Mesogenic heterocycles formed by bis-pyrazoles and bis-1,3,4-oxadiazoles

Article abstract of DOI:10.1016/j.tet.2015.04.055

Three new series of compounds 1a-c containing multiple heterocyclic 2,5-pyrazoles, 1,3,4-oxadiazole and thiophene exhibiting mesophases were reported. Crystallographic data showed that the crystal 1a (n=8) is a linear-shaped molecule with a molecular leng

Full text of DOI:10.1016/j.tet.2015.04.055

Tetrahedron xxx (2015) 1e10
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Mesogenic heterocycles formed by bis-pyrazoles and bis-
1,3,4-oxadiazoles
y
*
Kuan-Ting Lin, Gene-Hsiang Lee , Chung K. Lai
Department of Chemistry, National Central University, Chung-Li, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new series of compounds 1aec containing multiple heterocyclic 2,5-pyrazoles, 1,3,4-oxadiazole
and thiophene exhibiting mesophases were reported. Crystallographic data showed that the crystal 1a
Received 1 March 2015
Received in revised form 14 April 2015
Accepted 18 April 2015
Available online xxx
ꢀ
(n¼8) is a linear-shaped molecule with a molecular length of ca. 40.57 A. However, three heterocyclic
rings were not coplanar. All compounds 1a formed smectic A/C phases, and all compounds 1c exhibited
columnar phases, which were confirmed by powder X-ray diffractometer (XRD). A Ncell and Rar value
ꢀ
ꢀ
equal to 3.91 and 18.29 A within a slice of 9.0 A thick were obtained for derivatives 1c (n¼8), indicating
that two molecule was correlated within columns in Colh phases. All derivatives showed good stabilities
at temperature below T¼383e426 ꢀC on thermogravimetric analysis (TGA). The PL spectra of all com-
pounds 1bec (n¼8) showed one intense peak at lmax¼428e496 nm, and these photoluminescent
emissions (PL) originated from 1,3,4-oxadiazole moiety.
Keywords:
Heterocycles
Bis-pyrazoles
Bis-1,3,4-oxadiazoles
Columnar
Ó 2015 Elsevier Ltd. All rights reserved.
Catenar
1. Introduction
quantum yields and good thermal and chemical stabilities. Many
studies have demonstrated their widespread uses as electron-
Recently, a variety of heterocyclic derivatives have been applied
to generate numerous organic electronic materials; these include
semiconductors with high carrier mobilities, light-emitting diodes,1
solar cells,2 field-effect transistors,3 and others. Heteroatoms, such
as nitrogen, oxygen, sulfur atoms incorporated into these hetero-
cyclic moieties, might impact some chemical or physical properties,
often not observed in their homologues all-carbon rings. All these
heteroatoms are more electronegative than carbons, and they all
have delocalized lone electrons. Mesogenic pyrazoles,4 1,3,4-
oxadiazoles,5 and thiophenes6 considered as a few of the most sta-
ble heterocyclic structures have been investigated in this group and
others during past years. Their inherent futures, such as electron
unsaturation, electron-deficiency, nonplanar core or lower sym-
metries, higher polarizabilities and others were often responsible
for their induction or formation of the mesophases. Pyrazoles are
particularly interested due to their capabilities and abilities of
forming H-bonds with neighboring molecules (eNH/N). H-Bonds,
though relatively weak in strength could be crucial, when a delicate
balance between intermolecular interactions is needed to induce
the mesophases. In contrast, it is well-known that 2,5-diary-1,3,4-
oxadiazoles derivatives (OXD) showed high photoluminescene
transporting/hole blocking materials, emitting layers in electrolu-
minescent diodes or for non-linear optical processes.
The effect of larger exocyclic angles caused by these heterocycles
was also crucial in determine the overall shapes needed to induce the
mesophases; ε w151ꢀ, w149ꢀ and w134ꢀ for 2,5-pyrazoles, 2,5-
thiophene, and 1,3,4-oxadiazoles, respectively. In general, the more
linear structures were favorable to the appearance of mesophases. A
few similar structures (see Scheme 1) of heterocyclic bis-pyrazoles I7
and bis-1,3,4-oxadiazoles II8eIII9 and IV10 were prepared and stud-
ied. Interestingly, crystallographic data of mesogenic compound Ia
(n¼14) and non-mesogenic Ib (n¼8) indicated that an extended H-
bonded structure was formed in both crystal lattices, giving a pseudo
1D-polymeric tape-like structure. Derivatives 1a exhibited smectic
A/C mesophases, in contrast, derivatives 1b were all nonmesogenic.
The difference in mesomorphic behavior was attributed to the be-
tween linear conformation and the coplanarity of the five rings over
than in Ia. Compound IIa formed crystal phases. Compounds IIb,
classified as hexacatenar-shaped liquid crystals (LCs) exhibited co-
lumnar phases, and a strong blue light emission (lmax w456 nm) was
observed with a higherquantumyields in dilute chloroform solution.
