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DEUTERON MAGNETIC RESONANCE STUDY OF MOLECULAR ORDER IN TBBA
B. Deloche, J. Charvolin, L. Liébert, L. Strzelecki
To cite this version:
B. Deloche, J. Charvolin, L. Liébert, L. Strzelecki. DEUTERON MAGNETIC RESONANCE STUDY OF MOLECULAR ORDER IN TBBA. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-21-C1- 26. �10.1051/jphyscol:1975103�. �jpa-00215880�
JOURNAL DE PHYSIQUE Colloque C1, suppliment au no 3, Tome 36, Mars 1975, page C1-21
Classification Physics Abstracts
7.130
DEUTERON MAGNETIC RESONANCE STUDY OF MOLECULAR ORDER IN TBBA
(*)B. DELOCHE, J. CHARVOLIN, L. LIEBERT and L. STRZELECKI Laboratoire de Physique des Solides (**), UniversitC Paris-Sud, Centre d'Orsay,
91405 Orsay, France
RBsumB. - Des expMences de resonance magnetique nucleaire sur les deuterons de mol6cules stlectivement deutkrks (TBd-BA) permettent de comparer les orientations relatives des diffkrentes parties de la molCcule dans plusieurs mesophases. Les spectres des groupes methyl, mkthylkne et aromatique sont parfaitement r6solus. L'ordre orientationnel des segments des chaines diffkre de celui du groupement aromatique. Les chaines ne sont pas dans un 6tat rigide ; cependant l'ordre observe change peu quand la temperature est abaisste de la phase nematique aux phases smectiques.
Ces points sont confirm& par des etudes analogues sur un systkme B chaine plus longue (BOBd-BA).
Des mesures, dans lesquelles on fait varier I'orientation de l'Cchantillon par rapport au champ magnktique, montrent aussi que la rotation de la moltcule autour de son axe long devient trks lente dans une phase smectique metastable B basse temperature.
Abstract. - Deuteron magnetic resonance experiments on selectively deuterated molecules (TBd-BA) yield the relative orientational orders of the different groups within a molecule in a meso- phase. The DMR spectra of methyl, methylene, and aromatic groups are perfectly resolved. The orientational order of the chain segments differs from that of the aromatic core. The chains are not in a rigid state ; however, the observed chain order does not change much by decreasing the tem- perature from the nematic to the smectic phases. Similar studies on a system with longer chains (BOBd-BA) confirm this point. Experiments where the orientation of the sample in the magnetic field is varied also show that the rotation of the molecule around its long axis is considerably reduced in a metastable low temperature smectic phase.
1 . Introduction. - Most of our understanding of macroscopic properties of thermotropic liquid crystals has been obtained assuming their molecules t o be rigid cylindrical rods. Such a simple model is now appearing to be inadequate for a detailed analysis of microscopic interactions. On the one hand, it ignores the dual character of the structure of mesogen molecules, alkyl or alkoxy end chains attached on an aromatic core ; on the other hand, the deviations of the core interaction potentials from cylindrical symmetry are totally discarded.
If the assumption of rigidity may be valid in most cases for the aromatic core, it is certainly too drastic for the alkyl or alkoxy end chains. This point can be inferred from the experience gained in the study of paraffinic chain flexibility in lyotropic liquid crystals [I]
and is being strengthened by recent works in thermo- tropic systems. Thus, changes of intramolecular confor-
(*) This work has been supported by D. R. M. E., contract no 721579.
(**) Laboratoire associk au C. N. R. S.
mations are suggested by calorimetric data [2] and EPR experiments [3], the occurrence of smectic phases is shown to depend upon chain length [4, 51 and a recent theoretical treatment [6], introducing chain isomerizations, accounts for thermodynamic properties at the clearing point.
The assumption of cylindrical symmetry also has to be thought of again following recent studies of biaxial smectic structures. In a recent model [7], macroscopic biaxiality is explained from microscopic interactions between electric dipole moments of the aromatic cores ; X-ray patterns of metastable smectic phases of TBBA show evidence of orientational order in the plane of the smectic layer [8].
As far as the chain behavior is concerned, the quoted experiments are not selective enough : results from calorimetric measurements are data integrated over the whole specimen, while EPR experiments give only direct information about a probe and not about the chain itself. However, as demonstrated with lyotropic systems, NMR of deuterated chains (or DMR) appears as a good way to get specific and accurate information
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975103
C1-22 B. DELOCHE, J. CHARVOLIN, L. LIEBERT AND L. STRZELECKI
about the chain conformations [I, 91. We have thus developed this technique to investigate, in the TBBA liquid crystal, the two aspects of the molecular behavior discussed above. This molecule has short alkyl chains (C,), thus a system with a longer end chain (C,) has also been studied in order to compare the evolution of the methylene order all along the chain with existing theory [6]. Lastly, DMR also indicates the freezing of the rotation of the molecule around its long axis in
one of the low temperature metastable smectic phases of TBBA [8].
