Mesogenic heterocycles derived from quinoxaline Schiff Bases

Article abstract of DOI:10.1016/j.tet.2016.07.076

Three new series of heterocyclic quinoxaline Schiff Bases 1–2 were prepared, characterized and their mesomorphic properties were investigated. These compounds 1 and 2 are in fact geometric isomers in which an imine moiety (e.g., [sbnd]C[dbnd]N) is inversely incorporated into quinoxaline, leading to an opposite local dipole. Two single crystallographic structures 1 (m=8, n=8) and 2a (m=12, n=8) were determined by X-ray crystallographic analysis in order to understand the effect of mesomorphic properties. Weak H-bonds, CH–π and π–π interactions were found in both crystals, which were attributed to the formation of mesomorphic behavior. Variable temperature FT-IR experiments were performed to confirm the induced H-bonds. All series?of compounds 1–2 exhibited N/SmC or SmC phases, which were identified by optical microscope and confirmed by powder X-ray diffraction experiments. Compounds 2a have a slightly wider range of mesophase temperatures than that of compounds 1 and 2b.

Full text of DOI:10.1016/j.tet.2016.07.076

Accepted Manuscript
Mesogenic heterocycles derived from quinoxaline Schiff Bases
Hsiu-Ming Kuo, Wan-Ping Ko, Gene-Hsiang Lee, Chung K. Lai
PII:
S0040-4020(16)30747-5
10.1016/j.tet.2016.07.076
TET 27972
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 15 May 2016
Revised Date: 17 July 2016
Accepted Date: 30 July 2016
Please cite this article as: Kuo H-M, Ko W-P, Lee G-H, Lai CK, Mesogenic heterocycles derived from
quinoxaline Schiff Bases, Tetrahedron (2016), doi: 10.1016/j.tet.2016.07.076.
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ACCEPTED MANUSCRIPT
Mesogenic Heterocycles Derived from Quinoxaline Schiff Bases
Hsiu−Ming Kuo, Wan−Ping Ko, Gene−Hsiang Lee, Chung K. Lai*
OCnH2n+1
N
N
H2m+1Cm
O
HO
N
1; m, n = 8, 10, 12
N
OCnH2n+1
N
H2m+1Cm
O
X
N
2a; X = OH, m = 12, n = 8, 12, 16
2b: X = H, m = 12, n = 8, 12, 16
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Mesogenic Heterocycles Derived from Quinoxaline Schiff Bases
Hsiu−Ming Kuo, Wan-Ping Ko, Gene−Hsiang Lee, Chung K. Lai*
Department of Chemistry, National Central University, Chung-Li, Taiwan, ROC
Abstract.
Three new series of heterocyclic quinoxaline Schiff Bases 1-2 were prepared, characterized and
their mesomorphic properties were investigated. These compounds 1 and 2 are in fact geometric
isomers in which an imine moiety (e.g. –C=N) is inversely incorporated into quinoxaline,
leading to an opposite local dipole. Two single crystallographic structures 1 (m = 8, n = 8) and
2a (m = 12, n = 8) were determined by X−ray crystallographic analysis in order to understand
the effect of mesomorphic properties. Weak H−bonds, CH−π and π−π interactions were found
in both crystals, which were attributed to the formation of mesomorphic behavior. Variable
temperature FT−IR experiments were performed to confirm the induced H−bonds. All series
series of compounds 1−2 exhibited N/SmC or SmC phases, which were identified by optical
microscope and confirmed by powder X−ray diffraction experiments. Compounds 2a have a
slightly wider range of mesophase temperatures than that of compounds 1 and 2b.
* 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. Introduction
An exquisite balance of molecular interactions in mesogenic structures is crucially
important to the formation of a preferred arrangement in order to generate practical materials
with cooperative properties. Dipole is one of the forces or interactions that have been commonly
utilized in the design of liquid crystalline materials. Such dipole or polarization might be
intrinsically existent or induced under an external influence. Intrinsic dipoles are generally
obtained from molecular structures when designed properly. A polar group incorporated
resulting in an asymetric structure is a common approach. This is particularly important in
rod−shaped molecules. Rod−like molecule has an elongated and anisotropic geometry which
allows for preferential alignments along one spatial direction. On the other hand, too strong or
too weak an interaction force often resulted in the formation of a solid or liquid state. However,
such a force balance seems to be almost impossible to control and predict.
Schiff bases, such as salicyladimine and enaminoketone1 were probably the most studied
among known mesogens due to their known chemistry and versatile structures. They were easily
prepared by condensations of ketones or aldehydes with amines. The Schiff bases were
constructed by an imine moiety (−C=N). The imine fragment not only plays a role of linking
group, but also holds a local dipole moment. On the other hand, this dipole might
macroscopically induce the overall liquid crystallinity if incorporated correctly. In addition, the
better planarity of salicylaldimines derivatives formed by intramolecular H−bond often
facilitated a more ordered or preferred arrangement in mesophases.
Quinoxalines, considered as a heterocyclic core moiety have been applied to produce photo
luminecent and/ or efficient electroluminescent materials2 due to their extended π−conjugated and
more rigid structures. It is an electron−transporting or hole−blocking layer in organic
light-emitting diodes3 (OLEDs) because of its relative ease of preparation and also higher
thermal stability. Known examples of mesogenic quinoxalines4 were relatively rare, and the
formation of mesophases was mostly attributed to the weak intermolecular π−π interactions. A
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good LC gelator obtained from dibenzophenazine and diphenylquinoxaline5 was observed and
studied. Interestingly, a double−decked, hydrophobic metallocyclophane6 was formed by
self−assemble of quinoxaline−pyridine hybrids with tetrahedral metals (i.e. Cu1+, Ag1+). A
similar series of N-salicylidene-2-hydroxyanilines7 I and quinoxaline−salicylaldimine
conjugates II were previously prepared in this group (Fig. 1). All compounds I formed an
enantitropic SmC phase, compounds II formed an enantiotropic columnar phase. This is the first
examples derived from N-salicylidene-2-hydroxy aniline that exhibited columnar phases. A
series of electron−accepting acene−type LC materials, dialkoxycyano pyrazinoquinoxaline8 III
were also developed, and its hole mobility in SmA phase was observed. In these highly
conjugated systems, CH−π and π−π interactions9 also played an important role.
Fig. 1 The molecular structures of similar compounds I−III.
In this work, three series of Schiff Bases 1−2 (see Scheme 1) incorporating heterocyclic
quinoxaline were prepared, characterized and their mesomorphic properties were investigated.
