Synthesis and Study of Azo-Dye Compounds: Various Molecular Stackings from Different Polarities of the Molecules

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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: 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.


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 N

ˆN 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 Et


N in CH





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) O(1) and

C(2) O(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.6.13 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) O(1) and C(2) O(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.5.36 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) C(1) between the layers A

'' and B'', as well as F(2) C(2) between the layers

B'' and E''.The distances F(1) ¥¥¥ H (at C(3)) and F(1) ¥¥¥ H (at C(4)) are 2.73 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


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).



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 3a


).Consequently, various molecular stacking in the aromatic

moieties was obtained and different mesogenic properties of compounds 1a ± 1c were

observed during the thermal process


).Compound 1a shows a SmB phase in the range

of 40 ± 95.2

8 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 ± 71.8

8) and

cooling process (73.0 ± 71.8

8) probably because of stronger intermolecular H-bonding


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


Fig.2. The molecular packing of compound 1b in the solid state. The H-atoms are omitted for clarity.

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.

2) 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.


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 [cm1]: 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 r.t.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-hydroxy4-[(3-fluoro-4-hydroxy-phenyl)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 CH2Cl2with 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 (CDCl

3): 0.85 (t, Jˆ 5.4, Me); 1.26 ± 1.53

(m, 6 CH2, Me); 1.69 ± 1.78 (m, CH2); 2.57 (t, Jˆ 7.8, CH2); 4.40 (q, Jˆ 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 (CDCl

3): 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.


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 (CDCl

3): 0.87 (t, Jˆ 5.4, Me), 1.26 ± 1.54

(m, 6 CH2; Me); 1.69 ± 1.80 (m, CH2); 2.62 (t, Jˆ 7.5, CH2); 4.40 (q, Jˆ 7.2, CH2); 7.30 (t, Jˆ 7.5, 1 arom. H); 7.74

(2d, Jˆ 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 (CDCl

3): 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.442.2268).

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 (CDCl

3): 0.87 (t, Jˆ 5.4, Me); 1.22 ± 1.55

(m, 6 CH2, Me); 1.63 ± 1.80 (m, CH2); 2.57 (t, Jˆ 7.5, CH2); 4.40 (q, Jˆ 7.2, CH2); 6.96 (2d, Jˆ 9.0, 2.1,

1 arom.H); 7.12 (2d, Jˆ 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 (CDCl

3): 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.442.2268).

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 MoKaradiation (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 F2value.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, aˆ 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, V ˆ 1195.6 ä3, Zˆ 2, q ˆ 1.55 ± 27.558, data collection at 293 K, reflections

measuredˆ 14191, reflections used ˆ 5462 ((Rint)ˆ (0.0593)), R ˆ 0.0655.

Compound 1b: C25H31FN2O4, P1≈, triclinic, aˆ 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, V ˆ 1213.9 ä3, Zˆ 2, q ˆ 1.58 ± 27.528, data collection at 293 K, reflections

meas-uredˆ 14215, reflections used ˆ 5494 ((Rint)ˆ (0.0573)), R ˆ 0.0473.

Compound 1c: C25H31FN2O4, P1≈, triclinic, aˆ 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, V ˆ 1241.6 ä3, Zˆ 2, q ˆ 1.63 ± 24.998, data collection at 150 K, reflections

measuredˆ 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


of charge on application to CCDC, 12 Union Road, Cambridge CB2, 1EZ, UK (fax:‡44(1223)-336-033; e-mail:

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.


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