Phase crossover in columnar tris-(1,3,4-oxadiazoles) with pendant quinoxalines

Article abstract of DOI:10.1016/j.tet.2016.10.003

Four new series of bis-(1,3,4-oxadiazoles) 1a–c and tris-(1,3,4-oxadiazoles) 1d containing peripheral quinoxalines were prepared and their mesomorphic behavior was reported. Except for compound 1c (n=12), all other compounds 1a–d formed columnar phases wh

Full text of DOI:10.1016/j.tet.2016.10.003

Accepted Manuscript
Phase crossover in columnar tris-(1,3,4-oxadiazoles) with pendant quinoxalines
Kuan-Ting Lin, Chung K. Lai
PII:
S0040-4020(16)31012-2
10.1016/j.tet.2016.10.003
TET 28144
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 20 August 2016
Revised Date: 1 October 2016
Accepted Date: 4 October 2016
Please cite this article as: Lin K-T, Lai CK, Phase crossover in columnar tris-(1,3,4-oxadiazoles) with
pendant quinoxalines, Tetrahedron (2016), doi: 10.1016/j.tet.2016.10.003.
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ACCEPTED MANUSCRIPT
Phase crossover in columnar tris-(1,3,4-oxadiazoles) with pendant quinoxalines
Kuan−Ting Lin and Chung K. Lai
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Phase crossover in columnar tris-(1,3,4-oxadiazoles) with pendant quinoxalines
Kuan−Ting Lin and Chung K. Lai
Department of Chemistry, National Central University
Chung−Li, Taiwan, ROC
Abstract. Four new series of bis−(1,3,4−oxadiazoles) 1a−c and tris−(1,3,4−oxadiazoles) 1d
containing peripheral quinoxalines were prepared and their mesomorphic behavior was reported.
Except for compound 1c (n = 12), all other compounds 1a−d formed columnar phases which were
confirmed by powder X−ray diffractometer (XRD). Compounds 1d have the highest clearing
temperatures and the widest temperature range of columnar phases than those of other compounds
1a−b. Compounds 1d exhibited a phase crossover between the rectangular and hexagonal
columnar phases. A value of Ncell = 2.85 and 2.16 Å within a column slice of 9.0 Å thick was
obtained for compound 1a (n = 12) and 1d (n = 12), indicating that two molecule was correlated
within columns in columnar phases. All compounds 1a−d (n = 12) showed good stabilities at
temperature below Tdec = 412.5−420.8 °C on thermogravimetric analysis. The PL spectra of all
compounds 1a−d (n = 12) showed one intense peak at λmax = 517−520 nm, and these
photoluminescent emissions originated from peripheral quinoxaline moiety.
*Corresponding author. Tel.: +886 03 4259207; fax: +886 03 4277972
e−Mail address: cklai@cc.ncu.edu.tw (C.K. Lai).
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1.0 Introduction
1,3,4−Oxadiazole, as an important 5−membered ring heterocycle has been investigated in the
field of materials chemistry due to their excellent thermal and chemical stabilities. Their potential
applications have been fully reviewed.1 2,5−Diary−1,3,4−oxadiazoles2 derivatives were well long
known to show high photoluminescence quantum yields. Many studies have demonstrated their
widespread uses as electron transporting/ hole blocking materials, emitting layers in electro−
luminescent diodes, or for non−linear optical processes. 1,3,4−Oxadiazole is a nonlinear structure,
stemming from its larger exocyclic bond angle3 ( 135°). Incorporating heteroatoms, such as
nitrogen, oxygen, sulfur atoms might impact some chemical or physical properties, which were
often not observed in their homologues all−carbon rings. Furthermore, these heteroatoms are more
electronegative than carbons, and they all have delocalized lone electrons. These inherently
molecular futures, such as electron unsaturation, electron−deficiency, nonplanar core, lower
symmetries, higher polarizabilities or others often favorable for their induction or formation of the
mesophases, have made them an excellent core in the design of mesogenic materials. The first
mesogenic oxadiazoles4 showing SmA and N phases were studied until 1993. A few examples
columnar 1,3,4−oxadiazoles5 and their metallomesogens6 were previously reported by this group.
In contrast, compounds containing bis− or tris−(1,3,4−oxadiazoles) (i.e. poly-1,3,4-oxadiazoles)
were less prepared and studied. Known example of mesogenic bis−(1,3,4−oxadiazoles)7 Ia−b or
tris−(1,3,4−oxadiazoles)8 III were very limited. Isomeric examples of mesogenic
bis-(1,2,4-oxadiazoles)9 were also explored.
Quinoxalines were also known as highly photoluminecent and/or efficient electroluminescent
materials. Quinoxaline were highly π−conjugated fused backbones, and were useful in many
applications10 such as dyes, organic light-emitting diodes, electroluminescence, organic thin film
transistors (TFT), and organic photovoltaics. They were considered as half−disc, elliptical or
round molecules, which could easily or spontaneously self−assemble into columns when π−π
interaction or dipole−dipole interactions are accessible in the solid or liquid crystalline states.
These molecules capable of forming columnar stacking arrangements might have a dramatic
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impact propensity. Examples of using bulky quinoxalines to improve mesomorphic behavior in
1,3,4−oxadiazole IIa−b11 as core structure has been reported in our previous studies.
In this work, we report the preparation and mesomorphic studies of two new series of
symmetrical bis−(1,3,4−oxadiazoles) 1a−c and another series of tris−(1,3,4−oxadiazoles) 1d in
which a peripheral quinoxaline was incorporated to better induce their columnar phases.
Interestingly, except for compounds 1c (n = 12) all other compounds 1a−b and 1d formed
columnar phases. Compound 1a formed hexagonal columnar phase and compounds 1b exhibited
lamellar columnar phases. In contrast, all compounds 1d (n = 8, 10, 12, 14) formed rectangular
columnar at lower temperature and hexagonal columnar phase over a wide range of temperature.
Their optical behavior was also studied.
