Synthesis and study of an N,N-disubstituted 4-[(4-aminophenyl)diazenyl]benzaldehyde

Article abstract of DOI:10.1002/1522-2675(200201)85:1<108::AID-HLCA108>3.0.CO;2-H

An N,N-disubstituted 4-[(4-aminophenyl)diazenyl]benzaldehyde ('azo dye') with nematic phase was synthesized by reaction of the phenylpiperazine derivative with the formylbenzenediazonium salt. The salt was prepared by a simplified reaction of poly(aminobenzaldehyde) with NaNO₂ in the presence of HBF₄. The azo dye was further studied by crystallography. On the basis of molecular stacking identified from crystallography, the molecular modeling was carried out to explore the molecular interactions, and it was found that the attraction of two coplanar molecules with side-to-side contact is favored, and a nematic phase is thus formed during the thermal process.

Full text of DOI:10.1002/1522-2675(200201)85:1<108::AID-HLCA108>3.0.CO;2-H

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Synthesis and Study of an N,N-Disubstituted
4-[(4-Aminophenyl)diazenyl]benzaldehyde
by Long-Li Lai*, Chia-Husan Ho, Yu-Jen Lin, and Eshin Wang
Department of Applied Chemistry, National Chi Nan University, Puli, Taiwan 545, R.O.C.
(Phone: 886-49-2910960 4975; fax: 886-49-2917956; e-mail: lilai@ncnu.edu.tw.)
and Yi-Hung Liu and Yu Wang
Instrumentation Center and Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C.
and Yang-Chu Lin and Kung-Lung Cheng
Union Chemical Laboratory, Industrial Research Institute Hsinchu, Taiwan 300, R.O.C.
An N,N-disubstituted 4-[(4-aminophenyl)diazenyl]benzaldehyde (−azo dye×) with nematic phase was
synthesized by reaction of the phenylpiperazine derivative with the formylbenzenediazonium salt. The salt was
prepared by a simplified reaction of poly(aminobenzaldehyde) with NaNO2 in the presence of HBF4. The azo
dye was further studied by crystallography. On the basis of molecular stacking identified from crystallography,
the molecular modeling was carried out to explore the molecular interactions, and it was found that the
attraction of two coplanar molecules with side-to-side contact is favored, and a nematic phase is thus formed
during the thermal process.
1. Introduction. Azo dye molecules have recently attracted a lot of attention in
both academia and industry [1][2]. For example, a dichroic dye is dissolved in a liquid
crystal to give better reflectance of the polarizer [3]. Furthermore, azo dyes have a
reasonable dichroic ratio and are suitable materials for the lower-power-consumption
reflective liquid-crystal devices. The three-layered guest-host (G-H) system of the
dichroic dye with subtractive color mixing of yellow, magenta, and cyan are expected to
be used for developing the full-color reflective displays [4]. Previously, we have
successfully synthesized several azo-dye materials of type 1 [5], which show smectic
phases. However, the nematic liquid crystals are more valuable in practice, as they
possess lower viscosity and faster response under applied electrical switching.
Therefore, we designed and synthesized a nematic azo dye containing a CHOfunction.
Such a material is useful for further synthesizing the chiral derivatives [6] that are
important in developing the ferroelectric and antiferroelectric displays [7]. To

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understand the molecular stacking of the nematic liquid crystals, the azo dye was
studied by single-crystal structure determination. Based on the molecular arrangement
identified from crystallography, molecular modeling was further used to investigate the
interaction between molecules. A possible model of forming a nematic phase for the
azo dye is thus raised because of the successful strategy of combining crystallography
and simple molecular modeling. Here, we report our preliminary results.
Results and Discussion. Compound 2 was prepared by reaction of 1-phenyl-
piperazine with octanoyl chloride in CH2Cl2 in the presence of Et3N in an almost
quantitative yield. The N,N-disubstituted 4-[(4-aminophenyl)diazenyl]benzaldehyde 4
was synthesized by reaction of compound 2 with diazonium salt 3 according to the
Scheme. Although the corresponding salt can be synthesized from freshly prepared 4-
aminobenzaldehyde [8], the storage of the salt precursor is a problem, as it gradually
loses water during the storage. We thus simplified the salt preparation by hydrolyzing
the commercially available poly(4-aminobenzaldehyde) in hot tetrafluoroboric acid
and then allowing the resulting ammonium benzaldehyde to react with 1.5 equivalents
of NaNO2 at 08 (Scheme).
Scheme
A nematic phase of compound 4 was characterized by the Schlieren texture through
polarizing optical microscopy. The nematic phase for compound 4 is from 125.4 to
158.78 in the heating process and 158.3 to 107.08 in the cooling process. The nematic
phase of compound 4 was confirmed by the powder X-ray diffraction (XRD) study. As
there is no layer structure in the range of the nematic phase for the liquid crystals, no
reflection was observed in the small angle area for compound 4 from the XRD study.
To understand the molecular stacking of the molecules in the solid state, compound
4 was studied by single-crystal structure determination. The packing diagram for the
intermolecular interaction of compound 4 in the solid state is shown in Figs. 1,a and b.
In a vertical layer (for example A layer in Fig. 1,a), the first and the third molecules are
regularly arranged by face-to-face with the head-to-head or tail-to-tail arrangement,

