
72
51
70
37
82
Scheme 1.: 1) KCN, EtOH/H2O, 63%; ii) CH3COONH4, CH3COOH; iii) I2, O2,
140 °C, 53%; iv) KOH, 2-propanol/H2O; HCl, 99%; iv) SOCl2, toluene.
H
OC3H7
H
OC4H9
The most convenient route to 2H-tetrazoles is the addition of
azide to nitriles, directly[22] or combined with the dehydration of
amides (Scheme 2). Alkylation of protocatechunitrile 5 and
addition of hydrazoic acid gives the 3,4-disubstituted
phenyltetrazoles 6a-i in 60 - 70% yield. The preparation of the
3,5-disubstituted isomers 11a-c succeeded via alkylation of
methyl 3,5-dihydroxybenzoate 7, conversion of the ester 8a-c to
primary amide 10a-c via alkaline hydrolysis, chlorination with
thionyl chloride, ammonolysis, and finally dehydration/addition of
azide using triazidochlorosilane.[23] This five-step sequence
provides the tetrazoles 11a-c in 25-30% overall yield.
H
OC6H13
OC7H15
OC8H17
OC10H21
OC12H25
OC14H29
OC16H33
OC10H21
OC12H25
OC14H29
H
H
12g
12h
12i
H
H
H
12j
H
N
12k
12l
OC10H21
OC12H25
OC14H29
CN
N
NH
i, ii
RO
RO
N
H
OH
5a-i
12m
H
OH
6a-i
[a] Yields refer to pure product after chromatography.
N
N
COOCH3
CONH2
Molecular Structure
v
NH
i - iv
RO
N
Single crystals of a p-propyloxy substituted TOPP (12a) were
obtained via slow evaporation of a solution in chloroform.
According to the single crystal analysis, the twofold ortho-
disubstitution on the pyrazine results in an anti-orientation of the
vicinal, nearly planar (torsion angles < 6°) 2,5-diphenyl-1,3,4-
oxadiazole wings.
These are twisted about 42° (at pyrazine-2,5) or 60°(at pyrazine-
3,6) out of the plane of the central ring, similar to
tetraphenylpyrazine.[24] The molecule adopts a highly three-
dimensional, saddle shaped conformation (Figure 1.). The π-π
interactions (d = 3.68 Å) of the anti-parallel propoxyphenyl-1,3,4-
oxadiazole units connect neighboring molecules.
HO
OH
RO
OR
OR
11a-c
7
10a-c
Scheme 2. Reagents and conditions: (i) Br-R, DMF, K2CO3, 80 °C, N2, 42 -
94%; (ii) NaN3, (Et)3N*HCl, xylene, reflux, 42 - 81%; (iii) KOH/2-propanol,
reflux, 80 - 99% (iv) SOCl2, toluene, reflux., 3. NH3(aq.), 0 °C, 65 - 87%; (v)
SiCl4, NaN3, acetonitrile, reflux, 33 - 76%.
The final reaction of the fourfold acid chloride with four tetrazoles
is the convergent thus critical step (Scheme 3). Two procedures
were tested; initially, the acid chloride was added carefully to a
suspension of tetrazole and collidine in toluene and the mixture
was gradually heated to 80 °C to give TOPPs in moderate yields
(ca. 40%). The addition of collidine to a stirred suspension of
acid chloride and tetrazole in toluene was rewarded with a
significant increase in yields, often 70 - 80%. Chromatography
on a silica gel column with a head of basic alumina gave the
TOPPs without traces of products with three or less arms.
R1
R1
N
O
N
N
N
R2
R2
O
R3
R3
R3
R3
N
N
Figure 1. Molecular structure[25] of tetrakis-(4-propoxyphenyl-1,3,4-
oxadiazolylphenyl)pyrazine 12a.
i
4 + 6 or 11
O
N
O
R2
R2
N
N
N
TOPP 12a - m
Thermal Properties
R1
R1
Scheme 3. Synthesis of TOPPs 12a-m (i) collidine, toluene, 80 °C, N2.
Polarization microscopy, the standard examination method for
the observation of mesophases, shows no indication of liquid
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10.1002/cphc.201800936
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crystallinity for propoxy-substituted TOPPs 12a and 12b. All
other compounds reveal the characteristic textures of discotic
LCs in their mesophase. TOPPs with a lateral 3,4-dialkoxy
substitution typically display fan-shaped or (pseudo)focal conical
textures, whereas mosaic structures have been observed in the
case of the 3,5-dialkoxy compounds (see Figure 2. and SI). A
comparison of the pyrazine-centered TOPPs with analogues
based on benzene as central ring is under investigation.
compounds solidify as glasses. With a melting point at -46 °C,
12h displays the broadest mesophase (246 °C) of the
investigated TOPPs. The broad range of melting enthalpy is
attributed to incomplete crystallization of the highly viscous
material.
