Rigid phenylene ethynylene units linked by a V-shaped centre. An approach to biaxial nematogens?

Article abstract of DOI:10.1080/15421400490435260

Encouraged by theoretical predictions that nematic phases formed from V-shaped compounds could be biaxial, shape-persistent phenylene ethynylene arms linked by 1,3-phenylene, 4,4′-diphenylenemethane and 3,6-fluorenone units have been synthesized. Crystall

Full text of DOI:10.1080/15421400490435260

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Rigid Phenylene Ethynylene
Units Linked by A V-shaped
Centre. An Approach to Biaxial
Nematogens?
Matthias Lehmann a & Jérémy Levin a
a Laboratoire de chimie des polymères , CP 206/1
Université Libre de Bruxelles , Boulevard du
Triomphe, Bruxelles, Belgium
Published online: 18 Oct 2010.
To cite this article: Matthias Lehmann & Jérémy Levin (2004) Rigid Phenylene
Ethynylene Units Linked by A V-shaped Centre. An Approach to Biaxial
Nematogens?, Molecular Crystals and Liquid Crystals, 411:1, 273-281, DOI:
10.1080/15421400490435260
To link to this article: http://dx.doi.org/10.1080/15421400490435260
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 273=[1315]–281=[1323], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435260
RIGID PHENYLENE ETHYNYLENE UNITS LINKED
BY A V-SHAPED CENTRE. AN APPROACH TO
BIAXIAL NEMATOGENS?
ꢀ
´ ´
Matthias Lehmann and Jeremy Levin
`
´
Laboratoire de chimie des polymeres, CP 206=1 Universite Libre de
Bruxelles, Boulevard du Triomphe, 1050 Bruxelles, Belgium
Encouraged by theoretical predictions that nematic phases formed from
V-shaped compounds could be biaxial, shape-persistent phenylene ethynylene
arms linked by 1,3-phenylene, 4,40-diphenylenemethane and 3,6-fluorenone
units have been synthesised. Crystallisation and micro-segregation is pre-
vented by 2,5-alkoxy substitution on the middle benzene ring of the arms.
The specific molecular engineering, i.e. variation of peripheral substituents
(H, CN) and different lengths of lateral alkoxy chains, lead to enantiotropic
nematic liquid crystals over a wide temperature range for fluorenone contain-
ing materials II3d and mixtures of II3d with non-mesogenic II3b. These meso-
phases solidify at ambient temperature and form nematic glasses.
Keywords: biaxial nematics; phenylene ethynylene scaffold; shape persistent compounds;
thermotropic liquid crystals; V-shaped molecules
INTRODUCTION
Thermotropic, biaxial nematic mesogens are of great theoretical and prac-
tical interest. Since the prediction of Freiser, there have been various con-
cepts to restrict the rotation around the long axis of nematic mesogens
which should lead to biaxiality of nematic liquid crystal phases [1]. How-
ever, although biaxial nematic systems are known for lyotropics [2] and side
chain polymers [3], the low molar mass biaxial nematics claimed so far have
2
been shown by H NMR to have uniaxial symmetry [4a]. Attempts have
been made to force lad like molecules into biaxial order by charge transfer
interactions [5]. This lead to biaxial SmAb phases. A monotropic nematic
ꢁ
´
We are grateful to the Communaute Francaise de Belgique for its financial support (ARC n
00=05-257), to Prof. Herbert Meier for the availability of FD Mass and elemental analysis at the
University of Mainz and to Prof. Yves Geerts for the generous support and helpful discussions.
ꢀCorresponding author. E-mail: matthias.lehmann@ulb.ac.be
273=[1315]