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and the second molecule is horizontally inserted at one of the ends. On further
consideration of the arrangement of compound 4 from the view along the OÀb
direction in Fig. 2,a, molecules in different vertical layers (for example A, B, and C
layers) are co-planar side-to-side to each other, as demonstrated in Fig. 1,b.
Fig. 1. a) The packing diagram of compound 4 in a view down the a-axis, H-atoms were omitted for clarity. b) The
packing diagram of compound 4 in a view down the b-axis, H-atoms were omitted for clarity.
By the resulting crystallographic data, we calculated the molecular interaction
between two representing molecules of 4, which are parallel to each other by a face-to-
face arrangement in the same vertical layer or co-planar to each other by side-to-side
arrangement in different vertical layers. Although the calculation results under such
conditions are more related to real molecules in the gas phase, we deem that these
results may be reasonably applied to the solid state, as the intrinsically attractive and
repulsive properties for molecules should remain consistent in gas, liquid, and solid
states. Therefore, the conformation of a single molecule of compound 4 was first
optimized by performing a geometry calculation in mechanics with augmented MM3
parameters and further refined by calculating the optimized geometry by MOPAC with
PM3 parameters. Based on the arrangement in Fig. 1,a, two kinds of the two-molecular
system shown in Figs. 2 and 3 were defined preceeding the molecular modeling. In the
system of Fig. 2, a molecule in one vertical layer is coplanar to another molecule in
another vertical layer with side-to-side and head-to-tail contact, which is similar to part
of the arrangement identified from crystallography (Fig. 1,a). The system was then
refined by calculating the optimized geometry by MOPAC with PM3 parameters to
seek the minimum energy of the system. To clearly estimate the extent of interaction

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Fig. 2. The close contact of the two-molecular system of compound 4 by side-to-side and head-to-tail
arrangement
Fig. 3. The close contact of the two-molecular system of compound 4 by face-to-face and head-to-tail
arrangement
between two molecules, after optimizing the two-molecular system, each molecule was
further refined by MOPAC with PM3 parameters. Therefore, the corresponding heats
of the formation of the two coplanar molecules, shown in Fig. 2, were calculated at
different distances, and the results were shown in Table 1.
Table 1. The Distances, Lowered Energies, and Other Properties with Respect to Fig. 2
Heat of formation
for the system
[kcal/mol]
Heat of formation
for individual molecule
[kcal/mol]
Distances [ä]
Lowered energy
[kcal/mol]
C(9A)ÀC(2E)
C(15A)ÀC(8E)
À 34.01
À 38.22
À 41.63
À 38.72
À 15.72, À 16.60
À 15.75, À 16.62
À 15.21, À 15.80
À 14.70, À 15.73
5.23
4.08
3.84
3.72
4.72a)
5.18
3.99
3.48
3.42
3.93a)
À 1.70
À 5.85
À 10.62
À 8.29
a) Obtained from X-ray crystal-structure analysis.