A change of the 3,4- to a 3,5-disubstitution dramatically affects
the width of the mesophase. Clearing temperatures drop by
about 100 °C while the melting points raise. The width of the
mesophase shrinks to ca 20-25 °C - about 1/10 of the width
observed for the 3,4-disubstituted isomers. TGA analysis of
TOPPs reveal a high thermal stability, and a significant weight
loss was observed only above 418 °C.
Two-dimensional wide-angle X-ray scattering (WAXS)
experiments on macroscopically aligned extruded fibers[26] were
carried out to determine the intra- and intercolumnar
organization of the two isomers 12h and 12l in their liquid crystal
phases (Figure 2). For 3,4-disubstituted 12h, the columns
arrange in a hexagonal lattice of ah = 4.45 nm in the mesophase
as determined from the equatorial reflections in the pattern of
Figure 3a. The intracolumnar π-stacking distance of 0.36 nm
between the individual discs is derived from the diffuse wide-
angle meridional reflections.[26] The 3,5-disubstituted TOPP 12l
a)
b)
Figure 2. POM-textures (crossed-polarizers) of TOPP 12j at 175 °C (a) and
12m at 55 °C (b).
Table 2. Phase transition temperatures and enthalpies of TOPPs.[a]
TOPP
12c
12d
12e
12f
Tm /°C
103[b]
125
77[c]
74[b]
76[c]
-46
ΔHM /kJ mol-1
Tc /°C
174
196
199
200
196
202
196
188
106
85
ΔHc /kJ mol-1
reveals in the mesophase also
a hexagonal columnar
-
4
5
7
arrangement, but with a slightly smaller lattice constant of ah =
3.95 nm. At the same time, the π-stacking distance is increased
to 0.38 nm. One possible reason for the denser columnar
hexagonal packing is the higher symmetry of 12l compared to
12h.[27] The average alkyl chain spacing of 0.44 nm for 12h and
0.45 nm for 12l is very similar. Both compounds display diffuse
halos in the wide-angle range, caused by the disordered alkyl
chains in the periphery of mesogens.
2[c]
-
9
9
12g
12h
12i
3[c]
72
52
84
4
9
11
11
12
1
-8
12j
19
12k
12l
91
61
4
4
12m
66[a]
-
84
6
[a] Tm: onset melting transition; ΔHM: melting enthalpy, Tc: onset clearing
transition; ΔHc: clearing transition enthalpy. [b] glass transition; [c] data
determined from the first heating curve.
To determine the transition temperatures and the enthalpies, all
mesomorphous TOPPs were investigated by dynamic
differential calorimetry. Unless otherwise specified, the second
heating curve was used for data evaluation. Whereas TOPP 12b
with a 3,4-dipropyloxy substitution melts at 204 °C without any
sign of mesomorphism, compounds with chain lengths from
butyl to hexadecyl display broad LC phases. The clearing points
of six out of eight homologous TOPPs 12d - i are around 200 °C,
and only the first and last with butyl (12c) resp. hexadecyl chains
(12j) clear at significantly lower temperatures (Table 2). The
small range of clearing points indicates very similar core-core
interaction within the columns. Contrary to the isotropization, the
melting points vary between 125 °C (butyl, 12d) and -46 °C
(dodecyl, 12h). Since the crystallization of these materials is
inhibited due to high viscosity of the LC phase, some
Figure 3. 2DWAXS patterns of a) 12h and b) 12l in their mesophases and the
corresponding integrations. The fiber samples were placed vertically toward
the 2D detector. Peaks are assigned by Miller indices, *: halo, **: stacking.
Optical Properties
All TOPPs are yellow and fluorescent, as solids and dissolved in
dichloromethane. The optical properties of some TOPPS as
representative compounds have been studied by UV-vis and
fluorescence spectroscopy in different environments. The results
are collected in Table 3.
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10.1002/cphc.201800936
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shows the emission spectrum of 12c as solution in THF
compared to 12c in poor solvents, THF/water mixtures and
heptane. A nearly fourfold fluorescence intensity results from the
Generally, photophysical properties of TOPPs are nearly
independent from alkyl chain length and substitution pattern.
Uniquely, the absorption maximum of TOPP 12a in cyclohexane
peaks at lower energies compared to 12e with the stronger
donor-acceptor character. This is leveled out in the more polar
solvent dichloromethane. The long-wavelength absorption
maximum of TOPPs with a 3,4-disubstitution peaks at λmax = 362
nm, only 2 nm more bathochromic then the 3,5-isomer, which
also show a second, more intense maximum at higher energies,
λmax ca. 316 nm. The violet-blue fluorescence (λFmax = 444 - 457
nm)is separated from the long-wavelength absorption maximum
effect of aggregation induced emission (AIE)[29]
.