These compounds; iminephenols 1 and aminophenols 2a are in fact geometric isomers in which
an inime moiety (e.g. –C=N) is inversely incorporated. Dipole induced in opposite direction
might impact the overall dipole generated in these two series of rod−like molecules. A
systematic investigation of the structure–property relationship by changing the terminal carbon
lengths and the dipole might provide us about microscopic and/or macroscopic information in
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understanding the relationship between the structures and the properties. Compound 2b was also
prepared to understand the effect of H−bonds formed by hydroxyl group on the formation of
mesomorphic behavior. Single crystal crystallographic data of two single crystals showed that
the intramolecular and intermolecular H−bonds were observed in the crystal phases. Variable
temperature FT−IR experiments were performed to confirm the H−bonds induced in crystal
phases. Results indicated that all compounds 1−2 were truly mesogenic, exhibiting
nematic/smectic C or smectic C phases, which were identified and confirmed by powder X−ray
diffraction. The UV–vis absorption and PL spectra of three compounds in CH2Cl2 solution at
room temperature were also studied.
2. Results and Discussion
2.1 Synthesis and Characterization
The synthetic procedures for compounds 1 are summarized in Scheme 1. 2-(4-Hydroxy
phenyl)-2-oxoacetaldehyde was prepared by the reaction of 1-(4-hydroxyphenyl)ethanone and
SeO2 in refluxing 1,4-dioxane/H2O. Methyl 2-(4-hydroxyphenyl)quinoxaline-6-carboxylate was
obtained by the reaction of 3,4-diaminobenzoate with 2-(4-hydroxyphenyl)-2-oxoacetaldehyde
in refluxing ethanol. Alkylation of methyl 2-(4-hydroxyphenyl)quinoxaline-6-carboxylate with
n-alkylbromides and K2CO3 in refluxing acetone gave 2-(4-alkoxyloxyphenyl)quinoxaline-
6-carboxylates, which were then further reduced by Pd/C and hydrazine monohydrate to
produce 2-(4-(alkoxy)phenyl)quinoxalin-6-amines. All amines were freshly prepared and
directly used for the final steps without any purification. The final compounds 1, (E)-5-(alkoxy)-
2-(((2-(4-(alkoxy)phenyl)quinoxalin-6-yl) imino)methyl)phenols were obtained by reaction of
2-hydroxy-4-(alkoxy)benzaldehydes and 2-(4-(alkoxy)phenyl)quinoxalin-6-amines in refluxing
dried THF. 1H and 13C NMR spectroscopy in CDCl3 were used to characterize all intermediates.
For instance, two characteristic singlet peaks occurred at δ 8.67−8.68 and 13.40−13.51 ppm, were
assigned for imine –CH=N and phenolic −OH groups in compounds 1.
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The synthetic procedures for compounds 2 are summarized in Scheme 2. Methyl 2-
(4-hydroxyphenyl)quinoxaline-6-carboxylates and methyl 2-(4-(alkoxy)phenyl)quinoxaline-
6-carboxylates were prepared similar to aforementioned procedures in Scheme 1 for methyl
2-(4-hydroxyphenyl)quinoxaline-6-carboxylate and 2-(4-alkoxyloxyphenyl)quinoxaline-
6-carboxylates. (2-(4-(Alkoxy)phenyl)quinoxalin-6-yl)methanols were obtained by reduction of
methyl 2-(4-(alkoxy)phenyl)quinoxaline-6-carboxylates with LAH stirring in THF. Then
2-(4-(alkoxy)phenyl)quinoxaline-6-carbaldehydes were prepared by oxidation of (2-(4-(alkoxy)
phenyl)quinoxalin-6-yl)methanols with pyridinium chlorochromate in stirring THF. Two
characteristic singlet peaks on 1H-NMR spectroscopic spectra occurred at δ 4.91and 10.23 ppm,
assigned for groups −CH2OH and –CHO. The final compounds 2a and 2b were obtained by
condensation reactions of 2-(4-(Alkoxy)phenyl)quinoxaline-6-carbaldehydes with appropriate
amines. Some characteristic chemical peaks on 1H−NMR spectra were summarized in Table 1.
Elemental analysis of all compounds 1−2 was performed.
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Table 1. The characteristic chemical peaksa on 1H NMR spectra for compounds 1−2
Compds
1
2a
2b
−OH (phenolic)
−N=CH− (imine)
−CH=N− (imine)
13.40−13.51
8.67−8.68
−
7.54
−
−
−
8.79−8.82
8.70
a: samples were all dissolved in CDCl3 at rt. Unit: δ (ppm).
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In order to identify the formation of H−bonds possibly formed in crystal, liquid crystal
or/and isotropic states, variable temperature FT−IR spectra of three compounds 1−2 (all m, n =
12) were performed (in Fig. 2). In this experiment, a thin disc grounded with KBr was prepared
and studied at a temperature of 30 to 250 °C on the heating process. In general, the IR frequency
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of intermolecular H−bonds formed by phenolic–OH group occurred at ca. 3,200–3,400 cm-1,
whereas, intramolecular H−bonds occurred at a lower region of ca. 2,800–3,100 cm-1. As seen
from Fig. 2, a characteristic broad and weak peak of H−bond occurred at ca. 3,435 cm-1, assigned
to the phenolic –OH group was observed for compound 2a (m = n = 12) only at temperature of T
= 30 °C. A very weak peak at T = 70 °C was barely observed. However, this weak peak was not
seen at higher temperatures or isotropic state. This indicated that an intermolecular H−bond was
only formed in crystal state for compounds 2a. Other compounds 1 and 2b have no
intermolecular H−bonds formed in either crystal or mesogenic states. Obviously, the H−bonds
were not the major force or interaction induing the mesophases in such system.
Heating (1)
Heating (2a)
Heating (2b)
250 °C
150 °C
250 °C
250 °C
150 °C
170 °C
90 °C
50 °C
70 °C
30 °C
70 °C
30 °C
3435
3435
3500
3000
3500
3000
3500
3000
Wavenumber (cm-1)
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 2 Variable−temperature IR spectra for compound 1 (left), 2a (middle) and 2b (right, all m =
n = 12) obtained in heating process using KBr pellets.
2.2 Single crystal structures of (E)-5-(octyloxy)-2-(((2-(4-(octyloxy)phenyl)quinoxalin-6-yl)
imino)methyl)phenol and (E)-2-(((2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)methylene)
amino)-5-(octyloxy)phenol
In order to understand the correlation between the molecular structures and the formation of
mesophases, two single crystals of compounds 1 (m = n = 8) and 2a (m = 12, n = 8) were
obtained and their single crystallographic data were investigated. Two light yellow crystals
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suitable for X−ray diffraction analysis were slowly grown from THF at room temperature. Fig. 3
shows the molecular structures with the atomic numberings, and Table 2 lists the crystallographic
and structural refinement data for the two compounds. The crystal 1 crystallizes in the monoclinic
space group P2(1)/c, with a = 27.627(2) Å, b = 10.3544(8) Å, c = 11.2292(9) Å, and Z = 4, in
contrast, the crystal 2a crystallizes in the triclinic space group P-1, with a = 5.7598(2)Å, b =
13.6324(4) Å, c = 24.1259(8) Å, and Z = 2. The overall structures of both crystals were quite
planar, and the molecular lengths were l = 37.95 Å (atom C29−C37) and l = 42.76 Å (atom
C29−C41), for crystal 1 and 2a, respectively. On the other hand, the overall molecular planes in
two crystals were quite planar, and the dihedral angles twisted from central plane 2 (see Fig. 4) to
other phenyl or others were smaller than 8.839°. The relatively smaller dihedral angles favored
the formation of mesophases induced in these two series of molecules. A summary of all dihedral
angles in two crystals was listed in Table 3. The bond length of imine −CH=N was ranged in d =
1.2865 Å in crystal 1 and d = 1.2751 Å in crystal 2a.