274=[1316]
M. Lehmann and J. Levin
phase of highly biaxial molecules, where a rod is linked to a disc by a flex-
ible spacer has been reported [6]. Recently, V-shaped molecules have been
proposed to be possible biaxial nematogens [4]. A unit linking two arms by
an angle between h ¼ 108.5–110ꢁ should be most promising [4a]. Limiting
cases are linear molecules (h ¼ 180ꢁ) and compounds with h ¼ 90ꢁ, for
which only uniaxial phases are proposed. The influence of various linking
units on the supramolecular order of banana-shaped molecules has been
studied, but only smectic phases were found [7]. Biaxial SmA phases for
neat banana-shaped compounds has been proofed by conoscopy [8]. First
evidence for a biaxial nematic in a small range at high temperature was
mentioned for a compound bearing a oxadiazole center [9]. Nematic phases
are rather rare for the family of V-shaped molecules and were mainly
obtained with lateral substituents [10]. 2,5-Alkoxy substitution of stilbe-
noid, V-shaped molecules I (Chart 1) lead exclusively to nematic phases
[11]. But, these compounds with a 1,3-phenylene centre, can possess vari-
ous conformations leading to different linking angles h. Shape persistent
molecules can be envisaged by establishment of a rigid phenylene ethyny-
lene scaffold, thus the effect of the linking unit can be studied indepen-
dently from conformational influence. In the present contribution,
structures II (Chart 1) were synthesised with two rigid arms linked by dif-
ferent centres, i.e. 1,3-phenylene, 4,40-diphenylenemethane and 3,6-fluore-
none, thus establishing angles from 120ꢁ, 109.47ꢁ and 85ꢁ. Analogous to I,
CHART 1 V-shaped molecules: stilbenoid system I and shape persistent phenylene
ethynylene structure II.

Rigid Phenylene Ethynylene Units Linked by a V-shaped Centre 275=[1317]
lateral alkoxy side chains are attached to V-shaped molecules II, which
should prevent the molecules from crystallisation and from micro-segre-
gation. As stressed by Luckhurst, normal nematic phases tend to crystallise
before reaching the temperature region where a second order transition to
a biaxial nematic could theoretically occur [4a]. Peripheral substituents Y
were introduced for mesophase stabilisation by specific interactions.
RESULTS AND DISCUSSION
Synthesis
The synthesis of the phenylene ethynylene scaffold was performed by a
convergent strategy on the basis of Hagihara coupling [12]. The preparation
of the arms is outlined in Scheme 1. The key compound 6 was obtained
according to established reaction procedures [13]. 2,5-Dialkoxy-4-bromoio-
dobenzenes 5 were obtained by oxidative iodination of 2,5-dialkoxybromo-
benzenes 4 [14]. The different reactivity of the iodo and the bromo side in
5 allowed to couple triisopropylsilylacetylene followed by trimethyl-
silylacetylene without intermediate purification. The selective cleavage
of the trimetylsilyl group under basic conditions led to mono deprotected
6 in high overall yields for the three steps. The different arms 7a-d
were obtained by Hagihara coupling of 6 with various arylhalogens and
SCHEME 1 Synthesis of shape-persistent phenylene ethynylene arms.

276=[1318]
M. Lehmann and J. Levin
succeeding cleavage of the triisopropylsilyl group using tetrabutyl-
ammonium fluoride. In a final twofold reaction, arms 7 were linked to
1,3-diiodobenzene (1), 4, 40-dibromodiphenylmethane (2) [15] and
3,6-dibromofluorenone (3)(Scheme 2) [16]. The yields of the obtained
V-shaped molecules II are summarised in Table 1. Generally, the more
reactive diiodo derivative 1 and the fluorenone 3 gave high yields, whereas
the slow coupling with the less activated central unit 2 had to compete with
the homocoupling of two arm segments, thus resulted in low amounts and
pretentious purification of the target molecules II2.
SCHEME 2 Synthesis of V-shaped phenylene ethynylene derivatives.
TABLE I Yields of V-shaped Phenylene Ethynylene Derivatives II
Compound
Y
R
Yield [%]
II1a
II1b
II1c
II1d
II2a
II2b
II2c
II2d
II3a
II3b
II3c
II3d
H
H
C3H7
C6H13
C3H7
C6H13
C3H7
C6H13
C3H7
C6H13
C3H7
C6H13
C3H7
C6H13
10
93
72
62
14
7
CN
CN
H
H
CN
CN
H
5
28
77
44
80
77
H
CN
CN