1200
12c in
THF
1000
THF /H2O 3/1
THF /H2O 6/1
800
heptane
600
400
200
0
by Stokes shifts of ca. Δ휈̃ = 5200 cm-1 in cyclohexane, but up
푆푡
푆푡
to Δ 휈̃ = 6100 cm-1 in dichloromethane. A weak negative
solvatochromism of the absorption of TOPPs has been noted.
Fluorescence quantum yields are also depending on the solvent.
The highest values of ΦF = 12 - 17% were obtained in toluene.
Spin-coated films show very similar optical properties, pristine as
well as after annealing.
400
450
500
550
600
wavelength / nm
Figure 4. Emission spectra of a solution of 12c (3*10-4 M) in THF solution, as
suspension in THF/water mixtures, and heptane
Table 3. Spectroscopic data of TOPPs in different solvents.
Conclusions
ΦF
Δ휈̃
푆푡
TO
PP
λF
max
solv.
cy
λmax /nm
log ε
/nm
449
449
447
448
451
456
448
452
457
447
455
453
451
454
444
445
442
445
440
/cm-1
The synthesis and characterization of novel St. Andrews cross-
shaped mesogens with central tetraphenylpyrazine nucleus and
alkoxyaryl-1,3,4-oxadiazole units as wing groups have been
successfully accomplished for the first time. The modular
synthesis is based on a fourfold Huisgen reaction as convergent
step. Due to the double ortho-disubstitution on the pyrazine, the
rigid part of TOPPs adopts a saddle-like shape with a height of 9
Å. Nevertheless, even with side chains as short as butyl, these
cruciforms are mesomorphous with phase widths up to 246 °C.
In the liquid crystalline phases the molecules assemble into
characteristic hexagonal columnar structures as confirmed by X-
ray scattering analysis on selected compounds. A higher
substitution symmetry leads to a tighter columnar packing, but
larger intracolumnar distance between individual molecules.
Nearly unbiased by the substitution pattern and environment,
absorption of the yellow TOPPs peaks around 363 nm and a
moderately efficient emission is centered around 450 nm.
366 (330)
364 (338)
4.54
4.85
4.88
4.91
4.99
4.97
4.92
4.99
4.97
4.96
5.00
5.00
-
4%
5051
5176
5719
5278
5400
6105
5202
5300
6105
5152
5570
5834
7767
7889
(5255)
(5306)
(5153)
(4850)
(4621)
12a tol
DCM 356 (328)
5%
cy
362
363
7%
12e tol
15%
7%
DCM 357
cy
363
364
6%
12h tol
17%
7%
DCM 357
cy
tol
363
363
12%
17%
9%
12i
DCM 358
film[a] 334
film[b] 334
Experimental Section
General: Reactions were carried out in dry solvents and under a nitrogen
atmosphere or with moisture exclusion. TLC: Polygram SIL G/UV254,
Macherey-Nagel. Column chromatography: silica gel 60, Macherey-
Nagel. Basic aluminium oxide: Merck. Melting points: Dr. Tottoli
apparatus. POM: Olympus BX51 polarization microscope with a heatable
sample chamber Linkam TMS94. Images: Olympus DP22. DSC: DSC7
(Perkin Elmer). Heating rate 2nd scan: 10 °C/min. NMR: Bruker AC300;
Bruker ARX400; Bruker AMX400; CDCl3 or DMSO-d6. IR: Jasco FT/IR
4100 Fourier Transform spectrometer, Zn-Se crystal. FD-MS: MAT 90
(Finnigan), HR-MSESI: Q-TOF-ULTIMA 3. Given masses refer to the
pure isotopes. UV-Vis spectra: Lambda 16 (Perkin-Elmer). Fluorescence:
LSB50 (Perkin-Elmer). Starting materials were prepared according or
analogous to literature procedures,[28] details are given in the SI.
-
cy
tol
316 (360)
318 (360)
4.95
4.94
4.96
-
12%
12%
8%
12l
DCM 314 (354)
film[a] 318 (366)
film[b] 318 (366)
-
cy: cyclohexane; tol: toluene; DCM: dichloromethane [a] pristine
spin-coated film; [b] spin-coated film in the liquid crystal phase.
푆푡
Values in brackets: second maximum and corresponding Δ휈̃ The
Two-dimensional wide-angle X-ray scattering (2D-WAXS): 2D-WAXS
measurements were performed using a custom setup consisting of the
Siemens Kristalloflex X-ray source (copper anode X-ray tube, operated at
fluorescence quantum yields of TOPPs in highly diluted solution
are only moderate, but increase upon aggregation. Figure 4
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10.1002/cphc.201800936
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FULL PAPER
35 kV/20 mA), Osmic confocal MaxFlux optics, two collimating pinholes
(1.0 and 0.5 mm Owis, Germany) and an antiscattering pinhole (0.7 mm–
Owis, Germany). The patterns were recorded on a MAR345 image plate
detector (Marresearch, Germany). The samples were prepared by
filament extrusion using a home-built mini-extruder.