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2.0 Results and Discussion
2.1 Synthesis and characterization
The synthetic routes utilized to prepare bis-(1,3,4-oxadiazoles) 1a−c and
tris-(1,3,4-oxadiazoles) 1d are listed in Scheme 1 and Scheme 2. The methyl 2,3−bis(3,4−bis
(alkoxy)phenyl)quinoxaline 6−carboxylates 4, isolated as yellow solids were similarly prepared by
our previous procedures. Further reaction of methyl 6-carboxylates with hydrazine monohydrate
in refluxing in THF/methanol gave yellowish 2,3−bis(3,4−bis(alkoxy)phenyl)quinoxaline−6−
carbohydrazides 3. A singlet peak appeared at ca. 4.23 ppm on 1H−NMR spectrum, assigned for
–NH2 confirmed its formation. Furthermore, the precursors 2a−c, quinoxaline−6−carbonyl)
oxalohydrazides were obtained by reaction of quinoxaline−6−carbohydrazides and oxalyl chloride
in the presence of triethylamine at ice bath in stirring THF. The 6-carbohydrazides 3, isolated as
yellow−green solids were directly used for next reaction without any further purification. The final
compounds 1a, 5,5'−bis(2,3−bis(3,4−bis(alkoxy)phenyl) quinoxalin−6−yl)−2,2'−bi
(1,3,4−oxadiazoles) were prepared by neat reactions of N'1,N'2−bis (2,3−bis(3,4−bis(alkoxy)
phenyl)quinoxaline−6−carbonyl)oxalohydrazides in refluxing phosphoryl chloride. The products
isolated as bright yellow to orange solids were obtained after silica gel chromatography eluting
with hexane/ethyl acetate and recrystallization from THF/methanol. All other compounds 1b−c
were obtained by above procedures. Four derivatives 1d (n = 8, 10, 12, 14) of
tris−(1,3,4−oxadiazoles) were similarly prepared according to Scheme 2. All final compounds
1a−d were characterized by 1H NMR, 13C NMR, mass, and elemental analysis.
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OR
RO
OR
RO
N
N
OR
OR
OR
RO
RO
O
N
H
O
N
OR
N
N
N
NH
HN
O
N
H
a
NHNH2
O
O
HN
NH
OR
O
RO
O
3
RO
RO
N
N
2d; R = CnH2n+1, n = 8, 10, 12, 14
OR
RO
b
OR
OR
RO
RO
N
N
N
O
N
OR
N
O
N
N
OR
O
N
N
N
RO
RO
N
N
OR
RO
1d; R = CnH2n+1, n = 8, 10, 12, 14
OR
RO
Scheme 2. Reagents and conditions: (a) benzene-1,3,5-tricarbonyl trichloride, triethylamine, stirring in dry THF at
rt. for 24 h; (b) Refluxing in POCl3 for 24 h, 35-40%.
2.2 Phase transitions, mesomorphic properties, and thermal stability of compounds 1a-d
The liquid crystalline behavior of compounds 1a−d was characterized and studied by
differential scanning calorimetry and polarized optical microscope. The phase transitions and
thermodynamic data are summarized in Table 1. Except compounds 1c, all three series of
compounds 1a−b and 1d exhibited columnar mesomorphic behavior under POM observation (Fig.
1). Compound 1c (n = 12) was not mesogenic, and only transition of crystal−to−isotropic at 158.7
°C with a large enthalpy of ∆H = 47.4 kJ/mol was observed on heating process. Both compound
1b (n = 12) and 1c (n = 12) were in fact molecular isomers. The lack of mesomorphism on
compound 1c (n = 12) might probably attributed to its overall molecular shape or/and molecular
symmetry. Furthermore, its higher clearing temperature at 157.8 °C than that (Tcl = 117.3 °C) of
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compound 1b (n = 12) indicated that the intermolecular interaction in compound 1c (n = 12) was
indeed stronger than that in compound 1b (n = 12). Interestingly, compound 1b (n = 12) formed a
lamella columnar phase (ColL). DSC data showed that it clearing temperature at Tcl = 117.3 °C
was observed on heating process, and its temperature was relatively short, ∆TCol = 15.1 °C on
cooling process. The shorter range of columnar phase indicated that the lamella columnar in such
system is kinetically not very stable. Under optical microscope they exhibited typical focal−conic
textures (Fig. 1), similar to hexagonal columnar textures, when cooled from its isotropic state.
These optical textures also showed slightly small area of homeotropic domains which were
commonly observed in columnar phases.
Fig. 1 Optical textures of columnar phases observed by compounds: 1a (n = 12) at 122 °C (top
left), 1b (n = 12) at 105 °C (top right), 1d (n = 8) at 166 °C (bottom left) and 1d (n = 12) at 155 °C
(bottom right).
In order to improve the induction of columnar phases in such discotic system derived from
poly(1,3,4−oxadiazoles), four 1,3,4−oxadiazoles 1d (n = 8, 10, 12, 14) with three pendant
quinoxalines were prepared and their mesomorphic properties were investigated. As expected, all
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tris−(1,3,4−oxadiazoles), as more rounded molecules formed better improved mesomorphic
behavior. All compounds 1d (n = 8, 10, 12, 14) exhibited a phase−crossover dependence on
temperature; they formed rectangular columnar phases at lower temperature and hexagonal
columnar phases at higher temperature. The phase crossover behavior between the Colrec and
Colhex phases was enantiotropic. DSC data appeared that melting temperatures increased with
carbon chain length; Tmp = 102.0 (n = 8) < 104.0 (n = 10) < 110.0 (n = 12) < 115.0 °C (n = 14) on
heating process. In contrast, the clearing temperature decreased with carbon lengths; Tcl = 175.0 (n
= 10) > 166.2 (n = 8) > 160.3 (n = 12) > 148.1 °C (n = 14) on heating process. Both enthalpies for
the transition of Colrec−to−Colhex phase and Colhex−to−I phase were relatively small; 0.60 (n =
8)−1.97 kJ/mol (n = 10) and 2.07 (n = 8)−5.25 kJ/mol (n = 14) on cooling process, respectively.
