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E. S. R. study of some
4-methoxybenzilidene-4’-amino-n-alkyl cinnamates
D. Sy, M. Ptak
To cite this version:
D. Sy, M. Ptak. E. S. R. study of some 4-methoxybenzilidene-4’-amino-n-alkyl cinnamates. Journal de Physique, 1974, 35 (6), pp.517-522. �10.1051/jphys:01974003506051700�. �jpa-00208177�
E. S. R. STUDY OF SOME 4-METHOXYBENZILIDENE-4’-AMINO-
n-ALKYL CINNAMATES
D. SY (*) and M. PTAK (*)
Centre de Biophysique Moléculaire, C. N. R. S., 45045 Orléans Cedex, France (Reçu le 28 novembre 1973 révisé le 25 février 1974)
Résumé. 2014 On étudie le degré d’ordre dans des mésophases orientées nématiques et smectiques
par E. S. R. de sondes nitroxydes dissoutes dans des composés de la série 4-méthoxybenzilidéne-4’- amino-n-alkyl cinnamates. Les résultats expérimentaux, comparés aux prédictions théoriques de McMillan, mettent en évidence le rôle important joué par la chaîne terminale alkyl lors de transitions
smectique A ~ nématique.
Abstract. 2014 Nitroxide spin probes dissolved in compounds of the homologous series 4-methoxy- benzilidene-4’-amino-n-alkyl cinnamate are used for E. S. R. studies of the order parameter in oriented nematic and smectic mesophases. Comparison between experimental results and McMillan’s theoretical predictions for the smectic A ~ nematic transition allows one to show the important
role taken by the alkyl end-chain at such a transition.
Classification
Physics Abstracts
7.130
1. Introduction. - Orientational order and rota- tional molecular motion in nematic phases are com- monly investigated by Electron Spin Resonance (E. S. R.) of dissolved paramagnetic probes [1]. Nitro-
xide radicals are well suited to such studies in ani-
sotropic mesophases. In oriented smectic phases [2],
valuable results can be obtained in the same way and this paper presents an E. S. R. study of a homo- logous series of Schiff’s bases with variable molecular
alkyl end chain length.
2. Polymesomorphism. - The structure of the 4- methoxybenzilidene-4’-amino-n-alkyl cinnamates stu-
died is given below :
where n = 2 (MBACE), n = 3 (MBACP), n = 4 (MBACB), n = 6 (MBACH).
These compounds were kindly supplied by J. Jac-
ques (Laboratoire de Chimie Organique des Hor-
mones, Collège de France, Paris) and used without any further purification. Transition temperatures (Table I), measured by optical heated-stage micros-
copy, are in agreement with the earlier results of Leclercq et al. [3] and of Gray et al. [4].
TABLE 1
Transition temperatures. ( ) data of Gray [4]
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01974003506051700
518
Judging by the simple fan texture and by miscibility
studies [5], the first smectic phase obtained on cooling the nematic is identified as a smectic A. Decreasing
the temperature, a second smectic phase, SmX, is observed. This monotropic mesophase shows uni-
axiality and a « fan shaped texture with diminished
number of lines of discontinuity » [5]. For MBACP,
the layer thickness in SmX, from X-ray diffraction
experiments, is the same as in SmA : d = 23.9 ± 0.1 A,
1 A less than the length of the extended molecule measured on a C. P. K. model. By another methôd
Chistyakov has found a hexagonal symmetry in the smectic plane of the monotropic mesophase of
MBACE [6]. In an extensive study of several series of substituted 4-p-benzilidene-amino cinnamates, Gray identified smectics A and smectics B among the three possible distinct smectic forms [7]. Thus
there is some evidence that, in the series studied here, the phase X is a smectic B. However, more detailed investigations are required for definitive
confirmation.
The general scheme of polymesomorphism for
the compounds studied is :
C : crystal, SmA and SmX : smectic phases, N :
nematic phase, 1 : isotropic liquid.
It must be pointed out that the solid phase shows polymorphism, of which account must be taken
when studying Sm C ---> SmA transition. The different transition temperatures, mainly the N H 1 one, show
alternating changes with the number n of carbon -atoms in the terminal alkyl chain.