2. Experimental. - 2 . l SAMPLES. - Two Schiff's base derivatives were synthesized from deuterated alkylaniline. The molecules and their respective phases are the following :
(i) Terephtalidene-di(p-d,-butyl-2.6-d,-aniline) or TBd-BA :
(The low temperatures monotropic phases have been classified according to ref. [8].) (ii) p-Buty1oxybenzilidene-p-d1,-octy1-2.6-d,-aniline, or BOBd-OA :
D
37"C_ 5 1 65 79
Cr
-
Sm, Sm, Nem IsoThe deuterations, obtained by catalytic exchange, were estimated to be better than 93
%
from high resolution NMR ; in both cases the nitrogen electronegativity probably does not favour the deuterations of the ortho sites relative to the alkyl radicals. The phase transitions of the above deuterated compounds were verified with DTA recordings. Then the samples were outgassed and sealed under vacuum in glass ampoules. In a magnetic field of several kilogauss, the molecules, in the nematic phase, orient themselves spontaneously with their long axis parallel to the field ; this orientation is maintained while cooling down the sample into the different smectic phases. Once aligned, the smectic planes conserve their original orientation on rotation of the magnetic field.2 . 2 NMR. - The 13 MHz (or 20 kG) spectra were obtained using a homemade NMR pulsed spectro- meter of classical type. The whole free precession is integrated while the magnetic field is swept through resonance ; the recorded signal is thus the Fourier transform of the free precession, i. e. the absorption spectrum if the phasing of the detection is-appropriate.
The temperature of the sample, controlled by heated air flow, is regulated to 0.2 OC and thermal gradients in the sample are of the same order.
2.3 RESULTS.
-
DMR spectra of the twocompounds in their nematic phase are shown in figure 1. Both spectra are composed of well resolved doublet structures. Each of them corresponds to deuterons in a determined electric field gradient ( I ) (or
(1) In presence of quadrupolar coupling, the D M R line is split into a symmetrical doublet structure. With an axial electric field gradient (e. f. g.) the splitting is given, in magnetic field units, by :
where e2 q Q / p ~ is the static quadrupolar coupling constant.
For example, its value is 276.5 G for the C-D bonds of per- deuterated benzene [lo] and 255.5 G for the C-D bonds of per- deuterated hexane [I 11.0 is the angle between the e. f. g. axis and the magnetic field. Motions of this e. f. g. axis modulate 0 and a time average has to be taken if the motion frequencies are larger than the characteristic frequency of the static interaction ; in particular, if the motions are anisotropic and admit an axis of symmetry (1) becomes :
where 0' and 9 are the angles made by the symmetry axis with the e. f. g. axis and the magnetic field respectively. An effective quadrupolar coupling constant is then measured which is :
where
(
c 0 S 2 ~ ' - I ) mav be defined as the order parameter of the axis of thee. f, g. relative to the axis of symmetry.DMR STUDY OF MOLECULAR ORDER IN TBBA C1-23
1 T B d - B A
Nematic phase T = 210°C
BOBd-OA
Nematic phase T= 74OC 1
I I
6 5 10 15 20 25 30 35 40 45 50 55 Magnetic field (gauss)from 20 kgauss
FIG. 1. - TWO 13 MHz DMR spectra obtained with two deuterated liquid crystals in their nematic phases. Only half the spectra, which are symmetrical around zero magnetic field, are represented. From their relative splittings and intensities, the so-called 1 line and 4 or 8 lines, were identified respectively with the first methylene and last methyl groups. The identification of
the other groups is discussed in the text.
e. f. g.) except for the so-called phenyl groups which appear somewhat more complex. The study of a TBBA molecule, the butylaniline of which was synthesized from perdeuterated benzene [12], permitted the identi- fication of this phenyl group and showed that its struc- ture arises mostly from a dipolar coupling between the phenyl deuterons and their neighbouring protons. This last point will not be considered any further.
Link number
FIG. 2. - Variations of quadrupolar splittings AHn of the methylene and methyl groups as their positions are varied along the chain. Except for the first methylene and last methyl groups, the doublets of which were identified unambiguously, the iden-
tification of the other groups is discussed in the text.
