Synthesis and liquid-crystalline properties of bromoalkyloxy-substituted terphenylenes

Article abstract of DOI:10.1246/cl.2006.652

A series of bromoalkyloxy-substituted terphenylenes were synthesized through a Palladium cross-coupling reaction between arylboronic acids and bromoaryl derivatives. By DSC it was observed that these intermediates show a rich thermotropism in a wide range of temperature (110-180°C) and by X-ray diffraction it was determined that the liquid-crystalline nature corresponds to tilted type smectic mesophases (SmC, SmF/SmI, and a two-dimensional ordered tilted mesophase SmX). Copyright

Full text of DOI:10.1246/cl.2006.652

652
Chemistry Letters Vol.35, No.6 (2006)
Synthesis and Liquid-crystalline Properties of Bromoalkyloxy-substituted Terphenylenes
ꢀ1
1
2
2
´
´
´
Leticia Larios-Lopez, Damaso Navarro-Rodrıguez, Bertrand Donnio, and Daniel Guillon
´
1
´
´
Centro de Investigacion en Quımica Aplicada, Blvd. Enrique Reyna 140, 25100, Saltillo, Coahuila, Mexico
2
´
Institut de Physique et Chimie des Materiaux de Strasbourg, 23 rue du Loess, 67037 Strasbourg Cedex, France
(Received March 24, 2006; CL-060362; E-mail: llarios@ciqa.mx)
A
series of bromoalkyloxy-substituted terphenylenes
Br
O(CH2)XBr
(1)
were synthesized through a Palladium cross-coupling reaction
between arylboronic acids and bromoaryl derivatives. By DSC
it was observed that these intermediates show a rich thermotrop-
ism in a wide range of temperature (110–180 ꢁC) and by X-ray
diffraction it was determined that the liquid-crystalline nature
corresponds to tilted type smectic mesophases (SmC, SmF/
SmI, and a two-dimensional ordered tilted mesophase SmX).
+
(OH)2B
O(CH2)yCH3 (2)
3a: x = 8, y = 13
3b: x = 12, y = 13
3c: x = 8, y = 15
3d: x = 12, y = 15
(iv) Pd(0)
There are hundreds of conjugated organic molecules show-
ing light-emitting (LE) properties,1 however, few of them have
demonstrated to posses a second property of interest for opto-
electronic applications, as is the case of the liquid-crystalline
(LC) properties of oligophenylenes.2 This functional duality
has recently attracted the attention of chemists to synthesize
calamitic structured conjugated molecules, like the alkyl- or
alkyloxy-substituted terphenylenes that show a light emission
in the blue region and a rich thermotropism in a wide tempera-
ture range.3 Actually, both properties are of high interest for ap-
plications in spontaneously molecular aligned thin films that pro-
duce a linearly polarized light emission.4 Alkyloxy-substituted
terphenylenes can be functionalized at one chain-end to obtain
intermediates that can be further grafted into a polymer back-
bone to produce novel macromolecular structures also showing
both LE and LC properties. It is to point out that some intermedi-
ates by themselves show LE and LC properties. Let us consider
the case of the non-symmetrical alkyloxy-substituted terphen-
ylenes modified at one chain end with bromine. In these mole-
cules, the alkyloxy chains are the solubilizing moieties, other-
wise phenyl-conjugated molecules would be practically insolu-
ble in most common solvents.5 The bromine atom is introduced
because alkylbromides readily react with some common mono-
mers like vinylpyridine6 that polymerize spontaneously produc-
ing liquid-crystalline polysalts.