Fig. 3 Two ORTEP drawings for compound 1 (top) and 2a (bottom) with the numbering scheme,
and thermal ellipsoids of non−hydrogen atoms are drawn at the 50% probability level.
Table 2. Crystallographic data collected for single crystals 1 and 2a.
Compd.
1 (m = 8, n = 8)
C37H47N3O3
581.78
2a (m = 12, n = 8)
C41H55N3O3
637.88
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
200(2)
200(2)
Monoclinic
P2(1)/c
Triclinic
P−1
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a(Å)
27.627(2)
10.3544(8)
11.2292(9)
90
5.7598(2)
b(Å)
13.6324(4)
24.1259(8)
102.2488(18)
93.2326(18)
99.4464(15)
2
c(Å)
α(o)
β(o)
94.875(2)
90
γ(o)
Z
4
Density (calculated, Mg/m3)
Crystal size (mm3)
θ range for data collection(o)
Reflections collected
Independent reflections
Final R1, wR2
1.207
1.165
0.50x0.28x0.10
0.74 to 27.50
20512
0.150x0.100x0.030
3.373 to 67.500
12270
7351 [R(int) = 0.0486] 6371 [R(int) = 0.0255]
0.0660, 0.1456 0.0576, 0.1545
Table 3. Dihedral anglesa twisted in 1 (m = 8, n = 8) and 2a (m = 12, n = 8)
Ring plane
1
2
3
4
1
+8.839o
+3.384o
-
-
+0.410o
+1.838o
+6.517o
+0.778o
2a
a: angles are measured relative to central plane 2.
As expected, intramolecular H−bonds were observed in both crystal lattices. A distance of
intramolecular H−bond, ca. d 1.833, 2.131 and 2.574 Å was obtained for the two crystals, as
shown Fig. 4. These intramolecular H−bonds kept the phenolic ring nearly coplanar to the
quinoxaline ring, and the better planar structure was favorable for a better packing both in the
solid or/and the liquid crystal state. In contrast, other intermolecular interactions, such as
H−bonds, CH−π, and π−π were also observed in two crystals. In general, the CH−π11, and π−π
interactions were much weaker than H−bonds. However, these weak intermolecular forces were
very crucial on the induction and/or formation of mesophases in such heterocyclic compounds. A
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distance of d 1.928−2.928 Å was observed for the intermolecular H−bonds in crystal 2a. In
contrast, other weaker intermolecular interactions of CH−π and π−π, at a distance of d
2.812−3.403 Å were observed in both crystals. A dimeric structure was first formed by the
intermolecular CH−π, and π−π interactions and the dimers were then packed into the lattice,
shown in Fig. 6. A layer distance of 30.33 Å was observed in lattice, which was smaller than
those of d 35−37 Å obtained by XRD diffraction at mesophases. In contrast, crystal 2a has no
strong H−bonds; a more coplanar core structure was instead facilitated by weaker intermolecular
H−bonds; CH−O and CH−N. In crystal 2a, the formation of a layered structure was formed
or/and induced by strong H−bonds, OH−N (1.928 Å) and weaker H−bonds, CH−O (2.719 Å) and
CH−N (2.928 Å), giving a head−to−tail arrangement in the lattice (Fig. 5). Neighboring layers
were formed by weaker intermolecular CH−π (2.812 Å), shown in Fig 6. A summary of H−bonds
and other interactions was listed in Table 4 and Table 5. All series of compounds 1 and 2
exhibited layered structure in mesophases. A layered structure correlated to the smectic phases in
both unit cells was shown in Fig. 7.
Fig. 4 Intramolecular H−bonds in crystals1 1 (top) and 2a (bottom).
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Fig. 5 Intermolecular H−bonds observed in crystal 2a (m = 12, n = 8).
Fig. 6 Intermolecular CH−π, π−π interactions observed in crystal 1 (top) and 2a (bottom).
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Table 4. Intra and intermolecular H−bonds observed in both crystals.
Compd. Type
no.
bond
angles/°
H−bonds
distance/Å
1
C−H…N
O−H…N
1
2
H1−N1 = 1.83
H1−N2 = 1.93
∠O1−H1−N1 = 147.00
∠O1−H1−N2 = 166.90
2a
Table 5. Weak interactions observed in 1 (m = 8, n = 8) and 2a (m = 12, n = 8)
Compd. Type
no.
bond
angles/°
H−bonds distance/Å
1
C−H…O
π…π
C−H…π
2
2
2
2.81
3.40
2.84
∠C1−H1A−O1 = 158.51
∠ring−H22B−C22 = 140.40
2a
C−H…O
C−H…O
C−H…N
C−H…N
C−H…π
1
2
1
2
2
H9−O1 = 2.13
H5−O3 = 2.72
H6−N3 = 2.57
H18−N1 = 2.93
ring−H22A = 2.81
∠C9−H9−O1 = 127.19
∠C5−H5−O3 = 172.78
∠C6−H6−N3 = 97.24
∠C18−H18−N1 = 162.34
∠ring−H22A−C22 = 154.23
* Interactions or distance measured from the center of ring to the atom.
** π−π interaction or distance measured from vertical distance from ring center of benzene to
plane by quinoxaline.
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Fig. 7 A layer structure correlated to the smectic phases in 1 (top) and 2a (bottom).
2.3. Thermal analysis and phase properties
The liquid crystalline behavior of Schiff−base derivatives 1−2 was characterized and studied by
differential scanning calorimetry (DSC) and polarized optical microscope (POM). The phase
transitions and thermodynamic data are summarized in Table 6. All three series of heterocyclic
derivatives 1−2 exhibited an enanotropic mesomorphic behavior under POM observation, giving
nematic/smectic C or smectic C phases. The sole shorter derivative 1 (m = n = 8) formed SmC
phase at lower temperature and N phase at higher temperature. The clearing temperatures of
compounds 1 decreased with carbon length of terminal alkoxy chains, i.e., Tcl = 274.8 (m = n = 8)
> 261.9 (m = 12, n = 8) > 239.9 °C (m = 12, n = 16). The temperature range of mesophase was
quite wide; ∆Tmeso = 155.0 (m = 12, n = 16) < 189.4 (m = 12, n = 8) < 204.3 °C (m = n = 8) on
cooling process. This wide range of mesophase temperature might be attributed to a better planar
core, discussed in the crystallographic data. The transition enthalpies of SmC−to−I transitions
were in the range of ∆HSmC→I = 10.6−17.8 kJ/mol. These compounds 1 have a higher clearing
temperature than that of compounds 2a and 2b, for example, Tcl = 261.9 (1; m = 12, n = 8) >
256.7 (2a; m = 12, n = 8) > 245.7 °C (2b; m = 12, n = 8). The polar imine –C=N group is
generally considered as an electron−donating group, however, the quinoxaline is an
electron−withdrawing moiety. The direction of polar imine group affected the overall molecular
dipole. Under optical microscope, they all exhibited typical Schlieren textures of four or/and two
brushes or focal−conics textures. Furthermore, the nematic phase showed slightly homeotropic
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domains, whereas, the smectic C phases showed no homeotropic textures.