Rigid Phenylene Ethynylene Units Linked by a V-shaped Centre 277=[1319]
The analytical data is consistent with the structural pure compounds
II [17].
Thermotropic Properties
The thermotropic properties of compounds II were studied by differential
scanning calorimetry (DSC) and polarised optical microscopy (POM).
Results are summarised in Table II.
Generally, as proposed, the 2,5-alkoxy substitution prevent the mole-
cules from crystallisation. Compounds II1a to II3b are isotropic liquids
or pass into a liquid phase after a glass transition during the second heating
of the sample (entry 1–9, Table II). Some molecules (II1a, II2a, II2d,
II3a) eventually show a cold crystallisation before they melt again to form
an isotropic liquid. Comparison of the melting temperatures of the com-
pounds, including the melting transitions of the first heating trace for mole-
cules, which are not crystallising, shows that with shorter alkoxy chains or
with CN groups transition temperatures to the isotropic phase are
increased as expected.
Only two exceptions from these observations were found. Molecule II3c,
which crystallises upon cooling from the isotropic phase, and II3d which
TABLE II Thermotropic Properties of II. Results of DSC and POM Investigations
DSC results of the second heating,
rate 10ꢁC=min
Compoundy
(Onset [ꢁC]=DH [kJ=mol]
II1a
II1b
II1c
II1d
II2az
II2b
II2d
II3a
I86.9= ꢂ 33.8 Cr 153.9=37.0 I
g ꢂ 10.2(Tg) I (m.p. 64.9ꢁC first heating)
g 51.0 (Tg) I (m.p. 130.2ꢁC first heating)
no transition# (m.p. 83.7ꢁC first heating)
g 34.7 (Tg) I 103.7= ꢂ5.4 Cr 166.5=5.9 I
no transition# (m.p. 86.4ꢁC first heating)
g 20.9 (Tg) I 69.7= ꢂ45.5 Cr 131.6=54.8 I
g 52.9 (Tg) I 95.9=* ꢂ 22.5 Cr1 132.3= ꢂ 2.1
Cr2 156.0=36.5 I
II3b
II3c
g 29.7 (Tg)I (m.p. 104.3ꢁC first heating)
Cr 213.5=20.8 I
g3 4.4 (Tg) N 178.0=0.7 I
II3d
II3b:II3d ¼ 1:1
g 22.0 (Tg) N 118.0=0.3 I
yall compounds except II2a, II3c and d show no crystallisation or formation of a mesophase
in the cooling process.
zpartial cold crystallisation.
# no transition down to ꢂ 10ꢁC. Cr crystal, g glass, N nematic phase, I isotropic phase.

278=[1320]
M. Lehmann and J. Levin
indeed forms an enantiotropic liquid crystalline phase. The enthalpy re-
lated to the transition from the mesophase to the isotropic liquid amounts
to 0.7 kJ=mol (~S=R ¼ 0.47), which is a typical value found for nematic
phases. Two- and four-brushed disclinations are observed in the Schlieren
texture of droplets below the clearing temperature (Fig. 1). The LC phase
shows a high fluidity and spontaneous homeotropic alignment is observed
between two glass slides (Fig. 2, right hand side). Thus II3d forms an uni-
axial nematic liquid crystalline phase. Uniaxiality of V-shaped molecules
with an angle h around 90ꢀ, which is close to the angle realised with the
fluorenone centre (h ffi 85ꢀ), was predicted theoretically [4]. It is worth not-
ing, that the temperature range extends to 144ꢀC, although slow crystallis-
ation is observed when the sample is cooled slowly or annealedꢀabove the
glass transition temperature. These crystals melt again at 140 C to form
the nematic phase. In order to prevent the nematic phase from crystallis-
ation, mixtures of II3b and II3d were investigated by means of contact
method and by DSC measurements of a 1:1 mixture. The interface of the
contact sample of II3b and II3d is presented in Figure 2. On the right hand
side, the spontaneous homeotropically aligned nematic phase of II3d
(black region) can be seen. On the lower left hand side, the isotropic liquid
phase of II3b is exposed. In the interface a new liquid crystal phase can be
observed. POM studies for the 1:1 mixture reveal typical Schlieren tex-
FIGURE 1 Schlieren texture of a nematic droplet obtained from II3d at 178.3ꢀC
between crossed polarizers. (See COLOR PLATE IV)

