INTRAMOLECULAR ROTATIONS IN TBBA

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INTRAMOLECULAR ROTATIONS IN TBBA

J. Charvolin, B. Deloche

To cite this version:

J. Charvolin, B. Deloche. INTRAMOLECULAR ROTATIONS IN TBBA. Journal de Physique Col-

loques, 1976, 37 (C3), pp.C3-69-C3-72. �10.1051/jphyscol:1976310�. �jpa-00216493�

INTRAMOLECULAR ROTATIONS IN TBBA

Abstract. — Mesogen molecules have been considered as rigid objects. However, recent deute- ron magnetic resonance experiments (DMR) have shown that the end chains of those molecules can undergo deformations through oscillations or isomeric rotations around the C—C. bonds. We discuss here the occurrence of similar deformations in the complex aromatic core of TBBA which has three phenyl groups. At the moment DMR fails providing a direct proof of such intramolecular motions but their existence cannot be eliminated owing to the weak value calculated for the rotatio- nal energy barrier around the para axis of the central phenyl group.

1. Introduction. — Relations between chemical struc- ture and liquid-crystalline polymorphism have been discussed by several authors during this conference : the steric features and the dipole moments of the chemical groups have been taken into account. The relative mobilities of those different groups have to be

considered as well : several molecular shapes and moments, arising from isomeric rotations, can be associated to a single chemical structure. Such can be the case for the well-known liquid-crystal terephthali- dene-di(p-butylaniline), or TBBA, which exhibits the following phases [1] :

Isomeric rotations in the aromatic core around the phenyl para axis, could lead to very different represen- tations of the molecules as shown in figure 1.

One can see that the orientations of the molecular groups with respect to the long axis of alignment of the molecule in the liquid crystal are different, the molecular width in the plane of the figure is smaller by about 8 % for (1) than for (2), the transverse dipole moment expected from the C = N bonds is smaller for (1) than for (2). Such changes could favour the forma- tion of one phase rather than another.

We have tried to look for those conformational changes using magnetic resonance which could be a very sensitive method to detect desorientations of internuclear vectors.

FIG. 1. — Two planar representations of TBBA. In (1) the transverse components of the C=N dipoles are antiparallel, while they are parallel in (2). The methine and phenyl CH bonds are assumed to be at 60° from the para axis, as usual. The angle a between the para axis and the alignment axis of the molecule in the liquid crystal is of the order of 10° for (1), and is null for (2).

(*) Laboratoire associe au C. N. R. S.

JOURNAL DE PHYSIQUE Colloque C3, supplément au n° 6, Tome 37, Juin 1976, page C3-69

J. C H A R V O L I N and B. D E L O C H E Laboratoire de Physique des Solides (*)

Université Paris-Sud, Centre d'Orsay, 91405 Orsay, France

Résumé. — Les molécules de cristaux liquides ont été souvent assimilées à des objets rigides. Des expériences récentes de résonance magnétique de molécules deutérées (DMR) montrent que leurs chaînes terminales peuvent se déformer par oscillations ou rotations isomériques autour des liaisons C—C. On discute ici la possibilité de déformations analogues au niveau des trois noyaux phényls du cœur aromatique de TBBA. Si la D M R ne fournit pas actuellement de preuve directe pour de tels mouvements on ne peut pas les éliminer si on considère les faibles valeurs fournies par les calculs de barrière de rotation autour de l'axe para du phényl central.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976310

C3-70 J. CHARVOLIN AND B. DELOCHE

2.

Method. -

In order to distinguish between diffe- rent chemical groups we have deuterated the TBBA molecule, either on the alkyl chains or on the lateral phenyl rings [2]

;

our results will be compared with those of Luz, Hewitt and Meiboom who have studied a complementary molecule deuterated on the azomethine and the central ring [3]. We recall that the doublet splitting measured by deuteron magnetic resonance (or DMR) gives access to the orientational order parameter < P,(cos 0) > of the C-D bond with respect to the external magnetic field, through modu- lation of the static quadrupolar coupling

(-

180 kHz) by the molecular motions. In most liquid crystal cases the molecules are considered undergoing deformations while rotating around their long axis. If those motions are uncoupled the measured order parameter can be shown to be the product of two terms related to the dynamics of the liquid crystal (as shown in the DMR study of the alkyl chain of

cr

40.8

⁾⁾

[4]). One concerns the collective dynamics, it is the usual order parameter describing the orientational fluctuations of the long molecular axis with respect to the anisotropy axis. The other concerns the intramolecular dynamics, it is the order parameter describing the motion of the C-D bond under interest relative to the long molecular axis.

3. Discussion. -

3.1

AROMATIC CORE. -

The DMR spectra obtained by Luz

et al.