3. E. S. R. study. - The E. S. R. probe is a nitro-
xide free radical : { 3-spiro-[2’N-oxyl-3’, 3 dimethy- loxazolidine]} 5-a-androstan-17-p-ol [8]. Its hyperfine
tensor A can be considered as approximately cylin-
drical around a z axis lying along the direction of
the Pz orbitals of the nitrogen atom and perpendicular
to the long axis of the molecule (Fig. 1). The g ani- sotropy is ~ 7 x 10-3 and can be neglected in first approximation. This rigid, rod-shaped molecule is oriented by the anisotropically ordered molecules of the solvent and reftects local molecular orientation and mobility. It is easily dissolved at concentrations
~ 10-’ M. The transition temperatures are slightly
FIG. 1. - The androstan E. S. R. probe [1-c].
lowered, but it can be accepted that the intrinsic
properties of the mesophases are preserved.
Samples, 1/10 mm thick, are prepared between glass plates cleaned successively with sulfonitric mix- ture, water and alcohol.
3. 1 WHEN INCREASING TEMPERATURE, THE GENERAL FEATURES OF E. S. R. SPECTRA ARE THE FOLLOWING. -
The nitroxide probe is excluded from the solid at
room temperature : a broad spectrum, without hyper-
fine lines, typical of exchange interaction in aggregates of paramagnetic species, is observed.
- A few degrees below the melting point, a clear hyperfine structure appears and the spectrum takes the typical features of radicals in fast isotropic motion.
In this pretransitional range of temperatures, the probe seems to diffuse easily in the surrounding
medium and can rotate randomly without strong contraints.
- The probe is well dissolved in the smectic
phase. The spectrum, in this non oriented mesophase,
is complex with reinforced outer lines, as in a powder spectrum but with non random distribution of the director.
- In the nematic phase, the probe exhibits a
fast anisotropic rotational motion. In the isotropic liquid phase, this motion becomes isotropic.
3.2 ORIENTATION OF THE SMECTIC PHASES. - We define :
u : director of the oriented mesophase,
n : unit vector normal to glass plates,
H : magnetic field
(P = (u, H) and (u, n) .
The sample being able to rotate about a vertical axis,
n lies in a horizontal plane as the magnetic field H
has fixed orientation.
According to previous data [1, 2, 8], it can be assum-
ed that, in nematic and smectic A phases, an androstan probe is oriented with its long axis parallel to the
director of the mesophases. In the working E. S. R.
magnetic field (~ 3.3 kG), the director u of the
nematic mesophase is aligned with H «p = 0°). The
E. S. R. line shape demonstrates that the probe undergoes a fast anisotropic rotational motion
(te 8 x 10-8 s). An oriented smectic A mesophase
is obtained by decreasing the temperature in a higher orienting magnetic field (~ 10 kG), at low speed (10 °C/hour), with y5 = 90° ; some cycles through
the N H SmA transition then allow us to get a nearly perfect smectic monodomain, as judged by optical examination and E. S. R. pattern, in which the smectic planes are vertical and perpendicular
to the glass plates. The spectrum in this phase, recorded
with a 3.3 kG magnetic field, is shown together
with the isotropic and nematic spectra in figure 2.
Even by varying the orienting field, the initial value of p and the sample thickness, it is impossible
u
FIG. 2. - E. S. R. spectra of androstan dissolved in MBACP : (a) isotropic liquid, (b) nematic, (c) oriented smectic A (qJ = 00).
to get a SmA monodomain with smectic planes parallel to the cleaned glass plates. This fact is due
to very strong surface effects and a high cooperativity
of molecular interactions in the smectic phase of
the studied compound. Indeed, perfectly homeotropic
SmA samples were obtained with plates covered
with a solution of hexadecyltrimethylammonium bro- mide ; the initial value of gi during the SmA - N
transition was then fixed at y5 = 0°.
At a given temperature, the angular dependence
of the hyperfine splitting ( a > is [2]
A 11, ÃJ. and 911’ 0.1 are the components of the hyperfine and g tensors averaged by fast molecular rotation.