The synthesis route to obtain the bromoalkoxy terphenyl-
enes is shown in Figure 1. First, the 4-bromophenol is reacted,
in a DMF/NaOH solution, with an excess of an ꢀ,!-dibromoal-
kane (Williamson reaction (i)) to produce the 4-(!-bromoalkyl-
oxy)bromobenzene 1.7 In a parallel reaction (ii) the 40-bromo-4-
biphenol is reacted, in a DMF/NaOH solution, with a bromo-
alkane to obtain the 40-alkyloxy-4-bromobiphenyl. Then, this
compound is further reacted with triisopropylborate in a cooled
(ꢂ40 ꢁC) BuLi/THF solution (iii) to obtain the 40-alkyloxy-4-bi-
phenylboronic acid 2. Finally, the bromoalkyloxyterphenylenes
3 were obtained through a Suzuki coupling reaction (iv) between
the corresponding bromoaryl derivatives 1 and aryl boronic acid
2, catalyzed with tetrakis(triphenylphosphine)palladium(0).8
This method is highly convenient because arylboronic acids
are stable during both storage and handling and also because
they readily react with arylbromides through a Pd complex-cat-
Br(CH2)XO
O(CH2)yCH3
(3)
Figure 1. Synthesis route for the preparation of terphenylenes
3a, 3b, 3c, and 3d.
alyzed coupling reaction, which proceeds under relatively mild
conditions. Terphenylenes were purified as described for similar
compounds.3 All spectroscopic studies and elemental analysis
were consistent with the proposed molecular structures.9
The LC properties of terphenylenes have been studied by
differential scanning calorimetry (DSC), optical microscopy
(POM) and X-ray diffraction. All compounds in Figure 1 were
synthesized and all of them exhibited multiple thermal transi-
tions (Table 1). This behavior was observed in both heating
and cooling scans as it is shown in Figure 2 for the compound
3b. The observed optical textures for these compounds, like
the schlieren (see inset in Figure 2) and focal-conic fan textures,
are typical of a liquid-crystalline state. X-ray diffraction was
used to ascertain the nature of the various phases observed by
DSC and POM, and to determine their structural parameters
(Figure 3).
In Figure 3, the smectic nature of the liquid-crystalline
phases for the compound 3b at high temperature (ꢃ175 ꢁC)
was characterized by the presence of a sharp reflection at low an-
gles, related to the stacking period of the smectic layers.10 The
stacking period (d001) measured experimentally (see Table 2)
was compared with the length of the most stretched conforma-
˚
tion of the molecule (L ¼ 51:1 A), which was calculated by us-
Table 1. Transition temperatures and enthalpy changes of
terphenylenes
Compound
T/ꢁC [ꢀH/kJ molꢂ1
]
3a
104.1 [25.3] 125.3 [4.4] 154.5 [3.2] 187.4 [7.6]
201.4 [14.6]
3b
3c
117.0 [61.6] 150.4 [7.4] 169.3 [4.3] 180.8 [14.2]
106.0 [29.1] 116.1 [0.5] 129.6 [4.4] 159.3 [3.9]
185.7 [8.2] 196.8 [15.7]
3d
116.0 [47.6] 139.0 [3.4] 146.6 [1.5] 168.8 [3.8]
178.3 [10.1]
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.6 (2006)
653
Table 2. Experimental stacking period d100 of smectic layers
measured at temperature T, and calculated length L of the
compound 3b in its most extended conformation
T/ꢁC
Mesophase
Tilt angle/ꢁ
˚
L/A
˚
d001/A
200
175
160
150
I
SmC
51.1
—
—
38
37
40
40.3
40.7
39.1
heating
cooling
SmF or SmI
SmX
In summary, the bromoalkyloxy-substituted terphenylenes
can be synthesized by using two well known basic reactions:
the Williamson reaction to introduce the alkyloxy chains into
the aryl groups and the Suzuki reaction to couple the alkyloxy-
phenyboronic acids with the bromoalkyloxyarylbromides. These
terphenylenes showed thermotropic liquid-crystal phases of the
smectic C and smectic I/F type characterized by a tilted arrange-
ment of molecules with respect to the normal of the smectic
layers.
80
100
120
140
160
180
200
Temperature /°C
Figure 2. Heating and cooling DSC traces at 5 ꢁC/min for com-
pound 3b, and optical texture at 160 ꢁC obtained on cooling.
We thank the CONACYT of Mexico for supporting this
work (projects 35163-U and 43741-Y). We also wish to thank
Claudia V. Reyes-Castan˜eda for helpful assistance.