Rigid Phenylene Ethynylene Units Linked by a V-shaped Centre 279=[1321]
FIGURE 2 Interface of a contact sample obtained from II3b and II3d at 155.0ꢀC
between crossed polarizers. Left hand side: isotropic phase of II3b. Right hand side:
homeotropic aligned nematic phase of II3d. (See COLOR PLATE V)
tures. The observed nematic phase does not align homeotropically between
glass slides. The enthalpy at the I-N transition amounts to 0.3 kJ=mol
(~S=R ¼ 0.31), which is a typical value found for I-Nu transitions of mole-
cules organising in uniaxial nematic phases. For a material forming a biaxial
nematic mesophase from the uniaxial nematic phase a second order tran-
sition has been theoretically predicted [18]. However, no second order tran-
sition besides the glass transition at 22ꢀC has been detected. Crystallisation
is slower for the mixture of II3b and II3d than for the neat II3d and could
not be observed during DSC measurements, thus the material can be frozen
to a nematic glass. The nature of the nematic phases have to be explored in
2
future studies, by conoscopy and H solid state NMR.
CONCLUSIONS
V-shaped, shape-persistent molecules II, with central units linking rigid
arms, based on a phenylene ethynylene scaffold, by angles h of approxi-
mately 120ꢀ, 109.47ꢀ and 85ꢀ, were synthesised in an efficient convergent
strategy. Lateral 2,5-alkoxy substitution of the middle benzene ring pre-
vents effectively the molecules from crystallisation. For compounds II1
and II2, with linking units realizing angles h for which biaxial nematic

280=[1322]
M. Lehmann and J. Levin
phases are theoretically expected, no supramolecular liquid crystalline or-
der is observed. In the series II3, where the central unit is formed by a lar-
ger fluorenone moiety, the tendency of crystallisation increases. An
enantiotropic nematic phase is observed only for II3d. Interestingly, it
extends over a large temperature range of 144ꢁC and forms a nematic glass
at room temperature. In accordance with theoretical predictions, the ne-
matic phase of II3d was found to be uniaxial, since the linking angle is
close to 90ꢁ. A 1:1 mixture of II3b and II3d forms a nematic phase which
could not be homeotropically aligned. The nature of this phase has to be
explored in future studies, which will include experiments applying electric
fields, in order to establish eventually phase biaxiality.
In conclusion, the question raised in the title can not yet be positively
answered. In the series of molecules II no biaxial nematogen was found,
but molecular shape and substitution pattern can still be optimised.
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[17] Analytical data for II3d : 1H NMR (300 MHz, CDCl3) d ¼ 0.91 (m, 12H), 1.38 (m, 8H),
1.57 (m, 8H), 1.88 (m, 8H), 4.06 (m, 8H), 7.03, 7.04 (2s, 4H), 7.49 (dd, ABC, 2H),
7.60, 7.65 (AA0BB0, 8H), 7.67 (m, ABC, 4H). 13C NMR (75 MHz, CDCl3) d ¼ 14.0, 14.1,
22.6, 22.7, 25.7, 25.8, 29.2, 31.5, 31.6, 53.4, 69.5, 69.6, 89.7, 90.3, 93.4, 94.7, 111.5,
113.6, 114.1, 116.7, 118.5, 123.1, 124.3, 128.2, 129.8, 131.96, 132.0, 132.7, 133.7, 143.7,
153.8, 153.9, 191.8. FT-IR (KBr) m [cm ꢂ 1] ¼ 3055, 2926, 2855, 2224, 2206, 1708, 1601,
1510, 1217, 1027, 919, 880, 854, 783, 689, 553. MS (FD) m=z (%) ¼ 1030.4 (100,
Mþ ). EA (C71H70N2O5) calculated %C 82.69, %H 6.84, %N 2.72, found %C 82.59,
%H 6.77, %N 2.63. Analytical data for II3b : 1H NMR (300 MHz, CDCl3) d ¼ 0.91 (m,
12H), 1.37 (m, 8H), 1.57 (m, 8H), 1.88 (m, 8H), 4.06 (m, 8H), 7.04, 7.05 (2 s, 4H),
7.36 (m, 6H), 7.49 (dd, ABC, 2H), 7.55 (m, 4H), 7.67 (m, ABC, 4H). 13C NMR
(75 MHz, CDCl3) d ¼ 14.0, 14.1, 22.6, 22.7, 25.7, 25.8, 29.3, 31.6, 31.6, 53.4, 69.6, 69.7,
85.8, 90.1, 94.3, 95.3, 113.0, 115.0, 116.8, 117.0, 123.1, 123.3, 124.3, 128.3, 128.3,
130.0, 131.6, 132.7, 133.6, 143.7, 153.6, 153.9, 191.9. FT-IR (KBr) m [cm ꢂ 1] ¼ 3054,
2925, 2855, 2200, 1716, 1610, 1599, 1506, 1216, 1026, 917, 876, 851, 782, 753, 689,
668. MS (FD) m=z (%) ¼ 980.6 (100, Mþ ). EA (C69H72O5) calculated %C 84.45, %H
7.40, found %C 83.92, %H 7.99.
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