A polar bromo group incorporated in compound IIc8c led to an
asymmetric structure and greatly enhanced the formation of smectic
phases. The twin-tapered bis-1,3,4-oxadiazole derivatives IIIa
formed smectic phases. In contrast, compounds IIIb formed a layer-
structured lyotropic LC and further formed a helical fibrous organo
* Corresponding author. Tel.: þ886 03 4259207; fax: þ886 03 4277972; e-mail
address: cklai@cc.ncu.edu.tw (C.K. Lai).
y
Instrumentation Center, National Taiwan University, Taipei 10660, Taiwan, ROC.
http://dx.doi.org/10.1016/j.tet.2015.04.055
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
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K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
NH
N
OR
RO
RO
OR
HN
N
N
N
NH
N
HN
N
Ia, SmA/SmA
Ib, Cr
Br
X
N
N
RO
RO
X
X
O
O
O
O
OR
OR
N
N
IIc; SmA/SmC
N
N
X
IIa; X = H, Cr
IIb; X = OR, Col
X
RO
X
N
N
O
X
O
N
N
OR
X
N
IIIa; X = H, SmC
IIIb; X = OR, Col
N
R1O
RO
OR
O
N
N
N
OR2
N
O
N
O
O
N
IVa, Cr
IVb, Cr
Scheme 1. Examples of known bis-heterocyclic structures IeIV.
gel in DMF at concentrations above 0.6 wt %. The self-assembly
process of IIIb in DMF was accompanied by a change in its fluores-
cence. The pitches of the helical fibers are non-uniform, and both
left- and right-handed helical fibers are observed in equal quantities.
diketones 2a were synthesized by double condensation of di-
methyl thiophene-2,5-dicarboxylate, 1-(4-(octyloxy)phenyl)etha-
none and NaH in refluxing dry THF for 6 h under nitrogen
atmosphere. The yields were ranged from 55 to 60%. On 1H NMR
Intermolecular
pe
p
interactions between aromatic segments have
spectra, two singlet peaks appeared at d 6.67 and d 7.96 ppm is
been demonstrated to be the driving force for aggregate formation. A
few star-shaped derivatives11 made by tris-1,3,4-oxadiazoles exhib-
iting hexagonal columnar phases were also know. However, meso-
genic bis-1,2,4-oxadiazoles12 were relatively rare; compounds IV
derived from 1,4-benzene or 1,3-benzene were not mesogenic.10
Here we wish to report the preparation, characterization, and
mesomorphic studies of three series of heterocyclic bis-pyrazoles
1a and bis-1,3,4-oxadiazoles 1bec. The mesogenic examples6 de-
rived from thiophenes were relatively rare. The central core of 5-
membered thiophene was incorporated to study the effect of me-
somorphic behavior exhibited by other all-carbon 6-membered
rings. An exocyclic angle6a of 149.2ꢀ and 130.3ꢀ, respectively, for
2,5-thiophere and 1,3-benzene of bent-core mesogens was ob-
tained by high-resolution solid state 13C NMR studies. All bis-
pyrazoles 1a were mesogenic, giving smectic A or/and smectic C
phases. All compounds 1c with six alkoxy chains exhibited hex-
agonal columnar phases, whereas, all compounds 1b formed glass
states. All bis-pyrazoles 1a have a relatively high clearing temper-
ature. Their optical properties are also discussed.
characteristically assigned for olefiniceH (eCOH]CH) and thio-
pheneeH. The bis-pyrazoles 1a, isolated as white solids were ob-
tained by the reactions of bis-diketones 2a with excess hydrazine
monohydrate in refluxing THF/methanol. The solubility of all bis-
pyrazoles 1a were poor in THF or chloroform at room tempera-
ture, therefore, most of final pyrazoles were only characterized by
element analysis.
The formation of final pyrazoles was confirmed by a character-
istic peak appeared at
(eNH]C). The peak of thiopheneeH was also shifted to
d
12.22 ppm assigned to the pyrazoleeH
7.27 ppm.
d
On the other hand, bis-13,4-oxadiazoles 1b were prepared by
condensation reactions of N02, N05-bis(4-(alkoxy)benzoyl)thio-
phene-2,5-dicarbohydrazides 2b was refluxed in phosphoryl chlo-
ride for 24 h. The final compounds 1b, isolated as bright white-
green solids were obtained after recrystallization from DCM/
MeOH. Compounds 1b were all characterized by either 1H or 13C
NMR spectroscopy in CDCl3. On 13C NMR spectra, the observation of
peaks occurred, for example, at 162.27 and 164.60 ppm, and also at
164.8 ppm was indicative of the formation of compounds 1b (n¼8)
and 1c (n¼8), respectively.
2. Results and discussions
2.2. Single crystal and molecular structures of 2,5-bis(5-(4-
2.1. Synthesis and characterization
(octyloxy)phenyl)-1H-pyrazol-3-yl)thiophene 1a (n[8)
The synthetic routes used to prepare bis-pyrazoles 1a and bis-
1,3,4-oxadiazoles 1bec are listed in Scheme 2. The yellow bis-
In order to understand the correlation between the molecular
structure and mesomorphic behavior, a single crystal of the
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K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
3
X
HO
RO
X
OCH3
O
O
d
a
X
RO
X
RO
NHNH2
O
O
O
e
b
X
X
O
OH
OH O
O
RO
H
S
S
H
N
OR
N
N
N
H
H
X
OR
RO
O
X
O
2a
2b; X = H
2c; X = OR
c
f
N N
O
N N
O
X
HN N
N NH
X
S
S
RO
OR
RO
OR
X
X
R = CnH2n+1
1b; X = H
1c; X = OR
1a; R = CnH2n+1
Scheme 2. Reactions and reagents. (a) RBr, K2CO3 and KI, refluxing in dry acetone, 24 h, 91e95%; (b) NaH, dimethyl thiophene-2,5-dicarboxylate, refluxing in dry THF, 6 h, 55e60%;
(c) hydrazine monohydrate, acetic acid, refluxing in THF, MeOH, 24 h, 83%; (d) hydrazine monohydrate, refluxing in EtOH, 48 h, 65e70%; (e) thiophene-2,5-dicarboxylic acid and
SOCl2, refluxing in dry THF, 2 h; then, triethylamine, stirring in dry THF at rt, 24 h, 82e86%; (f) refluxing in POCl3, 24 h, 78%.