In both cases, the following identification of the other doublets might be proposed. The doublet of smaller splitting has an intensity larger than the other doublets and it is obviously to be attributed to the methyl end group. The doublet of larger splitting has also a slightly larger linewidth and it may correspond to the less mobile methylene group, presumably, the first one on the chain. The remaining doublets arise from intermediate methylene groups (2 for TBd-BA, 6 for BOBd-OA). A reasonable assumption for their identifi- cation might be that the averaging effects due to the motions are more and more effective going from the phenyl ring towards the methyl end (as shown on Fig. 2). If such an assignment is a reasonable guess, no decisive argument supports it yet. A second possibility, giving a stronger alternating behavior, as calculated in reference [6], could be derived from the above one by permuting each odd methylene group with its preceding even neighbor. These points will be discussed else- where [13]. An exact labelling would need a step by step deuteration of each methylene group.
3. Chain ordering. - 3.1 NEMATIC PHASE. - AS developed in the first footnote (I), in case of uniaxial symmetry, the measured splitting is proportional to the order parameter of the C-D bond with respect to the symmetry axis of the sample. Variations of methylene splittings all along the chains are given in figure 2 for both systems in corresponding states. In both cases, the variations of the chain order relative to the macrosco- pic axis of symmetry (2) are similar qualitatively. After the brutal decrease going from the first methylene to the second, the splitting variation becomes slower, then drops abruptly for the methyl end group.
The small value of the observed splittings, compared with the static quadrupolar coupling constant of C-D bonds (I), reveals strong motional narrowing effects.
But the low variation of these splittings, in the middle of the chain, rules out the simple model of an isolated chain fixed at one end, distorted with trans-gauche isomerizations, for which each link is expected to have a greater orientational freedom than the preceding ones.
The already quoted theory of MarEelja [6] takes the many-chains problem into account through a self consistent molecular field approximation. Unfortu- nately, a direct comparison is not yet possible, the computation of the order parameter having been given for an alkoxy chain whereas the experiments were made with alkyl chains. Nevertheless, if the C, alkoxy and alkyl chain are compared, there is a satisfying agree- ment as far as the mean value of the order and the mean slope of decrease in the middle part of the chain are considered. However, one should notice that the calculated decrease of the order between the first and
( 2 ) The methylene order parameter would be better defined in
a frame of coordinates related to the rigid aromatic core ; extracting such information from our results requires the precise knowledge of the molecular axis of symmetry. This determination is underway for the two molecules under study.
C1-24 B. DELOCHE, J. CHARVOLIN, L. LIEBERT AND L. STRZELECKI
second carbon atoms (just after the oxygen atoms) of the alkoxy chain is faster than the observed one at the same positions of the alkyl chain, i. e. between the second and third carbon atoms., This could be due either to possible variations all along the chain of the molecular interaction constants introduced by Mar- Eelja, or to steric hindrance between the protons of the aromatic ring and the deuterons of the second methy- lene group of the chain. Lastly, the measured drop for the methyl end group^ is not apparent in theoretical results ; this is to be related to an even-odd effect and will be discussed elsewhere [13].
3 . 2 SMECTIC PHASES. - Chain ordering in the smectic phases has not been yet studied theoretically from this point of view. From the order and free volume changes at the transitions, one could expect some alterations of the molecular interaction constants.
The chain ordering would have to change accordingly going from nematic to smectic phases. The measured variations of the normalized methylene splittings in the different mesophases of TBd-13A are shown on the figure 3. Surprisingly, the general aspects of the
L ~ n k number
FIG. 3. - Evolutions of the normalized splittings of the methy- lene groups along the TBd-BA chain in different mesophases.
The horizontal scales relative to each curve have been shifted by constant amounts with respect to each other.
decreases are similar whatever the phase and so the chains appear hardly more mobile in the nematic phase than in the smectic H phase. The behavior of BOBd-OA chains, which is not represented here, is quite analo- gous. A common striking feature appears : with our proposed identifications of the spectral lines, there is a tendency, when decreasing the temperature, for odd methylene groups to have the same splitting as their preceding even group. This might also be an even-odd effect, the corresponding C-C bond being oriented nearly parallel to the molecular axis of symmetry preferentially.
4. Rotation in the smectic planes. - The molecules have been previously oriented with their long axis closely parallel to the external magnetic field. If this
orientation is maintained, rnolecular rotations around this direction will be hardly detectable. However, as shown in footnote (I), reorientation of the smectic sample in the magnetic field will yield much different angular dependences for the deuteron splittings if a fast molecular rotation is present or not. In this respect, we have investigated the behavior of the quadrupolar splittings in the smectic phases as the TBBA sample is oriented at various angles relative to the magnetic field (to obtain simple, easily interpretable spectra, the molecule used in this study has been the one deuterated only on the phenyl ring of the butylaniline).
4.1 UNIAXIAL SMECTIC A. - The rotation of the molecule around a long axis close to the phenyl para axis is evident from previous proton magnetic reso- nance studies [5, 141. This has been verified in this DMR study for the butylaniline group ; the angular variation of the splitting fits well the (3 cos2 9 - 1) law. This of course agrees with the uniaxiality of the mesophase.
