Columnar order in thermotropic mesophases of oligophenylenevinylene derivatives

Article abstract of DOI:10.1016/S0040-4039(00)01095-9

Oligophenylenevinylene derivatives substituted with long alkyl chains on one end and a carboxylic acid function on the other end have been prepared and the dimeric polycatenar calamitic supramolecules resulting from hydrogen bonding of their acid function

Full text of DOI:10.1016/S0040-4039(00)01095-9

Tetrahedron Letters 41 (2000) 6411±6414
Columnar order in thermotropic mesophases of
oligophenylenevinylene derivatives
Jean-Francois Eckert, Jean-Francois Nicoud, Daniel Guillon
and Jean-Francois Nierengarten*
Â
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Groupe des Materiaux Organiques, Institut de Physique et Chimie des Materiaux de Strasbourg,
Â
Universite Louis Pasteur et CNRS, 23 rue du Loess, 67037 Strasbourg, France
Received 25 March 2000; accepted 28 June 2000
Abstract
Oligophenylenevinylene derivatives substituted with long alkyl chains on one end and a carboxylic acid
function on the other end have been prepared and the dimeric polycatenar calamitic supramolecules
resulting from hydrogen bonding of their acid functions show columnar liquid crystalline phases. # 2000
Elsevier Science Ltd. All rights reserved.
Although conjugated oligomers and polymers have been extensively investigated in previous
years for their electronic properties (conductivity, optical properties, etc),1 structural studies of
these materials have received less attention and appear today as a very important issue in their
application. Of particular interest are derivatives exhibiting liquid crystalline properties.2,3 Such
materials are not only ordered assemblies easy to process for various applications, but many
properties associated with the delocalized p-electron system (electroluminescence, third order
optical nonlinearity, etc.) can be enhanced due to the orientational e€ect induced by the liquid
crystal ordering.2 A particularly appealing challenge is the preparation of liquid crystalline
conjugated oligomers displaying columnar mesophases with the aim of generating ®bers that
could act as conducting nanowires similar to those obtained from discotic derivatives.4 In this
paper, we report the preparation of new liquid crystalline all-trans-oligophenylenevinylene (OPV)
derivatives giving rise to columnar order in thermotropic mesophases.
The synthesis of the OPV derivatives is depicted in Scheme 1. Benzaldimines 1a5 and 1b6 were
subjected to the Siegrist reaction with stilbene 26 to give protected trimers 3a and 3b, respectively.
Subsequent deprotection of 3a and 3b with CF3CO2H in CH2Cl2/H2O a€orded aldehydes 4a and
4b, respectively. Reaction of 4a and 4b with methyl (triphenyphosphoranylidene)acetate under
Wittig conditions followed by the saponi®cation of the resulting esters yielded carboxylic acids 6a
and 6b, respectively. Reduction of 4a and 4b with LiAlH4 to the corresponding alcohols followed
by a Mitsunobu reaction with methyl p-hydroxybenzoate a€orded 8a and 8b, respectively.
* Corresponding author. Fax: 33 388 10 72 46; e-mail: niereng@michelangelo.u-strasbg.fr
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01095-9

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Scheme 1. Reagents and conditions: (i) t-BuOK, DMF, 80ꢀC, 2 h (3a: 91%; 3b: 91%); (ii) TFA, CH2Cl2, H2O, room
temp., 4 h (4a: 96%; 4b: 96%); (iii) methyl (triphenyphosphoranylidene)acetate, THF, Á (5a: 80%; 5b: 84%); (iv)
KOH, THF, H2O, Á, 48 h (6a: 90%; 6b: 90%); (v) LiAlH4, THF, 0ꢀC, 1 h (7a: 90%; 7b: 86%); (vi) methyl p-hydro-
xybenzoate, diethyl azodicarboxylate (DEAD), PPh3, THF, Á, 24 h (8a: 72%; 8b: 85%); (vii) KOH, THF, H2O, Á, 48
h (9a: 90%; 9b: 90%)
Subsequent treatment with KOH in THF/H2O yielded the corresponding carboxylic acids 9a and
9b. All of the spectroscopic studies and elemental analysis results were consistent with the
proposed molecular structures.7
The liquid crystalline properties of the OPV derivatives 6a±b and 9a±b have been studied by
di€erential scanning calorimetry (DSC), X-ray di€raction and optical investigations. Further-
more, thermogravimetric analyses (TGA) show that these oligomers remain stable up to 350ꢀC.
The results are summarized in Table 1.
Columnar mesophases have been observed with polycatenar calamitic molecules composed of
long rigid aromatic cores (containing in general ®ve rings) with two or three paranic chains at
both ends.8 In our case, the OPV derivatives 6a±b and 9a±b are all able to form dimers through
hydrogen bonding9 of their acid functions to give polycatenar calamitic supramolecules containing
up to seven and nine rings, respectively. In fact, the polymorphism of the resulting supramole-
cules appears to be similar to that of polycatenar calamitic molecules in spite of the increased
length of the rigid core. E€ectively, columnar mesophases have been observed for all the OPV
derivatives 6a±b and 9a±b. Fig. 1 shows an optical texture observed for one of the derivatives.
This optical microscopy texture contains developable units that are characteristic of a columnar
mesophase.10

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Table 1
Phase behavior of 6a±b and 9a±b (K, crystalline phase; ColH, hexagonal columnar phase; ColR,
rectangular columnar phase; I, isotropic phase)
Figure 1. Left: optical texture observed with a polarizing microscope at 100ꢀC for compound 6a; right: X-ray di€rac-
tion pattern of compound 6a recorded at 100ꢀC
The columnar structure of the mesophases was con®rmed by X-ray di€raction investigations
for all the OPV derivatives 6a±b and 9a±b. For example, the di€raction pattern recorded at 100ꢀC
for compound 6a is depicted in Fig. 1. Whereas a columnar phase with hexagonal symmetry was
found for both biforked derivatives 6b and 9b, a columnar phase with P2gg rectangular symmetry
and a pseudo-hexagonal lattice (a/b=30.5) was observed for the corresponding derivatives with
tridodecyloxyphenyl groups.
Two types of columnar mesophases have been observed for OPV derivatives substituted with
long alkyl chains on one end and a carboxylic acid function on the other end. Further structural
studies of these new OPV derivatives, their incorporation in light emitting devices and determination
of their conducting properties are now under investigation.
Acknowledgements
We thank B. Heinrich for his help with the X-ray measurements and L. Oswald for technical
assistance in the synthesis.

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1
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1
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