In order to understand the effect of the polar groups, i.e. imine (−CH=N) and hydroxyl (−OH)
incorporated on the formation of mesophases, the compound 2a and 2b were prepared and their
mesomorphic behavior was investigated. Compounds 1 and 2a are in fact geometric isomers.
However, the imine moiety (−CHN) in both series of compounds was reversely incorporated, and
the local dipoles might alter the overall molecular polarization. The DSC data showed that the
clearing temperatures of compounds 2a decreased with carbon length of alkoxy chains, and they
were slightly lower than those of compounds 1 by ca. ∆Tcl = 5.2 (m = 12, n = 8) − 8.3 °C (m = 12,
n = 12). All compounds 2a formed smectic C phases, and the temperature range of the SmC
phases was ∆T = 187.0−156.8 °C on cooling process. All compounds 2a exhibited typical focal
conics textures. Their transition enthalpies of SmC−to−I transitions were slightly smaller than
those of compounds 1, i.e. ∆H = 6.48−9.15 kJ/mol.
On the other hand, all compounds 2b exhibited an enantiotropic behavior, giving N and SmC
phases. Their clearing temperatures were slightly lower than those of compounds 1 and 2a.
Compounds 2a and 2b have the molecular structures in which the polar imine group is positioned
opposite direction of the quinoxaline ring. This might reduce the overall molecular dipole to
some extent, leading to a difference in polarization and/or anisotropics. Furthermore, a lack of
polar hydroxyl group incorporated in compounds 2b was apparently unable to induce the possible
intra or intermolcular H−bonds. DSC data indicated that all clearing temperatures of compounds
2b were lower than those of compounds 2a by ∆Tcl = 11.0 (m = 12, n = 8) − 6.5 °C (m = 12, n =
12). Also the temperature of mesophase phases were slightly narrower than those of compounds
2a by ca. ∆TSmC = 13.5 (m = 12, n = 12) − 17.0 °C (m = 12, n = 16). The lower clearing or/and
narrower temperature of SmC phases were possibly attributed to the absence of a polar hydroxyl
group. A similar texture identified as focal−conic and Schlieren textures for smectic C and N
phases was observed under optical microscope (Fig. 8). The bar graph in Fig. 9 showed the
temperature of mesophases of all compounds 1−2.
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Fig. 8 Optical textures observed. N phase at 270 oC by 1 (m = 8, n = 8; top left), SmC phase at
240 oC by 1 (m = 8, n = 8; top right), SmC phase at 270 oC by 2a (n = 8; middle left), SmC phase
at 200 oC by 2a (n = 12; middle right), SmC phase at 190 oC by 2a (n = 16; bottom left), and
SmC phase at 220 oC by 2b (n = 16, bottom right).
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Fig. 9 Bar graphs showing the phase behavior of compounds 1−2. All temperatures were taken
from cooling process.
2.4 Variable powder diffraction
Variable−temperature powder XRD diffraction experiments were performed to confirm the
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structures of the mesophases. Fig. 10 shows typical diffraction plots for three derivatives. A strong
diffraction with a d−spacing of ca. 35.6, Å, 37.1, Å and 36.4Å was obtained for compounds
compound 1 (m = 12, n = 12), 2a (m = 12, n = 12) and 2b (m = 12, n = 12) at 180 oC, respectively.
These three peaks corresponded to layer structures, assigned as Miller indices 001. These
d−spacings were slightly larger than those of values measured from crystallographic data. In
addition, a second diffraction peak at 17.9 Å, 18.5 Å and 18.1 Å was also observed, assigned for
indices 002. The diffraction patterns were typically characteristic of a layer structure observed for
a SmC phase. The structure of SmC phase is similar to that of the unstructured SmA phase except
that the long axes of the molecules are tilted with respect to the planes of the layers. The optical
character is positive biaxial. The much smaller d−spacing indicated that the molecules were tilted
or/and the terminal alkoxy chains were to some extent interdigitated in smectic C phase.
Furthermore, a very broad and weak peak at ~ 4.70 Å was observed, and this peak was assigned
to the molten alkoxy chains.
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Fig. 10 The powder X−ray diffraction plots of compound 1 (m = n =12, top), 2a (m = n = 12,
middle) and 2b (m = n = 12, bottom) at 180 °C
Table 7. Detailed indexation by powder XRD of smectic C phasesa in compounds 1−2.
Compds
Mesophases/temp
d−Spacing
Miller indices
1 (m =12, n = 12)
SmC at 180.0 °C
35.6
17.9
4.70
37.1
18.5
4.70
36.4
18.1
4.70
001
002
halo
001
002
halo
001
002
halo
2a (m =12, n = 12)
2b (m =12, n = 12)
SmC at 180.0 °C
SmC at 180.0 °C
a: temperature taken on the cooling process;
2.5 Optical properties
The UV–vis absorptions of three compounds in CH2Cl2 solution at room temperature are
presented in Figure 11. The UV–vis absorption spectra of three compounds 1−2 (all m = 12, n =
16) were quite similar. A longer conjugated length was extended by imine moiety in all three
compounds. The absorption λmax peaks occurred at ca. 394 (for 1), 407 (for 2a) and 390 nm (for
2b) were attributed to π−π* transitions10 arising from quinoxaline moiety. Other peaks at
259−333 nm were originated from n−π transitions, as summarized in Table 8. The imine –C=N
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group is polar and generally considered as an electron−donating group, however, the quinoxaline
is an electron−withdrawing moiety. The push−pull effect by these two groups in compound 1 has
caused absorption slightly red−shifted over other compounds 2. In contrast, the presence of a
hydroxyl group in compound 2a caused an apparent shift (ca. 17 nm) on the λmax absorption.
1
2a
1.0
2b
0.5
0.0
300
400
500
600
Wavelength (nm)
Fig. 11 Absorption spectra of the compounds 1 and 2a−b (all m = 12, n = 16).
Table 8. Absorption dataa of compounds 1 and 2a−b (all m = 12, n = 16).
Compd
Absorption/nm
1
259, 291, 394
2a
2b
258, 333, 407
253, 279, 330, 390
a: Measured in CH2Cl2 solution (1.0 x 10-5 M) at room temperature.
3.0 Conclusions
Three series of heterocyclic quinoxaline Schiff bases were prepared and their mesomorphic
properties studied. Inter or intramolecular H−bond, intermolecular CH−π and π−π played an
important role in unit cells of crystals 1 and 2a. They exhibited N/SmC or SmC phases over a
wide range of mesophases. The compounds 1 and 2a with a hydroxyl group incorporated have a
slightly higher clearing temperature that the others, i.e. compounds 2b. In addition, the polar
21

ACCEPTED MANUSCRIPT
direction of the imine moiety also affected the mesophase temperatures.