19.36 (CH2), 14.03 (CH3) ppm. IR (ATR): 휈̃ = 2956, 1606, 1495, 1464,
1388, 1272, 1213, 1139, 1064, 1008, 849, 810, 723, 701 cm-1. FD-MS:
m/z = 1537.1 [M]+.
2,3,5,6-Tetrakis(4-(5-(3,4-bis(hexyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)pyrazine 12d: According to general procedure B, 100 mg (0.178
mmol) of 3 and 297 mg (0,856 mmol) of 6c were used. Column
chromatography: toluene/ethyl acetate 8:1; Yield: 253 mg (0,144 mmol,
81 %); m.p. 125 °C, c.p. 196 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
General procedure for the synthesis of TOPPs: In
a nitrogen
atmosphere, 4 (ca. 200 mg) was suspended in 4 mL of thionyl chloride
and stirred for 6 hours under reflux and evaporated to dryness. Variant A:
The residue was suspended in 5 mL toluene and added dropwise to a
solution of 4,4 eq. tetrazole and 1 mL 2,4,6-collidine in 10 mL toluene,.
Variant B: The acid chloride X and 4,4 eq. of the respective tetrazole
were stirred in 10 mL toluene and after 2 h at ambient temperature, 1 mL
of 2,4,6-collidine was quickly added. Subsequently (A and B) the mixture
was stirred for 16 hours at 80 °C. After complete conversion (TLC), 30
mL 2 M hydrochloric acid was added. The solution was extracted with
chloroform, the combined organic layers washed with water and brine
and dried over MgSO4. The solvent was evaporated. The crude product
was purified on a silica gel column with a top layer of basic Al2O3.
Toluene followed by toluene/ethyl acetate was used as eluent
3
3
8.16 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.6 Hz, 8H, Ph), 7.74 –
3
7.56 (m, 8H, Ph), 6.96 (d, JH-H = 8.3 Hz, 4H, Ph), 4.24 – 3.97 (m, 16H,
α-OCH2), 1.98 – 1.78 (m, 16H, β-OCH2), 1.57 – 1.43 (m, 16H, γ-CH2),
1.43 – 1.20 (m, 32H, CH2), 1.03 – 0.78 (m, 24H, CH3) ppm. 13C NMR
(100 MHz, 25 °C, CDCl3): δ = 165.11 (Oxa), 163.80 (Oxa), 152.51 (Ph),
149.46 (Ph), 148.04 (Pyr), 140.69 (Ph), 130.72 (Ph), 127.16 (Ph), 124.88
(Ph), 120.62 (Ph), 116.16 (Ph), 112.94 (Ph), 111.70 (Ph), 69.55, 69.25
(α-OCH2), 31.70, 29.28, 29.19, 25.81, 22.74 (CH2), 14.17 (CH3) ppm. IR
(ATR): 휈̃ = 2931, 2857, 1605, 1495, 1389, 1271, 1138, 1009, 849, 723
cm-1. FD-MS: m/z = 1762.7 [M]+.
2,3,5,6-Tetrakis(4-(5-(3,4-bis(heptyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)pyrazine 12e: According to general procedure A, 150 mg (0.268
mmol) of 3 and 297 mg (1,29 mmol) of 6d were used. Column
chromatography: toluene/ethyl acetate 3:1; Yield: 125 mg (0,0625 mmol,
24 %); m.p. 77 °C, c.p. 199 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
2,3,5,6-Tetrakis(4-(5-(4-propoxyphenyl)-1,3,4-oxadiazol-2-
yl)phenyl)pyrazine 12a According to general procedure B, 150 mg
(0,268 mmol) of 3 and 262,0 mg (1,3 mmol) 5-(4-propoxyphenyl)-2H-
tetrazole) were used. Column chromatography: toluene/ethyl acetate 2:1;
Yield: 157 mg (0,013 mmol, 49 %); m.p. 298 °C; 1H NMR (400 MHz,
25 °C, CDCl3): δ = 8.15 (d, 3J = 8.0 Hz, 8H, 3‘-H, Ph), 8.06 (d, 3J = 8.9
Hz, 8H, 2‘‘‘-H, Ph), 7.87 (d, 3J = 8.2 Hz, 8H, 2‘-H, Ph), 7.01 (d, 3J = 8.9
Hz, 8H, 3‘‘‘-H, Ph), 4.00 (t, 3J = 6.5 Hz, 8H, α-OCH2), 2.26 – 1.67 (m, 8H,
β-OCH2), 1.06 (t, 3J = 7.4 Hz, 12H, CH3) ppm. 13C NMR (100 MHz, 25 °C,