[3] and by our- selves, are shown in figure 2.

symmetry plane of its phenyl rings, or a molecule with phenyl rings rotating around their para axis whatever the axis of combined rotation is. For instance, this eliminates the usual representation (1) for its long axis is different from the para axis, whereas the representa- tion obtained from (1) by a

7r -

rotation of the phenyl

2

planes around the para axis could do because the overall rotation axis is then in the symmetry plane of the phenyls. Representation (2) could be convenient for the axes are parallel, but phenyl and methine deuterons would exhibit similar doublets (the angles between C-D bonds and the para axis are almost equal) and this is not observed. Those limited exam- ples show the need for a discussion of rotations in the aromatic core, particularly around the para axis of the central ring.

Due to uncertainties in our knowledge of bond angles [5] and static quadrupolar coupling constants we can not rely too heavily upon a direct exploitation of DMR results to choose between situations where the rotation axes differ by no more than 150. We have then looked for information in calculations of energy barriers, when rotations around para axes take place, considering that if they are of the order of

k ,

T (about 1 kcal/mole in the mesophase range) the existence of different core conformations and exchanges between them might be foreseen. Semi empirical INDO and PCILO computations were used to determinate the conformational energy of the simple Schiff's base shown in figure 3 [6]. The barrier for the B rotation is I

Nernat~c phase T = 205OC

FIG. 2. - DMR spectra of the aromatic core of TBBA in nema- tic phase by Luz et al. (L) from reference [3], by Deloche et al.

(D) from reference [2]. Each doublet is the signal of a definite chemical group ; L1 azomethine, LZ central phenyl, Dl lateral phenyl. (The first spectrum (L) is the derivative of a n absorption signal and the structure observed for Lz is due to dipolar cou- plings between deuterons, the second spectrum (D) is a pure

absorption mode.)

One can see that the doublets of the deuterons of every phenyl are all similar and that the doublet of the methine deuterons is twice as split as that of the phenyl ones.

The equivalence of the phenyl deuterons, observed regardless of the phase, can be explained considering either a molecule with an overall rotation axis in the

FIG. 3. -The benzilidenaniline molecule and the rotation angles 0 and yl.

found to be 5 kcal/mole when phenyl and azomethine

are coplanar

[7].

and that of the

^q

rotation is 0.3 kcall

mole with INDO, 2 kcal/mole with PCILO, when

phenyl and azomethine are perpendicular. Moreover

the 0 and

p

rotations appear independent 161. The

barriers, particularly the second one are not very strong

compared to

k , T

but this is a computation for a

molecule which is coarsely half the TBBA one. In the

TBBA molecule, one might think that couplings

between C=N dipoles,

.n

electron conjugations, could

strengthen the barriers. We have therefore developed a

conformational analysis of the isolated TBBA mole-

cule using the semi-empirical CNDO method

[8].

We

have studied only the rotation around the C-C bond

between the central ring and one azomethine. The

chosen geometries for each part of the molecule, on

each side of the rotating bond, are those found in the

INTRAMOLECULAR ROTATIONS IN TBBA C3-71

crystalline phase [9] (this point is in fact unimportant term could help to explain an observation not inter- considering the independence of

8

and

cp

rotations in preted by Luz

et

al. [3] : the ratio R between the benziliden-aniline). The total energies and dipolar methine and phenyl splittings decreases when the moments, obtained for four conformations, are shown temperature increases, as shown in figure 4.

on table I, where

@

is the angle between azomethine This fact could be due to a rapid exchange between

planes. different molecular conformations, the extrema of

which are representations (1) and (2), in the liquid TABLE I crystalline phases. This motion modulates the orienta-

@

(degree) 7 9 tiins of the CD bonds relative to the long axis from

Energy 0 -0.1 0.1 0

90 60 +

a

to 600 for the methine deuteron and from (kcal/mole) 60

- a

to 60 +

^a

for the phenyl ones (as seen on Moment X

-

0.06

-

0.04 - 0.09

-

0.05 figure 1). The averaged value of the quadrupolar (Debye) Y 3.48 3.51 1.96

-

0.04 interaction, i. e. the doublet splitting, is then larger

Z -

0.60

-

0.10 1.82 0.20 for the methine than for the phenyl and R > 1. In this picture the variation of R with the temperature

he computation appears quite relevant if one considers the size and variations of the dipole moments.

On the other hand, the variations of the total energy, while changing the relative orientation of the two dipoles C=N, are almost negligible compared to k , T, and this is due to an almost exact compensation bet- ween electronic and nuclear energy terms. For an isolated TBBA molecule, in the temperature range of the mesophases, those computational data then sug- gest that the rotations around the para axis of the central ring are rather easy, almost free following the INDO and CNDO methods.

In the actual liquid crystal one has to take into account the presence of neighboring molecules and intermolecular constraints could intervene to modify the potential of internal rotations. F o r example mole- cule in representation (2) needs more lateral space than that in representation (1). In other words, the energy of (2) would be higher than that of (1) in dense phases owing to an intermolecular energy term. This

arises from the variations of the statistical weights of the conformations. When the temperature increases the intermolecular constraints become less stringent, conformations of type (2) become more probable and R decreases because the methine and phenyl splittings tend to be similar in such molecular states.