4.0 Experimental Section
4.1 General materials and methods
All chemicals and solvents were reagent grades 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 2 equipped with a hot stage system of
Mettler FP90/FP82HT. The UV−vis absorption spectra were obtained using a Jasco V−530
spectrometer. Elemental analyses were performed on a Heraeus CHN−O−Rapid elemental
analyzer. The powder diffraction data were collected from the Wiggler−A beam line of the
National Synchrotron Radiation Research Center (NSRRC) with a wavelength of 1.3263 Å. The
powder samples were charged in Lindemann capillary tubes (80 mm long and 0.01 mm thick)
with an inner diameter of 1.0 mm from Charles Supper Co. The compounds of 4-alkoxynitro
benzenes, 4-alkoxyanilines, 3-alkoxy-6-nitrophenols and 3-alkoxy-6-anilines were prepared by
literatures’ procedures.
4.2. 2-(4-Hydroxyphenyl)-2-oxoacetaldehyde
To a solution of SeO2 (1.11 g, 0.01 mol), 1,4-dioxane/H2O (20 : 1), 1-(4-hydroxyphenyl)ethanone
(1.36 g, 0.01 mol) was added and the mixture was refluxed under nitrogen for 4 h. The solid
residues were removed by filtration. The solvent of filtrate were removed under reduced pressure.
The suspension was added 20 mL more water and heated to reflux for 3 h. Charcoal was added to
the solution, and the solution was then refluxed for further 30 min. Upon cooling to room
temperature, the solids were filtered. The filtrate was extracted by ethyl acetate/H2O. The organic
layer was collected and evaporated to dryness. The products isolated as pale yellow powder were
obtained by recrystallization from CH2Cl2. Yield: 80% 1H NMR (300 MHz, CDCl3): δ 5.60−5.65
22

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(m, 2H, CH−OH), 6.68 (s, 1H, −CH), 6.79−6.93 (m, 2H, Ar−H), 7.90−8.00 (m, 2H, Ar−H), 10.44
(s, 1H, −CH), 10.76 (s, 1H, Ar−OH). 13C NMR (75 MHz, DMSO): δ 15.47, 18.72, 56.54, 62.64,
88.60, 94.73, 115.54, 115.58, 115.94, 116.03, 116.32, 125.35, 132.33, 162.67, 162.80, 193.01,
194.77.
4.3. Methyl 3,4-diaminobenzoate
To a solution of 3,4-diaminobenzoic acid (10.0 g, 0.07 mol) and 200 mL of MeOH, sulfuric acid
(30 mL) was added dropwise. After refluxing for 24 h, the solution was cooled to room
temperature. The organic solvent was removed with reduced pressure. The crude product was
neutralized by aqueous K2CO3 then extracted by ethyl acetate/H2O. The organic layers were
collect and the solvent was evaporated to dryness. The products isolated as brown powder was
obtained after recrystallization from MeOH. Yield: 78%. 1H NMR (300 MHz, CDCl3): δ 3.52 (s,
4H, NH2), 3.82 (s, 3H, −CH3), 6.64 (d, 1H, Ar−H, J = 8.1 Hz), 7.38 (d, 1H, Ar−H, J = 1.8 Hz),
7.44 (d, 1H, Ar−H, J = 9.9 Hz). 13C NMR (75 MHz, DMSO): δ 51.62, 114.86, 118.31, 121.03,
123.22, 133.08, 140.39, 167.33.
4.4. Methyl 2-(4-hydroxyphenyl)quinoxaline-6-carboxylate
To a solution of methyl 3,4-diaminobenzoate (10.0 g, 0.07 mmol) and 200 mL of EtOH,
2-(4-hydroxyphenyl)-2-oxoacetaldehyde (9.0 g, 0.06 mmol) was added, the mixture was refluxed
for 3 h. Upon cooling to room temperature, the crude product was collected by filtration. The
products isolated as yellow powder were obtained after recrystallization from hexane/methanol.
Yield: 90%. 1H NMR (300 MHz, DMSO): δ 3.94 (s, 3H, −OCH3), 6.97 (d, 2H, Ar−H, J = 8.7 Hz),
8.12 (d, 1H, Ar−H, J = 8.1 Hz), 8.23 (d, 2H, Ar−H, J = 12.3 Hz), 8.54 (d, 1H, Ar−H, J = 1.2 Hz),
9.56 (s, 1H, Ar−H), 10.18(s, 1H, Ar−H). 13C NMR (75 MHz, DMSO): δ 53.07, 116.55, 126.78,
129.76, 129.95, 130.06, 131.15, 140.05, 144.17, 145.19, 153.03, 160.95, 166.03.
4.5. Methyl 2-(4-(dodecyloxy)phenyl)quinoxaline-6-carboxylate (m = 12)
Under nitrogen atmosphere, the mixture of methyl 2-(4-hydroxyphenyl)quinoxaline-6-
carboxylate (0.5 g, 2 mmol), dry acetone (75 mL) , K2CO3 (0.37 g, 2 mmol) and KI (catalyst
23

ACCEPTED MANUSCRIPT
amount) was refluxed for 24 h. The solids were filtered and the filtrate was concentrated by
rotary evaporator. The crude product was extracted by ethyl acetate/H2O. Organic layers were
collected and dried over MgSO4. The solvents were removed. The products isolated as white
solids were obtained after recrystallization from hexane/MeOH. Yield 74 %. 1H NMR (300 MHz,
CDCl3): δ 0.86 (t, 3H, −CH3, J = 6.6 Hz), 1.25−1.49 (m, 18H, −CH2), 1.77−1.84 (m, 2H, −CH2),
4.00 (s, 3H, −OCH3), 4.04 (t, 2H, −OCH2, J = 6.45 Hz), 7.06 (d, 2H, Ar−H, J = 8.7 Hz), 8.13 (d,
1H, Ar−H, J = 8.4 Hz), 8.19 (d, 2H, Ar−H, J = 8.7 Hz), 8.33 (d, 1H, Ar−H, J = 8.7 Hz), 8.78 (s,
1H, Ar−H), 9.35 (s, 1H, Ar−H). 13C NMR (75 MHz, CDCl3): δ 14.07, 22.65, 25.99, 29.16, 29.31,
29.36, 29.56, 31.61, 31.88, 52.48, 68.25, 115.19, 128.31, 129.22, 129.46, 129.73, 130.19, 131.68,
140.28, 143.93, 144.38, 152.77, 161.61, 166.30. MS(FAB): calcd. For M+ C28H36N2O3: 448.27,
found: 448.2718
4.6. (2-(4-(Dodecyloxy)phenyl)quinoxalin-6-yl)methanol (m = 12)
Under nitrogen atmosphere, to the mixture of methyl 2-(4-(dodecyloxy)phenyl)quinoxaline-
6-carboxylate (0.5 g, 1 mmol) and 50 ml dry THF, lithium aluminium hydride (0.12 g, 3 mmol)
was added. The mixture was stirred at room temperature for 4 h. Hydrochloric acid (2.0 N) was
slowly added into the mixture for quenching the reaction. The mixture was concentrated by rotary
evaporator and the residues were extracted with dichloromethane/H2O. Organic layers were
collected and dried over MgSO4. The solvents were removed. The products isolated as white
solids were purified by column chromatography eluting with ethyl acetate and hexane. Yield 60%.