References and Notes
1
M. Kertesz, Handbook of Organic Conductive Molecules
and Polymers, ed. by H. S. Nalwa, John Wiley, New York,
1997, Vol. 4, pp. 147–172.
100 °C
140 °C
160 °C
´ ´
L. Oriol, M. Pin˜ol, J. L. Serrano, C. Martınez, R. Alcala,
2
3
175 °C
´
R. Cases, C. Sanchez, Polymer 2001, 42, 2737.
200 °C
5
10
15
20
2θ
25
30
´
´
´
L. Larios-Lopez, D. Navarro-Rodrıguez, E. Arias-Marın, I.
Moggio, C. V. Reyes-Castan˜eda, B. Donnio, D. Guillon,
Liq. Cryst. 2003, 30, 423.
Figure 3. XRD patterns of compound 3b at five different tem-
peratures.
4
5
6
7
8
9
H. Tokuhisa, E. Masanao, T. Tsutsui, Appl. Phys. Lett. 1998,
72, 2639.
´
ing molecular modeling software from ACD/3D Viewer (Ad-
vanced Chemistry Development in three Dimensions). This
comparison suggests that molecules are tilted with respect to
the normal of the smectic plane. The tilt angle ꢁ (cos ꢁ ¼
d001=L) for the different smectic phases was around 38ꢁ. Similar
tilt angle values were observed for smectic phases of analogous
terphenylenes3 and some other compounds showing a smectic C
phase.11 The internal arrangement of molecules within the smec-
tic layers was monitored by the number and shape of the scatter-
ing signals in the wide angle region of the X-ray diffraction pat-
tern. At high temperature (ꢃ175 ꢁC), the phase was character-
ized by the presence of one broad diffuse peak, which is typical
of the liquid-like ordering of the molecules within the layers. On
cooling from this phase (ꢃ160 ꢁC), this broad signal becomes
sharper, indicating an hexatic-type ordering of the molecules
within the smectic layers similar to that reported for a smectic
F or smectic I phase.12 On cooling further (ꢃ150 ꢁC), the pattern
shows a drastic change only in the wide angle region: The sharp
signal transforms into two signals, one broad signal and one
sharp peak, probably indicating a two-dimensional ordering
of molecules within the smectic layers (SmX). The crystalline
nature of phases at low temperature was demonstrated by the
presence of many sharp Bragg reflections in the pattern.
L. Athouel, J. Wery, B. Dulieu, J. P. Buisson, G. Froyer,
¨
Synth. Met. 1997, 84, 287.
´
D. Navarro-Rodrıguez, D. Guillon, A. Skoulios, Makromol.
Chem. 1992, 193, 3117.
M. B. Smith, J. March MARCH’S Advanced Organic
Chemistry, 5th ed., John Wiley, New York, 2001.
N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 1981,
11, 513.
Selected spectroscopic data for 3b: 1H NMR (JEOL
300 MHz, CDCl3): ꢂ 0.85–0.95 (t, 3H, CH3), 1.24–1.56
(m, 38H, (CH2)8 and (CH2)11), 1.75–1.93 (m, 6H,
2CH2CH2O and CH2CH2Br), 3.37–3.43 (t, 2H, CH2Br),
3.98–4.05 (m, 4H, 2CH2O), 6.95–7.6 (2m, 12H, Ar). Anal.
(Carlo Erba 1106) Calcd for C44H65O2Br (705.90): C,
74.87; H, 9.28%; Found: C, 75.42; H, 9.39%. Spectroscopic
data for compounds 3a, 3c, and 3d are similar to those of
compound 3b, the only difference is the intensity of signals.
10 G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic
Crystals, Wiley, New York, 1974.
11 B. Heinrich, D. Guillon, Mol. Cryst. Liq. Cryst. 1995, 268,
21.
12 D. Guillon, A. Skoulios, J. J. Benaltar, J. Phys. 1986, 47,
133.
Published on the web (Advance View) May 20, 2006; doi:10.1246/cl.2006.652