This enthalpy dependence reflected macroscopically their molecular entropies when molecules
changed from Colrec-−to−Colhex−to−I state. Under POM, a focal−conic or more leaf-like texture
(Fig. 1) with linear birefringent defects was clearly observed when cooling from their isotropic
liquids. These observed textures, accompanied by a larger area of homeotropic domain were
characteristic for hexagonal columnar phases. The Figure 2 is the bar graphs showing the phase
behavior of compounds 1a−d
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Table 1 The phase transitions and enthalpiesa of compounds 1a-1d.
134.3 (41.4)
I
I
1a; n = 8
n = 12
Cr
Cr
Cr
118.8 (27.9) Colh 120.9 (9.56)
113.6 (30.4)
104.5 (27.7)
125.8 (8.17)
122.9 (8.36)
Colh
93.3b
90.8b
117.3 (1.66)
105.9 (0.92)
157.8 (47.4)
142.6 (38.4)
1b; n = 12
1c; n = 12
ColL
Cr
I
I
102.0b
90.0b
104.0b
95.0b
110.0b
102.0b
115.0b
127.1 (1.78)
124.9 (1.97)
175.0 (2.10)
167.6 (2.07)
Colr
Colr
Colr
Colr
1d; n = 8
n = 10
n = 12
n = 14
Cr
Cr
Colh
Colh
Colh
I
I
I
I
132.8 (0.51)
130.7 (0.60)
166.2 (2.94)
160.2 (2.64)
133.8 (0.47)
131.7 (0.63)
160.3 (3.93)
155.9 (3.43)
Cr
Cr
129.1 (0.70)
127.2 (0.70)
148.1 (5.28)
143.6 (5.25)
Colh
103.0b
a: n = the carbon number of alkoxy chains. Cr = crystal; ColL = columnar lamellar
phase; Colr = columnar rectangular phase; Colh = columnar hexaganol phase; I =
isotropic phases.
b: observed by POM.
Fig. 2 Bar graphs showing the phase behavior of compounds 1a−d. All temperatures were taken
on the cooling process.
The thermal stability of compounds 1a−d (all n = 12) was also performed by thermo-
gravimetric analysis (TGA) under nitrogen atmosphere, shown in Fig. 3. All four compounds
showed good thermal stability at temperature below ca. Tdec = 410 °C, with a relative thermal
stability of 1b > 1d > 1a > 1c. The decomposition temperatures for a 5% weight loss were listed in
Table 2.
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100
75
50
25
0
1a (n = 12)
1b (n = 12)
1c (n = 12)
1d (n = 12)
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 3 The TGA thermographs of compounds 1a−d (all n = 12) under nitrogen gas at a heating rate
of 10.0 °Cmin-1.
Table 2. The decomposition temperaturesa of compounds 1a−d measured by TGA analysis.
Compd.
Tdec (°C)
1a (n = 12)
417.5
420.8
412.5
418.3
1b (n = 12)
1c (n = 12)
1d (n = 12)
a: temperatures taken with a 5% weight loss under nitrogen atmosphere.
2.3 Powder X−ray diffractions of compounds 1a−b and 1d.
In order to confirm the structure of the columnar phases exhibited by compounds 1,
variable−temperature powder X−ray diffraction experiments of three compounds 1a−b (n = 12)
and 1d (n = 12) were also conducted. Interestingly, compound 1a (n = 12) formed hexagonal
columnar phase, whereas, compound 1b (n = 12) exhibited lamellar columnar phases. For the Colh
phase, a diffraction pattern of a two dimensional hexagonal lattice with one very strong diffraction
peak at lower angle and another much weaker diffraction peak of compound 1a (n = 12) was
obtained at 120.0 °C under cooling process, shown in Fig. 4. A diffraction pattern with a
d−spacing at 32.63 Å, and a broad diffuse peak with a d = 4.66 Å at wide angle region was
obtained. The strong peak with a d = 32.63 Å corresponded to a Miller indices 110 in the
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hexagonal columnar arrangement. However, liquid−like correlations between the rigid cores
occurred at wide−angle regions of 4.66 Å. This diffraction pattern corresponded to an
intercolumnar distance or lattice constant (i.e., a parameter of the hexagonal lattice) of a = 37.67 Å.
The lack of any relatively peaks at wide angles excluded a more regular periodicity along the
columns.
Columnar mesogens were characterized by columns of molecules, and within the columns all
molecules were packed together to form a two−dimensional crystalline array. On the other hand,
for compound 1b (n = 12), the diffraction pattern was different; one strong diffraction peak and
one much weaker peak at lower angle, and one broad diffused peak at wide−angle were observed.
The d−spacings occurred at d = 37.85 Å and 19.11 Å and a broad diffused peak at 4.60 Å. This
type of diffraction pattern corresponded to lamellar phases (ColL) with Miller indices 001 and 002.
Higher diffraction peaks indexed as 003, and others were not observed in this discotic system.
25000
25000
20000
15000
10000
5000
0
32.6
37.8
20000
15000
10000
5000
0
19.1
4.6
4.6
0
10
20
30
0
10
20
30
2 theta
2 theta
Fig. 4 The powder X−ray diffraction plots of compound 1a (n = 12; left) measured at 120.0 oC and
1b (n = 12; right) measured at 105.0 oC when cooled from above their clearing temperatures.
15000
35.9
20000
35.5
15000
10000
5000
0
10000
5000
0
32.2
4.7
4.7
12
0
10
20
30
0
10
20
30
2 theta
2 theta

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Fig. 5 The powder X−ray diffraction plots of compound 1d (n = 12; left) measured at 150.0 oC and
1d (n = 12; right) measured at 125.0 oC when cooled from above their clearing temperatures.
Interestingly, compound 1d (n = 12) showed a different columnar behavior; rectangular
columnar phases at lower temperature (T = 125.0 °C) and hexagonal columnar phases at higher
temperature (T = 150.0 °C). This compound showed a diffraction pattern of two strong peaks with
a d−spacing of 35.87 and 32.21Å at 125 °C, and these two strong peaks corresponded to Miller
index 110 and 210. The rectangular columnar mesophases includes three different Colrec
mesophases depending on the planar space groups, P21/a, P2/a, and C2/m, which corresponded to
the plane groups p2gg, p2mg, and c2mm, respectively. The exact Colrec was not possible to
identify in this example due to its lesser diffraction peaks. In general, the molecules in Colrec
phases were tilted with respect to the column axis, leading to an elliptic cross section of the
column. As the cross section of the tilted columns is elliptical, the symmetries of the Colr phases
differ from a proper hexagonal symmetry.