Fig. 1. An ORTEP plot for compound 1a (n¼8). The thermal ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.
mesogenic compound 1a (n¼8) suitable for crystallographic anal-
ysis was obtained by slow vaporization from THF at room tem-
perature and its structure resolved. Fig. 1 shows its molecular
structure with the atomic numbering schemes. Table 1 lists its
crystallographic and structural refinement data for the crystal. It
crystallizes in a triclinic space group P-1 with a Z¼2. The overall
molecular shape of the crystal 1a (n¼8) was considered as a linear-
withdrawing nitrogen atoms (d d
ꢁ-N) in central ring and þ-H. A
summary of intermolecular H-bonded interactions observed in 1a
(n¼8) were listed in Table 2. All molecules were tilt arranged in the
unit cell (Fig. 3).
2.3. Mesomorphic properties and thermal stabilities
ꢀ
shaped, and the molecular length was ca. 40.57 A (C38eC30).
The thermal behavior of compounds 1aec was investigated by
polarized optical microscopy (POM) and differential scanning cal-
orimetry (DSC). The transition temperatures and enthalpies of the
compounds are listed in Table 3. All compounds 1a and 1c are
mesogenic, whereas, all compounds 1b were not mesogenic. All
compounds 1a exhibited smectic A or/and smectic C phases, which
were all kinetically stable enantiotropic. Most of reported meso-
genic pyrazolyl derivatives showed higher clearing temperatures
than other similar heterocyclic derivatives, such as isozazoles or
1,3,4-oxdiazoles. For example, mesogenic 1,4-bis(5-(4-(octyloxy)
phenyl)-1H-pyrazol-3-yl)benzene has a higher clearing tempera-
ture at Tcl¼374.5 ꢀC. This series of compounds 1a was no exception.
The relatively higher clearing temperatures were often attributed
to the capabilities and abilities of forming H-bonds with
However, three heterocyclic rings were not coplanar; a total of di-
hedral angles; w26.52ꢀ between the two pyrazolyl rings were ob-
tained. The cause for the noncoplanarity might be attributed to the
intermolecular interactions observed in the unit cell. Three H-
bonds were formed. Two weak H-bonds between atoms N2eH1
ꢀ
ꢀ
(dw2.18 A) and atoms O2eH3 (dw2.82 A) were observed on the
same layer. The other H-bond is formed between intermolecular
ꢀ
pyrazole N3eH4 (dw2.17 A). All molecules were arranged by these
three H-bonds to lead to a pseudo polymeric supramolecular
structure. All three H-bonds were quite weak, whereas they were
very important interaction in forming enantiotropic meso-
morphism in this type of pyrazolyl structure. Such interaction
might be induced by an interaction between the two electron-
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4
K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
ꢀ
ꢀ
Fig. 2. The correlated structures induced by intermolecular H-bonds observed viewed from different axes in 1a (n¼8). Top plot: H3/O2¼2.82 A, H1/N2¼2.18 A; bottom:
ꢀ
H4/N3¼2.17 A.
Table 1
Crystallographic and experimental data for crystal 1a (n¼8)
between Tm¼295.5 ꢀC (n¼8) e 260.7 ꢀC (n¼16), while clearing
temperatures were ranged between Tcl¼346.0 ꢀC (n¼8) e 301.0 ꢀ
C
Compd
1a (n¼8)
(n¼16). On the other hands, the temperature range of mesophase
Empirical formula
Formula weight
T/K
C38H48N4O2S
remained at
D
TM¼41.1e52.7 ꢀC during the cooling process. Under
624.86
200(2)
Triclinic
P-1
11.5430(11)
11.5688(11)
14.8260(15)
71.586(2)
68.200(2)
89.975(2)
1728.7(3)
2
optical microscope, compounds 1a (n¼8) exhibited fan-shaped
textures at higher temperatures and Schlieren textures at lower
temperatures (Fig. 4). These two textures were characteristic of
a layer smectic A and C phase expected for the rod-like molecules.
Under optical microscope, a large area of homeotropic domains was
also observed for SmA phase (Fig. 5).
Crystal system
Space group
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
ꢀ
ꢀ
a
/
b
/
Interestingly, all compounds 1b were in fact nonmesogenic and
two phase transitions of glass-to-isotropic occurred at Tcl¼244.1 ꢀC
(n¼8) and 230.4 ꢀC (n¼12) were observed. They formed glass states
upon cooling from their isotropic states. Both series of compounds
are structurally similar and the lack of H-bonds by 1b needed to
induce their mesophases was probably the main reason. Also, the
clearing temperatures were much lower than those of compounds
1a by 101.9e92.6 ꢀC. In contrast, increasing the numbers of chains
to six alkoxy in series of compounds 1c led to the formation of
columnar phase. These two compounds are not discotic in overall
molecular shapes, but more catenar-shaped molecules. The for-
mation of columnar phases formed by tetra-catenar or hexa-
catenar heterocyclic derivatives was also known and reported in
this group and others. All clearing temperatures were dramatically
lowered to Tcl¼69.3e76.9 ꢀC. However, the temperature range of
ꢀ
/
g
3
ꢀ
U/A
Z
F(000)
672
1.200
Dc/mg mꢁ3
Crystal size/mm3
Range for data collection/0
Reflection collected
Data, restraints, parameters
Independent reflection
Final R1, wR2
0.32*0.12*0.10
1.57 to 25.00
18,614
6098, 4, 414
6098 [R(int)¼0.0647]
0.0744, 0.1431
Table 2
A summary of intermolecular H-bond interactions observed in 1a (n¼8)
columnar phases were ranged from
D
TCol¼73.3 (n¼12)>44.7 (n¼8)
Intermolecular H-bonds
on the cooling process (Fig. 2). An optical texture, described as
focal-conic texture with linear birefringent defects when slowly
cooled from their isotropic liquids was observed, as shown in Fig. 4.
