Synthesis and study of azo-dye compounds: Various molecular stackings from different polarities of the molecules

Article abstract of DOI:10.1002/1522-2675(200205)85:5<1517::AID-HLCA1517>3.0.CO;2-6

One non-F-containing and two F-containing azo-dye compounds were prepared to investigate their molecular stackings by X-ray crystallography. Introduction of the F-atom into the aromatic moiety of azo-dye compounds leads to a variation of the charge distribution and consequently to different molecular stackings. Accordingly, the mesogenic behaviors in the solid state are different.

Full text of DOI:10.1002/1522-2675(200205)85:5<1517::AID-HLCA1517>3.0.CO;2-6

Helvetica Chimica Acta Vol.85 (2002)
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Synthesis and Study of Azo-Dye Compounds: Various Molecular Stackings
from Different Polarities of the Molecules
by Long-Li Lai*, Feng-Ya Su, Yu-Jen Lin, Chia-Husan Ho, and Eshin Wang
Department of Applied Chemistry, National Chi Nan University, Puli, Taiwan 545, R.O.C.
(Tel.: 886-49-2910960 ꢀ 4975; fax: 886-49-2917956; e-mail: lilai@ncnu.edu.tw)
and Chen-Hsiung Hung
Department of Chemistry, National Changhua University of Education, Changhua, Taiwan 500, R.O.C.
and Yi-Hung Liu and Yu Wang
Instrumentation Center and Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C.
One non-F-containing and two F-containing azo-dye compounds were prepared to investigate their
molecular stackings by X-ray crystallography.Introduction of the F-atom into the aromatic moiety of azo-dye
compounds leads to a variation of the charge distribution and consequently to different molecular stackings.
Accordingly, the mesogenic behaviors in the solid state are different.
Introduction. Recently, the supramolecular aggregate by molecular self-assembly
has been an important issue in the field of structural chemistry [1].In addition to
electrostatic interaction, noncovalent forces also play a significant role in determi-
nation of the structural stacking and properties of molecules [2].The interaction
between the aromatic moiety of the molecules is found to be critical for the molecular
packing in crystallization [3], and furthermore the stability and phase behavior of
mesogenic molecules have been reported to arise therefrom [4].Particularly, the
electronegativity of H- and F-atoms are obviously different (Pauling scale: H 2.1,
F 4.0), and, thus, the replacement of the H- by the F-atom in aromatic moiety may
result in significant variation of molecular polarity.Although many studies have
disclosed the molecular packings with replacement of H- by F-atom, most of them were
focused only on small molecules, in which the shape and size of the molecules are
effective on the molecular stacking during the crystallization.Therefore, the polarity of
the molecules does not have significant influence in the crystal packing [5].However,
the molecular size of liquid crystals is rather big (molecular length ca.20 40 ä) and its
polarity may significantly influence the molecular packing in the crystal engineering.
Particularly, it has been shown that pure benzene adopts an edge-to-face structure in
the solid state [6], and the structure of benzene/hexafluorobenzene materials at low
temperatures consists of face-to-face stacks of alternative molecular arrangement [7].
Coates et al., thus, successfully utilized this phenyl/perfluorophenyl-stacking interaction
to prepare olefinic compounds by photopolymerization [8].Thus, the replacement of
H- by F-atoms in the aromatic moiety does, to some extent, cause different molecular
stacking in the solid state.However, the perfluorophenyl derivative is not inexpensive.