The formation of discotic or columnar mesophases by round or discotic molecules was often
strongly side chain dependent. More side chains are generally needed for larger core centers in
order to stabilize the mesogen. A phase crossover between rectangular and hexagonal columnar
phases was in fact observed for compound 1d (n =12), as shown in Fig. 5. A phase crossover
between Colhex and Colr phases was less observed, and these phase transitions were often related
or dependent to the temperature or/and side chain length of compounds. The mesophase
crossover12 from Colrec to Colhex has been observed in columnar systems, and was generally
attributed to the greater core interaction necessary for the formation of the Colr phases. The tilted
Colr phase reduced the interactions between the bulky side chains and allowed closer contacts
between the cores. This pseudohexagonal lattice constant (i.e. a rectangular lattice with an axial
ratio of b/a from the ideal hexagonal of √3; Fig. 6) for this series of compound 1d (n = 12) was
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equal to 1.49 (i.e. 64.42/43.19). This valve indicated that the structural departure from the ideal
hexagonal lattice was about 33.0% in this system. A summary of the diffraction peaks and lattice
for these three compounds 1a−b and 1d (all n = 12) is listed in Table 3.
Fig. 6 The structural departure from the ideal hexagonal lattice in compounds 1d (n = 12)
Table 3. Detailed indexation by powder XRD of columnar phasesa for compounds 1a, 1b, 1d
Compds.
Mesophases
temp.
d-Spacing
obs.(calcd.)
Lattice const.
(Å)
Miller
Indices
1a (n = 12)
1b (n = 12)
Colh at 120.0°C
32.63 (32.63)
4.66 (br)
a = 37.67
100
halo
100
200
halo
100
halo
110
200
halo
ColL at 105.0°C
37.85 (37.85)
19.11 (18.93)
4.60 (br)
1d (n = 12)
1d (n = 12)
Colh at 150.0°C
Colr at 125.0 °C
35.54 (35.54)
4.72 (br)
a = 41.03
35.87 (35.87)
32.21 (32.21)
4.66 (br)
a = 64.42
b = 43.19
a: temperature taken on the cooling process;
In order to understand the possible molecular packing in the columnar phases, the number of
molecules within a portion of columnar height h was calculated by use of a known model.13 By
this simple model, two important parameters; Ncell and Rar were then calculated from powder
X−ray diffraction data. Ncell is the number of molecules within a 9.0 Å column, and the Rar defined
as the diameter of the aromatic or hard columnar part was calculated by Rar = (4Sar/π)1/2. A value
of Ncell = 2.85 and 2.16 was obtained for compounds 1a (n = 12) at 120 °C and 1d (n = 12) at 150
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°C. On the other hand, an average number of ca. 2.16−2.85 molecules for compounds 1a or 1d
were stacked within a height of 9.0 Å in the columnar phases (Table 4). Therefore, a more
disc-like correlated structure constructed by two molecules lying up−and−down was generated
within the column. The possible molecular arrangement in columnar phases was proposed in Fig.
7 and Fig. 8.
Table 4. Geometric parametersa of compound 1a (n = 12) and 1d (n = 12).
Compd.
S(Å2)
Vcell(Å3)
Vm(Å3)
Temp(°C) Ncell Sar(Å2) Rar(Å)
1a (n = 12)
1228.68 11058.08
3873.83
6069.02
120.0
150.0
2.85 329.86 20.49
2.16 406.14 22.74
1d (n = 12) 1457.67 13119.05
a: S(Å2) = columnar cross section area, Vcell(Å3) = volume of the column stratum 9 Å thick,
Vm(Å3) = molecular volume, Ncell = the number of molecules contained in each columnar stratum
9−Å thick, Sar(Å2) = the surface area of hard columnar core, Rar (Å2) = the diameter of the
aromatic part.
Fig. 7 A schematic representation of molecular organization proposed in columnar phase by
compounds 1a.
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Fig. 8 A schematic representation of molecular organization proposed in columnar phase by
compounds 1d.
2.4 Optical properties
1,3,4−Oxadiazole has been long known to be an excellent blue-emitter and has potential
applications in OLED devices. The UV−vis absorption and PL spectra of the compounds 1a−d (all
n = 12) excited at λ = 342 cm-1 measured in CH2Cl2 solution at room temperature are presented in
Fig. 9 and data presented in Table 5. These compounds containing bis− and tris−1,3,4−oxadiazoles
are particularly interesting due to the presence of multiple chromophores and possible
π−conjugations between the peripheral structures and central core. The absorption λmax peaks of
compounds 1a−d occurred at ca. ~342 and 403−410 nm, which were attributed to π−π* transitions,
arising from 1,3,4−oxadiazole and quinoxaline, respectively. Compound 1a (n = 12) has a slightly
red−shifted λmax than those of others, which might be due to its better coplanar core. Single
crystallographic data14 of similar 1,3,4−oxadiazoles have indicated that a dihedral angle between
the benzene and 1,3,4−oxadiazole was often ranged of 0−15.5°.In contrast, the PL spectra of all
compounds 1a−d showed one intense and broad peak occurred at λmax = 517−529 nm, respectively,
and these photo luminescent emissions originated from quinoxaline moieties, similar to our
previous systems containing 1,3,4-oxaziazole and/or quinoxalinez. The λmax in compounds 1a and
1d are slightly red−shifted due to their better conjugation lengths. In general, a red-shift emission
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is often expected in this type of conjugated systems. All three bis-1,3,4-oxadiazoles 1a−c have a
much red−shifted λmax = 517−529 nm than those of our previous studied bis−1,3,4−oxadiazole,
2,5-bis(5-(3,4,5-tris (dodecyloxy)phenyl)- 1,3,4-oxadiazol-2-yl)thiophene and others. An apparent
red−shifted λmax in this system than those of 1,3,4-oxadiazoles might be attributed to the more
electron−donating group of peripheral quinoxalines. All quantum yields of luminescent materials
were ranged from 30 to 35%. However, an emission often seen by 1,3,4−oxadiazoles at ca.
