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A partially bilayered smectic B (Bd) phase of a thermochromic mesogen
C.G. Bazuin, D. Guillon, A. Skoulios, R. Zana
To cite this version:
C.G. Bazuin, D. Guillon, A. Skoulios, R. Zana. A partially bilayered smectic B (Bd) phase of a thermochromic mesogen. Journal de Physique, 1986, 47 (6), pp.927-930.
�10.1051/jphys:01986004706092700�. �jpa-00210287�
A PARTIALLY BILAYERED SMECTIC B (Bd) PHASE OF A THERMOCHROMIC MESOGEN C.G. BAZUIN, D. GUILLON, A. SKOULIOS and R. ZANA
Institut Charles Sadron (CRM-EAHP), ULP-CNRS, 6, rue Boussingault, 67083 Strasbourg Cedex, France
(Regu Ze 14 janvier 1986, révisé Ze 17 mars 1986, accepté Ze 20 mars 1986)
Résumé.- On a étudié l’iodure de N-hexa-
décyl-p-cyanopyridinium par diffraction des rayons X, enthalpimétrie différentielle et
microscopie en lumière polarisée. Les premières observations montrent que le matériau est thermochrome: il passe du
jaune au rouge à 104°C, température d’ap- parition d’une phase liquide-cristalline.
Cette dernière est de type smectique B,
avec une épaisseur de couches comprise
entre une et deux longueurs moléculaires.
Dans le modèle de structure proposé les molécules, orientées de manière antiparal- lèle, sont disposées aux noeuds d’un réseau bidimensionnel hexagonal en nids d’abeille.
Le centre des nids est occupé par des
paires de molécules associées par leur extrémité cyanée. Les ions iode se placent, deux à deux, dans le prolongement des molé-
cules non appariées, au-delh des groupes cyano. Cependant, les données expérimen-
tales ne permettent pas d’exclure un modèle dans lequel molécules et paires de molé-
cules seraient reparties aléatoirement aux noeuds d’un réseau hexagonal simple.
Abstract.- N-hexadecyl-p-cyanopyridinium
iodide was studied using X-ray scattering,
differential scanning calorimetry and pol-
arized light microscopy. Preliminary re-
sults reveal that the material is thermo-
chromic, turning from yellow to red upon
entering a liquid crystal phase at 104°C.
The mesophase is of the smectic B type,
where the lamellar spacing is between one and two molecular lengths. A structural
model is proposed that can be described as
a honeycomb arrangement of unpaired mol-
ecules with their dipolar moments alter- nating around the cells; a paired molecule
lies in the center of each cell and two iodide ions are located axially at the
cyano end of each unpaired molecule. It is noted, however, that the experimental data
do not exclude the possibility of a model
in which the paired and unpaired molecules
are arranged randomly in the more usual hexagonal network.
Classification
Physics Abstracts
61.30E - 78.20N
Among the mesogenic compounds
studied to date, those incorporating termi-
nal cyano groups are known to give unusual thermotropic liquid crystals (1,2). They
have been shown to give rise, for example,
to re-entrant smectic-nematic phenomena, to partial bilayering of smectic A mesophases,
and to in-plane modulation of smectic A and smectic C layers. The origin of these peculiar properties lies in the strong dipolar interactions between cyano groups.
Cyano-containing amphiphiles have
not yet been investigated. It may be expected that the presence of the ionic
species, in addition to the strong cyano
dipoles, could contribute to particularly interesting liquid crystalline behaviour.
Accordingly, four such amphiphiles, all
based on N-alkyl-cyanopyridinium iodide,
are currently being examined in this labor- atory. Three of them are hexadecyl com- pounds with the cyano group in the para, meta and ortho positions, respectively,
relative to the alkyl chain; the fourth is
a para-substituted dodecyl compound. Such compounds are used as photometric probes in T-jump studies (3) and as electrochemical
probes in polarographic studies (4) of
micellar solutions.
In this communication, attention
is focussed on the N-hexadecyl-p-cyano- pyridinium iodide (16-p-CPI):
It may be compared to a similar compound, recently reported on (5), where the cyano group is replaced by a methyl group. The latter has been shown to give rise to a
smectic A mesophase with the aromatic groups interdigitated in such a way that the halide ions (either iodide or bromide)
are sandwiched by pyridinium rings.
This lette.r reports preliminary
results concerning the thermotropic behav-
iour of 16-p-CPI obtained from X-ray dif- fraction, differential scanning calorimetry
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01986004706092700
928
(DSC), and polarized microscopy. Moreover, it proposes a structural model of the meso-
phase observed that accounts for the exper- imental details. Because the product
appears to undergo decomposition at high temperatures, the preliminary investiga-
tions have been confined to temperatures below 150°C.