1H NMR (300 MHz, CDCl3): δ 0.86 (t, 3H, −CH3, J = 6.9 Hz) 1.24−1.62 (m, 18H, −CH2), 1.81
(quint, 2H, −CH2, J = 6.3 Hz), 4.02 (t, 2H, −CH2, J = 6.3 Hz), 4.91 (s, 2H, −CH2), 7.03 (d, 2H,
Ar−H, J = 8.0 Hz), 7.74 (d, 1H, Ar−H, J = 7.7 Hz), 8.02 (s, 1H, Ar−H), 8.06 (d, 1H, Ar−H, J =
8.7 Hz), 8.12 (d, 2H, Ar−H, J = 7.7 Hz), 9.23 (s, 1H, Ar−H). 13C NMR (125 MHz, CDCl3): δ
14.11, 22.68, 26.02, 29.20, 29.34, 29.39, 29.59, 29.63, 31.91, 64.77, 68.22, 115.12, 126.08,
128.87, 129.27, 129.54, 141.01, 141.81, 142.15, 143.20, 151.43, 161.09.
2-(4-(Dodecyloxy)phenyl)quinoxaline-6-carbaldehyde (m = 12)
24

ACCEPTED MANUSCRIPT
Under nitrogen atmosphere, to the mixture of (2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)
methanol (0.5 g, 2 mmol) and 50 mL dry THF, pyridinium chlorochromate (0.75 g, 3.5 mmol)
was added. The mixture was stirred under room temperature for 3 h. The mixture was then
condensed by rotary evaporator, purified by column chromatography with ethyl acetate and
hexane. White powder was obtained with 85% yield. 1H NMR (300 MHz, CDCl3): δ 0.86 (t, 3H,
−CH3, J = 6.9 Hz) 1.25−1.59 (m, 18H, −CH2), 1.82 (quint, 2H, −CH2, J = 6.6 Hz), 4.04 (t, 2H,
−CH2, J = 6.5 Hz), 7.06 (d, 2H, Ar−H, J = 8.7 Hz), 8.19 (d, 1H, Ar−H, J = 2.5 Hz), 8.22 (d, 2H,
Ar−H, J = 1.7 Hz), 8.53 (s, 1H, Ar−H), 9.38 (s, 1H, Ar−H), 10.23 (s, 1H, CO−H). 13C NMR (125
MHz, CDCl3): δ 14.06, 22.63, 25.96, 29.11, 29.33, 29.53, 29.59, 31.86, 68.25, 115.23, 127.11,
128.14, 129.30, 130.52, 134.48, 136.06, 140.69, 144.20, 145.57, 153.30, 161.75, 191.25.
4.9. 4-(6-Nitroquinoxalin-2-yl)phenol
A solution of 2–(4–hydroxyphenyl)–2–oxoacetaldehyde (0.2 g, 4.19 mmol), 4-nitrobenzene-
1,2-diamine (0.64 g，4.19 mmol) and 75 mL EtOH was refluxed for 30 min under nitrogen. The
mixture was extracted by CH2Cl2 and water. The organic layers were collected, dried over MgSO4 and
then concentrated under reduced pressure. The products isolated as pale yellow powder were obtained
after recrystallization from hexanes and MeOH. Yield: 76%. 1H NMR (300 MHz, CDCl3): δ 7.03 (d,
2H, Ar−H, J = 8.7 Hz), 8.14−8.22 (m, 3H, Ar−H), 8.52 (dd, 1H, Ar−H, 3J = 9.3 Hz, 4J = 2.4 Hz),
8.98 (d, 1H, Ar−H, J = 2.4 Hz), 9.24 (s, 1H, Ar−H). 13C NMR (125 MHz, DMSO): δ 116.15,
116.22, 123.78, 124.69, 124.88, 125.91, 129.66, 129.97, 130.60, 130.73, 139.08, 144.47, 145.94,
146.49, 153.42, 161.00.
4.10. 2-(4-(Octyloxy)phenyl)-6-nitroquinoxaline (m = 8)
Under nitrogen atmosphere, a mixture of 4-(6-nitroquinoxalin-2-yl)phenol (0.5 g, 2 mmol) and
K2
CO (0.37 g, 2.0 mmol) was mixed in 75 mL dry acetone. The solution was then refluxed for 30
3
min. The mixture was then added 1–bromooctane (0.39 g, 2.0 mmol) and further refluxed for 24 h.
The residual solids were filtered off. The filtrate was extract by ethyl acetate/water. The organic
layers were then collected and concentrated and then concentrated by rotary evaporator. The
25

ACCEPTED MANUSCRIPT
products were purified by column chromatography eluting with ethyl acetate and hexane. Yellow
powder; yield 70%. 1H NMR (300 MHz, CDCl3): δ 0.88 (t, 3H, −CH3, J = 6.6 Hz), 1.17−1.55 (m,
10H, −CH2), 1.82 (quint, 2H, −CH2, J = 7.8 Hz), 4.05 (t, 2H, −OCH2, J = 6.5 Hz), 7.07 (d, 2H,
Ar−H, J = 8.9 Hz), 8.12−8.30 (m, 3H, Ar−H), 8.50 (dd, 1H, Ar−H, 3J = 9.15 Hz, 4J = 2.5 Hz),
8.97 (d, 1H, Ar−H, 2.1 Hz), 9.42 (s, 1H, Ar−H). 13C NMR (125 MHz, CDCl3): δ 14.23, 22.78,
26.14, 29.28, 29.36, 29.47, 31.93, 68.48, 115.49, 123.81, 125.73, 127.88, 129.65, 130.91, 139.95,
145.31, 147.10, 153.97, 162.25.
4.11. 2-(4-(Decyloxy)phenyl)-6-nitroquinoxaline (m = 10)
Yellow solids; yield 72%. 1H NMR (300 MHz, CDCl3): δ 0.87 (t, 3H, −CH3, J = 4.2 Hz),
1.18−1.52 (m, 14H, −CH2), 1.73−1.86 (m, 2H, −CH2), 4.05 (t, 2H, −OCH2, J = 6.3 Hz), 7.07 (d,
2H, Ar−H, J = 4.5 Hz), 8.17−8.23 (m, 3H, Ar−H), 8.50 (dd, 1H, Ar−H, 3J = 9 Hz, 4J = 2.4 Hz),
8.96 (d, 1H, Ar−H, 2.1 Hz), 9.42 (s, 1H, Ar−H). 13C NMR (125 MHz, CDCl3): δ 14.10, 22.66,
25.99, 29.14, 29.30, 29.37, 29.55, 31.88, 68.33, 115.34, 123.67, 125.59, 127.73, 129.51, 130.76,
139.80, 145.17, 146.95, 153.82, 162.10.