The mesophases of both compounds 1c (n¼8) was identified by
powder XRD experiments as hexagonal columnar phase.
No. of H-bonds (CeH/N)
Bond distance, angles
2
ꢀ
ꢀ
ꢀ
N2eH1¼2.18 A, :N1eH1eN2¼137.86
ꢀ
N3eH4¼2.17 A, :N3eH4eN4¼131.11
No. of H-bonds (CeH/O)
Bond distance, angle
1
ꢀ
ꢀ
O2eH3¼2.82 A, :O2eH3eC3¼154.41
The thermal stability of compounds 1aec (n¼8) and 2a (n¼8)
was also performed by thermogravimetric analyses (TGA) under
nitrogen atmosphere, shown in Fig. 6. All four compounds showed
good thermal stability at temperature below ca. 360.0 ꢀC, with
a relative thermal stability of 1a>1c>1b>2a. The decomposition
temperatures for a 5% weight loss were listed in Table 4. The de-
composition temperature of 1a is much higher than that of 1b by ca.
neighboring molecules, as confirmed by single crystal structural
analysis discussed above. Both melting and clearing temperatures
were relatively high, and both crystallization temperatures and
clearing temperatures of compounds 1a decreased with carbon
lengths of alkoxy chains. The melting temperatures were ranged
D
T¼43.4 ꢀC, which is quite consistent with the DSC data of these
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K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
5
Fig. 3. The molecular arrangements in the unit cell by 1a (n¼8).
Table 3
The phase transitions and enthalpiesa of compounds 1ae1c
346.0 (10.1)
344.7 (9.25)
295.5 (33.0)
340.4 (1.01)
340.0 (2.30)
Cr
SmC
I
I
I
I
SmA
SmC
1a; n = 8
n = 10
292.0 (30.8)
333.3 (12.9)
330.4 (6.66)
284.8 (34.6)
281.2 (26.0)
Cr
Cr
275.1 (25.4)
272.2 (23.6)
323.0 (10.2)
322.2 (8.07)
n = 12
SmC
SmC
SmC
G
309.8 (8.99)
305.0 (6.15)
268.0 (21.9)
263.9 (20.1)
n = 14
Cr
Cr
260.7(30.7)
258.0 (28.8)
301.0 (10.7)
300.5 (10.2)
I
n = 16
1b; n = 8
n = 12
244.1 (32.5)
236.9 (32.3)
I
I
230.4 (39.0)
226.3 (39.0)
G
69.3 (1.79)
66.4 (1.80)
45.9 (20.8)
21.7 (10.0)
1c; n = 8
n = 12
Cr
Cr
Col
Col
I
I
76.9 (3.36)
75.3 (3.19)
57.3 (90.3)
2.0 (12.3)
a: n = the carbon numbers of alkoxy chains; Cr = crystal; G = glass, SmA = smectic A, SmC =
smectic C, Col = columnar, I = isotropic phases measured at a scan rate of 100C/min.
two compounds. The clearing temperature of compound 1a (n¼8)
observed for this compound, as shown in Fig. 7. In contrast, com-
pounds 1c formed columnar phases. On XRD plot, a typical dif-
fraction pattern of a two-dimensional hexagonal lattice with one
very strong diffraction peak at lower angle and three much weaker
diffraction peaks of compound 1c (n¼8) was obtained at 65.0 ꢀC
under cooling process, shown in Fig. 7. A diffraction pattern with
is higher than that of compound 1b (n¼8) by
D
Tcl¼101.9 ꢀC.
2.4. Powder X-ray diffractions and packing arrangements
Variable-temperature powder XRD diffraction experiments
were performed to confirm the structure of the mesophases of
compounds 1a (n¼14) and 1c (n¼8), and a summary of diffraction
dataꢀwas listed in Table 5. For example, for compounds 1a (n¼14) at
ꢀ
a d-spacing at 27.13, 15.97, and 13.89 A, and a broad diffuse peak
ꢀ
4.35 A at wide-angle region was obtained. This diffraction pattern
corresponded to a hexagonal columnar arrangement with a d-
spacing ratio of 1, (1/3)1/2, and (1/4)1/2, corresponding to Miller
indices: 100, 110 and 200, respectively. This diffraction pattern
corresponded to an intercolumnar distance or lattice constant (i.e.,
ꢀ
265 C a diffraction pattern with one strong peak at 39.22 A and one
ꢀ
very weak peak at 19.81 A at small angle region, was observed, as
shown in Fig. 7. These two diffraction peaks at lower angle were
assigned as 001 and 002, which were typically characteristics of
layer structures observed for a SmC phase. A layer d-spacing of
ꢀ
a parameter of the hexagonal lattice) of 31.33 A. The lack of any
relatively peaks at wide angles excluded a more regular periodicity
along the columns. However, liquid-like correlations between the
ꢀ
ꢀ
39.22 A, compared to 58.0 A obtained for the molecular length of
the dimeric structure calculated by MM2 indicated that all mole-
cules were possibly tilt or/and the alkoxy carbon chains were in-
ꢀ
rigid cores occurred at wide angle regions of 4.35 A.