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Helvetica Chimica Acta Vol.85 (2002)
We, thus, introduced a single F-atom into our azo dye system and expected similar
results, i.e., different molecular stackings between fluoro and nonfluoro liquid crystals.
As the NˆN moiety of liquid crystals seems to preferably form smectic phase [9], we
applied this model to change the molecular stacking and consequently modify the
mesogenic behaviors of the azo-dye molecules systematically in the future.Now, we
report our primary results.
Results and Discussion. Compounds 1a 1c were synthesized according to the
Scheme.Reaction of diazonium salt 2 with phenol, 2-fluorophenol, and 3-fluorophenol,
respectively, in EtOH gave the corresponding azo compounds 3a 3b in ca.50% yield,
which were further treated with decanoyl chloride in the presence of Et3N in CH2Cl2 to
yield the desired products 1a 1c, respectively.These compounds have similar
molecular compositions except that an aromatic H-atom in 3a was replaced by a F-
atom at different positions to give 1b and 1c.The packing diagrams for the
intermolecular interaction of compounds 1a 1c in the solid state, obtained from
crystallographic studies, are shown in Figs. 1 3, respectively.In Fig. 1, each molecule of
1a is regularly arranged head-to-head or head-to-tail to adjacent molecules in the same
or different layers.Based on the crystallographic studies, the molecules are found to be
parallel to the neighboring molecules in the same layer (e.g., molecules a1 and a2 in
layer A), and the distances between two corresponding atoms in the same layers (e.g.,
Na and Nb and Ca and Cb) are all ca.5.57 ä.The molecules in different layers are also
approximately parallel to others, and the closest distance between two adjacent layers
(not including the H-atoms) is 3.36 ä, which is, for example, found for C(1)ÀO(1) and
C(2)ÀO(2) between B and E layers.The distances between O(1) ¥¥¥ H (at C(1)) and
O(2) ¥¥¥ H (at C(2)) are found to be both 2.58 ä, which are shorter than the sum of the
Van der Waals radii of H- and O-atoms (Bondi radii: H 1.20, F 1.47, O 1.52) [10a], and
the H-bond interaction arising therefrom should influence the crystallizing process.The
distances of the O(3) ¥¥¥ H (at C(3)) and O(4) ¥¥¥ H (at C(4)) are both 3.0 ä, which are
in the range of normal H-bond lengths [10b].
As shown in Fig. 2, the molecules of 1b are regularly arranged similar to those of
compound 1a.The distances between two corresponding atoms of two parallel
molecules in the same layer (not including the H-atoms) as described for Fig. 1 are all
ca.61.3 ä.The closest distance between two adjacent layers (not including the H-
atoms) is 3.62 ä, which is, e.g., found for C(1)ÀO(1) and C(2)ÀO(2) between the
layers A' and B'.The distance of O(1) ¥¥¥ H (at C(5)) is 2.40 ä, which is shorter than the
sum of the Van der Waals radii of H- and O-atoms.The distances of O(3) ¥¥¥ H (at
C(3)), O(4) ¥¥¥ H (at C(3)), and F(1) ¥¥¥ H (at C(4)) are 2.95, 3.05, and 3.11 ä,
respectively, which are in the range of normal H-bond lengths.
The molecules of 1c are regularly arranged as shown in Fig. 3.The distances
between two corresponding atoms of two parallel molecules in the same layer (not
including the H-atoms) as described for Fig. 1 are all ca.53. 6 ä.The closest distance
between two adjacent layers (not including the H-atoms) is 3.01 ä, which is, e.g., found
for F(1)ÀC(1) between the layers A'' and B'', as well as F(2)ÀC(2) between the layers
B'' and E''.The distances F(1) ¥¥¥ H (at C(3)) and F(1) ¥¥¥ H (at C(4)) are 27.3 and
2.76 ä, respectively. The distances of O(2) ¥¥¥ H (at C(5)) and O(3) ¥¥¥ H (at C(6)) are
both 2.57 ä, and the distance O(4) ¥¥¥ H (at C(7)) is 2.58 ä. It is interesting to note that

Helvetica Chimica Acta Vol.85 (2002)
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Scheme
Fig.1. The molecular packing of compound 1a in the solid state. The H-atoms are omitted for clarity.
the H-bonding interaction in compound 1c is much stronger than that of 1a and 1b.The
H ¥¥¥ F and H ¥¥¥ O distances of compound 1c are approximately equal to or shorter than
the sum of the Van der Waals radii of H- and F-atoms (or O-atom).