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The optimum distances of C(9A)ÀC(2E) and C(15A)ÀC(8E) according to our
calculation are 3.84 and 3.48 ä, respectively, wherein the heat of formation for the two-
molecular system is À 41.63 kcal/mol, and the heat of formation for each corresponding
molecule is À 15.21 and À 15.80 kcal/mol, respectively. The enthalpy difference is used
to investigate whether the two-molecular system shown in Fig. 2 is more stable than two
isolated molecules. Therefore, the sum of the heats of formation for each corresponding
molecule was subtracted from the heat of formation of the two-molecular unit and
consequently gave an energy difference of À 10.62 kcal/mol. The result indicates that
the arrangement in Fig. 2 is favored on the basis of the stabilization point. If two
molecules were moved closer, the energy calculated was lower (À 8.29 kcal/mol),
wherein the distances of C(9A)ÀC(2E) and C(15A)ÀC(8E) were calculated to be 3.72
and 3.42 ä, respectively. When two molecules were further separated, the calculated
energy was also lower (À 5.85 kcal/mol), wherein the distances of C(9A)ÀC(2E) and
C(15A)ÀC(8E) were calculated to be 4.08 and 3.99 ä, respectively. We thus suggest
that the optimum location of molecules shown in Fig. 2 is as previously described,
wherein the distances of C(9A)ÀC(2E) and C(15A)ÀC(8E) are in the range of 3.72
4.08 and 3.42 3.99 ä, respectively. When two molecules are moved closer together, the
repulsive force between the molecules will increase. The attractive force between two
molecules will decrease when the molecules are further separated. As the conformation
of the individual molecule in Fig. 2 slightly changes during the optimization of the two-
molecular system, the heat of formation for one molecule may be different from
another. However, the variation is not significant and is ca. 1 kcal/mol (Table 1).
In another system as shown in Fig. 3, two molecules in the same vertical layer are
orientated face-to-face together with head-to-tail arrangement, which is also similar to
part of the arrangement identified from crystallography (Fig. 1,a). The interaction of
the system was also simulated in a similar manner. The energy between the molecules
at the optimized distances is only lowered by ca. À 0.01 kcal/mol (Table 2). Therefore,
we assume that the attraction between the molecules in the same vertical layer, which
are in a face-to-face together with head-to-tail arrangement, can be neglected. During
the thermal process, two molecules in the same vertical layer of Fig. 3 dissociates easily,
but two co-planar molecules of Fig. 2 remain as a stable unit for forming the nematic
phase during the thermal process. According to the calculation results shown in Table 1,
the stabilization energy of the two-molecular system shown in Fig. 2 at optimum
distance is 10.62 kcal/mol and rather large when compared with Van der Waals forces
(1 5 kcal/mol) [9]. Also, the optimum distance between two molecules is smaller than
the corresponding distance obtained from crystallography (Table 1). This may result
from the simplicity of our calculation model. Owing to the limitation of the calculation
program and the size of the molecule (63 atoms in compound 4), the interaction of only
two molecules is calculated. The more molecules are involved to calculate their
interaction, the more accurate results should be obtained. When the distances of
C(9A)ÀC(2E) and C(15A)ÀC(8E) are fixed and close to the optimum distances
obtained from crystallography (4.72 and 3.93 ä, resp.), the stabilization according to
our present calculation should be greater than 1.74 but less than 5.85 kcal/mol, which is
consistent with Van der Waals forces of the molecules (1 5 kcal/mol). Additionally, the
calculation results clearly demonstrate that two coplanar molecules in a side-to-side
together with head-to-tail arrangement form a stable unit, and the mesogenic phase