CDCl3): δ = 165.00 (C-5‘‘, Oxa), 163.74 (C-2‘‘, Oxa), 162.24 (C-4‘‘‘, Ph),
148.07 (C-2, C-3, Pyr), 140.68 (C-1‘, Ph), 130.73 (C-2‘, Ph), 128.90 (C-
2‘‘‘, Ph), 127.13 (C-3‘, Ph), 124.87 (C-4‘, Ph), 116.08 (C-1‘‘‘, Ph), 115.16
(C-3‘‘‘, Ph), 69.90 (α-OCH2), 22.61 (β-OCH2), 10.63 (CH3) ppm. IR
3
3
8.16 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H, Ph), 7.76 –
3
7.52 (m, 8H, Ph), 6.96 (d, JH-H = 8.2 Hz, 4H, Ph), 4.30 – 3.87 (m, 16H
α-OCH2), 1.95 – 1.76 (m, 16H, β-OCH2), 1.54 – 1.17 (m, 64H, CH2), 1.06
– 0.38 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.13 (Oxa), 163.81 (Oxa), 152.55 (Ph), 149.49 (Ph), 148.05 (Pyr),
140.71 (Ph), 130.74 (Ph), 127.19 (Ph), 124.88 (Ph), 120.67 (Ph), 116.16
(Ph), 112.98 (Ph), 111.76 (Ph), 69.59, 69.27 (α-OCH2), 31.95, 29.35,
29.25, 29.22, 26.12, 26.10, 22.76 (CH2), 14.25 (CH3) ppm. IR (ATR): 휈̃ =
2926, 1607, 1496, 1468, 1390, 1274, 1216, 1140, 1911, 850, 789, 757
cm-1. HRMS (ESI): calc. for C119H155N10O12 [M + H]+: 1917.1854; found
1917.1852.
̃
(ATR): 흂 = 2960, 2926, 2875, 1610, 1585, 1551, 1494, 1471, 1389, 1308,
2,3,5,6-Tetrakis(4-(5-(3,4-bis(octyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)pyrazine 12f: According to general procedure A, 150 mg (0.268
mmol) of 3 and 517 mg (1.29 mmol) of 6e were used. Column
chromatography: toluene/ethyl acetate 5:1; Yield: 193 mg (0,097 mmol,
36 %); Tg 74 °C, m.p. 200 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
1254, 1173, 1093, 1064, 1011, 975, 909, 836, 708 cm-1; HR-ESI: calcd.
for C72H60N10O8 + H+: 1193,4668, found: 1193,4671.
2,3,5,6-Tetrakis(4-(5-(3,4-dipropoxyphenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)pyrazine 12b: According to general procedure B, 150 mg (0.268
mmol) of 3 and 309 mg (1,18 mmol) of 6a were used. Column
chromatography: toluene/ethyl acetate 6:1; Yield: 266 mg (0,186 mmol,
70 %); m.p. 204 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ = 8.16 (d,
3JH-H = 8.6 Hz, 8H, Ph), 7.88 (d, JH-H = 8.7 Hz, 8H, Ph), 7.73 – 7.56 (m,
8H, Ph), 6.97 (d, JH-H = 8.2 Hz, 4H, Ph), 4.15 – 3.90 (m, 16H, α-OCH2),
1.89 (m, 16H, β-OCH2), 1.07 (t, 3JH-H = 7.4 Hz, 24H, CH3) ppm. 13C NMR
(75 MHz, 25 °C, CDCl3): δ = 165,12 (Oxa), 163,82 (Oxa), 152.54 (Ph),
149.46 (Ph), 148.05 (Pyr), 140.71 (Ph), 130.74 (Ph), 127.17 (Ph), 124.87
(Ph), 120.69 (Ph), 116.18 (Ph), 113.03 (Ph), 111.83 (Ph), 71.03, 70.69
(α-OCH2), 22.70, 22.62 (CH2), 10.64, 10.60 (CH3) ppm. IR (ATR): 휈̃ =
2965, 2939, 2878, 1607, 1559, 1496, 1470, 1391, 1273, 1219, 1141,
1106, 1010, 978, 853, 720 cm-1. FD-MS: m/z = 1425.0 [M]+.
3
3
8.16 (d, JH-H = 8.4 Hz, 8H, Ph), 7.88 (d, JH-H = 8.4 Hz, 8H, Ph), 7.69 –
3
7.62 (m, 8H, Ph), 6.96 (d, JH-H = 8.3 Hz, 4H, Ph), 4.32 – 3.85 (m, 16H,
α-OCH2), 1.99 – 1.75 (m, 16H, β-OCH2), 1.61 – 1.16 (m, 80H, CH2), 1.18
– 0.44 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.10 (Oxa), 163.78 (Oxa), 152.56 (Ph), 149.49 (Ph), 148.02 (Pyr),
140.71 (Ph), 130.73 (Ph), 127.16 (Ph), 124.85 (Ph), 120.66 (Ph), 116.10
(Ph), 112.97 (Ph), 111.75 (Ph), 69.58, 69.27 (α-OCH2), 31.95, 29.49,
29.40, 29.33, 29.23, 26.16, 26.13, 22.81, 22.79 (CH2), 14.25 (CH3) ppm.