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Fig.2. The molecular packing of compound 1b in the solid state. The H-atoms are omitted for clarity.
Fig.3. The molecular packing of compound 1c in the solid state. The H-atoms are omitted for clarity.
Apparently, the driving force of the molecular stacking of compounds 1a 1c all
arise from the mutual attraction of polar functional groups together with the interaction
of the H-bonds.However, introduction of a single F-atom into the aromatic moieties of
1b and 1c changes the contribution of molecular polarity, and, thus, the mutual
attraction between aromatic moieties for 1b and 1c are more effective in the solid state
when compared with 3a1).Consequently, various molecular stacking in the aromatic
moieties was obtained and different mesogenic properties of compounds 1a 1c were
observed during the thermal process2).Compound 1a shows a SmB phase in the range
of 40 95.28 in the heating process and 88.4 408 in the cooling process.Compound 1b
is monotropic and shows a nematic phase (50.3 43.58) in the cooling process.Only
compound 1c shows a highly ordered SmX phase in the heating (73.0 71.88) and
cooling process (73.0 71.88) probably because of stronger intermolecular H-bonding
interaction.
In summary, three azo-dye compounds were prepared and studied by X-ray
crystallography.The consistent results imply that variation of the polarity of the liquid-
crystalline molecules effectively change the way of molecular stacking in the solid state,
and may provide a guide to modify the mesogenic behaviors of the liquid-crystalline
molecules.
1
)
Each single molecule of compounds 1a 1c was optimized by semi-empirical calculations in the gas phase.
The conformation of these molecules are found quite similar, and the details of the study will be published
elsewhere.
The SmB phase of compound 1a was characterized by polarizing optical microscopy in a stepped-drop
texture, and further confirmed with the hexagonal arrangement of the molecules with no tilt angle from the
study of powder-XRD and single-crystal structure determination.The nematic phase of compound 1b was
characterized in a schlieren texture.The mesogenic behaviors were recorded in the second heating or
cooling by DSC-6 at a rate of 108/min.
2
)