When g anisotropy is neglected
When the director u lies along H (ç = 00), ’a > = Â Il :
the splitting reaches its minimum value. (If afl radicals
are perfectly oriented with their long axis parallel
to the director then A il = A1.) The maximum value
of a > occurs for (p = 90°, a ) = Ã1.. When
rotating the sample from 0° to 1800 in the 3.3 kG
field, there is no reorientation of the molecules. The excellent agreement between theoretical and expe- rimental values of a ), as shown on figure 3, gives
evidence of cylindrical symmetry. The director u
FIG. 3. - Angular dependence of hyperfine splitting for MBACP
smectic A mesophase : o experiment ; - theoretical curve.
FIG. 4. - Orientation of the smectic A mesophase of MBACP.
keeps the same orientation for any temperature in the smectic A range and a > remains minimum for t/J = 900.
In E. S. R. spectra of SmX mesophase, two addi-
tional quasi symmetrical lines of comparable width
appear. Rotation of the sample from 0 = 0° to 0 = 900
shows a corresponding splitting variation from à l.
to a minimum value greater than A Il at a given tem- perature, according to eq. (3.2). Samples observed
under a microscope are homogeneous and oriented
520
FIG. 5. - E. S. R. spectra of androstan dissolved in MBACB : Smectic A H Smectic X transition (ç = 0°).
as in the smectic A mesophase. The relative intensity
of these additional lines does not depend on sample preparation. Surface effects and magnetic field effects up to 10 kG have no effect on them. This suggests that this new probe orientation does not reflect a new averaged orientation of the solvent molecules.
On the other hand, a slow reorientation of probe
molecules is unlikely : the compactness of the liquid crystal matrix does not allow a wobbling between
two extrema, parallel and perpendicular positions,
which otherwise should be observed on E. S. R.
spectra in SmA at higher températures ; moreover only rapid motions can account for narrow linewidth
at any angle. We propose the following assumption :
due to higher packing in SmX than in SmA, a part
ce
of the probe molecules is excluded from the smectic
layer rigid region constituted by the assembly of
aromatic rings (It was previously pointed out that,
in crystals, the probe molecules are wholly excluded
and form aggregates). These excluded androstan molecules migrate to the smectic layer aliphatic region. Their solubility here is higher with increasing
chain length : the corresponding percentage of spins depends the liquid crystal solvent, increasing with
the number of carbon atoms of the alkyl end chain (0 % for n = 2, - 20 % for n = 3, - 40 % for n = 4, predominant for n = 6). Their orientation in this
new type of site is roughly parallel to the smectic layer, as shown by the angular dependence of the
E. S. R. splitting, assuming a low degree of order in
such an environment. The local smectic organization is
somewhat disturbed by that probe molecule inclusion.
No relation between androstan orientation in the
layer and the hexagonal symmetry [6] of the liquid crystal molecular positions was found ; such a relation
would require isomorphism between probe molecule length and smectic lattice unit cell parameters. For the cholestan probe with a terminal aliphatic chain
instead of an OH group, E. S. R. spectra exhibit only three lines in smectic X, as in smectic A; the relative solubility of cholestan in the smectic X rigid region probably remains sufficient for the molecules not to be excluded.
Such probe effects have not been previously des-
cribed and are still under investigation, using different probe molecules, as a possible test of molecular
interactions inside smectic layers.
p _
FIG. 6. - Temperature dependence of the degree of orientational order for the members of the series studied.
3.3 DEGREE OF ORDER. - In nematic and smectic
mesophases the averaged orientation of the rapidly rotating probe, and, in a wider sense, of the solvent
molecules, is defined by the degree of order
From the spatially averaged static Hamiltonian,
the measured hyperfine splitting a> = A Il (qJ = 0°)
can be deduced, in a first order theory [9] :
where a = 1/3(A ll + 2 A 1.) is the isotropic hyperfine
splitting, b = A /1 - A 1. = 2(A Il
- a) is the hyperfine
tensor anisotropy, 0 is the angle between the z axis
of cylindrical symmetry of the hyperfine tensor and
the director, g anisotropy is neglected and the z axes
are assumed to be uniformly distributed around the director.