IR (ATR): 휈̃ = 3062, 2923, 2854, 1607, 1556, 1496, 1467, 1390, 1272,
1220, 1141, 1104, 1010, 850, 810, 723 cm-1. FD-MS: m/z = 1986.5 [M]+.
3
3
2,3,5,6-Tetrakis(4-(5-(3,4-bis(decyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)pyrazine 12g: According to general procedure A, 150 mg (0.268
mmol) of
3 and 540 mg (1,18 mmol) of 6f were used. Column
2,3,5,6-Tetrakis(4-(5-(3,4-dibutyloxyphenyl)-1,3,4-oxadiazol-2-yl)-
phenyl)pyrazine 12c: According to general procedure B, 100 mg (0.178
mmol) of 3 and 248 mg (0,856 mmol) of 6b were used. Column
chromatography: toluene/ethyl acetate 8:1; Yield: 200 mg (0,130 mmol,
73 %); Tg 103 °C, c.p. 174 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
chromatography: toluene/ethyl acetate 3:1; Yield: 188 mg (0.0850 mmol,
32 %); m.p. 76 °C, c.p. 196 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
3
3
8.16 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H, Ph), 7.68 –
3
7.60 (m, 8H, Ph), 6.96 (d, JH-H = 8.9 Hz, 4H, Ph), 4.15 – 3.97 (m, 16H,
α-OCH2), 1.95 – 1.76 (m, 16H, β-OCH2), 1.55 – 1.15 (m, 112H, CH2),
1.07 – 0.44 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.10 (Oxa), 163.79 (Oxa), 152.51 (Ph), 149.47 (Ph), 148.02 (Pyr),
140.69 (Ph), 130.72 (Ph), 127.15 (Ph), 124.87 (Ph), 120.62 (Ph), 116.16
(Ph), 112.95 (Ph), 111.71 (Ph), 69.56, 69.26 (α-OCH2), 32.05, 29.85,
29.76, 29.72, 29.56, 29.54, 29.49, 29.33, 29.23, 26.16, 26.13, 22.84
(CH2), 14.26 (CH3) ppm. IR (ATR): 휈̃ = 3734, 3628, 2923, 2853, 1734,
1496, 1466, 1274, 1011, 796, 721, 668 cm-1. FD-MS: m/z = 2211.6 [M]+.
3
3
8.16 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H, Ph), 7.71 –
3
7.59 (m, 8H, Ph), 6.96 (d, JH-H = 8.7 Hz, 4H, , Ph), 4.19 – 3.89 (m, 16H,
α-OCH2), 2.03 – 1.71 (m, 16H, β-OCH2), 1.59 – 1.44 (m, 16H, CH2), 1.05
– 0.93 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.11 (Oxa), 163.81 (Oxa), 152.54 (Ph), 149.49 (Ph), 148,04 (Pyr),
140.69 (Ph), 130.72 (Ph), 127.16 (Ph), 124.88 (Ph), 120.64 (Ph), 116.19
(Ph), 113.00 (Ph), 111.77 (Ph), 69.29, 68.98 (α-OCH2), 31.37, 31.27,
This article is protected by copyright. All rights reserved.

10.1002/cphc.201800936
ChemPhysChem
FULL PAPER
2,3,5,6-Tetrakis(4-(5-(3,4-bis(dodecyloxy)phenyl)-1,3,4-oxadiazol-
2-yl)-phenyl)pyrazine 12h: According to general procedure B, 150 mg
(0.268 mmol) of 3 and 661 mg (1,29 mmol) of 6g were used. Column
chromatography: toluene/ethyl acetate 8:1; Yield: 466 mg (0,191 mmol,
71 %); m.p. -46°C, c.p. 202 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
mg (0.268 mmol) of 3 and 661 mg (1.29 mmol) of 11a were used.