The simple notion of molecular rotation around a long axis is no more obvious now. When only one conformation is present, perhaps (1) at the bottom of smectic H, there is no difficulty speaking of combined rotation around the long axis. On the contrary there is no sense speaking in these terms when internal rotations take place on times shorter than that of the overall rotation, such a situation could be met in the high temperature phases.

3.2 ALKYL

^CHAINS.

- We shall just recall a few experimental results [2]. DMR spectrum clearly distinguishes between the different methylenes and methyl of the butyl chain as shown in figure 5.

Temperature ( O C )

FIG. 4. - Quadrupolar splittings of the deuteron resonance as function of the temperature. The lower curve applies to central ring deuterons,,the middle one to the methine deuterons, and the

top curve plots their ratio R (from ref. [3]).

Nematic phase T = 210 OC

Frequency (kHz) from 13 MHz

FIG. 5 .

-

13 MHz DMR spectrum obtained from a TBBA molecule with deuterated alkyl chains in the nematic phase. Only half the spectrum which is symmetrical around zero frequency

is shown.

The degree of motion averaging increases from the (CD,),, attached on the ring towards the methyl end.

Similar conclusions have been got from proton NMR

C3-72 J. CHARVOLIN AND B. DELOCHE

studies

[lo].

This indicates that the chains are not in a

rigid all-trans conformation in the liquid crystalline phases but, in the lack of any accurate model, we cannot define which type of deformation takes place, oscillations and isomerizations around the C-C bonds.

DMR and EPR studies

[ l l ]

show also that chain deformations appear at the crystal-liquid crystal transition. At last, if one refers to energy considera- tions and spectroscopic studies of ethylbenzene

[12]

one can think that the chain sticks out of the phenyl plane preferentially, because the energy increases by 5 kcal/mole when the ethyl group is in the phenyl plane.

4. Conclusion.

-

In this paper, we discussed mainly the occurrence of deformations in the aromatic core of TBBA in liquid crystalline phases. Despite the absence of any direct proof we think we have presented some

~ircunstancial evidences favourable to the idea of

intramolecular rotations around the para axes of the aromatic core. Following this idea, the molecule can be no more assimilated to a rigid cylindrical rod the shape of which is unvariant throughout the liquid crystal range. Instead it has to be thought of as an averaged molecule which geometry can change in order to fulfil the requirements for the existence of a determined phase, and the average is to be taken over all conformations compatible with the structure of the phase. In this respect the rich polymorphism of TBBA could owe to the complexity of its double Schiff's base structure and to the weakness of the intramolecular potentials of rotations which could yield easily to the intermolecular constraints.

Acknowledgments. -

B. Mely (Physique des Solides, Orsay) is gratefully acknowledged for his decisive intervention in the exploitation of the CNDO program

References [I] DOUCET, J., LEVELUT, A. M. and LAMBERT, M., Phys. Rev.

Lett. 32 (1974) 301.

[2] DELOCHE, B., CHARVOLIN, J., LIEBERT, L. and STRZE- LECKI, L., J. Physique Colloq. 36 (1975) C 1-21.

[3] Luz, Z., HEWITT, R. C. and MEIBOOM, S. L., J. Chem. Phys.

61 (1974) 1758, where the results are interpreted assuming an aromatic core in a conformation of type (1) with flip-flop of the central phenyl around its para axis.

[4] DELOCHE, B., CHARVOLIN, J., Specialized colloque AmpBre, Budapest (1975) to be published.

[5] BURGI, H. B. and DUNITZ, J. D., Chem. Commun. (1969) 472, suggests that in the crystalline phases of Schiff's base the angle between the methine CH bond and the para axis could be different from 600.

[6] BERGES, J., PERRIN, H., JAFFRAIN, M., C . R. Hebd. Sian.

Acad. Sci. 281 (1975) C 441.

[7] The non-planarity of these groups in the liquid crystal terephthalidene-di(p-fluoroaniline) was previously sug- gested by : BRAVO, N., DOANE, J. W., ARORA, S. L. and FERGASON, J. L., J. Chem. Phys. SO (1969) 1398.

[8] POPLE, J. A., SANTRY, D. P. and SEGAL, G., A. J. Chem.

Phys. 43 (196.5) 5129.

[9] DOUCET, J., LEVELUT, A. M., Private communication, to be published.

[lo] BODEN, N., LEVINE, Y. K., LIGHTOWLERS, D. and SQUIRES, R.

T., Chem. Phys. Lett. 34 (1975) 63.

[ l l ] POLDY, F., DVOLAITSKY, M. and TAUPIN, C., J. Physique Colloq. 36 (1975) C 1-27.

[12] SIMONE-ITA, M., CALCAGNO, B., SANTI, R. et SCHWARTZ, P., Chim. Znd. 55 (1973) 223.