380−418 nm was not observed. Both quinoxaline and 1,3,4−oxadiazoles were considered as
π−acceptors and fluorophore, and a red−shift emission often attributed to donor−acceptor transfer.
Whether a fluorescence resonance energy transfer (FRET) involved in this system was uncertain at
this moment, and more experimental data might be needed. FRET states a mechanism in which the
resonance energy transfers from an excited state of a donor fluorophore to the ground state of an
acceptor fluorophore via a non-radiative ‘dipole-dipole coupling’. In order to have this photo
effect, both fluorescent chromophores need to be within a certain distance,
1.0
0.8
0.6
0.4
0.2
0.0
800
700
600
500
400
300
200
100
0
1a (n = 12)
1b (n = 12)
1c (n = 12)
1d (n = 12)
1a (n = 12)
1b (n = 12)
1c (n = 12)
1d (n = 12)
400
500
600
700
800
400
500
Wavelength (nm)
Wavelength (nm)
Fig. 9 Absorption (left), and PL spectra (right) of the compounds 1a−d.
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Table 5. The UV−vis absorption and PL−emission dataa of compounds 1a−1d.
Compds
Absorption
Emissionb
ΦF
λ (nm)
λmax (nm)
1a (n = 12)
1b (n = 12)
1c (n = 12)
1d (n = 12)
342, 410
342, 403
342, 405
342, 405
529
518
517
520
0.33
0.35
0.30
0.33
a: All data measured in CH2Cl2 at room temperature. Anthracene is used as standard with a Φfs =
0.27 in hexane solution.
b: the excitation wavelength is 342 cm-1.
3. Conclusions
Quinoxaline moieties was incorporated into bis-(1,3,4-oxadiazoles) 1a−b and
tris−(1,3,4−oxadiazoles) 1d to better induce their columnar phases. The formation of columnar
phases was sensitive to the molecular conformations. Compounds 1a (n = 8, 12) formed Colh
phases, while compounds 1b (n = 12) exhibited ColL phases. In contrast, all compound 1d formed
Colr at lower temperature and Colh phases at higher temperature. The phase crossover in such a
tris-(1,3,4-oxadiazoles) might be attributed to the kinetically unstable conformations resulted from
the bulky quinoxalines pendant around the central core. The PL spectra of all compounds 1a−d
showed one intense peak at λmax = 517−520 nm, which was originating from the fused quinoxaline.
The lack of blue emissions at ca. λmax = 390−418 nm by 1,3,4−oxadiazoles might be probably due
to the effect of fluorescence resonance energy transfer.
4.0 Experimental Section
4.1. General materials and methods
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 calibrated with a pure indium
sample. All phase transitions are determined by a scan rate of 10.0 °C/min. Optical polarized
microscopy was carried out on Zeiss Axioplan two equipped with a hot stage system of Mettler
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FP90/FP82HT. The UV−vis absorption and fluorescence spectra were obtained using a Jasco
V−530 and Hitachi F−4500 spectrometer. Elemental analyses were performed on a Heraeus
CHN−Rapid elemental analyzer. The powder diffraction data were collected from the Wiggle A
beam line of the National Synchrotron Radiation Research Center (NSRRC) with a wavelength of
1.3223 Å. The powder samples were charged in Lindeman capillary tubes (80 mm long x 0.01 mm
thick) purchased from Charles Supper Co. with an inner diameter of 1.0 mm.
4.2 1, 2−Bis(dodecyloxy)benzene
1H NMR (300 MHz, CDCl3): δ 0.85 (t, 6H, −CH3, J = 6.9 Hz), 1.24−1.45 (m, 36H, −CH2),
1.74−1.84 (m, 4H, −CH2), 3.97 (t, 4H, −OCH2, J = 6.6 Hz), 6.86 (s, Ar−H, 4H). 13C NMR (75
MHz, CDCl3): δ 14.14, 22.72, 26.07, 29.38, 29.47, 29.67, 29.73, 31.95, 69.29, 114.12, 121.00,
149.25.
4.3 1, 2−Bis(3,4−bis(dodecyloxy)phenyl)−1, 2−ethanedione 5 (n = 12)
1H NMR (300 MHz, CDCl3): δ 0.83−0.88 (m, 12H, −CH3), 1.24−1.45 (m, 72H, −CH2), 1.77−1.86
(m, 8H, −CH2), 4.01−4.05 (m, 8H, −OCH2), 6.81 (d, 2H, Ar−H, J = 8.4 Hz), 7.39−7.42 (m, 2H,
Ar−H), 7.54 (d, 2H, Ar−H, J = 1.8 Hz). 13C NMR (75 MHz, CDCl3): δ 14.14, 22.72, 25.94, 26.00,
28.93, 29.08, 29.40, 29.66, 31.95, 69.11, 69.22, 111.54, 112.19, 126.17, 149.29, 154.97, 193.83.
4.4 Methyl 2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxaline−6−carboxylate 4 (n = 12)
The solution 1, 2−bis(3, 4−bis(dodecyloxy)phenyl)−1, 2−ethanedione (4.0 g, 4.0 mmol) dissolved
in 100 mL of THF was stirred with 0.5 mL of glacial acetic acid for 10 min. To the solution,
methyl 3,4−diaminobenzoate (0.70 g, 4.0 mmol) was added and stirred for 24 h. The product,
isolated as yellow solids was obtained after recrystallization from THF/MeOH. Yield 93%, 1H
NMR (300 MHz, CDCl3): δ 0.86 (t, 12H, −CH3, J = 6.6 Hz), 1.25−1.42 (m, 72H, −CH2),
1.68−1.83 (m, 8H, −CH2), 3.80−3.81 (m, 4H, −OCH2), 3.98−4.00 (m, 7H, −OCH2, −OCH3), 6.82
(d, 2H, Ar−H, J = 8.4 Hz), 7.07−7.15 (m, 4H, Ar−H), 8.12 (d, 1H, Ar−H, J = 8.7 Hz), 8.28 (d, 1H,
Ar−H, J = 8.4 Hz), 8.82 (s, 1H, Ar−H). 13C NMR (75 MHz, CDCl3): δ 14.05, 22.64, 25.99, 29.11,
29.19, 29.34, 29.39, 29.64, 31.89, 52.42, 69.15, 113.09, 115.30, 122.84, 122.96, 128.96, 129.11,
130.67, 131.31, 131.67, 140.12, 142.96, 148.72, 150.13, 150.26, 154.07, 154.78, 166.39.