Observations on a heat bench showed that 16-p-CPI changes colour from its initial yellow to red with heating; it
remains red on cooling (seemingly indefi- nitely), but returns to yellow when reheat-
ed to a relatively low temperature. More detailed observations using a Mettler FP84
TA Microscopy Cell combined with DSC data revealed that the change from yellow to red
occurs at a transition which peaks at 104°C
and that the change from red to yellow in a
second heating run occurs at about 70°C, at the location of an apparent crystal-crystal
transition. Parallel observations with a polarizing microscope showed a texture change at 104°C that indicates the forma- tion of a liquid crystalline phase.
X-ray diffraction patterns of the phases below the 104°C transition indicate
crystalline structures. The patterns recorded above 104°C are typical of an
ordered smectic mesophase. At low angles, five sharp reflections with reciprocal Bragg spacings in the ratio 1:2:3:4:5 were
registered at 130*C; they indicate a lamel- lar structure whose period is 41.7 A (Fig.
1).
Fig. 1 X-ray pattern of the smectic B
phase of 16-p-CPI at 130°C.
The intensity of the reflections is strongly dependent on their order, qual- itatively varying as follows with increas-
ing order: very strong, extremely weak,
very weak, medium, weak. It is thus ob- vious that the intensity exhibits a secon- dary maximum at a Bragg spacing of about 10 A. This indicates that there is additional diffraction from within the layers, the
most reasonable source being the
heavy
iodide ions located in planes that are 10 Ä apart.
At large angles, one narrow re-
flection is found, corresponding to a Bragg spacing of 4.3 A. This is commonly ob-
served for smectic B mesophases and is interpreted to denote a two-dimensional hexagonal packing of molecules, 5.0
apart, within the layers (6). In addition to the narrow reflection, there is a dif-
fuse band at about the same spacing, indic-
ative of the disordered state of the aliph-
atic chains.
To describe the structure of the
mesophase, the model represented in Fig. 2
is tentatively proposed. The molecules in
a layer are arranged in the hexagonal net- work characteristic of a smectic B struc- ture, but in a manner (Fig. 2a) based on a particular arrangement of paired and un- paired molecules. Fig. 2b illustrates this arrangement for a line of molecules in the plane of a layer (for example, those along
the dotted line of Fig. 2a). Two mol- ecules, paired through the cyano groups, alternate in a regular fashion with two
unpaired molecules whose dipoles point in opposite directions. Two iodide ions,
sandwiched by pyridine rings, are located along the axis of each of the unpaired mol-
ecules. As accented in Fig. 2a, the resul- ting network within a plane can be des-
cribed as a honeycomb structure of unpaired
molecules having alternating dipolar direc- tions, with the dimerized molecules located one-to-one in the centers of the honeycomb cells.
It remains to justify the model.
First of all, it is to be noted that the experimentally determined layer spacing, d,
has a value intermediate to one and two calculated molecular lengths, L. From CPK Molecular Models, L - 32 A when an iodide ion is included along the molecular axis
(Fig. 2c). A comparison of d with L pre- dicts a degree of association, r, of about 0.47, using the equation established ear-
lier for bilayer smectic A systems (2,7):
This result evidently correlates well with the requirement of the model that half of the molecules be paired.
Second, as is clear from Fig. 2c,
the location of the iodide ions accounts for the 10 A intralayer Bragg spacing, the X-ray diffraction pattern recording the
average position of the iodide pairs. In
this connection, it is worthv of note that
Fig. 2 Structural model of the smectic B
phase of 16-p-CPI:
a. view of the plane perpendicular
to the molecular long axis; P, paired molecule; U, unpaired molecule; U circled, unpaired
molecule with dipole moment pointing in opposite direction
from U uncircled.
Fig. 2 b. view of the plane parallel to
the molecular long axis (e.g. the plane indicated by the broken line in a).
the diameter of the iodide ions (4.4 A) is
comparable to that of the aromatic groups, thus allowing for a practically complete filling of space in the molecular packing proposed.
Third, it has already been men- tioned that the 4.3 A reflection at large
Fig. 2 c. specification of molecular parameters in the model.