4.12. 2-(4-(Dodecyloxy)phenyl)-6-nitroquinoxaline (m = 12)
Yellow solids; yield 73%. 1H NMR (300 MHz, CDCl3): δ 0.86 (t, 3H, −CH3, J = 6.6 Hz)
1.19−1.68 (m, 18H, −CH2), 1.82 (quint, 2H, −CH2, J = 6.6 Hz), 4.05 (t, 2H, −CH2, J = 6.6 Hz),
7.07 (d, 2H, Ar−H, J = 4.2 Hz), 8.16−8.23 (m, 3H, Ar−H), 8.50 (d, 1H, Ar−H, J = 9.3 Hz), 8.96 (s,
1H, Ar−H), 9.41 (s, 1H, Ar−H). 13C NMR (125 MHz, CDCl3): δ 14.10, 22.68, 26.00, 29.15, 29.37,
29.58, 29.64, 31.91, 68.35, 115.35, 123.67, 125.59, 129.51, 130.77, 139.81, 145.16, 146.97,
153.82, 162.13.
4.13. (E)-5-(octyloxy)-2-(((2-(4-(octyloxy)phenyl)quinoxalin-6-yl)imino)methyl)phenol
1 (m = 8, n = 8)
To a solution of 2-hydroxy-4-(octyloxy)benzaldehyde (1.5 g, 6.0 mmol) dissolved in 50 mL THF,
2-(4-(octyloxy)phenyl)quinoxalin-6-amine (2.0 g, 5.7 mmol) was added, and the mixture was
refluxed overnight. Methanol was slowly added to give brown solids. The crude product was
26

ACCEPTED MANUSCRIPT
collected and then purified by recrystallization from dichloromethane and methanol. Yellow
crystal, yield: 90%. 1H NMR (300 MHz, CDCl3): δ 0.86 (t, 6H, −CH3, J = 4.2 Hz) 1.23−1.65 (m,
20H, −CH2), 1.73−1.89 (m, 4H, −CH2), 3.94−4.10 (m, 4H, −OCH2), 6.48−6.56 (m, 2H, Ar−H),
7.05 (d, 2H, Ar−H, J = 4.8 Hz), 7.29 (d, 1H, Ar−H, J = 4.8 Hz), 7.68 (d, 1H, Ar−H, J = 5.4 Hz),
7.83 (s, 1H, Ar−H), 8.09 (d, 1H, Ar−H, J = 5.1 Hz), 8.14 (d, 2H, Ar−H, J = 4.8 Hz), 8.67 (s, 1H,
−CH=N), 9.25 (s, 1H, Ar−H), 13.40 (s, 1H, Ar−H). 13C NMR (125 MHz, CDCl3): δ 13.96, 22.58,
25.96, 26.02, 29.07, 29.16, 29.22, 29.27, 29.31, 31.77, 68.30, 68.39, 101.73, 107.98, 113.08,
115.22, 118.31, 125.78, 128.79, 129.09, 130.35, 133.90, 141.19, 141.93, 143.56, 149.16, 150.86,
161.16, 162.73, 164.06, 164.22, 176.61. Anal. Calcd for C37H47N3O3: C, 76.38; H, 8.14. Found C,
76.28; H, 8.11.
4.13.1 (E)-5-(decyloxy)-2-(((2-(4-(decyloxy)phenyl)quinoxalin-6-yl)imino)methyl)phenol
1 (m = 10, n = 10)
1H NMR (300 MHz, CDCl3): δ 0.87 (t, 6H, −CH3, J = 6.6 Hz), 1.15−1.56 (m, 28H, −CH2),
1.70−1.86 (m, 4H, −CH2), 3.95−4.0 (m, 4H, −OCH2), 6.43−6.58 (m, 2H, Ar−H), 7.05 (d, 2H,
Ar−H, J = 8.7 Hz), 7.31 (d, 1H, Ar−H, J = 9.3 Hz), 7.70 (dd, 2H, Ar−H, 3J = 8.7 Hz, 4J = 2.1
Hz), 7.84 (d, 1H, Ar−H, J = 2.1 Hz), 8.07−8.19 (m, 3H, Ar−H), 8.68 (s, 1H, −CH=N), 9.27 (s, 1H,
Ar−H), 13.51 (s, 1H, Ar−H). 13C NMR (125 MHz, CDCl3): δ 14.03, 22.67, 26.02, 26.08, 29.14,
29.31, 29.37, 29.41, 29.56, 31.31, 28.38, 68.46, 101.81, 108.07, 113.15, 115.30, 118.38, 125.87,
128.87, 129.18, 130.44, 133.97, 141.27, 142.02, 143.66, 149.28, 150.96, 161.24, 162.84, 164.13,
164.31. Anal. Calcd for C41H55N3O3: C, 77.20; H, 8.69. Found C, 77.12; H, 8.74.
4.13.2 (E)-2-(((2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)imino)methyl)-5-(octyloxy)phenol
1 (m = 12, n = 8)
1H NMR (500 MHz, CDCl3): δ 0.86 (t, 3H, −CH3, J = 6.7 Hz), 1.25−1.44 (m, 28H, −CH2),
1.75−1.83 (m, 4H, −CH2), 3.98 (m, 4H, −OCH2), 6.50 (m, 2H, Ar−H), 7.04 (d, 2H, Ar−H, J = 8.5
Hz), 7.31 (d, 1H, Ar−H, J = 8.7 Hz), 7.69 (d, 1H, Ar−H, J = 7.9 Hz), 7.83 (s, 1H, Ar−H), 8.09 (d,
1H, Ar−H, J = 8.8), 8.13 (d, 2H, Ar−H, J = 8.5 Hz), 8.67 (s, 1H, Ar−H), 9.26 (s, 1H, Ar−H),
27

ACCEPTED MANUSCRIPT
13.52 (s, 1H, OH). 13C NMR (75 MHz, CDCl3): δ 14.08, 14.10, 22.63, 22.67, 25.96, 26.00, 29.02,
29.18, 29.19, 29.29, 29.33, 29.38, 29.55, 29.57, 29.61, 29.64, 31.78, 31.89, 68.19, 68.32, 101.51,
108.00, 112.92, 115.10, 118.23, 125.91, 128.78, 128.89, 130.30, 133.93, 141.11, 141.79, 143.60,
149.04, 161.04, 162.77, 163.96, 164.10. Anal. Calcd for C41H55N3O3: C, 77.20; H, 8.69. Found C,
76.50; H, 8.66.