Helvetica Chimica Acta Vol.85 (2002)
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Experimental Part
General.Chemicals used were commercially available from ACROS.The mesogenic behaviors and phase
transitions were characterized by polarizing optical microscopy and differential scanning calorimetry (Perkin-
Elmer DSC 6).FT-IR Spectra [cm À1]: Perkin Elmer RX-I. 1H-NMR: Varian 300 FT-NMR spectrometer;
13C-NMR spectra: Bruker 200 FT-NMR spectrometer; d in ppm; J values in Hz.HR-EI-MS: VG70-250; 70 eV.
Synthesis of Compounds 1a 1c. A soln. of PhOH (1.88 g, 20 mmol) and KOH (1.12 g, 20 mmol) in H2O
(20 ml) was added to a soln.of [4-(ethoxycarbonyl)benzene]diazonium tetrafluoroborate (1; 5.17 g) in EtOH.
The soln.was then stirred at rt..for 2 h.A suitable amount of aq.HCl was added to maintain the resulting soln.
slightly acidic (pH 5 6).The product precipitated was filtered off and purified by chromatography to give
2.45 g (45.4%) of ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate (3a).Similarly, ethyl 4-[(3-fluoro-4-hydroxy-
phenyl)diazenyl]benzoate (3b) and ethyl 4-[(2-fluoro-4-hydroxyphenyl)diazenyl]benzoate (3c) were obtained
in 57.7% (3.3 g) and 48.5% (2.79 g) yields, resp. Reaction of compounds 3a 3c (2 mmol) with decanoyl chloride
(2 mmol) in CH2Cl2 with excess Et3N yielded the desired corresponding products 1a 1c in ca.85% yield after
normal workup.
Data of Ethyl 4-{[4-(Decanoyloxy)phenyl]diazenyl}benzoate (1a): 84.7% (0.72 g). IR: 3130, 2955, 2921,
2849, 2363, 1942, 1753, 1713, 1601, 1581, 1491, 1466, 1413. 1H-NMR (CDCl3): 0.85 (t, J ˆ 5.4, Me); 1.26 1.53
(m, 6 CH2, Me); 1.69 1.78 (m, CH2); 2.57 (t, J ˆ 7.8, CH2); 4.40 (q, J ˆ 7.2, CH2); 7.24 (AA'BB', J ˆ 8.7,
2 arom.H); 7.91 ( AA'BB', J ˆ 8.7, 2 arom. H); 7.97 (AA'BB', J ˆ 8.7, 2 arom. H); 8.17 (AA'BB', J ˆ 8.7,
2 arom.H). 13C-NMR (CDCl3): 171.84; 165.98 (CˆO); 154.97; 153.32; 150.06; 132.19; 130.53; 124.33; 122.57;
122.31; 61.22; 34.41; 31.83; 29.38; 29.23; 29.08; 24.86; 22.63; 14.30; 14.07. HR-MS: 424.2361 (C25H32N2O4; calc.
424.2362).
Data of Ethyl 4-{[4-(Decanoyloxy)-3-fluorophenyl]diazenyl}benzoate (1b): 73.6% (0.65 g). IR: 2956, 2917,
2350, 1750, 1714, 1601, 1598, 1507, 1493, 1490, 1428, 1410. 1H-NMR (CDCl3): 0.87 (t, J ˆ 5.4, Me), 1.26 1.54
(m, 6 CH2; Me); 1.69 1.80 (m, CH2); 2.62 (t, J ˆ 7.5, CH2); 4.40 (q, J ˆ 7.2, CH2); 7.30 (t, J ˆ 7.5, 1 arom. H); 7.74
(2d, J ˆ 9.0, 2.4, 1 arom. H); 7.81 (2d, J ˆ 9.0, 2.4, 1 arom. H); 7.92 (AA'BB', J ˆ 8.7, 2 arom. H); 8.17
(AA'BB', J ˆ 8.7, 2 arom. H). 13C-NMR (CDCl3): 170.87; 165.87 (CˆO); 156.21; 154.62; 152.87; 150.86;
140.94; 140.76; 132.59; 130.56; 124.03; 122.74; 121.57; 109.34; 109.07; 61.27; 33.89; 31.82; 29.36; 29.20; 28.98;
24.84; 22.63; 14.28; 14.06. HR-MS: 442.2268 (C25H31FN2O4; calc.4422. 268).
Data of Ethyl 4-{[4-(Decanoyloxy)-2-fluorophenyl]diazenyl}benzoate (1c): 68.2% (0.60 g). IR: 3117, 2956,
2920, 2850, 1758, 1714, 1610, 1587, 1485, 1468, 1427, 1412. 1H-NMR (CDCl3): 0.87 (t, J ˆ 5.4, Me); 1.22 1.55
(m, 6 CH2, Me); 1.63 1.80 (m, CH2); 2.57 (t, J ˆ 7.5, CH2); 4.40 (q, J ˆ 7.2, CH2); 6.96 (2d, J ˆ 9.0, 2.1,
1 arom.H); 7.12 (2 d, J ˆ 10.8, 2.1, 1 arom. H); 7.81 (t, J ˆ 8.7, 1 arom. H); 7.95 (AA'BB', J ˆ 8.7, 2 arom. H); 8.18
(AA'BB', J ˆ 8.7, 2 arom. H). 13C-NMR (CDCl3): 171.35; 165.91 (CˆO); 162.24; 158.78; 155.06; 154.25; 154.11;
138.28; 138.19; 132.55; 130.54; 122.84; 118.71; 117.82; 111.11; 110.80; 61.26; 34.34; 31.82; 29.36; 29.21; 29.03;
24.76; 22.63; 14.29; 14.06. HR-MS: 442.2267 (C25H31FN2O4; calc.4422. 268).
X-Ray Crystal-Structure Analysis.Crystals of compounds 1a 1c 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). No significant decay was observed during the
data collection.Data were processed on a PC with SHELXTL software package [13].The structures of 1a 1c
were solved by direct methods and refined by full-matrix least-squares on F2 value.All non-H-atoms were
refined anisotropically.The H-atoms were fixed at calculated positions and refined using a riding model.Other
data for compounds 1a 1c were as follows.
≈
Compound 1a: C25H32N2O4, P1, triclinic, a ˆ 5.5711(6), b ˆ 8.4652(10), c ˆ 26.459(3) ä, a ˆ 94.993(2), b ˆ
92.682(2), g ˆ 105.334(2)8, Vˆ 1195.6 ä3, Z ˆ 2, q ˆ 1.55 27.558, data collection at 293 K, reflections
measured ˆ 14191, reflections used ˆ 5462 ((Rint) ˆ (0.0593)), R ˆ 0.0655.
≈
Compound 1b: C25H31FN2O4, P1, triclinic, a ˆ 6.1291(7), b ˆ 8.1812(9), c ˆ 25.910(3) ä, a ˆ 83.859(2), b ˆ
87.415(2), g ˆ 70.006(2)8, Vˆ 1213.9 ä3, Z ˆ 2, q ˆ 1.58 27.528, data collection at 293 K, reflections meas-
ured ˆ 14215, reflections used ˆ 5494 ((Rint) ˆ (0.0573)), R ˆ 0.0473.
≈
Compound 1c: C25H31FN2O4, P1, triclinic, a ˆ 5.3563(9), b ˆ 6.3360(11), c ˆ 37.519(6) ä, a ˆ 87.162(3),
b ˆ 87.465(3), g ˆ 77.650(2)8, Vˆ 1241.6 ä3, Z ˆ 2, q ˆ 1.63 24.998, data collection at 150 K, reflections
measured ˆ 7219, reflections used ˆ 3984 ((Rint) ˆ (0.0400)), R ˆ 0.0817. Crystallographic data for the com-
pounds 1a 1c reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication No.CCDC-163831, 163832, and 17380, resp.Copies of the data can be obtained free