Column chromatography: toluene/ethyl acetate 10:1; Yield: 236 mg
(0,0981 mmol, 37 %); m.p. 61°C, c.p. 85 °C; 1H NMR (400 MHz, 25 °C,
3
3
CDCl3): δ = 8.17 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H,
Ph), 7.25 (d, 4J = 2.2 Hz, 8H, Ph), 6.62 (t, 4JH-H = 2.2 Hz, 4H, Ph), 4.01 (t,
3JH-H = 6.5 Hz, 16H, α-OCH2), 2.10 – 1.70 (m, 16H, β-OCH2), 1.53 – 1.19
(m, 144H, CH2), 0.92 – 0.84 (m, 24H, CH3) ppm. 13C NMR (100 MHz,
25 °C, CDCl3): δ = 164.94 (Oxa), 164.05 (Oxa), 160.74 (Ph), 147.93 (Pyr),
140.72 (Ph), 130.62 (Ph), 127.15 (Ph), 125.03 (Ph), 124.59 (Ph), 105.23
(Ph), 105.14 (Ph), 68.45 (α-OCH2), 31.93, 29.65, 29.58, 29.39, 29.36,
29.19, 26.02, 22.70 (CH2), 14.14 (CH3) ppm. IR (ATR): 휈̃ = 2922, 2852,
1734, 1598, 1546, 1441, 1388, 1297, 1166, 1055, 1011, 851, 757, 729,
679 cm-1. FD-MS: m/z = 2435.7 [M]+.
3
3
8.16 (d, JH-H = 8.1 Hz, 8H, Ph), 7.88 (d, JH-H = 8.2 Hz, 8H, Ph), 7.73 –
3
7.62 (m, 8H, Ph), 6.96 (d, JH-H = 8.9 Hz, 4H, Ph), 4.22 – 3.94 (m, 16H,
α-OCH2), 1.94 – 1.79 (m, 16H, β-OCH2), 1.56 – 1.17 (m, 144H, CH2),
0.98 – 0.68 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.11 (Oxa), 163.79 (Oxa), 152.54 (Ph), 149.49 (Ph), 148.03 (Pyr),
140.70 (Ph), 130.73 (Ph), 127.16 (Ph), 124.87 (Ph), 120.63 (Ph), 116.14
(Ph), 112.96 (Ph), 111.73 (Ph), 69.58, 69.27 (α-OCH2), 32.07, 29.85,
29.81, 29.77, 29.55, 29.52, 29.49, 29.34, 29.24, 26.17, 26.14, 22.84
(CH2), 14.27 (CH3) ppm. IR (ATR): 휈̃ = 2922, 2852, 1734, 1607, 1496,
1466, 1389, 1272, 1216, 1141, 1010, 850, 755, 668 cm-1. FD-MS: m/z =
2435.7 [M]+.
2,3,5,6-Tetrakis(4-(5-(3,5-bis(tetradecyloxy)phenyl)-1,3,4-oxadiazol-
2-yl)phenyl)pyrazine 12m: According to general procedure B, 75 mg
(0.13 mmol) of 3 and 367 mg (0,642 mmol) of 11a were used. Column
chromatography: toluene/ethyl acetate 12:1; Yield: 252 mg (0,107 mmol,
82 %); Tg. 66 °C, c.p. 88 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ = 8.17
2,3,5,6-Tetrakis(4-(5-(3,4-bis(tetradecyloxy)phenyl)-1,3,4-oxadiazol-
2-yl)phen-yl)pyrazine 12i: According to general procedure B, 75 mg
(0.13 mmol) of 3 and 367 mg (0,642 mmol) of 6h were used. Column
chromatography: toluene/ethyl acetate 8:1; Yield: 251 mg (0,945 mmol,
72 %); m.p. -8 °C, c.p. 196 °C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
3
3
(d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H, Ph), 7.25 (d,
4JH-H = 2.2 Hz, 8H, Ph), 6.62 (t, JH-H = 2.3 Hz, 4H, Ph), 4.01 (t,
4
3JH-H = 6.5 Hz, 16H, α-OCH2), 1.87 – 1.70 (m, 16H, β-OCH2), 1.52 – 1.13
(m, 176H, CH2), 0.96 – 0.67 (m, 24H, CH3) ppm. 13C NMR (100 MHz,
25 °C, CDCl3): δ = 165.08 (Oxa), 164.19 (Oxa), 160.89, (Ph), 148.07
(Pyr), 140.86 (Ph), 130.75 (Ph), 127.29 (Ph), 125.19 (Ph), 124.74 (Ph),
105.37 (Ph), 105.28 (Ph), 68.59 (α-OCH2), 32.07, 29.84, 29.81, 29.75,
29.72, 29.53, 29.51, 29.34, 26.17, 22.84 (CH2), 14.27 (CH3) ppm. IR
(ATR): 휈̃ = 2921, 2851, 1597, 1547, 1442, 1388, 1164, 1011, 850, 728,
678 cm-1. FD-MS: m/z = 2260,8 [M]+.