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4.5 2,3−Bis(3,4−bis(dodecyloxy)phenyl)quinoxaline−6−carbohydrazide 3 (n = 12)
The solution of methyl 2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxaline−6−carboxylate
(4.0 g, 4.0 mmol) dissolved in 100 mL of THF and 50 mL of methanol. To the solution, hydrazine
monohydrate (2 g, 40.0 mmol) was added and stirred for 12 h. The product, isolated as yellow
solids was obtained after recrystallization from THF/MeOH. Yield 90%, 1H NMR (300 MHz,
CDCl3): δ 0.85 (t, 12H, −CH3, J = 6.3 Hz), 1.24−1.42 (m, 72H, −CH2), 1.68−1.81 (m, 8H, −CH2),
3.78 (m, 4H, −OCH2), 3.97 (m, 4H, −OCH2), 4.23 (s, 2H, −NH2), 6.84 (d, 2H, Ar−H, J = 8.4 Hz),
7.03 (d, 2H, Ar−H, J = 6.3 Hz), 7.09 (d, 2H, Ar−H, J = 8.1 Hz), 8.09 (s, 2H, Ar−H), 8.42 (s, 1H,
Ar−H). 13C NMR (75 MHz, CDCl3): δ 14.06, 22.65, 26.00, 29.12, 29.20, 29.34, 29.42, 29.66,
31.89, 69.19, 113.06, 115.30, 122.83, 122.95, 127.59, 129.61, 131.20, 133.20, 140.10, 142.33,
148.70, 150.18, 150.26, 154.16, 154.58, 167.80.
4.6 N'1,N'2−Bis(2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxaline−6−carbonyl)oxalohydrazide
2a (n = 12)
The solution of 2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxaline−6−carbohydrazide (1.0 g, 1.0
mmol) dissolved in 125 mL of THF was added dropwise 0.14 mL of triethylamine (1.0 mmol) at
ice bath. Oxalyl chloride (0.03 g, 0.5 mmol) was slowly added to the solution and stirred at r.t. for
24 h. The solution was concentrated to give yellow−green solids. The solids were dissolved in 150
mL of dichloromethane and extracted with 100 mL of water. The resulting crude product 2a was
directly used for next reaction. All other three compounds 2b−d were similarly prepared as above
procedures.
4.7 5,5'−Bis(2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxalin−6−yl)−2,2'−bi(1,3,4−oxadiazole)
1a (n = 12)
The solution of crude product N'1,N'2−bis(2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxaline−6−
carbonyl)oxalohydrazide (1.0 g, 4.0 mmol) dissolved in 20 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 icy water. After stirring
for 2 h, the orange solids were filtered and collected. The solids were dissolved in 150 mL of
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dichloromethane and extracted with 150 mL of 1.0 M NaOH(aq) for 2 h. The yellow−brown solids
were collected. The products isolated as bright yellow solids were obtained after silica gel
chromatography eluting with hexane/ethyl acetate and recrystallization from THF/methanol. Yield
25%, mp. 113.6 °C. 1H NMR (300 MHz, CDCl3): δ 0.83−0.88 (m, 24H, −CH3), 1.25−1.42 (m,
144H, −CH2), 1.62−1.84 (m, 16H, −CH2), 3.84 (t, 8H, −OCH2, J = 6.6 Hz), 4.00 (t, 8H, −OCH2, J
= 6.6 Hz), 6.84 (d, 4H, Ar−H, J = 8.1 Hz), 7.12−7.19 (m, 8H, Ar−H), 8.27 (d, 2H, Ar−H, J = 8.7
Hz), 8.51 (dd, 2H, Ar−H, J = 2.1 Hz, J = 8.8 Hz), 9.00 (d, 2H, Ar−H, J = 1.8 Hz). 13C NMR (75
MHz, CDCl3): δ 14.09, 22.68, 26.04, 29.16, 29.22, 29.37, 29.45, 29.69, 31.92, 69.23, 113.07,
115.26, 122.85, 123.06, 127.00, 129.36, 130.52, 131.02, 131.11, 140.54, 142.82, 148.79, 150.38,
150.48, 153.29, 154.68, 155.14, 165.93. IR (KBr): 2955, 2921, 2850, 2363, 2329, 1600, 1578,
1560, 1541, 1514, 1466, 1423, 1390, 1351, 1423, 1390, 1350, 1423, 1389, 1265, 1252, 1225, 1186,
1142, 1066, 1028, 1003, 932, 889, 817, 727, 627, 488 cm-1. Anal. Calcd for C140H218N8O10: C,
77.37; H, 10.11; N, 5.16. Found C, 77.23; H, 10.11; N, 5.06.
4.7.1 5,5'−Bis(2,3−bis(3,4−bis(octyloxy)phenyl)quinoxalin−6−yl)−2,2'−bi(1,3,4−oxadiazole)
1a (n = 8)
Mp. 134.3 °C; 1H NMR (300 MHz, CDCl3): δ 0.85−0.89 (m, 24H, −CH3), 1.28−1.43 (m, 80H,
−CH2), 1.64−1.84 (m, 16H, −CH2), 3.84 (t, 8H, −OCH2, J = 6.9 Hz), 4.00 (t, 8H, −OCH2, J = 6.6
Hz), 6.84 (d, 4H, Ar−H, J = 8.4 Hz), 7.12−7.19 (m, 8H, Ar−H), 8.27 (d, 2H, Ar−H, J = 8.7 Hz),
8.51 (dd, 2H, Ar−H, J = 1.8 Hz, J = 8.7 Hz), 9.00 (d, 2H, Ar−H, J = 2.1 Hz). 13C NMR (75 MHz,
CDCl3): δ 14.07, 22.66, 26.01, 29.20, 29.26, 29.31, 29.37, 31.82, 69.17, 69.22, 113.05, 115.24,
122.84, 123.05, 126.99, 129.34, 130.50, 131.01, 131.10, 140.53, 142.81, 148.78, 150.36, 150.47,
153.28, 154.67, 155.12, 165.91. IR (KBr): 2957, 2921, 2854, 2364, 1598, 1560, 1514, 1469, 1424,
1394, 1347, 1308, 1260, 1186, 1136, 1068, 1024, 970, 929, 897, 858, 839, 815, 727, 671, 628, 420
cm-1. Anal. Calcd for C108H154N8O10: C, 75.22; H, 9.00; N, 6.50. Found C, 75.22; H, 9.08; N, 6.41.