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of charge on application to CCDC, 12 Union Road, Cambridge CB2, 1EZ, 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 90-2113-M-260-002) for
financial support.The National Center of High-Performing Computing and the Institute of Chemistry, Academia
Sinica are also highly appreciated for providing the Beilstein database system as well as the most helpful library
service.
REFERENCES
[1] a) J.-M. Lehn, −Supramolecular Chemistry×, VCH, Weinheim, 1995; b) A. D. Hamilton, −Perspectives in
Supramolecular Chemistry×, John Wiley & Sons, 1996.
[2] a) C.A.Hunter, Angew. Chem., Int. Ed. 1993, 32, 1584; b) C.A.Hunter, Chem. Soc. Rev. 1994, 23, 101.
[3] a) G.R.Desiraju, −Crystal Engineering: The Design of Organic Solids×, Elsevier, New York, 1989; b) G.R.
Desiraju, Chem. Commun. 1997, 1475.
[4] a) K.M.Lee, C.K.Lee, I.J.B.Lin,
Angew. Chem., Int. Ed. 1997, 36, 1850; b) J.Chang, J.S.Moore, J. Am.
Chem. Soc. 1994, 116, 2655.;
[5] A.Nangia, New. J. Chem. 2000, 24, 1049.
[6] E.G.Cox, D.W.J.Cruickshank, J.A.S.Smith,
[7] J.H.Williams, J.K.Cockcroft, A.N.Fitch,
Proc. R. Soc., London, Ser. A. 1958, 247, 1.
Angew. Chem., Int. Ed. 1992, 31, 1850.
[8] G.W.Coates, A.R.Dunn, L.M.Henling, J.W.Ziller, E.B.Lobkovsky, R.H.Grubbs,
J. Am. Chem. Soc.
1998, 120, 3641.
[9] a) L.L.Lai, H-.C.Lin, Liq. Cryst. 2000, 27, 707; b) L.L.Lai, E.Wang, L.J.Lee, J.J.Chen, D.W.Yang,
Cryst. 2001, 28, 157; c) L.L.Lai, L.J.Lee, E.Wang, F.Y.Su, Liq. Cryst. 2001, 28, 381; d) L.L.Lai, L.J.Lee,
Liq. Cryst. 2001, 28, 1513; e) L.L.Lai, E.Wang, Helv. Chim. Acta
Liq.
G.H.Lee, Y.Wang, K.L.Lu, S.J.Lee,
2001, 84, 3581.
[10] a) A.Bondi, J. Phys. Chem. 1964, 68, 441; b) F.H.Allen, J.M.P.Lommerse, V.J.Hoy, J.A.K.Howard,
G.R.Desiraju, Acta Crystallgr., Sect. B 1996, 52, 734.
[11] G.M.Sheldrick, SHELXTL version 50. 3, Siemens Analytical X-Ray Instruments Inc. Madison, Wisconsin,
1994.
Received December 31, 2001