Both values of Ncell and Rar were also obtained from lattice pa-
rameters of X-ray diffraction as shown in Table 6. These two values
ꢀ
terdigitated. A broad halo peak appeared at 4.80 A was also
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6
K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
Fig. 4. Optical textures of observed by compounds: SmA phase by 1a (n¼8, top left) at 344 ꢀC; SmC phase by 1a (n¼8, top right) at 335 ꢀC; columnar phase by 1c (n¼8, bottom left)
at 60 ꢀC and columnar phase by 1c (n¼12, bottom right) at 63 ꢀC.
Table 4
The decomposition temperaturesa of compounds 1aec and 2a by TGA analysis
Compds
Tdec (ꢀC)
1a (n¼8)
1b (n¼8)
1c (n¼8)
2a (n¼8)
426.7
383.3
387.5
367.5
a
Temperatures taken with a 5% weight loss under nitrogen atmosphere.
Table 5
Detailed indexation by powder XRD diffraction for compounds 1a, 1c
Compd
Mesophases tempa
d-Spacing obs.
(calcd)
Lattice
const. (A)
Miller
indices
ꢀ
1a (n¼14)
SmC at 260.0 ꢀ
Col at 65 ꢀC
C
39.22 (39.22)
19.81 (19.61)
4.80 (br)
27.13 (27.63)
15.97 (15.94)
13.89 (13.81)
4.35 (br)
001
002
Halo
100
110
200
Halo
Fig. 5. Bar graphs showing the phase behavior of compounds 1aec. All temperatures
were taken on the cooling process.
1c (n¼8)
a¼31.33
a
Temperature taken on the cooling process.
100
1a (n = 8)
were useful to understand the possible molecular packing in the
columnar phases. The detail calculations were not discussed here. A
value of Ncell¼3.91 was obtained for compounds 1c (n¼8) within
1b (n = 8)
1c (n = 8)
2a (n = 8)
75
50
25
0
ꢀ
a height of 9.0 A in the columnar phase. The overall molecular
shapes of compounds 1c were more catenar-shaped. Therefore,
a more disc-like correlated structure constructed by two molecules
lying side-by-side was generated within the column. The possible
molecular arrangement in columnar phases was proposed in Fig. 8.
2.5. Optical properties
100
200
300
400
500
600
700
The UVevis absorption and PL spectra of the compounds 1aec
(all n¼8) measured in CH2Cl2 solution at room temperature are
presented in Fig. 9 and data presented in Table 7. These compounds
containing three adjacent heterocyclic moieties are particularly
Temperature(oC)
Fig. 6. The TGA thermographs of compounds 1aec and 2a (all n¼8) under nitrogen gas
at a heating rate of 10.0 ꢀC minꢁ1
.
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K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
7
3000
2000
1000
compounds IIa and IIc, respectively. An apparent red-shift of ca.
w68 nm by compound 1c over 1b was also observed, which might
be attributed to the more electron-donating group of six alkoxy
chains in 1c. All quantum yields of luminescent materials were
ranged from 14 to 25%.
39.2
19.8
3. Conclusions
Three new series of bis-pyrazoles 1a and bis-1,3,4-oxadiazoles
1bec were prepared and studied; all compounds 1a formed
smectic A and/or C phases, and other compounds 1c exhibited
hexagonal columnar phases. Compounds 1a exhibited an improved
mesomorphic behavior than other similar bis-pyrazoles Ib, which
might be attributed to the larger exocyclic angle by central 2,5-
thiophene (εw149ꢀ) than homologues replaced by 1,3-benzene
(εw120ꢀ) in Ib. An apparent red-shift in PL wavelength by such
bis-1,3,4-oxadiazoles 1bec compared to other similar mono-1,3,4-
oxadiazoles due to their conjugated structures.
4.8
0
0
10
20
30
2 theta
9000
27.1
6000
3000
0
4. Experimental section
4.1. General materials and methods
15.7
13.6
All chemicals and solvents were reagent grade from Aldrich
Chemical Co., and solvents were dried by standard techniques. 1H
and 13C NMR spectra were measured on a Bruker DRS-300. DSC
thermographs were carried out on a Mettler DSC-822 and cali-
brated with a pure indium sample. All phase transitions are de-
4.5
0
10
20
30
termined by
a
scan rate of 10.0 ꢀC/min. Optical polarized
2 theta
microscopy was carried out on Zeiss Axioplan 2 equipped with
a hot stage system of Mettler FP90/FP82HT. The UVevis absorption
and fluorescence spectra were obtained using a Jasco V-530 and
Hitachi F-4500 spectrometer. All spectra were measured in CH2Cl2
at room temperature, and the excitation wavelengths of the fluo-
rescent spectra were 303 nm (for 1a), 354 nm (for 1b) and 365 nm
(for 1c). Elemental analyses were performed on a Heraeus CHN-O-
Rapid elemental analyzer. The powder diffraction data were col-
lected from the Wiggler-A beam line of the National Synchrotron
Fig. 7. Powder X-ray diffraction plots of compounds; 1a (n¼14; top plot) at 260.0 ꢀC
and 1c (n¼8; bottom plot) at 65.0 ꢀC when cooled from above their clearing
temperatures.
Table 6
Geometric parametersa of compound 1c (n¼8)
2
3
ꢀ
3
2
ꢀ
S (A )
Vcell (A ) Vm (A ) Temp (ꢀC) Ncell Sar (A ) Rar (A)
ꢀ
ꢀ
ꢀ
Compd
ꢀ
Radiation Research Center (NSRRC) with a wavelength of 1.3223 A.