4.13.3. (E)-5-(dodecyloxy)-2-(((2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)imino)methyl)
phenol 1 (m = 12, n = 12)
1H NMR (300 MHz, CDCl3): δ 0.87(t, 6H, −CH3, J = 4.2 Hz) 1.19−1.68 (m, 36H, −CH2),
1.70−1.91 (m, 4H, −CH2), 3.92−4.16 (m, 4H, −OCH2), 6.48−6.52 (m, 2H, Ar−H), 7.05 (d, 2H,
Ar−H, J = 8.1 Hz), 7.31 (d, 1H, Ar−H, J = 9.3 Hz), 7.70 (d, 2H, Ar−H, J = 6.9 Hz), 7.84 (s, 1H,
Ar−H), 8.07−8.19 (m, 3H, Ar−H), 8.68 (s, 1H, −CH=N), 9.27(s, 1H, Ar−H), 13.50 (s, 1H, Ar−H).
13C NMR (125 MHz, CDCl3): δ 22.83, 26.18, 16.23, 29.29, 29.44, 29.49, 29.52, 29.57, 29.76,
29.79, 29.82, 32.09, 68.53, 68.61, 101.96, 108.21, 113.30, 115.45, 118.53, 126.00, 129.02, 129.32,
130.58, 134.12, 141.42, 142.15, 143.78, 149.39, 151.09, 161.39, 162.96, 164.28, 164.46. Anal.
Calcd for C45H63N3O3: C, 77.88; H, 9.15. Found C, 77.82; H, 9.15.
4.13.4 (E)-2-(((2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)imino)methyl)-5-(hexadecyloxy)
phenol 1 (m = 12, n = 16)
1H NMR (300 MHz, CDCl3): δ 0.83 (t, 3H, −CH3, J = 6.6 Hz), 1.24−1.53 (m, 44H, −CH2),
1.74−1.86 (m, 4H, −CH2), 3.98−4.06 (m, 4H, −OCH2), 6.49−6.52 (m, 2H, Ar−H), 7.05 (d, 2H,
Ar−H, J = 8.8 Hz), 7.31 (d, 1H, Ar−H, J = 9.3 Hz), 7.70 (dd, 1H, Ar−H, 3J = 8.9 Hz, 4J = 2.3 Hz),
7.84 (d, 1H, Ar−H, J = 2.3), 8.1 (d, 1H, Ar−H, J = 9.0), 8.14 (d, 2H, Ar−H, J = 8.9 Hz), 8.68 (s,
1H, Ar−H), 9.27 (s, 1H, Ar−H), 13.50 (s, 1H, OH). 13C NMR (75 MHz, CDCl3): δ 14.18, 22.82,
26.18, 26.23, 29.29, 29.50, 29.51, 29.56, 29.75, 29.81, 29.85, 32.09, 68.54, 68.62, 101.96, 108.22,
113.30, 115.46, 118.52, 126.01, 129.02, 129.33, 130.58, 134.11, 141.42, 142.16, 143.08, 149.43,
151.11, 161.39, 162.98, 164.28, 164.46. Anal. Calcd for C49H71N3O3: C, 78.46; H, 9.54. Found C,
78.48; H, 9.54.
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ACCEPTED MANUSCRIPT
4.14 (E)-2-(((2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)methylene)amino)-5-(octyloxy)
phenol 2a (m = 12, n = 8)
To a solution of 2-amino-5-(octyloxy)phenol (1.0 g, 4.5 mmol) dissolved in 50 mL of EtOH,
2-(4-(dodecyloxy)phenyl)quinoxaline-6-carbaldehyde (2.0 g, 4.5 mmol) was added and the
mixture was refluxed overnight. Upon cooling to room temperature, the crude product was
collected by filtration and then purified by recrystallization from CH2Cl2/CH3OH. Yellow solids;
yield 90%. 1H NMR (300 MHz, CDCl3): δ 0.88 (t, 3H, −CH3, J = 6.9 Hz), 1.25−1.54 (m, 28H,
−CH2), 1.73−1.86 (m, 4H, −CH2), 3.96 (t, 2H, −OCH2, J = 6.6 Hz), 4.04 (t, 2H, −OCH2, J = 6.6
Hz), 6.49 (dd, 1H, Ar−H, 3J = 8.6 Hz, 4J = 2.7 Hz), 6.58 (d, 1H, Ar−H, J = 2.7 Hz), 7.06 (d, 2H,
Ar−H, J = 8.9 Hz), 7.37 (d, 1H, Ar−H, J = 9.0 Hz), 7.54 (s, 1H, −OH), 8.13 (d, 1H, Ar−H, J =
8.8), 8.17 (d, 2H, Ar−H, J = 9.0 Hz), 8.32 (s, 1H, Ar−H), 8.41 (d, 1H, Ar−H, J = 8.8 Hz), 8.82 (s,
1H, −CH=N), 9.31 (s, 1H, Ar−H). 13C NMR (75 MHz, CDCl3): δ 14.11, 22.68, 26.04, 29.21,
29.36, 29.41, 29.60, 31.82, 31.92, 68.28, 68.34, 100.54, 107.48, 116.36, 127.88, 128.01, 128.69,
129.09, 129.99, 130.76, 136.80, 141.23, 143.75, 144.04, 151.72, 152.05, 154.27, 160.89, 161.43.
Anal. Calcd for C41H55N3O3: C, 77.20; H, 8.69. Found C, 77.28; H, 8.73.
4.14.1(E)-5-(dodecyloxy)-2-(((2-(4-(dodecyloxy)phenyl)quinoxalin-6-yl)methylene)amino)
phenol 2a (m = 12, n = 12)
Yellow solids, 88%.1H NMR (300 MHz, CDCl3): δ 0.86 (t, 3H, −CH3, J = 6.9 Hz), 1.25−1.46 (m,
36H, −CH2), 1.73−1.86 (m, 4H, −CH2), 3.95 (t, 2H, −OCH2, J = 6.6 Hz), 4.01 (t, 2H, −OCH2, J =
6.6 Hz), 6.48 (dd, 1H, Ar−H, 3J = 8.7 Hz, 4J = 2.4 Hz), 6.57 (d, 1H, Ar−H, J = 2.7 Hz), 7.05 (d,
2H, Ar−H, J = 8.7 Hz), 7.36 (d, 1H, Ar−H, J = 9.0 Hz), 7.54 (s, 1H, −OH), 8.10 (d, 1H, Ar−H, J =
8.7), 8.17 (d, 2H, Ar−H, J = 8.4 Hz), 8.32 (s, 1H, Ar−H), 8.39 (d, 1H, Ar−H, J = 9.0 Hz), 8.79 (s,
1H, −CH=N), 9.29 (s, 1H, Ar−H). 13C NMR (75 MHz, CDCl3): δ 14.12, 22.69, 26.03, 29.21,
29.36, 29.40, 29.60, 29.65, 31.93, 68.28, 68.34, 100.54, 107.47, 115.22, 127.87, 128.00, 128.67,
129.07, 129.98, 130.76, 136.78, 141.23, 143.74, 144.03, 151.70, 152.04, 154.27, 160.89, 161.42.
Anal. Calcd for C41H55N3O3: C, 77.88; H,9.15. Found C, 77.66; H,9.12.
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