4.8 1,3−Bis(5−(2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxalin−6−yl)−1,3,4−oxadiazol−
2−yl)benzene 1b (n = 12)
Mp. 93.3 °C. 1H NMR (300 MHz, CDCl3): δ 0.83−0.87 (m, 24H, −CH3), 1.24−1.42 (m, 144H,
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−CH2), 1.68−1.83 (m, 16H, −CH2), 3.83 (t, 8H, −OCH2, J = 6.6 Hz), 3.99 (t, 8H, −OCH2, J = 6.6
Hz), 6.84 (d, 4H, Ar−H, J = 8.4 Hz), 7.10 (d, 4H, Ar−H, J = 1.8 Hz), 7.16 (d, 4H, Ar−H, J = 8.4
Hz), 7.77 (t, 1H, Ar−H, J = 8.1 Hz), 8.26 (d, 2H, Ar−H, J = 8.7 Hz), 8.38 (d, 2H, Ar−H, J = 7.5
Hz), 8.51 (d, 2H, Ar−H, J = 8.7 Hz), 8.91 (s, 2H, Ar−H), 8.98 (s, 1H, Ar−H). 13C NMR (75 MHz,
CDCl3): δ 14.07, 22.66, 26.03, 29.16, 29.22, 29.36, 29.45, 29.68, 31.91, 69.21, 113.10, 115.29,
123.02, 124.13, 125.04, 125.23, 127.03, 128.15, 130.01, 130.28, 131.20, 140.59, 142.44, 148.76,
150.26, 150.35, 154.50, 154.71, 164.04, 164.57. IR (KBr): 2922, 2852, 2362, 2329, 1600, 1559,
1509, 1467, 1424, 1394, 1347, 1298, 1261, 1185, 1138, 1068, 1013, 976, 857, 840, 812, 765, 727,
700, 680, 627, 591, 522,501, 485, 469, 462, 447, 437, 417 cm-1. Anal. Calcd for C146H222N8O10: C,
77.96; H, 9.95; N, 4.98. Found C, 77.87; H, 9.95; N, 4.97.
4.9 1,4−Bis(5−(2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxalin−6−yl)−1,3,4−oxadiazol−
2−yl)benzene 1c (n = 12)
Mp. 157.8 °C. 1H NMR (300 MHz, CDCl3): δ 0.84−0.88 (m, 24H, −CH3), 1.25−1.41 (m, 144H,
−CH2), 1.65−1.82 (m, 16H, −CH2), 3.83 (q, 8H, −OCH2, J = 4.8 Hz), 4.00 (t, 8H, −OCH2, J = 3.6
Hz), 6.82−6.87 (m, 4H, Ar−H, J = 8.4 Hz), 7.08−7.10 (m, 4H, Ar−H), 7.13−7.18 (m, 4H, Ar−H),
8.25 (d, 2H, Ar−H, J = 8.7 Hz), 8.38 (s, 4H, Ar−H), 8.50 (dd, 2H, Ar−H, J = 2.1 Hz, J = 8.7 Hz),
8.88 (d, 2H, Ar−H, J = 1.8 Hz). 13C NMR (75 MHz, CDCl3): δ 14.08, 22.68, 26.04, 29.16, 29.23,
29.37, 29.45, 29.69, 31.92, 69.24, 113.14, 115.32, 122.90, 123.05, 124.12, 126.65, 127.02, 127.71,
128.12, 130.31, 131.16, 131.23, 140.60, 142.51, 148.77, 150.30, 150.39, 154.57, 154.78, 164.14,
164.56. IR (KBr): 2921, 2851, 2360, 2340, 1600, 1578, 1561, 1516, 1466, 1304, 1262, 1185, 1139,
1063, 1013, 894, 850, 816, 728, 688, 669, 629, 518, 488, 401 cm-1. Anal. Calcd for C146H222N8O10:
C, 77.96; H, 9.95; N, 4.98. Found C, 77.71; H, 9.97; N, 4.93.
4.10 1,3,5−Tris(5−(2,3−bis(3,4−bis(octyloxy)phenyl)quinoxalin−6−yl)−1,3,4−
oxadiazol−2−yl)benzene 1d (n = 8)
Mp. 102.0 °C. 1H NMR (300 MHz, CDCl3): δ 0.85−0.89 (m, 24H, −CH3), 1.28−1.43 (m, 80H,
−CH2), 1.64−1.84 (m, 16H, −CH2), 3.84 (t, 8H, −OCH2, J = 6.9 Hz), 4.00 (t, 8H, −OCH2, J = 6.6
Hz), 6.84 (d, 4H, Ar−H, J = 8.4 Hz), 7.12−7.19 (m, 8H, Ar−H), 8.27 (d, 2H, Ar−H, J = 8.7 Hz),
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ACCEPTED MANUSCRIPT
8.51 (dd, 2H, Ar−H, J = 1.8 Hz, J = 8.7 Hz), 9.00 (d, 2H, Ar−H, J = 2.1 Hz). 13C NMR (75 MHz,
CDCl3): δ 14.07, 22.66, 26.01, 29.20, 29.26, 29.31, 29.37, 31.82, 69.17, 69.22, 113.05, 115.24,
122.84, 123.05, 126.99, 129.34, 130.50, 131.01, 131.10, 140.53, 142.81, 148.78, 150.36, 150.47,
153.28, 154.67, 155.12, 165.91. IR (KBr): 3072, 2926,2 853, 2581, 2366, 1733, 1598, 1514, 1467,
1423, 1392, 1346, 1297, 1261, 1186, 1136, 1068, 1022, 971, 933, 896, 863, 841, 809, 780, 730,
675, 628, 485, 423 cm-1. Anal. Calcd for C168H234N12O15: C, 75.81; H, 8.86; N, 6.31. Found C,
74.81; H, 8.83; N, 6.10.