1c (n¼8) 849.90 7649.12 1955.39 65.0
3.91 262.61 18.29
The powder samples were charged in Lindemann capillary tubes
(80 mm long and 0.01 mm thick) from Charles Supper Co. with an
inner diameter of 1.0 mm.
a
2
3
ꢀ
ꢀ
ꢀ
S (A )¼columnar cross section area, Vcell (A )¼volume of the column stratum 9 A
3
ꢀ
thick, Vm (A )¼molecular volume, Ncell¼the number of molecules contained in each
2
ꢀ
ꢀ
columnar stratum 9-A thick, Sar (A )¼the surface area of hard columnar core, Rar
2
ꢀ
(A )¼the diameter of the aromatic part.
4.2. Synthesis of the compounds 1aec
4.2.1. 1-(4-(Octyloxy)phenyl)ethanone. 1H NMR (300 MHz, CDCl3):
interesting due to the presence of multiple chromophores and
d
0.82 (t, 3H, eCH3, J¼6.9 Hz), 1.21e1.77 (m, 10H, eCH2), 1.74e1.84
possible
p-conjugations between the peripheral structures and
(m, 2H, eCH2), 2.50 (s, 3H, eCH3), 3.97 (t, 2H, eOCH2), 6.88 (d,
AreH, 2H, J¼8.7 Hz), 7.89 (d, AreH, 2H, J¼8.7 Hz). 13C NMR
central core. The lmax peaks of UVevis absorption and the lmax of PL
spectra were listed in Table 7. The absorption lmax peaks of com-
pounds 1aec occurred at ca. 303e365 nm, which were attributed
(75 MHz, CDCl3):
d 13.99, 22.58, 25.87, 26.07, 29.02, 29.25, 29.45,
to
contrast, absorption lmax peaks at ca. 261e275 nm were attributed
to the * transitions arising from the thiophene. The lmax for
pep
* transitions arising from the pyrazoles and oxadiazole.13 In
31.79, 68.10, 113.98, 129.98, 163.00, 196.30.
pep
4.3. (2Z,20Z)-3,30-(Thiophene-2,5-diyl)bis(3-hydroxy-1-(4-(oc-
compound 1a occurred at 380 nm, and its intensity was relatively
weak. The PL spectra of all compounds 1bec showed one intense
and broad peak occurred at lmax¼428 and 496 nm, respectively,
and these photoluminescent emission originated from heterocyclic
bis-pyrazoles 1a and bis-1,3,4-oxadiazoles 1bec. Compounds 1bec
are potentially better blue-emitters. In general, a red-shift emission
is often expected in this type of conjugated systems. However, the
two lmax peaks observed by bis-oxadiazoles1bec were much dif-
ferent from those of other similar mono-1,3,4-oxadiazoles. Central
thiophene is often considered as a more electron-donating group;
lmax for compounds 1b and 1c was slightly red shifted by ca.
w31and w40 nm when compared with the similar all-carbon
tyloxy)phenyl)prop-2-en-1-one) 2a (n[8)
The solution 1-(4-(octyloxy)phenyl)ethanone (1.0 g, 0.004 mol)
and NaH (0.16 g, 0.012 mol) dissolved in 50 mL of dry THF heated to
40 ꢀC for 30 min under nitrogen atmosphere. The solution turned
deep red. Then dimethyl thiophene-2,5-dicarboxylate (0.41 g,
0.004 mol) was added and then the solution was refluxed for 6 h
under nitrogen atmosphere. The solution was cooled and neutral-
ized with aqueous 1.0 M hydrochloric acid. The solution was twice
extracted with 100 mL of dichloromethane, and the organic layers
were collected and dried. The product, isolated as yellow solids was
obtained after recrystallization from THF/MeOH. Yield 60%.
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8
K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
Fig. 8. A schematic representation of molecular organization proposed in columnar phase by compounds 1c (n¼8).
Table 7
0.8
0.6
0.4
0.2
0.0
The UVevis absorption and PL emission dataa of compound 1
1a
1b
1c
Compds
Absorption lmax (nm)
Emission lmax (nm)
FF
1a (n¼8)
261, 303
266, 354
275, 365
329, 339
270, 340
335
380
428
496
397
456
436
14.0
25.2
24.1
1b (n¼8)
1c (b¼8)
IIa (n¼8)8d
IIb (n¼8)8b
IIc (n¼8)8c
a
Measured in CH2Cl2 solution (5.0ꢂ10ꢁ6 M) at rt. The standard is anthracene
(
F
fs¼0.27 in hexane solution) and
n is reflective indices of solvents;
nCH2Cl2¼1.42440, nhexane¼1.37506. Also see Refs. 8bed for optical data of com-
pounds IIaec.
300
400
500
600
Wavelength (nm)
CDCl3): d 14.60, 23.72, 24.96, 25.22, 25.49, 25.76, 26.02, 27.13, 30.30,
30.41, 30.50, 32.98, 69.18, 93.05, 115.44, 127.76, 130.27, 131.25,
147.91, 164.46, 180.99, 184.34.
