Self-assembled columnar mesophase from a new disclike polar mesogen based on a 3,5-dicyano-2,4,6-tristyrylpyridine core

Article abstract of DOI:10.1080/15421400490478803

We report on the design and synthesis of a new disc-like Liquid crystal based on a 3,5-dicyano-2,4,6-tristyrylpyridine core. The mesogenic behavior was investigated by differential scanning calorimetry, polarized-light optical microscopy, and X-ray diffra

Full text of DOI:10.1080/15421400490478803

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Self-assembled Columnar
Mesophase from a New Disclike
Polar Mesogen Based on a 3,5-
Dicyano-2,4,6-Tristyrylpyridine
Core
André-Jean Attias a b , Chantal Cavalli b , Bertrand
Donnio c , Daniel Guillon c , Philippe Hapiot d
Jacques Malthête e
&
a Université Pierre et Marie Curie-UMR 7610-Chimie
des Polymères , Paris Cedex 05, France
b ONERA-Département des Matériaux et Systèmes
Composites , Châtillon Cedex, France
c Groupe des Matériaux Organiques , Institut de
Physique et Chimie des Matériaux de Strasbourg ,
Strasbourg Cedex, France
d 1-UMR 6510-Synthèse et Electrosynthèse
Organique , Université de Rennes , Rennes Cedex,
France
e Section de recherche , Institut Curie , Paris Cedex
05, France
Published online: 18 Oct 2010.
To cite this article: André-Jean Attias , Chantal Cavalli , Bertrand Donnio , Daniel
Guillon , Philippe Hapiot & Jacques Malthête (2004) Self-assembled Columnar
Mesophase from a New Disclike Polar Mesogen Based on a 3,5-Dicyano-2,4,6-
Tristyrylpyridine Core, Molecular Crystals and Liquid Crystals, 415:1, 169-177, DOI:
10.1080/15421400490478803

To link to this article: http://dx.doi.org/10.1080/15421400490478803
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Mol. Cryst. Liq. Cryst., Vol. 415, pp. 169–177, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490478803
SELF-ASSEMBLED COLUMNAR MESOPHASE FROM
A NEW DISCLIKE POLAR MESOGEN BASED ON
A 3,5-DICYANO-2,4,6-TRISTYRYLPYRIDINE CORE
´
Andre-Jean Attias
´
`
Universite Pierre et Marie Curie-UMR 7610-Chimie des Polymeres,
4, Place Jussieu-Case Courrier 185, F 75252-Paris Cedex 05, France
and
´
´
`
ONERA-Departement des Materiaux et Systemes Composites,
29, avenue de la Division Leclerc, B. P. 72,
ˆ
F-92322-Chatillon Cedex, France
Chantal Cavalli
´
´
`
ONERA-Departement des Materiaux et Systemes Composites,
29, avenue de la Division Leclerc, B. P. 72,
ˆ
F-92322-Chatillon Cedex, France
Bertrand Donnio and Daniel Guillon
´
Institut de Physique et Chimie des Materiaux de Strasbourg,
Groupe des Materiaux Organiques, UMR 7504 (CNRS-ULP),
´
23, rue du Loess, F-67037-Strasbourg Cedex, France
Philippe Hapiot
Universite de Rennes 1-UMR 6510-Synthese et Electrosynthese
´
Organique, Campus de Beaulieu, F-35042-Rennes Cedex, France
`
`
ˆ
Jacques Malthete
Institut Curie, Section de recherche, UMR 168, 26, rue d’Ulm,
F-75248 Paris Cedex 05, France
We report on the design and synthesis of a new disc-like liquid crystal based on
a 3,5-dicyano-2,4,6-tristyrylpyridine core. The mesogenic behavior was inves-
tigated by differential scanning calorimetry, polarized-light optical
microscopy, and X-ray diffraction. The disc-like structure self-assembles into
´
Address correspondence to A.-J. Attias, Universite Pierre et Marie Curie, Laboratoire de
´
Chimie Macromoleculaire, 4, Place Jussieu, F 75252-Paris Cedex 05 France. E-mail: attias@
ccr.jussieu.fr
169