3
3
8.16 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H, Ph), 7.73 –
3
7.58 (m, 8H, Ph), 6.96 (d, JH-H = 9.1 Hz, 4H, Ph), 4.14 – 3.98 (m, 16H,
α-OCH2), 1.91 – 1.78 (m, 16H, β-OCH2), 1.55 – 1.17 (m, 176H, CH2),
0.97 – 0.77 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.11 (Oxa), 163.80 (Oxa), 152.52 (Ph), 149.49 (Ph), 148.03 (Pyr),
140.69 (Ph), 130.72 (Ph), 127.16 (Ph), 124.89 (Ph), 120.61 (Ph), 116.18
(Ph), 112.95 (Ph), 111.72 (Ph), 69.57, 69.27 (α-OCH2), 32.08, 32.07,
29.87, 29.83, 29.78, 29.58, 29.56, 29.53, 29.35, 29.25, 26.18, 26.15,
22.85 (CH2), 14.28 (CH3) ppm. IR (ATR): 휈̃ = 2918, 2849, 1597, 1557,
1466, 1393, 1285, 1166, 1065, 844, 745, 721, 675 cm-1. HRMS (ESI):
calc. for C172H262N10O12 [M+2H+]: 1330.0075; found 1330.0099.
Acknowledgements
2,3,5,6-Tetrakis(4-(5-(3,4-bis(hexadecyloxy)phenyl)-1,3,4-oxadiazol-
2-yl)phenyl)pyrazine 12j: According to general procedure B, 40 mg
(0.071 mmol) of 3 and 215 mg (0,343 mmol) of 6i were used. Column
chromatography: toluene/ethyl acetate 12:1; Yield: 105 mg (0.0364 mmol,
51 %); m.p. 19 °C, c.p. 188°C; 1H NMR (400 MHz, 25 °C, CDCl3): δ =
T. M. and W. P. acknowledge the Polish National Research
Centre under the project code UMO-2015/18/E/ST3/00322; H.D.
acknowledges the Deutsche Forschungsgemeinschaft (DFG)
for generous financial support, M. Müller (DSC) and Dr. D.
Schollmeyer (XRD) for many measurements.
3
3
8.16 (d, JH-H = 8.5 Hz, 8H, Ph), 7.88 (d, JH-H = 8.5 Hz, 8H, Ph), 7.77 –
3
7.51 (m, 8H, Ph), 6.96 (d, JH-H = 9.0 Hz, 4H, Ph), 4.22 – 3.93 (m, 16H,
α-OCH2), 2.01 – 1.73 (m, 16H, β-OCH2), 1.55 – 1.10 (m, 208H, CH2),
0.97 – 0.64 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
165.11 (Oxa), 163.79 (Oxa), 152.52 (Ph), 149.48 (Ph), 148.02 (Pyr),
140.69 (Ph), 130.72 (Ph), 127.16 (Ph), 124.88 (Ph), 120.61 (Ph), 116.17
(Ph), 112.95 (Ph), 111.72 (Ph), 69.57, 69.27 (α-OCH2), 32.08, 29.87,
29.82, 29.78, 29.58, 29.52, 29.35, 29.25, 26.18, 26.15, 22.85 (CH2),
14.28 (CH3) ppm. IR (ATR): 휈̃ = 2917, 2849, 1607, 1496, 1467, 1389,
1272, 1218, 1141, 1010, 849, 721 cm-1. FD-MS: m/z = 2885.9 [M]+.
Conflict of interest
The authors declare no conflict of interest.
Keywords: pyrazine • liquid crystals • WAXS • POM •
solvatochromism
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Hz, 8H, Ph), 6.62 (t, JH-H = 2.3 Hz, 4H, Ph), 4.01 (t, JH-H = 6.5 Hz, 16H,
α-OCH2), 1.89 – 1.72 (m, 16H, β-OCH2), 1.53 – 1.18 (m, 114H, CH2),
0.94 – 0.83 (m, 24H, CH3) ppm. 13C NMR (100 MHz, 25 °C, CDCl3): δ =
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1465, 1441, 1389, 1298, 1165, 1098, 1054, 1011, 907, 850, 730, 679 cm-
1. FD-MS: m/z = 2211.7 [M]+.
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This article is protected by copyright. All rights reserved.

10.1002/cphc.201800936
ChemPhysChem
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10.1002/cphc.201800936
ChemPhysChem
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Tetraphenylpyrazine is the core of a
Discotic Liquid Crystals
RO
OR
new series of electron deficient
fluorophores. Fourfold Huisgen
reaction with tetrazoles generates the
RO
OR
Nico Röder, Tomasz Marszalek, Daniel
Limbach, Wojciech Pisula* and Heiner
Detert*
O
O
O
O
N
N
N
N
diaryl-1,3,4-oxadiazole arms with
alkoxy chains in the periphery. The
saddle-shaped molecules assemble to
columns. When alkoxy chains of 4 -
16 carbons are attached, the
molecules form broad mesophases
up to 246 °C and typically self-
assemble in hexagonal columnar
superstructures.
N
N
N
N
N
N
Page No. – Page No.
Tetrakis(oxadiazolylphenyl)pyrazines:
New St. Andrew's Cross-Shaped
Liquid Crystals
RO
OR
RO
OR
This article is protected by copyright. All rights reserved.