1a
1b
1c
800
700
600
500
400
300
200
100
0
4.3.1. 2,5-Bis(5-(4-(octyloxy)phenyl)-1H-pyrazol-3-yl)thiophene 1a
(n¼8). The solution of (2Z,20Z)-3,30-(thiophene-2,5-diyl)bis(3-
hydroxy-1-(4-(octyloxy)phenyl)prop-2-en-1-one)
(1.00
g,
2.0 mmol) dissolved in 50 mL THF was slowly added 0.5 mL of
glacial acetic acid and the solution was stirred for 5 min. Then
hydrazine (1.0 mL) was added and then refluxed for 24 h. The so-
lution was concentrated to dryness. The product, isolated as white
solid was obtained after recrystallization from THF/MeOH. Yield
83%, 1H NMR (300 MHz, d-THF):
d 0.87e0.90 (m, 6H, eCH3),
300
400
500
600
700
800
1.31e1.49 (m, 20H, eCH2), 1.75e1.81 (m, 4H, eCH2), 3.99 (t, 4H,
eOCH2, J¼6.3 Hz), 6.74 (s, 2H, eCH]C), 6.95 (d, 4H, AreH,
J¼8.4 Hz), 7.27 (s, 2H, thiopheneeH), 7.63 (d, 4H, AreH, J¼8.4 Hz),
12.22 (s, 2H, eNH]C). Anal. Calcd for C38H48N4O2S: C, 73.04; H,
7.74. Found C, 72.95; H, 7.74. MS (HRFAB): calcd for Mþ: 624.3498.
Found: 624.3502.
Wavelength (nm)
Fig. 9. Absorption (top), PL spectra (central) and PL normalized spectra (bottom) of the
compounds 1aec.
1H NMR (300 MHz, CDCl3):
d 0.87e0.92 (m, 6H, eCH3), 1.31e1.51
(m, 20H, eCH2), 1.77e1.83 (m, 4H, eCH2), 4.06 (t, 4H, eOCH2,
J¼6.3 Hz), 6.67 (s, 2H, eCH]C), 7.01 (d, 4H, AreH, J¼7.2 Hz), 7.96 (s,
2H, thiopheneeH), 8.03 (d, 4H, AreH, J¼7.2 Hz). 13C NMR (75 MHz,
4.3.2. 2,5-Bis(5-(4-(decyloxy)phenyl)-1H-pyrazol-3-yl)thiophene 1a
(n¼10). NMR data are not available due to its poor solubility. Anal.
Please cite this article in press as: Lin, K.-T.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.055

K.-T. Lin et al. / Tetrahedron xxx (2015) 1e10
9
Calcd for C42H56N4O2S: C, 74.08; H, 8.29. Found C, 73.89; H, 8.28. MS
(HRFAB): calcd for Mþ: 680.4124. Found: 680.4128.
4.6.1. 2,5-Bis(5-(4-(octyloxy)phenyl)-1,3,4-oxadiazol-2-yl)thiophene
1b (n¼8). The solution of N02, N05-bis(4-(octyloxy)benzoyl)thio-
phene-2,5-dicarbohydrazide (1.0 g, 2.0 mmol) dissolved in 40 mL of
phosphoryl chloride was refluxed for 24 h. The solution slowly
turned reddish-black in color. The solution was cooled at room
temperature, and then the solution was slowly poured into 300 mL
of ice water. After stirring for 2 h, the orange-red solids were fil-
tered and collected. The solids were dissolved in 25 mL of
dichloromethane and extracted with 150 mL of 1.0 M NaOH(aq). The
products, isolated as yellow-green solids were obtained after re-
crystallization from DCM/MeOH. Yield 80%, 1H NMR (300 MHz,
4.3.3. 2,5-Bis(5-(4-(dodecyloxy)phenyl)-1H-pyrazol-3-yl)thiophene
1a (n¼12). NMR data are not available due to its poor solubility.
Anal. Calcd for C46H64N4O2S: C, 74.96; H, 8.75. Found C, 74.85; H,
8.77. MS (HRFAB): calcd for Mþ: 736.4750. Found: 736.4745.
4.3.4. 2,5-Bis(5-(4-(tetradecyloxy)phenyl)-1H-pyrazol-3-yl)thio-
phene 1a (n¼14). NMR data are not available due to its poor sol-
ubility. Anal. Calcd for C50H72N4O2S: C, 75.71; H, 9.15. Found C,
75.41; H, 9.12. MS (HRFAB): calcd for Mþ: 792.5376. Found:
792.5377.
CDCl3):
d
0.87 (t, 6H, eCH3, J¼6.9 Hz), 1.27e1.46 (m, 20H, eCH2),
1.75e1.82 (m, 4H, eCH2), 4.01 (t, 4H, eOCH2, J¼6.3 Hz), 7.00 (d, 4H,
AreH, J¼9 Hz), 7.81 (s, 2H, thiopheneeH), 8.02 (d, 4H, AreH,
4.3.5. 2,5-Bis(5-(4-(hexadecyloxy)phenyl)-1H-pyrazol-3-yl)thio-
phene 1a (n¼16). NMR data are not available due to its poor sol-
ubility. Anal. Calcd for C54H80N4O2S: C, 76.37; H, 9.49. Found C,
76.25; H, 9.46.
J¼9 Hz). 13C NMR (75 MHz, CDCl3):
d 14.08, 22.64, 25.98, 29.10,
29.21, 29.32, 31.79, 68.10, 115.06, 115.45, 128.83, 129.71, 159.43,
162.27, 164.61. Anal. Calcd for C36H44N4O4S: C, 68.76; H, 7.05. Found
C, 67.96; H, 7.01.
4.6.2. 2,5-Bis(5-(4-(dodecyloxy)phenyl)-1,3,4-oxadiazol-2-yl)thio-
4.4. 4-(Octyloxy)benzohydrazide
phene 1b (n¼12). 1H NMR (300 MHz, CDCl3):
d 0.86 (t, 6H, eCH3,
J¼6.9 Hz), 1.25e1.46 (m, 36H, eCH2), 1.78e1.81 (m, 4H, eCH2), 4.03
(t, 4H, eOCH2, J¼6.3 Hz), 7.01 (d, AreH, 4H, J¼9 Hz), 7.84 (s, 2H,
thiopheneeH), 8.04 (d, AreH, 4H, J¼9 Hz). 13C NMR (75 MHz,
The solution of methyl-4-(dodecyloxy)benzoate (5.0 g,
0.02 mol) dissolved in 100 mL of MeOH was added hydrazine
monohydrate (3.1 g, 0.2 mol). The solution was refluxed for 12 h.