170
A.-J. Attias et al.
a hexagonal columnar mesophase and, due to the permanent dipole exhibited
by the isolated molecule, the stacking along the columnar axis is antiparallel.
Besides two-dimensional charge transfer and fluorescence, the disk-like
molecule exhibits a high electron affinity due to the strong acceptor character
of the 3,5-dicyanopyridine core.
Keywords: columnar mesophase; discotic liquid crystals; polar mesogen; 3,5-dicyano-2,4,6-
tristyrylpyridine
INTRODUCTION
Columnar mesophases of disk-like mesogens with an extended flat,
aromatic, central core have attracted increasing interest as potential one-
dimensional conducting systems [1]. Typically, the aromatic cores are
stacked one upon another to build columns which are arranged in varied
two-dimensional lattices. Very recently, it has been reported that deriva-
tives of hexabenzocoronene [2] exhibit extremely high charge carrier
mobilities (up to 1 cm2=vs) as a consequence of a long-range order in a
room-temperature hexagonal columnar mesophase. These high electronic
properties associated with long-range columnar order make discotic liquid
crystals promising as organic semi-conductors for applications in the
domain of molecular electronics and optoelectronics [2–5], more parti-
cularly in the area of photoconductivity [3], for photovoltaic [4], and
electro-luminescent devices [5].
In this context we were interested in designing and synthesizing a new
class of columnar disc-like liquid crystal molecule (i) with high electron
affinity to favor electrons injection and (ii) based on a core having a perma-
nent electric dipole perpendicular to the column axis to favor strong
dipolar interactions and consequently long-range intra-columnar order.
We report here on the synthesis and characterization of a new disk-
like molecule, the 3,5-dicyano-2,4,6-tris(3,4-didecyloxystyryl)pyridine (4)
(Scheme 1). 3,5-Dicyano-2,4,6-trimethyl-pyridine (2) has been chosen as
starting material, the 3,5-dicyanopyridine structure acting as an acceptor
core. Moreover, 2 has a permanent dipole because the two cyanide groups
are at the 3 and 5 positions of the pyridine ring. According to the synthetic
strategy described, 2 results, when reacting with aromatic aldehydes bear-
ing electron-donor groups (3), in a highly p-conjugated molecule, 4, which
possesses also a permanent dipole.
SYNTHESIS
The general aspects of the synthetic routes are shown in Scheme 1. (4) was
synthesized in 3 steps from collidine (0). Bromation of collidine with a

Self-assembled Columnar Mesophase from a New Disclike Polar Mesogen
171
SCHEME 1
slight excess of elemental bromine afforded the 2,4,6-tristyryl-3,5-dibromo-
pyridine (1), which was converted into the 3,5-dicyano derivative (2) in
the presence of copper cyanide. The overall yield for the conversion of 0
to 2 was ca. 65%. Toluenesulfonic acid-catalyzed Knoevenagel type
condensation of 2 with 3 equiv. of 3 afforded the resulting 30,40 -didecy-
loxy-2,4,6-tristyryl-3,5-dicyanopyridine, which was isolated in 42% yield
as a yellow-orange powder.
The structure and purity of all the compounds were confirmed by
using 1H NMR, 13C NMR, and elemental analyses. The 1H and 13C
NMR spectra of 1, 2, and 4 were consistent with the proposed structure.
As example, concerning the 1H NMR spectrum of 4, (i) there is a
complete disappearance of the methyl protons present in 2, (ii) two
doublets appear at 8.15 and 7.87 ppm with a three bond coupling
3
constant JH,H ¼ 15.55 and 16.12 Hz respectively, indicative of a trans
carbon-carbon double bond. These resonances are assigned to the two
kinds of trans vinylene units at the 2 (6) and 4 positions of the pyridine
ring respectively. In addition, the ratio of the integrated aromatic
resonances versus the aliphatic ones is equal to 15=44 as expected.