Thus
For androstan, maximum order 8 = - 0.5 occurs
when molecules are fixed with 0 = n/2. In general,
- 0.5 8 0. Figure 6 shows temperature depen-
dence of the degree of order for the four compounds
studied.
In nematic phases, the values of 8 lie between
- 0.1 and - 0.2 for any alkyl chain length. Expe-
rimental temperature gradients prevent accurate deter- mination of ASNI at the transition N H I. However, ASNI does not seem to change markedly with
n (Table II). The transition entropy ASN,, from diffe- rential scanning calorimetry measurements [3],
increases with n. Such a trend is in better agreement with Marcelja’s (S. Marcelja, Solid State Commun.
13 (1973) 759) theoretical predictions than with those
of Saupe and McMillan [10] : with a rigid molecule
model ASNI remains constant. The E. S. R. androstan TABLE Il
Transition entropies and orientational order discontinuities
(*) (cal/mole OK).
(**) From Leclercq data [3].
probe is unable to detect any small possible variations
of 8 in nematic phases due to changes in end alkyl
chain length.
In SmA mesophase, 8 greatly depends on n value
and parity : 8 ri - 0.45 for n = 2, 8 = - 0.25 for
n = 6. The discontinuity ASSN at the SmA --> N tran- sition decreases as n increases. The same general
trend is found for the transition entropy ASSN (Table II). Hasty generalization should not be made
from such a restricted set of observations, but our data suggest that ASSN does not obey the law pro-
posed by McMillan [10] with a rigid molecules basic model, namely, ASSN increasing with alkyl end-chain length. Moreover, it seems that the orientational order 8 is preponderant in the entropy (Fig. 7).
Anyway, a paramagnetic probe is insensitive to
translational order and other types of investigation
are required to specify the dependence of entropy S on the order in a smectic phase.
In SmX phases, 8 approaches its maximum value - 0.5. The degree of orientational order alone is unable to describe smectic-smectic transitions : the experimental curves - 8 = f (T) present only
slope variations (Fig. 6) for such transitions which
are however first order as clearly seen by differential
scanning calorimetry thermograms.
FIG. 7. - Smectic A - Nematic transition. Transitipn entropy AS (in units of R N 1.99 cal/mole OK) versus discontinuity of
orientational order parameter A(S for different alkyl chain lengths.
522
4. Conclusion. - 4.1 Measurements of the orien- tational order by means of dissolved paramagnetic probes are of value in detecting polymesomorphic
transitions in liquid crystals. The orientation of smectic samples between glass plates in a magnetic field is easily studied by this method. The degree of orien-
tational order is shown to be temperature dependent
in a smectic A mesophase and nearly constant and
maximum in a low temperature smectic. In this later
phase, an androstan probe can be oriented in two
different ways by the solvent molecules. Synthesis
of probes with a better chemical similarity with ther- motropic liquid crystals and refinement of relaxation process models could improve understanding of the
local molecular mobility. Nevertheless, comparison
with other methods such as optical microscopy,
N. M. R., and dielectric relaxation would be necessary to reach a more quantitative interpretation.
4.2 We have confirmed the existence of two smectic forms in a series of 4-methoxy-benzilidene-4’-amino- n-alkylcinnamates and identified the higher tempe-
rature form as smectic A. At the SmA - N tran-
sition, entropy and orientational order show parallel discontinuities, when the alkyl end chain length changes. These end chains probably take part in the ordering process and define a region in which the
solubility can be different from the one in other parts of the smectic layer.
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46 (1967) 55.
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c) LUCKHURST, G. R. and SANSON, A., Mol. Phys. 24 (1972)
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[2] LUCKHURST, G. R. and SANSON, A., Mol. Cryst. and Liq.
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[4] GRAY, G. W. and HARRISON, K. J., Mol. Cryst. and Liq.
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[7] GRAY, G. W. and HARRISON, K. J., Faraday Symposium n° 5 (1971).
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