4.10.1 1,3,5−Tris(5−(2,3−bis(3,4−bis(decyloxy)phenyl)quinoxalin−6−yl)−1,3,4−
oxadiazol−2−yl)benzene 1d (n = 10)
Mp. 104.0 °C. 1H NMR (300 MHz, CDCl3): δ 0.84−0.88 (m, 36H, −CH3), 1.26−1.43 (m, 168H,
−CH2), 1.71−1.84 (m, 24H, −CH2), 3.83 (t, 12H, −OCH2, J = 6.3 Hz), 3.99 (t, 12H, −OCH2, J =
6.3 Hz), 6.84 (d, 6H, Ar−H, J = 8.1 Hz), 7.12 (s, 6H, Ar−H), 7.15−7.20 (m, 6H, Ar−H), 8.29 (d,
3H, Ar−H, J = 8.7 Hz), 8.55 (d, 3H, Ar−H, J = 9 Hz), 8.98 (s, 3H, Ar−H) , 9.14 (s, 3H, Ar−H). 13C
NMR (75 MHz, CDCl3): δ 14.07, 22.67, 26.03, 29.16, 29.23, 29.38, 29.44, 29.62, 29.68, 31.91,
69.22, 113.07, 115.28, 123.03, 123.82, 126.30, 127.03, 127.58, 128.49, 130.37, 131.10, 131.22,
140.60, 142.52, 148.76, 150.30, 150.37, 154.54, 154.82, 163.20, 165.07. IR (KBr): 3076, 2955,
2922, 2853, 2361, 1600, 1510, 1466, 1423, 1395, 1347, 1261, 1186, 1137, 1069, 1016, 982, 897,
840, 810, 781, 730, 674, 629, 488, 419 cm-1. Anal. Calcd for C192H282N12O15: C, 76.91; H, 9.48; N,
5.61. Found C, 76.52; H, 9.59; N, 5.50.
4.10.1 1,3,5−Tris(5−(2,3−bis(3,4−bis(dodecyloxy)phenyl)quinoxalin−6−yl)−1,3,4−
oxadiazol−2−yl)benzene 1d (n = 12)
Mp. 110.0 °C. 1H NMR (300 MHz, CDCl3): δ 0.84 (t, 36H, −CH3, J = 6.3 Hz), 1.25−1.43 (m,
216H, −CH2), 1.69−1.84 (m, 24H, −CH2), 3.83 (t, 12H, −OCH2, J = 6.3 Hz), 4.00 (t, 12H, −OCH2,
J = 6.3 Hz), 6.84 (d, 6H, Ar−H, J = 8.4 Hz), 7.12 (s, 6H, Ar−H), 7.15−7.20 (m, 6H, Ar−H), 8.30
(d, 3H, Ar−H, J = 8.7 Hz), 8.55 (d, 3H, Ar−H, J = 8.7 Hz), 8.98 (s, 3H, Ar−H) , 9.17 (s, 3H,
Ar−H). 13C NMR (75 MHz, CDCl3): δ 14.08, 22.67, 26.04, 29.17, 29.23, 29.37, 29.46, 29.69,
31.92, 69.22, 113.07, 115.29, 123.03, 123.83, 126.30, 127.02, 127.58, 128.49, 130.38, 131.10,
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131.23, 140.61, 142.52, 148.77, 150.30, 150.38, 154.53, 154.81, 163.20, 165.07. IR (KBr): 3076,
2923, 2544, 2363, 2329, 1602, 1510, 1467, 1424, 1394, 1347, 1261, 1186, 1136, 1071, 1017, 979,
897,841, 809, 730, 677, 628, 440, 413 cm-1. Anal. Calcd for C216H330N12O15: C, 77.79; H, 9.97; N,
5.04. Found C, 77.46; H, 9.96; N, 4.98.
4.10.2 1,3,5−Tris(5−(2,3−bis(3,4-bis(tetradecyloxy)phenyl)quinoxalin−6−yl)−1,3,4−
oxadiazol−2−yl)benzene 1d (n = 14)
Mp. 115.0 °C. 1H NMR (300 MHz, CDCl3): δ 0.83−0.88 (m, 36H, −CH3), 1.24−1.41 (m, 264H,
−CH2), 1.67−1.84 (m, 24H, −CH2), 3.84 (t, 12H, −OCH2, J = 6.3 Hz), 4.00 (t, 12H, −OCH2, J =
6.6 Hz), 6.84 (d, 6H, Ar−H, J = 8.4 Hz), 7.12 (s, 6H, Ar−H), 7.15−7.21 (m, 6H, Ar−H), 8.30 (d,
3H, Ar−H, J = 8.7 Hz), 8.56 (d, 3H, Ar−H, J = 7.8 Hz), 8.99 (s, 3H, Ar−H) , 9.17 (s, 3H, Ar−H).
13C NMR (75 MHz, CDCl3): δ 14.08, 22.67, 26.05, 29.17, 29.25, 29.36, 29.47, 29.72, 31.92, 69.22,
113.08, 115.29, 123.04, 123.83, 126.36, 127.04, 127.61, 128.50, 130.39, 131.11, 131.24, 140.62,
142.54, 148.78, 150.31, 150.38, 154.56, 154.83, 163.22, 165.10. IR (KBr): 3080, 2922, 2853,
2361, 1599, 1518, 1468, 1424, 1396, 1346, 1261, 1186, 1136, 1026, 897, 810, 730, 419 cm-1. Anal.
Calcd for C240H378N12O15: C, 78.51; H, 10.38; N, 4.58. Found C, 78.27; H, 10.38; N, 4.53.
Acknowledgements
We thank the Ministry of Science and Technology, Taiwan, ROC (MOST
104−2119−M−008−011) in generous support of this work. Also we deeply appreciated Prof.
Hwo-Shuenn Sheu for performing powder XRD experiments at the National Synchrotron
Radiation Research Center.
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