angles is typical of a smectic B hexagonal
structure. The proposition that this
structure may be described by filled honey- comb cells raises a question. The larger repeat pattern of the honeycomb structure compared to that of the classic hexagonal pattern associated with smectic B phases
should give rise to an additional reflec- tion at intermediate angles, specifically
the (1,0) reflection of the honeycomb
structure located at a spacing of 1//5 of
the location of the (1,1) reflection at 4.3
A. However, an examination of the electron densities of the two molecular species
involved (paired and unpaired) and the cor- responding intensities that can be calcula- ted* show that the intensity of the (1,0)
reflection relative to that of the (1,1)
reflection is far too weak to be observed in normal X-ray scattering experiments. It is noted, at this point, that the experi-
mental data obtained do not exclude the
possibility that the paired and unpaired molecules are arranged randomly on the
classic hexagonal lattice. The existence of
the (1,0) reflection is crucial to disting- uishing between the two possibilities. It
is therefore hoped that neutron diffractior
experiments which are in the planning will
*It is sufficient to compare intensities with respect to the lamellar planes com- posed of non-aliphatic material only. If A represents the number of electrons of the paired species in this plane and B that of
the unpaired species (including the axial
iodide ions), the intensities of the
(1,1
and
(1,0)2
reflections should be (A-43B)and (A-B) , respectively. In the non- aliphatic portion of the unpaired species,
there are 160 electrons per molecule. In order that the same molecular length (15.7
1, see Fig. 2c) be used in the calculations for the paired species, about five carbon atoms of the aliphatic chain must be in-
cluded : the result is then 148 electrons per paired species. It is thus clear that
the intensity of the (1,0) reflection must be 115 times less than that of the.(1,1)
reflection.
930
provide the necessary information (calcula- tions indicate that the (1,0) reflection
should be eight times more intense than the (1,1) reflection, if the proposed model is correct).
Fourth, the location of the iodide ions allows for charge transfer effects
that probably account for the colour change
observed on entering the mesophase. That the cyano group must also play a signifi-
cant role is emphasized by the fact that thermochromic behaviour was not observed in the similar methyl-terminated alkyl pyridi-
nium halides of ref. 5. It is postulated
that in the unpaired molecules some of the negative charge from the large electron- donating axial iodide ions is transferred to the positively-charged pyridinium ring through the strongly electronegative cyano
groups. This must then be accompanied by a certain amount of delocalisation of -the electrons over the pyridinium ring, cyano group and iodide ions. However, it should be noted that even considerable electron delocalisation is not inconsistent with
high dipole moments in the unpaired mol-
ecules due to the large distance between
the residual positive and negative charges.
Finally, attention is drawn to the electrostatic interactions in the proposed
model. As was similarly observed for the
materials of ref. 5, the iodide ions are
stabilized laterally by the neighbouring pyridinium rings. The major part of this stability is most likely provided by the rings of the paired molecules, since their
positive charge is undiminished by axial charge-transfer effects. A closer inspec-
tion of Fig. 2a shows that each unpaired molecule, and hence the iodide pair, is, in fact, surrounded by three of the paired
molecules. A smaller amount of lateral stabilization of the iodide ions is un-
doubtedly provided by the pyridinium rings
of the three neighbouring unpaired mol- ecules. On the other hand, each positively charged pyridinium ring in the paired mol-
ecules is additionally sandwiched by three unpaired pyridinium rings, also at least partly positively charged. The normally destabilizing effect of such close prox-
imity of like charges is, however, dimin- ished by the axial charge-transfer effects
in the unpaired molecules as described
above. A final aspect of the electrostatic interactions concerns the arrangement of the unpaired molecules. Fig. 2a clearly
shows that their dipole moments alternate
in anti-parallel fashion around the honey- comb cells. This type of arrangement is also known to be that favoured by antiferro- magnetic and -electric materials in two- dimensional hexagonal crystal form, in order to most effectively reduce repulsive interactions. This lends still further support to the validity of the proposed
model. In that connection, the 16-p-CPI should display magnetic properties as well, since the axial charge transfer must in-
volve the displacement of lone electrons
and hence (according to the model) there
must be an arrangement of spins in anti- parallel fashion.
In conclusion, this communication presents preliminary structural data for the first example of a partially bilayered
smectic B (or Bd) phase, as well as the
first thermochromic single-component meso- gen, ever reported to our knowledge. A honeycomb model involving partial associa-
tion of the molecules is proposed for the phase structure; such a model is partic- ularly attractive from the point of view of
the physics of two-dimensional systems.
At first sight, it may be sur-
prising that the model involves the place-
ment of two negative iodide ions in pairs,
since this would normally result in strong repulsion. But, as has been argued above, this unfavourable situation is attenuated by the axial charge-transfer effects and by
the favourable lateral electrostatic inter- actions. These effects remain to be proven
experimentally, however. In particular, it
must be shown that the cyanopyridinium species is actually electrochemically re- duced by an iodide ion, as is suggested by the axial charge transfer postulated.
Currently, a more complete study
of the 16-p-CPI, as well as its meta and ortho homologues, is in progress in this laboratory. As mentioned, neutron diffrac- tion experiments are planned. Finally, the correspondence between the thermochromic behaviour observed and the appearance of the liquid crystalline phase is also being
determined more precisely.
Acknowledgement: One of us (CGB) wishes to thank FCAR (Quebec) and NSERC (Canada) for financial support during this work.
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