4.2 BIAXIAL SMECTIC C AND H. - The direction of a molecular rotation and its description are of primary importance for the understanding of those phases. As already said, their tilt angle and biaxiality might be explained from interactions between dipole moments of the molecules 171. In the smectic C phase, the angular variations of the spectra are unfortunately dominated by the displacements of the molecules at a constant tilt angle in the smectic planes in order to minimize their magnetic energy [14, 151. This may be no longer the case in the much more viscous smectic H phase where the overall splitting obeys coarsely the (3 cos2 9 - 1) law. However, in both mesophases, if the molecules remain aligned along the magnetic field, it has been proposed [16] that some features of the spectral splittings and linewidths may be interpreted as a result of biased rotation around the long molecular axis.
4 . 3 METASTABLE SMECTIC VI. - This mesophase is characterized by the existence of an orientational
T B B A
Smectlc Kl pt,asc
q;&N=cHacH=N+$cp9 T= 80°C
D D 0 D
Magnetic field (gauss) from 20 kgauss
FIG. 4. -The TBBA sample is in the smectic VI metastable phase. Evolution of the DMR spectrum of the outer phenyl rings when the magnetic field is turned from parallel (a) to perpen-
dicular (b) to the long molecular axis.
DMR STUDY OF MOLECULAR ORDER IN TBBA C1-25
order which can be described as a herring-bone type arrangement of the molecules within a smectic plane [8]. As shown in figure 4, the angular variation of the splitting no longer obeys the (3 cos2 cp - 1) law, on the contrary, the splitting increases and a structure appears when the magnetic field is turned from parallel to perpendicular to the long molecular axis. Such an observation is inconsistent with an assumption of a fast rotational averaging effect around this axis. It can be interpreted assuming either, that the molecules are slowly rotating ( I ) or, that they are immobilized with a distribution of their azimuthal orientation in the smectic layers of the sample. Such a distribution, suggested by the structure observed at cp = 900, might arise from the fact that if the molecules are parallel, the smectic planes are not [14, 151. This second interpreta- tion, assuming the freezing of the molecular rotation, is also consistent with the structure obtained from X-ray data in the metastable smectic VI phase of TBBA.
5. Conclusion. - Deuteron magnetic resonance experiments on selectively deuterated TBd-BA mole- cules have been presented. The spectra are characte- rized by an interesting resolution due to the negligible dipolar interactions between deuterons ; methyl, methylene and aromatic groups are resolved in doublet structures of quadrupolar origin. In phases with uni- axial symmetry, the observed splitting, AH, is propor- tional to the orientational order parameter of the corresponding C-D bond relative to the macroscopic symmetry axis. All our results for TBd-BA have been extracted from the figure 5 where the splitting varia- tions with temperature are shown. Besides the general features relative to different phase transitions, we can find here the main points of our discussion. The phenyl and first methylene splittings vary with an almost constant ratio and this confirms the identification of the latter. On the contrary, the splittings of the other methylene and methyl groups decrease more rapidly than the phenyl one, when the temperature increases ; this determines how much the rigid rod picture is misleading for the TBBA molecule. This is confirmed by studying the BOBd-OA molecule which has a longer alkyl chain (C8 instead of C,). A striking feature, in both cases, is that only very weak changes in the chain order are observed through the different mesophases.
are not so conclusive, probably because of the small effect expected from biasing or partial freezing of the rotation in these phases. A direct investigation of the methine group would be of great importance in the understanding of these much discussed biaxial phases.
Finally, from a more general point .of view, all the phase transitions presented by TBBA appear, on figure 5, to be first order except for the Sm,-Sm, one which might be suggested to be second order within the limit of our experimental accuracy.
Temperature ( O C )
FIG. 5. - Variations of the methyl, methylene, phenyl deuteron splittings as the temperature is varied across the different meso- phases of TBd-BA. The splittings of the first methylene group on the chain (V) are not given at low temperatures, but they present also discontinuous changes at Smx-Smc and S ~ A - N e m transitions. Two splittings are observed for the deuterons of the phenyl group which correspond to the quadruplet structure due to a dipolar coupling between these deuterons and their neigh-
bowing protons (see text and Fig. 1).
A strong support for molecular rotation freezing out
can be presented for one metastable smectic TBBA Acknowledgments. - We thank Dr. G. Emptoz phase ; on the other hand, the experimental data (Laboratoire de Chimie organomCtallique, Orsay) wbo actually obtained in the smectic C and smectic H phases controlled the deuteration steps.
References
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1121 We are very grateful to Dr H. Hervet (Physique de la Matiere condens&, College de France, Paris) for supplying this sample.
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