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during the thermal process is thus formed because of the attractive forces between
molecules. On the other hand, two molecules in the same vertical layer by face-to-face
together with head-to-tail arrangement are not attracted to each other, and will easily
disassociate during the thermal process.
Table 2. The Distances, Lowered Energies, and Other Properties with Respect to Fig. 3
Heat of formation
for the system
[kcal/mol]
Heat of formation
for individual molecule
[kcal/mol]
Distances [ä]
Lowered energy
[kcal/mol]
N(1B)ÀC(22C)
C(8B)ÀN(2C)
À 32.06
À 32.01
À 31.65
À 29.43
À 15.60, À 16.49
À 15.54, À 16.46
À 15.54, À 16.46
À 15.06, À 16.52
5.02
4.50
3.96
3.74
3.89a)
4.99
4.49
3.92
3.60
3.87a)
‡ 0.03
À 0.01
‡ 0.35
‡ 2.15
a) Obtained from X-ray crystal-structure analysis.
In conclusion, this work presents a convenient synthesis of a nematic azo-dye
molecule with a CHOgroup, which is useful for further synthesis of chiral liquid
crystals. The results from the crystallographic study provide the details of the crystal
packing in the solid state and the correlative position of the molecules. The calculation
of the interaction between molecules is performed on the basis of arrangement
identified from crystallography, and the results show that two coplanar molecules in
side-to-side together with head-to-tail arrangement are favored during the thermal
process on the basis of the stabilization point. Noteworthy is that the successful strategy
of combining crystallography and simple molecular modeling to study the details of the
molecular interaction provides a useful method in material chemistry.
Experimental Part
General. Chemicals used were available from ACROS. IR Spectra: in cmÀ1; Bio-Rad; Digilab ITS-60. 1H-
and 13C-NMR spectra: Bruker 200 FT NMR spectrometer. High-resolution (HR) MS: VG70-250; EI, 70 eV).
The mesogenic behaviors and phase transitions were characterized by polarizing optical microscopy and
differential scanning calorimetry (Perkin-Elmer DSC 6). Powder X-ray diffraction (XRD) patterns were
obtained from a Siemens D-5000 X-ray diffractometer equipped with a TTK 450 temp. controller and Cu
radiation with the wavelength of l ˆ 1.5406 ä. Semi-empirical calculation was carried out with the CAChe
program, which was provided by Fujitsu (version 3.2).
Synthesis of 4-{[4-(Octanoylpiperazin-1-yl)phenyl]diazenyl}benzaldehyde (4). Poly(4-aminobenzalde-
hyde) (1.21 g, 10 mmol) was added to HBF4 (50wt-% in H2O; 8 ml), which was then diluted with hot H2O
(ca. 908; 7 ml). NaNO2 (1.03 g, 15 mmol) was gradually added at 08, and the soln. was then kept in the
refrigerator overnight. Diazonium salt 3, which had precipitated, was filtered off, washed with dry Et2Oand then
dried to give quant. yield. The phenylpiperazine derivative 2 (2 mmol) was added to salt 3 (2.4 mmol) in AcOH
(10 ml), and the soln. was stirred for 2 h at r.t. H2O(100 ml) was added, and the mixture was extracted with
CH2Cl2 (2 Â 100 ml). The combined extracts were dried (Na2SO4), and the solvent was removed at reduced
pressure. The residue was chromatographed on silica gel to give 4 (40.8%, 0.342 g), which was characterized as
the correct compound by spectroscopy. IR: 2948, 2919, 2847, 2731, 1694, 1643, 1595, 1561, 1542, 1509, 1457, 1428,
1391, 1370. 1H-NMR (CDCl3): 0.85 (t, J ˆ 6.6, Me); 1.29 1.36 (m, 4 CH2); 1.57 1.66 (m, CH2); 2.33 (t, J ˆ 7.8,
CH2); 3.40 (br. s, 2 CH2); 3.66 (br. s, CH2); 3.80 (br. s, CH2); 6.95 (AA'BB', J ˆ 8.8, 2 arom. H); 7.92 (AA'BB',
J ˆ 8.8, 2 arom. H); 7.97 (br. s, 4 arom. H); 10.05 (s, CHO). 13C-NMR (CDCl3): 191.56 (CHO); 171.77 (CˆO);

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156.31; 153.10; 145.48; 136.52; 130.59; 125.29; 122.78; 114.43; 47.67; 47.44; 44.93; 40.89; 33.20; 31.60; 29.33;
29.00; 25.18; 22.51; 14.00. HR-MS: 420.2526 (C25H32N4O2; calc. 420.2525).
X-Ray Crystal-Structure Analysis. Crystals of compound 4 were grown from CH2Cl2/hexane 1 :1 at r.t.
Single crystal of suitable quality was mounted on a glass fiber and used for measurement of precise cell constants
and intensity-data collection. Diffraction measurement was made on
a Siemens SMART 1K CCD
diffractometer with graphite-monochromated MoKa radiation (l ˆ 0.71073 ä), operated at 150(2) K over the
q range of 1.54 23.268. No significant decay was observed during the data collection. 2269 Reflections were
observed with I ꢀ 2s (I) among the 3188 unique reflections, and 2987 reflections were used in the refinement.
Data were processed on a PC with the SHELXTL software package [10]. The structure of 4 was solved by direct
methods and refined by full-matrix least square on F2 value. All non-H-atoms were refined anisotropically. The
H-atoms were fixed at calculated positions and refined using a riding model. The final indices were R1 ˆ 0.0441,
wR2 ˆ 0.0901 with goodness-of-fit on F2 ˆ 1.000. Further data: formula: C25H32N4O2; formula weight: 420.55;
crystal size: 0.02  0.10  0.25 mm3; unit-cell parameters: a ˆ 7.4709(7), b ˆ 11.2442(10), c ˆ 26.389(3) ä, a ˆ
b ˆ g ˆ 908; crystal system: orthorhombic; space group: P212121 ; Vˆ 2216.8(4) ä3; cell units: Z ˆ 4; calc. density
1.260 g cmÀ3; linear absorption coefficient 0.081 (m/cmÀ1). Crystallographic data for the structure 4 has been
deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication No. CCDC-
164793. Copies of the data can be obtained, free of charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK fax: (‡44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.
We thank the National Chi Nan University and the National Science Council (NSC 89-2113-M-260-005) for
financial support. The National Center of High-Performing Computing and the Institute of Chemistry, Academia
Sinica, are also acknowledged for providing the Beilstein database system, as well as the most helpful library
service and the XRD apparatus, respectively.
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Received June 13, 2001