172
A.-J. Attias et al.
Moreover, the elemental analysis data of 1, 2, and 4 show good agree-
ment with the proposed structures.
RESULTS AND DISCUSSION
Mesomorphic Behavior
The thermal behavior of compound 4 was examined by DSC (10ꢀC=min)
and phase assignments were based on powder X-ray diffraction (XRD)
and polarized-light optical microscopy (POM). The transition tempera-
tures, phase sequence and some thermodynamic data are listed in
Table 1. Placed between glass and coverslide, 4 displays a beautiful fan-
shaped texture, characteristic of a hexagonal columnar mesophase. Tem-
perature- dependent X-ray diffraction (XRD) measurements were carried
out every 10ꢀC starting in the solid state at 30 up to 160ꢀC in the isotropic
liquid. A good agreement was found between the transition temperatures
derived from both POM and DSC techniques with those determined by
XRD. Figure 1 shows a representative X-ray diffractogram of the meso-
phase recorded at 100ꢀC. The pattern consists of (i) a broad diffuse scatter-
ing halo (II) in the wide angle region, corresponding to the liquid-like
˚
disorder of the aliphatic chains, at ca 4.5 A, (ii) another broad diffuse band
˚
(III) at ca 3.5 A, which is a characteristic distance usually found between
flat aromatic cores (and corresponding to the stacking period in a column
˚
[6]), (iii) a third less intense broad band at ca 7.1 A which may be an indi-
cation of some ‘‘dimerisation’’, resulting from an antiparallel stacking of the
molecules [7] (the double of III), and finally (iv) a single sharp and intense
˚
reflection (27.1 A) in the small angle region. Identical X-ray patterns were
obtained at all other temperatures of the mesomorphic range, with no
change of position of the fundamental reflection. The disc-like structure
of the molecules must lead preferably to the formation of columns, and
thus of columnar phases, by their long range order stacking rather than
to layered structures or nematic arrangements. Such stacking is also facili-
tated by the antiparallel arrangement of the molecular dipoles (estimated
TABLE 1 Thermodynamic Data and Phase Sequence of 4
Transition temperatures (ꢀC) – [Enthalpy Changes DH (kJ.molꢁ1)]a
1st Heating
1st Cooling
M1 60[36] X 70.5[ꢁ4] M2 74[19] ColH 133.5[2.2]I
I 133.5 [ꢁ1.5]ColH61 [ꢁ6.3]M3
aDetermined by DSC. M1, M2 and M3: Intermediate phases; ColH ¼ hexagonal columnar
mesophase; X: exothermic peak; I ¼ isotropic phase.

Self-assembled Columnar Mesophase from a New Disclike Polar Mesogen
173
FIGURE 1 X-ray diffraction patterns of 4 in the mesophase at 100ꢀC.
at 3.9 D by means of a calculation on the AM1-optimized geometry) as
sketched in Figure 2a and confirmed by the presence of the diffuse halos
I and III [7]. These facts suggest that the mesophase is columnar hexag-
onal (ColH). The structural parameters of the ColH mesophase (Fig. 2b),
i.e. The columnar repeat distance h and the first-order spacing d10, are
equal to 3.5 (estimated from the center position of the halo III) and
˚
27.1 A respectively. The molecular diameter of the hard dis-like core /Ar
˚
(without the aliphatic chains) has been estimated at 18.7 A by means of
a calculation on the AM1-optimized geometry.
Electrochemical Study
In view of the potential application of such molecules in optoelectronics, it
is important to consider the relative ionization potential (IP) and electron
affinity (EA), parameters which indicate the facility to inject holes and
electrons respectively in the material. These values can be estimated from
the first oxidation and first reduction potentials in solution which are asso-
ciated with the highest occupied and the lowest unoccupied molecular
orbitals (HOMO and LUMO energy levels respectively) [8]. For this
purpose, the electrochemical behaviors of 4 was investigated by cyclic
voltammetry (CV), to measure the redox potentials, and thus to estimate

174
A.-J. Attias et al.
FIGURE 2 Schematic representation of (a) the disc-shaped structure formed with
the dimer of 4. /Ar is the diameter of the dimer’s aromatic part. (b) the hexagonal
columnar lattice. Structural parameters h, s, and d10 are discussed in the text.