The solution was concentrated to give off-white solids. The prod-
ucts, isolated as white solids were obtained after recrystallization
CDCl3): d 14.11, 22.68, 25.98, 29.10, 29.35, 29.63, 31.91, 68.33, 115.06,
115.44, 128.83, 129.71, 159.43, 162.27, 164.61. Anal. Calcd for
44H60N4O4S: C, 71.32; H, 8.16. Found C, 71.25; H, 8.16.
from hexane/MeOH. Yield 90%, 1H NMR (300 MHz, CDCl3):
d 0.86 (t,
C
3H, eCH3, J¼5.7 Hz), 1.24e1.42 (m, 10H, eCH2), 1.73e1.78 (m, 4H,
eCH2), 4.00 (t, 2H, eOCH2, J¼6.3 Hz), 6.87 (d, 2H, AreH, J¼8.4 Hz),
7.69 (d, 1H, AreH, J¼8.4 Hz), 7.77 (s, 1H, NeH). 13C NMR (75 MHz,
4.6.3. 2,5-Bis(5-(3,4,5-tris(octyloxy)phenyl)-1,3,4-oxadiazol-2-yl)
thiophene 1c (n¼8). 1H NMR (300 MHz, CDCl3):
d 0.87 (t,18H, eCH3,
J¼6.9 Hz), 1.23e1.49 (m, 60H, eCH2), 1.76e1.86 (m, 12H, eCH2),
CDCl3): d 14.08, 22.65, 25.94, 29.07, 29.32, 29.54, 29.60, 31.87, 68.16,
4.01e4.10 (m, 12H, eOCH2), 7.29 (s, 4H, AreH), 7.87 (s, 2H, thio-
114.34, 124.51, 128.63, 129.23, 162.05, 168.36.
pheneeH). 13C NMR (75 MHz, CDCl3):
d 14.08, 22.66, 26.06, 29.34,
29.50, 30.33, 31.81, 31.88, 69.43, 73.65, 105.55, 117.79, 128.81,
129.89, 141.75, 153.66, 159.70, 164.80. Anal. Calcd for C68H108N4O8S:
C, 71.54; H, 9.53. Found C, 71.61; H, 9.60.
4.5. N02, N05-Bis(4-(octyloxy)benzoyl)thiophene-2,5-
dicarbohydrazide 2b (n[8)
The solution of thiophene-2,5-dicarboxylic acid (1.0 g,
6.0 mmol) and 2.0 mL of thionyl chloride (0.016 mol) was gently
refluxed in dry THF for 4 h under nitrogen atmosphere. The excess
of thionyl chloride was removed under reduced vacuum. The
compound prepared was then used directly for the following step.
The solution of 4-(octyloxy)benzohydrazide (3.2 g, 0.012 mol) dis-
solved in 60 mL of THF was dropwise added 0.14 mL of triethyl-
amine (1.0 mmol) at ice bath. The solution was stirred at rt for 24 h.
The solution was extracted twice with 100 mL of CH2Cl2/H2O. The
organic layers were combined and concentrated to give white
solids. The products, isolated as white solids were obtained after
recrystallization from THF/MeOH. Yield 83%, 1H NMR (300 MHz,
4.6.4. 2,5-Bis(5-(3,4,5-tris(dodecyloxy)phenyl)-1,3,4-oxadiazol-2-yl)
thiophene 1c (n¼12). 1H NMR (300 MHz, CDCl3):
d 0.86 (t, 18H,
eCH3, J¼6.9 Hz), 1.24e1.57 (m, 108H, eCH2), 1.70e1.88 (m, 12H,
eCH2), 4.01e4.08 (m, 12H, eOCH2), 7.29 (s, 4H, AreH), 7.87 (s, 2H,
thiopheneeH). 13C NMR (75 MHz, CDCl3):
d 14.10, 22.68, 26.07,
29.39, 29.64, 29.69, 30.34, 31.92, 69.42, 73.65,105.54,117.78,128.82,
129.88, 141.75, 153.66, 159.70, 164.80. Anal. Calcd for C92H156N4O8S:
C, 74.75; H, 10.64. Found C, 74.73; H, 10.61.
Acknowledgements
We thank the Ministry of Science and Technology, Taiwan, ROC
(MOST 103-2113-M-008-002) for generous support of this work.
CDCl3):
d
0.86 (t, 6H, eCH3, J¼6.3 Hz), 1.27e1.42 (m, 20H, eCH2),
1.70e1.75 (m, 4H, eCH2), 4.04 (t, 4H, eOCH2, J¼6 Hz), 7.04 (d, 4H,
AreH, J¼8.4 Hz), 7.87 (d, 4H, AreH, J¼9 Hz), 7.90 (s, 2H, thio-
pheneeH), 10.4 (s, 2H, NeH), 10.7 (s, 2H, NeH). 13C NMR (75 MHz,
Supplementary data
CDCl3): d 13.99, 22.11, 25.49, 28.58, 28.68, 28.74, 31.26, 67.77,106.52,
114.23, 124.26, 129.41, 141.34, 160.33, 161.63, 165.41.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2015.04.055.
4.6. N02, N05-Bis(3,4,5-tris(octyloxy)benzoyl)thiophene-2,5-
dicarbohydrazide 2c (n[8)
References and notes
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Please cite this article in press as: Lin, K.-T.; et al., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.04.055