Self-assembled Columnar Mesophase from a New Disclike Polar Mesogen
175
IP and EA. In reduction, the studied compound presents a mono-electronic
transfer. The reduction process was reversible at scan rates higher than
10Vsꢁ1 , showing a relatively good chemical stability of the radical-anion
(life-time in the order of 1 s). From this reversible voltammogram, the
formal potential E0red was immediately derived as the half-sum between
the forward and the reverse scan peak potentials [9] (E0red ¼ 1.28 V vs
SCE). In oxidation, only one irreversible process has been observed, indi-
cating that the life-time of the produced radical cation is shorter than
1ms in our experimental conditions. It is noticeable that the radical-cation
has a much lower chemiꢀcal stability than the corresponding radical-anion.
In our determination, Eox value was approximated by the corresponding
peak potential (Eox ¼ ꢁ1.19 V vs SCE). To a first approximation, the
reduction potential can be related to the electron affinity and thus to
the LUMO energy level [8], according to the following equation:
IP ¼ (Eox þ 4.75). The estimated ionization potential ranges in the 6.0 eV.
Optical Properties in Solution
The photophysical properties of 4 in dichloromethane (i.e., absorption and
emission maxima, Stokes shift, and quantum yield of fluorescence) are
reported in Table 2. A single structureless and broad absorption peak is
observed at 400 nm (3.10 eV). It is attributed to a p ꢁ pꢂ transition due
to the large molar coefficient absorption (e > 80000 L molꢁ1 cmꢁ1) indica-
tive of a highly p-conjugated system. The photoluminescence spectrum of 4
results in a broad peak at 550 nm (2.26 eV) with a shoulder around 620 nm
(2.00 eV). Additionally, it should be noted that the Stokes shift is of the or-
der of 150 nm. Since the Stokes shift is usually dependent on the structural
relaxation of the excited molecule, such a large value (0.84 eV or
1580 cmꢁ1) results from a significant conformational change that occurs
TABLE 2 Optical and Electrochemical Data of 4 in Dichloromethane and in a
Toluene=Acetonitrile Mixture (50=50 v=v) Respectively
Optical properties
Electrochemical properties
Absorption
Emission
Potential (V)a
b
kmax (nm)
k
E
max (nm)
Stokes shift (nm)
150
Uf (%)
Oxidation Eox
Reduction E0red
Emax(eV)
max(eV)
400
3.1
550
2.26
40
ꢁ1.19
1.28
aVersus SCE.
bPeak potential.

176
A.-J. Attias et al.
upon excitation of 4. The excited state could be described as being
quinoid-like, thus resulting in a planar conformation, and leading to an
intra-molecular two-dimensional charge-transfer (ICT) state emission.
CONCLUSION
In conclusion, the design, synthesis, and mesophase characterization of a new
class of disc-like mesogen containing a permanent dipole have been reported.
Based on 3,5-dicyano-2,4,6-tristyrylpyridine core, the highly p-conjugated
derivatives align in an antiparallel arrangement and form disc-shape ‘‘dimeric’’
units which self assemble over a broad temperature range (80 to 135ꢀC) into
columns themselves packed together in a two-dimensional hexagonal lattice.
In addition, electrochemical as well photophysical properties of the individual
disc-like molecule have been investigated, in solution. Besides two-
dimensional charge transfer and photoluminescence, the electron accepting
character, evidenced by a high electron affinity, combined with a columnar
self-organization seems promising for electron transport and therefore opens
the way to applications in optoelectronics.
Because of the versatility of the synthetic pathway, the influence of
the strength of the dipolar permanent moment (by varying the nature of
the donor group) on mesogenic and electronic properties are currently
investigated.
EXPERIMENTAL PART
Full details of the synthesis and characterizations of all the compounds
have been reported elsewhere [10].
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